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Electric Current Activation of Sintering: A Review of the Pulsed Electric Current Sintering Process Zuhair A. Munir w and Dat V. Quach Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 Manshi Ohyanagi Department of Materials Chemistry, Ryukoku University, Ohtsu 520-2134, Japan The phenomenal increase during the past decade in research utilizing pulsed electric current to activate sintering is attributed generally to the intrinsic advantages of the method relative to conventional sintering methods and to the observations of the enhanced properties of materials consolidated by this method. This review focuses on the fundamental aspects of the process, discussing the reported observations and simulation studies in terms of the basic aspects of the process and identifying the intrinsic benefits of the use of the parameters of current (and pulsing), pressure, and heating rate. I. Introduction S INTERING as a process to consolidate powders is an ancient art that has been practiced for more than 6000 years in the making of bricks and pottery 1 and in the consolidation of pre- cious metals in pre-Columbian South America. 2 The ability to achieve consolidation without melting is made possible by the thermal activation of mass transport processes driven by reduc- tion of surface and grain boundary energies. To optimize ther- mal activation and attain high density with concomitant strength, sintering is carried out at high temperatures, relative to the melting point of the material. For practical as well as economic reasons, significant efforts have been, and continue to be directed, toward other means of activation to achieve high density at lower temperatures or in shorter times. Among these is the use of an electric current to activate sintering. The recent widespread use of this form of activation has been referred to variously as spark plasma sintering (SPS), pulsed electric current sintering (PECS), field-activated sintering technique, and cur- rent-activated pressure-assisted densification. Research using field activation in sintering has increased dramatically in the past decade and has drawn attention to this process at both the fundamental and the applied levels. In a previous review, we provided a historical perspective for the use of a current to activate sintering. 3 A recent review of patents on activated sintering attributes the first use of current in sintering to Bloxam, who obtained a patent in 1906. 4 However, little and typically unnoticed work on current activated sintering was carried out during the next eight decades. Between 1900 and the first half of 2008, more than 640 patents were issued worldwide 4 ; the majority of these (86%) were issued since 1990. The topics of these patents cover a wide range of properties and utility of materials, as can be seen in Fig. 1. 4 The most dominant coverage in these patents deals with the functional aspect of materials, including magnetic, thermo- electric, and electronic properties, Fig. 1(a), while coverage for structural properties and use is dominated by utilization as cut- ting tools and composites, Fig. 1(b). The marked increase in the number of patents issued since 1990 is mirrored by a corre- sponding increase in the number of publications on field-acti- vated sintering. Figure 2(a) shows the number of published papers since 1993; statistics for earlier years are reported in the previous review. 3 The increase in the number of published papers worldwide has an exponential trend, reflecting the strong interest in and the utilization of this method of sintering. Nearly 450 papers were published in 2009. Initially, most of the pub- lications came from Japanese investigators, a reflection of the fact that until relatively recently the equipment for field-acti- vated sintering was manufactured exclusively in Japan. How- ever, as Fig. 2(b) shows, other countries have become more active in this area, with the largest number of recent publications now coming from China. Several reviews of this process, with different emphasis on aspects of the process or on specific materials, have been pub- lished previously. 3–9 In this review, we will attempt to emphasize the fundamental aspects related to the PECS. In doing so, it will neither be possible nor advisable to provide a comprehensive discussion of all published papers on PECS. Aside from being a daunting undertaking, inclusion of all papers on PECS would add little to the emphasis on fundamentals, which is the aim of this review. The rapid increase in the use of PECS can be attributed largely to two broad considerations: (a) the intrinsic advanta- ges of the method relative to conventional sintering methods and (b) the observations of enhanced properties of materials Feature D. J. Green—contributing editor w Author to whom correspondence should be addressed. e-mail: [email protected] Manuscript No. 28308. Received September 20, 2010; approved September 21, 2010 J ournal J. Am. Ceram. Soc., 94 [1] 1–19 (2011) DOI: 10.1111/j.1551-2916.2010.04210.x r 2010 The American Ceramic Society

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Electric Current Activation of Sintering: A Review of the Pulsed ElectricCurrent Sintering ProcessZuhair A.Munirwand Dat V. QuachDepartment of Chemical EngineeringandMaterials Science, Universityof California,Davis, California 95616ManshiOhyanagiDepartment of Materials Chemistry,RyukokuUniversity, Ohtsu520-2134,JapanThe phenomenal increaseduringthe past decadeinresearchutilizing pulsed electric current to activate sintering is attributedgenerallytotheintrinsicadvantagesofthemethodrelativetoconventional sinteringmethodsandtotheobservationsof theenhancedpropertiesofmaterialsconsolidatedbythismethod.Thisreviewfocusesonthefundamentalaspectsoftheprocess,discussingthereportedobservationsandsimulationstudiesinterms of thebasicaspects of theprocess andidentifyingtheintrinsicbenetsoftheuseoftheparametersofcurrent(andpulsing),pressure,andheatingrate.I. IntroductionSINTERINGasaprocesstoconsolidatepowdersisanancientartthathasbeenpracticedformorethan6000yearsinthemakingofbricksandpottery1andintheconsolidationofpre-ciousmetalsinpre-ColumbianSouthAmerica.2Theabilitytoachieveconsolidationwithoutmeltingismadepossiblebythethermal activation of mass transport processes driven by reduc-tionofsurfaceandgrainboundaryenergies.Tooptimizether-mal activation and attain high density with concomitantstrength, sinteringiscarriedoutathightemperatures, relativetothemeltingpoint of thematerial. Forpractical aswell aseconomic reasons, signicant efforts have been, and continue tobedirected, towardothermeansofactivationtoachievehighdensity at lower temperatures or in shorter times. Among theseis the use of an electric current to activate sintering. The recentwidespreaduseofthisformofactivationhasbeenreferredtovariously as spark plasma sintering (SPS), pulsed electric currentsintering(PECS), eld-activatedsinteringtechnique, andcur-rent-activated pressure-assisted densication. Research usingeldactivationinsinteringhas increaseddramaticallyinthepast decade and has drawn attention to this process at both thefundamentaland theappliedlevels.Inaprevious review, we providedahistorical perspectivefortheuseofacurrenttoactivatesintering.3Arecentreviewof patents on activated sintering attributes the rst use ofcurrent in sintering to Bloxam, who obtained a patent in1906.4However, little and typically unnoticed work oncurrent activated sintering was carried out during the next eightdecades. Between 1900 and the rst half of 2008, more than 640patents wereissuedworldwide4; the majorityof these (86%)were issued since 1990. The topics of these patents cover a widerangeofpropertiesandutilityofmaterials, ascanbeseeninFig. 