Review ArticleConverting a Microwave Oven into a Plasma Reactor A Review

Victor J Law and Denis P Dowling

School of Mechanical and Materials Engineering University College Dublin Belfield D04 V1W8 Dublin 4 Ireland

Correspondence should be addressed to Victor J Law viclaw66gmailcom

Received 21 February 2018 Accepted 17 April 2018 Published 21 May 2018

Academic Editor Michael Harris

Copyright copy 2018 Victor J Law and Denis P Dowling is is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in anymedium provided the original work isproperly cited

is paper reviews the use of domestic microwave ovens as plasma reactors for applications ranging from surface cleaning topyrolysis and chemical synthesis is review traces the developments from initial reports in the 1980s to todayrsquos converted ovensthat are used in proof-of-principle manufacture of carbon nanostructures and batch cleaning of ion implant ceramics In-formation sources include the US and Korean patent office peer-reviewed papers and web references It is shown that themicrowave oven plasma can induce rapid heterogeneous reaction (solid to gas and liquid to gassolid) plus the much slowerplasma-induced solid state reaction (metal oxide to metal nitride) A particular focus of this review is the passive and active natureof wire aerial electrodes igniters and thermalchemical plasma catalyst in the generation of atmospheric plasma In addition tothe development of the microwave oven plasma a further aspect evaluated is the development of methodologies for calibrating theplasma reactors with respect to microwave leakage calorimetry surface temperature DUV-UV content and plasma ion densities

1 Introduction

Since the 1990s tabletop domestic microwave ovens havebeen converted into plasma reactors and used for a widerange of manufacturing applications e common featurein these reactors is that they contain a multimode resonantcavity (MRC) which is illuminated through one sidewall ofthe cavity using a rectangular transverse electric (TE10)waveguide with an interior waveguide aspect ratio of 2 1that houses a packaged cavity magnetron operating in the245GHz range Using this configuration no furtherimpedance-matching apparatus is used between the mag-netron and MRC

As these types of microwave oven plasma reactors ex-ploit dielectric heating and plasma chemistry it is worthnoting that dielectric heating of organic materials has a longand established history ranging from medical therapeuticuse (short-wave diathermy) in the 1900s [1] and demon-strations of food cooking at the 1933 Chicago Worldrsquos Fair[2] to the first microwave cooking of foodstuff with patentapplication being filed in 1945 [3] followed by the firstcommercial microwave cooker built and sold by Raytheon in1947 and Amana in 1967 [2 4] ese ovens were of limited

commercial success due to their bulkiness and cost butcommercial success came later when the cost-effectivepackaged cavity magnetron became available [5 6] Al-though a combination of microwave heating and chemicalreactions were reported in the early 1980s no large-scaleoven production was done until rapid synthesis of organiccompounds in microwave ovens was performed in 1986[7 8] More recently (2017) carbothermic reduction of zincoxide and zinc ferrites has also been reported [9] Once thefirst conversion of a microwave oven into a plasma reactorwas reported in 1978 [10] plasma-induced synthesis ofinorganic compounds became available [11ndash13] followed byplasma modification of polymer surfaces [14] Interest in theconversion of microwave oven for plasma processing hasalso been reported for plasma pyrolysis paper [15 16] andin-liquid plasma decomposition to produce hydrogen gasand carbon films [17ndash21] More recently initial studies ofmarine diesel exhaust gas abatement within a convertedmicrowave oven have been reported [22] however little gas-line or reactor conversion details were given

e success of the packaged cavity magnetron andthe rectangular TE10 waveguide as found in the standarddomestic oven has lead to their reuse in more advanced

HindawiInternational Journal of Chemical EngineeringVolume 2018 Article ID 2957194 12 pageshttpsdoiorg10115520182957194

microwave plasma systems that are employed for microwavechemical deposition of diamond-like films [23] in thesemiconductor industry [24] and in microwave plasmasystems that are designed for the dissociation of hydrogenfrom water [25] Plasma reactors based on the microwaveoven have also been built for plasma cleaning of contami-nated ion implant ceramics [26 27] and used for plasmaremoval of photoresistant substances [28] In 2009 the USPatent US 20090012223 A1 describing a cylindrical cavitydriven by a magnetron that generated atmospheric plasmafor the fast food industry was also published [29]

Outside the peer-reviewed journals microwave ovenexperiments as performed in schools have been reportedwhich range from using plasma balls to explore eggs and thecreation of soup sculptures [30] Semiamateur studies on thesubject of microwave oven plasma reactors have also beenwritten One particular article by Hideaki Page in theSummerFall issue of the Bell Jar provides a useful discussionon the practical problems encountered in converting do-mestic microwave ovens into plasma reactors operating atsubatmospheric pressure [31] Two of the problems en-countered are as follows (1) finding a suitable location forcutting into the thin (typically 075 to 1mm) metal sheetwalls of the MRC without causing the metal to bend andbuckle and (2) achieving sufficient vacuum in the jam jars orinverted bowls for the plasma to strike Video postings onhttpswwwyoutubecom also provide graphic informationon domestic microwave oven plasma cleaning experiments[32] Most of the other postings indicate that you would notwant to do them yourself at home Indeed Stanley [33] goesas far as to exemplify many YouTube postings as ldquowacky anddownright dangerousrdquo For completeness five such postingsare given here [34ndash38]

e aim of this paper is to review the technology ofmicrowave oven plasma reactors the plasma chemical en-gineering and the process measurements used Within theworks reviewed here plasma processes have been reportedquoting different pressure values and pressure unitstherefore to ease comparison between the processes theoriginal pressure values along with the equivalent SI unit ofpressure (Pascal) are presented is review paper is con-structed as follows Section 2 presents the technology used inmicrowave oven conversion Section 3 looks at a purpose-built microwave plasma reactor that is based on the mi-crowave oven Section 4 describes the measurements thatare used to calibrate the microwave MRC in terms of mi-crowave leakage calorimetric surface temperature near-field E-probe and plasma ion density measurement Section 5provides a look at the cavity magnetron control drive circuitand finally Section 6 provides a conclusion to this review

2 Microwave Oven Conversions

21 Converted Microwave Oven Plasma Reactors By way ofintroduction it is useful to list the 10 claims in Ribnerrsquos 1989patent [10] that relates to the oven conversion process(Figure 1(a)) In brief the claims are as follows (1) posi-tioning a vacuum chamber within the MRC with embodi-ments for admitting gas into the vacuum chamber and

through the cavity and for extracting gas by-products fromthe vacuum chamber (2) relating to claim 1 where a meansof regulating the gas to generate uniform plasma in thevacuum chamber (3) a moving antenna for a means ofgenerating a time average of uniform plasma (4) a rotatingantenna for a means of generating a time average of uniformplasma (5) a means of reducing microwave leakage aroundeach feedthrough (6) a means of water cooling substrateswithin the vacuum chamber during plasma etching withoutthermal damage to the substrate during the plasma etchprocess (7) relating to claim 6 where the water tubes havea heat transfer relationship with the vacuum chamber witha means of microwave leakage prevention (8) a means ofcontrolling microwave power (9) relating to claim 8 a po-tentiometer in series with the primary transfer side of themagnetron transformer for controlling the maximum powerin the oven (10) relating to claim 9 when plasma etching oforganics from substrate and finally (11) relating to claim 10the use of water cooling of the substrate

Beyond Ribnerrsquos patent some studies [11ndash22] showthat the microwave oven plasma reactor can be used fora multitude of processes and in many levels of oven re-construction e following sections describe the changesrequired to conventional domestic microwave ovens thatrange from minimal to major

211 e Use of Replaceable Reaction Vessels An example isthe rapid synthesis of phase pure K3C60 [11] and alkali-metalfullerides [12] in replaceable reaction vessels Only minorchanges to the conventional oven are required such as theprovision of providing supports for positioning the reactionvessel at the node or antinode of the microwave field nor theneed for a rotating table or a moving (or rotating) antenna asthe objective of the plasma process is to focus the microwaveenergy onto the sample (Figure 1(b)) In this case the sampleswere prepared in an argon-filled Pyrex vessel and then po-sitioned using fire bricks at the node or antinode of themicrowave field e plasma process time is however limiteddue to the fixed amount of residual gas in the reaction vessel

212e Use of Replaceable Desiccators Ginn and Steinbock[14] have reported oxygen plasma cleaning of poly(dime-thylsiloxane) surfaces within a replaceable desiccator thatincorporated a steel electrode to promote plasma ignition(Figure 1(c)) e samples are prepared outside of the mi-crowave oven and then placed in the desiccator which ispurged with oxygen for 2 minutes and then evacuated toa pressure of about 10minus3 Torr (0133 Pascal) When placed inthe oven and the microwave power (1100W) is turned onthe steel wire electrode generates a spark to initiate theoxygen plasma Here again the plasma process time islimited but the use of a steel electrode is found to promotethe reaction to end e subjection of the wire electrode isdiscussed further in Section 217

213 Pumping through the Wall In 2010 Singh and Jarvisreported the generation of carbon nanostructures fromwithin

2 International Journal of Chemical Engineering

a continuously pumped 3-port reaction flask (made fromborosilicate glass and 1000ml volume) that was held withinthe microwave oven [17] To support the vessel and facilitateaccess to it the oven door was replaced with an aluminumplate of the same size that has three apertures one for eachflask port With the flask supported the flask was evacuatedfrom the outside using one port while the other two ports areused for the carrier gas and the selected hydrocarbon pre-cursor gases (either ethanol xylene or toluene) To enhance

the reaction a 2mmdiameter aerial electrodemade fromNiloKreg (Ni 29 Fe 53 and Co 17) wasmounted on a stainlesssteel base within the reaction flask (Figure 1(d)) As novacuum pressure or microwave power was reported it mustbe assumed that the flask was subatmospheric and the mi-crowave power was at a maximum (1000W) Neverthelessusing this approach no other modifications to the oven wereneeded Two variants of this approach which preserve thedoor access are found in the work of Page [31] who drilled

Patent US 4804 431

Timer

Power

Antenna

Vacuumchamber

Gas in

Gas out

(a)

Replaceable reaction vessels

Timer

Power

Reactionvessel

Fire bricks

(b)Desiccator with steel wire

Timer

PowerDesiccator

(c)

3-port reaction flask fit to door plus steel aerial electrode

Timer

Power

Reactionflask

(d)

Coaxial narrow

Timer

PowerReactiontube

Reaction tube

(e)Waveguide reactor

Timer

PowerWave guide

Dum

my

load

(f)

Figure 1 Front view schematic of converted microwave ovens Each oven is scaled to the US 4804431 patent oven and for clarity theauxiliary gas lines and vacuum systems outside the ovens are not shown (a) Patent US 4804 431 (b) Replaceable reaction vessels(c) Desiccator with a steel wire (d) ree-port reaction flask fitted to door plus steel aerial electrode (e) Coaxial narrow reaction tube(f) Waveguide reactor

International Journal of Chemical Engineering 3

through the bottom of the cavity and Tallaire who drilledthrough the side of the cavity In the latter case TallairersquosYouTube posting provides an example of plasma cleaning ofa microscope glass side [32]

214 Coaxial Narrow Tube Reactor Khongkrapan et alhave reported a converted microwave oven for pyrolysis ofpaper to produce gaseous waste by-products at 800W[15 16] In their reactor the process occurs inside a cylin-drical quartz tube (internalexternal diameters of 2730mmand length of 250mm) that coaxially passes verticallythrough the MRC Air or argon is used as the precursor gasat a nominal atmospheric pressure (1013 kmiddotPa) with the gasflowing from bottom to top of theMRCe shredded paper(5 g) is suspended in the center of the tube (Figure 1(e)) In[16] Khongkrapan et al state that an igniter was placedwithin the tube to generate the plasma but no direct detailswere given Upon further reading of their reference list(reference 17 in their paper) a simple cartoon showing theigniter positioned within the tube is given again without textexplanation e subject of igniters in the form of a metalantenna is discussed in Section 217

215 Internal Waveguide In 2004 Brooks and Douthwaitepresented their internal waveguide fitted to an 800W do-mestic microwave oven for plasma-induced processing ofpowered metal oxides (Ga2O3 TiO2 and V2O5) into binarymetal nitrides formed in ammonia (NH3) plasma [13] Inthis design a slot is cut at the rear of the MRC to allowa 20mm internal diameter U-shaped tube containing thesolid-state sample within an alumina boat to be positionedwithin the microwave field (Figure 1(f)) Outside of theMRC one end of the U-shaped tube is fitted to a vacuumpump and the other end is fitted to the carrier and processgases To prevent microwave leakage at the rear of the ovenextensive gasket and Faraday shielding were arranged Aninternal waveguide is then fitted to the MRC iris in sucha way to focus the microwave energy in the vicinity of thesample Furthermore to prevent reflected power damagingthe cavity magnetron and overheating the waveguidea water-cooled dummy load is fitted to the waveguide outputaperture With these extensive oven conversions the plasmaregion may be considered to be operating in the coherentmode rather than in the multimode Typically plasma pa-rameters used to convert the metal oxides to nitrides are anNH3 gas flow rate of 113 cm3middotminminus1 pressure of 20mbar(2000 Pascal) and microwave power of 900W for a plasmaexposure time of 25 to 6 hours

216 Liquid Plasma Vessels Microwave in-liquid plasmadecomposition of n-dodecane (molecular formula C12H26(I))to simultaneously produce hydrogen gas and carbide inthe hydrocarbon liquid has been achieved using a convertedmicrowave oven at a reportedmicrowave power level of 500 to750W [18ndash20] A typical representation of these reactors isshown in Figure 2 e reaction is performed in a closedvolume Pyrex reaction vessel containing 500ml of n-dodecane

liquid with one or more electrodes where the electrode(s) canbe either single-tip steel wire electrodes or copper U-shapeddual-tip aerial electrodes Also two siliconPTFE tubes areinserted from the top of the cavity one tube is used for sendingthe carrying gas (argon) as the precursor gas and the secondtube is used to collect the spent argon and the by-product gasat a working pressure close to atmospheric pressure

To understand the purpose of these electrodes the re-action efficiency of both types of electrodes is examined asa function of the geometry and the number of electrodes inthe context of their electromagnetic design and heteroge-neous reaction kinetics

First consider the single-tip electrodes [18ndash20] esemetal electrodes have a dimensional length of L 21mm anda diameter of 15mm and they are fixed vertically in a singlearray (Figure 3(a)) or in a multiple array (Figure 3(b)) with 1electrode in the center and up to 6 electrodes circum-ferentially spaced at a gap separation of λm4 where λm is thewavelength of the microwave radiation passing through themedium e wavelength calculation is given in the fol-lowing equation

λm simC

fmiddot1εr

radic (1)

e approximate expression in (1) is used as the oper-ation frequency of the free running cavity magnetron isfrequency pulled by changing SWR conditions in therectangular TE10 waveguide in which the magnetron ismounted All other symbols have their normal meaning C isthe speed of light (299792times108mmiddotsminus1) f is the magnetronoperating frequency (245GHz) and εr is the medium inwhich the radiation is passing through us for the liquidn-dodecane (εr 178 to 2) λm approximates to 885 cm andλm4 approximates to 22 cm

Based on the works [18ndash20] and the work of Pongsoponet al [21] it is generally considered that the electrodeshave three well-defined roles to confine the plasma to theimmediate proximity of the electrode(s) tip to function as

Timer

Power

Tubes (silicon)

Antenna

Stand (heat resistance glass)

Liquid

Reaction vessel(heat resistance

lass)

Figure 2 A typical front view schematic of a converted microwaveoven for liquid processing For clarity the auxiliary gas linesoutside the ovens are not shown

4 International Journal of Chemical Engineering

a catalytic source for plasma heterogeneous reaction and inthe case of manufacturing carbon nanomaterials to providea substrate on which the carbon material can grow In thefirst of these roles increasing the number of electrodes from1 to 6 has revealed that the efficiency of plasma de-composition of n-dodecane does increase but beyond 6-7electrodes the reaction efficiency becomes rate-limited ismay be due to electromagnetic power loss by the resonantstructure of the electrodes [20] or simply that the addition ofmore than 7 electrodes and their associated surroundingreaction zones (cylindrical volume around each electrodeFigure 3(c)) within a fixed closed volume simply producesa loading effect within the heterogeneous reaction [39] atis to say as the percentage of the combined electrode re-action zones approaches the total fixed volume the amountof fresh reactant flowing to the electrode reaction zonebecomes reduced erefore mass transport in and out of

each electrode reaction zone rather than plasma de-composition may become the rate-limiting step To clarifythese observations further investigation is needed

For the dual-tipped aerial electrode Toyota et al [20]have shown that the U-shaped aerial electrodes have distinctoptimum lengths of L sim 2λm 3λm2 λm and λm2 ey alsoshow that the use of the approximation sign in (1) is justifiedby experimentally determining the λ2 FHHW length ofthe U-shaped dual-tip aerial electrode to be 44 to 47 cm forn-dodecane

217 Igniter e description of construction and use ofwire aerial electrodes for plasma ignition is now used as anaid to outline the construction of the plasma igniter [16] andthe drawing in [40] (Figure 4) Assuming that the drawing in[40] may be scaled the plasma igniter may be constructed intwo ways Firstly the igniter may be constructed using twowire electrodes opposing each other and bent at 45deg so thattheir tips are aligned with the gas flow and the fixing lo-cation is formed using an insulating ring e second andmore practical arrangement is that the igniter is preformedfrom a 30mm diametertimes 05mm thick steel steel disc anda plurality of electrodes are punched from the centralportion of the disc and bent to a 45deg angle For the purposeof this second option the construction of a 4-electrodeigniter is exemplified using the 2730mm internalexternaldiameter glass tube in [16] as a reference tube (Figure 1(e))A schematic of the manufacturing stages of the igniter isgiven in Figure 4 where it is shown that the first stage is topunch out the form of the igniter the second stage is tobend the electrodes and the third stage is to align theigniter to the glass tube Using this method of constructionthe lip of the preform can self-align to enable the 4 aerialelectrodes to suit the plasma ignition criteria as describedin Section 215

218 Production of Plasmoids (Fireballs) e production ofplasmoids sometimes called fireballs or ball-lightningwithin domestic microwave ovens has been posted onYouTube postings [34ndash38] Perhaps the simplest way ofproducing a fireball without modification to the microwaveoven is to place a partially sliced grape (that has its two halvesconnected via a thin piece of skin) in the microwave ovenand then turn on the microwave power for 3ndash10 secondse YouTube posting [34] shows that arc-like plasmoids aregenerated at the thin skin bridge that connects the two grapehalves with the discharge emission continuing until eitherthe power is turned off or the grapes have shriveled up isaction may be understood by considering that the twofreshly cut grape halves have a characteristic dimension of15 to 2 cm and are partially filled with a conducting elec-trolyte the combination of which creates an organic con-ducting dipole antenna not unlike the metal antennasdiscussed in Sections 215 and 216 Given this un-derstanding it is reasonable to assume that as the freeelectrons are pushed back and forth through the narrow thinskin bridge of the grape heat is generated due to the

Coppersheet Glass and so on

Metal electrode

L

(a)

λ4

λ4

(b)

Electrode reaction zone

(c)

Figure 3 Typical single-tip electrode arrangement (a) multipleelectrode arrangements (b) electrode reaction zone (c)

International Journal of Chemical Engineering 5

resistance and burns away the skin In addition themovement of electrons through the grape electrolyte inducesa rapid increase in temperature causing vaporization of theelectrolyte to a cloud of electrons and ions thus forming thelocalized plasmoid e plasmoid continues to be sustainedas long as the free electrons are available from the dimin-ishing volume of the grape electrolyte

Moving away from the organic source for generatingplasmoids a lighted safety match supported by a wine corkcovered with a glass jar and placed within the center of theMRC can also be used [35] Upon turning on the micro-wave power a plasma discharge is generated that rises to thetop of the jar thus forming a buoyant plasmoid Warren[36] used a similar approach but this time using a glassjar supported by three wine corks and a lighted cigaretteplaced in the gap provided by the corks In this work andthe previous example the plasmoids are maintained whenthe thermal source is extingusihed It is only when themicrowave power is turned-off does the plasmoid becomeextinguished Plasmoids can also be generated withinelectric light bulbs and fluorescence tubes as shown in [37]this example also appears to be the basis for the near-fieldE-probe (Section 44)

A more dangerous approach to generating plasmoids isdemonstrated in [38] where a cavity magnetron connectedto a food tin can is used to lunch microwaves at a domesticlight bulb to produce a plasmoid within the bulb From thisexperiment it would appear that the electric filament acts asthe initiating electrode

Before finishing this section it is worth noting that thecylindrical plasma reactor produced for the fast food in-dustry [29] employed a patented passive plasma catalyst inthe form of an electrode to ignite the atmospheric plasma[41] where the passive plasma catalyst can include anyobject capable of inducing plasma by deforming the localelectric field On the other hand the patent states that anactive plasma catalyst produces particles or a high-energywave packet capable of transferring a sufficient amount ofenergy to a gaseous atom (or molecule) to remove at leastone electron from the gaseous atom (or molecule) in thepresence of electromagnetic radiation Given these twodefinitions it is reasonable to assume that the safety matchflame [35] cigarette [36] and grape [34] can be classed as anactive plasma catalyst and the metal electrode as a passiveplasma catalyst

219 Plasmoid Food Cooking e Korean patents [42 43]and conference paper [44] report on a form of tuning withinthe TE10 waveguide that fall outside the scope of this reviewbut they are listed for three reasons Firstly the phenomenaof plasmoids extend the cooking range of the domesticmicrowave oven from one of dielectric heating of food stuffto one that provides surface browning and imparting textureand flavor that is similar to the traditional flame-cookingprocess Secondly Jerby et al [44] have noted that plasmoidsproduced in this way require wire antenna electrode to ignitethe plasmoid and therefore may contain nanoparticleswhich might be harmful for the food quality and even makeit inedible irdly the additional use of plasma dischargethat generates ozone and ions for the removal of odor-producing materials from the cooking chamber [45] doesprovide one possible technical route forward in the futuredevelopment of the domestic microwave oven

3 Purpose-Built Microwave OvenPlasma Reactor

is section describes the methodology used in the con-struction of a purpose-built microwave oven plasma reactorOf particular importance in this regard is the MRC seriesof plasma reactors that were built in the mid-1990s atCambridge Fluid Systems Ltd (England UK) e designconcept behind these plasma reactors was to build a simplereliable and cost-effective table-top plasma reactor that couldbe sold to research laboratories and low-volume productionsunits eir main use was for surface engineering enhance-ment in the microelectronic semiconductor sector and themanufacture of bodyshell of Formula One racing cars

e design of the plasma reactor is similar to microwaveovens where the cavity magnetron antenna is locatedwithin a TE10 waveguide that is used to illuminate the MRCthrough a single iris e cutoff frequency (fc)mn of theTE10 waveguide is calculated using the following equation

fc( 1113857mn C

2

m

a1113874 1113875

2+

n

b1113874 1113875

21113971

(2)

where c is the speed of the light and a and b are the internaldimensions (width and height) of the waveguide in this case80 and 38mm are used respectively which equate a cutofffrequency of 1875GHz

30 mm

(1) Punch stage (2) Bend stage (3) Glass tube fitting

Lip

28 mm Glass tube

FIGURE 4 Manufacturing stages of the disc igniter that is suitable for a narrow glass tube reactor

6 International Journal of Chemical Engineering

With the cavity magnetron antenna positioned 26mmfrom the end of the waveguide the frequency and bandwidthof the magnetron are allowed to be free running us thenoncoherent reflected power passing through the iris travelsback to the magnetron thus altering the SWR of the coherentwave within the TE10 waveguide resulting in varying theoutput power of the magnetron

e MRC reactor design differs from the domesticmicrowave oven plasma reactor in the following ways (alsocf Figure 1 with Figure 5)

(i) e chassis MRC and waveguide are constructed asone welded component using 14mm thick mild steelsheet Before each of the three components is weldedtogether they have all the necessary holes punchedand clasp nuts fixed Once welded the structure isnickel plated to produce a metal structure that isrobust with sufficient stiffness to support all theadditional components (the front and rear stainlessflanges gas lines DC power supply pressure gaugeetc) Using this construction approach the MRC hasa theoretical maximum unloadedQ-factor (Qu) in theTE mode that is dependent on the ratio of storedenergy in the cavity (Vc) to the energy loss to thecavity walls (δ times Ac)

Qu 2Vc

δAc (3)

where δ is the electrical skin depth at the cavity wall per cycleand Ac is the cavity wall area

For this reactor the main cavity has an approximatelyQuof 20000 at a resonant frequency of 245GHz

(ii) A cylindrical Pyrex glass chamber (190mm di-ameter 300mm length and 5mm wall thicknessproducing a volume of 3 liters) is located within themultimode cavity with its longitudinal axis per-pendicular to the microwave iris and with the frontand rear of the chamber housed within metal flangesthat form the part of the multimode cavity wall erear flange contains welded vacuum and pressuregauge ports and the front flanges contain the accessdoor is design maximizes the chamber volumeand removes all fragile glass fittings plastic tubeconnectors and feedthrough microwave leakagegaskets

(iii) e gas lines are fitted within the chassis and to theside of theMRC thereby enabling the process gassesto be injected throughmultiple equally spaced radialports in the front flange thus reducing the possibilityof precursor gas being preionized prior to chamberentry and maximizing uniform gas flow and plasmauniformity along process chamber longitudinal axis

31 Plasma Cleaning of Ion Implant Ceramic InsulatorsIon implantation is one of the key processes in the highvolume (220 wafers per hour) manufacture of silicon

semiconductor devices ese ion implant machines how-ever cost between $18M and $3M ese machines are alsohighly maintenance-intensive systems with high capitalcost therefore availability and cost of ownership aremajor factors to be considered Many of the parts changedduring regular maintenance and ion source changes areceramic insulators In this section an overview of theplasma cleaning of ion implant ceramic is described forfull details of the process see [26 27] e plasma cleaningprocess has been performed in the MRC series of plasmareactor using a gas mixture of 5ndash10 O2 in CF with anadmixture of 50 by flow of argon e argon admix isused to stabilize the microwave plasma by moderating theelectron energy distribution and to provide a uniformexcited species throughout the plasma volume eplasma etch chemistry at the surface of the ceramic may beconsidered to proceed by the following representativeheterogeneous reaction

3CF4(g) + 15O2(g) + 2X(s)rarr 2XF3(g) + 3COF3(g)

(4)

in which the addition of O2 scavenges carbon from the CF4through the formation of COFx species to enhance thesteady-state concentration of F atoms in the plasma vo1umee element X in reaction (4) represents the group V ele-ment (As P and Sb) on the ceramic surface and the XF3 arethe etch products Given sufficient microwave power theetch rates of these products are therefore controlled by the

Gas lines CavityMagnetron timer and

power supply

Front view

Pressuremeter

(a)

Gaugeport

Vacuumport

Doo

r

Side view

(b)

Figure 5 Typical front and side view schematic of theMRC plasmareactor chassis and cavity Photograph of the microwave MRC-200plasma reactor

International Journal of Chemical Engineering 7

production of F atoms (O2 balance) product volatility andceramic microscopic surface area

For the MRC-100 reactor typical plasma process pa-rameters obtained were 104W and 10mbar with an etchtime of 45 minutes where the surface temperature of theceramics reached 80plusmn 5K In the case of the MRC-200reactor the process parameters were as follows 200Wand 10mbar (1000 Pascal) with an etch time of 20 to 25minutes where the surface temperature of the ceramicsreached 125plusmn 5K

