n型Bi2Te3系热电材料成型与性能研究

华中科技大学硕士学位论文n型Bi,2Te,3系热电材料成型与性能研究姓名：陈柔刚申请学位级别：硕士专业：材料学指导教师：杨君友20050701IBi2Te3BridgmanBi2Te3nBi2Te3nBi2Te3XRDDTA4Bi2Te33404Z1.710-3/KAgSn0.20wt.%AgZ1.310-3/K1.710-3/KSnSnTeBi2Te3Bi2Te3II450400183610IIIAbstractThermoelectric(TE)materials,whichcanachievesolid-statemutualdirectconversionofelectrical-thermalenergy,areofgreatadvantageforsomespecialapplication.BismuthtelluridebasedmaterialsarethemostcommonlyusedTEmaterial,theyareusuallypreparedbycrystalgrowthmethods(e.g.Bridgmanmethod),buttheirmachinabililyisquitelowduetoitsbrittleness.Sopowdermetallurgyrouteiscommonlyusedtoovercomethisdemerit.Traditionalpowdermetallurgicalmethodneedstopremelttoobtainanalloyingot,andthenpulverizetheingotandsintertoformanequiaxialmicrostructure.Thewholeprocessiscomplicatedandofquitehighcost.Inthefirstpartofthisthesis,thedevelopmentofthermoelectricmaterialsissummarizedfirst,andpowdermetallurgyofBi2Te3-basedmaterialsisthenintroduced.Mechanicalalloying(MA)isapowerfulmethodinthepreparationofpowders,whichcandirectlyalloytherawmaterialsintotargetpowdersinarelativelylowcostway.Inthiswork,MAmethodisusedtopreparen-typeBi2Te3-basedalloypowders,andhotpress(HP)methodisusedtoconsolidethepowdersintobulkmaterial.InordertoimprovetheTEpropertiesoftheasHP-edmaterials,someexploringworkofhotextrusion(HE)hasbeendone.TheMAprocessofthen-typeBi2Te3-basedmaterialisstudiedbyusingXRDandDTAanalysismethod.4hourMAisprovedtobeenoughtoobtainthesinglephaseBi2Te3basedalloypowders.Withtheobtainedalloypowder,hotpresssinteringexperimentsareconducted.Itshowsthatwiththeincreaseofsinteringtemperature,TEpropertiesincrease.SEManalysisshowsthatthe340HP-edsampleisnotsinteredwellenough.ProlongingHPtimeishelpfultoimproveTEproperties,butovertimeHPsinteringmaycauseabnormalgrowthofgrains,whicharenotfavorablefortoTEproperties.The4-hour-HPedsamplehasthehighestTEproperties,whichshowsafigure-of-meritofZ=1.710-3/K.InordertomodulatetheTEpropertiesofHP-edsamples,AgandSndopingaretried.Itshowsthat,adopingcontentof0.20wt.%Agadditioncouldimprovethefigure-of-meritIVfrom1.310-3/Kfortheundopedsampleto1.710-3/K,whileforSn-dopedmaterials,heavyadditionoftinproducesanegativeeffect,becausesomeTe-richsecondphaseappears,whichcausesthedeteriorationofTEproperties.Athoughgoodmechanicalpropertycanbeachievedthroughpowdermetallurgybylittlegrainsize,yettheanisotropyofBi2Te3-basedmaterialsislostduetoitspolycrystallinemicrostructure.Thus,hotextrusion(HE)processofBi2Te3-basedmaterialisconductedtoimprovetheTEpropertiesoftheHP-edsamples.Theappliedforceanalysis,stressfieldanalysisbyusingplasticmoldingsimulationsoftwareandexperimentresultsareusedtofindthesourceofthebadsurfacequalityofHE-edsample,andtheHEprocessaremended.MicrostructureobservationandXRDanalysisshowthatHEprocesscanachieveanorientedmicrostructure,andtheTEpropertiesincrease.AfurtherefforttoimprovetheTEpropertiesisannealingtreatment.InourexperimentofannealingoftheHP-edandtheHE-edsamples,inthecaseofatemperaturehigherthan450,airholeinthesamplesexpandsandthedensitydecreasessharply,sodotheTEproperties.Withanannealingtemperatureof400,theTEpropertiesoftheannealedsampleshowa-shapedchangeastheannealingtimeprolonged.ForHE-edsampleannealedfor18hours,thepowerfactorshowsanincreaseof36%.Pulseactivatesintering(PAS)hasbeentriedtosintertheMA-edpowderalso.Theresultshowsthatthesamples’densityisquitehighevenwithasinteringtimeof10minutes,andtheTEpropertiesofthePAS-edsamplesareclosetothatoftheHP-edsample.Micrographanalysisshowssomedegreeoforientationhasbeenaquiredwithashortsinteringtime.Keywords:thermoelectricmaterial;mechanicalalloying;hotpress;doping;hotextrusion;annealing;pulseactivatesintering11[1-5]1.11.1.1SeebeckPeltierThompson[6]KelvinSeebeckPeltier1823SeebeckSeebeck1.1a12PeltierPeliterSeebeck1838LenzPeltier1.1b1855ThomsonSeebeckPeltierThomsonThomson2(a)(b)1.1aSeebeckbPeltierFig.1.1Schematicdrawingfor(a)Seebeckeffectand(b)Peltiereffect1911AltenkirchSeebeckFigureofmerit32Zask=1.1asSeebeckkWiedemann-FranzZaa10μV/K0.6%30Seebeck100μV/K1947Telkes5%1949IoffeIoffeBi2Te3PbTeSiGe5060Bi2Te3ZTT1Ioffe[7,8]41977RTG[9]12spot-cooling[10]34[11-13]1.1.2SkutteruditeCoSb3[13](clathrate)[14]-Zn4Sb3[15]Half-Heusler[16][17]Ca-Co-O[18,19,20]Na-Co-O[21,22,23]Seebeck[24]5[25][26]spot-coolingRTIRamaVenkatasubramanian[8]MOCVDZT2.4Bi/Te32KJPLG.J.Snyder[27]Bi2Te3,Bi2Te3/Sb2Te3[28][6]ZT=4ZT=1ZT31.21.2.1JnJqTeTTeTJn1122∇⎟⎟⎠⎞⎜⎜⎝⎛+∇⎟⎠⎞⎜⎝⎛-=safs1.2TkeJTJnq∇-=a1.36JnJqsfkaSeebeckeTSeebeckTVSΔΔ=1.4ZZT(figureofmerit)PF(PowerFactor)sa2=PF1.5krkLekkk+=1.61.2.2SeebeckPeltiernpp-n(a)(b)1.2Fig1.2SchematicDiagramsforthermoelectricpowergenerationandrefrigeration7npnp1.2(b)np[3]21212111T/TTZTZTTTmax++-+-=h1.7T2T12/)(21TTT+=⎟⎟⎠⎞⎜⎜⎝⎛+-=ppnnpnZkrkraa1.8knkpnprnrpnpZT3ZT1.34SeebeckaaZTnn=1019/cm3nWiedemann-Franz232ebTLTekks⎛⎞=-=⎜⎟⎝⎠(1.9)kbBoltzmanLLorenz2.4510-8W/K2[29]8cage-likestructure[30][31]1.3Bi2Te3Bi2Te3pnZT1TEPbTeSiGeBiSb1.3.1Bi2Te3Bi2Te3VBVIB585[32]Bi2Te31.3Bi2Te31.3Bi-TeFig.1.3BinaryphasediagramofBi2Te39BiBi40.065%7.6828103kg.m-37.8587103kg.m-3Bi2Te3BiBiTeBi2Te3PPPbCdSnpBi2Te3TeIBrSeSbI3AgICuBrBiI3nB