1.4The most dominant coverage in these patents deals withthefunctionalaspectofmaterials,includingmagnetic,thermo-electric,andelectronicproperties,Fig.1(a),whilecoverageforstructural properties and use is dominated by utilization as cut-ting tools and composites, Fig. 1(b). The marked increase in thenumber of patents issuedsince 1990is mirroredbyacorre-spondingincreaseinthenumberofpublicationsoneld-acti-vatedsintering. Figure 2(a) shows the number of publishedpapers since 1993; statistics for earlier years are reportedinthepreviousreview.3Theincreaseinthenumberofpublishedpapers worldwide has an exponential trend, reecting the stronginterest in and the utilization of this method of sintering. Nearly450paperswerepublishedin2009. Initially, mostofthepub-licationscamefromJapaneseinvestigators, areectionofthefact that until relativelyrecentlytheequipment for eld-acti-vatedsinteringwasmanufacturedexclusivelyinJapan. How-ever, as Fig. 2(b) shows, other countries have become moreactive in this area, with the largest number of recent publicationsnowcoming from China.Several reviewsof thisprocess, withdifferent emphasis onaspectsoftheprocessoronspecicmaterials,havebeenpub-lished previously.39In this review, we will attempt to emphasizethe fundamental aspects related to the PECS. In doing so, it willneitherbepossiblenoradvisabletoprovideacomprehensivediscussion of all published papers on PECS. Aside from being adauntingundertaking, inclusionofall papersonPECSwouldadd little to the emphasis on fundamentals, which is the aim ofthis review.The rapidincrease inthe use of PECScanbe attributedlargelytotwobroadconsiderations: (a)theintrinsicadvanta-ges of themethodrelativetoconventional sinteringmethodsand(b) theobservations of enhancedproperties of materialsFeatureD.J. GreencontributingeditorwAuthor towhom correspondence shouldbe addressed. e-mail: [email protected] September20, 2010; approvedSeptember 21,2010JournalJ. Am.Ceram.Soc.,94 [1]119 (2011)DOI: 10.1111/j.1551-2916.2010.04210.xr 2010 The American Ceramic Societyconsolidated by this method. Here, we refer to selected examplesof both categories and plan to discuss some in more detail in thesubsequent sections ofthis paper.II. Reported Advantages of PECSThe literature contains numerous publications demonstratingthe advantages of PECS over other methods of consolidation. Acommonadvantageistheshorter timeneededtoconsolidatepowdersrelativetoconventionalmethods,includinghotpress-ing. For example, to obtain a density of B95% for ultrane Nipowders (100 nm), it took 150 min at 7001C when hot pressingwas used, while it took only 1 min at an even lower temperature(5001C), using approximately the same pressure.10Similarobservations of a shorter sintering time andlower sinteringtemperatureshavebeenmadeinotherstudies.11,12Attainmentof higher densities at the same temperature has also beenreported.13,14Thus, the difference betweenPECSandothermethods has ramications in process efciency and energy sav-ingsaswellasmicrostructuralandcompositionalimplications.The energy efciency of PECS relative to hot pressing has beendemonstratedinthesinteringofcomposites.15Withrespecttocompositional andmicrostructural changes, sinteringat lowertemperatures andforshortertimes minimizes materialloss duetovaporization,1619undesirable phase transformation,20andsuppressionof graingrowth.21,22While the benets cited above provided a signicant impetusfortheincreasedinterestinPECS, theclaimsofbetterorim-provedproperties of materials sinteredbythis methodhavegenerated an even stronger push for its use. Improvements in avarietyofpropertieshavebeenreported,2329includingcleanergrainboundariesinsinteredceramicmaterials,30aremarkableincreaseinthesuperplasticityof ceramics,31,32higher permit-tivity in ferroelectrics,33improved magnetic properties,34re-duced impurity segregation at grain boundaries,32higherchemical stability,25higherhydrogenstoragecapacityinBCCsolidmetallicsolutions,35betterthermoelectricproperties,36,37improvedmechanical properties,38andbetter optical proper-ties.39Inthemost recent workonimpuritysegregation, Mi-tzuguchi et al.12showed that ZrB2 sintered by PECS had lowerlevelsof impuritiesinthegrainboundariesandinthegrainsthan samples sintered by hot pressing. Furthermore, PECS sam-ples had a higher density and a lower grain size than those con-solidatedby ahotpress.Inadditiontotheabove, manyreportshaveindicatedun-usualaccomplishmentsinmaterialsprocessingusingthePECSmethod. The consolidation of mechanically alloyed amorphousAl-based AlNiTi intermetallic was investigated recently usingthree methods: PECS, hot pressing, and pressureless sintering.40The work demonstrated that the consolidation was best accom-plishedwithPECStoproduceauniformdistributionofinter-metallic nanoparticles in an amorphous matrix. As will bediscussedinalatersectioninthispaper,arelatedobservationwasmadeearlierinastudyontheeffectofanelectriceldonthe crystallization of bulk metallic glass.41Also, recently, Ericks-sonet al.16succeededinconsolidating lead-free ferroelectricniobateceramicsusingthePECSmethodavoidingvolatiliza-tion, anddemonstratedthat theceramichadimprovedferro-electric properties with an unusually high remnant polarization.In a recent investigation, Wang et al.42produced a glass phase ofzeolitefromcrystallinepowdersusingeld-activatedsintering.The approachis basedonanorderdisorder transformationunder a pulsed-current heating and a uniaxial pressure. In a se-ries of related studies, disorderorder transformation was showntoenhancethedensicationofSiCundersimilarconditions.43Similar observations were alsomade inthe consolidationofcarbonwithanamorphous-graphitetransformation.44UnderPECSconditions,thetransformationresultedinanunconven-tional alignment of lattice planes in the resulting graphite (nor-mallythe c-axis of sinteredgraphite aligns parallel withthepressuredirection, but that resultingfromthetransformationalignswith thec-axisperpendicular to thepressure direction).The combination of prior mechanical activation (high energymilling) onpowders witha subsequent sintering or reactivesinteringby the PECSprocess has beenutilizedtosimulta-neously synthesize anddensify nanostructured, intermetallic,1993199419951996199719981999200020012002200320042005200620072008050100150200250300350400450500Number of PapersChinaJapanS. KoreaUSAFranceItalySwedenGermanySingapore020040060080010001200Number of Papers(a)(b)Fig. 2. Number ofpublishedpapers (a)from 1994to 2009and(b)bycountry.91011122626306071OpticalFGMSputteringCNTElectricBiomaterialsElectronicsThermoelectricMagnetic7111315162426350 5 10 15 20 25 30 35 40PorousHigh temp.AbrasiveMetalMechanicalStructuralCompositeCutting(a)(b)Fig. 1. Numberofpulsedelectriccurrentsinteringpatentspublishedfrom1900 to 2008 applied to (a) functional and (b) structural materials.42 Journalof theAmericanCeramic SocietyMuniret al. Vol. 94,No.1and composite materials.4547For example, using this approach,it was shown that MoSi2could be microalloyed successfully withMg,48alongsoughtaftergoal, facilitatingareductionintheductile-brittletransitiontemperatureinaccordancewiththeo-retical predictions.49Accomplishmentswerealsomadeintheformationof functionally graded materials (FGMs)5052and inthejoining(bonding)of materials.5355At the endof the SectionI, we pointedthat increasedinterest inthePECSprocess was generallymotivatedbytheintrinsic advantages of the method relative to conventionalsintering methods and by the observations of enhanced proper-ties of materials consolidated by this method. It shouldbepointedout, however, that theseconsiderationsarenot inde-pendent of each other and that the observed property enhance-mentismostlikely(inalargesegmentoftheseobservations)relatedtothe reductioninthe sinteringtemperature. Lowersinteringtemperatures affect compositionandmicrostructureandthesechangesinturnresultindifferentproperties. How-ever, itshouldbeemphasizedthatnotall theobservationsofproperty enhancement could be attributed solely to the effect oflowertemperature.Aswewilldiscussinmoredetail inasub-sequent section, it is likely that other parameters of the processcan play arole,specically thecurrent and pressure.III. ThePECSProcessInitssimplestform, thePECSprocessinvolvestheconsolida-tion of powders under the simultaneous action of a current anda unixial pressure. The current provides the heat to achieve thedesired sintering temperature and its application constitutes themain difference between hot pressing and the PECS process. Aswill be seen later, the application of the current gives rise to thehigh heating rates that can be accomplished in the PECS and toother nonthermal contributions including current effect on masstransport.Figure3showsaschematicofthePECSapparatus.Typically,apulsedDCcurrentisappliedwitharelativelylowvoltage (B10 V). The pulsing pattern is made up of a sequenceof pulses with the current, followed by the absence of a current.Thus, a pulse pattern of 122 means that 12 pulses are applied,followed by a duration of two pulses where the current isnotapplied. Pulsesinatypical PECSapparatusare3.3msinduration.Thesimultaneous output of temperatureanddisplacement(shrinkage) makesitpossibletogainaninsightintosinteringkinetics or the reaction mechanism in the PECS method. How-ever, the observed displacement represents an overall character-ization of shrinkage as it also includes contributions fromthe dieandsystem.Withcalibration(toobtainbaselinedisplacement)andaccurate measurements of temperature, it is possible toobtain valid shrinkagedata, as is demonstratedin Fig. 4(a)forthe densication of zirconia.56Using such an analysis, the PECSmethod can provide information on reactivity, as has beendemonstratedinAnselmi-Tamburini et al.56ThisisshowninFig. 