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1、Unit4ApplicationsandProcessingofCeramicsTypesandApplicationsofCeramicsGlassesTheglassesareafamiliargroupofceramics;containers,lenses,andfiberglassrepresenttypicalapplications.Asalreadymentioned,theyarenoncrystallinesilicatescontainingotheroxides,notablyCaO,Na2O,K2O,andAl2O3,whichinfluencetheglas
2、sproperties.Atypicalsoda–limeglassconsistsofapproximately70wt%SiO2,thebalancebeingmainlyNa2O(soda)andCaO(lime).Possiblythetwoprimeassetsofthesematerialsaretheiropticaltransparencyandtherelativeeasewithwhichtheymaybefabricated.Glass-ceramicsMostinorganicglassescanbemadetotransformfromanoncrystall
3、inestatetoonethatiscrystallinebytheproperhigh-temperatureheattreatment.Thisprocessiscalledcrystallization,andtheproductisafine-grainedpolycrystallinematerialwhichisoftencalledaglass–ceramic.Theformationofthesesmallglass-ceramicgrainsis,inasense,aphasetransformation,whichinvolvesnucleationandcrys
4、talgrowthstages.Asaconsequence,thekinetics(i.e.,therate)ofcrystallizationmaybedescribedusingthesameprinciplesthatwereappliedtophasetransformationsformetalsystems.Thedependenceofdegreeoftransformationontemperatureandtimemaybeexpressedusingisothermaltransformationandcontinuouscoolingtransformation
5、diagrams.Thecontinuouscoolingtransformationdiagramforthecrystallizationofalunarglassispresented.Thebegin-andend-transformationcurvesonthisplothavethesamegeneralshapeasthoseforaniron–carbonalloyofeutectoidcomposition.Alsoincludedaretwocontinuouscoolingcurves,whicharelabeled“1”and“2”;thecoolingrat
6、erepresentedbycurve2ismuchgreaterthanthatforcurve1.Asalsonotedonthisplot,forthecontinuouscoolingpathrepresentedbycurve1,crystallizationbeginsatitsintersectionwiththeuppercurve,andprogressesastimeincreasesandtemperaturecontinuestodecrease;uponcrossingthelowercurve,alloftheoriginalglasshascrystall
7、ized.Theothercoolingcurve(curve2)justmissesthenoseofthecrystallizationstartcurve.Itrepresentsacriticalcoolingrate(forthisglass,100ºC/min)thatis,theminimumcoolingrateforwhichthefinalroom-temperatureproductis100%glass;forcooli
