9781420038903%2Ech10

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1、10ANewUpperLimittotheThermoelectricFigure-of-Merit10.1Introduction...................................................................10-110.2NewMaterials................................................................10-210.3Low-DimensionalSystems............................................10-310.4T

2、heMaximumZTforLow-DimensionalStructures........................................................................10-310.5Solid-StateThermionicConverters..............................10-510.6PossibilityofIndependentElectronandPhononCurrents...........................................................10-6

3、H.J.Goldsmid10.7VacuumThermoelements.............................................10-8UniversityofNewSouthWales10.8Conclusions....................................................................10-810.1IntroductionThermodynamicsdoesnotplaceanyupperlimitonthedimensionlessthermoelectricfigure-of-merit,Z

4、T,butformanyyearsitwasimpossibletofindvaluessignificantlygreaterthanunity.Empirically,onecanpredictanupperboundbycombiningthebestelectronicpropertiesofanyknowncrystallinematerialwithaphononconductivitythatistypicalofglass.Bythismeans,onearrivesatahighestvalueofZTequaltoaboutfour.Recently,however,ithas

5、beendemonstratedthatcertainlow-dimensionalstructurescanhavevaluesofthefigure-of-meritthatexceedthosefoundinbulkmaterials.ThisallowsustoupgradeourpredictedlimitforZTto,perhaps,20.Ifthisvaluewereeventuallyachieved,thermoelectricenergyconverterswouldhavenolessthanabout50%oftheefficiencyofaCarnotcycle.The

6、performancewouldthenbecomparablewiththebestthatislikelytobeachievedusingconvertersthatarebasedonthevacuumdiode.Severalattemptshavebeenmadeinthepasttoestimatethemaximumthermoelectricfigure-of-meritthatwilleverbereached.ThereisnorestrictionplacedonthevalueofZTbythermodynamicssince,ifthisquantityeverbec

7、ameinfinite,thethermoelectricefficiencywouldstillbenogreaterthanthatoftheCarnotcycle.Realisticconsiderationssuggestthatthermocoupleswillneverapproachthisidealefficiency.Atroomtemperature,thebestthermoele