4(b), where the small increase in temperature represents theexothermicreactionof theformationof B4Cfromelementalpowders. As themeasuringthermocouplealsoprovides feed-backtothesystem, thedipinthepowercurverepresentsthecontrol step taken by the system to account for the small, albeittransient, increase in temperature due to the reaction. Similarly,thedecreaseinthedisplacement (i.e., shrinkage) signies thedecrease in themolar volumeaccompanying thereaction.It is the application of the pulsed current that has beenclaimedtobe the mainadvantage of the PECSprocess. Amore specic discussion on this will be presented below. But theconcept of using an electric spark in the sintering of powders isnot new. As was detailed in a previous review paper,3in a recentcomprehensive review,8and as has been discussed in a paper onPECS patents,4the concept of electric spark was utilized in var-ious forms for more than a century. However, the recent surge inthe use of this approach is, in large part, the consequence of theavailability of commercial units, manufactured initially by com-panies in Japan. More recently, PECS equipment has been man-ufactured in Germany,theU.S.,Korea, andChina.(1) Nature andInuenceof PulsingA persisting source of controversy regardingthe benets of thePECSprocessistheoft-repeatedclaimthatthepulsingofthecurrent creates a plasma that activates the surfaces of the pow-der particles, through the removal of surface layers (e.g., oxides).Conicting results have been provided to argue for the existenceor absence of the plasma,5761but the more convincing evidencepoints to itsabsenceunderPECS conditions.However, aside from the concept of plasma, the role of puls-ing pattern in sintering and reactivity in the PECS has been thesubject of several investigations. Nanko and coinvestigatorsreported earlier an absence of pulse current effects on the sinte-ringofcastironandNi-20Crpowders.61,62Xieetal.63investi-gatedtheeffect of thefrequencyof sinteringof Al powders.TheydensiedthepowderinthePECSunderdifferent pulsefrequencies: 0and300Hz, and10and40kHz, withpatternsshowninFig. 5, andconcludedthat pulsefrequencyhadnoeffect onthedensicationandmicrostructureof thesinteredpowders. Similarly, Dang et al.64studied the effect of the wave-form of the pulsed current on the sintering of a-Al2O3 using 300Hz and 16 kHz, with pulse patterns (on-off) of 122 and 26 forthe former frequency and 4010 and 1020 for the latter. TheirresultsaresummarizedinFigs. 6(a) and(b), whichshowtheeffect ondensity andgrainsize, respectively. As the guresshow, neither thefrequencynor thepulsepattern had aneffecton thedensication or thegrain growthofalumina.Aspartofaseriesofinvestigationsonthefundamental as-pects of the SPS process, we studied the nature and effect of thepulsingpattern.65,66Figure7(a)showsapulsepatternof82,i.e., eight pulses of 3.3 ms on, followed by two pulses off. As canbeseenfromthisgure, thepeaksdonot correspondtoonevoltageandinfact theexact pulsenumberisnot alwaysfol-lowed,as is evidentin thesecond sequenceof onpulses, wherethere are nine instead of eight pulses. A Fourier transformof thepattern of Fig. 7(a) is shown in Fig. 7(b). The transform exhibitsa peak at about 350 Hz and smaller ones at higher frequencies.However, itisseenthatthebulkofthepowerinthePECSisprovided by the component at zero frequency, i.e. DC power. Asthecontributionofagivenfrequencytotheheatingispropor-tional to the square of its amplitude, we plot this as a function of Fig. 3. Schematicof a pulsed electriccurrentsintering apparatus.January 2011 Electric CurrentActivation of Sintering 3frequency in Fig. 7(c). From this, it can be seen that most of theheatingin thePECS is generatedby frequenciesof o100Hz.As we have seen above, various studies have shown that pulsepatternhadnoeffectondensicationorgraingrowth.6164Toinvestigatetheeffectofpulsepatternonreactivity(andhencemasstransport)inthePECS, astudywascarriedoutusingathree-layer sample to determine the effect on product formationattheinterfacesbetweenthelayers. Awaferofp-typeSi wasplacedbetweentwofoilsofMoandannealedatconstanttem-peratures under different pulsing patterns.65The use of a three-Fig. 4. (a)Correcteddisplacementduringthedensicationofnanometriczirconiaunderapressureof106MPaand(b)temperature, power, anddisplacement proles during thesynthesis of B4Cfrom the elements inPECS(pressure 550MPa andheating rate 51001C/min).56Fig. 5. Measured waveforms duringthepulsedelectric current sintering processof aluminum powderwithdifferentpulse frequencies.634 Journalof theAmericanCeramic SocietyMuniret al. Vol. 94,No.1layer system was planned to determine the possible effect of thedirectionof theDCcurrent. AscanbeseenfromFig. 8, thepulse pattern had no effect on the thickness of the product (pri-marily MoSi2) formed at the two interfaces. Moreover, thedirectionof the current alsohadnoeffect. As will be seenlaterinthispaper,thislatterndingisnotsurprisingininter-actions whereacompoundis formed.The results discussedabove lead to the conclusion that puls-ingpatternandpulsefrequencyhavenomeasurableeffectondensication, grain growth, andmasstransport.IV. PECSParameters andtheirEffect on ProcessingThe parameters that are typicallyassociatedwiththe PECSprocessincludethecurrent, theapplieduniaxial pressure, andheatingrate. Typically, thecurrent andsinteringtemperatureare dependent parameters as Joule heating is the source of ther-mal activation, whether in the graphite die only (when the sam-ple is nonelectrically conducting) or in the die and sample (whenthe sample is electrically conducting). The maximum (DC) cur-rentavailabledependsonthespecicPECSapparatus; inthecaseoftheSumitomoModel2050apparatus,forexample,themaximum current is 5000 A. As seen later, it is possible to makethe current and temperature independent parameters in the SPSwithcertaindesignmodications. While the current is com-monly (but not accurately) associated with Joule heating only, itcan also play another intrinsic role in enhancing mass transport,aswillbeshown subsequently.Thepressurehasbeenshownrecentlytoplayacrucial roleinthe consolidationof materials, particularlynanostructuredpowders. Until recently, the maximum pressure that can be uni-axially applied in the PECS was limited by the mechanical prop-erty of the graphite die. Atheoretical upper limit of aboutFig. 6. Relative density (a) and grain size (b) of alumina pulsed electriccurrent sinteringedat different temperatures withvariousfrequenciesand pulsepatterns.64Fig. 7. (a) Typical pulse pattern of 8:2 (on:off) in the PECS; (b) Fouriertransformof thepatternof (a); (c) Fourier transformplottedas thesquare of themagnitude versus frequency.6605001000150020002500300035004000450050000 1000 2000 3000 4000 5000Square of Layer Thickness ( m2)Time (s)1270 C, pulses 12:21270 C, pulses 7:71170 C, pulses 12:21170 C, pulses 8:21070 C, pulses 8:21070 C, pulses 2:8Fig. 8. ThegrowthofMoSi2layerinthepulsedelectriccurrentsinte-ring atdifferent temperatures underdifferentpulsepatterns.65January 2011 Electric CurrentActivation of Sintering 5140 MPa is used as a guide, but in practice, the limit may be lower.Recent modications of die design (Fig. 9) have made it possibleto achieve much higher pressures.67With this design, it is possibleto apply pressures as high as 1 GPa. The importance of this will beseen when we discuss the role of the pressure, below.Heating rate is another parameter that distinguishes thePECS process from hot pressing. Because of the nature of heat-ing, much higher heating rates can be achieved in the PECS, ashighas about 20001C/min. The advantage of higher heatingrates is the bypassingof the nondensifyingsinteringmecha-nisms, e.g., surfacediffusion. However, fornonelectrically con-ducting samples, the heating rate can play a role in theoccurrence of thermal gradients, depending on the thermal con-ductivityandthesizeof thesample.68(1) Inuence of theCurrent(A) General ObservationsofCurrentEffectinMaterialsProcessing: As was stated above, the notion of the presence ofplasma(andhencethecommonnameoftheprocess)hasnotbeen demonstrated adequately and thus the (nonthermal) role ofthe current is likely to be in its effect on mass transport. That thecurrent couldhave aninuence onmass transport has beenshown clearly by numerous investigations. The current enhancesmasstransportthroughelectromigration,69pointdefectgener-ation,70and enhanced defect mobility.71Electromigrationstudiesonmultilayersystemshaveshownthat thecurrent increasestherateof product layerformationand decreases the incubation time for the nucleation of the newphase.7275The imposition of a current has also beenshown tohave other effects. It was shown to increase the solubility in liq-uidmetals andinuence the resulting microstructure of thesolidied product.76,77In a study on the crystallization ofbulk metallicglasses,theimpositionof acurrent was showntoinuencethegrainsizeandfractionoftheresultingnanocrys-tallites.41Conrad and colleagues have carried out extensiveinvestigations ontheeffect of anelectriceld(current) onavarietyofmaterials-relatedprocesses.Inarecentinvestigation,theyshowedthat theimpositionof amodestelectriceldre-ducedthe tensile owstress of MgO, Al2O3, andtetragonalZrO2.78They attributed the decrease to a reduction in the elect-rochemicalpotentialfortheformationofvacancies.Theyalsoreported aretardationof grain growthas a consequenceoftheeld. Itwasproposedthatgraingrowthretardationmightbeattributed to either a eld effect on solute ion segregation (Y inthecaseof ZrO2), adecreaseingrainboundaryenergy, oradecrease in ion mobility. The grain growth retardation was con-sistentwithearlierobservationsontheinuenceof aeldoncopper and tetragonal zirconia, as can be seen in Figs. 10 and 11,respectively.79,80Similarobservationsofgraingrowthretarda-tionin tetragonalyttria-stabilizedzirconia(YSZ)were recentlyreportedbyGhoshet al.81withtheapplicationof anelectriceldofabout4V/cm.Whiletheseobservationsofeldeffectsongraingrowthhaveimportantimplicationsintheprocessingofmaterials, theirunderlyingcauseisnotyetclear, acircum-stancethatdoesnotdiminishtheirimportancebutclearlysug-geststhatmoreresearchneeds to becarriedout.