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1、.~::::YS::CA'.i.V::'.WCONDENSEDMATTERTHIRDSERIES,VOLUME48,NUMBER91SEPTEMBER1993-IFreezingandmeltingoffluidsinporousglassesEricMolzDepartmentofPhysics,UniuersityofAlberta,Edmonton,Alberta,CanadaT6G2J1ApolloP.Y.Wong'andM.H.W.ChanDepartmentofPhysics,Pennsylvania
2、StateUniversity,104DaveyLaboratory,UniuersityPark,Pennsyluania16802J.R.Beamish1'DepartmentofPhysics,UniversityofAlberta,Edmonton,Alberta,CanadaT6G2J(Received5February1993)WehavestudiedthefreezingandmeltingofanumberofcryogenicAuids(hydrogen,neon,oxygen,andargo
3、n)confinedinporousglasses(Vycorandasilicaxerogel).acheat-capacitymeasurementsshowbroadenedlatent-heatpeaksassociatedwithbothfreezingandmeltingattemperaturessubstantiallybelowthebulkmeltingtemperatures.Thermalcyclingshowspronouncedhysteresis,withmeltingoccurri
4、ngatahighertemperaturethanfreezing.Also,thelatentheatoffreezingappearstobemuchsmallerthanthatofmelting.Thehysteresisintheargon-Vycorsystemwasstudiedindetailusinghigh-resolutionultrasonictechniqueswhichdirectlyprobetheshearmodulusofthematerialinthepores.Wefoun
5、dthattheonsetoffreezingisextremelysharp,despitetherandomporegeometry,andthatfreezingcontinuesoverarangeoftemperatures.Thefreezingprocessisextremelyirreversible,inthesensethat,oncethesolidappears,subsequentwarmingdoesnotcauseittomeltuntilamuchhighertemperature
6、.ThisistrueevenifonlyasmallfractionoftheQuidisinitiallyfrozen.Themeltingbranchofthehysteresisloopismorenearlyreversible.Inordertocorrectlymeasurethelatentheatoffreezinginthepresenceofsuchhysteresis,atechniqueshouldbeusedinwhichcoolingisperformedmonotoni-cally
7、,forexample,thermalrelaxationordifferentialscanningcalorimetry.INTRODUCTIONmore,thefreezingishighlyirreversible.Anysolidwhichforms,evenverysmallamounts,remainsfrozenuntilWereportheretheresultsofacombinedheat-capacitywarmedwellabovethetemperatureatwhichfreezin
8、gbe-andultrasonicstudyofthefreezingandmeltingofsimplegan.Meltingiscompleteatawell-definedtemperaturefluidsconfinedinporousglasses.Theheat-capacitymea-roughlyhalfwaybetweentheonsetoffreezi
