TailoringAerogelforThermalSprayApplicationsin Aero-Engines ...downloads.hindawi.com/journals/amse/2018/5670291.pdf · ResearchArticle TailoringAerogelforThermalSprayApplicationsin

Research ArticleTailoring Aerogel for Thermal Spray Applications inAero-Engines A Screening Study

Nadiir Bheekhun 1 Abd Rahim Abu Talib 2 and Mohd Roshdi Hassan 3

1Aerospace and Communication Technology Research Group Department of Engineering and TechnologyFaculty of Information Sciences and Engineering Management amp Science University University Drive Seksyen1340100 Shah Alam Selangor Darul Ehsan Malaysia2Aerodynamics Heat Transfer and Propulsion Research Group Department of Aerospace Engineering Faculty of EngineeringUniversiti Putra Malaysia 43400 Serdang Selangor Darul Ehsan Malaysia3Department of Mechanical and Manufacturing Engineering Faculty of Engineering Universiti Putra Malaysia 43400 SerdangSelangor Darul Ehsan Malaysia

Correspondence should be addressed to Abd Rahim Abu Talib abdrahimupmedumy

Received 16 March 2018 Revised 21 August 2018 Accepted 4 September 2018 Published 17 October 2018

Academic Editor Lijing Wang

Copyright copy 2018 Nadiir Bheekhun et alis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Irregular silica aerogel particles had been tailored to a regular spherical shape within the proper granulometric size range for thermalspraying Silica aerogel is an ultralow dense and highly porous nanomaterial with its thermal conductivity being the lowest than anysolids Although silica aerogels possess fascinating physical properties their implementation is limited to aerogel-doped blankets in theaerospace industry Due to space constraints these heat insulative and fireproof blankets are not encouraged by aero-enginemanufacturers and hence alternatives are being sought Although it was thought that an aerogel-based thermally sprayed coatingmaybe applicable aerogel powders are extremely challenging to be injected and deposited by thermal spray guns because of their in-appropriate granulometric and morphological properties Consequently this study intends to tailor the aerogel powders accordinglyAerogel-based slurries with yttria-stabilized zirconia as a secondary ceramic were prepared and spray-dried according to a modifiedTaguchi experimental design in order to appreciate the effect of both the slurry formulation and drying conditions such as the solidcontent the ratio of yttria-stabilized zirconia aerogel added the amount of dispersant and binder inlet temperature atomizationpressure and feeding rate on the aforementioned characteristics of the resulting spray-dried powder Uniformity was found to be themost influenced one (F-ratio 6240) by the overall spray-drying process Solid content had the most significant effect on medianparticle size (p value 0035) and volume fraction (p value 0010) but did not affect uniformity significantly (p value 0065)Furthermore a strong positive and significant correlation existed (Pearsonrsquos r 0930) between median particle size and volumefraction Based on the derived relationships an optimised condition to achieve the maximummedian particle size was then predictedand verified experimentally e optimised aerogel-based spray-dried powder had a median particle size volume fraction anduniformity of 2893plusmn 0726μm 6445plusmn 0535 and 0475plusmn 0002 respectively Finally the morphology of the optimised powder wasnoticed to have been changed from irregular shapes to spherical or donut-like granules whichmade themwithin the frame of thermallysprayable However when the optimised spray-dried powder was weighed the quantity was found to be 10 only from the total weightof ceramics within the slurry prior to spray-drying which makes it uneconomically reasonable for subsequent thermal spraying

1 Introduction

Silica aerogel [1] invented in 1931 sometimes considered asa state of matter on its own is a highly porous material withextraordinary physical properties which has attracted in-creasingly more attention since the turn of the millenniumdue to its potential in resolving various existing problems

e usage of aerogels was restricted until recently due totheir relatively high manufacturing cost that arises owing tothe expensive synthetic source of silica andor the super-critical drying during the synthesis process thereby makingthe material impracticable compared to conventional ma-terials Silica aerogel is prepared via the sol-gel method andinitially followed by supercritical drying [2 3] Nowadays

HindawiAdvances in Materials Science and EngineeringVolume 2018 Article ID 5670291 11 pageshttpsdoiorg10115520185670291

manufacturers of aerogels have modified the synthesistechnique such as modifying the sol-gel route applyingambient drying instead of subjecting on supercritical con-ditions and also using the cheaper natural source of silicasuch as rice husk in the place of synthetic matter to saveenergy and time and hence the overall manufacturing costIntensive research was carried out in the academia prior tothe commercialised ecofriendly andor cheaper aerogels[4ndash10] e actual trend which depicts an increasing interestin aerogels by manufacturers is not because of the newadvances in the nanotechnology of the material itself butinstead to meet the tailored demands by consumers re-quiring innovative and yet cost-effective solutions As a re-sult the aerogels market has recently seen a surge in thenumber of patents filed on aerogels in the recent historyPublished patents started to rise rapidly from about 80published in 2000 to in excess of 400 in 2009 [11] isdemonstrates the increase in nanomaterials research inrecent years in the field of aerogels One of the mostpromising implementations of these nanostructured mate-rials is high-performance thermal insulation because of itsrelatively low thermal conductivity sim0012WmK and ul-tralow density which can go as low as 0003 gcm3 [12]Comparatively the same property of air is approximately00012 gcm3 that is thrice lower than that of silica aerogelwhich may be considered as significant In the powder formsilica aerogel is normally denser and may range from 006 to03 gcm3 or even a higher value depending on the source ofsilica and drying process [13 14] is illustrates the po-tential substantial weight savings that can be attained ifaerogels are used for thermal insulation It is neverthelessnoticed that powdered aerogels are preferred over monolithsand are being integrated into various innovative solutionsFor instance thick thermally insulated aerogel-basedblankets of millimetre order are being continuously stud-ied and used in aerospace [15] while thin aerogel-dopedinsulative paints and granulate glazing systems are currentlybeing employed for architectural purposes [16] e issueschallenges and applications of aerogels in aerospace havebeen addressed elsewhere [17]

However up to now silica aerogel has not been appliedas a heat- or fire-resistive coating It is thought that thermalspraying is a potential class of coating technology that can beopted to achieve so e technique is one in which thefeedstock in the powder form is propelled onto a substrate athigh velocity and temperature to melt the particles partiallyso as they adhere on the surface to form a coating Typicallythermal spray coatings are typically produced with a thick-ness between 30 μm and 2mm [18] One vital requisiteamongst others while considering conventional thermalspraying is that the particles should be spherical rangingwithin a certain range of particle size typically 10ndash100 μm[18] with a median particle size within 20ndash60 μm[19]

However the as-received aerogel powder intended to bethermal sprayed had irregular shapes granule particle range of2ndash300μm and median particle size of 81μm which made itinappropriate for thermal spraying In consequence the ob-jective of this paper is to tailor the particles of silica aerogelobtained from the manufacturer into spherical ones with

granulometric properties via spray-drying as an intermediateprocess for subsequent air plasma spraying (APS) which will beconducted and reported later APS is regarded as the mostversatile thermal spray process which allows deposition even oncritical substrates such as polymer matrix composites [20 21]Spray-drying has been chosen because of its superiority inproducing thermal-spray-graded spherical free-flowing powderswithin a wide range of sizes economically compared to thoseprepared as precipitation [22] sinteringcrushing [23] andhollow-spherical powder [24] It is a process by which a fluidfeed material is transformed into a dry powder by spraying thelatter which is a water-based suspension with air as drying gas[25] Some spray-dryers operate with alcohol-based suspensionsbut instead use nitrogen as the drying gas It is a well-establishedstatement that the significant factors to influence the charac-teristics of spray-dried powders are suspension compositiondrying temperature atomising pressure and feeding rate [26]Considering the fact that no academic study has investigated thefeasibility of spray-drying aerogel powders this investigationtakes into consideration all these factors as its independentvariables and studies their effect on three measurable responsesnamely median particle size volume fraction and uniformitye optimised condition to achieve the maximum medianparticle size and volume fraction was subsequently predictedafter which an assessment on themorphology of on the resultingspray-dried powders was carried out