(B) Current Effects inPECSProcessing: Various ob-servationsmadewhileprocessingmaterialsonthePECShavebeendirectlyorindirectlyattributedtotheroleofthepulsedcurrent.UsingastartingpowderofPb(Mg1/3Nb2/3)O3-PbTiO3withagrainsizeof110 mm,Chenetal.82obtainedasinteredbodywithagrainsizeintherange20100nm. This uniqueobservationof makingananostructuredceramicfrommicro-structured powders during sintering in the PECS was attributedto the role of the current. It is proposed that the pulsed currentinduced thermo-mechanical fatigue, which resulted in thebreakupof the microstructuredgrains intonanograins. Thisobservation is of signicant interest and its application to othermaterials is needed before it can be considered as a general phe-nomenoninPECSprocessing.Nagaeetal.83studiedthesinte-ring of aluminumpowders by the PECS and hot-pressingmethodsandreportedalowerelectrical resistivityof samplessintered by the PECS, which they attributed to the effect of thepulsedcurrentonthedestructionofthesurfaceoxidethroughlocalizedJouleheatingatthecontactpointsbetweenparticles.Anotherobservation, whichwasattributedtotheroleof theeld (current), was made during the sintering of the spinelMgAl2O4. Mussi etal.84observedadifferent(inversion)occu-pationof the tetrahedral andoctahedral sites whenpowdersweresinteredinthePECSrelativetoobservations whenthespinel was sinteredbypressureless sinteringandhot-isostaticpressing.Theyexplainedtheirndingonthebasisof theeffectof the PECS process (presumably the electric eld) on thespacecharge. Whilethereisnodirect evidenceof changesinthespacechargelayerduringSPSsintering, theoccurrenceofspacechargeanditsassociateddistributionofdefectsinmag-nesium aluminum spinel and alumina have been demon-strated.85,86Asinthecaseoftheobservationofgraingrowthretardation,theconceptofachangeinthespacechargeince-ramics due to exposure to PECS conditions remains to be dem-onstrateddirectly.The effect of current onreactivityinthe PECShas beeninvestigated.56,8789The reaction between Mo foils and Si waferswas investigatedunder PECSconditions withandwithout acurrent passing through the multilayer ensemble.56As wasstatedabove,65the reactionbetweenthese elements was notaffected by the pulse pattern. However, the reaction rate to pro-duce the interface product, primarily MoSi2with minoramountsofMo5Si3, wasmarkedlyinuencedbythepresenceof a current, as can be seen in Fig. 12. The kinetics of growth ofMoSi2 were determined from the results under both conditions,ascanbeseeninFig.13,inwhichtherateconstantisplottedagainstthereciprocal oftheabsolutetemperature. Thecalcu-latedactivationenergiesforthegrowthofMoSi2forthecaseswith andwithoutacurrent(B600A/cm2)are168and175kJ/mol,respectively.Theeffectofthedirectionof theDCcurrentonthegrowthoftheproductwasmadepossiblebythethree-layergeometry(Si issandwichedbetweentwoMofoils). Theresults, depicted in Fig. 14, showthat there is no effect of currentGraphiteTungsten carbideSilicon carbideSampleFig. 9. Schematicofadoubleactingdiethatallowstheuseofmuchhigherpressures.676 Journalof theAmericanCeramic SocietyMuniret al. Vol. 94,No.1direction on the growth of the product layer. This nding is notsurprisingas the growthof the product, MoSi2inthis case,requires the diffusion of both Mo and Si. Thus, if one isenhancedbyelectromigration,thegrowthratewillstilldependontheslowerdiffusingspecies. Avalidationofthisisseenincases where no product forms. This was demonstrated clearly bythe electromigration study on the CuNi system that forms con-tinuoussolidsolutions.75Inthatstudy,acleardependenceonthedirectionof thecurrent oninterdiffusivity wasseen.Otherobservationsontheeffectofcurrentonreactivityun-der PECS conditions include reactions between carbon andNb,87Mo,88and Ti and Zr.89Figure15 shows thetimedepen-dence of the growth of the product layer thickness (b-Mo2C) oncurrent density for samples annealed at 1842 K.88The effect ofthe current density is best shown in Fig. 16, in which theannealingtimeisconstantat20min. Thegureshowsanap-parent threshold value of current before an effect is observed; inthiscase,thevalueisapproximately500A/cm2.Incontrasttothe case of the reaction between Mo and Si,56the activation en-ergy for the growth of the product has a strong dependence oncurrent density, as can be seen in Fig. 17. Relative to annealingwithoutacurrent,adecrease of about44%wasseen when thesamples were annealed at 1476 A/cm2. In the cases of the growthofTiCandZrClayers,theactivationenergywasbasicallyun-alteredwiththeapplicationofacurrent,althoughthegrowthrate was enhanced in the presence of a current.89Similar resultswereobtainedwiththesystemNbC,exceptthatinthiscase,twocarbide phases were formed: NbCandNb2C.87Again,growth enhancement was observed, but no change in activationenergy. The difference in behavior between the MoC case andtheothers(MoSi,TiC,ZrC,andNbC)isnotwellunder-stood. It may be the consequence of the kinetics of the ab phasetransformationofMo2Crelativetosimilartransformationsintheotherbinary systems.Fig. 11. Mean linear intercept grain size

d of tetragonal zirconia versustemperature in the grip tab (e0) and near the fracture surface (e1.0)withandwithout an electric eld.80050010001500200025000 1000 2000 3000 4000 5000Square of Layer Thickness (m2)Time (s)1270 C current1270 C no current1200 C current1200 C no current1150 C current1150 C no current1100 C currentFig. 12. GrowthratesoftheMoSi2layeratdifferenttemperaturesinthepresence andabsence of currentowing throughthesample.65Fig. 10. LoggrainsizeDversuslogtimetforannealingofaCufoil(thickness 518mm) at 15011951Cwithandwithout anelectriceld:(a)topside of thefoil and(b)bottomside.79January 2011 Electric CurrentActivation of Sintering 7The results presentedabove clearlyshowthe effect of thecurrentonreactivity(masstransport)underPECSconditions.However, as the PECS method is used primarily for the consol-idationofpowders, amoreconvincinginvestigationwouldbethe direct demonstration of current effect on sintering. Such aninvestigation was carried out using the sintering of copperspheres to copper plates as the model.90Figure 18 shows a sche-maticofthearrangementusedintheSPS.Inordertodemon-stratetheeffect of thecurrent, it was necessarytodeviseanexperimental setup in which the current can be varied while thetemperature is held constant.Thisis important as under anormal PECSoperation, thecurrent and temperature are interdependent parameters. The useof carbon foil layers made it possible to achieve the desired goal.Thecurrent(I)dependenceonthenumberof graphitefoillay-ers, x,is derived as90:I 1x1=2P4Rco2Rgf 1=2(1)where P is the power and R is the resistance, where the subscriptco refers to contact resistance and gf refers to graphite foilresistance.Figure19showstheexperimentallydeterminedcur-rent as afunctionof the number of graphite foil layers forT59001Cand15min. The gure alsoshows the predictedrelationship from Eq. (1) as a solid line. The effect of current onthesinteringof thecopperspherestocopperplatesisshownqualitativelyinFig. 20. ThegureshowsSEMimagesofthefracture surface of the necks that formed between them on sinte-ring at 9001C for 60 min under different current values, rangingfrom0to1040A. Theseimagesclearlyshowtheeffectofthecurrentbytheincreaseinthediameteroftheneckwithanin-creaseincurrent. Quantitativeinterpretations of theseresultsweremadefrommeasurementsoftheradiioftheneck,x,andexpressing themin theform,xR nBtRm(2)where R is the radius of the sphere, B is a constant that containsthediffusioncoefcient,andtheexponentsnandmaremech-anism-dependent constants. Eq. (2) is for the initial stagesof sinteringwithx/Rr0.3. Aplot of(x/R) versust is showninFig. 21, in which the effect of the current is clearly demonstrated.An important observation made in that study is the presenceof features onthe surface of the copper plates indicative ofevaporation. The features appear as white rings surroundingneckregionsunderopticalmicroscopy(Fig.22)butareshowntobesurfaceledgestructureswhenobservedbyscanningelec-tronmicroscopy, Fig. 23. Theextentoftheseareasisdirectly2.01.81.61.41.21.00.80.60.40.20.06.4 6.6 6.8 7 7.2 7.4ln (k)104/ T (K1)with currentwithout currentFig. 