4 Microwave Cavity Calibration

is section describes a range of different techniques whichare used for the evaluation of the microwave efficiency aswell as leakage

41 Microwave Leakage Measurement e European Di-rective 200440EC and ICNIRP (1998) guidelines recom-mend that industrial microwave ovens have surfacemicrowave (3ndash300GHz) radiation power density levelsgt5mWmiddotcmminus2 and 5 times less for domestic microwave ovensintended for general use Applying a quadratic law based onplane wave theory an operator standing at a distance of20 cm from an industrial oven the operator will receive themaximum allowable power density level of 3mWmiddotcmminus2 Fordomestic oven environment this equates to a power densitylevel of 03mWmiddotcmminus2 In the context of converted ovensintended for plasma many feedthrough and apertures in theMRC require considerable care in design and constructionto prevent microwave leakage

42 Calorimetric Magnetron Power Calibration e mag-netron power entering the MRC may be calibrated using thewater open-dish load method see for example BritishStandard 75091995 and IEC 13071994 Hence given theheat capacity of water is 4184 J(gmiddotK) the calculated appliedpower (P) within the cavity may be obtained by placinga known mass of water (m) within the cavity and heating itfor a short period of time (t) making sure the water does notboil With the knowledge of themeasured water temperaturechange (ΔT final temperature ndash initial temperature) themicrowave power entering into the cavity is calibratedfor a given power setting using the following equation(see also [40])

P mCΔT

t (5)

e calibration however must be considered to be anupper value for plasma processing as its dielectric volumewill be different from the water calibration (Note when theprocess chamber volume or the geometric shape does notallow the open dish method to be used the alternative flowmethod can be used as outlined in [46])

Given (4) the microwave power density (Wmiddotcmminus3) ofa system may be calculated by dividing through by theprocess chamber volume e following open-dish loadmethod for the MRC 100 is given here as an illustrative

example the MRC-100 and MRC-200 reactors have a cal-culated magnetron applied power of 104W which equatesto a power density of 0116Wmiddotcmminus3 For the MRC-200 re-actor the calorimetric measurements produce magnetronapplied power values of 200W (0022Wmiddotcmminus3) and 450W(005Wmiddotcmminus3)

43 Surface Temperature Measurement Knowledge of thesurface temperature of materials immersed in the plasma isuseful in understanding the heterogeneous plasma-surfaceinteraction is is of particular importance when localdielectric heating has the potential of thermal runawaybecause most materials increase their dielectric loss withtemperature [12] Two simple means of estimating localsurface temperature of materials that are immersed in mi-crowave plasma have been used For surface temperaturesbelow 180K liquid crystal temperature-sensitive (20plusmn 5K to180plusmn 5K) strips attached to the plasma immersed surfacecan be used [27] For higher temperatures salts of knownmelting point (KCl 1043K and NaCl 1074K) sealed insilica capillaries have been used [13]

44 Near-Field Plasma E-Probe Measurement Attempts tostrike a plasma outside the ignition pressure limits of 01 to20mbar (10 to 2000 Pascal) result in the microwave radi-ation being stored ldquoper cyclerdquo in the empty-cavity modeunder these conditions the rate of energy loss to the cavitywall can substantially heat the MRC structure and in theextreme case the microwave radiation leakage can becomea health risk Additionally if the MRC is loaded with ma-terials (semiconductor wafers and low dielectric strengthmaterial) these can be electrically or mechanically damagedIt therefore becomes necessary to have an automatic powercutoff device to prevent microwave leakage and damage toboth the reactor and loaded materials e near-field plasmaE-probe as described by Law [47] is one such device thathelps monitor such events In this circuit a neon dischargelamp a photodiode and a reference voltage are connected asshown in Figure 6 with one leg of the neon being usedprotruding into the cavity to act as a near-field E-probe Inthe original circuit design a strip chart is used to record thevoltage-time-series data but with todayrsquos analogue-to-digital converters and software (such as LabView) theplasma ignition state-space and plasma state-space may bemonitored with trigger levels set to give a binary GoNocontrol output [24 47]

45 Bebesonde Electrostatic Probe Measurement For plasmaprocesses it is generally considered that the ion flux arrivingand leaving the surface determines the plasma processHowever the use of electrostatic probe techniques to de-termine the ion density and temperature of plasma driven bymodulated power source in the presence of sputtered in-sulating material is problematic Such is the case of plasmacontaining CF4 is section describes a probe techniquethat is tolerant of drive modulation and sputtered insulatingmaterials e following measurements were performed at

8 International Journal of Chemical Engineering

the Oxford Research Unit Open University on the MRC-100 reactor e probe used is an RF-biased ion flux probe(known colloquially as ldquoBebesone or BBsrdquo) For full detailsof the probe and measurement see [48]

For good probe measurements visual observations ofthe plasma volume as a whole and around the probe itselfare usually required For the MRC reactors it was foundnecessary to replace the standard front flange door witha flange incorporating a probe port and inspection portcovered by an open mesh Given this modification argonplasma within the MRC-100 visually exhibits littlestructure with a slight brightening close to the dielectricannulus boundary Nevertheless there is no evidence ofmicrowave structure in the optical emission which in-dicates that energy is rapidly homogenized in the electronpopulation

Following these visual baseline inspections theBebesonde was used to determine the ion flux in the lowpower and high setting with argon gas flow varying froma maximum (5 lmiddotminminus1) to a minimum for which plasmacould be sustained (2 lmiddotminminus1) For the MRC-200 reactorargon ion densities are measured to be in the range of2times1011middotcmminus3 at 50mbar (5 kmiddotPascal) to 3times l012middotcmminus3 at1mbar (100 Pascal) and the electron temperature is in therange of l to l5 eV for an input power of 0022Wmiddotcmminus3 thehigher plasma density at low pressure corresponds to a longermean free path and increased electron power transfer

46 Ultraviolet Fluorescence Microwave Probe For manyprocessing plasmas photon energy (E hcλ) varies from10 eV to 1 eV with preferential intensity at discrete spectralwavelengths e spectral characteristic is determined by thenature of gas excitation and relaxation processes both ofwhich are sensitive to the local electron temperature and gasexcitation cross sections Gas composition and mode ofplasma production therefore has a major impact on UVproduction and plasma chemistry

e ultraviolet fluorescence probe utilizes activated rare-earth salts Y2SiO5Ce (absorptionlt 200 nm) and Zn2SiO4Mn and Y2O3Eu3+ (absorptionlt 300 nm) For full detailsof these probes see [49] At these energies the host lattice(H) undergoes electronic excitation via the production of

electron-hole pair separation (6) followed by the lowestenergetic deactivation pathway fluorescence at wavelengthslonger than the initial radiation In general the fluorescencespectrum is composed of a narrow band with the precisewavelength determined by the intimate relation between theactivator (Ce Mn and Eu3+) and the host lattice and by theirradiating photon radiation

H A + hvprime rarrH Alowast

hvprime gt 4ev( 1113857 (6)

H Alowast rarrH A + hvPrime hvPrime lt 4ev( 1113857 (7)

emost simple form of the probes comprises a syntheticDUV grade fused silica capsule (12mm diametertimes 20mm)containing one of three activated salts at a nominal reducedpressure of 10mbare fused silica has transmittance T 05at 170 nm When placed in the plasma volume the probecollects 4π steradians of incoming DUV photon radiatione emitted fluorescence is viewed through an optical crownglass viewport (T 05 at 380 nm) Due to the dielectriccapsule photoluminescence rather than electroluminescenceis considered to be the prime mechanism of fluorescence inthese probes e dielectric also acts as a wavelength dis-criminator and provides the upper working limit of the probeto be 1100K

Using these knowledge the salts Zn2SiO4Mn and Y2O3Eu3+ (placed in their own capsule) integrates DUV plasmawhile Y2SiO5Ce when placed directly in the plasma volumeintegrates VUV (lt200 nm) plasma thus their fluorescenceappearing in the green red and blue respectively

5 Magnetron Oven Control Circuits

Microwave ovens generally employed one of two types ofmagnetron drive circuits For microwave ovens with low-power outputs (typically lt500W) the output power isachieved by pulse-width modulation of a single incidentpower to the magnetron with the cooking time set between0 and 30 minutes Between the 500W and 1100W ratingcontinuous application of the microwave power is usedwhere the power level is set by the magnetron drive capacitorvalue

P 12

CV2

1113872 1113873f (8)

e choice of either of these two drive circuits can havean impact on the magnetron control circuit ability to carryout the selected plasma processis choice is exemplified bycomparing the short and low power requirements of rapidplasma syntheses of organic compounds [7 8] cleaning ofglass slides and polymers [14] to that of the high power andlong processing times of solid-state metal oxides processing[13] and the plasma cleaning of ion implant ceramic in-sulators [26 27]

51 Cavity Magnetron Capacitor-Controlled Drive CircuitIn this section the choice of the capacitor controlled drivecircuit is considered along with its safety control circuit thatis used in the MRC-100200 plasma reactors A schematic of

Cavity wall

E-field probe

Neon

Photodiode

Shield

Vref

AD

Figure 6 Schematic of the E-probe circuit showing a neon dis-charge lamp a photodiode and an analogue-to-digital converter

International Journal of Chemical Engineering 9

the capacitor controlled drive circuit is shown in Figure 7 Asimilar cavity magnetron capacitor controlled drive circuit isreported in [23] Chaichumporn et al have also reportedfurther refinement of the magnetron anode voltage (33 to66 kV) in [40]

e transformer comprises two winding circuits efirst provides a winding ratio to produce 240V at 50ndash60Hzto approximately 35V at 50ndash60Hz for the cathode heaterand the second provides the step-up voltage (240V at 50ndash60Hzto 2-3 kV at 50ndash60Hz) e HV capacitor (06 to 15 μF)and the HV diode are used to bias the cathode negativewith respect to the anode block which contains themagnetronrsquos cavity structure Using this arrangement thecapacitor value and diode determines the DC powerdissipated in the magnetron cavity structure For safetyreasons both passive and active control components areincorporated on either side of the setup transformer eseinclude the following the circuit is fused on both sides ofthe transformer plus an emergency stop button a chassisinterlock a magnetron thermoswitch (135degC) and a chassecooling fan In addition 24 V DC circuit is used to provideremote passive and active control of the drive circuit (seeSection 52)

52 24V DC Control Circuit In purpose-built microwaveoven plasma reactors the auxiliary equipment (vacuumpumps vacuum valves pressure gauge gas lines purge linesprocess timer and microwave power) is synchronized toplasma process in a safe manner e role of the 24V DCcontrol circuit is to synchronize the auxiliary componentsto the plasma process and shut down the system in the caseof an emergency failure this is especially important whenusing flammable corrosive and toxic precursor gases andby-products e circuit is designed so that all chasse controlsare isolated from the cavity magnetron capacitor-controlleddrive circuit using relays and solenoids It should be notedthat when converting a domestic microwave oven intoa plasma reactor the synchronization has to be built fromscratch

6 Conclusions

is work has reviewed the conversation of the domes-tic microwave oven into a source for cleaning as well aschemical reactions e conversion of domestic systems intoplasma reactor is described as is the construction ofpurpose-built microwave oven plasma reactors Calibrationof the MRC has been discussed along with identifying thetwo types of magnetron drive circuitry used e pro-fessional and armature use has also been presented in thelatter case mainly limited to kitchen top experiments eproof of principle and small batch processes established inthese plasma reactors range from plasma cleaning of glassand polymer surface and removal of toxic metals fromceramic surfaces to the manufacture of carbon nano-structures and the pyrolysis of paper to produce gaseouswaste by-products In all cases the power source is thepackage cavity magnetron operating below 1100W outputpower and pressures ranging from a few 01 s Pascal toa nominal atmospheric pressure (1013 Pascal)

At or close to atmospheric pressure single or multiplewire antenna electrodes that have a physical length ap-proximating to 14 or 12 of the microwave length in whichthat it is immersed have been found to play a catalytic rolein instigating plasma production and in the case ofmanufacturing carbon nanomaterials provide a substrate onwhich carbon material can grow With regard to reactionrate increasing with the number (1 to 6) of electrodes be-yond which the reaction becomes rate-limited a geometricalloading effect around the wire antenna is proposed in thispaper Whether this or electromagnetic power loss is re-sponsible for the effect further work is required In additionthis work has reconstructed a preformed disc aerial elec-trode (igniter) suitable for a narrow tube reactor [16 40](Figure 4)

e safety match flame lighted cigarette sliced grapeand metal antenna electrode have been observed to havea catalyst role in the production of plasma and plasmoids Todistinguish between the metal antenna and the thermal-chemical based catalyst it has been put forward that metal

Anode

Cathode

24 V

dcCo

ntro

l circ

uit

Step

-up

tran

sform

er

HV diode

35 V

Chassisinterlock

Magnetronthermoswitch Fuse

Filtered240 V

50ndash60 Hz

HVFuse

Coolingfan

HVCapacitor

25 kV

Emergencystop

Figure 7 Cavity magnetron capacitor-controlled dive circuit e 24V DC control circuit represented by the dashed box is discussed inSection 52

10 International Journal of Chemical Engineering

antennas may be classified as a passive catalyst as they onlysupply a surface that generates free electrons whilst thesafety match flame lighted cigarette and sliced grape may beclassified as an active catalyst as they supply energy in theform of heat and free electrons from an electrolyte

Finally this review also has highlighted plasmoid foodcooking within a MRC and the use of plasma discharge forremoving food odor frommicrowave ovens Given that foodsafety issues are addressed it is reasonable to envisagemicrowave oven plasma reactors incorporating both plas-moid cooking of food stuff and plasma deodorization ofcooking by-products may be realized in the near future

Conflicts of Interest

e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

e authors would like to acknowledge the support of SFIthrough the I-Form Advanced Manufacturing ResearchCenter 16RC3872

References

[1] F Nagelschmidt Diathermy Text Book for Physicians andStudents Springer-Verlag Berlin Heidelberg 1921

[2] Davis A History of the Microwave Oven e IEEE NewsSource e IEEE News Source Piscataway NJ USA httpswwwtheinstituteieeeorgtech-historytechnology-historya-history-of-the-microwave-oven

[3] P L Spencer ldquoMethod of treating foodstuffsrdquo US Patent2495429 1950

[4] J M Osepchuk ldquoe history of the microwave oven a criticalreviewrdquo in Proceedings of Digest IEEE International Micro-wave Symposium pp 1397ndash1400 Boston MA USA June2009

[5] J R Mims ldquoMicrowave magnetronrdquo US Patent 37392251973

[6] T Koinuma ldquoMagnetronrdquo US Patent 3809590 1974[7] R N Gedye F Smith and K C Westaway ldquoe rapid

synthesis of organic compounds in microwave ovensrdquo Ca-nadian Journal of Chemistry vol 6 no 1 pp 17ndash26 1988

[8] R N Gedye W Rank and K C Westaway ldquoe rapidsynthesis of organic compounds in microwave ovens IIrdquoCanadian Journal of Chemistry vol 69 no 4 pp 706ndash7111991

[9] M Omran T Fabritius E-P Heikkinen and G ChenldquoDielectric properties and carbothermic reduction of zincoxide and zinc ferrite by microwave heatingrdquo Royal SocietyOpen Science vol 4 no 9 p 170710 2017

[10] A Ribner ldquoMicrowave plasma etching machine and methodof etchingrdquo US Patent 4804431 1989

[11] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of phase pure K3C6o using a microwave-induced argon plasmardquo Journal of the Chemical SocietyChemical Communications no 18 pp 2027-2028 1994

[12] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of alkali-metal fullerides using a microwave-induced argon plasmardquo Chemistry of Materials vol 8 no 2pp 394ndash400 1996

[13] D J Brooks and R E Douthwaite ldquoMicrowave-inducedplasma reactor based on a domestic microwave oven forbulk solid state chemistryrdquo Review of Scientific Instrumentsvol 75 no 12 pp 5277ndash5279 2004

[14] B T Ginn and O Steinbock ldquoPolymer surface modificationusing microwave-oven-generated plasmardquo Langmuir vol 19no 19 pp 8117-8118 2003

[15] P Khongkrapan N Tippayawong and T Kiatsiriroatldquoermochemical conversion of waste papers to fuel gas ina microwave plasma reactorrdquo Journal of Clean EnergyTechnologies vol 1 no 2 pp 80ndash83 2013

[16] P Khongkrapan P anompongchart N Tippayawong andT Kiatsiriroat ldquoFuel gas and char from pyrolysis of wastepaper in a microwave plasma reactorrdquo IJEE vol 4 no 6pp 969ndash974 2013

[17] R Singh and A L L Jarvis ldquoMicrowave plasma-enhancedchemical vapour deposition growth of carbon nano-structuresrdquo South African Journal of Science vol 106 no 5-6p 4 2010

[18] S Nomura H Toyota S Mukasa H Yamashita T Maeharaand A Kawashima ldquoProduction of hydrogen in a conven-tional microwave ovenrdquo Journal of Applied Physics vol 106no 7 p 073306 2009

[19] S Nomura H Yamashita H Toyota S Mukasa andY Okamura ldquoSimultaneous production of hydrogen andcarbon nanotubes in a conventional microwave ovenrdquo inProceedings of International Symposium on Plasma Chemistry(ISPC19) vol 65 Bochum Germany July 2009

[20] H Toyota S Nomura and S Mukasa ldquoA practical electrodefor microwave plasma processesrdquo International Journal ofMaterials Science and Applications vol 2 no 3 pp 83ndash882013

[21] R Pongsopon T Chim-Oye and M Fuangfoong ldquoMicro-wave plasma reactor based on microwave ovenrdquo in PIERSProceedings pp 2723ndash2726 Guangzhou China August 2014

[22] N Manivannan W Balachandran R Beleca and M AbbodldquoMicrowave plasma system design and modelling for marinediesel exhaust gas abatement of NOx and SOxrdquo InternationalJournal of Environmental Science and Development vol 6no 2 pp 151ndash154 2015

[23] M C Savadori V P Mammana O G Martins andF T Degasperi ldquoPlasma-assisted chemical vapour depositionin a tunable microwave cavityrdquo Plasma Sources Science andTechnology vol 4 no 3 pp 489ndash494 1995

[24] V J Law and N Macgearailt ldquoVisualization of a dual fre-quency plasma etch processrdquo Measurement Science andTechnology vol 18 no 3 pp 645ndash649 2007

[25] Y H Jung S O Jang and H J You ldquoHydrogen generationfrom the dissociation of water using microwave plasmasrdquoChinese Physics Letters vol 30 no 6 p 065204 2013

[26] V J Law and D Tait ldquoContaminated ceramic plasmacleaningrdquo European Semiconductor vol 19 no 9 pp S38ndashS41 1997

[27] V J Law andD Tait ldquoMicrowave plasma cleaning of ion implantceramic insulatorsrdquo Vacuum vol 49 no 4 pp 273ndash278 1998

[28] T M Burke E H Linfield D A Ritchie M Peper andJ H Burroughs ldquoHydrogen radical surface cleaning of GaAsfor MBE regrowthrdquo Journal of Crystal Growth vol 175-176pp 416ndash421 1997

[29] D Tasch D J Brosky S Conrad S Kumar and D KumarldquoMicrowave plasma cookingrdquo US Patent 20090012223 A12009

[30] H Stanley ldquoMicrowave experiments at schoolrdquo Science inSchool vol 12 pp 31ndash33 2009

International Journal of Chemical Engineering 11

through the bottom of the cavity and Tallaire who drilledthrough the side of the cavity In the latter case TallairersquosYouTube posting provides an example of plasma cleaning ofa microscope glass side [32]

214 Coaxial Narrow Tube Reactor Khongkrapan et alhave reported a converted microwave oven for pyrolysis ofpaper to produce gaseous waste by-products at 800W[15 16] In their reactor the process occurs inside a cylin-drical quartz tube (internalexternal diameters of 2730mmand length of 250mm) that coaxially passes verticallythrough the MRC Air or argon is used as the precursor gasat a nominal atmospheric pressure (1013 kmiddotPa) with the gasflowing from bottom to top of theMRCe shredded paper(5 g) is suspended in the center of the tube (Figure 1(e)) In[16] Khongkrapan et al state that an igniter was placedwithin the tube to generate the plasma but no direct detailswere given Upon further reading of their reference list(reference 17 in their paper) a simple cartoon showing theigniter positioned within the tube is given again without textexplanation e subject of igniters in the form of a metalantenna is discussed in Section 217

215 Internal Waveguide In 2004 Brooks and Douthwaitepresented their internal waveguide fitted to an 800W do-mestic microwave oven for plasma-induced processing ofpowered metal oxides (Ga2O3 TiO2 and V2O5) into binarymetal nitrides formed in ammonia (NH3) plasma [13] Inthis design a slot is cut at the rear of the MRC to allowa 20mm internal diameter U-shaped tube containing thesolid-state sample within an alumina boat to be positionedwithin the microwave field (Figure 1(f)) Outside of theMRC one end of the U-shaped tube is fitted to a vacuumpump and the other end is fitted to the carrier and processgases To prevent microwave leakage at the rear of the ovenextensive gasket and Faraday shielding were arranged Aninternal waveguide is then fitted to the MRC iris in sucha way to focus the microwave energy in the vicinity of thesample Furthermore to prevent reflected power damagingthe cavity magnetron and overheating the waveguidea water-cooled dummy load is fitted to the waveguide outputaperture With these extensive oven conversions the plasmaregion may be considered to be operating in the coherentmode rather than in the multimode Typically plasma pa-rameters used to convert the metal oxides to nitrides are anNH3 gas flow rate of 113 cm3middotminminus1 pressure of 20mbar(2000 Pascal) and microwave power of 900W for a plasmaexposure time of 25 to 6 hours

216 Liquid Plasma Vessels Microwave in-liquid plasmadecomposition of n-dodecane (molecular formula C12H26(I))to simultaneously produce hydrogen gas and carbide inthe hydrocarbon liquid has been achieved using a convertedmicrowave oven at a reportedmicrowave power level of 500 to750W [18ndash20] A typical representation of these reactors isshown in Figure 2 e reaction is performed in a closedvolume Pyrex reaction vessel containing 500ml of n-dodecane

liquid with one or more electrodes where the electrode(s) canbe either single-tip steel wire electrodes or copper U-shapeddual-tip aerial electrodes Also two siliconPTFE tubes areinserted from the top of the cavity one tube is used for sendingthe carrying gas (argon) as the precursor gas and the secondtube is used to collect the spent argon and the by-product gasat a working pressure close to atmospheric pressure

To understand the purpose of these electrodes the re-action efficiency of both types of electrodes is examined asa function of the geometry and the number of electrodes inthe context of their electromagnetic design and heteroge-neous reaction kinetics

First consider the single-tip electrodes [18ndash20] esemetal electrodes have a dimensional length of L 21mm anda diameter of 15mm and they are fixed vertically in a singlearray (Figure 3(a)) or in a multiple array (Figure 3(b)) with 1electrode in the center and up to 6 electrodes circum-ferentially spaced at a gap separation of λm4 where λm is thewavelength of the microwave radiation passing through themedium e wavelength calculation is given in the fol-lowing equation

λm simC

fmiddot1εr

radic (1)

e approximate expression in (1) is used as the oper-ation frequency of the free running cavity magnetron isfrequency pulled by changing SWR conditions in therectangular TE10 waveguide in which the magnetron ismounted All other symbols have their normal meaning C isthe speed of light (299792times108mmiddotsminus1) f is the magnetronoperating frequency (245GHz) and εr is the medium inwhich the radiation is passing through us for the liquidn-dodecane (εr 178 to 2) λm approximates to 885 cm andλm4 approximates to 22 cm

Based on the works [18ndash20] and the work of Pongsoponet al [21] it is generally considered that the electrodeshave three well-defined roles to confine the plasma to theimmediate proximity of the electrode(s) tip to function as

Timer

Power

Tubes (silicon)

Antenna

Stand (heat resistance glass)

Liquid

Reaction vessel(heat resistance

lass)

Figure 2 A typical front view schematic of a converted microwaveoven for liquid processing For clarity the auxiliary gas linesoutside the ovens are not shown

4 International Journal of Chemical Engineering

a catalytic source for plasma heterogeneous reaction and inthe case of manufacturing carbon nanomaterials to providea substrate on which the carbon material can grow In thefirst of these roles increasing the number of electrodes from1 to 6 has revealed that the efficiency of plasma de-composition of n-dodecane does increase but beyond 6-7electrodes the reaction efficiency becomes rate-limited ismay be due to electromagnetic power loss by the resonantstructure of the electrodes [20] or simply that the addition ofmore than 7 electrodes and their associated surroundingreaction zones (cylindrical volume around each electrodeFigure 3(c)) within a fixed closed volume simply producesa loading effect within the heterogeneous reaction [39] atis to say as the percentage of the combined electrode re-action zones approaches the total fixed volume the amountof fresh reactant flowing to the electrode reaction zonebecomes reduced erefore mass transport in and out of

each electrode reaction zone rather than plasma de-composition may become the rate-limiting step To clarifythese observations further investigation is needed

For the dual-tipped aerial electrode Toyota et al [20]have shown that the U-shaped aerial electrodes have distinctoptimum lengths of L sim 2λm 3λm2 λm and λm2 ey alsoshow that the use of the approximation sign in (1) is justifiedby experimentally determining the λ2 FHHW length ofthe U-shaped dual-tip aerial electrode to be 44 to 47 cm forn-dodecane

217 Igniter e description of construction and use ofwire aerial electrodes for plasma ignition is now used as anaid to outline the construction of the plasma igniter [16] andthe drawing in [40] (Figure 4) Assuming that the drawing in[40] may be scaled the plasma igniter may be constructed intwo ways Firstly the igniter may be constructed using twowire electrodes opposing each other and bent at 45deg so thattheir tips are aligned with the gas flow and the fixing lo-cation is formed using an insulating ring e second andmore practical arrangement is that the igniter is preformedfrom a 30mm diametertimes 05mm thick steel steel disc anda plurality of electrodes are punched from the centralportion of the disc and bent to a 45deg angle For the purposeof this second option the construction of a 4-electrodeigniter is exemplified using the 2730mm internalexternaldiameter glass tube in [16] as a reference tube (Figure 1(e))A schematic of the manufacturing stages of the igniter isgiven in Figure 4 where it is shown that the first stage is topunch out the form of the igniter the second stage is tobend the electrodes and the third stage is to align theigniter to the glass tube Using this method of constructionthe lip of the preform can self-align to enable the 4 aerialelectrodes to suit the plasma ignition criteria as describedin Section 215

218 Production of Plasmoids (Fireballs) e production ofplasmoids sometimes called fireballs or ball-lightningwithin domestic microwave ovens has been posted onYouTube postings [34ndash38] Perhaps the simplest way ofproducing a fireball without modification to the microwaveoven is to place a partially sliced grape (that has its two halvesconnected via a thin piece of skin) in the microwave ovenand then turn on the microwave power for 3ndash10 secondse YouTube posting [34] shows that arc-like plasmoids aregenerated at the thin skin bridge that connects the two grapehalves with the discharge emission continuing until eitherthe power is turned off or the grapes have shriveled up isaction may be understood by considering that the twofreshly cut grape halves have a characteristic dimension of15 to 2 cm and are partially filled with a conducting elec-trolyte the combination of which creates an organic con-ducting dipole antenna not unlike the metal antennasdiscussed in Sections 215 and 216 Given this un-derstanding it is reasonable to assume that as the freeelectrons are pushed back and forth through the narrow thinskin bridge of the grape heat is generated due to the