2 Materials and Methods

21 Materials Ecological GEATM 0125 aerogel was suppliedby Green Earth Aerogel Technologies whilst commercialisedyttria-stabilised zirconia (YSZ) ZrO28Y2O3 (Metco 240NS-G)was purchased fromOerlikonMetco (previously SulzerMetco)Polyvinyl alcohol (Mowiolreg 4-88) and polyethylene glycol werebought from Sigma-Aldrich Darvan 821-A was provided byVanderbilt Minerals LLC Distilled water was available at thefacility Advanced Materials Research Centre (AMRECSIRIM) All the materials were used as obtained with no furthertreatment YSZ was added as a secondary ceramic material inorder to improve the flowability of the slurry through the nozzleof the spray-dryer as aerogel in ultralight Also YSZ is a ther-mal-spray-graded powder which is used as a heat insulativecoating in aero-engines PVA which is a binder was optedbecause of its water soluble nature and its ability to be readilyball-milledwithout degradation hence creating extremely stablesolutions e uniformity or polydispersity index (PDI) of thepowders of aerogels which gives an indication of the hetero-geneity of sizes of the granules was measured using particle sizeanalysis via the Mastersizer 2000EHydro 2000MU It wasfound that the uniformity of the GEATM 0125 aerogel powderswas 1993 which is relatively high for a thermal spray powder ascompared to the uniformity of the thermal-spray-graded YSZhaving a value of 0433 YSZ had an apparent density of 2300plusmn0200 gcm3 while the aerogel was considerably less dense witha value of 0129plusmn0010 gcm3

22 Slurry Formulation and Granulation A strict mixingprocedure was carried out whereby the amount of each

2 Advances in Materials Science and Engineering

aforementioned material and the amount of each afore-mentioned chemical were added according to the formu-lation with respect to Table 1 For a constant slurry volume of1000 cm3 firstly Darvan 821-A as a dispersant was firstdropped to distilled water at 95degC and stirred for 10minewater-soluble polyvinyl alcohol PVA of molecular weightsim31000 was then added into the mixture and mixed at thesame temperature for 20 minutes until being completelydissolved followed by a specific amount of plasticizerpolyethylene glycol PEG of 05 wt for an additional10min at 95degC e solution was left to be cooled until roomtemperature and was weighed e loss of water during theheating was compensated Yttria-stabilised zirconia was thenmixed gradually and the suspension was blended for an-other 15min at room temperature Finally aerogel wasadded and mixed for an additional 15min e resultingmixture was ball-milled in stainless steel jars and zirconiaballs for 25 hours to attain a well-stabilized and dispersedslurry e ratio of slurry ball was kept 1 2 at all times eball-milled slurry was then weighed again so that the amountof the spray-dried powder collected could be correlated withthe amount of the slurry being granulated in other wordsthe yield It is the ratio of the amount of powder collectedafter every spray-drying experiment to the initial amount ofsolids in the feed solution and expressed in percentage

A LabPlant SD-05 spray-dryer with a two-fluid nozzle wasused in this investigation In this type of nozzle a spray isobtained by using two fluids the feed and the compressed gase compressed gas in this case air atomised is breakingdown the feed solution into individual spheres as it emergedfrom the jet to form the required fine spraye jet nozzle hadan internal diameter of 05mm in order to obtain the desiredparticle size distribution of 20ndash60 μm e operating pro-cedure of the spray-dryer was followed in accordance with themanual Heated air was blown through the main chamberevaporating the liquid content of the atomised spray leavingthe solid particles of the material to be separated from theexhaust air flow by a cyclone and collected in the samplecollection bottle During operation the values of the designparameters under study were ensured to remain constant atall times and the suspension was kept on stirringmagneticallyin the beaker until completion e spray-dried aerogels werethen transferred into a clean container and labelled accord-ingly Afterwards the components of the spray-dryer weremeticulously cleaned before the next run

23Design of Experiment e Taguchi design of experimentwas employed as the statistical tool to minimize effort andtime required without compromising the process It isa powerful tool being used within the scientific communityfor the design of high-quality systems through screening andoptimisation by using a strategically designed experimentbeing derived mathematically and empirically In this workthe influence of the spray-drying process and the effect ofeach spray-drying parameter on the responses werescreened and their significances were appraised accordinglyto enable optimization A modified orthogonal array L18was used to examine a seven-factor system with a combined

number of levels wherein one of the factors consists of sixlevels and the remaining were with three levels as shown inTable 1 As a note L denotes the Latin square and 18represents the number of runs to be undertaken e in-dependent variables were powder content (vol) mass ratioof YSZ aerogel dispersant (wt) binder (wt) inlettemperature (degC) pressure (bar) and feeding rate (mlmin)e first four factors are related to the composition while theremaining ones are attributed to the operating settings of thespray-dryer On the contrary the responses were medianparticle size (D50) volume fraction representing the amountof particles within the range of 20ndash60 μm[19] which isconsidered to be the appropriate granule size for air plasmaspraying (APS) and uniformity It is to be noted that thevolume fraction is obtained from the particle size distri-bution generated by the Mastersizer 2000EHydro 2000MUEach run was performed in triplicate and the average wascalculated for each response e statistical analysis of thedata collected was done via the Minitabreg software to de-termine any significance correlations and optimization

24 Characterisation A laser particle size analyser (Master-sizer 2000E by Malvern Instruments) was used to obtain thegranulometric properties while a Hitachi TM3000BenchtopScanning Electron Microscope was employed for morpho-logical analysis of the as-received aerogel powders anda Philips XL-30 Scanning Electron Microscope was used toobserve the spray-dried aerogel-based powders

3 Results

e comparison amongst the prepared spray-dried YSZ-aerogel samples highlights diagrammatically how each re-sponse that is median particle size volume fraction anduniformity was affected from treatment 1 to treatment 18when subjected to different conditions according to Table 2e effect of the spray-drying variables on the responses isdiscussed in the next section

31 Effect of Spray-Drying Variables on Responses e re-gression coefficients derived from ANOVA were employedto analyse the effect of the independent spray-drying vari-ables on the targeted responses is coefficient is by def-inition the mean change in the response variable for oneunit of change in the independent variable while holdingother predictors in the model constant [27] Figures 1ndash3

Table 1 Modified L18 Taguchi design

Symbol FactorsLevels

1 2 3 4 5 6A Powder content (vol) 15 20 25 30 35 40B YSZ aerogel (wt) 1 1 1 3 1 5C Dispersant (wt) 03 06 09D Binder (wt) 4 6 8E Inlet temperature (degC) 150 175 200F Atomization pressure (bar) 09 10 11G Feeding rate (mlmin) 18 21 24Note each experiment was carried out in triplicate

Advances in Materials Science and Engineering 3

present the effect of each control factor viz the solidcontent the weight ratio YSZ aerogel dispersant binderinlet temperature atomisation pressure and feeding rate onthe median particle size volume fraction and uniformity ofthe prepared spray-dried aerogel-YSZ powders

In general the granulation process could be divided intothree steps (1) atomisation where the droplets were formedafter the slurry had passed by the nozzle which was posi-tioned at the top of the drying chamber (2) drying gas anddroplet contact where evaporation took place and the liquidfeed was turned into droplets in the drying chamber and (3)powder recovery where the dried feedstock was separatedfrom the drying gas stream in the collecting chamber [19]While empirical models provide an accurate and practicalapproach to describe the relationships that exist betweenoperating variables during spray-drying and various processresponses the droplet size and solution solid content areregarded to have a direct correlation to the final size of the

dried particle as shown in Equation (1) [28] Droplet size inturn is a function of (1) atomizer geometry (2) spray-solution attributes and (3) atomisation parameters Inthis study the atomizer geometry was kept constant with anorifice diameter of 05mm whilst the solid content (volYSZ aerogel) spray-solution attributes (dispersant andbinder) and atomisation parameters (inlet temperatureatomisation pressure and feeding rate) were altered

Dparticle Ddroplet times

xsolids timesρdropletρparticle

3

1113971

(1)

where Dparticle is the diameter of the dried particle Ddroplet isthe diameter of the droplet ρparticle is the density of the driedparticle and ρdroplet is the density of the spray solution