13. Arrheniusplotofthetemperaturedependenceofthegrowthrate of MoSi2 in the presence and absence of current owing through thesample.650102030405060700 10 20 30 40 50 60 70Bottom Layer Thickness (m)Top Layer Thickness (m)Fig. 14. ComparisonofMoSi2layerthicknessatthetwo(MoSiandSiMo)interfacesrelative to thedirection of thecurrent.650.00.51.01.52.02.53.03.54.04.55.00 500 1000 1500 2000Square of Layer Thickness (108m2)Holding Time (s)0 [A.cm2]521 [A.cm2]732 [A.cm2]1145 [A.cm2]1476 [A.cm2]Fig. 15. Inuence of current density on the growth of the b-Mo2C layerat 1842K.88501001502000 500 1000 1500Product Layer Thickness (m)Current Density (A.cm2)Fig. 16. Inuenceofcurrentdensityonthethicknessoftheb-Mo2Clayer (T51842K; t 520 min).888 Journalof theAmericanCeramic SocietyMuniret al. Vol. 94,No.1relatedtothestrengthoftheimposedcurrent, ascanbeseenfrom Fig. 22. The occurrence of these ledge structures and theirproximity to the neck regions suggest an effect of the current onevaporation. While there are no published accounts on the effectof a current on evaporation, there is indirect evidence from pre-vious electromigrationstudiesonthesystemAgZn.91AcomparativestudyshowingthecontributionofthePECSconditions to sintering relative to hot pressing was carried out byFuetal.92Inthisstudy,thesinteringbetweenspheres(CuNiandFeCu) was investigated underthe two conditions. In con-trast to the study on copper spheres and copper plates,90in thisstudy,thedifferencewasnotrelateddirectlytothelevelofthecurrent, but only to a specic temperature and hold time of an-nealing.However,theresultsunambiguouslyshowaneffectofthe PECS conditions on sintering, as can be seen from Table I.92The table shows the effect of temperature and hold time on the(x/R)ratioforsamplessinteredinPECSandinthehotpress.Also shown in Table I are calculated diffusion coefcients basedon neck formation.93The results show a signicant sintering en-hancement under PECS conditions relative to hot pressing, bothin termsof neck formation and in terms of the diffusion coef-cient.ThediffusioncoefcientsofNi,calculatedfromconcen-tration prolesacross necks between nickel andcopper spheresfor samples sinteredunder the twoconditions, areshowninFig. 24.92The values obtained when sintering in PECSaregreaterbyafactorof23thanthoseobtainedduringhotpressing.More recently, N. Toyofuku, T. Kuramoto, T. Imai, M.Ohyanagi andZ. A. Munir (unpublishedresults) investigatedthe effect of current on the sintering of W wires to W plates, inanarrangementsimilartothatdescribedaboveforthecopperspheresandplates. ThetimedependenceoftheneckradiusisshowninFig. 25for thecases of sinteringwithandwithoutcurrent at 17001C. After 30 min of sintering, the neck radius witha current is 1.5 times that of the neck formed in the absence of acurrent.Evidenceofapossibleeffectofthecurrentonevapo-ration is also provided. However, in this case, the effect is likelydue to the reduction of surface oxide, as has been observed in thesintering of tungsten.94,95The investigations discussed briey above (N. Toyofuku,T. Kuramoto, T. Imai, M. Ohyanagi andZ. A. Munir un-published results)90,92provide evidence of sintering enhancementunder PECS conditions, with a direct correlation to the level ofthecurrent,aswasshowninthesinteringofcopperspherestoplates. The enhancement of mass transport is believed to be theconsequence of electromigration. The increase in the ux, Ji, of adiffusing species, i, is a result of the momentum transfer fromtheelectronwind effect, ascanbeseenfromthefollowingrelationship96:Ji DiCiRTRTq lnCi@xFzE (3)where Jiis the ux of the diffusing ith species, Di is the diffusivityofthespecies,Ciistheconcentrationofthespecies,FisFara-days constant, zis the effective charge on the diffusing species,E is the eld, R is the gas constant, andT is the temperature.Theabovestudiesshowtheeffectofcurrentonneckforma-tion, i.e. during the initial stage of sintering. We are not aware ofany study showing such an effect directly during the intermediateand nal stages of sintering.(2) TheEffectofPressureExperimental observations showingthebenet of pressureindensicationarenumerous,with hotpressingbeingacommonexample.97Theroleof pressureinsinteringhasbeeninvesti-gatedextensively. Theeffectofpressureonvariousapplicablemechanisms in sintering has been discussed in a review by Ger-man.98Inthepresent review, wewill focus primarilyontheeffects of pressure in the PECS process. In typical PECS exper-iments, using graphite dies, there is a practical upper limit for theapplied pressure (B140 MPa) dictatedby themechanical prop-erties of graphite. Higher pressures were possible throughamodicationof die design.66As was indicatedinapreviousreview,3the pressure has intrinsic and extrinsic effects on sinte-ring;atafundamentallevel,theformerinvolvesanincreaseinthechemical potential, as indicated by thefollowing99:mI moi snOI(4)where mIisthechemicalpotential ataparticleinterfaceunderstress, mioisthestandardchemical potential,snisthenormalFig. 17. Calculatedvaluesoftheactivationenergyforthegrowthofb-Mo2Cas afunction of currentdensity.88Fig. 18. Schematic of a sample of a Cu sphere to plate sintering geom-etry in thepulsedelectric current sintering apparatus.90Fig. 19. Dependenceof total current onthenumberof graphitefoillayersat 9001C.90January 2011 Electric CurrentActivation of Sintering 9stress at the interface, and OI is the atomic volume of the diffus-ingspecies. Inadditiontoinuencingdiffusion-relatedmasstransport, thepressureinuencesotherprocessesintrinsically,including viscous ow, plastic ow, and creep. Extrinsically, thepressureinuencesparticlerearrangementandthedestructionof agglomerates in powders, the latter playing an important rolein theconsolidation of nanopowders, as will be seen below.Makinoet al.100investigatedtheeffect of pressureonthesinteringof ultranea-aluminapowders under PECScondi-tions.Theyshowedthatdensicationunderalowpressure(30MPa) wasinuencedbythegrainsizeofthestartingpowder(100and230 nmpowders)but that suchaninuencewassup-pressed when densication was carried out under a high pressure(100 MPa). In addition, the authors reported an inuence of thepressure oncrystallite growth; graingrowthsuppression in-creasedwhenthepowdersweresinteredat thehighpressure,as can be seen in Fig.26.Guillard et al.101investigated the role of the PECS parametersin the densication of SiC and showed that the effect of pressuredependedonthetemperatureatwhichitisapplied. Theysin-tered SiCunder two different conditions: in the rst, theyappliedthepressure(75MPa) at theultimatetemperatureofsintering(18001C), andinthe secondcase, the pressure wasapplied at a lower temperature (10001C). Their results show thatsamples in which the pressure was applied at the lower temper-ature(i.e., case2)hadlowerdensities,whichtheyattributed tothe difculty in removing closed porosity after the application ofthe pressure. Ina similar investigation, ChaimandShen102showednoeffect onthedensityof thetemperatureat whichthepressurewasapplied in thesinteringof Ndyttriumalumi-num garnet (YAG) nanopowders. However, the effect on grainsize was more complex: the grain size was lower when the pres-sure was applied at the sintering temperature if the latter (T) wasless thanabout 13751Cbut it was larger whensinteringwascarried out at higher temperatures. The grain size was indepen-dent of the sintering temperature when the pressure was appliedat a lower temperature (12001C). The authors explain theirresultsin termsoftherole of particle coarsening.Theysuggestthat the process of coarsening during the heating up period has astronginuenceongraingrowth. Thisprocessmayleadtoavariation in the particle size distribution, which, in turn, affectsthe grain boundary curvature before pressure application. Theyconcludethat applicationof pressurebeforesignicant coars-eningofthenanoparticleswouldbebenecialforthesuppres-sion of graingrowth in thedensecompact.Todemonstrate theeffect of theappliedpressure(aswell asothersinteringparameters),ChaimandMargulis103developedSPSdensicationmaps for nanocrystalline MgO usinghot-iso-static pressing as a model. As can be seen in Fig. 27, an increasein pressure at a constant relative density changes the mechanismFig. 20. SEMimagesshowingtheeffectofcurrentontheneckformationbetweencopperspheresandcopperplatessinteredat9001Cfor60min:(a)zero current,(b)700 A, (c)850A, and(d)1040 A.90Fig. 21. Time dependence of neck growth between copper spheres andcopperplatesat9001Cunderdifferentcurrents. Thenecksizeatzerotime refersto thevalue obtained during ramp up to temperature.9010 Journalof theAmericanCeramic SocietyMuniret al. Vol. 94,No.1fromdiffusiontoplasticowcontrol. Orforanygivenpres-sures, thesinteringis dominatedinitiallybyplasticowandnally by diffusion for the case of MgO. Agreement withexperimental observations was taken as support for the validityoftheassumedmodel.Theeffect ofpressureonthedensicationofnanopowderswasdemonstratedinthecaseofsinteringof8mol%YSZ.104Figure28showstheeffectofthepressureonthetemperatureneeded to obtain 95% dense YSZ in 5 min. The gure shows anexponential decrease in temperature from about 13501 to about8901C as the uniaxial pressure is increased from 30 to 800 MPa.Thegrain size decreased initially from about 200 to 15nmandthenremained constantfor pressureshigher than 150MPa. Aswaspointedoutabove,inordertoreachhighpressuresinthePECS, we have designed a die conguration, as shown in Fig. 9.Theresults of amoredetaileddeterminationof theeffect ofpressure ondensity andgrainsize for YSZare depictedinFig. 