Coppersheet Glass and so on

Metal electrode

L

(a)

λ4

λ4

(b)

Electrode reaction zone

(c)

Figure 3 Typical single-tip electrode arrangement (a) multipleelectrode arrangements (b) electrode reaction zone (c)

International Journal of Chemical Engineering 5

resistance and burns away the skin In addition themovement of electrons through the grape electrolyte inducesa rapid increase in temperature causing vaporization of theelectrolyte to a cloud of electrons and ions thus forming thelocalized plasmoid e plasmoid continues to be sustainedas long as the free electrons are available from the dimin-ishing volume of the grape electrolyte

Moving away from the organic source for generatingplasmoids a lighted safety match supported by a wine corkcovered with a glass jar and placed within the center of theMRC can also be used [35] Upon turning on the micro-wave power a plasma discharge is generated that rises to thetop of the jar thus forming a buoyant plasmoid Warren[36] used a similar approach but this time using a glassjar supported by three wine corks and a lighted cigaretteplaced in the gap provided by the corks In this work andthe previous example the plasmoids are maintained whenthe thermal source is extingusihed It is only when themicrowave power is turned-off does the plasmoid becomeextinguished Plasmoids can also be generated withinelectric light bulbs and fluorescence tubes as shown in [37]this example also appears to be the basis for the near-fieldE-probe (Section 44)

A more dangerous approach to generating plasmoids isdemonstrated in [38] where a cavity magnetron connectedto a food tin can is used to lunch microwaves at a domesticlight bulb to produce a plasmoid within the bulb From thisexperiment it would appear that the electric filament acts asthe initiating electrode

Before finishing this section it is worth noting that thecylindrical plasma reactor produced for the fast food in-dustry [29] employed a patented passive plasma catalyst inthe form of an electrode to ignite the atmospheric plasma[41] where the passive plasma catalyst can include anyobject capable of inducing plasma by deforming the localelectric field On the other hand the patent states that anactive plasma catalyst produces particles or a high-energywave packet capable of transferring a sufficient amount ofenergy to a gaseous atom (or molecule) to remove at leastone electron from the gaseous atom (or molecule) in thepresence of electromagnetic radiation Given these twodefinitions it is reasonable to assume that the safety matchflame [35] cigarette [36] and grape [34] can be classed as anactive plasma catalyst and the metal electrode as a passiveplasma catalyst

219 Plasmoid Food Cooking e Korean patents [42 43]and conference paper [44] report on a form of tuning withinthe TE10 waveguide that fall outside the scope of this reviewbut they are listed for three reasons Firstly the phenomenaof plasmoids extend the cooking range of the domesticmicrowave oven from one of dielectric heating of food stuffto one that provides surface browning and imparting textureand flavor that is similar to the traditional flame-cookingprocess Secondly Jerby et al [44] have noted that plasmoidsproduced in this way require wire antenna electrode to ignitethe plasmoid and therefore may contain nanoparticleswhich might be harmful for the food quality and even makeit inedible irdly the additional use of plasma dischargethat generates ozone and ions for the removal of odor-producing materials from the cooking chamber [45] doesprovide one possible technical route forward in the futuredevelopment of the domestic microwave oven

3 Purpose-Built Microwave OvenPlasma Reactor

is section describes the methodology used in the con-struction of a purpose-built microwave oven plasma reactorOf particular importance in this regard is the MRC seriesof plasma reactors that were built in the mid-1990s atCambridge Fluid Systems Ltd (England UK) e designconcept behind these plasma reactors was to build a simplereliable and cost-effective table-top plasma reactor that couldbe sold to research laboratories and low-volume productionsunits eir main use was for surface engineering enhance-ment in the microelectronic semiconductor sector and themanufacture of bodyshell of Formula One racing cars

e design of the plasma reactor is similar to microwaveovens where the cavity magnetron antenna is locatedwithin a TE10 waveguide that is used to illuminate the MRCthrough a single iris e cutoff frequency (fc)mn of theTE10 waveguide is calculated using the following equation

fc( 1113857mn C

2

m

a1113874 1113875

2+

n

b1113874 1113875

21113971

(2)

where c is the speed of the light and a and b are the internaldimensions (width and height) of the waveguide in this case80 and 38mm are used respectively which equate a cutofffrequency of 1875GHz

30 mm

(1) Punch stage (2) Bend stage (3) Glass tube fitting

Lip

28 mm Glass tube

FIGURE 4 Manufacturing stages of the disc igniter that is suitable for a narrow glass tube reactor

6 International Journal of Chemical Engineering

With the cavity magnetron antenna positioned 26mmfrom the end of the waveguide the frequency and bandwidthof the magnetron are allowed to be free running us thenoncoherent reflected power passing through the iris travelsback to the magnetron thus altering the SWR of the coherentwave within the TE10 waveguide resulting in varying theoutput power of the magnetron

e MRC reactor design differs from the domesticmicrowave oven plasma reactor in the following ways (alsocf Figure 1 with Figure 5)

(i) e chassis MRC and waveguide are constructed asone welded component using 14mm thick mild steelsheet Before each of the three components is weldedtogether they have all the necessary holes punchedand clasp nuts fixed Once welded the structure isnickel plated to produce a metal structure that isrobust with sufficient stiffness to support all theadditional components (the front and rear stainlessflanges gas lines DC power supply pressure gaugeetc) Using this construction approach the MRC hasa theoretical maximum unloadedQ-factor (Qu) in theTE mode that is dependent on the ratio of storedenergy in the cavity (Vc) to the energy loss to thecavity walls (δ times Ac)

Qu 2Vc

δAc (3)

where δ is the electrical skin depth at the cavity wall per cycleand Ac is the cavity wall area

For this reactor the main cavity has an approximatelyQuof 20000 at a resonant frequency of 245GHz

(ii) A cylindrical Pyrex glass chamber (190mm di-ameter 300mm length and 5mm wall thicknessproducing a volume of 3 liters) is located within themultimode cavity with its longitudinal axis per-pendicular to the microwave iris and with the frontand rear of the chamber housed within metal flangesthat form the part of the multimode cavity wall erear flange contains welded vacuum and pressuregauge ports and the front flanges contain the accessdoor is design maximizes the chamber volumeand removes all fragile glass fittings plastic tubeconnectors and feedthrough microwave leakagegaskets

(iii) e gas lines are fitted within the chassis and to theside of theMRC thereby enabling the process gassesto be injected throughmultiple equally spaced radialports in the front flange thus reducing the possibilityof precursor gas being preionized prior to chamberentry and maximizing uniform gas flow and plasmauniformity along process chamber longitudinal axis

31 Plasma Cleaning of Ion Implant Ceramic InsulatorsIon implantation is one of the key processes in the highvolume (220 wafers per hour) manufacture of silicon

semiconductor devices ese ion implant machines how-ever cost between $18M and $3M ese machines are alsohighly maintenance-intensive systems with high capitalcost therefore availability and cost of ownership aremajor factors to be considered Many of the parts changedduring regular maintenance and ion source changes areceramic insulators In this section an overview of theplasma cleaning of ion implant ceramic is described forfull details of the process see [26 27] e plasma cleaningprocess has been performed in the MRC series of plasmareactor using a gas mixture of 5ndash10 O2 in CF with anadmixture of 50 by flow of argon e argon admix isused to stabilize the microwave plasma by moderating theelectron energy distribution and to provide a uniformexcited species throughout the plasma volume eplasma etch chemistry at the surface of the ceramic may beconsidered to proceed by the following representativeheterogeneous reaction

3CF4(g) + 15O2(g) + 2X(s)rarr 2XF3(g) + 3COF3(g)

(4)

in which the addition of O2 scavenges carbon from the CF4through the formation of COFx species to enhance thesteady-state concentration of F atoms in the plasma vo1umee element X in reaction (4) represents the group V ele-ment (As P and Sb) on the ceramic surface and the XF3 arethe etch products Given sufficient microwave power theetch rates of these products are therefore controlled by the

Gas lines CavityMagnetron timer and

power supply

Front view

Pressuremeter

(a)

Gaugeport

Vacuumport

Doo

r

Side view

(b)

Figure 5 Typical front and side view schematic of theMRC plasmareactor chassis and cavity Photograph of the microwave MRC-200plasma reactor

International Journal of Chemical Engineering 7

production of F atoms (O2 balance) product volatility andceramic microscopic surface area

For the MRC-100 reactor typical plasma process pa-rameters obtained were 104W and 10mbar with an etchtime of 45 minutes where the surface temperature of theceramics reached 80plusmn 5K In the case of the MRC-200reactor the process parameters were as follows 200Wand 10mbar (1000 Pascal) with an etch time of 20 to 25minutes where the surface temperature of the ceramicsreached 125plusmn 5K

4 Microwave Cavity Calibration

is section describes a range of different techniques whichare used for the evaluation of the microwave efficiency aswell as leakage

41 Microwave Leakage Measurement e European Di-rective 200440EC and ICNIRP (1998) guidelines recom-mend that industrial microwave ovens have surfacemicrowave (3ndash300GHz) radiation power density levelsgt5mWmiddotcmminus2 and 5 times less for domestic microwave ovensintended for general use Applying a quadratic law based onplane wave theory an operator standing at a distance of20 cm from an industrial oven the operator will receive themaximum allowable power density level of 3mWmiddotcmminus2 Fordomestic oven environment this equates to a power densitylevel of 03mWmiddotcmminus2 In the context of converted ovensintended for plasma many feedthrough and apertures in theMRC require considerable care in design and constructionto prevent microwave leakage

42 Calorimetric Magnetron Power Calibration e mag-netron power entering the MRC may be calibrated using thewater open-dish load method see for example BritishStandard 75091995 and IEC 13071994 Hence given theheat capacity of water is 4184 J(gmiddotK) the calculated appliedpower (P) within the cavity may be obtained by placinga known mass of water (m) within the cavity and heating itfor a short period of time (t) making sure the water does notboil With the knowledge of themeasured water temperaturechange (ΔT final temperature ndash initial temperature) themicrowave power entering into the cavity is calibratedfor a given power setting using the following equation(see also [40])

P mCΔT

t (5)

e calibration however must be considered to be anupper value for plasma processing as its dielectric volumewill be different from the water calibration (Note when theprocess chamber volume or the geometric shape does notallow the open dish method to be used the alternative flowmethod can be used as outlined in [46])

Given (4) the microwave power density (Wmiddotcmminus3) ofa system may be calculated by dividing through by theprocess chamber volume e following open-dish loadmethod for the MRC 100 is given here as an illustrative

example the MRC-100 and MRC-200 reactors have a cal-culated magnetron applied power of 104W which equatesto a power density of 0116Wmiddotcmminus3 For the MRC-200 re-actor the calorimetric measurements produce magnetronapplied power values of 200W (0022Wmiddotcmminus3) and 450W(005Wmiddotcmminus3)

43 Surface Temperature Measurement Knowledge of thesurface temperature of materials immersed in the plasma isuseful in understanding the heterogeneous plasma-surfaceinteraction is is of particular importance when localdielectric heating has the potential of thermal runawaybecause most materials increase their dielectric loss withtemperature [12] Two simple means of estimating localsurface temperature of materials that are immersed in mi-crowave plasma have been used For surface temperaturesbelow 180K liquid crystal temperature-sensitive (20plusmn 5K to180plusmn 5K) strips attached to the plasma immersed surfacecan be used [27] For higher temperatures salts of knownmelting point (KCl 1043K and NaCl 1074K) sealed insilica capillaries have been used [13]

44 Near-Field Plasma E-Probe Measurement Attempts tostrike a plasma outside the ignition pressure limits of 01 to20mbar (10 to 2000 Pascal) result in the microwave radi-ation being stored ldquoper cyclerdquo in the empty-cavity modeunder these conditions the rate of energy loss to the cavitywall can substantially heat the MRC structure and in theextreme case the microwave radiation leakage can becomea health risk Additionally if the MRC is loaded with ma-terials (semiconductor wafers and low dielectric strengthmaterial) these can be electrically or mechanically damagedIt therefore becomes necessary to have an automatic powercutoff device to prevent microwave leakage and damage toboth the reactor and loaded materials e near-field plasmaE-probe as described by Law [47] is one such device thathelps monitor such events In this circuit a neon dischargelamp a photodiode and a reference voltage are connected asshown in Figure 6 with one leg of the neon being usedprotruding into the cavity to act as a near-field E-probe Inthe original circuit design a strip chart is used to record thevoltage-time-series data but with todayrsquos analogue-to-digital converters and software (such as LabView) theplasma ignition state-space and plasma state-space may bemonitored with trigger levels set to give a binary GoNocontrol output [24 47]

45 Bebesonde Electrostatic Probe Measurement For plasmaprocesses it is generally considered that the ion flux arrivingand leaving the surface determines the plasma processHowever the use of electrostatic probe techniques to de-termine the ion density and temperature of plasma driven bymodulated power source in the presence of sputtered in-sulating material is problematic Such is the case of plasmacontaining CF4 is section describes a probe techniquethat is tolerant of drive modulation and sputtered insulatingmaterials e following measurements were performed at

8 International Journal of Chemical Engineering

the Oxford Research Unit Open University on the MRC-100 reactor e probe used is an RF-biased ion flux probe(known colloquially as ldquoBebesone or BBsrdquo) For full detailsof the probe and measurement see [48]

For good probe measurements visual observations ofthe plasma volume as a whole and around the probe itselfare usually required For the MRC reactors it was foundnecessary to replace the standard front flange door witha flange incorporating a probe port and inspection portcovered by an open mesh Given this modification argonplasma within the MRC-100 visually exhibits littlestructure with a slight brightening close to the dielectricannulus boundary Nevertheless there is no evidence ofmicrowave structure in the optical emission which in-dicates that energy is rapidly homogenized in the electronpopulation

Following these visual baseline inspections theBebesonde was used to determine the ion flux in the lowpower and high setting with argon gas flow varying froma maximum (5 lmiddotminminus1) to a minimum for which plasmacould be sustained (2 lmiddotminminus1) For the MRC-200 reactorargon ion densities are measured to be in the range of2times1011middotcmminus3 at 50mbar (5 kmiddotPascal) to 3times l012middotcmminus3 at1mbar (100 Pascal) and the electron temperature is in therange of l to l5 eV for an input power of 0022Wmiddotcmminus3 thehigher plasma density at low pressure corresponds to a longermean free path and increased electron power transfer

46 Ultraviolet Fluorescence Microwave Probe For manyprocessing plasmas photon energy (E hcλ) varies from10 eV to 1 eV with preferential intensity at discrete spectralwavelengths e spectral characteristic is determined by thenature of gas excitation and relaxation processes both ofwhich are sensitive to the local electron temperature and gasexcitation cross sections Gas composition and mode ofplasma production therefore has a major impact on UVproduction and plasma chemistry

e ultraviolet fluorescence probe utilizes activated rare-earth salts Y2SiO5Ce (absorptionlt 200 nm) and Zn2SiO4Mn and Y2O3Eu3+ (absorptionlt 300 nm) For full detailsof these probes see [49] At these energies the host lattice(H) undergoes electronic excitation via the production of

electron-hole pair separation (6) followed by the lowestenergetic deactivation pathway fluorescence at wavelengthslonger than the initial radiation In general the fluorescencespectrum is composed of a narrow band with the precisewavelength determined by the intimate relation between theactivator (Ce Mn and Eu3+) and the host lattice and by theirradiating photon radiation

H A + hvprime rarrH Alowast

hvprime gt 4ev( 1113857 (6)

H Alowast rarrH A + hvPrime hvPrime lt 4ev( 1113857 (7)

emost simple form of the probes comprises a syntheticDUV grade fused silica capsule (12mm diametertimes 20mm)containing one of three activated salts at a nominal reducedpressure of 10mbare fused silica has transmittance T 05at 170 nm When placed in the plasma volume the probecollects 4π steradians of incoming DUV photon radiatione emitted fluorescence is viewed through an optical crownglass viewport (T 05 at 380 nm) Due to the dielectriccapsule photoluminescence rather than electroluminescenceis considered to be the prime mechanism of fluorescence inthese probes e dielectric also acts as a wavelength dis-criminator and provides the upper working limit of the probeto be 1100K

Using these knowledge the salts Zn2SiO4Mn and Y2O3Eu3+ (placed in their own capsule) integrates DUV plasmawhile Y2SiO5Ce when placed directly in the plasma volumeintegrates VUV (lt200 nm) plasma thus their fluorescenceappearing in the green red and blue respectively

5 Magnetron Oven Control Circuits

Microwave ovens generally employed one of two types ofmagnetron drive circuits For microwave ovens with low-power outputs (typically lt500W) the output power isachieved by pulse-width modulation of a single incidentpower to the magnetron with the cooking time set between0 and 30 minutes Between the 500W and 1100W ratingcontinuous application of the microwave power is usedwhere the power level is set by the magnetron drive capacitorvalue

P 12

CV2

1113872 1113873f (8)

e choice of either of these two drive circuits can havean impact on the magnetron control circuit ability to carryout the selected plasma processis choice is exemplified bycomparing the short and low power requirements of rapidplasma syntheses of organic compounds [7 8] cleaning ofglass slides and polymers [14] to that of the high power andlong processing times of solid-state metal oxides processing[13] and the plasma cleaning of ion implant ceramic in-sulators [26 27]

51 Cavity Magnetron Capacitor-Controlled Drive CircuitIn this section the choice of the capacitor controlled drivecircuit is considered along with its safety control circuit thatis used in the MRC-100200 plasma reactors A schematic of

Cavity wall

E-field probe

Neon

Photodiode

Shield

Vref

AD

Figure 6 Schematic of the E-probe circuit showing a neon dis-charge lamp a photodiode and an analogue-to-digital converter

International Journal of Chemical Engineering 9

the capacitor controlled drive circuit is shown in Figure 7 Asimilar cavity magnetron capacitor controlled drive circuit isreported in [23] Chaichumporn et al have also reportedfurther refinement of the magnetron anode voltage (33 to66 kV) in [40]

e transformer comprises two winding circuits efirst provides a winding ratio to produce 240V at 50ndash60Hzto approximately 35V at 50ndash60Hz for the cathode heaterand the second provides the step-up voltage (240V at 50ndash60Hzto 2-3 kV at 50ndash60Hz) e HV capacitor (06 to 15 μF)and the HV diode are used to bias the cathode negativewith respect to the anode block which contains themagnetronrsquos cavity structure Using this arrangement thecapacitor value and diode determines the DC powerdissipated in the magnetron cavity structure For safetyreasons both passive and active control components areincorporated on either side of the setup transformer eseinclude the following the circuit is fused on both sides ofthe transformer plus an emergency stop button a chassisinterlock a magnetron thermoswitch (135degC) and a chassecooling fan In addition 24 V DC circuit is used to provideremote passive and active control of the drive circuit (seeSection 52)

52 24V DC Control Circuit In purpose-built microwaveoven plasma reactors the auxiliary equipment (vacuumpumps vacuum valves pressure gauge gas lines purge linesprocess timer and microwave power) is synchronized toplasma process in a safe manner e role of the 24V DCcontrol circuit is to synchronize the auxiliary componentsto the plasma process and shut down the system in the caseof an emergency failure this is especially important whenusing flammable corrosive and toxic precursor gases andby-products e circuit is designed so that all chasse controlsare isolated from the cavity magnetron capacitor-controlleddrive circuit using relays and solenoids It should be notedthat when converting a domestic microwave oven intoa plasma reactor the synchronization has to be built fromscratch

6 Conclusions

is work has reviewed the conversation of the domes-tic microwave oven into a source for cleaning as well aschemical reactions e conversion of domestic systems intoplasma reactor is described as is the construction ofpurpose-built microwave oven plasma reactors Calibrationof the MRC has been discussed along with identifying thetwo types of magnetron drive circuitry used e pro-fessional and armature use has also been presented in thelatter case mainly limited to kitchen top experiments eproof of principle and small batch processes established inthese plasma reactors range from plasma cleaning of glassand polymer surface and removal of toxic metals fromceramic surfaces to the manufacture of carbon nano-structures and the pyrolysis of paper to produce gaseouswaste by-products In all cases the power source is thepackage cavity magnetron operating below 1100W outputpower and pressures ranging from a few 01 s Pascal toa nominal atmospheric pressure (1013 Pascal)

At or close to atmospheric pressure single or multiplewire antenna electrodes that have a physical length ap-proximating to 14 or 12 of the microwave length in whichthat it is immersed have been found to play a catalytic rolein instigating plasma production and in the case ofmanufacturing carbon nanomaterials provide a substrate onwhich carbon material can grow With regard to reactionrate increasing with the number (1 to 6) of electrodes be-yond which the reaction becomes rate-limited a geometricalloading effect around the wire antenna is proposed in thispaper Whether this or electromagnetic power loss is re-sponsible for the effect further work is required In additionthis work has reconstructed a preformed disc aerial elec-trode (igniter) suitable for a narrow tube reactor [16 40](Figure 4)

e safety match flame lighted cigarette sliced grapeand metal antenna electrode have been observed to havea catalyst role in the production of plasma and plasmoids Todistinguish between the metal antenna and the thermal-chemical based catalyst it has been put forward that metal

Anode

Cathode

24 V

dcCo

ntro

l circ

uit

Step

-up

tran

sform

er

HV diode

35 V

Chassisinterlock

Magnetronthermoswitch Fuse

Filtered240 V

50ndash60 Hz

HVFuse

Coolingfan

HVCapacitor

25 kV

Emergencystop

Figure 7 Cavity magnetron capacitor-controlled dive circuit e 24V DC control circuit represented by the dashed box is discussed inSection 52

10 International Journal of Chemical Engineering

antennas may be classified as a passive catalyst as they onlysupply a surface that generates free electrons whilst thesafety match flame lighted cigarette and sliced grape may beclassified as an active catalyst as they supply energy in theform of heat and free electrons from an electrolyte

Finally this review also has highlighted plasmoid foodcooking within a MRC and the use of plasma discharge forremoving food odor frommicrowave ovens Given that foodsafety issues are addressed it is reasonable to envisagemicrowave oven plasma reactors incorporating both plas-moid cooking of food stuff and plasma deodorization ofcooking by-products may be realized in the near future

Conflicts of Interest

e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

e authors would like to acknowledge the support of SFIthrough the I-Form Advanced Manufacturing ResearchCenter 16RC3872

References

[1] F Nagelschmidt Diathermy Text Book for Physicians andStudents Springer-Verlag Berlin Heidelberg 1921

[2] Davis A History of the Microwave Oven e IEEE NewsSource e IEEE News Source Piscataway NJ USA httpswwwtheinstituteieeeorgtech-historytechnology-historya-history-of-the-microwave-oven

[3] P L Spencer ldquoMethod of treating foodstuffsrdquo US Patent2495429 1950

[4] J M Osepchuk ldquoe history of the microwave oven a criticalreviewrdquo in Proceedings of Digest IEEE International Micro-wave Symposium pp 1397ndash1400 Boston MA USA June2009

[5] J R Mims ldquoMicrowave magnetronrdquo US Patent 37392251973

[6] T Koinuma ldquoMagnetronrdquo US Patent 3809590 1974[7] R N Gedye F Smith and K C Westaway ldquoe rapid

synthesis of organic compounds in microwave ovensrdquo Ca-nadian Journal of Chemistry vol 6 no 1 pp 17ndash26 1988

[8] R N Gedye W Rank and K C Westaway ldquoe rapidsynthesis of organic compounds in microwave ovens IIrdquoCanadian Journal of Chemistry vol 69 no 4 pp 706ndash7111991

[9] M Omran T Fabritius E-P Heikkinen and G ChenldquoDielectric properties and carbothermic reduction of zincoxide and zinc ferrite by microwave heatingrdquo Royal SocietyOpen Science vol 4 no 9 p 170710 2017

[10] A Ribner ldquoMicrowave plasma etching machine and methodof etchingrdquo US Patent 4804431 1989

[11] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of phase pure K3C6o using a microwave-induced argon plasmardquo Journal of the Chemical SocietyChemical Communications no 18 pp 2027-2028 1994

[12] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of alkali-metal fullerides using a microwave-induced argon plasmardquo Chemistry of Materials vol 8 no 2pp 394ndash400 1996

[13] D J Brooks and R E Douthwaite ldquoMicrowave-inducedplasma reactor based on a domestic microwave oven forbulk solid state chemistryrdquo Review of Scientific Instrumentsvol 75 no 12 pp 5277ndash5279 2004

[14] B T Ginn and O Steinbock ldquoPolymer surface modificationusing microwave-oven-generated plasmardquo Langmuir vol 19no 19 pp 8117-8118 2003

[15] P Khongkrapan N Tippayawong and T Kiatsiriroatldquoermochemical conversion of waste papers to fuel gas ina microwave plasma reactorrdquo Journal of Clean EnergyTechnologies vol 1 no 2 pp 80ndash83 2013

[16] P Khongkrapan P anompongchart N Tippayawong andT Kiatsiriroat ldquoFuel gas and char from pyrolysis of wastepaper in a microwave plasma reactorrdquo IJEE vol 4 no 6pp 969ndash974 2013

[17] R Singh and A L L Jarvis ldquoMicrowave plasma-enhancedchemical vapour deposition growth of carbon nano-structuresrdquo South African Journal of Science vol 106 no 5-6p 4 2010

[18] S Nomura H Toyota S Mukasa H Yamashita T Maeharaand A Kawashima ldquoProduction of hydrogen in a conven-tional microwave ovenrdquo Journal of Applied Physics vol 106no 7 p 073306 2009

[19] S Nomura H Yamashita H Toyota S Mukasa andY Okamura ldquoSimultaneous production of hydrogen andcarbon nanotubes in a conventional microwave ovenrdquo inProceedings of International Symposium on Plasma Chemistry(ISPC19) vol 65 Bochum Germany July 2009

[20] H Toyota S Nomura and S Mukasa ldquoA practical electrodefor microwave plasma processesrdquo International Journal ofMaterials Science and Applications vol 2 no 3 pp 83ndash882013

[21] R Pongsopon T Chim-Oye and M Fuangfoong ldquoMicro-wave plasma reactor based on microwave ovenrdquo in PIERSProceedings pp 2723ndash2726 Guangzhou China August 2014

[22] N Manivannan W Balachandran R Beleca and M AbbodldquoMicrowave plasma system design and modelling for marinediesel exhaust gas abatement of NOx and SOxrdquo InternationalJournal of Environmental Science and Development vol 6no 2 pp 151ndash154 2015

[23] M C Savadori V P Mammana O G Martins andF T Degasperi ldquoPlasma-assisted chemical vapour depositionin a tunable microwave cavityrdquo Plasma Sources Science andTechnology vol 4 no 3 pp 489ndash494 1995

[24] V J Law and N Macgearailt ldquoVisualization of a dual fre-quency plasma etch processrdquo Measurement Science andTechnology vol 18 no 3 pp 645ndash649 2007

[25] Y H Jung S O Jang and H J You ldquoHydrogen generationfrom the dissociation of water using microwave plasmasrdquoChinese Physics Letters vol 30 no 6 p 065204 2013

[26] V J Law and D Tait ldquoContaminated ceramic plasmacleaningrdquo European Semiconductor vol 19 no 9 pp S38ndashS41 1997

[27] V J Law andD Tait ldquoMicrowave plasma cleaning of ion implantceramic insulatorsrdquo Vacuum vol 49 no 4 pp 273ndash278 1998

