材料的磁性能优秀PPT.ppt

材料的磁性能你现在浏览的是第一页，共20页ContentsOpticalPropertiesElectricalPropertiesThermalPropertiesMagneticProperties你现在浏览的是第二页，共20页OtherPropertiesofMaterials(ForSelf-Study)ElectricalProperties：Thermoelectric,PyroelectricandMagnetoelectricProperty (热电、热释电、磁电性能)MagneticProperties：MagnetostrictionandMagnetoresistance (磁致伸缩、磁电阻)OpticalProperties:Electrooptic,PhotoelectricandMagnetoopticProperty (电光、光电、磁光性能)AcousticProperties:Propagation,AbsorptionandElectroacousticProperty (声音的传播、吸收、电声性能)Elasticity:AnelasticityandInternalFriction (滞弹性与内耗)你现在浏览的是第三页，共20页4.MagneticProperties4.1BASICCONCEPTSMagneticDipoles(磁偶极子):smallbarmagnetscomposedofnorthandsouth.Magneticforces(fields)aregeneratedbymovingelectricallychargedparticles.MagneticfieldlinesofforcearoundacurrentloopandabarmagnetThemagneticmomentasdesignatedbyanarrow你现在浏览的是第四页，共20页Magneticfieldstrength(磁场强度)H:theexternallyappliedmagneticfield.H=NI/l (4.1)A cylindrical coil(or solenoid):N closely spaced turns,lengthl,current ofmagnitudeI.UnitofH:amperespermeter.Magneticfieldstrengthwithinacoildependenceonnumberofturns,appliedcurrent,andcoillength.Magnetic induction(磁感应强度)/magnetic flux density(磁通密度)B:themagnitudeoftheinternalfieldstrengthwithinasubstancethatissubjectedtoanHfield.B=H (4.2)Permeability(磁导率):ThepropertyofthespecificmediumthroughwhichtheHfieldpassesandinwhichBismeasured.Thepermeabilityhasdimensionsofwebersperampere-meter(Wb/Am)orhenriespermeter(H/m).BothBandHarefieldvectors.Magnetic flux density in a material dependence on permeability andmagneticfieldstrength.你现在浏览的是第五页，共20页In a vacuum:B0=0H (4.3)Permeabilityofavacuum(真空磁导率)0:auniversalconstant,whichhasavalueof4 x10-7(1.257x10-6)H/m.B0:thefluxdensitywithinavacuum.r=/0 (4.4)Relativepermeability(相对磁导率)r:a measure of the degree to which the material can be magnetized,or the ease with which a B field can be induced in the presence of an external H field.AcoilandtheH,B你现在浏览的是第六页，共20页Magnetization(磁化强度)M:B=0 H+0 M (4.5)Magneticsusceptibility(磁化率)m,(unitless):M=mH (4.6)Magnetizationofamaterialdependenceonsusceptibilityandmagneticfieldstrength.Relationshipbetweenmagneticsusceptibilityandrelativepermeability:m=r -1 (4.7)OriginsofMagneticMomentsEach electron in an atom has magnetic moments that originate from two sources:orbitalmomentsandspinmoments.Ineachindividualatom,orbitalmomentsofsomeelectronpairscanceleachother;thisalsoholdsforthespinmoments.The net magnetic moment for an atom is the sum of the magnetic moments of each of theconstituent electrons,including both orbital and spin contributions,and taking into accountmomentcancellation.For an atom having completely filled electron shells or subshells,there is totalcancellationofbothorbitalandspinmoments,andisnotcapableofbeingpermanentlymagnetized(He,Ne,Ar,etc.).你现在浏览的是第七页，共20页4.2DIAMAGNETISMANDPARAMAGNETISMDiamagnetism(抗磁性/逆磁性/反磁性)Veryweakformofmagnetismthatisnonpermanent,andpersistsonlywhileanexternalfieldisbeingapplied.Inducedbyachangeintheorbitalmotionofelectronsduetoanappliedmagneticfield.Themagnitudeoftheinducedmagneticmomentisextremelysmall,andinadirectionoppositetothatoftheappliedfield.r islessthanunity(onlyveryslightly),andm isnegative;B islessthanthatinavacuum,m isontheorderof-10-5.AtomicdipoleconfigurationforadiamagneticmaterialWhenplacedbetweenthepolesofastrongelectromagnet,diamagneticmaterialsareattractedtoward regions where the field isweak.This form of magnetism is of nopracticalimportance.