29.105Whensinteringwascarriedout atarelativelylowtemperature(9801C),thepressurehadamarkedeffectonden-sity; the density increased from about 78% to 96% as the pres-sure was increased from150 to 700 MPa. However, whensintering was carried out at a relatively high temperature(11801C), thepressurehadamarginaleffect.The difference in behavior seen in these results reects the roleof temperature relative to that of the pressure, as can be seen inthefollowingrelationship:dr1 r dt B ggxP (5)where r is the fractional density, B is a parameter that includesthe diffusion coefcient (of the slowest species) and temperature,g is a geometric constant, g is the surface energy, x is a param-eter representing a size scale that is related to particle size, t is thetime, andPis the effective pressure. The effective pressureexertedonpores varies accordingtothe pore geometryandthestageofsintering, butwecanassumeitsvaluetobepro-portional tothe macroscopic appliedpressure. At the lowertemperature, mass transport through diffusion is less signicant(BinEq.(1)isrelativelysmall)andthepressureplaysadom-inantrole.Atthehighertemperature,therelativecontributionof the pressure becomes less signicant. This was demonstratedby Quach et al.105through simplecomparativecalculations.Animportant,extrinsic,contributionofthepressurerelatestoitseffect onagglomeratesinpowders, especiallyfornano-powders. Nanopowders are susceptible to the formation ofagglomeratesduetovanderWaal bondsbetweenparticles.106When compacted, agglomerates produce an inhomogeneousstructureinthegreenbodyandthisleadstoalowgreenden-sity.107TheroleofthepressureintheparticlerearrangementsandthebreakupofagglomeratesisillustratedschematicallyinFig. 22. Optical micrographs of neck images on a copper plate showingthehaloformationaroundtheperimetersofnecks:(a)zerocurrent,(b)700 A, and (c)1040 A.90Fig. 23. SEM image near the edge of a neck showing the formation ofledges (AIP).90TableI. Neck Ratios (x/R)andCalculatedDiffusionCoefcientsof NiinthePulsedElectricCurrentSintering(PECS) andHot-PressSinteringof Ni/CuSpheres92Sinteringprocess T(1C) Holdtime (s) x/R DNi ( 108)(m2/s)PECS 1000 300 0.595 3.5641100 300 0.705 9.239Hotpress 1000 2700 0.548 1.1111100 2700 0.626 2.142January 2011 Electric CurrentActivation of Sintering 11Fig. 30.108The effect of pressure on the pore size distribution ofa reactive mixture of ZrO2 and Y2O3 is shown in Fig. 31.108Aswill be discussed below, the destruction of agglomerates throughtheapplicationofhighpressurewasthekeytoproducing,forthe rst time, dense bulk cubic YSZ with a grain size o20 nm.104Another aspect of the inuence of pressure relates tothetemperature difference between the sample and the die.109Grassoetal.109investigatedtheeffect of theappliedpressureonthe difference inthe temperature at these two locationsexperimentallyandthroughsimulationforagraphitedieandsample.Theyshowedthatanincreaseinpressureresultedinamarkedlylowerdifferenceintemperatureatthetwolocations,and attributed this to a decrease in electrical and thermal contactresistancesatthepunch/dieinterfaceduetoPoissondeforma-tion of the punch with a higher pressure. Contact resistance hasbeen identied previously as a reason for the temperature differ-encesbetweenthesurfaceofthegraphitedieandthecenterofthesample.110,111Another aspect of thepressure, therateatwhich it is applied, was investigated by Xu et al.112in the dens-ication of YSZ. They found that the displacement ratesaffected the densication rates and the nal density of the sam-plessignicantly. Higherratesresultedinhigherdensicationrates, as can be seenfrom Fig. 32.(3) TheEffect of Heating RateOne of the main differences between the PECS process and hotpressingis the heatingrate. Heatingrates as highas about2025303540455055605 10 15 20 25 30 35Neck radius (m)Time (min)With currentWithout currentFig. 25. Time dependence of neck growth between W wires and platesannealed at 17001Cwith and without current (N. Toyofuku, T. Kuramoto,T. Imai,M.Ohyanagiand Z. A. Munirunpublishedresults).404550556065900 1000 1100 1200 1300 1400 1500Crystallite Size (nm)Consolidating Temperature (C)AA-alumina (100MPa)AA-alumina (30 MPa)TM-alumina (100 MPa)TM-alumina (30 MPa)Fig. 26. Dependenceof crystallite sizeon pulsedelectriccurrent sinte-ring(PECS)temperatureandappliedpressurefortwodifferent com-mercial powdersof alumina(TM-alumina andAA-alumina).10001234561060 1080 1100 1120 1140 1160 1180DNi (1010 cm2/s)Temperature (K)PECSHPFig. 24. Temperature dependence of the diffusion coefcients ofNiattheinterfaceforpulsedelectriccurrentsintering(PECS)andhotpressing (HP).92Fig. 27. Densication map for 20 nmparticle size nanocrystalline MgOat 7501Cin pulsedelectric current sintering (PECS).103Fig. 28. Relationshipbetweenholdtemperatureandtheappliedpres-sure required to obtain samples with a relative density of 95%in the caseof nanometric fully stabilized zirconia (8% YSZ). Hold time: 5 min. Thegrain sizeof thematerials is alsoshown.10412 Journalof theAmericanCeramic SocietyMuniret al. Vol. 94,No.120001C/mincanbeachievedinthePECS. Highheatingratesreducethetimethatpowdersdwellatthelowertemperatures,wherenondensifying(graincoarsening)mechanisms(e.g., sur-facediffusion)aredominant. Inaddition, higherheatingratescreateanadditional drivingforceduetolargethermal gradi-ents.113However, despite theoretical expectations and the resultsof simulations,114experimental observations on the effect of theheatingrate in theSPS haveprovidedconicting results.For example, inthecaseof alumina, inonestudy, it wasfound that the heating rate (507001C/min) had no effect on thenal density,115andinanotherstudy, theeffect wasnegative(i.e.,thedensitydecreasedwith anincreaseintheheatingrate)whentheheatingrateswerehigh(43501C/min).116Inbothofthese studies, the heating rate had an effect on grain size; higherheating rates resulted in smaller grain sizes. Similar observationsweremademorerecentlybyotherinvestigatorsinastudyonalumina117and on cubic zirconia.118Figure 33 shows the effectof the heating rate on the densication and grain size of 8 mol%YSZsinteredunderahighpressure(500MPa).105Ascanbeseen in thegure, theheatingrate hadno effect on density buthad an effect on grain size, consistent with earlier observations.The inconsistency in some of the results on the effect of heat-ing rate on densication in the PECS is likely the consequence ofdifferences inthe materials properties andalsoexperimentaluncertainties. The latter include differences in the effective ther-mal and electrical conductivities of the samples (and thustemperaturegradients) onthecontact resistancesbetweenthesampleandthedieandbetweenpartsofthedieassembly,andthe timing and rate of pressure application. And the effect of theheating rate on grain size relates to the bypassing of grain coars-eningprocessesandontheeffectivetimeforsintering: higher7075808590951001050 200 400 600 800% Theoretical DensityApplied Pressure, MPa980 C1180 CFig. 29. Effect oftheappliedpressureonthenal densityof c-YSZsamples pulsed electric current sinteringed at 9801 and 11801Cfor5 min.105Fig. 30. Schematic on the role of applied pressure in particle re-arrangements and breakup of agglomerates in the reactive mixture of ZrO2 and Y2O3whenthepressure is (a)4MPa (b)8MPa (c)95 MPa(d)400 MPa.108January 2011 Electric CurrentActivation of Sintering 13heatingrateshaveashorterdwell timeandarethusexpectedtoresult insmaller graingrowth. Nevertheless, this area ofinvestigationhas not receivedadequate attentionexperimen-tally. Simulationsstudieshavebeenperformedwithclearpre-dictions,aswill be discussedbelow.114V. Simulation Studieson thePECSProcessSimulationstudiesonthePECSprocesshavebeencarriedoutgenerally for two purposes: (1) to verify the role of the assumedparameters of the process andtomake predictions ontheireffects and (2) to explain observations made in the processing ofvarious materials by this method. Thus, an important contribu-tion of all these studies is providing a basic understanding of thePECS process, and in doing so, removing the black boxstigmathat,webelieve,hascontributedtotheslowestablish-ment ofthis method in theUnitedStates.Asignicantnumberofsimulationinvestigationshavebeenconductedinrecentyears.Matsugiandcolleaguesinvestigatedthevoltage,temperature,anddensity distributionsof titanium.They found that the largest heat source was in the punch of thedieandthusheatowwasmainlyfromthepunchtothesam-ple.119McWilliamsandZavaliangos120investigatedthedensityevolutionin relationto theconduction path of thecurrentandshowedthatvariationinthelocaldensity insampleduringthePECS process can inuenceitssintering behavior.OlevskyandFroyenhavemadesignicantcontributionsinthe area of simulations. They examined the role of the LudwigSoret effect of thermal diffusion during SPS sintering andreported it