From Figures 1ndash3 it can be seen that each control factorhad an effect on the spray-dried granules at different degreesof significance and inclination on the responsese effect of

Table 2 Mean responses of the orthogonal experiment for the spray-drying process

Treatment

Independent variables Responses (meanplusmn SD)Powder

content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)

Atomizationpressure (bar)

Feeding rate(mlmin)

D50(μm)

Volumefraction Uniformity

1 15 1 1 03 4 150 09 18 16531plusmn0483

3589plusmn1356

0423plusmn0003

2 15 1 3 06 6 180 10 21 17761plusmn1022

4119plusmn1267

0417plusmn0002

3 15 1 5 09 8 200 11 24 16882plusmn0971

3739plusmn1124

0417plusmn0001

4 20 1 1 03 6 180 11 24 20309plusmn0481

5127plusmn1159

0431plusmn0003

5 20 1 3 06 8 200 09 18 20793plusmn1145

5309plusmn2050

0416plusmn0003

6 20 1 5 09 4 150 10 21 16173plusmn0590

3443plusmn1083

0427plusmn0002

7 25 1 1 06 4 200 10 24 21227plusmn0484

5430plusmn1024

0442plusmn0004

8 25 1 3 09 6 150 11 18 19803plusmn0749

4944plusmn2101

0414plusmn0002

9 25 1 5 03 8 180 09 21 19464plusmn0281

4822plusmn1040

0448plusmn0002

10 30 1 1 09 8 180 10 18 22892plusmn1592

5779plusmn1294

0458plusmn0002

11 30 1 3 03 4 200 11 21 21484plusmn0932

5523plusmn1842

0437plusmn0002

12 30 1 5 06 6 150 09 24 20252plusmn0994

5111plusmn1134

0418plusmn0002

13 35 1 1 06 8 150 11 21 22587plusmn0826

5752plusmn1443

0450plusmn0007

14 35 1 3 09 4 180 09 24 21702plusmn1442

5507plusmn2002

0454plusmn0002

15 35 1 5 03 6 200 10 18 22003plusmn0545

5460plusmn0644

0488plusmn0016

16 40 1 1 09 6 200 09 21 27272plusmn0666

6039plusmn1111

0511plusmn0007

17 40 1 3 03 8 150 10 24 20916plusmn0641

5196plusmn1261

0473plusmn0004

18 40 1 5 06 4 180 11 18 20599plusmn0540

5172plusmn2216

0466plusmn0002

Each experiment was carried out in triplicate Values of responses are reported by Mastersizer 2000E analyser in 3 significant figures SD standard deviation


the control factors on the two primary responses that ismedian particle size and volume fraction has been discussedsimultaneously because of their strong correlation which hasbeen proven statistically later Atomization pressure andfeeding rate contribute in the first step of spray-dryingAtomization is the process of breaking up bulk liquidsinto droplets and hence the pressure being applied to do thetask refers to atomization pressure It was observed that theparticle size was affected in such a way that an increase inpressure led to a decrease in D50 (Figure 1) Figure 4 depictsthe schematic diagram of a two-fluid nozzle with the externalmix and the sequence involved during the formation ofdroplets First the slurry was forced into the two-fluid nozzleby the aid of the compressed air which caused the formationof an air cone covered by the liquid membrane dictated bythe cone angle or spray angle θ For a constant feeding ratean increase in the atomization pressure would enlarge thespray angle causing the thickness of the liquid membrane todecrease Consequently the diameter of the droplet woulddecrease which in turn would reduce the diameter of thespray-dried particle in other words D50 as observed inFigure 1 Increasing the feeding rate of the slurry from18mlmin to 21mlmin on the contrary would increase thethickness of the membrane leading to the formation of largerparticle size as pointed out in the corresponding diagram(Figure 1) However when the feeding rate was augmentedto 24mlmin D50 shrank A higher feed rate would cause anincrease in the drying load as the sample is sucked into thedrying chamber and sprayed at a faster rate In order wordsthe thickness of the liquid membrane did not have enoughtime to be formed hence shrinking the droplet Volumefraction seemed to be also affected by both atomizationpressure and feeding rate Increasing the pressure had

a similar dual effect as in D50 whilst an increase in thefeeding rate had an inverse effect compared to D50 ismeans that pumping more slurry in a faster speed from18mlmin to 21mlmin produced spray-dried powder in therange of 20ndash60 μm (volume fraction) in a larger quantitythan when increasing the rate from 21mlmin to 24mlminbecause higher amount of the extract was not dried due toaccumulation in the drying chamber resulting in the de-creased yield is phenomenon is different for differentpowders as Bai et al observed that increasing the feeding rateincreases both the volume fraction and D50 [19] whileMaskat et al observed the opposite [29]

e second step in the spray-drying process is related tothe slurry formulation and inlet temperature It could beobserved (Figures 1 and 2) that increasing the solid contentin the slurry had a strong influence on both D50 and volumefraction is could be explained by the fact that a rise in thepowder content results in an increase of suspension vis-cosity as proposed by Cao et al [26] In addition the particlesize is intrinsically the summation of the hollow centre partand the wall thickness It was reported in the quoted studythat the lower the solid content in the slurry the larger theballooning but the thinner the wall thickness Increasing theconcentration of the solid content in the slurry caused a risein the volume fraction of the spray-dried powder as it couldbe expected However when the amount of YSZ and aerogelwas too high (gt40 vol) a lesser volume of spray-driedpowder was recovered In this case the increase in thevolume of the powder content which was done mainly byadding aerogel powder which is much lighter than commonceramics experienced clotting at the nozzle thereby de-creasing the amount of spray-dried powder collectedFurthermore the decrease in D50 and volume fraction with

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vol YSZ aerogel Dispersant (wt)

Binder (wt) Inlet temperature (degC) Atomization pressure (bar)

Feeding rate (mlmin)

Main effects plot for meansData means

D50

(μm

)

Figure 1 Effect of slurry formulation and granulation variables on median particle size of YSZ-aerogel spray-dried powder


an increase in the ratio of YSZ aerogel could be possiblyjustified by the fact that the suspension was less viscousthereby inhibiting the liquid feed to be turned into dropletsin the drying chamber on time e required amount ofpowder according to the ratio of YSZ aerogel considerablequantity of aerogel powder had to be added due to itsrelatively ultralow density causing the suspension to bemuch thicker YSZ had an apparent density of 2300plusmn0200 gcm3 while the aerogel was considerably less densewith a value of 0129plusmn 0010 gcm3 ese phenomena maynot be observed in the spray-drying process of commonceramics Changing the amount of the dispersant from03wt to 09 wt increased the median particle sizenegligibly compared to the other three slurry formulationparameters that is solid content YSZ aerogel and binderBesides it should be pointed out that ball-milling wascarried out for a minimum of 24 hours which could haveenhanced the dispersion of the particles making thema homogeneous suspension Mahdjoub et al [30] studied theeffect of the slurry formulation upon the morphology ofspray-dried YSZ particles by measuring the relative sedi-ment height (RSH) which denotes the state of dispersion ofthe suspension and is an influential factor controlling thedroplet drying It was found that granules with hollow shapesare formed in the case of dispersed slurries (low ratio sedimentheight (RSH)lt 53) whilst full spherical granules were ob-tained with flocculated slurries (high RSHgt 62) Addingbinder in the slurry had two effects D50 was increased whenpolyvinyl alcohol (PVA) was added with a quantity of 04wtto 06wt whilst a decrease was observed when the amountwas brought to 08wt Binder is practically required in orderto produce solid compact granules It was shown in [31] thatthe RSH value can be drastically modified by the addition ofa binder subsequently affecting the state of dispersion of the

suspension being stable or unstable and hence the overallgranule morphology upon spray-drying When particles arecompletely dispersed in a suspension with no interactionbetween each other which is the case of a simple slurry(without binder or dispersant) they are very mobile Duringspray-drying the water migrates outside the granules andevaporatesWhen a binder is added the water particles and thebinder are attracted to form a dense shell leaving behind aninternal void is is because PVA is water-soluble and henceit is attracted with water Because of the difference in pressurebetween the internal void and the ambient atmosphere onepart of the granule collapses generating hollow powder Incontrast when the suspension is flocculated the particles arealmost immovable the shell does not appear and granules areshaped Hence the dual effect due to an addition of PVA in theaerogel-based slurry can be appreciated