[28] T M Burke E H Linfield D A Ritchie M Peper andJ H Burroughs ldquoHydrogen radical surface cleaning of GaAsfor MBE regrowthrdquo Journal of Crystal Growth vol 175-176pp 416ndash421 1997

[29] D Tasch D J Brosky S Conrad S Kumar and D KumarldquoMicrowave plasma cookingrdquo US Patent 20090012223 A12009

[30] H Stanley ldquoMicrowave experiments at schoolrdquo Science inSchool vol 12 pp 31ndash33 2009

International Journal of Chemical Engineering 11

a catalytic source for plasma heterogeneous reaction and inthe case of manufacturing carbon nanomaterials to providea substrate on which the carbon material can grow In thefirst of these roles increasing the number of electrodes from1 to 6 has revealed that the efficiency of plasma de-composition of n-dodecane does increase but beyond 6-7electrodes the reaction efficiency becomes rate-limited ismay be due to electromagnetic power loss by the resonantstructure of the electrodes [20] or simply that the addition ofmore than 7 electrodes and their associated surroundingreaction zones (cylindrical volume around each electrodeFigure 3(c)) within a fixed closed volume simply producesa loading effect within the heterogeneous reaction [39] atis to say as the percentage of the combined electrode re-action zones approaches the total fixed volume the amountof fresh reactant flowing to the electrode reaction zonebecomes reduced erefore mass transport in and out of

each electrode reaction zone rather than plasma de-composition may become the rate-limiting step To clarifythese observations further investigation is needed

For the dual-tipped aerial electrode Toyota et al [20]have shown that the U-shaped aerial electrodes have distinctoptimum lengths of L sim 2λm 3λm2 λm and λm2 ey alsoshow that the use of the approximation sign in (1) is justifiedby experimentally determining the λ2 FHHW length ofthe U-shaped dual-tip aerial electrode to be 44 to 47 cm forn-dodecane

217 Igniter e description of construction and use ofwire aerial electrodes for plasma ignition is now used as anaid to outline the construction of the plasma igniter [16] andthe drawing in [40] (Figure 4) Assuming that the drawing in[40] may be scaled the plasma igniter may be constructed intwo ways Firstly the igniter may be constructed using twowire electrodes opposing each other and bent at 45deg so thattheir tips are aligned with the gas flow and the fixing lo-cation is formed using an insulating ring e second andmore practical arrangement is that the igniter is preformedfrom a 30mm diametertimes 05mm thick steel steel disc anda plurality of electrodes are punched from the centralportion of the disc and bent to a 45deg angle For the purposeof this second option the construction of a 4-electrodeigniter is exemplified using the 2730mm internalexternaldiameter glass tube in [16] as a reference tube (Figure 1(e))A schematic of the manufacturing stages of the igniter isgiven in Figure 4 where it is shown that the first stage is topunch out the form of the igniter the second stage is tobend the electrodes and the third stage is to align theigniter to the glass tube Using this method of constructionthe lip of the preform can self-align to enable the 4 aerialelectrodes to suit the plasma ignition criteria as describedin Section 215

218 Production of Plasmoids (Fireballs) e production ofplasmoids sometimes called fireballs or ball-lightningwithin domestic microwave ovens has been posted onYouTube postings [34ndash38] Perhaps the simplest way ofproducing a fireball without modification to the microwaveoven is to place a partially sliced grape (that has its two halvesconnected via a thin piece of skin) in the microwave ovenand then turn on the microwave power for 3ndash10 secondse YouTube posting [34] shows that arc-like plasmoids aregenerated at the thin skin bridge that connects the two grapehalves with the discharge emission continuing until eitherthe power is turned off or the grapes have shriveled up isaction may be understood by considering that the twofreshly cut grape halves have a characteristic dimension of15 to 2 cm and are partially filled with a conducting elec-trolyte the combination of which creates an organic con-ducting dipole antenna not unlike the metal antennasdiscussed in Sections 215 and 216 Given this un-derstanding it is reasonable to assume that as the freeelectrons are pushed back and forth through the narrow thinskin bridge of the grape heat is generated due to the

Coppersheet Glass and so on

Metal electrode

L

(a)

λ4

λ4

(b)

Electrode reaction zone

(c)

Figure 3 Typical single-tip electrode arrangement (a) multipleelectrode arrangements (b) electrode reaction zone (c)

International Journal of Chemical Engineering 5

resistance and burns away the skin In addition themovement of electrons through the grape electrolyte inducesa rapid increase in temperature causing vaporization of theelectrolyte to a cloud of electrons and ions thus forming thelocalized plasmoid e plasmoid continues to be sustainedas long as the free electrons are available from the dimin-ishing volume of the grape electrolyte

Moving away from the organic source for generatingplasmoids a lighted safety match supported by a wine corkcovered with a glass jar and placed within the center of theMRC can also be used [35] Upon turning on the micro-wave power a plasma discharge is generated that rises to thetop of the jar thus forming a buoyant plasmoid Warren[36] used a similar approach but this time using a glassjar supported by three wine corks and a lighted cigaretteplaced in the gap provided by the corks In this work andthe previous example the plasmoids are maintained whenthe thermal source is extingusihed It is only when themicrowave power is turned-off does the plasmoid becomeextinguished Plasmoids can also be generated withinelectric light bulbs and fluorescence tubes as shown in [37]this example also appears to be the basis for the near-fieldE-probe (Section 44)

A more dangerous approach to generating plasmoids isdemonstrated in [38] where a cavity magnetron connectedto a food tin can is used to lunch microwaves at a domesticlight bulb to produce a plasmoid within the bulb From thisexperiment it would appear that the electric filament acts asthe initiating electrode

Before finishing this section it is worth noting that thecylindrical plasma reactor produced for the fast food in-dustry [29] employed a patented passive plasma catalyst inthe form of an electrode to ignite the atmospheric plasma[41] where the passive plasma catalyst can include anyobject capable of inducing plasma by deforming the localelectric field On the other hand the patent states that anactive plasma catalyst produces particles or a high-energywave packet capable of transferring a sufficient amount ofenergy to a gaseous atom (or molecule) to remove at leastone electron from the gaseous atom (or molecule) in thepresence of electromagnetic radiation Given these twodefinitions it is reasonable to assume that the safety matchflame [35] cigarette [36] and grape [34] can be classed as anactive plasma catalyst and the metal electrode as a passiveplasma catalyst

219 Plasmoid Food Cooking e Korean patents [42 43]and conference paper [44] report on a form of tuning withinthe TE10 waveguide that fall outside the scope of this reviewbut they are listed for three reasons Firstly the phenomenaof plasmoids extend the cooking range of the domesticmicrowave oven from one of dielectric heating of food stuffto one that provides surface browning and imparting textureand flavor that is similar to the traditional flame-cookingprocess Secondly Jerby et al [44] have noted that plasmoidsproduced in this way require wire antenna electrode to ignitethe plasmoid and therefore may contain nanoparticleswhich might be harmful for the food quality and even makeit inedible irdly the additional use of plasma dischargethat generates ozone and ions for the removal of odor-producing materials from the cooking chamber [45] doesprovide one possible technical route forward in the futuredevelopment of the domestic microwave oven

3 Purpose-Built Microwave OvenPlasma Reactor

is section describes the methodology used in the con-struction of a purpose-built microwave oven plasma reactorOf particular importance in this regard is the MRC seriesof plasma reactors that were built in the mid-1990s atCambridge Fluid Systems Ltd (England UK) e designconcept behind these plasma reactors was to build a simplereliable and cost-effective table-top plasma reactor that couldbe sold to research laboratories and low-volume productionsunits eir main use was for surface engineering enhance-ment in the microelectronic semiconductor sector and themanufacture of bodyshell of Formula One racing cars

e design of the plasma reactor is similar to microwaveovens where the cavity magnetron antenna is locatedwithin a TE10 waveguide that is used to illuminate the MRCthrough a single iris e cutoff frequency (fc)mn of theTE10 waveguide is calculated using the following equation

fc( 1113857mn C

2

m

a1113874 1113875

2+

n

b1113874 1113875

21113971

(2)

where c is the speed of the light and a and b are the internaldimensions (width and height) of the waveguide in this case80 and 38mm are used respectively which equate a cutofffrequency of 1875GHz

30 mm

(1) Punch stage (2) Bend stage (3) Glass tube fitting

Lip

28 mm Glass tube

FIGURE 4 Manufacturing stages of the disc igniter that is suitable for a narrow glass tube reactor

6 International Journal of Chemical Engineering

With the cavity magnetron antenna positioned 26mmfrom the end of the waveguide the frequency and bandwidthof the magnetron are allowed to be free running us thenoncoherent reflected power passing through the iris travelsback to the magnetron thus altering the SWR of the coherentwave within the TE10 waveguide resulting in varying theoutput power of the magnetron

e MRC reactor design differs from the domesticmicrowave oven plasma reactor in the following ways (alsocf Figure 1 with Figure 5)

(i) e chassis MRC and waveguide are constructed asone welded component using 14mm thick mild steelsheet Before each of the three components is weldedtogether they have all the necessary holes punchedand clasp nuts fixed Once welded the structure isnickel plated to produce a metal structure that isrobust with sufficient stiffness to support all theadditional components (the front and rear stainlessflanges gas lines DC power supply pressure gaugeetc) Using this construction approach the MRC hasa theoretical maximum unloadedQ-factor (Qu) in theTE mode that is dependent on the ratio of storedenergy in the cavity (Vc) to the energy loss to thecavity walls (δ times Ac)

Qu 2Vc

δAc (3)

where δ is the electrical skin depth at the cavity wall per cycleand Ac is the cavity wall area

For this reactor the main cavity has an approximatelyQuof 20000 at a resonant frequency of 245GHz

(ii) A cylindrical Pyrex glass chamber (190mm di-ameter 300mm length and 5mm wall thicknessproducing a volume of 3 liters) is located within themultimode cavity with its longitudinal axis per-pendicular to the microwave iris and with the frontand rear of the chamber housed within metal flangesthat form the part of the multimode cavity wall erear flange contains welded vacuum and pressuregauge ports and the front flanges contain the accessdoor is design maximizes the chamber volumeand removes all fragile glass fittings plastic tubeconnectors and feedthrough microwave leakagegaskets

(iii) e gas lines are fitted within the chassis and to theside of theMRC thereby enabling the process gassesto be injected throughmultiple equally spaced radialports in the front flange thus reducing the possibilityof precursor gas being preionized prior to chamberentry and maximizing uniform gas flow and plasmauniformity along process chamber longitudinal axis

31 Plasma Cleaning of Ion Implant Ceramic InsulatorsIon implantation is one of the key processes in the highvolume (220 wafers per hour) manufacture of silicon

semiconductor devices ese ion implant machines how-ever cost between $18M and $3M ese machines are alsohighly maintenance-intensive systems with high capitalcost therefore availability and cost of ownership aremajor factors to be considered Many of the parts changedduring regular maintenance and ion source changes areceramic insulators In this section an overview of theplasma cleaning of ion implant ceramic is described forfull details of the process see [26 27] e plasma cleaningprocess has been performed in the MRC series of plasmareactor using a gas mixture of 5ndash10 O2 in CF with anadmixture of 50 by flow of argon e argon admix isused to stabilize the microwave plasma by moderating theelectron energy distribution and to provide a uniformexcited species throughout the plasma volume eplasma etch chemistry at the surface of the ceramic may beconsidered to proceed by the following representativeheterogeneous reaction

3CF4(g) + 15O2(g) + 2X(s)rarr 2XF3(g) + 3COF3(g)

(4)

in which the addition of O2 scavenges carbon from the CF4through the formation of COFx species to enhance thesteady-state concentration of F atoms in the plasma vo1umee element X in reaction (4) represents the group V ele-ment (As P and Sb) on the ceramic surface and the XF3 arethe etch products Given sufficient microwave power theetch rates of these products are therefore controlled by the

Gas lines CavityMagnetron timer and

power supply

Front view

Pressuremeter

(a)

Gaugeport

Vacuumport

Doo

r

Side view

(b)

Figure 5 Typical front and side view schematic of theMRC plasmareactor chassis and cavity Photograph of the microwave MRC-200plasma reactor

International Journal of Chemical Engineering 7

production of F atoms (O2 balance) product volatility andceramic microscopic surface area

For the MRC-100 reactor typical plasma process pa-rameters obtained were 104W and 10mbar with an etchtime of 45 minutes where the surface temperature of theceramics reached 80plusmn 5K In the case of the MRC-200reactor the process parameters were as follows 200Wand 10mbar (1000 Pascal) with an etch time of 20 to 25minutes where the surface temperature of the ceramicsreached 125plusmn 5K

4 Microwave Cavity Calibration

is section describes a range of different techniques whichare used for the evaluation of the microwave efficiency aswell as leakage

41 Microwave Leakage Measurement e European Di-rective 200440EC and ICNIRP (1998) guidelines recom-mend that industrial microwave ovens have surfacemicrowave (3ndash300GHz) radiation power density levelsgt5mWmiddotcmminus2 and 5 times less for domestic microwave ovensintended for general use Applying a quadratic law based onplane wave theory an operator standing at a distance of20 cm from an industrial oven the operator will receive themaximum allowable power density level of 3mWmiddotcmminus2 Fordomestic oven environment this equates to a power densitylevel of 03mWmiddotcmminus2 In the context of converted ovensintended for plasma many feedthrough and apertures in theMRC require considerable care in design and constructionto prevent microwave leakage

42 Calorimetric Magnetron Power Calibration e mag-netron power entering the MRC may be calibrated using thewater open-dish load method see for example BritishStandard 75091995 and IEC 13071994 Hence given theheat capacity of water is 4184 J(gmiddotK) the calculated appliedpower (P) within the cavity may be obtained by placinga known mass of water (m) within the cavity and heating itfor a short period of time (t) making sure the water does notboil With the knowledge of themeasured water temperaturechange (ΔT final temperature ndash initial temperature) themicrowave power entering into the cavity is calibratedfor a given power setting using the following equation(see also [40])

P mCΔT

t (5)

e calibration however must be considered to be anupper value for plasma processing as its dielectric volumewill be different from the water calibration (Note when theprocess chamber volume or the geometric shape does notallow the open dish method to be used the alternative flowmethod can be used as outlined in [46])

Given (4) the microwave power density (Wmiddotcmminus3) ofa system may be calculated by dividing through by theprocess chamber volume e following open-dish loadmethod for the MRC 100 is given here as an illustrative

example the MRC-100 and MRC-200 reactors have a cal-culated magnetron applied power of 104W which equatesto a power density of 0116Wmiddotcmminus3 For the MRC-200 re-actor the calorimetric measurements produce magnetronapplied power values of 200W (0022Wmiddotcmminus3) and 450W(005Wmiddotcmminus3)

43 Surface Temperature Measurement Knowledge of thesurface temperature of materials immersed in the plasma isuseful in understanding the heterogeneous plasma-surfaceinteraction is is of particular importance when localdielectric heating has the potential of thermal runawaybecause most materials increase their dielectric loss withtemperature [12] Two simple means of estimating localsurface temperature of materials that are immersed in mi-crowave plasma have been used For surface temperaturesbelow 180K liquid crystal temperature-sensitive (20plusmn 5K to180plusmn 5K) strips attached to the plasma immersed surfacecan be used [27] For higher temperatures salts of knownmelting point (KCl 1043K and NaCl 1074K) sealed insilica capillaries have been used [13]

44 Near-Field Plasma E-Probe Measurement Attempts tostrike a plasma outside the ignition pressure limits of 01 to20mbar (10 to 2000 Pascal) result in the microwave radi-ation being stored ldquoper cyclerdquo in the empty-cavity modeunder these conditions the rate of energy loss to the cavitywall can substantially heat the MRC structure and in theextreme case the microwave radiation leakage can becomea health risk Additionally if the MRC is loaded with ma-terials (semiconductor wafers and low dielectric strengthmaterial) these can be electrically or mechanically damagedIt therefore becomes necessary to have an automatic powercutoff device to prevent microwave leakage and damage toboth the reactor and loaded materials e near-field plasmaE-probe as described by Law [47] is one such device thathelps monitor such events In this circuit a neon dischargelamp a photodiode and a reference voltage are connected asshown in Figure 6 with one leg of the neon being usedprotruding into the cavity to act as a near-field E-probe Inthe original circuit design a strip chart is used to record thevoltage-time-series data but with todayrsquos analogue-to-digital converters and software (such as LabView) theplasma ignition state-space and plasma state-space may bemonitored with trigger levels set to give a binary GoNocontrol output [24 47]

45 Bebesonde Electrostatic Probe Measurement For plasmaprocesses it is generally considered that the ion flux arrivingand leaving the surface determines the plasma processHowever the use of electrostatic probe techniques to de-termine the ion density and temperature of plasma driven bymodulated power source in the presence of sputtered in-sulating material is problematic Such is the case of plasmacontaining CF4 is section describes a probe techniquethat is tolerant of drive modulation and sputtered insulatingmaterials e following measurements were performed at

8 International Journal of Chemical Engineering

the Oxford Research Unit Open University on the MRC-100 reactor e probe used is an RF-biased ion flux probe(known colloquially as ldquoBebesone or BBsrdquo) For full detailsof the probe and measurement see [48]

For good probe measurements visual observations ofthe plasma volume as a whole and around the probe itselfare usually required For the MRC reactors it was foundnecessary to replace the standard front flange door witha flange incorporating a probe port and inspection portcovered by an open mesh Given this modification argonplasma within the MRC-100 visually exhibits littlestructure with a slight brightening close to the dielectricannulus boundary Nevertheless there is no evidence ofmicrowave structure in the optical emission which in-dicates that energy is rapidly homogenized in the electronpopulation

Following these visual baseline inspections theBebesonde was used to determine the ion flux in the lowpower and high setting with argon gas flow varying froma maximum (5 lmiddotminminus1) to a minimum for which plasmacould be sustained (2 lmiddotminminus1) For the MRC-200 reactorargon ion densities are measured to be in the range of2times1011middotcmminus3 at 50mbar (5 kmiddotPascal) to 3times l012middotcmminus3 at1mbar (100 Pascal) and the electron temperature is in therange of l to l5 eV for an input power of 0022Wmiddotcmminus3 thehigher plasma density at low pressure corresponds to a longermean free path and increased electron power transfer

46 Ultraviolet Fluorescence Microwave Probe For manyprocessing plasmas photon energy (E hcλ) varies from10 eV to 1 eV with preferential intensity at discrete spectralwavelengths e spectral characteristic is determined by thenature of gas excitation and relaxation processes both ofwhich are sensitive to the local electron temperature and gasexcitation cross sections Gas composition and mode ofplasma production therefore has a major impact on UVproduction and plasma chemistry

e ultraviolet fluorescence probe utilizes activated rare-earth salts Y2SiO5Ce (absorptionlt 200 nm) and Zn2SiO4Mn and Y2O3Eu3+ (absorptionlt 300 nm) For full detailsof these probes see [49] At these energies the host lattice(H) undergoes electronic excitation via the production of

electron-hole pair separation (6) followed by the lowestenergetic deactivation pathway fluorescence at wavelengthslonger than the initial radiation In general the fluorescencespectrum is composed of a narrow band with the precisewavelength determined by the intimate relation between theactivator (Ce Mn and Eu3+) and the host lattice and by theirradiating photon radiation

H A + hvprime rarrH Alowast

hvprime gt 4ev( 1113857 (6)

H Alowast rarrH A + hvPrime hvPrime lt 4ev( 1113857 (7)

emost simple form of the probes comprises a syntheticDUV grade fused silica capsule (12mm diametertimes 20mm)containing one of three activated salts at a nominal reducedpressure of 10mbare fused silica has transmittance T 05at 170 nm When placed in the plasma volume the probecollects 4π steradians of incoming DUV photon radiatione emitted fluorescence is viewed through an optical crownglass viewport (T 05 at 380 nm) Due to the dielectriccapsule photoluminescence rather than electroluminescenceis considered to be the prime mechanism of fluorescence inthese probes e dielectric also acts as a wavelength dis-criminator and provides the upper working limit of the probeto be 1100K

Using these knowledge the salts Zn2SiO4Mn and Y2O3Eu3+ (placed in their own capsule) integrates DUV plasmawhile Y2SiO5Ce when placed directly in the plasma volumeintegrates VUV (lt200 nm) plasma thus their fluorescenceappearing in the green red and blue respectively

5 Magnetron Oven Control Circuits

Microwave ovens generally employed one of two types ofmagnetron drive circuits For microwave ovens with low-power outputs (typically lt500W) the output power isachieved by pulse-width modulation of a single incidentpower to the magnetron with the cooking time set between0 and 30 minutes Between the 500W and 1100W ratingcontinuous application of the microwave power is usedwhere the power level is set by the magnetron drive capacitorvalue

P 12

CV2

1113872 1113873f (8)

e choice of either of these two drive circuits can havean impact on the magnetron control circuit ability to carryout the selected plasma processis choice is exemplified bycomparing the short and low power requirements of rapidplasma syntheses of organic compounds [7 8] cleaning ofglass slides and polymers [14] to that of the high power andlong processing times of solid-state metal oxides processing[13] and the plasma cleaning of ion implant ceramic in-sulators [26 27]

51 Cavity Magnetron Capacitor-Controlled Drive CircuitIn this section the choice of the capacitor controlled drivecircuit is considered along with its safety control circuit thatis used in the MRC-100200 plasma reactors A schematic of

Cavity wall

E-field probe

Neon

Photodiode

Shield

Vref

AD

Figure 6 Schematic of the E-probe circuit showing a neon dis-charge lamp a photodiode and an analogue-to-digital converter

International Journal of Chemical Engineering 9

the capacitor controlled drive circuit is shown in Figure 7 Asimilar cavity magnetron capacitor controlled drive circuit isreported in [23] Chaichumporn et al have also reportedfurther refinement of the magnetron anode voltage (33 to66 kV) in [40]

e transformer comprises two winding circuits efirst provides a winding ratio to produce 240V at 50ndash60Hzto approximately 35V at 50ndash60Hz for the cathode heaterand the second provides the step-up voltage (240V at 50ndash60Hzto 2-3 kV at 50ndash60Hz) e HV capacitor (06 to 15 μF)and the HV diode are used to bias the cathode negativewith respect to the anode block which contains themagnetronrsquos cavity structure Using this arrangement thecapacitor value and diode determines the DC powerdissipated in the magnetron cavity structure For safetyreasons both passive and active control components areincorporated on either side of the setup transformer eseinclude the following the circuit is fused on both sides ofthe transformer plus an emergency stop button a chassisinterlock a magnetron thermoswitch (135degC) and a chassecooling fan In addition 24 V DC circuit is used to provideremote passive and active control of the drive circuit (seeSection 52)

52 24V DC Control Circuit In purpose-built microwaveoven plasma reactors the auxiliary equipment (vacuumpumps vacuum valves pressure gauge gas lines purge linesprocess timer and microwave power) is synchronized toplasma process in a safe manner e role of the 24V DCcontrol circuit is to synchronize the auxiliary componentsto the plasma process and shut down the system in the caseof an emergency failure this is especially important whenusing flammable corrosive and toxic precursor gases andby-products e circuit is designed so that all chasse controlsare isolated from the cavity magnetron capacitor-controlleddrive circuit using relays and solenoids It should be notedthat when converting a domestic microwave oven intoa plasma reactor the synchronization has to be built fromscratch

6 Conclusions

is work has reviewed the conversation of the domes-tic microwave oven into a source for cleaning as well aschemical reactions e conversion of domestic systems intoplasma reactor is described as is the construction ofpurpose-built microwave oven plasma reactors Calibrationof the MRC has been discussed along with identifying thetwo types of magnetron drive circuitry used e pro-fessional and armature use has also been presented in thelatter case mainly limited to kitchen top experiments eproof of principle and small batch processes established inthese plasma reactors range from plasma cleaning of glassand polymer surface and removal of toxic metals fromceramic surfaces to the manufacture of carbon nano-structures and the pyrolysis of paper to produce gaseouswaste by-products In all cases the power source is thepackage cavity magnetron operating below 1100W outputpower and pressures ranging from a few 01 s Pascal toa nominal atmospheric pressure (1013 Pascal)

At or close to atmospheric pressure single or multiplewire antenna electrodes that have a physical length ap-proximating to 14 or 12 of the microwave length in whichthat it is immersed have been found to play a catalytic rolein instigating plasma production and in the case ofmanufacturing carbon nanomaterials provide a substrate onwhich carbon material can grow With regard to reactionrate increasing with the number (1 to 6) of electrodes be-yond which the reaction becomes rate-limited a geometricalloading effect around the wire antenna is proposed in thispaper Whether this or electromagnetic power loss is re-sponsible for the effect further work is required In additionthis work has reconstructed a preformed disc aerial elec-trode (igniter) suitable for a narrow tube reactor [16 40](Figure 4)

e safety match flame lighted cigarette sliced grapeand metal antenna electrode have been observed to havea catalyst role in the production of plasma and plasmoids Todistinguish between the metal antenna and the thermal-chemical based catalyst it has been put forward that metal

Anode

Cathode

24 V

dcCo

ntro

l circ

uit

Step

-up

tran

sform

er

HV diode

35 V

Chassisinterlock

Magnetronthermoswitch Fuse

Filtered240 V

50ndash60 Hz

HVFuse

Coolingfan

HVCapacitor

25 kV

Emergencystop

Figure 7 Cavity magnetron capacitor-controlled dive circuit e 24V DC control circuit represented by the dashed box is discussed inSection 52

10 International Journal of Chemical Engineering

antennas may be classified as a passive catalyst as they onlysupply a surface that generates free electrons whilst thesafety match flame lighted cigarette and sliced grape may beclassified as an active catalyst as they supply energy in theform of heat and free electrons from an electrolyte

Finally this review also has highlighted plasmoid foodcooking within a MRC and the use of plasma discharge forremoving food odor frommicrowave ovens Given that foodsafety issues are addressed it is reasonable to envisagemicrowave oven plasma reactors incorporating both plas-moid cooking of food stuff and plasma deodorization ofcooking by-products may be realized in the near future

Conflicts of Interest

e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

e authors would like to acknowledge the support of SFIthrough the I-Form Advanced Manufacturing ResearchCenter 16RC3872

References

[1] F Nagelschmidt Diathermy Text Book for Physicians andStudents Springer-Verlag Berlin Heidelberg 1921

[2] Davis A History of the Microwave Oven e IEEE NewsSource e IEEE News Source Piscataway NJ USA httpswwwtheinstituteieeeorgtech-historytechnology-historya-history-of-the-microwave-oven

[3] P L Spencer ldquoMethod of treating foodstuffsrdquo US Patent2495429 1950

[4] J M Osepchuk ldquoe history of the microwave oven a criticalreviewrdquo in Proceedings of Digest IEEE International Micro-wave Symposium pp 1397ndash1400 Boston MA USA June2009

[5] J R Mims ldquoMicrowave magnetronrdquo US Patent 37392251973

[6] T Koinuma ldquoMagnetronrdquo US Patent 3809590 1974[7] R N Gedye F Smith and K C Westaway ldquoe rapid

synthesis of organic compounds in microwave ovensrdquo Ca-nadian Journal of Chemistry vol 6 no 1 pp 17ndash26 1988

[8] R N Gedye W Rank and K C Westaway ldquoe rapidsynthesis of organic compounds in microwave ovens IIrdquoCanadian Journal of Chemistry vol 69 no 4 pp 706ndash7111991

[9] M Omran T Fabritius E-P Heikkinen and G ChenldquoDielectric properties and carbothermic reduction of zincoxide and zinc ferrite by microwave heatingrdquo Royal SocietyOpen Science vol 4 no 9 p 170710 2017