to be signicant, especially for small particle sizes.121Usingamodelthatincorporatesthermaldiffusion,theyfoundtheirpredictedresultsonthesinteringofAl2O3tobeinqual-itativeagreementwithexperimental observations,116ascanbeseeninFig.34.Olevskyetal.114alsoinvestigatedtheeffectofheatingrateondensicationand, as was pointedout above,their results show anenhancement of consolidation with an in-crease in the heating rate. Figure 35 shows the predicted effect ofheatingrate onshrinkage of aluminumpowders. Inanotherstudy, Olevsky and Froyen122incorporated electromigrationintoaconstitutive model for PECSsinteringandconcluded2030405060708090929496981001020 100 200 300 400Grain Size, nm% Theoretical DensityHeating Rate, C/min.Fig. 33. Effect of the heating rate on the density and grain size of eightYSZheated up to 11801Cwithno holding time under500MPa.105Fig. 31. Pore size distribution curves determined from gas desorption measurements for a reactive mixture of ZrO2 and Y2O3 obtained by compactionunder(a)4MPa (b)8MPa (c)95 MPaand (d)400 MPa.108Fig. 32. Timeevolutionoftherelativedensityofeight YSZsamplesprocessed using varying displacement controlrates at 12001C.11214 Journalof theAmericanCeramic SocietyMuniret al. Vol. 94,No.1thatmasstransportduetothiseffectcanprovideasignicantcontributiontothesinteringprocess. Thisprovidesanalyticalsupport for the experimental observations discussed above:(a)thedirect sintering enhancement dueto theactionof acur-rent for the case of copper spheres to copper plates90and tung-sten wires to tungsten plates (N. Toyofuku, T. Kuramoto,T. Imai, M. Ohyanagi andZ. A. Munirunpublishedresults)and (b) for the observed reactivity enhancements in the PECS bytheactionof thecurrent.8789Numerous other simulation studies have focused on the tem-peratureandcurrentdistributionsforconductingandnoncon-ducting materials under PECS conditions.65,110,111,123129Inadditiontoelucidatingthetemperatureandelectricalpotentialdistributions, Wangetal.130alsosimulatedthestressdistribu-tioninSPSexperimentsandfoundthat stress gradients,whichdepended on materials properties (coefcient of thermal expan-sion, CTE, andmodulus), werelargerthanthermal gradients.Stress gradients, both in the vertical and in the radial directions,were found to be signicant, especially for materials with a highCTE, whichindicatesthatsimplecalculationsofstressonthebasis oftheforce andsampleareamay not bevalid. The inu-ence of percolation on the sintering of ZrO2TiN composites bySPS was investigated experimentally and by simulation by Van-meensel et al.131,132The inuence of addingTiN to zirconia onthetemperature and current distribution wassimulated.Theeffect of pressureonthehomogeneityof SPS-sinteredtungstencarbidewasinvestigatedexperimentallyandbysimu-lation by Grasso et al.133Taking into account the electrothermalcontact resistance change due to sample shrinkage and the con-comitant punch sliding, and using a moving-mesh nite elementmodel,129theauthors showedtheeffect of pressureongraingrowth,residualporosity,andhardnessdistributionsalongthesample radius. Increasing sintering pressure resulted in a reduc-tioninthesinteringtemperature, aconclusionconsistentwithpreviously reported observationson oxideceramics.104VI. Consolidation of Functional MaterialsInSectionII,,weprovidedexamplesofthereportedadvan-tages of the PECS process. In this section, we highlight selectedaccomplishments focusing onfunctional materials, includingtransparent materials,electroceramics,and porous materials.(1) TransparentMaterialsEffortsaimedatobtainingtransparentmaterialsfocusontheparameters of density and grain size. Pores and grain boundariesare light-scatteringregions, but as has beenshown, porosityplaysamoredeterminingrole.134Adecreaseinporesize(tonanometricscale) leadstoadecreaseinscattering, acircum-stance that has led to the motivation to prepare nanostructuresasameansofobtainingtransparency.134Successinobtainingtransparent nanostructuredtetragonal andcubiczirconiawasdemonstrated, Fig. 36.135The effect of sinteringpressure ontransparency for bothmodications of zirconiais showninFig. 37; for any given temperature, increasing the pressurechangedthesamples from translucent to transparent.Successhasbeenshowninthepreparationof avarietyoftransparent materials by PECS processing. A transparentMgAl2O4spinel waspreparedbyreducingporosityandgrainsizebycontrollingtheheatingrate(o101C/min).39Moritaandcolleagues found that high heating rates enhanced the formationofclosedporosityduringtheheatingprocess, withtheclosedpores remaining at grainboundary junctions. Figure 38 depictstheSEMimagesshowingtheeffectoftheheatingrateonthemicrostructure of samples heated at 100 and 101C/min.39Othertransparent materialshavealsobeenpreparedsuccessfullybyusing the PECS method. These include alumina, AlN ceramics,mullite,YAG, cubic zirconia,spinel,and others.136144Fig. 34. Porositykinetics duringpulsedelectric current sinteringofaluminapowder.121Comparisonof thedevelopedmodel takingintoaccounttheimpactofthermaldiffusionwiththeexperimentaldataofShen etal.116Fig. 35. Simulations on the shrinkage kinetics of aluminum powder.114Fig. 36. (a) Sample of 1-mm-thick YSZ 8%densied at 10001Cunder apressureof600MPa; (b)sampleof1-mm-thickYSZ3%densiedat10001Cunderapressureof800MPa.Forbothsamples,theholdtimewas 5 min.135100200300400500600700800900850 900 950 1000 1050Pressure (MPa)Temperature (C)100200300400500600700800900900 950 1000 1050 1100Pressure (MPa)Temperature (C)YSZ 3% YSZ 8%TransparentTransparentTranslucentTranslucent(a) (b)Fig. 37. Combinedtemperatureandpressureconditionsrequiredforoptical transparency in (a) YSZ 3% and (b) YSZ 8% sintered powders.Transparencylimitsetat 10%of transmittanceatawavelengthof 600nm fora 1-mm-thicksample.135January 2011 Electric CurrentActivation of Sintering 15(2) Porous MaterialsTypically, the use of the PECS method is directed towardobtainingdensematerials; however, themethodhasalsobeenused to obtain porous materials. Kunetal.145obtainedporousstainless steel and found it to have compressive strength superiortothatofsamplespreparedbyhotpressing. Throughmodi-cationofdiedesign,itispossibletoobtainatemperaturegra-dient alongthevertical axisof thedie, andwiththis, obtainFGMs. Suk et al.146used this approach to obtain porous struc-tures of tungsten FGM with porosity and pore size distribution,as shown in Fig. 39. Using TiH2 powder as an additive to formpores upon decomposition, Zhao and Taya147prepared porousNiTialloysbyPECS.OthernanocrystallinealloysofTiwerepreparedinporous formfor biomedical applications,148andother porous materials for biomedical (implant) applicationshave alsobeenpreparedsuccessfully using the PECSmeth-ods.149,150Porousalumina, boronnitride, andothermaterialshave alsobeenpreparedsuccessfully.151,152(3) ElectroceramicsTheconcept of thesizeeffectinelectroceramicshasbeenthefocus of a number of investigations. The aimis to demonstratetheeffect of grainsizeontheelectrical properties of variousoxide ceramics. 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Munir, Pres-sureEffectsandGrain GrowthKineticsintheConsolidationofNanostructuredFullyStabilized ZirconiabyPulsedElectricCurrentSintering,ActaMater.,58,502230 (2010).106R. J. Stokes andD. Evans, Fundamentals of Interfacial Engineering, pp.2433. Wiley, NewYork,NY,1996.107G. Y. Onoda and J. Toner, Fractal Dimensions of Model Particle PackingsHaving Multiple Generations of Agglomerates, J. Am. Ceram. Soc., 69, C2789(1986).108M. A. C. G. Van de Graaf, J. H. H. Ter Maat, and A. J. Burgraaf, Micro-structureandSinteringKineticsofHighlyReactiveZrO2Y2O3,J.Mater.Sci.,20,140718 (1985).109S. Grasso, Y. Sakka, and G. Maizza, Pressure Effects on Temperature Dis-tributionDuringSpark PlasmaSinteringwithGraphiteSample,Mater.Trans.,50 [8] 21114 (2009).110A. Zavaliangos, J. Zhang, M. Krammer, andJ. R. Groza, TemperatureEvolutionDuring Field ActivatedSintering, Mater. Sci. Eng., A379, 2182(2004).111K.Vanmeensel,A.Laptev,J.Hennicke,J.Vleugels,andO.VanderBiest,Modellingof theTemperatureDistributionDuringFieldAssistedSintering,Acta Mater.,53,437988 (2005).112J. Xu, S. R. Casolco, and J. E. Garay, Effect of Varying Displacement RatesontheDensicationof NanostructuredZirconiaby Current Activation,J. Am.Ceram.Soc.,92 [7] 150613(2009).113R. M. German, Sintering Theory and Practice, p. 482. Wiley, New York, NY,1996.114E.A. Olevsky, S.Kandukuri, andL.Froyen, ConsolidationEnhancementinSpark-plasmaSintering:ImpactofHighHeatingRates,J.Appl.Phys.,102,114913,12pp (2007).115L. A. Stanciu,V. Y. Kodash,andJ. R. Groza, Effects of Heating Rate onDensicationandGrainGrowthDuringField-assistedSinteringof a-Al2O3andMoSi2Powders,Metall. Mater.Trans. A,32A,26338 (2001).116Z. Shen, M. Johnsson, Z. Zhao, and M. Nygren, Spark Plasma Sintering ofAlumina,J. Am.Ceram. Soc.,85 [8] 19217 (2002).117Y. Zhou, K. Hirao, Y. Yamauchi, and S. Kanzaki, Effects of Heating Rateand Particle Size on Pulse Electric Current Sintering of Alumina, Scr. Mater., 48,16316 (2003).118U. Anselmi-Tamburini, J. E. Garay, Z. A. Munir, A. Tacca, F. Maglia, andG. Spinolo, SparkPlasmaSinteringandCharacterizationofBulkNanostruc-tured Fully StabilizedZirconia: PartI. Densication Studies,J. Mater.Res.,19[11]325562(2004).119K. Matsugi, H. Kuramoto, O. Yanagisawa, and M. Kiritani, A Case StudyforProductionofPerfectlySinteredComplexCompactsinRapidConsolidationby Spark Sintering,Mater. Sci. Eng.,A354,23442 (2003).120B. McWilliams and A. Zavaliangos, Multi-Phenomena Simulation of Elec-tricFieldAssisted Sintering,J. Mater.Sci.,43,50315 (2008).121E. A. Olevsky and L. Froyen, Impact of Thermal Diffusion on DensicationDuringSPS,J. Am.Ceram.Soc.,92 [S1] S12232(2009).122E. Olevsky and L. Froyen, Constitutive Modeling of Spark-plasma Sinteringof Conductive Materials, Scr. Mater.,55,11758 (2006).123J. Rathel, M. Herrmann, andW. Beckert, TemperatureDistributionforElectrically Conductive and Non-conductive Materials During Field-AssistedSintering (FAST), J. Eur. Ceram.Soc.,29,141925(2009).124D. Tiwari, B. Basu, andK. Biswas, SimulationsofThermal andElectricField Evolution During Spark Plasma Sintering, Ceram. Int., 35, 699708(2009).125A. Cincotti, A. M. Locci, R. Orru, andG. Cao, ModelingofSPSAppa-ratus: Temperature, Current and Strain Distribution with no Powders, A. I. Ch.E.J.,53 [3]70319 (2007).126R. S. Dobedoe, G. D. West, andM. H. Lewis, SparkPlasmaSinteringof Ceramics: UnderstandingTemperatureDistributionEnables MoreRealisticComparisonwithConventional Processing, Adv. Appl. Ceram., 104[3] 1106(2005).127N.Chennou, G. Majkic, Y. C. Chen,andK.Salama, Temperature, Cur-rent, and Heat Loss Distributions in Reduced Electrothermal Loss Spark PlasmaSintering,Metall. Mater.Trans. A,40A,24019 (2009).128X. Liu, X. Song, J. Zhang, andS. Zhao, TemperatureDistributionandNeck Formation of WC-Co Combined Particles During Spark Plasma Sintering,Mater.Sci.Eng.,A488,17(2008).129G.Maizza,S.Grasso,andY.Sakka, MovingFinite-elementMeshModelfor Aiding Spark Plasma Sintering in Current Control Mode of Pure Ultrane WCPowder, J. Mater.Sci.,44,121936(2009).18 Journalof theAmericanCeramic SocietyMuniret al. Vol. 94,No.1130X. Wang, S. R. Casolco, G. Xu, and J. E. Garay, Finite Element Modelingof Electric Current-activated Sintering: The Effect of Coupled Electrical Potential,Temperatureand Stress, ActaMater.,55,361122(2007).131K. Vanmeensel, A. Laptev, O. Van der Biest, and J. Vleugels, Field AssistedSintering of Electro-Conductive ZrO2-based Composites, J. Eur. Ceram. Soc., 27,97985 (2007).132K. Vanmeensel, A. Laptev, O. Van der Biest, and J. Vleugels, The Inuenceof PercolationDuringPulsedElectricCurrent Sinteringof ZrO2TiNPowderCompacts withVarying TiNContent,Acta Mater.,55,18011 (2007).133S.Grasso,Y.Sakka,G.Maizza,andC.Huz,PressureEffectontheHo-mogeneityofSparkPlasma-SinteredTungstenCarbidePowder,J.Am.Ceram.Soc.,92 [10] 241821(2009).134R. Apetz and P. B. van Bruggen, Transparent Alumina: A Light-scatteringModel,J. Am.Ceram.Soc.,86 [3]4806(2003).135U. Anselmi-Tamburini, J. N. Woolman, andZ. A. Munir, TransparentNanometricCubicandTetragonal ZirconiaObtainedbyHigh-pressurePulsedElectric Current Sintering,Adv.Funct.Mater.,17,326773(2007).136B-N. Kim, K. Hiraga, K. Morita, H. Yoshida, T. Miyazaki, and Y. Kagawa,MicrostructureandOptical PropertiesofTransparentAlumina, ActaMater.,57,131926(2009).137Y. Xiong,Z. Fu, Y. Wang, and F. Quan,Fabrication of Transparent AlNCeramics,J. Mater.Sci.,41,25379 (2006).138D. Jiang, D. M. Hulbert, U. Anselmi-Tamburini, T. Ng, D. Land, and A. K.Mukherjee, Optically Transparent Polycrystalline Al2O3Produced by SparkPlasmaSintering, J. Am.Ceram.Soc.,91 [1] 1514 (2008).139G. Zhang, Y. Wang, Z. Fu, H. Wang, W. Wang, J. Zhang, S. W. Lee, and K.Nihara,TransparentMulliteCeramicfromSingle-PhaseGel bySparkPlasmaSintering,J. Eur. Ceram.Soc.,29,270511(2009).140R. Chaim, Z. Shen, and M. Nygren, Transparent Nanocrystalline MgO byRapidandLow-Temperature SparkPlasmaSintering, J. Mater. Res., 19[9]252731(2004).141K. Morita, B-N. Kim, K. Hiraga, andH. Yoshida, FabricationofHigh-strength Transparent MgAl2O4SpinelPolycrystals by Optimizing Spark-Plasma-SinteringConditions, J. Mater.Res.,24 [9] 286372 (2009).142R. Chaim, R. Marder, and C. Estournes, Optically Transparent Ceramics bySpark Plasma Sintering of Oxide Nanoparticles, Scripta Mater., 63, 2114 (2010).143R. Chaim, M. Kalina, andJ. Z. Shen, Transparent YttriumAluminumGarnet (YAG)CeramicsbySparkPlasmaSintering, J. Eur. Ceram. Soc., 27,33317(2007).144J. E. Alaniz, F. G. Perez-Gutierrez, g. Aguilar, andJ. E. Garay, OpticalProperties of Transparent Nanocrystalline Yttria Stabilized Zirconia, Opt.Mater.,32,628 (2009).145W.Kun,F.Zhengyi,W. Weimin,W.Yucheng,Z.Jinyong,andZ.Qingjie,StudyonFabricationandMechanisminof PorousMetalsbySparkPlasmaSintering,J. Mater.Sci.,42,3026(2007).146M-J. Suk, W-S. Seo, and Y-S. Kwon, Fabrication of Graded Porous Struc-turewithPoresizeDistributionbySPSProcess, Mater. Sci. Forum, 534536,9658(2007).147Y.ZhaoandM.Taya,ProcessingofPorousNiTibySpark PlasmaSinte-ring, Proc.SPIE,6170,617013,6pp (2006).148R. Nicula, F. Luethen, M. Stir, B. Nebe, andE. Burkel, SparkPlasmaSinteringSynthesis of Porous NanocrystallineTitaniumAlloys for BiomedicalApplications, Biomol. Eng.,24,5647(2007).149D. Kawagoe, R. Sawai, andT. Ishiduka, PreparationofPorousHydrox-yapatite Ceramics by Spark Plasma Sintering, Trans. Mater. Res. Soc. Jpn., 33 [4]9114(2008).150F. Zhang, K. Lina, J. Changa, J. Lua, and C. Ning, Spark Plasma Sinteringof MacroporousCalciumPhosphateScaffoldsfromNanocrystallinePowders,J. Eu.Ceram. Soc.,28,53945 (2008).151D. Chakravarty, H. Ramesh, and T. N. Rao, High Strength PorousAlumina by Spark PlasmaSintering,J. Eur.Ceram. Soc.,29,13619 (2009).152P. Dibandjo, L. Bois, C. Estournes, B. Durand, and P. Miele, Silica, Carbonand BoronNitride Monoliths with Hierarchical Porosity Prepared by SparkPlasmaSinteringProcess,Micropor. Mesopor.Mater.,111,6438(2008).153S. Kim, U. Anselmi-Tamburini, H. J. Park, M. Martin, andZ. A. Munir,Unprecedented Room-temperature Electrical Power Generation UsingNanoscale Fluorite-structured Oxide Electrolytes, Adv. Mater., 20, 5569(2008). &Zuhair A. Munir is a Distin-guishedResearchProfessorintheDepartment of Chemical Engi-neering and Materials Scienceandformer Deanof the Collegeof Engineering at the University ofCalifornia, Davis (UC Davis). Hereceivedall his degrees fromtheUniversity of California, Berkeley.Hisresearchisin thegeneralareaof kinetics and thermodynamics ofmaterialsprocessingwithempha-sisoneld-activated processes. Professor Munir is therecipientof numerousawardsandhonorsincludingtheSocietysJohnJeppson Award and the Outstanding Educator Award. Hereceived the Gold Medal from the Russian Academy of Sciencesfor contribution to the science of self-propagating high-tempera-ture synthesis (SHS), andreceivedthe UCDavis Prize, thecampus highest award for scholarly distinction and outstandingteaching.ProfessorMunirhaspublishedmorethan475papersand holds 13 U.S. patents. He is listed as a Highly Cited Authorin Materials Science. His service for the Society includes Associ-ate Editor and member of the Task Force on Globalization andthe Phase Equilibria Subcommittee. He also served as Editor-in-ChieffortheJournalofMaterialsSynthesisandProcessingasPrincipalEditorfortheJournalofMaterialsResearch,andasEditorfortheJournal ofMaterialsScience.Dr. Dat V. Quachis aPostdoc-toral ResearchFellowintheDe-partment of Chemical EngineeringandMaterialsScienceattheUni-versityof California, Davis. Hismain research focuses on chemicalchanges and phase stability of na-nostructured materials processedunder electric eld applicationsandonthe inuence of externalelds on consolidation. He is alsointerestedinresearchonalternative energy sources andhasworked on template-based synthesis of thin lms and nanowiresofskuterruditesforthermoelectricapplications. Quachwasabronzemedalistatthe29thInternational ChemistryOlympiadinMontreal,Canada. HegraduatedwithacombinedB.S. inMechanical Engineering +Materials Science and recentlyreceivedaPh.D.inMaterials ScienceandEngineering.Manshi Ohyanagi isDeanoftheFacultyof Science andTechnol-ogy and Director of InnovativeMaterials and Processing Re-search Center at Ryukoku Uni-versity in Ohtsu, Japan. HereceivedhisdoctorateinAppliedChemistry at WasedaUniversityin 1988 and spent a year as apostdoctoral fellow at Caltech.Afterthat, hejoinedtheDepart-ment of Materials Chemistry atRyukokuUniversityas Assistant Professor. In2001, he waspromotedtoFull Professor. In2002-2003, he was aVisitingProfessor at the Universityof California, Davis. His currentresearch focuses on Spark Plasma Sintering (SPS) of nanostruc-tured ceramics, with emphasis on the sintering of high tempera-ture materialswith stackingdisorder-ordertransformation.Hisresearch also includes the processing of hydrogen storage mate-rials. He servedas coorganizer of several symposiaonSPS,associatedwithPacRimconferences. Hehaspublished4110papersandholds 10 U.S.patents.January 2011 Electric CurrentActivation of Sintering 19


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