你现在浏览的是第八页，共20页Paramagnetism(顺磁性)Each atom possesses a permanent dipole moment by virtue of incompletecancellationofelectronspinand/ororbitalmagneticmoments.Intheabsenceofanexternalmagneticfield,theorientationsoftheseatomicmagneticmoments arerandom,suchthatapieceofmaterialpossesses nonetmacroscopicmagnetization.Atomicdipolesarefreetorotate,andactedonindividuallywithnomutualinteractionbetweenadjacentdipoles.r isgreaterthanunity,andtoarelativelysmallbutpositivemagneticsusceptibility.mrangefromabout10-5to10-2.Bothdiamagneticandparamagneticmaterialsareconsiderednonmagnetic.AtomicdipoleconfigurationforaparamagneticmaterialSchematicrepresentationofB-H fordiamagneticandparamagneticmaterials你现在浏览的是第九页，共20页4.3FERROMAGNETISMFerromagnetism(铁磁性):Apermanentmagneticmomentintheabsenceofanexternalfield,andmanifestverylargeandpermanentmagnetizations.Fe(as-ferrite),Co,Ni,andsomeoftherareearthmetalssuchasGd.m ashighas106,andH M:B 0 M (4.8)Permanentmagneticmomentsresultfromatomicmagneticmomentsduetouncancelledelectronspinsandasmallerorbitalmagneticmoment.Couplinginteractionscausenetspinmagneticmomentsofadjacentatomstoalignwithoneanother,evenintheabsenceofanexternalfield.MutualalignmentofatomicdipolesforaferromagneticmaterialDomain(磁畴):Mutualspinalignmentexistsoverrelativelylarge-volumeregionsofthecrystal.Saturation magnetization(饱和磁化强度)Ms:Themaximumpossiblemagnetization.Thereisalsoacorrespondingsaturationfluxdensity(饱和磁通密度)Bs.你现在浏览的是第十页，共20页4.4ANTIFERROMAGNETISMANDFERRIMAGNETISMAntiferromagnetism(反铁磁性)Thealignmentofthespinmomentsofneighboringatomsorionsinexactlyoppositedirections;Reason:Magnetic moment coupling between adjacent atoms or ions results in anantiparallelalignment;Thesolidasawholepossessesnonetmagneticmoment.AntiparallelalignmentofspinmagneticmomentsforantiferromagneticMnO你现在浏览的是第十一页，共20页Ferrimagnetism(亚铁磁性/铁氧体磁性)Thereareantiparallelspin-couplinginteractions,similarincharactertoantiferromagnetism.The net ferrimagnetic moment arises from the incomplete cancellation of spinmoments.The saturation magnetizations for ferrimagnetic materials arenot as high as forferromagnets.Ferrite(铁氧体)inthemagneticsenseshouldnotbeconfusedwiththeferrite(铁素体)-iron.Ferrites,beingceramicmaterials,aregoodelectricalinsulators.Forsomemagneticapplications,suchashigh-frequencytransformers,alowelectricalconductivityismostdesirable.SpinmagneticmomentconfigurationforFe2+andFe3+ionsinFe3O4你现在浏览的是第十二页，共20页4.5THEINFLUENCEOFTEMPERATUREONMAGNETICBEHAVIORRaisingthetemperatureresultsinanincreaseinthemagnitudeofthethermalvibrationsofatoms.Withrisingtemperature,theincreasedthermalmotionoftheatomstendstorandomizethedirectionsofanymoments.The saturation magnetization is a maximum at 0 K.With increasing temperature,thesaturationmagnetizationdiminishesgraduallyandthenabruptlydropstozeroattheCurietemperature(居里温度)Tc.For temperatures aboveTc,bothferromagneticandferrimagneticmaterialsareparamagnetic.Antiferromagnetismisalsoaffected by temperature;thisbehavior vanishes at theNeltemperature(尼 尔 温 度).Attemperaturesabovethispoint,antiferromagneticmaterialsalsobecomeparamagnetic.SaturationmagnetizationasafunctionoftemperatureforFeandFe3O4你现在浏览的是第十三页，共20页4.6DOMAINSANDHYSTERESISEachdomainismagnetizedtoitssaturationmagnetization;Adjacentdomainsareseparatedbydomainboundariesorwalls,acrosswhichthedirectionofmagnetizationgraduallychanges;Normally,domainsaremicroscopicinsize,andforapolycrystallinespecimen,eachgrainmayconsistofmorethanasingledomain;The magnitude of theM field for the entire solid is the vector sum of themagnetizationsofallthedomains;For an unmagnetized specimen,the vector sum of the magnetizations of all thedomainsiszero.DomainsinaferromagneticorferrimagneticmaterialThegradualchangeinmagneticdipoleorientationacrossadomainwall你现在浏览的是第十四页，共20页FluxdensityB