[10] A Ribner ldquoMicrowave plasma etching machine and methodof etchingrdquo US Patent 4804431 1989

[11] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of phase pure K3C6o using a microwave-induced argon plasmardquo Journal of the Chemical SocietyChemical Communications no 18 pp 2027-2028 1994

[12] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of alkali-metal fullerides using a microwave-induced argon plasmardquo Chemistry of Materials vol 8 no 2pp 394ndash400 1996

[13] D J Brooks and R E Douthwaite ldquoMicrowave-inducedplasma reactor based on a domestic microwave oven forbulk solid state chemistryrdquo Review of Scientific Instrumentsvol 75 no 12 pp 5277ndash5279 2004

[14] B T Ginn and O Steinbock ldquoPolymer surface modificationusing microwave-oven-generated plasmardquo Langmuir vol 19no 19 pp 8117-8118 2003

[15] P Khongkrapan N Tippayawong and T Kiatsiriroatldquoermochemical conversion of waste papers to fuel gas ina microwave plasma reactorrdquo Journal of Clean EnergyTechnologies vol 1 no 2 pp 80ndash83 2013

[16] P Khongkrapan P anompongchart N Tippayawong andT Kiatsiriroat ldquoFuel gas and char from pyrolysis of wastepaper in a microwave plasma reactorrdquo IJEE vol 4 no 6pp 969ndash974 2013

[17] R Singh and A L L Jarvis ldquoMicrowave plasma-enhancedchemical vapour deposition growth of carbon nano-structuresrdquo South African Journal of Science vol 106 no 5-6p 4 2010

[18] S Nomura H Toyota S Mukasa H Yamashita T Maeharaand A Kawashima ldquoProduction of hydrogen in a conven-tional microwave ovenrdquo Journal of Applied Physics vol 106no 7 p 073306 2009

[19] S Nomura H Yamashita H Toyota S Mukasa andY Okamura ldquoSimultaneous production of hydrogen andcarbon nanotubes in a conventional microwave ovenrdquo inProceedings of International Symposium on Plasma Chemistry(ISPC19) vol 65 Bochum Germany July 2009

[20] H Toyota S Nomura and S Mukasa ldquoA practical electrodefor microwave plasma processesrdquo International Journal ofMaterials Science and Applications vol 2 no 3 pp 83ndash882013

[21] R Pongsopon T Chim-Oye and M Fuangfoong ldquoMicro-wave plasma reactor based on microwave ovenrdquo in PIERSProceedings pp 2723ndash2726 Guangzhou China August 2014

[22] N Manivannan W Balachandran R Beleca and M AbbodldquoMicrowave plasma system design and modelling for marinediesel exhaust gas abatement of NOx and SOxrdquo InternationalJournal of Environmental Science and Development vol 6no 2 pp 151ndash154 2015

[23] M C Savadori V P Mammana O G Martins andF T Degasperi ldquoPlasma-assisted chemical vapour depositionin a tunable microwave cavityrdquo Plasma Sources Science andTechnology vol 4 no 3 pp 489ndash494 1995

[24] V J Law and N Macgearailt ldquoVisualization of a dual fre-quency plasma etch processrdquo Measurement Science andTechnology vol 18 no 3 pp 645ndash649 2007

[25] Y H Jung S O Jang and H J You ldquoHydrogen generationfrom the dissociation of water using microwave plasmasrdquoChinese Physics Letters vol 30 no 6 p 065204 2013

[26] V J Law and D Tait ldquoContaminated ceramic plasmacleaningrdquo European Semiconductor vol 19 no 9 pp S38ndashS41 1997

[27] V J Law andD Tait ldquoMicrowave plasma cleaning of ion implantceramic insulatorsrdquo Vacuum vol 49 no 4 pp 273ndash278 1998

[28] T M Burke E H Linfield D A Ritchie M Peper andJ H Burroughs ldquoHydrogen radical surface cleaning of GaAsfor MBE regrowthrdquo Journal of Crystal Growth vol 175-176pp 416ndash421 1997

[29] D Tasch D J Brosky S Conrad S Kumar and D KumarldquoMicrowave plasma cookingrdquo US Patent 20090012223 A12009

[30] H Stanley ldquoMicrowave experiments at schoolrdquo Science inSchool vol 12 pp 31ndash33 2009

International Journal of Chemical Engineering 11

resistance and burns away the skin In addition themovement of electrons through the grape electrolyte inducesa rapid increase in temperature causing vaporization of theelectrolyte to a cloud of electrons and ions thus forming thelocalized plasmoid e plasmoid continues to be sustainedas long as the free electrons are available from the dimin-ishing volume of the grape electrolyte

Moving away from the organic source for generatingplasmoids a lighted safety match supported by a wine corkcovered with a glass jar and placed within the center of theMRC can also be used [35] Upon turning on the micro-wave power a plasma discharge is generated that rises to thetop of the jar thus forming a buoyant plasmoid Warren[36] used a similar approach but this time using a glassjar supported by three wine corks and a lighted cigaretteplaced in the gap provided by the corks In this work andthe previous example the plasmoids are maintained whenthe thermal source is extingusihed It is only when themicrowave power is turned-off does the plasmoid becomeextinguished Plasmoids can also be generated withinelectric light bulbs and fluorescence tubes as shown in [37]this example also appears to be the basis for the near-fieldE-probe (Section 44)

A more dangerous approach to generating plasmoids isdemonstrated in [38] where a cavity magnetron connectedto a food tin can is used to lunch microwaves at a domesticlight bulb to produce a plasmoid within the bulb From thisexperiment it would appear that the electric filament acts asthe initiating electrode

Before finishing this section it is worth noting that thecylindrical plasma reactor produced for the fast food in-dustry [29] employed a patented passive plasma catalyst inthe form of an electrode to ignite the atmospheric plasma[41] where the passive plasma catalyst can include anyobject capable of inducing plasma by deforming the localelectric field On the other hand the patent states that anactive plasma catalyst produces particles or a high-energywave packet capable of transferring a sufficient amount ofenergy to a gaseous atom (or molecule) to remove at leastone electron from the gaseous atom (or molecule) in thepresence of electromagnetic radiation Given these twodefinitions it is reasonable to assume that the safety matchflame [35] cigarette [36] and grape [34] can be classed as anactive plasma catalyst and the metal electrode as a passiveplasma catalyst

219 Plasmoid Food Cooking e Korean patents [42 43]and conference paper [44] report on a form of tuning withinthe TE10 waveguide that fall outside the scope of this reviewbut they are listed for three reasons Firstly the phenomenaof plasmoids extend the cooking range of the domesticmicrowave oven from one of dielectric heating of food stuffto one that provides surface browning and imparting textureand flavor that is similar to the traditional flame-cookingprocess Secondly Jerby et al [44] have noted that plasmoidsproduced in this way require wire antenna electrode to ignitethe plasmoid and therefore may contain nanoparticleswhich might be harmful for the food quality and even makeit inedible irdly the additional use of plasma dischargethat generates ozone and ions for the removal of odor-producing materials from the cooking chamber [45] doesprovide one possible technical route forward in the futuredevelopment of the domestic microwave oven

3 Purpose-Built Microwave OvenPlasma Reactor

is section describes the methodology used in the con-struction of a purpose-built microwave oven plasma reactorOf particular importance in this regard is the MRC seriesof plasma reactors that were built in the mid-1990s atCambridge Fluid Systems Ltd (England UK) e designconcept behind these plasma reactors was to build a simplereliable and cost-effective table-top plasma reactor that couldbe sold to research laboratories and low-volume productionsunits eir main use was for surface engineering enhance-ment in the microelectronic semiconductor sector and themanufacture of bodyshell of Formula One racing cars

e design of the plasma reactor is similar to microwaveovens where the cavity magnetron antenna is locatedwithin a TE10 waveguide that is used to illuminate the MRCthrough a single iris e cutoff frequency (fc)mn of theTE10 waveguide is calculated using the following equation

fc( 1113857mn C

2

m

a1113874 1113875

2+

n

b1113874 1113875

21113971

(2)

where c is the speed of the light and a and b are the internaldimensions (width and height) of the waveguide in this case80 and 38mm are used respectively which equate a cutofffrequency of 1875GHz

30 mm

(1) Punch stage (2) Bend stage (3) Glass tube fitting

Lip

28 mm Glass tube

FIGURE 4 Manufacturing stages of the disc igniter that is suitable for a narrow glass tube reactor

6 International Journal of Chemical Engineering

With the cavity magnetron antenna positioned 26mmfrom the end of the waveguide the frequency and bandwidthof the magnetron are allowed to be free running us thenoncoherent reflected power passing through the iris travelsback to the magnetron thus altering the SWR of the coherentwave within the TE10 waveguide resulting in varying theoutput power of the magnetron

e MRC reactor design differs from the domesticmicrowave oven plasma reactor in the following ways (alsocf Figure 1 with Figure 5)

(i) e chassis MRC and waveguide are constructed asone welded component using 14mm thick mild steelsheet Before each of the three components is weldedtogether they have all the necessary holes punchedand clasp nuts fixed Once welded the structure isnickel plated to produce a metal structure that isrobust with sufficient stiffness to support all theadditional components (the front and rear stainlessflanges gas lines DC power supply pressure gaugeetc) Using this construction approach the MRC hasa theoretical maximum unloadedQ-factor (Qu) in theTE mode that is dependent on the ratio of storedenergy in the cavity (Vc) to the energy loss to thecavity walls (δ times Ac)

Qu 2Vc

δAc (3)

where δ is the electrical skin depth at the cavity wall per cycleand Ac is the cavity wall area

For this reactor the main cavity has an approximatelyQuof 20000 at a resonant frequency of 245GHz

(ii) A cylindrical Pyrex glass chamber (190mm di-ameter 300mm length and 5mm wall thicknessproducing a volume of 3 liters) is located within themultimode cavity with its longitudinal axis per-pendicular to the microwave iris and with the frontand rear of the chamber housed within metal flangesthat form the part of the multimode cavity wall erear flange contains welded vacuum and pressuregauge ports and the front flanges contain the accessdoor is design maximizes the chamber volumeand removes all fragile glass fittings plastic tubeconnectors and feedthrough microwave leakagegaskets

(iii) e gas lines are fitted within the chassis and to theside of theMRC thereby enabling the process gassesto be injected throughmultiple equally spaced radialports in the front flange thus reducing the possibilityof precursor gas being preionized prior to chamberentry and maximizing uniform gas flow and plasmauniformity along process chamber longitudinal axis

31 Plasma Cleaning of Ion Implant Ceramic InsulatorsIon implantation is one of the key processes in the highvolume (220 wafers per hour) manufacture of silicon

semiconductor devices ese ion implant machines how-ever cost between $18M and $3M ese machines are alsohighly maintenance-intensive systems with high capitalcost therefore availability and cost of ownership aremajor factors to be considered Many of the parts changedduring regular maintenance and ion source changes areceramic insulators In this section an overview of theplasma cleaning of ion implant ceramic is described forfull details of the process see [26 27] e plasma cleaningprocess has been performed in the MRC series of plasmareactor using a gas mixture of 5ndash10 O2 in CF with anadmixture of 50 by flow of argon e argon admix isused to stabilize the microwave plasma by moderating theelectron energy distribution and to provide a uniformexcited species throughout the plasma volume eplasma etch chemistry at the surface of the ceramic may beconsidered to proceed by the following representativeheterogeneous reaction

3CF4(g) + 15O2(g) + 2X(s)rarr 2XF3(g) + 3COF3(g)

(4)

in which the addition of O2 scavenges carbon from the CF4through the formation of COFx species to enhance thesteady-state concentration of F atoms in the plasma vo1umee element X in reaction (4) represents the group V ele-ment (As P and Sb) on the ceramic surface and the XF3 arethe etch products Given sufficient microwave power theetch rates of these products are therefore controlled by the

Gas lines CavityMagnetron timer and

power supply

Front view

Pressuremeter

(a)

Gaugeport

Vacuumport

Doo

r

Side view

(b)

Figure 5 Typical front and side view schematic of theMRC plasmareactor chassis and cavity Photograph of the microwave MRC-200plasma reactor

International Journal of Chemical Engineering 7

production of F atoms (O2 balance) product volatility andceramic microscopic surface area

For the MRC-100 reactor typical plasma process pa-rameters obtained were 104W and 10mbar with an etchtime of 45 minutes where the surface temperature of theceramics reached 80plusmn 5K In the case of the MRC-200reactor the process parameters were as follows 200Wand 10mbar (1000 Pascal) with an etch time of 20 to 25minutes where the surface temperature of the ceramicsreached 125plusmn 5K

4 Microwave Cavity Calibration

is section describes a range of different techniques whichare used for the evaluation of the microwave efficiency aswell as leakage

41 Microwave Leakage Measurement e European Di-rective 200440EC and ICNIRP (1998) guidelines recom-mend that industrial microwave ovens have surfacemicrowave (3ndash300GHz) radiation power density levelsgt5mWmiddotcmminus2 and 5 times less for domestic microwave ovensintended for general use Applying a quadratic law based onplane wave theory an operator standing at a distance of20 cm from an industrial oven the operator will receive themaximum allowable power density level of 3mWmiddotcmminus2 Fordomestic oven environment this equates to a power densitylevel of 03mWmiddotcmminus2 In the context of converted ovensintended for plasma many feedthrough and apertures in theMRC require considerable care in design and constructionto prevent microwave leakage

42 Calorimetric Magnetron Power Calibration e mag-netron power entering the MRC may be calibrated using thewater open-dish load method see for example BritishStandard 75091995 and IEC 13071994 Hence given theheat capacity of water is 4184 J(gmiddotK) the calculated appliedpower (P) within the cavity may be obtained by placinga known mass of water (m) within the cavity and heating itfor a short period of time (t) making sure the water does notboil With the knowledge of themeasured water temperaturechange (ΔT final temperature ndash initial temperature) themicrowave power entering into the cavity is calibratedfor a given power setting using the following equation(see also [40])

P mCΔT

t (5)

e calibration however must be considered to be anupper value for plasma processing as its dielectric volumewill be different from the water calibration (Note when theprocess chamber volume or the geometric shape does notallow the open dish method to be used the alternative flowmethod can be used as outlined in [46])

Given (4) the microwave power density (Wmiddotcmminus3) ofa system may be calculated by dividing through by theprocess chamber volume e following open-dish loadmethod for the MRC 100 is given here as an illustrative

example the MRC-100 and MRC-200 reactors have a cal-culated magnetron applied power of 104W which equatesto a power density of 0116Wmiddotcmminus3 For the MRC-200 re-actor the calorimetric measurements produce magnetronapplied power values of 200W (0022Wmiddotcmminus3) and 450W(005Wmiddotcmminus3)

43 Surface Temperature Measurement Knowledge of thesurface temperature of materials immersed in the plasma isuseful in understanding the heterogeneous plasma-surfaceinteraction is is of particular importance when localdielectric heating has the potential of thermal runawaybecause most materials increase their dielectric loss withtemperature [12] Two simple means of estimating localsurface temperature of materials that are immersed in mi-crowave plasma have been used For surface temperaturesbelow 180K liquid crystal temperature-sensitive (20plusmn 5K to180plusmn 5K) strips attached to the plasma immersed surfacecan be used [27] For higher temperatures salts of knownmelting point (KCl 1043K and NaCl 1074K) sealed insilica capillaries have been used [13]

44 Near-Field Plasma E-Probe Measurement Attempts tostrike a plasma outside the ignition pressure limits of 01 to20mbar (10 to 2000 Pascal) result in the microwave radi-ation being stored ldquoper cyclerdquo in the empty-cavity modeunder these conditions the rate of energy loss to the cavitywall can substantially heat the MRC structure and in theextreme case the microwave radiation leakage can becomea health risk Additionally if the MRC is loaded with ma-terials (semiconductor wafers and low dielectric strengthmaterial) these can be electrically or mechanically damagedIt therefore becomes necessary to have an automatic powercutoff device to prevent microwave leakage and damage toboth the reactor and loaded materials e near-field plasmaE-probe as described by Law [47] is one such device thathelps monitor such events In this circuit a neon dischargelamp a photodiode and a reference voltage are connected asshown in Figure 6 with one leg of the neon being usedprotruding into the cavity to act as a near-field E-probe Inthe original circuit design a strip chart is used to record thevoltage-time-series data but with todayrsquos analogue-to-digital converters and software (such as LabView) theplasma ignition state-space and plasma state-space may bemonitored with trigger levels set to give a binary GoNocontrol output [24 47]

45 Bebesonde Electrostatic Probe Measurement For plasmaprocesses it is generally considered that the ion flux arrivingand leaving the surface determines the plasma processHowever the use of electrostatic probe techniques to de-termine the ion density and temperature of plasma driven bymodulated power source in the presence of sputtered in-sulating material is problematic Such is the case of plasmacontaining CF4 is section describes a probe techniquethat is tolerant of drive modulation and sputtered insulatingmaterials e following measurements were performed at

8 International Journal of Chemical Engineering

the Oxford Research Unit Open University on the MRC-100 reactor e probe used is an RF-biased ion flux probe(known colloquially as ldquoBebesone or BBsrdquo) For full detailsof the probe and measurement see [48]

For good probe measurements visual observations ofthe plasma volume as a whole and around the probe itselfare usually required For the MRC reactors it was foundnecessary to replace the standard front flange door witha flange incorporating a probe port and inspection portcovered by an open mesh Given this modification argonplasma within the MRC-100 visually exhibits littlestructure with a slight brightening close to the dielectricannulus boundary Nevertheless there is no evidence ofmicrowave structure in the optical emission which in-dicates that energy is rapidly homogenized in the electronpopulation

Following these visual baseline inspections theBebesonde was used to determine the ion flux in the lowpower and high setting with argon gas flow varying froma maximum (5 lmiddotminminus1) to a minimum for which plasmacould be sustained (2 lmiddotminminus1) For the MRC-200 reactorargon ion densities are measured to be in the range of2times1011middotcmminus3 at 50mbar (5 kmiddotPascal) to 3times l012middotcmminus3 at1mbar (100 Pascal) and the electron temperature is in therange of l to l5 eV for an input power of 0022Wmiddotcmminus3 thehigher plasma density at low pressure corresponds to a longermean free path and increased electron power transfer

46 Ultraviolet Fluorescence Microwave Probe For manyprocessing plasmas photon energy (E hcλ) varies from10 eV to 1 eV with preferential intensity at discrete spectralwavelengths e spectral characteristic is determined by thenature of gas excitation and relaxation processes both ofwhich are sensitive to the local electron temperature and gasexcitation cross sections Gas composition and mode ofplasma production therefore has a major impact on UVproduction and plasma chemistry

e ultraviolet fluorescence probe utilizes activated rare-earth salts Y2SiO5Ce (absorptionlt 200 nm) and Zn2SiO4Mn and Y2O3Eu3+ (absorptionlt 300 nm) For full detailsof these probes see [49] At these energies the host lattice(H) undergoes electronic excitation via the production of

electron-hole pair separation (6) followed by the lowestenergetic deactivation pathway fluorescence at wavelengthslonger than the initial radiation In general the fluorescencespectrum is composed of a narrow band with the precisewavelength determined by the intimate relation between theactivator (Ce Mn and Eu3+) and the host lattice and by theirradiating photon radiation

H A + hvprime rarrH Alowast

hvprime gt 4ev( 1113857 (6)

H Alowast rarrH A + hvPrime hvPrime lt 4ev( 1113857 (7)

emost simple form of the probes comprises a syntheticDUV grade fused silica capsule (12mm diametertimes 20mm)containing one of three activated salts at a nominal reducedpressure of 10mbare fused silica has transmittance T 05at 170 nm When placed in the plasma volume the probecollects 4π steradians of incoming DUV photon radiatione emitted fluorescence is viewed through an optical crownglass viewport (T 05 at 380 nm) Due to the dielectriccapsule photoluminescence rather than electroluminescenceis considered to be the prime mechanism of fluorescence inthese probes e dielectric also acts as a wavelength dis-criminator and provides the upper working limit of the probeto be 1100K

Using these knowledge the salts Zn2SiO4Mn and Y2O3Eu3+ (placed in their own capsule) integrates DUV plasmawhile Y2SiO5Ce when placed directly in the plasma volumeintegrates VUV (lt200 nm) plasma thus their fluorescenceappearing in the green red and blue respectively

5 Magnetron Oven Control Circuits

Microwave ovens generally employed one of two types ofmagnetron drive circuits For microwave ovens with low-power outputs (typically lt500W) the output power isachieved by pulse-width modulation of a single incidentpower to the magnetron with the cooking time set between0 and 30 minutes Between the 500W and 1100W ratingcontinuous application of the microwave power is usedwhere the power level is set by the magnetron drive capacitorvalue

P 12

CV2

1113872 1113873f (8)

e choice of either of these two drive circuits can havean impact on the magnetron control circuit ability to carryout the selected plasma processis choice is exemplified bycomparing the short and low power requirements of rapidplasma syntheses of organic compounds [7 8] cleaning ofglass slides and polymers [14] to that of the high power andlong processing times of solid-state metal oxides processing[13] and the plasma cleaning of ion implant ceramic in-sulators [26 27]

51 Cavity Magnetron Capacitor-Controlled Drive CircuitIn this section the choice of the capacitor controlled drivecircuit is considered along with its safety control circuit thatis used in the MRC-100200 plasma reactors A schematic of

Cavity wall

E-field probe

Neon

Photodiode

Shield

Vref

AD

Figure 6 Schematic of the E-probe circuit showing a neon dis-charge lamp a photodiode and an analogue-to-digital converter

International Journal of Chemical Engineering 9

the capacitor controlled drive circuit is shown in Figure 7 Asimilar cavity magnetron capacitor controlled drive circuit isreported in [23] Chaichumporn et al have also reportedfurther refinement of the magnetron anode voltage (33 to66 kV) in [40]

e transformer comprises two winding circuits efirst provides a winding ratio to produce 240V at 50ndash60Hzto approximately 35V at 50ndash60Hz for the cathode heaterand the second provides the step-up voltage (240V at 50ndash60Hzto 2-3 kV at 50ndash60Hz) e HV capacitor (06 to 15 μF)and the HV diode are used to bias the cathode negativewith respect to the anode block which contains themagnetronrsquos cavity structure Using this arrangement thecapacitor value and diode determines the DC powerdissipated in the magnetron cavity structure For safetyreasons both passive and active control components areincorporated on either side of the setup transformer eseinclude the following the circuit is fused on both sides ofthe transformer plus an emergency stop button a chassisinterlock a magnetron thermoswitch (135degC) and a chassecooling fan In addition 24 V DC circuit is used to provideremote passive and active control of the drive circuit (seeSection 52)

52 24V DC Control Circuit In purpose-built microwaveoven plasma reactors the auxiliary equipment (vacuumpumps vacuum valves pressure gauge gas lines purge linesprocess timer and microwave power) is synchronized toplasma process in a safe manner e role of the 24V DCcontrol circuit is to synchronize the auxiliary componentsto the plasma process and shut down the system in the caseof an emergency failure this is especially important whenusing flammable corrosive and toxic precursor gases andby-products e circuit is designed so that all chasse controlsare isolated from the cavity magnetron capacitor-controlleddrive circuit using relays and solenoids It should be notedthat when converting a domestic microwave oven intoa plasma reactor the synchronization has to be built fromscratch

6 Conclusions

is work has reviewed the conversation of the domes-tic microwave oven into a source for cleaning as well aschemical reactions e conversion of domestic systems intoplasma reactor is described as is the construction ofpurpose-built microwave oven plasma reactors Calibrationof the MRC has been discussed along with identifying thetwo types of magnetron drive circuitry used e pro-fessional and armature use has also been presented in thelatter case mainly limited to kitchen top experiments eproof of principle and small batch processes established inthese plasma reactors range from plasma cleaning of glassand polymer surface and removal of toxic metals fromceramic surfaces to the manufacture of carbon nano-structures and the pyrolysis of paper to produce gaseouswaste by-products In all cases the power source is thepackage cavity magnetron operating below 1100W outputpower and pressures ranging from a few 01 s Pascal toa nominal atmospheric pressure (1013 Pascal)

At or close to atmospheric pressure single or multiplewire antenna electrodes that have a physical length ap-proximating to 14 or 12 of the microwave length in whichthat it is immersed have been found to play a catalytic rolein instigating plasma production and in the case ofmanufacturing carbon nanomaterials provide a substrate onwhich carbon material can grow With regard to reactionrate increasing with the number (1 to 6) of electrodes be-yond which the reaction becomes rate-limited a geometricalloading effect around the wire antenna is proposed in thispaper Whether this or electromagnetic power loss is re-sponsible for the effect further work is required In additionthis work has reconstructed a preformed disc aerial elec-trode (igniter) suitable for a narrow tube reactor [16 40](Figure 4)

e safety match flame lighted cigarette sliced grapeand metal antenna electrode have been observed to havea catalyst role in the production of plasma and plasmoids Todistinguish between the metal antenna and the thermal-chemical based catalyst it has been put forward that metal

Anode

Cathode

24 V

dcCo

ntro

l circ

uit

Step

-up

tran

sform

er

HV diode

35 V

Chassisinterlock

Magnetronthermoswitch Fuse

Filtered240 V

50ndash60 Hz

HVFuse

Coolingfan

HVCapacitor

25 kV

Emergencystop

Figure 7 Cavity magnetron capacitor-controlled dive circuit e 24V DC control circuit represented by the dashed box is discussed inSection 52

10 International Journal of Chemical Engineering

antennas may be classified as a passive catalyst as they onlysupply a surface that generates free electrons whilst thesafety match flame lighted cigarette and sliced grape may beclassified as an active catalyst as they supply energy in theform of heat and free electrons from an electrolyte

Finally this review also has highlighted plasmoid foodcooking within a MRC and the use of plasma discharge forremoving food odor frommicrowave ovens Given that foodsafety issues are addressed it is reasonable to envisagemicrowave oven plasma reactors incorporating both plas-moid cooking of food stuff and plasma deodorization ofcooking by-products may be realized in the near future

Conflicts of Interest

e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

e authors would like to acknowledge the support of SFIthrough the I-Form Advanced Manufacturing ResearchCenter 16RC3872

References

[1] F Nagelschmidt Diathermy Text Book for Physicians andStudents Springer-Verlag Berlin Heidelberg 1921

[2] Davis A History of the Microwave Oven e IEEE NewsSource e IEEE News Source Piscataway NJ USA httpswwwtheinstituteieeeorgtech-historytechnology-historya-history-of-the-microwave-oven

[3] P L Spencer ldquoMethod of treating foodstuffsrdquo US Patent2495429 1950

[4] J M Osepchuk ldquoe history of the microwave oven a criticalreviewrdquo in Proceedings of Digest IEEE International Micro-wave Symposium pp 1397ndash1400 Boston MA USA June2009

[5] J R Mims ldquoMicrowave magnetronrdquo US Patent 37392251973

[6] T Koinuma ldquoMagnetronrdquo US Patent 3809590 1974[7] R N Gedye F Smith and K C Westaway ldquoe rapid

synthesis of organic compounds in microwave ovensrdquo Ca-nadian Journal of Chemistry vol 6 no 1 pp 17ndash26 1988

[8] R N Gedye W Rank and K C Westaway ldquoe rapidsynthesis of organic compounds in microwave ovens IIrdquoCanadian Journal of Chemistry vol 69 no 4 pp 706ndash7111991