Increasing the inlet temperature resulted in larger par-ticle size and more spray-dried powder as observed byWang et al [32] When the aqueous aerogel-based sus-pension was forced into the drying chamber the compressedair converted it into small droplets which shrunk as rapidevaporation of the water in the suspension took place due tothe high temperature of the air During that process ofmoistness moving from the inside to the outside of the dropthe solid content binder and dispersant were also carriedalong Overall for granulation increasing the drying tem-perature causes a greater loss of water of the resultant spray-dried powder as the higher rate of heat transfers into par-ticles the faster is the water removal [33] However if theinlet temperature was set above 200degC in this study it couldbe anticipated that the evaporation rate of moisture insidethe droplet would become higher than the diffusion ratethrough the droplet surface which would cause a decrease inthe resulting spray-dried powder [19]

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Volu

me f

ract

ion

Figure 2 Effect of slurry formulation and granulation variables on volume fraction of YSZ-aerogel spray-dried powder


Increasing the powder content caused a rise in the uni-formity of the particles as in the case of D50 and volumefraction However it could be observed that it had a dual effectfor uniformity with a change in the ratio of YSZ aerogelAdding more aerogel from a ratio of 1 1 to 1 3 the uni-formity of the spray-dried particles decreased but whenadditional aerogel was added up to a ratio of 1 5 the uni-formity increased again e amount of dispersant hada greater effect than that of the binder but in an oppositemanner e three settings of the spray-dryer contributed aswell in the alteration of the uniformity Particles tend to bemore uniform at higher temperature On the contrary toomuch pressure (11 bar) can lead to a higher PDI A similarchange happened with a rise in the feeding rate e sig-nificance of each of these variables towards uniformity isprovided in Table 3

Table 3 displays the statistical significance of each in-dependent variable of the spray-drying process on the threespray-dried powder characteristics under study to furthersupport the above justificationse volume of the solid in thesuspension had the most significant effect (p 0035) on themedian particle size followed by the amount of YSZ aerogel(p 0048) Conversely the amount of dispersant and binderadded the inlet temperature and the atomisation pressure ofthe spray-dryer did not significantly affect that response(pgt 005) As for the volume fraction again the powdercontent followed by the ratio of YSZ aerogel contributedsignificantly (p 0010 and p 0019 respectively) com-pared to the inlet temperature (p 0025) e other in-dependent variables were insignificant towards volumefraction Uniformity was not influenced significantly by any ofthe spray-drying variables (pgt 005)

32 Correlation amongst Responses A correlation coefficientmeasures the extent to which two variables tend to changetogether e coefficient describes both the strength and thedirection of the relationship In this study the Pearsoncorrelation was used It is one which evaluates the linear

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048

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042





Uni

form

ity

Figure 3 Effect of slurry formulation and granulation variables on uniformity of YSZ-aerogel spray-dried powder

Two-fluid nozzle

Liquid membrane

Air cone

Spraying angle

θ

Compressed air

Slurry

Figure 4 Schematic diagram of two-fluid nozzle with the externalmix and formation of droplets (adapted from [19])


relationship between two continuous variables A relation-ship is linear when a change in one variable is associated witha proportional change in the other variable

Table 4 shows the correlation (Pearsonrsquos r coefficient)which existed between the three responses which were thephysical characteristics of the spray-dried YSZ-aerogelpowder and their significance degree (p value) of correla-tion It can be seen that there was a strong positive significantcorrelation (Pearsonrsquos r 0930 p value 0001) between themedian particle size D50 and volume fraction Bai et al [19]observed a similar correlation but did not provide anystatistical evaluation

e median particle size was also correlated with theuniformity of the spray-dried powder but with a lesserpositive linear interaction (Pearsonrsquos r 0754) and therelationship was still statistically significant (p 0001) Apositive correlation between volume fraction and uniformitywas drawn (Pearsonrsquos r 0599) but was nevertheless in-significant (p 0009)

33 Precision of the Spray-Drying Process e consistency ofthe spray-drying process carried out to reconstitute the aerogelparticles was evaluated for each response It was found that thethree characteristics median particle size volume fraction anduniformity were all consistent (CVlt 4) as shown in Table 5with an indication from the most consistent data to leastconsistent data in the order of uniformitygt volume frac-tiongtmedian particle size It can therefore be argued that theoverall process undertaken is repeatable

34 Optimisation of YSZ-Aerogel Spray-Dried Powdere Taguchi design provides an optimisation techniquewherein the conditions are limited to only the levels in-corporated into the experimental design and hence it isconsidered to provide only an approximation of the opti-misation points and not the exact ones in between theselevels because the design is discrete and not continuousFrom the regression analysis by ANOVA which was also

Table 3 Significant effect of each independent variable on the spray-dried powder characteristics

ResponsesSpray-drying variables

Powder content(vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature (degC)


Feeding rate(mlmin)

Median particlesize

p value 0035lowast 0048lowast 0424 0113 0063 0289 gt09F-ratio 2787 1971 136 784 1481 246 mdash

Volume fractionp value 0010lowast 0019lowast 0124 0056 0025 0237 gt09F-ratio 10044 5044 709 1674 3846 322 mdash

Uniformityp value 0065 0219 0248 0770 0208 0269 gt09F-ratio 1459 357 304 030 382 271 mdash

Values are generated by Minitabreg Significance indicators are derived from ANOVA Higher F-ratio and lower p value refer to more significance and viceversa lowastStatistically significant results (plt 005)

Table 4 Pearsonrsquos r correlation and significance degree of cor-relation among the physical characteristics of spray-dried powder

Variable D50 Volume fractionVolume fraction

Pearsonrsquos r 0930 mdashp value 0001

UniformityPearsonrsquos r 0754 0599p value 0001 0009lowast

Values are generated by Minitabreg D50 median particle size Pearsonrsquos rcoefficient of 1 means perfect linear relationship results lowastStatistically in-significant (pgt 005)

Table 5 Precision of responses for the spray-drying process

TreatmentCoefficient of variation ()

D50 Volume fraction Uniformity1 2924 3779 06962 5756 3076 05183 5751 3006 03394 2370 2261 06565 5509 3861 07086 3649 3146 05067 2282 1885 09778 3782 4249 03949 1443 2156 048210 6954 2240 047211 4338 3334 037412 4907 2219 051713 3655 2509 148514 6645 3636 047615 2064 1179 320516 2442 1840 139317 3066 2427 075218 2619 4285 0464Average CV () 3898 2838 0801D50 median particle size CV coefficient of variation Higher CV refers toless consistency or lower precision and vice versa Low CV (lt4) indicateshigh precision of data


presented earlier (Figures 1ndash3) three optimal treatmentswere possible as tabulated in Table 6

Considering that all three responses had to bemaximisedbecause (1) the median particle size should be at least withinthe range of 20ndash60 μm (2) the volume fraction of particleswithin that range should be larger and (3) the uniformityshould be close enough to the sprayable YSZ powder witha value of 0433plusmn 0001 the-larger-the-better option wasopted From Figure 1 a maximummedian particle size couldbe predicted (treatment 19) by using a powder content of40 vol a 1 1 ratio of YSZ aerogel 09 wt of dispersant6 wt of binder an inlet temperature of 200degC an atom-ization pressure of 09 bar and a feeding rate at 21mlminTreatments 20 and 21 followed the settings from Figures 2and 3 to obtain the maximum volume fraction and uni-formity respectively

