材料科学与工程导论双语课件教学课件 作者 陈克正 王玮 刘春延 等编Chapter 8.ppt

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1、INTRODUCTIONTOMATERIALSSCIENCEANDENGINEERINGQingdaoUniversityofScience&TechnologyKezhengChenWeiWangChuntingLiu国家级双语教学示范课程ElectricalpropertiesThermalpropertiesMagneticpropertiesOpticalproperties8.48.18.28.3CHAPTER8ELECTRICAL,THERMAL,MAGNETICANDOPTICALPROPERTIES

2、OFMATERIALS8.1ELECTRICALPROPERTIESElectricalconductionHowmanymoveableelectronsarethereinamaterial(carrierdensity)?Howeasilydotheymove(mobility)?BASICLAWSWhenanelectricalpotentialV[volts,J/C]isappliedacrossapieceofmaterial,acurrentofmagnitudeI[amperes,C/s]flows

3、.Inmostmetals,atlowvaluesofV,thecurrentisproportionaltoV,andcanbedescribedbyOhm'slaw:I=V/RwhereRistheelectricalresistance[ohms].Rdependsontheintrinsicresistivityofthematerialandonthegeometry(lengthlandareaAthroughwhichthecurrentpasses):R=l/AInmostmaterials(e

4、.g.metals),thecurrentiscarriedbyelectrons(electronicconduction).Inioniccrystals,thechargecarriersareions(ionicconduction).ELECTRICALPROPERTIESOFMETALSTheelectricalconductivity(theabilityofasubstancetoconductanelectriccurrent)istheinverseoftheresistivity:=1/E

5、lectricalconductivityvariesbetweendifferentmaterialsbyover27ordersofmagnitude,thegreatestvariationofanyphysicalpropertyENERGYBANDSTRUCTURESINSOLIDSInanisolatedatomelectronsoccupywelldefinedenergystates,asdiscussedinChapter2.Whenatomscometogethertoformasolid,t

6、heirvalenceelectronsinteractwitheachotherandwithnucleiduetoCoulombforces.Inaddition,twospecificquantummechanicaleffectshappen.First,byHeisenberg'suncertaintyprinciple,constrainingtheelectronstoasmallvolumeraisestheirenergy,thisiscalledpromotion.Thesecondeffect

7、,duetothePauliexclusionprinciple,limitsthenumberofelectronsthatcanhavethesameenergy.ENERGYBANDSTRUCTURESINSOLIDSAsaresultoftheseeffects,thevalenceelectronsofatomsformwideelectronenergybandswhentheyformasolid.Thebandsareseparatedbygaps,whereelectronscannotexist

8、.ENERGYBANDSTRUCTURESINSOLIDSConductionband-apartiallyfilledoremptyenergybandValenceband–thehighestpartiallyorcompletelyfilledbandInsemiconductorsandinsulators,thevalencebandisfill