[9] M Omran T Fabritius E-P Heikkinen and G ChenldquoDielectric properties and carbothermic reduction of zincoxide and zinc ferrite by microwave heatingrdquo Royal SocietyOpen Science vol 4 no 9 p 170710 2017

[10] A Ribner ldquoMicrowave plasma etching machine and methodof etchingrdquo US Patent 4804431 1989

[11] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of phase pure K3C6o using a microwave-induced argon plasmardquo Journal of the Chemical SocietyChemical Communications no 18 pp 2027-2028 1994

[12] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of alkali-metal fullerides using a microwave-induced argon plasmardquo Chemistry of Materials vol 8 no 2pp 394ndash400 1996

[13] D J Brooks and R E Douthwaite ldquoMicrowave-inducedplasma reactor based on a domestic microwave oven forbulk solid state chemistryrdquo Review of Scientific Instrumentsvol 75 no 12 pp 5277ndash5279 2004

[14] B T Ginn and O Steinbock ldquoPolymer surface modificationusing microwave-oven-generated plasmardquo Langmuir vol 19no 19 pp 8117-8118 2003

[15] P Khongkrapan N Tippayawong and T Kiatsiriroatldquoermochemical conversion of waste papers to fuel gas ina microwave plasma reactorrdquo Journal of Clean EnergyTechnologies vol 1 no 2 pp 80ndash83 2013

[16] P Khongkrapan P anompongchart N Tippayawong andT Kiatsiriroat ldquoFuel gas and char from pyrolysis of wastepaper in a microwave plasma reactorrdquo IJEE vol 4 no 6pp 969ndash974 2013

[17] R Singh and A L L Jarvis ldquoMicrowave plasma-enhancedchemical vapour deposition growth of carbon nano-structuresrdquo South African Journal of Science vol 106 no 5-6p 4 2010

[18] S Nomura H Toyota S Mukasa H Yamashita T Maeharaand A Kawashima ldquoProduction of hydrogen in a conven-tional microwave ovenrdquo Journal of Applied Physics vol 106no 7 p 073306 2009

[19] S Nomura H Yamashita H Toyota S Mukasa andY Okamura ldquoSimultaneous production of hydrogen andcarbon nanotubes in a conventional microwave ovenrdquo inProceedings of International Symposium on Plasma Chemistry(ISPC19) vol 65 Bochum Germany July 2009

[20] H Toyota S Nomura and S Mukasa ldquoA practical electrodefor microwave plasma processesrdquo International Journal ofMaterials Science and Applications vol 2 no 3 pp 83ndash882013

[21] R Pongsopon T Chim-Oye and M Fuangfoong ldquoMicro-wave plasma reactor based on microwave ovenrdquo in PIERSProceedings pp 2723ndash2726 Guangzhou China August 2014

[22] N Manivannan W Balachandran R Beleca and M AbbodldquoMicrowave plasma system design and modelling for marinediesel exhaust gas abatement of NOx and SOxrdquo InternationalJournal of Environmental Science and Development vol 6no 2 pp 151ndash154 2015

[23] M C Savadori V P Mammana O G Martins andF T Degasperi ldquoPlasma-assisted chemical vapour depositionin a tunable microwave cavityrdquo Plasma Sources Science andTechnology vol 4 no 3 pp 489ndash494 1995

[24] V J Law and N Macgearailt ldquoVisualization of a dual fre-quency plasma etch processrdquo Measurement Science andTechnology vol 18 no 3 pp 645ndash649 2007

[25] Y H Jung S O Jang and H J You ldquoHydrogen generationfrom the dissociation of water using microwave plasmasrdquoChinese Physics Letters vol 30 no 6 p 065204 2013

[26] V J Law and D Tait ldquoContaminated ceramic plasmacleaningrdquo European Semiconductor vol 19 no 9 pp S38ndashS41 1997

[27] V J Law andD Tait ldquoMicrowave plasma cleaning of ion implantceramic insulatorsrdquo Vacuum vol 49 no 4 pp 273ndash278 1998

[28] T M Burke E H Linfield D A Ritchie M Peper andJ H Burroughs ldquoHydrogen radical surface cleaning of GaAsfor MBE regrowthrdquo Journal of Crystal Growth vol 175-176pp 416ndash421 1997

[29] D Tasch D J Brosky S Conrad S Kumar and D KumarldquoMicrowave plasma cookingrdquo US Patent 20090012223 A12009

[30] H Stanley ldquoMicrowave experiments at schoolrdquo Science inSchool vol 12 pp 31ndash33 2009

International Journal of Chemical Engineering 11

With the cavity magnetron antenna positioned 26mmfrom the end of the waveguide the frequency and bandwidthof the magnetron are allowed to be free running us thenoncoherent reflected power passing through the iris travelsback to the magnetron thus altering the SWR of the coherentwave within the TE10 waveguide resulting in varying theoutput power of the magnetron

e MRC reactor design differs from the domesticmicrowave oven plasma reactor in the following ways (alsocf Figure 1 with Figure 5)

(i) e chassis MRC and waveguide are constructed asone welded component using 14mm thick mild steelsheet Before each of the three components is weldedtogether they have all the necessary holes punchedand clasp nuts fixed Once welded the structure isnickel plated to produce a metal structure that isrobust with sufficient stiffness to support all theadditional components (the front and rear stainlessflanges gas lines DC power supply pressure gaugeetc) Using this construction approach the MRC hasa theoretical maximum unloadedQ-factor (Qu) in theTE mode that is dependent on the ratio of storedenergy in the cavity (Vc) to the energy loss to thecavity walls (δ times Ac)

Qu 2Vc

δAc (3)

where δ is the electrical skin depth at the cavity wall per cycleand Ac is the cavity wall area

For this reactor the main cavity has an approximatelyQuof 20000 at a resonant frequency of 245GHz

(ii) A cylindrical Pyrex glass chamber (190mm di-ameter 300mm length and 5mm wall thicknessproducing a volume of 3 liters) is located within themultimode cavity with its longitudinal axis per-pendicular to the microwave iris and with the frontand rear of the chamber housed within metal flangesthat form the part of the multimode cavity wall erear flange contains welded vacuum and pressuregauge ports and the front flanges contain the accessdoor is design maximizes the chamber volumeand removes all fragile glass fittings plastic tubeconnectors and feedthrough microwave leakagegaskets

(iii) e gas lines are fitted within the chassis and to theside of theMRC thereby enabling the process gassesto be injected throughmultiple equally spaced radialports in the front flange thus reducing the possibilityof precursor gas being preionized prior to chamberentry and maximizing uniform gas flow and plasmauniformity along process chamber longitudinal axis

31 Plasma Cleaning of Ion Implant Ceramic InsulatorsIon implantation is one of the key processes in the highvolume (220 wafers per hour) manufacture of silicon

semiconductor devices ese ion implant machines how-ever cost between $18M and $3M ese machines are alsohighly maintenance-intensive systems with high capitalcost therefore availability and cost of ownership aremajor factors to be considered Many of the parts changedduring regular maintenance and ion source changes areceramic insulators In this section an overview of theplasma cleaning of ion implant ceramic is described forfull details of the process see [26 27] e plasma cleaningprocess has been performed in the MRC series of plasmareactor using a gas mixture of 5ndash10 O2 in CF with anadmixture of 50 by flow of argon e argon admix isused to stabilize the microwave plasma by moderating theelectron energy distribution and to provide a uniformexcited species throughout the plasma volume eplasma etch chemistry at the surface of the ceramic may beconsidered to proceed by the following representativeheterogeneous reaction

3CF4(g) + 15O2(g) + 2X(s)rarr 2XF3(g) + 3COF3(g)

(4)

in which the addition of O2 scavenges carbon from the CF4through the formation of COFx species to enhance thesteady-state concentration of F atoms in the plasma vo1umee element X in reaction (4) represents the group V ele-ment (As P and Sb) on the ceramic surface and the XF3 arethe etch products Given sufficient microwave power theetch rates of these products are therefore controlled by the

Gas lines CavityMagnetron timer and

power supply

Front view

Pressuremeter

(a)

Gaugeport

Vacuumport

Doo

r

Side view

(b)

Figure 5 Typical front and side view schematic of theMRC plasmareactor chassis and cavity Photograph of the microwave MRC-200plasma reactor

International Journal of Chemical Engineering 7

production of F atoms (O2 balance) product volatility andceramic microscopic surface area

For the MRC-100 reactor typical plasma process pa-rameters obtained were 104W and 10mbar with an etchtime of 45 minutes where the surface temperature of theceramics reached 80plusmn 5K In the case of the MRC-200reactor the process parameters were as follows 200Wand 10mbar (1000 Pascal) with an etch time of 20 to 25minutes where the surface temperature of the ceramicsreached 125plusmn 5K

4 Microwave Cavity Calibration

is section describes a range of different techniques whichare used for the evaluation of the microwave efficiency aswell as leakage

41 Microwave Leakage Measurement e European Di-rective 200440EC and ICNIRP (1998) guidelines recom-mend that industrial microwave ovens have surfacemicrowave (3ndash300GHz) radiation power density levelsgt5mWmiddotcmminus2 and 5 times less for domestic microwave ovensintended for general use Applying a quadratic law based onplane wave theory an operator standing at a distance of20 cm from an industrial oven the operator will receive themaximum allowable power density level of 3mWmiddotcmminus2 Fordomestic oven environment this equates to a power densitylevel of 03mWmiddotcmminus2 In the context of converted ovensintended for plasma many feedthrough and apertures in theMRC require considerable care in design and constructionto prevent microwave leakage

42 Calorimetric Magnetron Power Calibration e mag-netron power entering the MRC may be calibrated using thewater open-dish load method see for example BritishStandard 75091995 and IEC 13071994 Hence given theheat capacity of water is 4184 J(gmiddotK) the calculated appliedpower (P) within the cavity may be obtained by placinga known mass of water (m) within the cavity and heating itfor a short period of time (t) making sure the water does notboil With the knowledge of themeasured water temperaturechange (ΔT final temperature ndash initial temperature) themicrowave power entering into the cavity is calibratedfor a given power setting using the following equation(see also [40])

P mCΔT

t (5)

e calibration however must be considered to be anupper value for plasma processing as its dielectric volumewill be different from the water calibration (Note when theprocess chamber volume or the geometric shape does notallow the open dish method to be used the alternative flowmethod can be used as outlined in [46])

Given (4) the microwave power density (Wmiddotcmminus3) ofa system may be calculated by dividing through by theprocess chamber volume e following open-dish loadmethod for the MRC 100 is given here as an illustrative

example the MRC-100 and MRC-200 reactors have a cal-culated magnetron applied power of 104W which equatesto a power density of 0116Wmiddotcmminus3 For the MRC-200 re-actor the calorimetric measurements produce magnetronapplied power values of 200W (0022Wmiddotcmminus3) and 450W(005Wmiddotcmminus3)

43 Surface Temperature Measurement Knowledge of thesurface temperature of materials immersed in the plasma isuseful in understanding the heterogeneous plasma-surfaceinteraction is is of particular importance when localdielectric heating has the potential of thermal runawaybecause most materials increase their dielectric loss withtemperature [12] Two simple means of estimating localsurface temperature of materials that are immersed in mi-crowave plasma have been used For surface temperaturesbelow 180K liquid crystal temperature-sensitive (20plusmn 5K to180plusmn 5K) strips attached to the plasma immersed surfacecan be used [27] For higher temperatures salts of knownmelting point (KCl 1043K and NaCl 1074K) sealed insilica capillaries have been used [13]

44 Near-Field Plasma E-Probe Measurement Attempts tostrike a plasma outside the ignition pressure limits of 01 to20mbar (10 to 2000 Pascal) result in the microwave radi-ation being stored ldquoper cyclerdquo in the empty-cavity modeunder these conditions the rate of energy loss to the cavitywall can substantially heat the MRC structure and in theextreme case the microwave radiation leakage can becomea health risk Additionally if the MRC is loaded with ma-terials (semiconductor wafers and low dielectric strengthmaterial) these can be electrically or mechanically damagedIt therefore becomes necessary to have an automatic powercutoff device to prevent microwave leakage and damage toboth the reactor and loaded materials e near-field plasmaE-probe as described by Law [47] is one such device thathelps monitor such events In this circuit a neon dischargelamp a photodiode and a reference voltage are connected asshown in Figure 6 with one leg of the neon being usedprotruding into the cavity to act as a near-field E-probe Inthe original circuit design a strip chart is used to record thevoltage-time-series data but with todayrsquos analogue-to-digital converters and software (such as LabView) theplasma ignition state-space and plasma state-space may bemonitored with trigger levels set to give a binary GoNocontrol output [24 47]

45 Bebesonde Electrostatic Probe Measurement For plasmaprocesses it is generally considered that the ion flux arrivingand leaving the surface determines the plasma processHowever the use of electrostatic probe techniques to de-termine the ion density and temperature of plasma driven bymodulated power source in the presence of sputtered in-sulating material is problematic Such is the case of plasmacontaining CF4 is section describes a probe techniquethat is tolerant of drive modulation and sputtered insulatingmaterials e following measurements were performed at

8 International Journal of Chemical Engineering

the Oxford Research Unit Open University on the MRC-100 reactor e probe used is an RF-biased ion flux probe(known colloquially as ldquoBebesone or BBsrdquo) For full detailsof the probe and measurement see [48]

For good probe measurements visual observations ofthe plasma volume as a whole and around the probe itselfare usually required For the MRC reactors it was foundnecessary to replace the standard front flange door witha flange incorporating a probe port and inspection portcovered by an open mesh Given this modification argonplasma within the MRC-100 visually exhibits littlestructure with a slight brightening close to the dielectricannulus boundary Nevertheless there is no evidence ofmicrowave structure in the optical emission which in-dicates that energy is rapidly homogenized in the electronpopulation

Following these visual baseline inspections theBebesonde was used to determine the ion flux in the lowpower and high setting with argon gas flow varying froma maximum (5 lmiddotminminus1) to a minimum for which plasmacould be sustained (2 lmiddotminminus1) For the MRC-200 reactorargon ion densities are measured to be in the range of2times1011middotcmminus3 at 50mbar (5 kmiddotPascal) to 3times l012middotcmminus3 at1mbar (100 Pascal) and the electron temperature is in therange of l to l5 eV for an input power of 0022Wmiddotcmminus3 thehigher plasma density at low pressure corresponds to a longermean free path and increased electron power transfer

46 Ultraviolet Fluorescence Microwave Probe For manyprocessing plasmas photon energy (E hcλ) varies from10 eV to 1 eV with preferential intensity at discrete spectralwavelengths e spectral characteristic is determined by thenature of gas excitation and relaxation processes both ofwhich are sensitive to the local electron temperature and gasexcitation cross sections Gas composition and mode ofplasma production therefore has a major impact on UVproduction and plasma chemistry

e ultraviolet fluorescence probe utilizes activated rare-earth salts Y2SiO5Ce (absorptionlt 200 nm) and Zn2SiO4Mn and Y2O3Eu3+ (absorptionlt 300 nm) For full detailsof these probes see [49] At these energies the host lattice(H) undergoes electronic excitation via the production of

electron-hole pair separation (6) followed by the lowestenergetic deactivation pathway fluorescence at wavelengthslonger than the initial radiation In general the fluorescencespectrum is composed of a narrow band with the precisewavelength determined by the intimate relation between theactivator (Ce Mn and Eu3+) and the host lattice and by theirradiating photon radiation

H A + hvprime rarrH Alowast

hvprime gt 4ev( 1113857 (6)

H Alowast rarrH A + hvPrime hvPrime lt 4ev( 1113857 (7)

emost simple form of the probes comprises a syntheticDUV grade fused silica capsule (12mm diametertimes 20mm)containing one of three activated salts at a nominal reducedpressure of 10mbare fused silica has transmittance T 05at 170 nm When placed in the plasma volume the probecollects 4π steradians of incoming DUV photon radiatione emitted fluorescence is viewed through an optical crownglass viewport (T 05 at 380 nm) Due to the dielectriccapsule photoluminescence rather than electroluminescenceis considered to be the prime mechanism of fluorescence inthese probes e dielectric also acts as a wavelength dis-criminator and provides the upper working limit of the probeto be 1100K

Using these knowledge the salts Zn2SiO4Mn and Y2O3Eu3+ (placed in their own capsule) integrates DUV plasmawhile Y2SiO5Ce when placed directly in the plasma volumeintegrates VUV (lt200 nm) plasma thus their fluorescenceappearing in the green red and blue respectively

5 Magnetron Oven Control Circuits

Microwave ovens generally employed one of two types ofmagnetron drive circuits For microwave ovens with low-power outputs (typically lt500W) the output power isachieved by pulse-width modulation of a single incidentpower to the magnetron with the cooking time set between0 and 30 minutes Between the 500W and 1100W ratingcontinuous application of the microwave power is usedwhere the power level is set by the magnetron drive capacitorvalue

P 12

CV2

1113872 1113873f (8)

e choice of either of these two drive circuits can havean impact on the magnetron control circuit ability to carryout the selected plasma processis choice is exemplified bycomparing the short and low power requirements of rapidplasma syntheses of organic compounds [7 8] cleaning ofglass slides and polymers [14] to that of the high power andlong processing times of solid-state metal oxides processing[13] and the plasma cleaning of ion implant ceramic in-sulators [26 27]

51 Cavity Magnetron Capacitor-Controlled Drive CircuitIn this section the choice of the capacitor controlled drivecircuit is considered along with its safety control circuit thatis used in the MRC-100200 plasma reactors A schematic of

Cavity wall

E-field probe

Neon

Photodiode

Shield

Vref

AD

Figure 6 Schematic of the E-probe circuit showing a neon dis-charge lamp a photodiode and an analogue-to-digital converter

International Journal of Chemical Engineering 9

the capacitor controlled drive circuit is shown in Figure 7 Asimilar cavity magnetron capacitor controlled drive circuit isreported in [23] Chaichumporn et al have also reportedfurther refinement of the magnetron anode voltage (33 to66 kV) in [40]

e transformer comprises two winding circuits efirst provides a winding ratio to produce 240V at 50ndash60Hzto approximately 35V at 50ndash60Hz for the cathode heaterand the second provides the step-up voltage (240V at 50ndash60Hzto 2-3 kV at 50ndash60Hz) e HV capacitor (06 to 15 μF)and the HV diode are used to bias the cathode negativewith respect to the anode block which contains themagnetronrsquos cavity structure Using this arrangement thecapacitor value and diode determines the DC powerdissipated in the magnetron cavity structure For safetyreasons both passive and active control components areincorporated on either side of the setup transformer eseinclude the following the circuit is fused on both sides ofthe transformer plus an emergency stop button a chassisinterlock a magnetron thermoswitch (135degC) and a chassecooling fan In addition 24 V DC circuit is used to provideremote passive and active control of the drive circuit (seeSection 52)

52 24V DC Control Circuit In purpose-built microwaveoven plasma reactors the auxiliary equipment (vacuumpumps vacuum valves pressure gauge gas lines purge linesprocess timer and microwave power) is synchronized toplasma process in a safe manner e role of the 24V DCcontrol circuit is to synchronize the auxiliary componentsto the plasma process and shut down the system in the caseof an emergency failure this is especially important whenusing flammable corrosive and toxic precursor gases andby-products e circuit is designed so that all chasse controlsare isolated from the cavity magnetron capacitor-controlleddrive circuit using relays and solenoids It should be notedthat when converting a domestic microwave oven intoa plasma reactor the synchronization has to be built fromscratch

6 Conclusions

is work has reviewed the conversation of the domes-tic microwave oven into a source for cleaning as well aschemical reactions e conversion of domestic systems intoplasma reactor is described as is the construction ofpurpose-built microwave oven plasma reactors Calibrationof the MRC has been discussed along with identifying thetwo types of magnetron drive circuitry used e pro-fessional and armature use has also been presented in thelatter case mainly limited to kitchen top experiments eproof of principle and small batch processes established inthese plasma reactors range from plasma cleaning of glassand polymer surface and removal of toxic metals fromceramic surfaces to the manufacture of carbon nano-structures and the pyrolysis of paper to produce gaseouswaste by-products In all cases the power source is thepackage cavity magnetron operating below 1100W outputpower and pressures ranging from a few 01 s Pascal toa nominal atmospheric pressure (1013 Pascal)

At or close to atmospheric pressure single or multiplewire antenna electrodes that have a physical length ap-proximating to 14 or 12 of the microwave length in whichthat it is immersed have been found to play a catalytic rolein instigating plasma production and in the case ofmanufacturing carbon nanomaterials provide a substrate onwhich carbon material can grow With regard to reactionrate increasing with the number (1 to 6) of electrodes be-yond which the reaction becomes rate-limited a geometricalloading effect around the wire antenna is proposed in thispaper Whether this or electromagnetic power loss is re-sponsible for the effect further work is required In additionthis work has reconstructed a preformed disc aerial elec-trode (igniter) suitable for a narrow tube reactor [16 40](Figure 4)

e safety match flame lighted cigarette sliced grapeand metal antenna electrode have been observed to havea catalyst role in the production of plasma and plasmoids Todistinguish between the metal antenna and the thermal-chemical based catalyst it has been put forward that metal

Anode

Cathode

24 V

dcCo

ntro

l circ

uit

Step

-up

tran

sform

er

HV diode

35 V

Chassisinterlock

Magnetronthermoswitch Fuse

Filtered240 V

50ndash60 Hz

HVFuse

Coolingfan

HVCapacitor

25 kV

Emergencystop

Figure 7 Cavity magnetron capacitor-controlled dive circuit e 24V DC control circuit represented by the dashed box is discussed inSection 52

10 International Journal of Chemical Engineering

antennas may be classified as a passive catalyst as they onlysupply a surface that generates free electrons whilst thesafety match flame lighted cigarette and sliced grape may beclassified as an active catalyst as they supply energy in theform of heat and free electrons from an electrolyte

Finally this review also has highlighted plasmoid foodcooking within a MRC and the use of plasma discharge forremoving food odor frommicrowave ovens Given that foodsafety issues are addressed it is reasonable to envisagemicrowave oven plasma reactors incorporating both plas-moid cooking of food stuff and plasma deodorization ofcooking by-products may be realized in the near future

Conflicts of Interest

e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

e authors would like to acknowledge the support of SFIthrough the I-Form Advanced Manufacturing ResearchCenter 16RC3872

References

[1] F Nagelschmidt Diathermy Text Book for Physicians andStudents Springer-Verlag Berlin Heidelberg 1921

[2] Davis A History of the Microwave Oven e IEEE NewsSource e IEEE News Source Piscataway NJ USA httpswwwtheinstituteieeeorgtech-historytechnology-historya-history-of-the-microwave-oven

[3] P L Spencer ldquoMethod of treating foodstuffsrdquo US Patent2495429 1950

[4] J M Osepchuk ldquoe history of the microwave oven a criticalreviewrdquo in Proceedings of Digest IEEE International Micro-wave Symposium pp 1397ndash1400 Boston MA USA June2009

[5] J R Mims ldquoMicrowave magnetronrdquo US Patent 37392251973

[6] T Koinuma ldquoMagnetronrdquo US Patent 3809590 1974[7] R N Gedye F Smith and K C Westaway ldquoe rapid

synthesis of organic compounds in microwave ovensrdquo Ca-nadian Journal of Chemistry vol 6 no 1 pp 17ndash26 1988

[8] R N Gedye W Rank and K C Westaway ldquoe rapidsynthesis of organic compounds in microwave ovens IIrdquoCanadian Journal of Chemistry vol 69 no 4 pp 706ndash7111991

[9] M Omran T Fabritius E-P Heikkinen and G ChenldquoDielectric properties and carbothermic reduction of zincoxide and zinc ferrite by microwave heatingrdquo Royal SocietyOpen Science vol 4 no 9 p 170710 2017

[10] A Ribner ldquoMicrowave plasma etching machine and methodof etchingrdquo US Patent 4804431 1989

[11] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of phase pure K3C6o using a microwave-induced argon plasmardquo Journal of the Chemical SocietyChemical Communications no 18 pp 2027-2028 1994

[12] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of alkali-metal fullerides using a microwave-induced argon plasmardquo Chemistry of Materials vol 8 no 2pp 394ndash400 1996

[13] D J Brooks and R E Douthwaite ldquoMicrowave-inducedplasma reactor based on a domestic microwave oven forbulk solid state chemistryrdquo Review of Scientific Instrumentsvol 75 no 12 pp 5277ndash5279 2004

[14] B T Ginn and O Steinbock ldquoPolymer surface modificationusing microwave-oven-generated plasmardquo Langmuir vol 19no 19 pp 8117-8118 2003

[15] P Khongkrapan N Tippayawong and T Kiatsiriroatldquoermochemical conversion of waste papers to fuel gas ina microwave plasma reactorrdquo Journal of Clean EnergyTechnologies vol 1 no 2 pp 80ndash83 2013

[16] P Khongkrapan P anompongchart N Tippayawong andT Kiatsiriroat ldquoFuel gas and char from pyrolysis of wastepaper in a microwave plasma reactorrdquo IJEE vol 4 no 6pp 969ndash974 2013

[17] R Singh and A L L Jarvis ldquoMicrowave plasma-enhancedchemical vapour deposition growth of carbon nano-structuresrdquo South African Journal of Science vol 106 no 5-6p 4 2010

[18] S Nomura H Toyota S Mukasa H Yamashita T Maeharaand A Kawashima ldquoProduction of hydrogen in a conven-tional microwave ovenrdquo Journal of Applied Physics vol 106no 7 p 073306 2009

[19] S Nomura H Yamashita H Toyota S Mukasa andY Okamura ldquoSimultaneous production of hydrogen andcarbon nanotubes in a conventional microwave ovenrdquo inProceedings of International Symposium on Plasma Chemistry(ISPC19) vol 65 Bochum Germany July 2009

[20] H Toyota S Nomura and S Mukasa ldquoA practical electrodefor microwave plasma processesrdquo International Journal ofMaterials Science and Applications vol 2 no 3 pp 83ndash882013

[21] R Pongsopon T Chim-Oye and M Fuangfoong ldquoMicro-wave plasma reactor based on microwave ovenrdquo in PIERSProceedings pp 2723ndash2726 Guangzhou China August 2014

[22] N Manivannan W Balachandran R Beleca and M AbbodldquoMicrowave plasma system design and modelling for marinediesel exhaust gas abatement of NOx and SOxrdquo InternationalJournal of Environmental Science and Development vol 6no 2 pp 151ndash154 2015

[23] M C Savadori V P Mammana O G Martins andF T Degasperi ldquoPlasma-assisted chemical vapour depositionin a tunable microwave cavityrdquo Plasma Sources Science andTechnology vol 4 no 3 pp 489ndash494 1995

[24] V J Law and N Macgearailt ldquoVisualization of a dual fre-quency plasma etch processrdquo Measurement Science andTechnology vol 18 no 3 pp 645ndash649 2007

[25] Y H Jung S O Jang and H J You ldquoHydrogen generationfrom the dissociation of water using microwave plasmasrdquoChinese Physics Letters vol 30 no 6 p 065204 2013

[26] V J Law and D Tait ldquoContaminated ceramic plasmacleaningrdquo European Semiconductor vol 19 no 9 pp S38ndashS41 1997

[27] V J Law andD Tait ldquoMicrowave plasma cleaning of ion implantceramic insulatorsrdquo Vacuum vol 49 no 4 pp 273ndash278 1998

[28] T M Burke E H Linfield D A Ritchie M Peper andJ H Burroughs ldquoHydrogen radical surface cleaning of GaAsfor MBE regrowthrdquo Journal of Crystal Growth vol 175-176pp 416ndash421 1997