It was found that treatment 19 surpassed theremaining two (treatments 20 and 21) in terms of medianparticle size (28932 plusmn 0726 μm) and volume fraction(64450 plusmn 0535) e D50 was within the targeted range of20ndash60 μmOn the contrary the uniformity was lower (0475plusmn0002) than that obtained by treatment 20 (0481plusmn 0001) butnevertheless higher than that obtained by treatment 21(0468plusmn 0004) A polydispersed powder of that extent(0475plusmn 0002) could be regarded as acceptable for APS as theuniformity of the sprayable YSZ is actually 0433plusmn 0001Hence the YSZ-aerogel spray-dried powder obtained undertreatment 19 was considered to be the optimised powder tocarry out APS e morphological analysis carried out af-terwards to appreciate the shape of the spray-dried granulescollected indicated an improvement from the irregular shapes(Figure 5) to a more regular morphology having sphericaldonut-like forms as shown in Figure 6 Such granules can beconsidered as sprayable by APS [31] It is worthy to attest thatthe hollow centre part was formed as the evaporation rate ofmoisture inside the droplet was higher than the diffusion ratethrough the droplet surface [19]

Having obtained an optimised aerogel-based powder tocarry out APS a drawback was however encountered theamount of spray-dried YSZ-aerogel collected was relativelylow More precisely only 10 of the amount of powdercontent added in the slurry was obtained as spray-driede rest was trapped in the spray chamber is could beprobably because of the ultralight weight of the aerogelwhen the amount of aerogel was more than that of YSZthat is for ratios 1 3 and 1 5 the volume fraction waslower Hence most of the aerogel powder was seemingly tobe trapped It was therefore thought that the spray-dryingintermediate route was not economically feasible eventhough the reconstitution of the GEATM 0125 aerogelpowder succeeded

4 Conclusion

e integration of nanostructured silica aerogel in aero-propulsion systems has continuously intrigued engineersdue to its extremely low thermal conductivity non-flammable and low density characteristics inter alia

Aerogel-doped blankets have been developed but nostudy has been brought forward yet to investigate thepossibility of developing an aerogel-based thermallysprayed coating for aerospace applications One of thereasons is because of the inappropriateness of silicaaerogel powders in terms of their granulometric andmorphological characteristics for thermal spraying isstudy addresses this issue in attempting to reconstitutesilica aerogel powders via spray-drying A detailed sta-tistical analysis was carried out prior to optimization toappreciate the effect of the spray-drying process (slurrypreparation and granulation) and the effect of the spray-drying variables on the three characteristics under studynamely median particle size volume fraction and uni-formity Correlations amongst the responses were alsoderived It was found that the spray-drying technique hadthe most significant effect on uniformity (F-ratio 6240)followed by volume fraction (F-ratio 5594) and lastlymedian particle size (F-ratio 1854) Out of the sevenspray-drying independent variables which comprised thepowder content ratio of YSZ aerogel and amount ofdispersant and binder as the slurry formulation and inlettemperature atomization pressure and feeding rate as thesettings for granulation (spray-dryer) it was found thatthe first two factors contributed significantly (plt 005)towards the median particle size and volume fraction Astrong correlation (Pearsonrsquos r 0930) between D50 andvolume fraction was obtained Although the optimisedspray-dried powder had an improved median particle sizeof 28932 plusmn0726 μm with a volume fraction of 64450 plusmn053 having uniformity of 0475 plusmn 0002 and an improvedmorphology than the starting aerogel the yield of thegranulated aerogel-based powder was only 10 which wasthought to be uneconomically feasible to be used forsubsequent air plasma spraying at this stage If the yield isincreased the spray-dried YSZ-aerogel can be tried to bedeposited using the thermal spray if a mechanically robustsuperinsulative coating is sought is feasibility study hasnevertheless provided an insight into the reconstitution ofsilica aerogel powders via the spray-drying process priorto thermal spraying Based on this study air plasmaspraying using as-received aerogel powders will be triedeven though it is practically challenging

Figure 5 Irregular morphology of GEATM 0125 aerogel measuredby SEM with a top surface view with a magnification of times500


Data Availability

e data used to support the findings of this study areavailable from the first author upon request

Conflicts of Interest

e authors have no conflicts of interest to declare

Acknowledgments

Part of this work was partially supported by the Funda-mental Research Grant Schemes (FRGS) Funding Nos5524611 and 5524896 e first author wishes to express hisappreciation to the Ministry of Education of Malaysia fortheir financial support SEM of the as-received GEATM 0125was carried out at Laboratory of Science of Ceramic Pro-cessing and Surface Treatments University of Limogesunder the observation of Prof Dr Lech Pawlowski who alsoshared his valuable opinions

References

[1] S S Kistler ldquoCoherent expanded aerogels and jelliesrdquoNaturevol 127 no 3211 p 741 1931

[2] J Fricke ldquoAerogelsmdashhighly tenuous solids with fascinatingpropertiesrdquo Journal of Non-Crystalline Solids vol 100no 1ndash3 pp 169ndash173 1988

[3] A SoleimaniDorcheh and M H Abbasi ldquoSilica aerogelsynthesis properties and characterizationrdquo Journal of Ma-terials Processing Technology vol 199 no 1ndash3 pp 10ndash262008

[4] A Hilonga J -K Kim P B Sarawade and H T Kim ldquoLow-density TEOS-based silica aerogels prepared at ambientpressure using isopropanol as the preparative solventrdquoJournal of Alloys and Compounds vol 487 no 1-2pp 744ndash750 2009

[5] P B Sarawade J -K Kim A Hilonga and H T KimldquoInfluence of aging conditions on textural properties of water-glass-based silica aerogels prepared at ambient pressurerdquoKorean Journal of Chemical Engineering vol 27 no 4pp 1301ndash1309 2010

[6] P B Sarawade J -K Kim A Hilonga and H T KimldquoProduction of low-density sodium silicate-based hydro-phobic silica aerogel beads by a novel fast gelation process andambient pressure drying processrdquo Solid State Sciences vol 12no 5 pp 911ndash918 2010

[7] S K Hong M Y Yoon and H J Hwang ldquoFabrication ofspherical silica aerogel granules from water glass by ambientpressure dryingrdquo Journal of the American Ceramic Societyvol 94 no 10 pp 3198ndash3201 2011

[8] C J Lee G S Kim and S H Hyun ldquoSynthesis of silicaaerogels from waterglass via new modified ambient dryingrdquoJournal of Materials Science vol 37 no 11 pp 2237ndash22412002

[9] F Shi L Wang and J Liu ldquoSynthesis and characterization ofsilica aerogels by a novel fast ambient pressure drying pro-cessrdquo Materials Letters vol 60 no 29-30 pp 3718ndash37222006

[10] T YWei T F Chang S Y Lu and Y C Chang ldquoPreparationof monolithic silica aerogel of low thermal conductivity byambient pressure dryingrdquo Journal of the American CeramicSociety vol 90 no 7 pp 2003ndash2007 2007

[11] Patent Insight Pro httpwwwpatentinsightprocom[12] S B Riffat and G Qiu ldquoA review of state-of-the-art aerogel

applications in buildingsrdquo International Journal of Low-Carbon Technologies vol 8 no 1 pp 1ndash6 2013

[13] A Tadjarodi M Haghverdi and V Mohammadi ldquoPrepa-ration and characterization of nano-porous silica aerogel fromrice husk ash by drying at atmospheric pressurerdquo MaterialsResearch Bulletin vol 47 no 9 pp 2584ndash2589 2012

[14] J Nayak and J Bera ldquoPreparation of silica aerogel by ambientpressure drying process using rice husk ash as raw materialrdquo

Table 6 Optimisation conditions and responses for the spray-drying process

Treatment

Independent variables ResponsesPowder

content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


19lowast 40 1 1 09 6 200 09 21 28932plusmn0726

64450plusmn0535

0475plusmn0002

20 40 1 1 06 6 200 10 21 24478plusmn0526

59520plusmn0751

0481plusmn0001

21 40 1 1 03 6 200 10 21 26719plusmn0271

63090plusmn0660

0468plusmn0004

lowastOptimised treatment for YSZ-aerogel for maximumD50 volume fraction and uniformity Values of responses are reported byMastersizer 2000E analyser in3 significant figures D50 median particle size

H times12k 50 μm

Figure 6 Optimised spray-dried YSZ-aerogel powder measured bySEM with a top surface view with a magnification of times12k


