材料物理性能1yt

8.5.6POLYCRYSTALLINEMATERIALSANDTHEMVERSUSHBEHAVIOR

Themajorityofthemagneticmaterialsusedinengineeringispolycrystallineandthereforehaveamicrostructurethatconsistsofmanygrainsofvarioussizesandorientationsdependingonthepreparationandthermalhistoryofthecomponent.Inanunmagnetizedpolycrystallinesample,eachcrystalgrainwillpossessdomains,asdepictedinFigure8.30.1Thedomainstructureineachgrainwilldependonthesizeandshapeofthegrainand,tosomeextent,onthemagnetizationsinneighboringgrains.Althoughverysmallgrainsperhapssmallerthan0.1µm,maybesingledomains,inmostcasesthemajorityofthegrainswillhavemanydomains.Overall,thestructurewillpossessnonetmagnetization,providedthatitwasnotpreviouslysubjectedtoanappliedmagneticfield.WecanassumethatthecomponentwasheatedtoatemperatureabovetheCuriepointandthenallowedtocooltoroomtemperaturewithoutanappliedfield.2Supposethatwestartapplyingaverysmallexternalmagneticfield(µ0H)alongsomedirection,whichwecanarbitrarilylabelas+x.Thedomainwallswithinvariousgrainsbegintomovesmalldistances,andfavorablyorienteddomains(thosewithacomponentofMalong+x)growalittlelargerattheexpenseofthosepointingswayfromthefield,asindicatedbypointaninFigure8.31.Thedomainwallsthatarepinnedbyimperfectionstendtobowout.Thereisaverysmallbutnetmagnetizationalongthefield,asindicatebytheOaregioninthemagnetizationversusmagnetizingfield(MversusH)behaviorinFigure8.31.3Asweincreasethemagnetizingfield,thedomainmotionextendlargerdistance,asshownforpointbinFigure8.31,andwallsencountervariousobstaclessuchascrystalimperfections,impurities,secondphase,andsoonwhichtendtoattractthewallsandtherebyhindertheirmotions.Adomainwallthatisstuck(orpinned)atanimperfectionatagivenfieldcannotmoveuntilthefieldincreasessufficientlytoprovidethenecessaryforcetosnapthewallsnapsfree,whichthensuddenlysurgesforwardtothenextobstacle.4Asawallsuddenlysnapsfreeandshootsforwardtothenextobstacle,essentiallytwocausesleadtoheatgeneration:Suddenchangeinthelatticedistortion,duetomagnetostriationcreatelatticewavesthatcarryoffsomeoftheenergy.SuddenchangesinthemagnetizationinduceeddycurrentsthatdissipateenergyviaJouleheating(domainshaveafiniteelectricalresistance).Theseprocessesinvolveenergyconversiontoheatandareirreversible.Suddenjerksinthewallmotionsleadtosmalljumpsinthemagnetizationofthespecimenasthemagnetizingfieldisincreased;thephenomenonisknownastheBarkhauseneffect.Ifwecouldexaminethemagnetizationpreciselywithahighlysensitiveinstrument,wewouldseejumpsintheMversusHbehavior,asshowintheinsetinFigure8.31.58.5.7DEMAGNETIZATIONTheB-Hhysteresiscurves,asisFigure8.32b,thatarecommonlygivenformagneticmaterialsrepresentBversusHbehaviorobservedunderrepeatedcycling.TheappliedfieldintensityHiscycledbackandforwardbetweenthe+xand–xdirections.IfweweretotryanddemagnetizeaspecimenwitharemnantmagnetizationatpointeinFigure8.34byapplyingreversefieldintensity,thenthemagnetizationwouldmovealongfrompointetopointf.Ifatpointfweweretosuddenlyswitchofftheappliedfield,wewouldfindthatBdoesnotactuallyremainzerobutrecoversalongftopointe’andattainssomevalueBr’.

6ThesimplestmethodtodemagnetizethesampleisfirsttocycleHwithamplemagnitudetoreachsaturationandthentocontinuecyclingHbutwithagraduallydecreasingmagnitude,asdepictedinFigure8.35.AsHiscycledwithadecreasingmagnitude,thesampletracesoutsmallerandsmallerB-HloopsuntiltheB-HloopsaresosmallthattheyendupattheoriginwhenHreacheszero.ThedemagnetizationprocessinFigure8.35iscommonlyknownasdiapering.Undesirablemagnetizationofvariousmagneticdevicessuchasrecordingheadsistypicallyremovedbythisdiaperingprocess(forexample,ademagnetizinggunbroughtclosetoamagnetizedrecordingheadimplementsdiaperingbyapplyingacycledHwithdecreasingmagnitude).78.6SOFTANDHARDMAGNETICMATERIALS

8.6.1DEFINITIONS

BasedontheirB-Hbehavior,engineeringmaterialsaretypicallyclassifiedintosoftandhardmagneticmaterials.TheirtypicalB-HhysteresiscurvesareshowninFigure8.37.Softmagneticmaterialsareeasytomagnetizeanddemagnetizeandhencerequirerelativelylowmagneticfieldintensities.Putdifferently,theirB-Hloopsarenarrow,asshowninFigure8.37.Thehysteresisloophasasmallarea,sothehysteresispowerlosspercycleissmall.Softmagneticmaterialsaretypicallysuitableforapplicationwhererepeatedcycleofmagnetizationanddemagnetizationareinvolved,asinelectricmotors,transformers,andinductors,wherethemagneticfieldvariescyclically.Theseapplicationsalsorequirelowhysteresislosses,orsmallhysteresislooparea.Electromagneticrelaysthathavetobeturnedonandoffrequiretherelayirontobemagnetizedanddemagnetizedandthereforeneedsoftmagneticmaterials.8Hardmagneticmaterials,ontheotherhand,aredifficulttomagnetizeanddemagnetizeandhencerequirerelativelylargemagneticfieldintensities,asapparentinFigure8.37.TheirB-Hcurvesarebroadandalmostrectangular.Theypossessrelativelylargecoercivities,whichmeanthattheyneedlargeappliedfieldstobedemagnetized.Thecoercivefieldforhardmaterialscanbemillionsoftimesgreaterthanthoseforsoftmagneticmaterials.Theircharacteristicsmakehardmagneticmaterialsusefulaspermanentmagnetinavarietyofapplications.

Itisalsocleanthatthemagnetizationcanbeswitchedfromoneverypersistentdirectiontoanotherverypersistentdirection,from+Brto–Br,byasuitablylargemagnetizingfieldintensity.Asthecoercivityisstrong,boththestates+Brand–Brpersistuntilasuitable(large)magneticfieldintensityswitchesthefieldfromonedirectiontotheother.Itisapparentthathardmagneticmaterialscanalsobeusedinmagneticstorageofdigitaldata,wherethe