[29] D Tasch D J Brosky S Conrad S Kumar and D KumarldquoMicrowave plasma cookingrdquo US Patent 20090012223 A12009

[30] H Stanley ldquoMicrowave experiments at schoolrdquo Science inSchool vol 12 pp 31ndash33 2009

International Journal of Chemical Engineering 11

production of F atoms (O2 balance) product volatility andceramic microscopic surface area

For the MRC-100 reactor typical plasma process pa-rameters obtained were 104W and 10mbar with an etchtime of 45 minutes where the surface temperature of theceramics reached 80plusmn 5K In the case of the MRC-200reactor the process parameters were as follows 200Wand 10mbar (1000 Pascal) with an etch time of 20 to 25minutes where the surface temperature of the ceramicsreached 125plusmn 5K

4 Microwave Cavity Calibration

is section describes a range of different techniques whichare used for the evaluation of the microwave efficiency aswell as leakage

41 Microwave Leakage Measurement e European Di-rective 200440EC and ICNIRP (1998) guidelines recom-mend that industrial microwave ovens have surfacemicrowave (3ndash300GHz) radiation power density levelsgt5mWmiddotcmminus2 and 5 times less for domestic microwave ovensintended for general use Applying a quadratic law based onplane wave theory an operator standing at a distance of20 cm from an industrial oven the operator will receive themaximum allowable power density level of 3mWmiddotcmminus2 Fordomestic oven environment this equates to a power densitylevel of 03mWmiddotcmminus2 In the context of converted ovensintended for plasma many feedthrough and apertures in theMRC require considerable care in design and constructionto prevent microwave leakage

42 Calorimetric Magnetron Power Calibration e mag-netron power entering the MRC may be calibrated using thewater open-dish load method see for example BritishStandard 75091995 and IEC 13071994 Hence given theheat capacity of water is 4184 J(gmiddotK) the calculated appliedpower (P) within the cavity may be obtained by placinga known mass of water (m) within the cavity and heating itfor a short period of time (t) making sure the water does notboil With the knowledge of themeasured water temperaturechange (ΔT final temperature ndash initial temperature) themicrowave power entering into the cavity is calibratedfor a given power setting using the following equation(see also [40])

P mCΔT

t (5)

e calibration however must be considered to be anupper value for plasma processing as its dielectric volumewill be different from the water calibration (Note when theprocess chamber volume or the geometric shape does notallow the open dish method to be used the alternative flowmethod can be used as outlined in [46])

Given (4) the microwave power density (Wmiddotcmminus3) ofa system may be calculated by dividing through by theprocess chamber volume e following open-dish loadmethod for the MRC 100 is given here as an illustrative

example the MRC-100 and MRC-200 reactors have a cal-culated magnetron applied power of 104W which equatesto a power density of 0116Wmiddotcmminus3 For the MRC-200 re-actor the calorimetric measurements produce magnetronapplied power values of 200W (0022Wmiddotcmminus3) and 450W(005Wmiddotcmminus3)

43 Surface Temperature Measurement Knowledge of thesurface temperature of materials immersed in the plasma isuseful in understanding the heterogeneous plasma-surfaceinteraction is is of particular importance when localdielectric heating has the potential of thermal runawaybecause most materials increase their dielectric loss withtemperature [12] Two simple means of estimating localsurface temperature of materials that are immersed in mi-crowave plasma have been used For surface temperaturesbelow 180K liquid crystal temperature-sensitive (20plusmn 5K to180plusmn 5K) strips attached to the plasma immersed surfacecan be used [27] For higher temperatures salts of knownmelting point (KCl 1043K and NaCl 1074K) sealed insilica capillaries have been used [13]

44 Near-Field Plasma E-Probe Measurement Attempts tostrike a plasma outside the ignition pressure limits of 01 to20mbar (10 to 2000 Pascal) result in the microwave radi-ation being stored ldquoper cyclerdquo in the empty-cavity modeunder these conditions the rate of energy loss to the cavitywall can substantially heat the MRC structure and in theextreme case the microwave radiation leakage can becomea health risk Additionally if the MRC is loaded with ma-terials (semiconductor wafers and low dielectric strengthmaterial) these can be electrically or mechanically damagedIt therefore becomes necessary to have an automatic powercutoff device to prevent microwave leakage and damage toboth the reactor and loaded materials e near-field plasmaE-probe as described by Law [47] is one such device thathelps monitor such events In this circuit a neon dischargelamp a photodiode and a reference voltage are connected asshown in Figure 6 with one leg of the neon being usedprotruding into the cavity to act as a near-field E-probe Inthe original circuit design a strip chart is used to record thevoltage-time-series data but with todayrsquos analogue-to-digital converters and software (such as LabView) theplasma ignition state-space and plasma state-space may bemonitored with trigger levels set to give a binary GoNocontrol output [24 47]

45 Bebesonde Electrostatic Probe Measurement For plasmaprocesses it is generally considered that the ion flux arrivingand leaving the surface determines the plasma processHowever the use of electrostatic probe techniques to de-termine the ion density and temperature of plasma driven bymodulated power source in the presence of sputtered in-sulating material is problematic Such is the case of plasmacontaining CF4 is section describes a probe techniquethat is tolerant of drive modulation and sputtered insulatingmaterials e following measurements were performed at

8 International Journal of Chemical Engineering

the Oxford Research Unit Open University on the MRC-100 reactor e probe used is an RF-biased ion flux probe(known colloquially as ldquoBebesone or BBsrdquo) For full detailsof the probe and measurement see [48]

For good probe measurements visual observations ofthe plasma volume as a whole and around the probe itselfare usually required For the MRC reactors it was foundnecessary to replace the standard front flange door witha flange incorporating a probe port and inspection portcovered by an open mesh Given this modification argonplasma within the MRC-100 visually exhibits littlestructure with a slight brightening close to the dielectricannulus boundary Nevertheless there is no evidence ofmicrowave structure in the optical emission which in-dicates that energy is rapidly homogenized in the electronpopulation

Following these visual baseline inspections theBebesonde was used to determine the ion flux in the lowpower and high setting with argon gas flow varying froma maximum (5 lmiddotminminus1) to a minimum for which plasmacould be sustained (2 lmiddotminminus1) For the MRC-200 reactorargon ion densities are measured to be in the range of2times1011middotcmminus3 at 50mbar (5 kmiddotPascal) to 3times l012middotcmminus3 at1mbar (100 Pascal) and the electron temperature is in therange of l to l5 eV for an input power of 0022Wmiddotcmminus3 thehigher plasma density at low pressure corresponds to a longermean free path and increased electron power transfer

46 Ultraviolet Fluorescence Microwave Probe For manyprocessing plasmas photon energy (E hcλ) varies from10 eV to 1 eV with preferential intensity at discrete spectralwavelengths e spectral characteristic is determined by thenature of gas excitation and relaxation processes both ofwhich are sensitive to the local electron temperature and gasexcitation cross sections Gas composition and mode ofplasma production therefore has a major impact on UVproduction and plasma chemistry

e ultraviolet fluorescence probe utilizes activated rare-earth salts Y2SiO5Ce (absorptionlt 200 nm) and Zn2SiO4Mn and Y2O3Eu3+ (absorptionlt 300 nm) For full detailsof these probes see [49] At these energies the host lattice(H) undergoes electronic excitation via the production of

electron-hole pair separation (6) followed by the lowestenergetic deactivation pathway fluorescence at wavelengthslonger than the initial radiation In general the fluorescencespectrum is composed of a narrow band with the precisewavelength determined by the intimate relation between theactivator (Ce Mn and Eu3+) and the host lattice and by theirradiating photon radiation

H A + hvprime rarrH Alowast

hvprime gt 4ev( 1113857 (6)

H Alowast rarrH A + hvPrime hvPrime lt 4ev( 1113857 (7)

emost simple form of the probes comprises a syntheticDUV grade fused silica capsule (12mm diametertimes 20mm)containing one of three activated salts at a nominal reducedpressure of 10mbare fused silica has transmittance T 05at 170 nm When placed in the plasma volume the probecollects 4π steradians of incoming DUV photon radiatione emitted fluorescence is viewed through an optical crownglass viewport (T 05 at 380 nm) Due to the dielectriccapsule photoluminescence rather than electroluminescenceis considered to be the prime mechanism of fluorescence inthese probes e dielectric also acts as a wavelength dis-criminator and provides the upper working limit of the probeto be 1100K

Using these knowledge the salts Zn2SiO4Mn and Y2O3Eu3+ (placed in their own capsule) integrates DUV plasmawhile Y2SiO5Ce when placed directly in the plasma volumeintegrates VUV (lt200 nm) plasma thus their fluorescenceappearing in the green red and blue respectively

5 Magnetron Oven Control Circuits

Microwave ovens generally employed one of two types ofmagnetron drive circuits For microwave ovens with low-power outputs (typically lt500W) the output power isachieved by pulse-width modulation of a single incidentpower to the magnetron with the cooking time set between0 and 30 minutes Between the 500W and 1100W ratingcontinuous application of the microwave power is usedwhere the power level is set by the magnetron drive capacitorvalue

P 12

CV2

1113872 1113873f (8)

e choice of either of these two drive circuits can havean impact on the magnetron control circuit ability to carryout the selected plasma processis choice is exemplified bycomparing the short and low power requirements of rapidplasma syntheses of organic compounds [7 8] cleaning ofglass slides and polymers [14] to that of the high power andlong processing times of solid-state metal oxides processing[13] and the plasma cleaning of ion implant ceramic in-sulators [26 27]

51 Cavity Magnetron Capacitor-Controlled Drive CircuitIn this section the choice of the capacitor controlled drivecircuit is considered along with its safety control circuit thatis used in the MRC-100200 plasma reactors A schematic of

Cavity wall

E-field probe

Neon

Photodiode

Shield

Vref

AD

Figure 6 Schematic of the E-probe circuit showing a neon dis-charge lamp a photodiode and an analogue-to-digital converter

International Journal of Chemical Engineering 9

the capacitor controlled drive circuit is shown in Figure 7 Asimilar cavity magnetron capacitor controlled drive circuit isreported in [23] Chaichumporn et al have also reportedfurther refinement of the magnetron anode voltage (33 to66 kV) in [40]

e transformer comprises two winding circuits efirst provides a winding ratio to produce 240V at 50ndash60Hzto approximately 35V at 50ndash60Hz for the cathode heaterand the second provides the step-up voltage (240V at 50ndash60Hzto 2-3 kV at 50ndash60Hz) e HV capacitor (06 to 15 μF)and the HV diode are used to bias the cathode negativewith respect to the anode block which contains themagnetronrsquos cavity structure Using this arrangement thecapacitor value and diode determines the DC powerdissipated in the magnetron cavity structure For safetyreasons both passive and active control components areincorporated on either side of the setup transformer eseinclude the following the circuit is fused on both sides ofthe transformer plus an emergency stop button a chassisinterlock a magnetron thermoswitch (135degC) and a chassecooling fan In addition 24 V DC circuit is used to provideremote passive and active control of the drive circuit (seeSection 52)

52 24V DC Control Circuit In purpose-built microwaveoven plasma reactors the auxiliary equipment (vacuumpumps vacuum valves pressure gauge gas lines purge linesprocess timer and microwave power) is synchronized toplasma process in a safe manner e role of the 24V DCcontrol circuit is to synchronize the auxiliary componentsto the plasma process and shut down the system in the caseof an emergency failure this is especially important whenusing flammable corrosive and toxic precursor gases andby-products e circuit is designed so that all chasse controlsare isolated from the cavity magnetron capacitor-controlleddrive circuit using relays and solenoids It should be notedthat when converting a domestic microwave oven intoa plasma reactor the synchronization has to be built fromscratch

6 Conclusions

is work has reviewed the conversation of the domes-tic microwave oven into a source for cleaning as well aschemical reactions e conversion of domestic systems intoplasma reactor is described as is the construction ofpurpose-built microwave oven plasma reactors Calibrationof the MRC has been discussed along with identifying thetwo types of magnetron drive circuitry used e pro-fessional and armature use has also been presented in thelatter case mainly limited to kitchen top experiments eproof of principle and small batch processes established inthese plasma reactors range from plasma cleaning of glassand polymer surface and removal of toxic metals fromceramic surfaces to the manufacture of carbon nano-structures and the pyrolysis of paper to produce gaseouswaste by-products In all cases the power source is thepackage cavity magnetron operating below 1100W outputpower and pressures ranging from a few 01 s Pascal toa nominal atmospheric pressure (1013 Pascal)

At or close to atmospheric pressure single or multiplewire antenna electrodes that have a physical length ap-proximating to 14 or 12 of the microwave length in whichthat it is immersed have been found to play a catalytic rolein instigating plasma production and in the case ofmanufacturing carbon nanomaterials provide a substrate onwhich carbon material can grow With regard to reactionrate increasing with the number (1 to 6) of electrodes be-yond which the reaction becomes rate-limited a geometricalloading effect around the wire antenna is proposed in thispaper Whether this or electromagnetic power loss is re-sponsible for the effect further work is required In additionthis work has reconstructed a preformed disc aerial elec-trode (igniter) suitable for a narrow tube reactor [16 40](Figure 4)

e safety match flame lighted cigarette sliced grapeand metal antenna electrode have been observed to havea catalyst role in the production of plasma and plasmoids Todistinguish between the metal antenna and the thermal-chemical based catalyst it has been put forward that metal

Anode

Cathode

24 V

dcCo

ntro

l circ

uit

Step

-up

tran

sform

er

HV diode

35 V

Chassisinterlock

Magnetronthermoswitch Fuse

Filtered240 V

50ndash60 Hz

HVFuse

Coolingfan

HVCapacitor

25 kV

Emergencystop

Figure 7 Cavity magnetron capacitor-controlled dive circuit e 24V DC control circuit represented by the dashed box is discussed inSection 52

10 International Journal of Chemical Engineering

antennas may be classified as a passive catalyst as they onlysupply a surface that generates free electrons whilst thesafety match flame lighted cigarette and sliced grape may beclassified as an active catalyst as they supply energy in theform of heat and free electrons from an electrolyte

Finally this review also has highlighted plasmoid foodcooking within a MRC and the use of plasma discharge forremoving food odor frommicrowave ovens Given that foodsafety issues are addressed it is reasonable to envisagemicrowave oven plasma reactors incorporating both plas-moid cooking of food stuff and plasma deodorization ofcooking by-products may be realized in the near future

Conflicts of Interest

e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

e authors would like to acknowledge the support of SFIthrough the I-Form Advanced Manufacturing ResearchCenter 16RC3872

References

[1] F Nagelschmidt Diathermy Text Book for Physicians andStudents Springer-Verlag Berlin Heidelberg 1921

[2] Davis A History of the Microwave Oven e IEEE NewsSource e IEEE News Source Piscataway NJ USA httpswwwtheinstituteieeeorgtech-historytechnology-historya-history-of-the-microwave-oven

[3] P L Spencer ldquoMethod of treating foodstuffsrdquo US Patent2495429 1950

[4] J M Osepchuk ldquoe history of the microwave oven a criticalreviewrdquo in Proceedings of Digest IEEE International Micro-wave Symposium pp 1397ndash1400 Boston MA USA June2009

[5] J R Mims ldquoMicrowave magnetronrdquo US Patent 37392251973

[6] T Koinuma ldquoMagnetronrdquo US Patent 3809590 1974[7] R N Gedye F Smith and K C Westaway ldquoe rapid

synthesis of organic compounds in microwave ovensrdquo Ca-nadian Journal of Chemistry vol 6 no 1 pp 17ndash26 1988

[8] R N Gedye W Rank and K C Westaway ldquoe rapidsynthesis of organic compounds in microwave ovens IIrdquoCanadian Journal of Chemistry vol 69 no 4 pp 706ndash7111991

[9] M Omran T Fabritius E-P Heikkinen and G ChenldquoDielectric properties and carbothermic reduction of zincoxide and zinc ferrite by microwave heatingrdquo Royal SocietyOpen Science vol 4 no 9 p 170710 2017

[10] A Ribner ldquoMicrowave plasma etching machine and methodof etchingrdquo US Patent 4804431 1989

[11] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of phase pure K3C6o using a microwave-induced argon plasmardquo Journal of the Chemical SocietyChemical Communications no 18 pp 2027-2028 1994

[12] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of alkali-metal fullerides using a microwave-induced argon plasmardquo Chemistry of Materials vol 8 no 2pp 394ndash400 1996

[13] D J Brooks and R E Douthwaite ldquoMicrowave-inducedplasma reactor based on a domestic microwave oven forbulk solid state chemistryrdquo Review of Scientific Instrumentsvol 75 no 12 pp 5277ndash5279 2004

[14] B T Ginn and O Steinbock ldquoPolymer surface modificationusing microwave-oven-generated plasmardquo Langmuir vol 19no 19 pp 8117-8118 2003

[15] P Khongkrapan N Tippayawong and T Kiatsiriroatldquoermochemical conversion of waste papers to fuel gas ina microwave plasma reactorrdquo Journal of Clean EnergyTechnologies vol 1 no 2 pp 80ndash83 2013

[16] P Khongkrapan P anompongchart N Tippayawong andT Kiatsiriroat ldquoFuel gas and char from pyrolysis of wastepaper in a microwave plasma reactorrdquo IJEE vol 4 no 6pp 969ndash974 2013

[17] R Singh and A L L Jarvis ldquoMicrowave plasma-enhancedchemical vapour deposition growth of carbon nano-structuresrdquo South African Journal of Science vol 106 no 5-6p 4 2010

[18] S Nomura H Toyota S Mukasa H Yamashita T Maeharaand A Kawashima ldquoProduction of hydrogen in a conven-tional microwave ovenrdquo Journal of Applied Physics vol 106no 7 p 073306 2009

[19] S Nomura H Yamashita H Toyota S Mukasa andY Okamura ldquoSimultaneous production of hydrogen andcarbon nanotubes in a conventional microwave ovenrdquo inProceedings of International Symposium on Plasma Chemistry(ISPC19) vol 65 Bochum Germany July 2009

[20] H Toyota S Nomura and S Mukasa ldquoA practical electrodefor microwave plasma processesrdquo International Journal ofMaterials Science and Applications vol 2 no 3 pp 83ndash882013

[21] R Pongsopon T Chim-Oye and M Fuangfoong ldquoMicro-wave plasma reactor based on microwave ovenrdquo in PIERSProceedings pp 2723ndash2726 Guangzhou China August 2014

[22] N Manivannan W Balachandran R Beleca and M AbbodldquoMicrowave plasma system design and modelling for marinediesel exhaust gas abatement of NOx and SOxrdquo InternationalJournal of Environmental Science and Development vol 6no 2 pp 151ndash154 2015

[23] M C Savadori V P Mammana O G Martins andF T Degasperi ldquoPlasma-assisted chemical vapour depositionin a tunable microwave cavityrdquo Plasma Sources Science andTechnology vol 4 no 3 pp 489ndash494 1995

[24] V J Law and N Macgearailt ldquoVisualization of a dual fre-quency plasma etch processrdquo Measurement Science andTechnology vol 18 no 3 pp 645ndash649 2007

[25] Y H Jung S O Jang and H J You ldquoHydrogen generationfrom the dissociation of water using microwave plasmasrdquoChinese Physics Letters vol 30 no 6 p 065204 2013

[26] V J Law and D Tait ldquoContaminated ceramic plasmacleaningrdquo European Semiconductor vol 19 no 9 pp S38ndashS41 1997

[27] V J Law andD Tait ldquoMicrowave plasma cleaning of ion implantceramic insulatorsrdquo Vacuum vol 49 no 4 pp 273ndash278 1998

[28] T M Burke E H Linfield D A Ritchie M Peper andJ H Burroughs ldquoHydrogen radical surface cleaning of GaAsfor MBE regrowthrdquo Journal of Crystal Growth vol 175-176pp 416ndash421 1997

[29] D Tasch D J Brosky S Conrad S Kumar and D KumarldquoMicrowave plasma cookingrdquo US Patent 20090012223 A12009

[30] H Stanley ldquoMicrowave experiments at schoolrdquo Science inSchool vol 12 pp 31ndash33 2009

International Journal of Chemical Engineering 11

the Oxford Research Unit Open University on the MRC-100 reactor e probe used is an RF-biased ion flux probe(known colloquially as ldquoBebesone or BBsrdquo) For full detailsof the probe and measurement see [48]

For good probe measurements visual observations ofthe plasma volume as a whole and around the probe itselfare usually required For the MRC reactors it was foundnecessary to replace the standard front flange door witha flange incorporating a probe port and inspection portcovered by an open mesh Given this modification argonplasma within the MRC-100 visually exhibits littlestructure with a slight brightening close to the dielectricannulus boundary Nevertheless there is no evidence ofmicrowave structure in the optical emission which in-dicates that energy is rapidly homogenized in the electronpopulation

Following these visual baseline inspections theBebesonde was used to determine the ion flux in the lowpower and high setting with argon gas flow varying froma maximum (5 lmiddotminminus1) to a minimum for which plasmacould be sustained (2 lmiddotminminus1) For the MRC-200 reactorargon ion densities are measured to be in the range of2times1011middotcmminus3 at 50mbar (5 kmiddotPascal) to 3times l012middotcmminus3 at1mbar (100 Pascal) and the electron temperature is in therange of l to l5 eV for an input power of 0022Wmiddotcmminus3 thehigher plasma density at low pressure corresponds to a longermean free path and increased electron power transfer

46 Ultraviolet Fluorescence Microwave Probe For manyprocessing plasmas photon energy (E hcλ) varies from10 eV to 1 eV with preferential intensity at discrete spectralwavelengths e spectral characteristic is determined by thenature of gas excitation and relaxation processes both ofwhich are sensitive to the local electron temperature and gasexcitation cross sections Gas composition and mode ofplasma production therefore has a major impact on UVproduction and plasma chemistry

e ultraviolet fluorescence probe utilizes activated rare-earth salts Y2SiO5Ce (absorptionlt 200 nm) and Zn2SiO4Mn and Y2O3Eu3+ (absorptionlt 300 nm) For full detailsof these probes see [49] At these energies the host lattice(H) undergoes electronic excitation via the production of

electron-hole pair separation (6) followed by the lowestenergetic deactivation pathway fluorescence at wavelengthslonger than the initial radiation In general the fluorescencespectrum is composed of a narrow band with the precisewavelength determined by the intimate relation between theactivator (Ce Mn and Eu3+) and the host lattice and by theirradiating photon radiation

H A + hvprime rarrH Alowast

hvprime gt 4ev( 1113857 (6)

H Alowast rarrH A + hvPrime hvPrime lt 4ev( 1113857 (7)

emost simple form of the probes comprises a syntheticDUV grade fused silica capsule (12mm diametertimes 20mm)containing one of three activated salts at a nominal reducedpressure of 10mbare fused silica has transmittance T 05at 170 nm When placed in the plasma volume the probecollects 4π steradians of incoming DUV photon radiatione emitted fluorescence is viewed through an optical crownglass viewport (T 05 at 380 nm) Due to the dielectriccapsule photoluminescence rather than electroluminescenceis considered to be the prime mechanism of fluorescence inthese probes e dielectric also acts as a wavelength dis-criminator and provides the upper working limit of the probeto be 1100K

Using these knowledge the salts Zn2SiO4Mn and Y2O3Eu3+ (placed in their own capsule) integrates DUV plasmawhile Y2SiO5Ce when placed directly in the plasma volumeintegrates VUV (lt200 nm) plasma thus their fluorescenceappearing in the green red and blue respectively

5 Magnetron Oven Control Circuits

Microwave ovens generally employed one of two types ofmagnetron drive circuits For microwave ovens with low-power outputs (typically lt500W) the output power isachieved by pulse-width modulation of a single incidentpower to the magnetron with the cooking time set between0 and 30 minutes Between the 500W and 1100W ratingcontinuous application of the microwave power is usedwhere the power level is set by the magnetron drive capacitorvalue

P 12

CV2

1113872 1113873f (8)

e choice of either of these two drive circuits can havean impact on the magnetron control circuit ability to carryout the selected plasma processis choice is exemplified bycomparing the short and low power requirements of rapidplasma syntheses of organic compounds [7 8] cleaning ofglass slides and polymers [14] to that of the high power andlong processing times of solid-state metal oxides processing[13] and the plasma cleaning of ion implant ceramic in-sulators [26 27]

51 Cavity Magnetron Capacitor-Controlled Drive CircuitIn this section the choice of the capacitor controlled drivecircuit is considered along with its safety control circuit thatis used in the MRC-100200 plasma reactors A schematic of

Cavity wall

E-field probe

Neon

Photodiode

Shield

Vref

AD

Figure 6 Schematic of the E-probe circuit showing a neon dis-charge lamp a photodiode and an analogue-to-digital converter

International Journal of Chemical Engineering 9

the capacitor controlled drive circuit is shown in Figure 7 Asimilar cavity magnetron capacitor controlled drive circuit isreported in [23] Chaichumporn et al have also reportedfurther refinement of the magnetron anode voltage (33 to66 kV) in [40]

e transformer comprises two winding circuits efirst provides a winding ratio to produce 240V at 50ndash60Hzto approximately 35V at 50ndash60Hz for the cathode heaterand the second provides the step-up voltage (240V at 50ndash60Hzto 2-3 kV at 50ndash60Hz) e HV capacitor (06 to 15 μF)and the HV diode are used to bias the cathode negativewith respect to the anode block which contains themagnetronrsquos cavity structure Using this arrangement thecapacitor value and diode determines the DC powerdissipated in the magnetron cavity structure For safetyreasons both passive and active control components areincorporated on either side of the setup transformer eseinclude the following the circuit is fused on both sides ofthe transformer plus an emergency stop button a chassisinterlock a magnetron thermoswitch (135degC) and a chassecooling fan In addition 24 V DC circuit is used to provideremote passive and active control of the drive circuit (seeSection 52)

52 24V DC Control Circuit In purpose-built microwaveoven plasma reactors the auxiliary equipment (vacuumpumps vacuum valves pressure gauge gas lines purge linesprocess timer and microwave power) is synchronized toplasma process in a safe manner e role of the 24V DCcontrol circuit is to synchronize the auxiliary componentsto the plasma process and shut down the system in the caseof an emergency failure this is especially important whenusing flammable corrosive and toxic precursor gases andby-products e circuit is designed so that all chasse controlsare isolated from the cavity magnetron capacitor-controlleddrive circuit using relays and solenoids It should be notedthat when converting a domestic microwave oven intoa plasma reactor the synchronization has to be built fromscratch

6 Conclusions

is work has reviewed the conversation of the domes-tic microwave oven into a source for cleaning as well aschemical reactions e conversion of domestic systems intoplasma reactor is described as is the construction ofpurpose-built microwave oven plasma reactors Calibrationof the MRC has been discussed along with identifying thetwo types of magnetron drive circuitry used e pro-fessional and armature use has also been presented in thelatter case mainly limited to kitchen top experiments eproof of principle and small batch processes established inthese plasma reactors range from plasma cleaning of glassand polymer surface and removal of toxic metals fromceramic surfaces to the manufacture of carbon nano-structures and the pyrolysis of paper to produce gaseouswaste by-products In all cases the power source is thepackage cavity magnetron operating below 1100W outputpower and pressures ranging from a few 01 s Pascal toa nominal atmospheric pressure (1013 Pascal)