andfieldintensityH arenotproportionalforferromagnetsandferrimagnets.Initialpermeability(初始磁导率)i:TheslopeoftheB-H curveatH=0.Hysteresis(磁滞现象):TheB fieldlagsbehindtheappliedH field.Remanence/remanentfluxdensity(剩磁)Br:ResidualB fieldexistsatzeroH field.Hysteresisbehaviorandpermanentmagnetizationmaybeexplainedbythemotionofdomainwalls.Coercivity/Coerciveforce(矫顽力)Hc:,A fieldappliedinadirectionoppositetotheoriginalfieldtoreducetheB fieldtozero.TheB-H behaviorforaferromagneticorferrimagneticmaterialthatwasinitiallyunmagnetizedTheB-H behaviorforaferromagneticmaterialthatissubjectedtoforwardandreversesaturations你现在浏览的是第十五页，共20页ItisnotnecessarytoincreasetheHfieldtosaturationbeforereversingthefielddirection.It is possible to reverse thedirection of the field at anypoint along the curve andgenerateotherhysteresisloops.DifferenthysteresiscurvesB-H behaviorsforferromagnetic/ferromagneticanddiamagnetic/paramagneticmaterialsThebehaviorofatypicalferromagnetic/ferrimagneticmaterialsisnonlinear,andthatofparamagneticanddiamagneticislinear.Paramagnetics and diamagnetics are nonmagnetic materials:the flux density B of theparamagneticanddiamagneticmaterialsitisontheorderof10-5tesla.你现在浏览的是第十六页，共20页4.7MAGNETICANISOTROPYThemagnetichysteresiscurveshavedifferentshapesdependingonvariousfactors:(1)asinglecrystalorpolycrystalline;(2)ifpolycrystalline,anypreferredorientationofthegrains;(3)poresorsecond-phaseparticles;and(4)otherfactors,suchastemperatureandthestressstate.Magnetic(ormagnetocrystalline)anisotropy(磁/磁晶各向异性):Thedependenceofmagneticbehavioroncrystallographicorientation.Easymagnetizationdirection(易磁化方向):Acrystallographicdirectioninwhichmagnetizationiseasiestthatis,saturation(ofM)isachievedatthelowestH field.MagnetizationcurvesforsinglecrystalsofcobaltMagnetizationcurvesforsinglecrystalsofironandnickel你现在浏览的是第十七页，共20页4.8SOFTMAGNETICMATERIALSSoftmagneticmaterials(软磁材料)：Smallhysteresisloop(thinandnarrow)HighinitialpermeabilityandalowcoercivityReachsaturationmagnetizationwitharelativelylowappliedfield(i.e.,iseasilymagnetizedanddemagnetized)LowhysteresisenergylossesSchematicmagnetizationcurvesforsoftandhardmagneticmaterialsThesaturationfieldormagnetizationisdetermined only by the composition of thematerial.The susceptibility and coercivity,which influence theshapeofthehysteresiscurve,aresensitivetostructuralvariablesratherthantocomposition.Eddycurrents(涡流):Electricalcurrentsinducedina magnetic material by a magnetic field thatvariesinmagnitudeanddirectionwithtime.Increasingtheelectricalresistivitytominimizeenergylossinsoftmagneticmaterials.你现在浏览的是第十八页，共20页4.9HARDMAGNETICMATERIALSHardmagneticmaterials(硬磁材料):highremanence,coercivityandsaturationfluxdensitylowinitialpermeabilityhighhysteresisenergylosses.Energyproduct(磁能积):(BH)maxThevalueoftheenergyproductisrepresentativeoftheenergyrequiredtodemagnetizeapermanentmagnet.Schematic magnetization curve that displays hysteresis.WithinthesecondquadrantaredrawntwoBH energyproduct rectangles;the area of the rectanglelabeled(BH)maxisthelargestConventionalHardMagneticMaterials：Magnetsteels,CuNiFealloys,AlNiCoalloys,Hexagonalferrites(BaO-6Fe2O3)High-EnergyHardMagneticMaterials：Sm-CoMagnets:SmCo5Nd-Fe-BMagnets:Nd2Fe14B你现在浏览的是第十九页，共20页4.10MAGNETICSTORAGESchematicdiagramofaharddiskdrivethatemploysthe perpendicular magnetic recording medium;alsoshownareinductivewriteandmagnetoresistive-readheadsTransmissionelectronmicrographshowingthe microstructureof theperpendicularmagneticrecordingmediumusedinhard-diskdrivesScanning electron micrographsshowingparticulatemediausedintapememorystorage你现在浏览的是第二十页，共20页