seebeck effectseebeck 效应.docx

seebeck effectseebeck 效应.docx

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seebeckeffectseebeck效应

Thermoelectriceffect

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Thisarticleisaboutthethermoelectriceffectasaphysicalphenomenon.Forapplicationsofthethermoelectriceffect,seethermoelectricmaterialsandthermoelectriccooling.

Thermoelectriceffect

Principles[hide]

Thermoelectriceffect（Seebeckeffect,

Peltiereffect,Thomsoneffect） ·Thermopower（Seebeckcoefficient） ·Ettingshauseneffect ·Nernsteffect

Applications[show]

Thermoelectricmaterials ·Thermocouple ·Thermopile ·Thermoelectriccooling ·Thermoelectricgenerator ·Radioisotopethermoelectricgenerator ·Automotivethermoelectricgenerator

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Thethermoelectriceffectisthedirectconversionoftemperaturedifferencestoelectricvoltageandvice-versa.Athermoelectricdevicecreatesavoltagewhenthereisadifferenttemperatureoneachside.Conversely,whenavoltageisappliedtoit,itcreatesatemperaturedifference.Attheatomicscale,anappliedtemperaturegradientcauseschargecarriersinthematerialtodiffusefromthehotsidetothecoldside,similartoaclassicalgasthatexpandswhenheated;henceinducingathermalcurrent.

Thiseffectcanbeusedtogenerateelectricity,measuretemperatureorchangethetemperatureofobjects.Becausethedirectionofheatingandcoolingisdeterminedbythepolarityoftheappliedvoltage,thermoelectricdevicesareefficienttemperaturecontrollers.

Theterm"thermoelectriceffect"encompassesthreeseparatelyidentifiedeffects:

theSeebeckeffect,PeltiereffectandThomsoneffect.TextbooksmayrefertoitasthePeltier–Seebeckeffect.ThisseparationderivesfromtheindependentdiscoveriesofFrenchphysicistJeanCharlesAthanasePeltierandEstonian-GermanphysicistThomasJohannSeebeck.Jouleheating,theheatthatisgeneratedwheneveravoltageisappliedacrossaresistivematerial,isrelatedthoughitisnotgenerallytermedathermoelectriceffect.ThePeltier–SeebeckandThomsoneffectsarethermodynamicallyreversible,[1]whereasJouleheatingisnot.

Contents

 [hide] 

∙1Seebeckeffect

o1.1Thermopower

o1.2Charge-carrierdiffusion

o1.3Phonondrag

∙2Peltiereffect

∙3Thomsoneffect

o3.1Thomsonrelations

∙4Figureofmerit

∙5Deviceefficiency

∙6Applications

o6.1Seebeckeffect

o6.2Peltiereffect

o6.3Temperaturemeasurement

∙7Seealso

∙8References

∙9Externallinks

[edit]Seebeckeffect

DiagramofthecircuitonwhichSeebeckdiscoveredtheSeebeckeffect.AandBaretwodifferentmetals.

TheSeebeckeffectistheconversionoftemperaturedifferencesdirectlyintoelectricityandisnamedforGerman-EstonianphysicistThomasJohannSeebeck,who,in1821discoveredthatacompassneedlewouldbedeflectedbyaclosedloopformedbytwometalsjoinedintwoplaces,withatemperaturedifferencebetweenthejunctions.Thiswasbecausethemetalsrespondeddifferentlytothetemperaturedifference,creatingacurrentloopandamagneticfield.Seebeckdidnotrecognizetherewasanelectriccurrentinvolved,sohecalledthephenomenonthethermomagneticeffect.DanishphysicistHansChristianØrstedrectifiedthemistakeandcoinedtheterm"thermoelectricity".Thevoltagecreatedbythiseffectisoftheorderofseveralmicrovoltsperkelvindifference.Onesuchcombination,copper-constantan,hasaSeebeckcoefficientof41microvoltsperkelvinatroomtemperature.[2]

ThevoltageVdevelopedcanbederivedfrom:

whereSAandSBarethethermopowers（Seebeckcoefficient）ofmetalsAandBasafunctionoftemperatureandT1andT2arethetemperaturesofthetwojunctions.TheSeebeckcoefficientsarenon-linearasafunctionoftemperature,anddependontheconductors'absolutetemperature,material,andmolecularstructure.IftheSeebeckcoefficientsareeffectivelyconstantforthemeasuredtemperaturerange,theaboveformulacanbeapproximatedas:

TheSeebeckeffectisusedinthethermocoupletomeasureatemperaturedifference;absolutetemperaturemaybefoundbysettingoneendtoaknowntemperature.Ametalofunknowncompositioncanbeclassifiedbyitsthermoelectriceffectifametallicprobeofknowncomposition,keptataconstanttemperature,isheldincontactwithit.Industrialqualitycontrolinstrumentsusethisasthermoelectricalloysortingtoidentifymetalalloys.Thermocouplesinseriesformathermopile,sometimesconstructedinordertoincreasetheoutputvoltage,sincethevoltageinducedovereachindividualcoupleissmall.Thermoelectricgeneratorsareusedforcreatingpowerfromheatdifferentialsandexploitthiseffect.

