Novel Approaches to Materials Synthesis and Processing with Microwaves

7/28/2019 Novel Approaches to Materials Synthesis and Processing with Microwaves

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Festschrift Symposium for Professor Ted White, Chemical EngineeringUniversity of Queensland, Australia, December 2001

Novel Approaches to Materials Synthesis

and Processing with Microwaves

Kuruvilla A. Cherian*1, Arne J. Fliflet

2, Suman Ganguly

1

and Rustum Roy3

1Center for Remote Sensing Inc., Fairfax, VA 22030, USA

2Radiation and Particle Beam Generation Section, Code 6793

Naval Research Laboratory, Washington, DC 20375

3Materials Research Laboratory, The Pennsylvania State University,

University Park, PA 16801, USA

____________________________________________________________________

Use of microwaves for materials synthesis and processing is fast emerging as a viable

technological option offering specific advantages over several other conventional

methods. Novel synthesis routes for several technologically important materials and

sintering of various ceramic, and even powder metal, bodies have been successfully

demonstrated using 0.915 or 2.45 GHz radiation. 83 GHz quasi-optical Gyrotron

radiation offers novel and unique materials processing possibilities not possible with

the other frequencies. This paper presents some details of recent research &

development work utilizing these microwave frequencies.

_____________________________________________________________________

Introduction

Innovative approaches and new developments in materials synthesis and processing

have been spurred on by [1]:

*Author for correspondence; current address: Chief Scientist & Vice President of

Research, QQC Inc., 12825 Ford Road, Dearborn, MI 48126, USA

Email: [email protected]


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a) need for new materials with unique properties (e.g. nano structured material formssuch as nano-particles, nano-fibers and nano-composites),

b) need for new and efficient routes to known materials, in known or new forms (e.g.

bulk and thin film forms of ultrahard phases such as diamond and diamond

films), and

c) need for new and improved technologies which are better, cheaper, faster and

greener (e.g. sintering, surface modification, etc).

Several approaches have been investigated to achieve these on a practical scale,

involving and exploiting unique characteristics of different types of electromagnetic

radiation. This article deals with novel approaches to materials synthesis and

processing using electromagnetic radiation in the microwave range: 0.915GHz,

2.45GHz & 83 GHz Gyrotron radiations.

Microwave Heating Characteristics

Microwaves are electromagnetic radiation with wavelengths in the 1 mm to 1 m range

in free space, or frequency between 300GHz to 300 MHz. 2.45 GHz microwaves are

currently used for a range of scientific, technological and industrial applications

including heating. There is a fundamental difference between microwave heating andconventional heating. In the microwave process, heating is a function of the material

being processed; depending upon the interacting material type, microwaves may be

transmitted, absorbed, or reflected. Heat is generated internally within the material

instead of originating from external heating sources.

Microwave characteristics that are advantageous for materials processing may be

summarized as follows:

a) Radiation Penetration and Heating

The depth of penetration in various materials depends on several factors, including

frequency of the radiation used and the dielectric properties of the material.

Generally, most ceramics absorb microwaves to varying degrees. The extent of

microwave coupling largely depends upon the dielectric properties of the material.

When microwaves penetrate and propagate through a dielectric material, the internal

electric field generated within the affected volume induces translational motions of


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the free or bound charges (e.g. electrons or ions) and rotates charge complexes suchas dipoles. The resistance of these induced motions due to inertial, elastic, and

frictional forces cause losses and attenuate the electric field. As a consequence of

these losses, energy is transferred and volumetric heating occurs within the solid,

leading to inside-out heating or an inverse heating gradient with the interior of the

solid hotter than the outside. This enables rapid volumetric heating without surface

overheating, and various practical advantages such as removal of gases and binder

material from interior of porous materials, or enabling more efficient infiltration and

condensation of material into porous bodies.

b) Temperature Dependent Dielectric Losses and Accelerated Heating

The rate of temperature rise in a microwave field varies widely for different materials;

however, when this phenomenon occurs the heating rate is very fast since the energy

is transferred directly to the desired solid without the need of heating its environment

first. Since the effective heater in the microwave system is the solid being heated and

the heating is a function of its ability to couple with the microwave energy, the

heating rate is a function of this material property and the frequency of the radiation

used. For the same material, higher frequencies could provide higher heating rates

whereas lower frequencies have higher penetration. Many materials exhibit

acceleration of dielectric losses above a critical temperature. When uncontrolled this

could lead to thermal runaway but when controlled this could facilitate very rapid

bulk heating, which may be used to advantage in various processes like annealing,

calcining, firing, melting, sintering, etc.

c) Field Distribution Control and Localized Heating

Using specific applicators electric field distributions can be focused and controlled.