At or close to atmospheric pressure single or multiplewire antenna electrodes that have a physical length ap-proximating to 14 or 12 of the microwave length in whichthat it is immersed have been found to play a catalytic rolein instigating plasma production and in the case ofmanufacturing carbon nanomaterials provide a substrate onwhich carbon material can grow With regard to reactionrate increasing with the number (1 to 6) of electrodes be-yond which the reaction becomes rate-limited a geometricalloading effect around the wire antenna is proposed in thispaper Whether this or electromagnetic power loss is re-sponsible for the effect further work is required In additionthis work has reconstructed a preformed disc aerial elec-trode (igniter) suitable for a narrow tube reactor [16 40](Figure 4)

e safety match flame lighted cigarette sliced grapeand metal antenna electrode have been observed to havea catalyst role in the production of plasma and plasmoids Todistinguish between the metal antenna and the thermal-chemical based catalyst it has been put forward that metal

Anode

Cathode

24 V

dcCo

ntro

l circ

uit

Step

-up

tran

sform

er

HV diode

35 V

Chassisinterlock

Magnetronthermoswitch Fuse

Filtered240 V

50ndash60 Hz

HVFuse

Coolingfan

HVCapacitor

25 kV

Emergencystop

Figure 7 Cavity magnetron capacitor-controlled dive circuit e 24V DC control circuit represented by the dashed box is discussed inSection 52

10 International Journal of Chemical Engineering

antennas may be classified as a passive catalyst as they onlysupply a surface that generates free electrons whilst thesafety match flame lighted cigarette and sliced grape may beclassified as an active catalyst as they supply energy in theform of heat and free electrons from an electrolyte

Finally this review also has highlighted plasmoid foodcooking within a MRC and the use of plasma discharge forremoving food odor frommicrowave ovens Given that foodsafety issues are addressed it is reasonable to envisagemicrowave oven plasma reactors incorporating both plas-moid cooking of food stuff and plasma deodorization ofcooking by-products may be realized in the near future

Conflicts of Interest

e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

e authors would like to acknowledge the support of SFIthrough the I-Form Advanced Manufacturing ResearchCenter 16RC3872

References

[1] F Nagelschmidt Diathermy Text Book for Physicians andStudents Springer-Verlag Berlin Heidelberg 1921

[2] Davis A History of the Microwave Oven e IEEE NewsSource e IEEE News Source Piscataway NJ USA httpswwwtheinstituteieeeorgtech-historytechnology-historya-history-of-the-microwave-oven

[3] P L Spencer ldquoMethod of treating foodstuffsrdquo US Patent2495429 1950

[4] J M Osepchuk ldquoe history of the microwave oven a criticalreviewrdquo in Proceedings of Digest IEEE International Micro-wave Symposium pp 1397ndash1400 Boston MA USA June2009

[5] J R Mims ldquoMicrowave magnetronrdquo US Patent 37392251973

[6] T Koinuma ldquoMagnetronrdquo US Patent 3809590 1974[7] R N Gedye F Smith and K C Westaway ldquoe rapid

synthesis of organic compounds in microwave ovensrdquo Ca-nadian Journal of Chemistry vol 6 no 1 pp 17ndash26 1988

[8] R N Gedye W Rank and K C Westaway ldquoe rapidsynthesis of organic compounds in microwave ovens IIrdquoCanadian Journal of Chemistry vol 69 no 4 pp 706ndash7111991

[9] M Omran T Fabritius E-P Heikkinen and G ChenldquoDielectric properties and carbothermic reduction of zincoxide and zinc ferrite by microwave heatingrdquo Royal SocietyOpen Science vol 4 no 9 p 170710 2017

[10] A Ribner ldquoMicrowave plasma etching machine and methodof etchingrdquo US Patent 4804431 1989

[11] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of phase pure K3C6o using a microwave-induced argon plasmardquo Journal of the Chemical SocietyChemical Communications no 18 pp 2027-2028 1994

[12] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of alkali-metal fullerides using a microwave-induced argon plasmardquo Chemistry of Materials vol 8 no 2pp 394ndash400 1996

[13] D J Brooks and R E Douthwaite ldquoMicrowave-inducedplasma reactor based on a domestic microwave oven forbulk solid state chemistryrdquo Review of Scientific Instrumentsvol 75 no 12 pp 5277ndash5279 2004

[14] B T Ginn and O Steinbock ldquoPolymer surface modificationusing microwave-oven-generated plasmardquo Langmuir vol 19no 19 pp 8117-8118 2003

[15] P Khongkrapan N Tippayawong and T Kiatsiriroatldquoermochemical conversion of waste papers to fuel gas ina microwave plasma reactorrdquo Journal of Clean EnergyTechnologies vol 1 no 2 pp 80ndash83 2013

[16] P Khongkrapan P anompongchart N Tippayawong andT Kiatsiriroat ldquoFuel gas and char from pyrolysis of wastepaper in a microwave plasma reactorrdquo IJEE vol 4 no 6pp 969ndash974 2013

[17] R Singh and A L L Jarvis ldquoMicrowave plasma-enhancedchemical vapour deposition growth of carbon nano-structuresrdquo South African Journal of Science vol 106 no 5-6p 4 2010

[18] S Nomura H Toyota S Mukasa H Yamashita T Maeharaand A Kawashima ldquoProduction of hydrogen in a conven-tional microwave ovenrdquo Journal of Applied Physics vol 106no 7 p 073306 2009

[19] S Nomura H Yamashita H Toyota S Mukasa andY Okamura ldquoSimultaneous production of hydrogen andcarbon nanotubes in a conventional microwave ovenrdquo inProceedings of International Symposium on Plasma Chemistry(ISPC19) vol 65 Bochum Germany July 2009

[20] H Toyota S Nomura and S Mukasa ldquoA practical electrodefor microwave plasma processesrdquo International Journal ofMaterials Science and Applications vol 2 no 3 pp 83ndash882013

[21] R Pongsopon T Chim-Oye and M Fuangfoong ldquoMicro-wave plasma reactor based on microwave ovenrdquo in PIERSProceedings pp 2723ndash2726 Guangzhou China August 2014

[22] N Manivannan W Balachandran R Beleca and M AbbodldquoMicrowave plasma system design and modelling for marinediesel exhaust gas abatement of NOx and SOxrdquo InternationalJournal of Environmental Science and Development vol 6no 2 pp 151ndash154 2015

[23] M C Savadori V P Mammana O G Martins andF T Degasperi ldquoPlasma-assisted chemical vapour depositionin a tunable microwave cavityrdquo Plasma Sources Science andTechnology vol 4 no 3 pp 489ndash494 1995

[24] V J Law and N Macgearailt ldquoVisualization of a dual fre-quency plasma etch processrdquo Measurement Science andTechnology vol 18 no 3 pp 645ndash649 2007

[25] Y H Jung S O Jang and H J You ldquoHydrogen generationfrom the dissociation of water using microwave plasmasrdquoChinese Physics Letters vol 30 no 6 p 065204 2013

[26] V J Law and D Tait ldquoContaminated ceramic plasmacleaningrdquo European Semiconductor vol 19 no 9 pp S38ndashS41 1997

[27] V J Law andD Tait ldquoMicrowave plasma cleaning of ion implantceramic insulatorsrdquo Vacuum vol 49 no 4 pp 273ndash278 1998

[28] T M Burke E H Linfield D A Ritchie M Peper andJ H Burroughs ldquoHydrogen radical surface cleaning of GaAsfor MBE regrowthrdquo Journal of Crystal Growth vol 175-176pp 416ndash421 1997

[29] D Tasch D J Brosky S Conrad S Kumar and D KumarldquoMicrowave plasma cookingrdquo US Patent 20090012223 A12009

[30] H Stanley ldquoMicrowave experiments at schoolrdquo Science inSchool vol 12 pp 31ndash33 2009

International Journal of Chemical Engineering 11

the capacitor controlled drive circuit is shown in Figure 7 Asimilar cavity magnetron capacitor controlled drive circuit isreported in [23] Chaichumporn et al have also reportedfurther refinement of the magnetron anode voltage (33 to66 kV) in [40]

e transformer comprises two winding circuits efirst provides a winding ratio to produce 240V at 50ndash60Hzto approximately 35V at 50ndash60Hz for the cathode heaterand the second provides the step-up voltage (240V at 50ndash60Hzto 2-3 kV at 50ndash60Hz) e HV capacitor (06 to 15 μF)and the HV diode are used to bias the cathode negativewith respect to the anode block which contains themagnetronrsquos cavity structure Using this arrangement thecapacitor value and diode determines the DC powerdissipated in the magnetron cavity structure For safetyreasons both passive and active control components areincorporated on either side of the setup transformer eseinclude the following the circuit is fused on both sides ofthe transformer plus an emergency stop button a chassisinterlock a magnetron thermoswitch (135degC) and a chassecooling fan In addition 24 V DC circuit is used to provideremote passive and active control of the drive circuit (seeSection 52)

52 24V DC Control Circuit In purpose-built microwaveoven plasma reactors the auxiliary equipment (vacuumpumps vacuum valves pressure gauge gas lines purge linesprocess timer and microwave power) is synchronized toplasma process in a safe manner e role of the 24V DCcontrol circuit is to synchronize the auxiliary componentsto the plasma process and shut down the system in the caseof an emergency failure this is especially important whenusing flammable corrosive and toxic precursor gases andby-products e circuit is designed so that all chasse controlsare isolated from the cavity magnetron capacitor-controlleddrive circuit using relays and solenoids It should be notedthat when converting a domestic microwave oven intoa plasma reactor the synchronization has to be built fromscratch

6 Conclusions

is work has reviewed the conversation of the domes-tic microwave oven into a source for cleaning as well aschemical reactions e conversion of domestic systems intoplasma reactor is described as is the construction ofpurpose-built microwave oven plasma reactors Calibrationof the MRC has been discussed along with identifying thetwo types of magnetron drive circuitry used e pro-fessional and armature use has also been presented in thelatter case mainly limited to kitchen top experiments eproof of principle and small batch processes established inthese plasma reactors range from plasma cleaning of glassand polymer surface and removal of toxic metals fromceramic surfaces to the manufacture of carbon nano-structures and the pyrolysis of paper to produce gaseouswaste by-products In all cases the power source is thepackage cavity magnetron operating below 1100W outputpower and pressures ranging from a few 01 s Pascal toa nominal atmospheric pressure (1013 Pascal)

At or close to atmospheric pressure single or multiplewire antenna electrodes that have a physical length ap-proximating to 14 or 12 of the microwave length in whichthat it is immersed have been found to play a catalytic rolein instigating plasma production and in the case ofmanufacturing carbon nanomaterials provide a substrate onwhich carbon material can grow With regard to reactionrate increasing with the number (1 to 6) of electrodes be-yond which the reaction becomes rate-limited a geometricalloading effect around the wire antenna is proposed in thispaper Whether this or electromagnetic power loss is re-sponsible for the effect further work is required In additionthis work has reconstructed a preformed disc aerial elec-trode (igniter) suitable for a narrow tube reactor [16 40](Figure 4)

e safety match flame lighted cigarette sliced grapeand metal antenna electrode have been observed to havea catalyst role in the production of plasma and plasmoids Todistinguish between the metal antenna and the thermal-chemical based catalyst it has been put forward that metal

Anode

Cathode

24 V

dcCo

ntro

l circ

uit

Step

-up

tran

sform

er

HV diode

35 V

Chassisinterlock

Magnetronthermoswitch Fuse

Filtered240 V

50ndash60 Hz

HVFuse

Coolingfan

HVCapacitor

25 kV

Emergencystop

Figure 7 Cavity magnetron capacitor-controlled dive circuit e 24V DC control circuit represented by the dashed box is discussed inSection 52

10 International Journal of Chemical Engineering

antennas may be classified as a passive catalyst as they onlysupply a surface that generates free electrons whilst thesafety match flame lighted cigarette and sliced grape may beclassified as an active catalyst as they supply energy in theform of heat and free electrons from an electrolyte

Finally this review also has highlighted plasmoid foodcooking within a MRC and the use of plasma discharge forremoving food odor frommicrowave ovens Given that foodsafety issues are addressed it is reasonable to envisagemicrowave oven plasma reactors incorporating both plas-moid cooking of food stuff and plasma deodorization ofcooking by-products may be realized in the near future

Conflicts of Interest

e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

e authors would like to acknowledge the support of SFIthrough the I-Form Advanced Manufacturing ResearchCenter 16RC3872

References

[1] F Nagelschmidt Diathermy Text Book for Physicians andStudents Springer-Verlag Berlin Heidelberg 1921

[2] Davis A History of the Microwave Oven e IEEE NewsSource e IEEE News Source Piscataway NJ USA httpswwwtheinstituteieeeorgtech-historytechnology-historya-history-of-the-microwave-oven

[3] P L Spencer ldquoMethod of treating foodstuffsrdquo US Patent2495429 1950

[4] J M Osepchuk ldquoe history of the microwave oven a criticalreviewrdquo in Proceedings of Digest IEEE International Micro-wave Symposium pp 1397ndash1400 Boston MA USA June2009

[5] J R Mims ldquoMicrowave magnetronrdquo US Patent 37392251973

[6] T Koinuma ldquoMagnetronrdquo US Patent 3809590 1974[7] R N Gedye F Smith and K C Westaway ldquoe rapid

synthesis of organic compounds in microwave ovensrdquo Ca-nadian Journal of Chemistry vol 6 no 1 pp 17ndash26 1988

[8] R N Gedye W Rank and K C Westaway ldquoe rapidsynthesis of organic compounds in microwave ovens IIrdquoCanadian Journal of Chemistry vol 69 no 4 pp 706ndash7111991

[9] M Omran T Fabritius E-P Heikkinen and G ChenldquoDielectric properties and carbothermic reduction of zincoxide and zinc ferrite by microwave heatingrdquo Royal SocietyOpen Science vol 4 no 9 p 170710 2017

[10] A Ribner ldquoMicrowave plasma etching machine and methodof etchingrdquo US Patent 4804431 1989

[11] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of phase pure K3C6o using a microwave-induced argon plasmardquo Journal of the Chemical SocietyChemical Communications no 18 pp 2027-2028 1994

[12] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of alkali-metal fullerides using a microwave-induced argon plasmardquo Chemistry of Materials vol 8 no 2pp 394ndash400 1996

[13] D J Brooks and R E Douthwaite ldquoMicrowave-inducedplasma reactor based on a domestic microwave oven forbulk solid state chemistryrdquo Review of Scientific Instrumentsvol 75 no 12 pp 5277ndash5279 2004

[14] B T Ginn and O Steinbock ldquoPolymer surface modificationusing microwave-oven-generated plasmardquo Langmuir vol 19no 19 pp 8117-8118 2003

[15] P Khongkrapan N Tippayawong and T Kiatsiriroatldquoermochemical conversion of waste papers to fuel gas ina microwave plasma reactorrdquo Journal of Clean EnergyTechnologies vol 1 no 2 pp 80ndash83 2013

[16] P Khongkrapan P anompongchart N Tippayawong andT Kiatsiriroat ldquoFuel gas and char from pyrolysis of wastepaper in a microwave plasma reactorrdquo IJEE vol 4 no 6pp 969ndash974 2013

[17] R Singh and A L L Jarvis ldquoMicrowave plasma-enhancedchemical vapour deposition growth of carbon nano-structuresrdquo South African Journal of Science vol 106 no 5-6p 4 2010

[18] S Nomura H Toyota S Mukasa H Yamashita T Maeharaand A Kawashima ldquoProduction of hydrogen in a conven-tional microwave ovenrdquo Journal of Applied Physics vol 106no 7 p 073306 2009

[19] S Nomura H Yamashita H Toyota S Mukasa andY Okamura ldquoSimultaneous production of hydrogen andcarbon nanotubes in a conventional microwave ovenrdquo inProceedings of International Symposium on Plasma Chemistry(ISPC19) vol 65 Bochum Germany July 2009

[20] H Toyota S Nomura and S Mukasa ldquoA practical electrodefor microwave plasma processesrdquo International Journal ofMaterials Science and Applications vol 2 no 3 pp 83ndash882013

[21] R Pongsopon T Chim-Oye and M Fuangfoong ldquoMicro-wave plasma reactor based on microwave ovenrdquo in PIERSProceedings pp 2723ndash2726 Guangzhou China August 2014

[22] N Manivannan W Balachandran R Beleca and M AbbodldquoMicrowave plasma system design and modelling for marinediesel exhaust gas abatement of NOx and SOxrdquo InternationalJournal of Environmental Science and Development vol 6no 2 pp 151ndash154 2015

[23] M C Savadori V P Mammana O G Martins andF T Degasperi ldquoPlasma-assisted chemical vapour depositionin a tunable microwave cavityrdquo Plasma Sources Science andTechnology vol 4 no 3 pp 489ndash494 1995

[24] V J Law and N Macgearailt ldquoVisualization of a dual fre-quency plasma etch processrdquo Measurement Science andTechnology vol 18 no 3 pp 645ndash649 2007

[25] Y H Jung S O Jang and H J You ldquoHydrogen generationfrom the dissociation of water using microwave plasmasrdquoChinese Physics Letters vol 30 no 6 p 065204 2013

[26] V J Law and D Tait ldquoContaminated ceramic plasmacleaningrdquo European Semiconductor vol 19 no 9 pp S38ndashS41 1997

[27] V J Law andD Tait ldquoMicrowave plasma cleaning of ion implantceramic insulatorsrdquo Vacuum vol 49 no 4 pp 273ndash278 1998

[28] T M Burke E H Linfield D A Ritchie M Peper andJ H Burroughs ldquoHydrogen radical surface cleaning of GaAsfor MBE regrowthrdquo Journal of Crystal Growth vol 175-176pp 416ndash421 1997

[29] D Tasch D J Brosky S Conrad S Kumar and D KumarldquoMicrowave plasma cookingrdquo US Patent 20090012223 A12009

[30] H Stanley ldquoMicrowave experiments at schoolrdquo Science inSchool vol 12 pp 31ndash33 2009

International Journal of Chemical Engineering 11

antennas may be classified as a passive catalyst as they onlysupply a surface that generates free electrons whilst thesafety match flame lighted cigarette and sliced grape may beclassified as an active catalyst as they supply energy in theform of heat and free electrons from an electrolyte

Finally this review also has highlighted plasmoid foodcooking within a MRC and the use of plasma discharge forremoving food odor frommicrowave ovens Given that foodsafety issues are addressed it is reasonable to envisagemicrowave oven plasma reactors incorporating both plas-moid cooking of food stuff and plasma deodorization ofcooking by-products may be realized in the near future

Conflicts of Interest

e authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

e authors would like to acknowledge the support of SFIthrough the I-Form Advanced Manufacturing ResearchCenter 16RC3872

References

[1] F Nagelschmidt Diathermy Text Book for Physicians andStudents Springer-Verlag Berlin Heidelberg 1921

[2] Davis A History of the Microwave Oven e IEEE NewsSource e IEEE News Source Piscataway NJ USA httpswwwtheinstituteieeeorgtech-historytechnology-historya-history-of-the-microwave-oven

[3] P L Spencer ldquoMethod of treating foodstuffsrdquo US Patent2495429 1950

[4] J M Osepchuk ldquoe history of the microwave oven a criticalreviewrdquo in Proceedings of Digest IEEE International Micro-wave Symposium pp 1397ndash1400 Boston MA USA June2009

[5] J R Mims ldquoMicrowave magnetronrdquo US Patent 37392251973

[6] T Koinuma ldquoMagnetronrdquo US Patent 3809590 1974[7] R N Gedye F Smith and K C Westaway ldquoe rapid

synthesis of organic compounds in microwave ovensrdquo Ca-nadian Journal of Chemistry vol 6 no 1 pp 17ndash26 1988

[8] R N Gedye W Rank and K C Westaway ldquoe rapidsynthesis of organic compounds in microwave ovens IIrdquoCanadian Journal of Chemistry vol 69 no 4 pp 706ndash7111991

[9] M Omran T Fabritius E-P Heikkinen and G ChenldquoDielectric properties and carbothermic reduction of zincoxide and zinc ferrite by microwave heatingrdquo Royal SocietyOpen Science vol 4 no 9 p 170710 2017

[10] A Ribner ldquoMicrowave plasma etching machine and methodof etchingrdquo US Patent 4804431 1989

[11] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of phase pure K3C6o using a microwave-induced argon plasmardquo Journal of the Chemical SocietyChemical Communications no 18 pp 2027-2028 1994

[12] R E Douthwaite M L H Green and M J RosseinskyldquoRapid synthesis of alkali-metal fullerides using a microwave-induced argon plasmardquo Chemistry of Materials vol 8 no 2pp 394ndash400 1996

[13] D J Brooks and R E Douthwaite ldquoMicrowave-inducedplasma reactor based on a domestic microwave oven forbulk solid state chemistryrdquo Review of Scientific Instrumentsvol 75 no 12 pp 5277ndash5279 2004

[14] B T Ginn and O Steinbock ldquoPolymer surface modificationusing microwave-oven-generated plasmardquo Langmuir vol 19no 19 pp 8117-8118 2003

[15] P Khongkrapan N Tippayawong and T Kiatsiriroatldquoermochemical conversion of waste papers to fuel gas ina microwave plasma reactorrdquo Journal of Clean EnergyTechnologies vol 1 no 2 pp 80ndash83 2013

[16] P Khongkrapan P anompongchart N Tippayawong andT Kiatsiriroat ldquoFuel gas and char from pyrolysis of wastepaper in a microwave plasma reactorrdquo IJEE vol 4 no 6pp 969ndash974 2013

[17] R Singh and A L L Jarvis ldquoMicrowave plasma-enhancedchemical vapour deposition growth of carbon nano-structuresrdquo South African Journal of Science vol 106 no 5-6p 4 2010

[18] S Nomura H Toyota S Mukasa H Yamashita T Maeharaand A Kawashima ldquoProduction of hydrogen in a conven-tional microwave ovenrdquo Journal of Applied Physics vol 106no 7 p 073306 2009

[19] S Nomura H Yamashita H Toyota S Mukasa andY Okamura ldquoSimultaneous production of hydrogen andcarbon nanotubes in a conventional microwave ovenrdquo inProceedings of International Symposium on Plasma Chemistry(ISPC19) vol 65 Bochum Germany July 2009

[20] H Toyota S Nomura and S Mukasa ldquoA practical electrodefor microwave plasma processesrdquo International Journal ofMaterials Science and Applications vol 2 no 3 pp 83ndash882013

[21] R Pongsopon T Chim-Oye and M Fuangfoong ldquoMicro-wave plasma reactor based on microwave ovenrdquo in PIERSProceedings pp 2723ndash2726 Guangzhou China August 2014

[22] N Manivannan W Balachandran R Beleca and M AbbodldquoMicrowave plasma system design and modelling for marinediesel exhaust gas abatement of NOx and SOxrdquo InternationalJournal of Environmental Science and Development vol 6no 2 pp 151ndash154 2015

[23] M C Savadori V P Mammana O G Martins andF T Degasperi ldquoPlasma-assisted chemical vapour depositionin a tunable microwave cavityrdquo Plasma Sources Science andTechnology vol 4 no 3 pp 489ndash494 1995

[24] V J Law and N Macgearailt ldquoVisualization of a dual fre-quency plasma etch processrdquo Measurement Science andTechnology vol 18 no 3 pp 645ndash649 2007

[25] Y H Jung S O Jang and H J You ldquoHydrogen generationfrom the dissociation of water using microwave plasmasrdquoChinese Physics Letters vol 30 no 6 p 065204 2013

[26] V J Law and D Tait ldquoContaminated ceramic plasmacleaningrdquo European Semiconductor vol 19 no 9 pp S38ndashS41 1997

[27] V J Law andD Tait ldquoMicrowave plasma cleaning of ion implantceramic insulatorsrdquo Vacuum vol 49 no 4 pp 273ndash278 1998

[28] T M Burke E H Linfield D A Ritchie M Peper andJ H Burroughs ldquoHydrogen radical surface cleaning of GaAsfor MBE regrowthrdquo Journal of Crystal Growth vol 175-176pp 416ndash421 1997

[29] D Tasch D J Brosky S Conrad S Kumar and D KumarldquoMicrowave plasma cookingrdquo US Patent 20090012223 A12009

[30] H Stanley ldquoMicrowave experiments at schoolrdquo Science inSchool vol 12 pp 31ndash33 2009

International Journal of Chemical Engineering 11

[31] H Page ldquoMicrowave oven plasma reactorrdquo SummerFallvol 10 no 3-4 pp 11ndash13 2001

[32] A Tallaire ldquoPlasma cleaning in modified microwave ovenat LSPM (CNRS)rdquo httpswwwyoutubecomchannelUCfG3h7mSltjtsKcXH0dFCHQ

[33] H Stanley ldquoPlasma balls creating the 4th state of matter withmicrowavesrdquo Science in School vol 12 pp 24ndash29 2009

[34] Soxfreak5243 ldquoGrapes making fireballs in the microwaverdquohttpswwwyoutubecomwatchvJrD6yzemDRw

[35] Stupideaproductions ldquoMicrowave plasma awesome experi-mentrdquo httpswwwyoutubecomwatchvG7lfzA7WzVI

[36] J P Warren ldquoMicrowave plasma experimentrdquo httpswwwyoutubecomwatchvCNMjCggFKzM

[37] W Sajado ldquo10 destructive science experiments with micro-wave Be really careful when using microwave httpswwwyoutubecomwatchv8Yv9o8aFTuk

[38] Kreosan ldquoWhat microwave oven is capable Generatedplasmardquo httpswwwyoutubecomwatchvRrOw03gIIQQ

[39] V J Law G A C Jones N Patel and M Tewordt ldquoLoadingeffects in CH4 and H2 Morie of GaAsrdquo Microelectronic En-gineering vol 11 no 1ndash4 pp 611ndash614 1990

[40] C Chaichumporn P Ngamsirijit N BrkoonklinK Eaiprasetsak and M Fuangfoong ldquoDesign and con-struction of 245GHz microwave plasma source at atmo-spheric pressurerdquo Procedia Engineering vol 8 pp 94ndash1002011

[41] D Kumar and S Kumar ldquoPlasma assisted joiningrdquo US Patent7309843 B2 2007

[42] K Y Gyeong and R J Gwan ldquoHeating device of microwaveovenrdquo KR Patent 20010004084 2001

[43] S C Bo L Y Woo and S S Wom ldquoHeater apparatus formicrowave ovenrdquo KR Patent 100766440 2007

[44] E Jerby Y Meir R Jaffe and I Jerby ldquoFood cooking bymicrowave-excited plasmoid in air atmosphererdquo in Pro-ceedings of 14th International Conference on Microwave andHigh Frequency Heating pp 17ndash30 Nottingham UK 2013

[45] W H Lee and H J Kim ldquoCooking device with de-odorizationrdquo US Patent 20090110592 A1 2009

[46] N F Alekseev D D Malairov and I B Bensen ldquoGenerationof high-power oscillations with a magnetron in the centimeterbandrdquo Proceedings of the IRE vol 32 no 3 pp 136ndash139 1944

[47] V J Law ldquoMicrowave near-field plasma proberdquo Vacuumvol 51 no 3 pp 463ndash468 1998

[48] N J Brathwaite J P Booth and G Gunge ldquoA novel elec-trostatic probe method for ion flux measurementsrdquo PlasmaSources Science and Technology vol 5 no 4 pp 677ndash6841996

[49] V J Law ldquoUltraviolet fluorescence microwave plasma proberdquoVacuum vol 49 no 3 pp 217ndash220 1998

12 International Journal of Chemical Engineering

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