Transactions of the Indian Ceramic Society vol 68 no 2pp 91ndash94 2009

[15] S Chakraborty A A Pisal V K Kothari andA Venkateswara Rao ldquoSynthesis and characterization of fibrereinforced silica aerogel blankets for thermal protectionrdquoAdvances in Materials Science and Engineering vol 2016Article ID 2495623 8 pages 2016

[16] P C apliyal and K Singh ldquoAerogels as promising thermalinsulating materials an overviewrdquo Journal of Materialsvol 2014 Article ID 127049 10 pages 2014

[17] N Bheekhun A R Abu Talib and M R Hassan ldquoAerogels inaerospace an overviewrdquo Advances in Materials Science andEngineering vol 2013 Article ID 406065 18 pages 2013

[18] L Pawlowski lteScience and Engineering of ltermal SprayCoatings John Wiley amp Sons Hoboken NJ USA 2008

[19] Y Bai J F Yang and S Lee ldquoOptimization of alumina slurryproperties and drying conditions in the spray drying processand characterization of corresponding coating fabricated byatmospheric plasma sprayrdquo in Key Engineering Materialsvol 484 pp 158ndash165 Trans Tech Publications ZurichSwitzerland 2011

[20] N Bheekhun A R Abu Talib and M R Hassan ldquoermalspray coatings for polymer matrix composites in gas turbineengines a literary previewrdquo International Review of AerospaceEngineering vol 7 pp 84ndash87 2014

[21] R Gonzalez H Ashrafizadeh A Lopera P Mertiny andA McDonald ldquoA review of thermal spray metallization ofpolymer-based structuresrdquo Journal of ltermal Spray Tech-nology vol 25 no 5 pp 897ndash919 2016

[22] J Xu K A Khor Z Dong Y Gu R Kumar and P CheangldquoPreparation and characterization of nano-sized hydroxy-apatite powders produced in a radio frequency (rf) thermalplasmardquoMaterials Science and Engineering A vol 374 no 1-2 pp 101ndash108 2004

[23] N Johari R Ibrahim M A Ahmad M J Suleiman Ahmadand A R A Talib ldquoe effect of sintering temperature onphysical properties of sintered inconel 718 for potentialaerospace industry applicationrdquo Advanced Materials Re-search vol 879 pp 139ndash143 2014

[24] N Kawahashi and E Matijevic ldquoPreparation of hollowspherical particles of yttrium compoundsrdquo Journal of Colloidand Interface Science vol 143 no 1 pp 103ndash110 1991

[25] K Masters Spray Drying Handbook Halstead Press ArdsleyNY USA 1979

[26] X Q Cao R Vassen S Schwartz W Jungen F Tietz andD Stoever ldquoSpray-drying of ceramics for plasma-spraycoatingrdquo Journal of the European Ceramic Society vol 20no 14-15 pp 2433ndash2439 2000

[27] N R Draper and H Smith Applied Regression Analysis JohnWiley amp Sons Hoboken NJ USA 2014

[28] D E Dobry D M Settell J M Baumann R J RayL J Graham and R A Beyerinck ldquoA model-based meth-odology for spray-drying process developmentrdquo Journal ofPharmaceutical Innovation vol 4 no 3 pp 133ndash142 2009

[29] M Y Maskat C Lung E Momeny M Khan and S SiddiquildquoTemperature and feed rate effects properties of spray driedHibiscus sabdariffa powderrdquo International Journal of DrugDevelopment and Research vol 6 pp 28ndash34 2014

[30] H Mahdjoub P Roy C Filiatre G Bertrand and C Coddetldquoe effect of the slurry formulation upon the morphology ofspray-dried yttria stabilised zirconia particlesrdquo Journal of theEuropean Ceramic Society vol 23 no 10 pp 1637ndash16482003

[31] G Bertrand C Filiatre H Mahdjoub A Foissy andC Coddet ldquoInfluence of slurry characteristics on the mor-phology of spray-dried alumina powdersrdquo Journal of theEuropean Ceramic Society vol 23 no 2 pp 263ndash271 2003

[32] W Wang C Dufour and W Zhou ldquoImpacts of spray-dryingconditions on the physicochemical properties of soy saucepowders using maltodextrin as auxiliary drying carrierrdquoCyTAmdashJournal of Food vol 13 pp 548ndash555 2015

[33] C K Tuyen M H Nguyen and P D Roach ldquoEffects of spraydrying conditions on the physicochemical and antioxidantproperties of the Gac (Momordica cochinchinensis) fruit arilpowderrdquo Journal of Food Engineering vol 98 no 3pp 385ndash392 2010


CorrosionInternational Journal of

Hindawiwwwhindawicom Volume 2018

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Polymer ScienceInternational Journal of



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Journal ofEngineeringVolume 2018

Applied ChemistryJournal of


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High Energy PhysicsAdvances in

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

TribologyAdvances in



ChemistryAdvances in


Advances inPhysical Chemistry


BioMed Research InternationalMaterials

Journal of


Na

nom

ate

ria

ls


Journal ofNanomaterials

Submit your manuscripts atwwwhindawicom

manufacturers of aerogels have modified the synthesistechnique such as modifying the sol-gel route applyingambient drying instead of subjecting on supercritical con-ditions and also using the cheaper natural source of silicasuch as rice husk in the place of synthetic matter to saveenergy and time and hence the overall manufacturing costIntensive research was carried out in the academia prior tothe commercialised ecofriendly andor cheaper aerogels[4ndash10] e actual trend which depicts an increasing interestin aerogels by manufacturers is not because of the newadvances in the nanotechnology of the material itself butinstead to meet the tailored demands by consumers re-quiring innovative and yet cost-effective solutions As a re-sult the aerogels market has recently seen a surge in thenumber of patents filed on aerogels in the recent historyPublished patents started to rise rapidly from about 80published in 2000 to in excess of 400 in 2009 [11] isdemonstrates the increase in nanomaterials research inrecent years in the field of aerogels One of the mostpromising implementations of these nanostructured mate-rials is high-performance thermal insulation because of itsrelatively low thermal conductivity sim0012WmK and ul-tralow density which can go as low as 0003 gcm3 [12]Comparatively the same property of air is approximately00012 gcm3 that is thrice lower than that of silica aerogelwhich may be considered as significant In the powder formsilica aerogel is normally denser and may range from 006 to03 gcm3 or even a higher value depending on the source ofsilica and drying process [13 14] is illustrates the po-tential substantial weight savings that can be attained ifaerogels are used for thermal insulation It is neverthelessnoticed that powdered aerogels are preferred over monolithsand are being integrated into various innovative solutionsFor instance thick thermally insulated aerogel-basedblankets of millimetre order are being continuously stud-ied and used in aerospace [15] while thin aerogel-dopedinsulative paints and granulate glazing systems are currentlybeing employed for architectural purposes [16] e issueschallenges and applications of aerogels in aerospace havebeen addressed elsewhere [17]

However up to now silica aerogel has not been appliedas a heat- or fire-resistive coating It is thought that thermalspraying is a potential class of coating technology that can beopted to achieve so e technique is one in which thefeedstock in the powder form is propelled onto a substrate athigh velocity and temperature to melt the particles partiallyso as they adhere on the surface to form a coating Typicallythermal spray coatings are typically produced with a thick-ness between 30 μm and 2mm [18] One vital requisiteamongst others while considering conventional thermalspraying is that the particles should be spherical rangingwithin a certain range of particle size typically 10ndash100 μm[18] with a median particle size within 20ndash60 μm[19]