This could help localize very high field strengths, offering a host of possibilities

including heating selected regions between two material to facilitate bonding, brazing

or welding, etc. Single mode systems, or high frequency Gyrotron systems with

quasi-optical beam characteristics, are more useful for these applications. The

generation of microwave assisted plasmas for sintering or chemical vapor deposition

is another intensely investigated possibility. Single mode systems are useful for

microwave plasma applications.


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d) Material Dependent Selective HeatingDifferent materials exhibit different degrees of coupling with microwaves. The

differential coupling characteristics can be used to advantage in various types of

processing. The strong microwave absorption characteristic of water, for example, is

exploited in food processing, drying or dehydrating several types of materials.

Similarly, selective microwave absorption and heating characteristics of specific

components may be exploited to advantage in processing asphalt, rubber and other

composite materials.

e) Self-limiting Heating.

In cases where one constituent phase of a composite materials system is selectively

heated, a phase change of the specific phase into a non microwave-absorbing phase as

a result of the processing could lead to cessation of heating and thus be self-limiting.

This characteristic could be used to advantage in the synthesis of certain phases that

are non-microwave absorbers, from starting phases that are microwave absorbers.

Microwave Processing Set-Ups

These characteristics outlined above can be used to advantage for the synthesis and

processing of a variety of materials. Several types of microwaves systems arepresently available and these include single mode and multi mode cavities and

travelling wave applicators, besides a variety of radio frequency and induction heating

systems. These could offer considerable flexibility in processing and also enable

tailoring of processes for specific materials synthesis and product processing. Several

reputed research groups worldwide have seized upon this opportunity, and detailed

reports of the varying degrees of successes obtained have been reported. [1-4].

The most commonly used systems for materials processing at present use 2.45

GHz radiation for which the microwave power sources are readily available. Since

lower frequencies offer greater depth of penetration, 0.915 GHz radiation has been

found beneficial for certain larger bulk materials processing applications. Typical

0.915 and 2.45 GHz microwave processing set-ups at the Penn State Materials

Research Laboratory (MRL) are shown schematically in figs. 1a & b. These normally

operate in air but adaptations have been made to operate in special atmospheres as


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well. Using these or similar set-ups, a host of useful materials and material systemshave been synthesized and/or processed. These include sintering of WC+Co and other

superhard material composites for tool applications. It was found, depending on the

Figure 1a & b. Schematic of 0.915 and 2.45GHz microwave processing systems at

Penn State MRL

precursor characteristics and processing regime adopted, that microwave processing

could yield better mechanical properties than those conventionally processed, fine and

uniform microstructure (~1 micron grains) with very little grain growth and nearly


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full density achieved without addition of grain growth inhibitors. Typical processparameters could include sintering at 1250-1320

oC for 10-30 minutes. Materials

synthesis utilizing reduced oxide precursors was another successful area, and this

include BaTiO3 (BT), Pb(Zr 0.52 Ti 0.48)O3 (PZT), Ba3(MgTa2)O9 (BMT), etc. Lesser

processing times and temperatures result in less loss of volatile components such as

Pb compounds [1]. Other notable results include synthesis of sub-micron diameter

Si3N4, [5], fabrication of transparent ceramics through minimization of grain growth

[1], etc. Fig. 2 shows some examples of components processed with microwaves at

Penn State MRL.

Figure 2. Various kinds of WC/Co parts sintered through 2.45GHz microwaveprocessing at Penn State MRL

Novel Approaches and Results with 2.45GHz radiation

Reduced Oxide Precursor Microwave Synthesis

In the area of microwave-assisted materials synthesis, it was found that pre-reduction

of oxide phases could yield highly microwave absorptive precursor materials which

could enhance reaction kinetics dramatically. The creation of a defect structure


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apparently makes microwave coupling more efficient at room temperatures. Forexample, conventional methods for the synthesis of technologically important oxides

such as BaTiO3 (BT), Pb(Zr0.52Ti0.48)O3 (PZT) and Ba3 (MgTa2)O9 (BMT) require

temperatures in the range of 900-1400oC with several hours of soaking time. Pure

stoichiometric metal oxides such as TiO2 and Ta2O5 do not couple with microwaves

efficiently unless heated to temperatures >1000oC where they become dielectrically

lossy. Partial reduction of these phases to oxygen defective states such as TiO2-x and