[edit]Thermopower

Mainarticle:

Thermopower

ThethermopowerorSeebeckcoefficient,representedbyS,ofamaterialmeasuresthemagnitudeofaninducedthermoelectricvoltageinresponsetoatemperaturedifferenceacrossthatmaterial,andtheentropyperchargecarrierinthematerial.[3]ShasunitsofV/K,thoughμV/Kismorecommon.ValuesinthehundredsofμV/K,regardlessofsign,aretypicalofgoodthermoelectricmaterials.Theterm"thermopower"isamisnomersinceitdoesnotmeasurepower,butmeasuresthevoltageinducedinresponsetoatemperaturedifference.Anappliedtemperaturedifferencecauseschargedcarriersinthematerialtodiffusefromthehotsidetothecoldside.Mobilechargedcarriersmigratingtothecoldsideleavebehindtheiroppositelychargednucleiatthehotsidethusgivingrisetoathermoelectricvoltage.Sinceaseparationofchargescreatesanelectricpotential,thebuildupofchargedcarriersontothecoldsideeventuallyceasesatsomemaximumvaluesincetheelectricfieldisatequilibrium.Anincreaseinthetemperaturedifferenceresumesabuildupofchargecarriersonthecoldside,leadingtoanincreaseinthethermoelectricvoltage,andviceversa.Thematerial'stemperatureandcrystalstructureinfluenceS;typicallymetalshavesmallthermopowersbecauseofhalf-filledbandscausedbyequalnegativeandpositivechargescancellingeachothercontributingtotheinducedthermoelectricvoltage.Incontrast,semiconductorscanbedopedwithexcesselectronsorelectronholes,causingthemagnitudeofStobelarge.Thesignofthethermopowerdetermineswhichchargedcarriersdominatetheelectrictransport.[citationneeded]

Ifthetemperaturedifference

betweenthetwoendsofamaterialissmall,thenthethermopowerofamaterialisdefinedapproximately[4]as:

andathermoelectricvoltageofΔVisseenattheterminals.

Thiscanbewritteninrelationtotheelectricfield

andthetemperaturegradient

bytheapproximate[4]equation:

Theabsolutethermopowerofthematerialofinterestisrarelypracticallymeasuredbecauseelectrodesattachedtoavoltmetermustbeplacedontothematerialinordertomeasurethethermoelectricvoltage,inducingathermoelectricvoltageacrossonelegofthemeasurementelectrodes.Themeasuredthermopowerthenincludesthethermopowerofthematerialofinterestandthematerialofthemeasurementelectrodesandiswrittenas:

SuperconductorshaveS=0sincethechargedcarriersproducenoentropy.Thisallowsadirectmeasurementoftheabsolutethermopowerofthematerialofinterest,sinceitisthethermopoweroftheentirethermocouple.Inaddition,ameasurementoftheThomsoncoefficient

ofamaterialyieldsthethermopowerthroughtherelation

Sisanimportantmaterialparameterthatdeterminestheefficiencyofathermoelectricmaterial;alargerinducedthermoelectricvoltageandahigherSmeanahigherefficiency.

[edit]Charge-carrierdiffusion

TheSeebeckcircuitconfiguredasathermoelectriccooler.

Thesamecircuitconfiguredasagenerator.

TheSeebeckeffectiscausedbytwothings:

charge-carrierdiffusionandphonondrag.Chargecarriersinthematerialswilldiffusewhenoneendofaconductorisatadifferenttemperaturethantheother.Hotcarriersdiffusefromthehotendtothecoldend,sincethereisalowerdensityofhotcarriersatthecoldendoftheconductor,andviceversa.Iftheconductorwerelefttoreachthermodynamicequilibrium,thisprocesswouldresultinheatbeingdistributedevenlythroughouttheconductor（seeheattransfer）.Themovementofheat（intheformofhotchargecarriers）fromoneendtotheotherisaheatcurrentandanelectriccurrentaschargecarriersaremoving.

Inasystemwherebothendsarekeptataconstanttemperaturedifference,thereisaconstantdiffusionofcarriers.Iftherateofdiffusionofhotandcoldcarriersinoppositedirectionsisequal,thereisnonetchangeincharge.Thediffusingchargesarescatteredbyimpurities,imperfections,andlatticevibrationsorphonons.Ifthescatteringisenergydependent,thehotandcoldcarrierswilldiffuseatdifferentrates,creatingahigherdensityofcarriersatoneendofthematerialandanelectrostaticvoltage.

Thiselectricfieldopposestheunevenscatteringofcarriers,andanequilibriumisreachedwherethenetnumberofcarriersdiffusinginonedirectioniscanceledbythenetnumberofcarriersmovingintheoppositedirection.Thismeansthethermopowerofamaterialdependsgreatlyonimpurities,imperfections,andstructuralchangesthatvarywithtemperatureandelectricfield;thethermopowerofamaterialisacollectionofmanydifferenteffects.[citationneeded]

Earlythermocouplesweremetallic,butmanymorerecentlydevelopedthermoelectricdevicesaremadefromalternatingp-typeandn-typesemiconductorelementsconnectedbymetallicconnectors.Semiconductorjunctionsarecommoninpowergenerationdevices,whilemetallicjunctionsaremorecommonintemperaturemeasurement.Chargeflowsthroughthen-typeelement,crossesametallicinterconnect,andpas