However the as-received aerogel powder intended to bethermal sprayed had irregular shapes granule particle range of2ndash300μm and median particle size of 81μm which made itinappropriate for thermal spraying In consequence the ob-jective of this paper is to tailor the particles of silica aerogelobtained from the manufacturer into spherical ones with

granulometric properties via spray-drying as an intermediateprocess for subsequent air plasma spraying (APS) which will beconducted and reported later APS is regarded as the mostversatile thermal spray process which allows deposition even oncritical substrates such as polymer matrix composites [20 21]Spray-drying has been chosen because of its superiority inproducing thermal-spray-graded spherical free-flowing powderswithin a wide range of sizes economically compared to thoseprepared as precipitation [22] sinteringcrushing [23] andhollow-spherical powder [24] It is a process by which a fluidfeed material is transformed into a dry powder by spraying thelatter which is a water-based suspension with air as drying gas[25] Some spray-dryers operate with alcohol-based suspensionsbut instead use nitrogen as the drying gas It is a well-establishedstatement that the significant factors to influence the charac-teristics of spray-dried powders are suspension compositiondrying temperature atomising pressure and feeding rate [26]Considering the fact that no academic study has investigated thefeasibility of spray-drying aerogel powders this investigationtakes into consideration all these factors as its independentvariables and studies their effect on three measurable responsesnamely median particle size volume fraction and uniformitye optimised condition to achieve the maximum medianparticle size and volume fraction was subsequently predictedafter which an assessment on themorphology of on the resultingspray-dried powders was carried out

2 Materials and Methods

21 Materials Ecological GEATM 0125 aerogel was suppliedby Green Earth Aerogel Technologies whilst commercialisedyttria-stabilised zirconia (YSZ) ZrO28Y2O3 (Metco 240NS-G)was purchased fromOerlikonMetco (previously SulzerMetco)Polyvinyl alcohol (Mowiolreg 4-88) and polyethylene glycol werebought from Sigma-Aldrich Darvan 821-A was provided byVanderbilt Minerals LLC Distilled water was available at thefacility Advanced Materials Research Centre (AMRECSIRIM) All the materials were used as obtained with no furthertreatment YSZ was added as a secondary ceramic material inorder to improve the flowability of the slurry through the nozzleof the spray-dryer as aerogel in ultralight Also YSZ is a ther-mal-spray-graded powder which is used as a heat insulativecoating in aero-engines PVA which is a binder was optedbecause of its water soluble nature and its ability to be readilyball-milledwithout degradation hence creating extremely stablesolutions e uniformity or polydispersity index (PDI) of thepowders of aerogels which gives an indication of the hetero-geneity of sizes of the granules was measured using particle sizeanalysis via the Mastersizer 2000EHydro 2000MU It wasfound that the uniformity of the GEATM 0125 aerogel powderswas 1993 which is relatively high for a thermal spray powder ascompared to the uniformity of the thermal-spray-graded YSZhaving a value of 0433 YSZ had an apparent density of 2300plusmn0200 gcm3 while the aerogel was considerably less dense witha value of 0129plusmn0010 gcm3

22 Slurry Formulation and Granulation A strict mixingprocedure was carried out whereby the amount of each







3 Results












3

1113971

(1)




Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


1 15 1 1 03 4 150 09 18 16531plusmn0483

3589plusmn1356

0423plusmn0003

2 15 1 3 06 6 180 10 21 17761plusmn1022

4119plusmn1267

0417plusmn0002

3 15 1 5 09 8 200 11 24 16882plusmn0971

3739plusmn1124

0417plusmn0001

4 20 1 1 03 6 180 11 24 20309plusmn0481

5127plusmn1159

0431plusmn0003

5 20 1 3 06 8 200 09 18 20793plusmn1145

5309plusmn2050

0416plusmn0003

6 20 1 5 09 4 150 10 21 16173plusmn0590

3443plusmn1083

0427plusmn0002

7 25 1 1 06 4 200 10 24 21227plusmn0484

5430plusmn1024

0442plusmn0004

8 25 1 3 09 6 150 11 18 19803plusmn0749

4944plusmn2101

0414plusmn0002

9 25 1 5 03 8 180 09 21 19464plusmn0281

4822plusmn1040

0448plusmn0002

10 30 1 1 09 8 180 10 18 22892plusmn1592

5779plusmn1294

0458plusmn0002

11 30 1 3 03 4 200 11 21 21484plusmn0932

5523plusmn1842

0437plusmn0002

12 30 1 5 06 6 150 09 24 20252plusmn0994

5111plusmn1134

0418plusmn0002

13 35 1 1 06 8 150 11 21 22587plusmn0826

5752plusmn1443

0450plusmn0007

14 35 1 3 09 4 180 09 24 21702plusmn1442

5507plusmn2002

0454plusmn0002

15 35 1 5 03 6 200 10 18 22003plusmn0545

5460plusmn0644

0488plusmn0016

16 40 1 1 09 6 200 09 21 27272plusmn0666

6039plusmn1111

0511plusmn0007

17 40 1 3 03 8 150 10 24 20916plusmn0641

5196plusmn1261

0473plusmn0004

18 40 1 5 06 4 180 11 18 20599plusmn0540

5172plusmn2216

0466plusmn0002






403530252015

22

20

18

151311 090603

864

22

20

18

200180150 111009

242118

22

20

18





D50

(μm

)






403530252015

56

48

40

151311 090603

864

56

48

40

200180150 111009

242118

56

48

40





Volu

me f

ract

ion






403530252015

048

045

042151311 090603

864

048

045

042200180150 111009

242118

048

045

042





Uni

form

ity


Two-fluid nozzle

Liquid membrane

Air cone

Spraying angle

θ

Compressed air

Slurry










Powder content(vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)



Feeding rate(mlmin)

Median particlesize


















4 Conclusion





Data Availability




Acknowledgments


References
















Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


19lowast 40 1 1 09 6 200 09 21 28932plusmn0726

64450plusmn0535

0475plusmn0002

20 40 1 1 06 6 200 10 21 24478plusmn0526

59520plusmn0751

0481plusmn0001

21 40 1 1 03 6 200 10 21 26719plusmn0271

63090plusmn0660

0468plusmn0004


H times12k 50 μm


























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls









3 Results












3

1113971

(1)




Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


1 15 1 1 03 4 150 09 18 16531plusmn0483

3589plusmn1356

0423plusmn0003

2 15 1 3 06 6 180 10 21 17761plusmn1022

4119plusmn1267

0417plusmn0002

3 15 1 5 09 8 200 11 24 16882plusmn0971

3739plusmn1124

0417plusmn0001

4 20 1 1 03 6 180 11 24 20309plusmn0481

5127plusmn1159

0431plusmn0003

5 20 1 3 06 8 200 09 18 20793plusmn1145

5309plusmn2050

0416plusmn0003

6 20 1 5 09 4 150 10 21 16173plusmn0590

3443plusmn1083

0427plusmn0002

7 25 1 1 06 4 200 10 24 21227plusmn0484

5430plusmn1024

0442plusmn0004

8 25 1 3 09 6 150 11 18 19803plusmn0749

4944plusmn2101

0414plusmn0002

9 25 1 5 03 8 180 09 21 19464plusmn0281

4822plusmn1040

0448plusmn0002

10 30 1 1 09 8 180 10 18 22892plusmn1592

5779plusmn1294

0458plusmn0002

11 30 1 3 03 4 200 11 21 21484plusmn0932

5523plusmn1842

0437plusmn0002

12 30 1 5 06 6 150 09 24 20252plusmn0994

5111plusmn1134

0418plusmn0002

13 35 1 1 06 8 150 11 21 22587plusmn0826

5752plusmn1443

0450plusmn0007

14 35 1 3 09 4 180 09 24 21702plusmn1442

5507plusmn2002

0454plusmn0002

15 35 1 5 03 6 200 10 18 22003plusmn0545

5460plusmn0644

0488plusmn0016

16 40 1 1 09 6 200 09 21 27272plusmn0666

6039plusmn1111

0511plusmn0007

17 40 1 3 03 8 150 10 24 20916plusmn0641

5196plusmn1261

0473plusmn0004

18 40 1 5 06 4 180 11 18 20599plusmn0540

5172plusmn2216

0466plusmn0002






403530252015

22

20

18

151311 090603

864

22

20

18

200180150 111009

242118

22

20

18





D50

(μm

)






403530252015

56

48

40

151311 090603

864

56

48

40

200180150 111009

242118

56

48

40





Volu

me f

ract

ion






403530252015

048

045

042151311 090603

864

048

045

042200180150 111009

242118

048

045

042





Uni

form

ity


Two-fluid nozzle

Liquid membrane

Air cone

Spraying angle

θ

Compressed air

Slurry










Powder content(vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)