Ta2O5-x radically enhances their ability to absorb microwave energy at lower and near

room temperatures. By using such pre-reduced precursor oxides, BaTiO3 (BT), Pb(Zr

0.52 Ti 0.48)O3 (PZT), Ba3 (MgTa2)O9 (BMT) could be synthesized at amazingly lower

temperatures, between 300-900oC in 5-12 minutes! This concept of reduced oxides

causing better microwave coupling has also been extended to densification by

sintering. An example is that of titania, where 98% density was achieved in 40

minutes, compared to 3 hours through conventional heating [1].

Microwave Hydrothermal Processing

A novel variation in processing approach is the development of the microwave

hydrothermal apparatus, initially used for rapid dissolution of rock and other samples

for chemical analysis. The Penn State MRL group has successfully used this for new

materials synthesis studies. Rapid and convenient precipitation of 1-micron metal

powders from ethylene glycol solutions and a wide variety of synthesis in oxide and

silicate systems were studied [1]. The microwave hydrothermal process have yielded

at least two new phases and, besides, has been found to be a powerful tool to

synthesize a range of useful materials such as ferrites, ferroelectrics, etc. Reaction

times were found to be lowered substantially and reaction times by nearly an order of

magnitude, compared to not only dry conditions but also non-microwave

hydrothermal conditions as well.

Full Sintering of Powdered Metal Bodies

A very novel and significant result achieved recently was sintering of powdered metal

bodies [6]. Bulk metals reflect microwaves and thus are not heated significantly in a

microwave field. Nevertheless in a powdered and unsintered form, all metals alloys

and intermetallics are reported to couple and heat up in a microwave field efficiently


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and effectively to yield well-sintered bodies with improved mechanical properties.Examples include PM green bodies of various metals and alloys (Fe-Ni-C and Fe-Cu-

C) sintered in a microwave field with total time cycle ~90 min for sintering

temperatures 1100-1300oC and soaking period varying from 5 to 60 minutes. Physical

properties, such as density, hardness and modulus of rupture were reported to be

better than those prepared through conventional routes. The ability to sinter metals

with microwaves should be beneficial in the preparation of high-performance metal

parts needed in many industries, the automotive industry for example.

Novel Approaches with 83 GHz Gyrotron Radiation

We have seen above that advanced materials processing technologies using

microwaves offer several advantages over conventional methods; the frequency most

commonly being investigated has been the conventional 2.45 GHz. Efforts have been

underway to use these microwaves for numerous materials processing applications at

various laboratories worldwide (see references in [1]). With the realization of the

frequency dependence of microwave-material interaction characteristics, other

processing approaches employing different frequencies have assumed greater

importance. For larger bulk processing, 0.915 GHz has been found to have certainadvantages over 2.45 GHz. For technologies involving extremely fast heating and

surface processing, however, higher frequencies offer unique advantages.

Gyrotron radiation is a microwave beam with a wavelength from one to 10 mm.

This is a new, concentrated beam source of energy and has broad, unique applicability

in the processing of various materials such as ceramics, composites, organics,

polymers, and semiconductors, and chemical synthesis. This beam is generated by a

Gyrotron, which is a generator of high-power concentrated electromagnetic radiation.

There are no other devices in existence today that can generate this specific

wavelength at power levels high enough for materials processing. The Gyrotron was

developed almost simultaneously in Russia and the U.S. and today all nuclear centers

throughout the world use Gyrotron devices in their fusion synthesis units, but

generally for this application only. The novel 83 GHz microwave beam produced by a

Gyrotron, with quasi-optical beam characteristics, offers unique advantages in


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materials processing over conventional 2.45 GHz microwaves. Higher frequencieshave different material interaction characteristics that could be exploited for unique

materials processing advantages.

