HEATCAPACITIESOFFe_(40)Ni_(40)B_(20)GLASSYAL LOYMEASUREDBYENTHALPYMETHOD¥JIANGQing;ZHAOM ing;LIJianchen(JilinUniversityofTechnology, Changchun,ChinaManuscriptreceived26December ,1994)Abstract:Theheatcapacities(C_p)ofglas syandcrystallineFe_(40)Ni_(40)B_(20)alloyha vebeenmeasuredbymeansofdifferentialscanning calorimeter(DSC).AlargerC_pvalueattheglasst ransitiontemperature(T_g)canbeobtained.Ther eproducibleC_pvaluesofrelaxedglassatcertain heatingratearelowerthanthatofasquenchedglas sbelow500K,andaremuchhigheratT_gthanthatoft hecrystal.TheheatingrateaffectsC_pvaluesoft heglass.Atlast,C_pvaluesoftheundercooledliq uidareestimated.Keywords:heatcapacity,glass transition,undercooledliquid,Fe_40Ni_40B_20 alloyTheas-quenchedglassyalloysusuallyshown oevidentendothermicpeakattheglasstransition temperature(Tg)byacontinuousheatingonDSC.Th eglasstransition,whichischaracterizedbyaste epincreaseofCpvaluesandanentropicorconfigur ationalchange,ismaskedbystructuralrelaxatio nrelatedtoloweringenergystateoftheglassandb ythestartofcrystallization.TheapparentCpval uesstronglydecreasenearTgandcrystallization temperatureTxduetoamassmentofstructuralrela xationinthistemperaturerange.Ontheotherhand ,atransitionofglasstonanostructurecrystal(N C)canbeobservedbyaslowheatingrate[1,2]orann ealing[3]nearTg,sotheCpvalueatTg(Cpg)isasum ofCpofNCandtheglass(CpvaluesofNCcouldbelarg erthanthatoftheglass[4]).Todistinguishasmal ldifferenceofCpvaluesamongtheas-quenchedgla ss,therelaxedglass,NCandthecrystallinewithl argegrainsize,themeasuringaccuracyofCpvalue sisnotenoughwithacontinuousheating.Therefor eabettermethodshouldbeusedforthiskindofmeas urements.TwomeasuringmethodsofCpvaluesonDSC canbeused,oneisthescanningmethodmadebyacont inuousheating(orcooling)[5].Cpvaluesareobta inedbymeasuringthedifferenceofheatflowbetwe enmeasurementrunswithandwithoutaspecimen.Th emeasuredheatflowvaluesarecalibratedbytheco mparisonwithameasurementofastandardspecimen .i.e.sapphire.Anothermethodiscalledenthalpy methodcharacterizedbyastepwiseheating(orcoo ling)[5-6].Aftereveryheatinganewquasi-balan cestateisestablisheddependingonthetimeforth ethermalarrest.EachsinglemeasurementforoneC pvaluecorrespondstoanenthalpymeasurementand isevaluatedbyintegrationofthemeasuredarea.T heartificialbaselinenecessaryforthisoperati onisconstructedbyaspline-function.Themeasur ementsofreferenceandcalibrationcurvesarecar riedouttogetherwiththemeasurementwithinonee xperimentbythesameparametersofthecalorimete r.Theaverageheatingrateisgivenbytheratiooft hetemperaturestepandthetimeforheatinguptoat emperatureandforthermalarrestbeforenextheat ingbegins.Wetakeindividualtemperaturestepsa s5or10K.Thetimeforheatingupbetweentwotemper aturestepsisfixedinthesemeasurementsas25%of thetotalone.IncontrasttoR=0.33-1.33Ks-1used tomeasureCpbythescanningmethod[6],R<0.083Ks -1,whichisnecessarytoreachaquasi-balancesta teofaglassyalloyforthermalarrest.Thismethod hasabettermeasuringaccuracyonCpvaluesofagla ssyalloythanthescanningmethod[6].Animportan tadvantageoftheenthalpymethodisthatthestruc turalrelaxationoftheas-quenchedglasshasmuch smallerinfluenceontheapparentCpvaluesbecaus eitoccursnotonlyattheheating,butalsoattheth ermalarrest.Sotherelaxationofglasslowersthe measuredcurveduringheatingandthebaselineatt hethermalarrest.Asaresulttheareabetweenthem easuredcurveandthebaseline,whichisusedtocal culateCpvalues,remainsconstant.However;ther elaxationprocess,whichisanexponentialfuncti onoftime,ispresentmoreatheatingthanattherma larrest(seethefollowingexperiments).Theglas syFe40Ni40B20alloyistakenasanexampletostudy theCpdifferenceofdifferentstatewiththismeth od,sincethisglasshasbeenwidelystudiedonDSCw iththescanningmethod[7-10],besidestheappear anceofNCbeforeTghasbeenfound[11].1.Experime ntalTheglassribbons,whichwerepreparedbymelt spinningandsuppliedbyVacuum-SchmelzeGmbH,Ha nau,Germany,are1mminwidthand24μminthickness .Thespecimenswerecutinto5mminlengthand35-45 mginweight.Theywereputinaluminiumpanswithcoverandwerepressedinacompactform.Themoreweightandthecompactformincreasethemeasuringaccuracy.ThemeasurementswerecarriedoutonDSCunde
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