SOLIDIFICATIONOFZn-PbALLOYUNDERSIMULANTMICRO GRAVITYCONDITIONOFORTHOGONALELECTRICANDMAGN ETICFIELDS¥Zhao,Jiuzhou;Guo,Jingjie;Jia,Jun ;Li,Qingchun(SchoolofMaterialsScienceandEng ineering,HarbinInstituteofTechnology,Harbin 150001)Abstract:Solidificationexperimentsha vebeendonewithZn-Pballoyunderthesimulantmic rogravityconditionoforthogonalelectricandma gneticfields.Computersimulationhasbeencarri edoutfortheprocessesofthecoarseningofthedis persiondropletsandtheformationofthegravitys egregationduringcoolingthiskindofalloyinthe irliquid-liquidtemperatureregions.Experimen tsandanalysesshowthatelectricandmagneticfie ldsproperlyselectedcandecreasethecoarsening rateofdispersiondropletsgreatlyandslowdownt hevelocityatwhichthegravitysegregationisfor med.Keywords:solidification,electricandmagn eticfields,computersimulation1INTRODUCTIONM onotecticalloyisabroadkindofmaterialandmany ofthemhaveparticularphysicalandmechanicalpr operties[1-4].Butuntilpresent,theyhavefewpr acticalapplicationsbecauseonlyingotwithseri ousgravitysegregationcanbeobtainedwhentheys olidifyundergeneralgroundconditions.Inrecen tyears,greatattentionshavebeenpaidtothestud iesofthiskindofmaterialandalotofresearchwor khasbeencarriedoutundermicrogravityconditio ninspace[5-8].Butuptodate,peoplearenotveryc learaboutthecoarseningprocessofthedispersio ndropletsandthephaseseparationofmonotectica lloy.Therefore,itisverymeaningfultodosomefu rtherresearchinthisfield.Solidificationexpe rimentswillbedonewithZn-Pbmonotecticalloyun derthesimulantmicrogravityconditionoforthog onalelectricandmagneticfieldsinthispaper,an dcomputersimulationwillbecarriedoutforthepr ocessofthecoarseningofthedispersiondroplets .2EXPERIMENTALANDRESULTSZn-Pbbinaryalloyhas beenusedastheexperimentalmaterial.Fig.1show stheschematicoftheexperimentalset[9].Theele ctricandmagneticfieldswerechargedwithtwoa.c .-d.c.converterssothattheirparameterscanbea djustiedseparately.Theexperimentalprocedure isasfolFig.1Principledrawingofexperimentals et-up1-electrode;2-magneticfield;3-melt;4-c rucible①SupportedbytheNationalNaturalScienc eFoundationofChina;ReceivedDec.6,1994lows:f irst,putthepreparedalloyintothecrucibleandh eatittotemperatureaboveitsimmiscibletempera turegap.Thenhomogenizethemeltbyholdingtempe raturefor30~40min.Finallycoolthemeltat1℃/si northogonalelectricandmagneticfields.Itwasf oundthatwhensolidifiedundergeneralgroundcon ditions,onlysamplewithseriousgravitysegrega tioncanbeobtained.Orthogonalelectricandmagn eticfieldscanchangethesolidificationstructu regreatly.Whenthedensitiesofelectricandmagn eticfieldsaresmaller,thereisstillapartofPbp hasedepositedtothebottomofsampleduringtheso lidification.Whenthedensitiesofelectricandm agneticfieldsareinacertainregion,sampleinwh ichPb-richparticlesdistributehomogeneouslyc anbeobtained,asshowninFig.2.Analyeseshowtha ttheaverageparticleradiiinZn-2%PbsampleandZ n4%Pbsampleare2.45and3.47μmrespectively.3DI SCUSSION3.1PrincipleofSimulatingMicrogravit yConditionbyElectricandMagneticFieldsOnekno wsthatwhencoolingahomogeneousmonotecticallo ymeltintoitsimmiscibletemperaturegap,itwill transformtoasuspensionofdispersiondropletin thematrix.Fromnowon,thesedispersiondroplets willgrowthroughdiffusionandcoagulatethrough collisionbetweendroplets.Orthogonalelectric andmagneticfieldschangetheresultantforcesuf feredbyadroplet[9].Ifthedensitiesofelectric andmagneticfieldsareproperlyselected,theres ultantforcesufferedbyadropletwilltendtozero .Underthisconditionthemeltwillseemtobeinspa ceandthereisnodepositionorfloatingofdroplet sinit.Certainly,itisverydifficulttofindthee lectricandmagneticfields'densitiesJ0andB0wi thwhichthespecificgravitydifferencebetweent hetwoliquidscanbecompensatedFig.2Structureo fZn-2%Pbsamplesolidifiedinproperelectricand magneticfieldscompletelyduringthesolidifica tionprocessbecausethephysicalparametersofan alloychangefromtimetotime.Generally,ifalloy meltssolidifiedinelectricandmagneticfieldsw ithdensitiesJandB,onecanuseaconstantKbindicatingtheextenttowhichthespecificgravitydifferencebetweenthetwoliquidsiscompensated.Kb=1-(J×B)/(J0×B0) (1)WhenKbequals1,thealloy
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