Figure 3. Gyrotron Beam Energy Source for Materials Processing

Fig 3 shows some of the Gyrotron beam characteristics. The unique advantages of

83 GHz Gyrotron microwaves are:

1. Quasi-optical beam characteristics that enable it to be focussed, spread, rastered

or directed on to specific parts of sample being processed

2. Efficient heating source for non-metallic materials: extremely rapid heating to

melting points in seconds or fraction of a second

3. Heating depth in mm to cm range: extremely useful dimensions for industrial

processing

4. Selective heating of materials: non absorbing material components not heated

unnecessarily during processing

. To exploit this for materials research, design and construction of a novel high

frequency (83 GHz) Gyrotron materials processing facility was necessary. The

combined efforts and resources of the Center for Remote Sensing (CRS), Inc., VA

and the Naval Research Laboratory (NRL) in Washington carried out this complex


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task. A program in HF microwave ceramic processing based on Gyrotron sources wasinitiated in 1995 at NRL, with the initial focus of the program being on bulk sintering

of nano-crystalline ceramics. In 1998, the resources of the NRL program and an ONR

Phase II SBIR grant awarded to CRS Inc. helped to establish a new 15kW CW 83

GHz Gyrotron beam facility. The facility was intended to be made available to

various commercial and Government users for test and development of new

techniques and processes.

Fig. 4a shows the schematic, and Fig. 4b the actual set up, of this Gyrotron beam

material processing facility. The millimeter wave radiation source is a CW Gaussian

beam generated by an industrial Gyrotron manufactured by Gycom, Ltd., operating at

83 GHz and power output up to 15 kW. This materials-processing facility is designed

to undertake various R & D activities to exploit the unique Gyrotron microwave (mm

wave) interaction characteristics with materials, to develop new or better processing

technologies. Application areas include coating of materials, soldering and brazing,

polymer treatments, semiconductor processing, ceramic processing and materials

synthesis.

Figure 4a. Schematic of the Gyrotron Processing Facility at the Naval ResearchLaboratory, Washington


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Figure 4b. The Gyrotron Beam Material Processing Facility at the Naval Research

Laboratory, Washington

Some of the specific materials processing possibilities with Gyrotron radiation

which have been identified may be summarized as follows:

Coating of materials

Coating of metallic objects (e.g. oil and gas pipelines, chemical equipment) with

polymer (polyethylene, Teflon, polypropylene and other nonpolar polymers) for

better corrosion resistance, with excellent adhesion.

Ceramic glaze on firebricks, with wide transition zone, strong adhesion, high

resistance to thermal shock.

Steel casting ladle linings, gates and pipes, with excellent performance conditions

under cyclical mechanical and thermal loads.

Metal surface hardening of metals, tool steels, etc. by high temperature surface

diffusion of hardening component and with lower overall internal heating.

Metallization of dielectric materials, PCB manufacture using thick film

technology instead of etching: copper on fiberglass laminate substrates possible


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with substrate temperatures rising to only 60-80C, fast copper fusing on ceramicsubstrates with excellent adhesion, without need for shielding gasses.

Soldering and brazing

Fast brazing of diamonds and cBN to metal tools; use of high temperature brazes

for increased mechanical bonding properties, without destructive phase

transformations.

Low pressure compaction of diamonds and cBN, using high temperature brazes

and without phase transformations.

Joining of ceramic articles and fabrication of complex shapes using high

temperature brazes; preferential heating of joint enable use of brazes with melting

points higher than the materials being joined.

Polymer treatment

Hermetical sealing of electronic components by selective and rapid heating of

polymers.

Manufacture of fiberglass reinforced polymer laminate sheets without use of

polymer solvents and large drying furnaces.

Manufacture of reinforced polymer structures with rotational symmetry, without

air bubble entrapment.Semiconductor processing

Homogenization of semiconductors, active dopants, form ohmic contacts, etc. by

selective fast diffusion processes under Gyrotron radiation.

Ceramic processing

Faster sintering process for ceramics with fine microstructure and UV/optical/IR

transparency.

Materials synthesis

More efficient synthesis routes for specific phases exploiting selective absorption

characteristics of materials to Gyrotron radiation.

Penn State MRL, CRS Inc. and NRL have been collaborating at various levels in

developing this new technology for practical applications.


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Figure 5. SiOx fibers synthesized through 83 GHz Gyrotron microwave processing

Figure 6. WCx coating on low carbon steel through 83 GHz Gyrotron microwaveprocessing


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Some preliminary results have been published [7-10], while others are undervarious stages of processing. Figs.5, 6 and 7 show some preliminary materials

processing results: rapid synthesis of SiOx fibers, coating of WCx on low carbon steel

and ceramic-metal joining. Detailed research & development efforts are underway at

Figure 7. Examples of ceramic-metal brazing done with the 83 GHz Gyrotron facility

the NRL facility to apply the Gyrotron radiation to the various possibilities that have

been identified.