Feeding rate(mlmin)

Median particlesize


















4 Conclusion





Data Availability




Acknowledgments


References
















Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


19lowast 40 1 1 09 6 200 09 21 28932plusmn0726

64450plusmn0535

0475plusmn0002

20 40 1 1 06 6 200 10 21 24478plusmn0526

59520plusmn0751

0481plusmn0001

21 40 1 1 03 6 200 10 21 26719plusmn0271

63090plusmn0660

0468plusmn0004


H times12k 50 μm


























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls









3

1113971

(1)




Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


1 15 1 1 03 4 150 09 18 16531plusmn0483

3589plusmn1356

0423plusmn0003

2 15 1 3 06 6 180 10 21 17761plusmn1022

4119plusmn1267

0417plusmn0002

3 15 1 5 09 8 200 11 24 16882plusmn0971

3739plusmn1124

0417plusmn0001

4 20 1 1 03 6 180 11 24 20309plusmn0481

5127plusmn1159

0431plusmn0003

5 20 1 3 06 8 200 09 18 20793plusmn1145

5309plusmn2050

0416plusmn0003

6 20 1 5 09 4 150 10 21 16173plusmn0590

3443plusmn1083

0427plusmn0002

7 25 1 1 06 4 200 10 24 21227plusmn0484

5430plusmn1024

0442plusmn0004

8 25 1 3 09 6 150 11 18 19803plusmn0749

4944plusmn2101

0414plusmn0002

9 25 1 5 03 8 180 09 21 19464plusmn0281

4822plusmn1040

0448plusmn0002

10 30 1 1 09 8 180 10 18 22892plusmn1592

5779plusmn1294

0458plusmn0002

11 30 1 3 03 4 200 11 21 21484plusmn0932

5523plusmn1842

0437plusmn0002

12 30 1 5 06 6 150 09 24 20252plusmn0994

5111plusmn1134

0418plusmn0002

13 35 1 1 06 8 150 11 21 22587plusmn0826

5752plusmn1443

0450plusmn0007

14 35 1 3 09 4 180 09 24 21702plusmn1442

5507plusmn2002

0454plusmn0002

15 35 1 5 03 6 200 10 18 22003plusmn0545

5460plusmn0644

0488plusmn0016

16 40 1 1 09 6 200 09 21 27272plusmn0666

6039plusmn1111

0511plusmn0007

17 40 1 3 03 8 150 10 24 20916plusmn0641

5196plusmn1261

0473plusmn0004

18 40 1 5 06 4 180 11 18 20599plusmn0540

5172plusmn2216

0466plusmn0002






403530252015

22

20

18

151311 090603

864

22

20

18

200180150 111009

242118

22

20

18





D50

(μm

)






403530252015

56

48

40

151311 090603

864

56

48

40

200180150 111009

242118

56

48

40





Volu

me f

ract

ion






403530252015

048

045

042151311 090603

864

048

045

042200180150 111009

242118

048

045

042





Uni

form

ity


Two-fluid nozzle

Liquid membrane

Air cone

Spraying angle

θ

Compressed air

Slurry










Powder content(vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)



Feeding rate(mlmin)

Median particlesize


















4 Conclusion





Data Availability




Acknowledgments


References
















Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


19lowast 40 1 1 09 6 200 09 21 28932plusmn0726

64450plusmn0535

0475plusmn0002

20 40 1 1 06 6 200 10 21 24478plusmn0526

59520plusmn0751

0481plusmn0001

21 40 1 1 03 6 200 10 21 26719plusmn0271

63090plusmn0660

0468plusmn0004


H times12k 50 μm


























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls







403530252015

22

20

18

151311 090603

864

22

20

18

200180150 111009

242118

22

20

18





D50

(μm

)






403530252015

56

48

40

151311 090603

864

56

48

40

200180150 111009

242118

56

48

40





Volu

me f

ract

ion






403530252015

048

045

042151311 090603

864

048

045

042200180150 111009

242118

048

045

042





Uni

form

ity


Two-fluid nozzle

Liquid membrane

Air cone

Spraying angle

θ

Compressed air

Slurry










Powder content(vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)



Feeding rate(mlmin)

Median particlesize


















4 Conclusion





Data Availability




Acknowledgments


References
















Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


19lowast 40 1 1 09 6 200 09 21 28932plusmn0726

64450plusmn0535

0475plusmn0002

20 40 1 1 06 6 200 10 21 24478plusmn0526

59520plusmn0751

0481plusmn0001

21 40 1 1 03 6 200 10 21 26719plusmn0271

63090plusmn0660

0468plusmn0004


H times12k 50 μm


























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls







403530252015

56

48

40

151311 090603

864

56

48

40

200180150 111009

242118

56

48

40





Volu

me f

ract

ion






403530252015

048

045

042151311 090603

864

048

045

042200180150 111009

242118

048

045

042





Uni

form

ity


Two-fluid nozzle

Liquid membrane

Air cone

Spraying angle

θ

Compressed air

Slurry










Powder content(vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)



Feeding rate(mlmin)

Median particlesize


















4 Conclusion





Data Availability




Acknowledgments


References
















Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


19lowast 40 1 1 09 6 200 09 21 28932plusmn0726

64450plusmn0535

0475plusmn0002

20 40 1 1 06 6 200 10 21 24478plusmn0526

59520plusmn0751

0481plusmn0001

21 40 1 1 03 6 200 10 21 26719plusmn0271

63090plusmn0660

0468plusmn0004


H times12k 50 μm


























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls







403530252015

048

045

042151311 090603

864

048

045

042200180150 111009

242118

048

045

042





Uni

form

ity


Two-fluid nozzle

Liquid membrane

Air cone

Spraying angle

θ

Compressed air

Slurry










Powder content(vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)



Feeding rate(mlmin)

Median particlesize


















4 Conclusion





Data Availability




Acknowledgments


References
















Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


19lowast 40 1 1 09 6 200 09 21 28932plusmn0726

64450plusmn0535

0475plusmn0002

20 40 1 1 06 6 200 10 21 24478plusmn0526

59520plusmn0751

0481plusmn0001

21 40 1 1 03 6 200 10 21 26719plusmn0271

63090plusmn0660

0468plusmn0004


H times12k 50 μm


























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls











Powder content(vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)



Feeding rate(mlmin)

Median particlesize


















4 Conclusion





Data Availability




Acknowledgments


References
















Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


19lowast 40 1 1 09 6 200 09 21 28932plusmn0726

64450plusmn0535

0475plusmn0002

20 40 1 1 06 6 200 10 21 24478plusmn0526

59520plusmn0751

0481plusmn0001

21 40 1 1 03 6 200 10 21 26719plusmn0271

63090plusmn0660

0468plusmn0004


H times12k 50 μm


























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls








4 Conclusion





Data Availability




Acknowledgments


References
















Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


19lowast 40 1 1 09 6 200 09 21 28932plusmn0726

64450plusmn0535

0475plusmn0002

20 40 1 1 06 6 200 10 21 24478plusmn0526

59520plusmn0751

0481plusmn0001

21 40 1 1 03 6 200 10 21 26719plusmn0271

63090plusmn0660

0468plusmn0004


H times12k 50 μm


























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls




Data Availability




Acknowledgments


References
















Treatment


content (vol)

YSZ aerogel

Dispersant(wt)

Binder(wt)

Inlettemperature

(degC)


Feeding rate(mlmin)

D50(μm)


19lowast 40 1 1 09 6 200 09 21 28932plusmn0726

64450plusmn0535

0475plusmn0002

20 40 1 1 06 6 200 10 21 24478plusmn0526

59520plusmn0751

0481plusmn0001

21 40 1 1 03 6 200 10 21 26719plusmn0271

63090plusmn0660

0468plusmn0004


H times12k 50 μm


























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls



























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls






Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls




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TailoringAerogelforThermalSprayApplicationsin Aero-Engines ...downloads.hindawi.com/journals/amse/2018/5670291.pdf · ResearchArticle TailoringAerogelforThermalSprayApplicationsin