The new CRS-NRL 83 GHz Gyrotron processing facility at the NRL, together

with the 0.915 GHz and various 2.45 GHz microwave processing facilities of the

Penn State MRL microwave processing group who are collaborators, offer unique

opportunities for developing novel advanced materials processing technologies

employing microwaves of various frequencies 0.915, 2.45 and 83 GHz. There are

numerous applications for multi-frequency microwave processing, and the

effectiveness of different frequencies for various processes are currently being

evaluated.

Conclusions

Significant advances and developments have taken place during recent years in the

application of 0.915 and 2.45 GHz microwaves to materials synthesis and processing.

Parameters leading to extremely fast reaction kinetics and new reaction pathways

producing specific material phases at lower temperatures than that possible by

conventional methods, have been established through novel approaches for several


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materials. Reduction in processing times compared to conventional routes, coupledwith better physical properties of the processed material, are leading to better and

cheaper products through microwave processing now becoming a reality

particularly specialty ceramics. Most significant recent developments include

sintering of powdered metals and fabrication of transparent ceramics in a single step

process.

The establishment of the 83 GHz Gyrotron materials processing facility offers

further possibilities. The unique Gyrotron beam characteristics enable new ways of

selectively heating parts of a component for specific processing advantages. It is

being developed as a valuable industrial tool for developing novel technological

solutions that were considered impossible or impractical, earlier.

Acknowledgements

The Gyrotron related work was performed through a SBIR contract from the Navy to

the Center For Remote Sensing, Inc. The gyrotron facility was developed by the

Center For Remote Sensing, Inc. and is currently available for various users; those

interested should contact www.cfrsi.com. KAC acknowledges that most of the

Gyrotron microwave materials processing work mentioned here were performedwhile associated with CRS Inc.

. Support for the research at Penn State MRL has been provided by the Office of

Naval Research and the Defense Advanced Research Projects Agency.

References

1. Roy, R., Agrawal, D., Cheng, J.P. and Mathis, M. 1997 Microwave processing: triumph ofapplications-driven science in WC-composites and ferroic titanates. Ceram. Trans. 80, 3-26.

2. Clark, D.and Sutton, W.H. 1996 Microwave processing of materials. Annu. Rev. Mater. Sci., 26, 299-331.

3. Schiffman, R.F. 1995 Commercializing microwave systems: paths to success or failure. Ceram. Trans.

59, 7-174. Vaidhyanathan, B. and Rao, K.J. 1997. Synthesis of Ti, Ga and V Nitrides: Microwave assisted

carbothermal reduction and nitridation, Chem. Mater., 9, 1196-1200

5. Gedevanishvili, S., Cherian, K.,Agrawal, D., and Roy, R. 1999. Synthesis of silicon nitride whiskersby microwave heating. Solid-State Chemistry of Inorganic Materials II, Mater. Res. Soc. Symp. Proc.

547, 413-417

6. Roy, R., Agrawal, D., Cheng, J., and Gedevanishvili, S., 1999. Full sintering of powdered-metal bodiesin a microwave field. Nature, 399, 668670.


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7. Fliflet, A.W., Bruce, R.W., Fischer, R.P., Kinkead, A.K., Gold, S.H., Ganguly, S. and Cherian, K.1999. Initial operation of a gyrotron-powered millimeter-wave beam facility for microwave processingof materials. Submitted to 24th Int. Conf. On IR&MMW, Monterey, California USA.

8. Ganguly, S. and Cherian, K. 1999. Multi-frequency microwave processing facility for advancedmaterials. Materials Technology: A Strategic Industry Report on Emerging Materials, RelatedInnovations, Technologies, Applications, Markets, Technology Transfer and Advanced Performance

Materials, 14 (4) 239-240

9. Bruce, R., Fliflet, A., Fischer, R., Kinkead, A., Gold, S., Ganguly, S. and Cherian, K., 1999. MaterialHeating Experiments with a Gyrotron-Powered Millimeter-Wave Beam. Presented at: 41st Annual

Meeting of the Division of Plasma Physics, APS, Seattle, USA.

10. Roy, R., Cherian, K.A., Badzian, A., Schriempf, J.T., and Petach, T.P., 2000. Solid State Synthesis &Processing by Microwave & Laser Photons. Presented at: Laser Applications in DoD Conference, The

Pennsylvania State University, USA

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Novel Approaches to Materials Synthesis and Processing with Microwaves