近代分析实验原理：第十四课-热分析

十四、热分析

（ThermalAnalysis）近代分析实验原理（Introductionofmodernanalyticalmethods）12Measurepropertiesorpropertychangesofmaterialsasafunctionoftemperature.热重分析差热分析法差示扫描量热法动态热机械分析热机械分析热膨胀法dimension,尺寸,mass,phaseandmechanicalbehavior.relativelysimple31.CommonCharacteristics1.1ThermalEventsmaterialreactionstothermalenergyflowinoroutofsolids.EnthalpyChangequantifiestheheatflowingintooroutofasolidunderconstantpressure.41.2InstrumentationGeneralinstrumentationforthermalanalysis.51.3ExperimentalParameterssampledimensionandmassheating(orcooling)ratesatmospheresurroundingthesamplethermalandmechanicalhistoryofthesampleshouldbesmallsamplesintheformofapowderwithsamplemasslessthan10mgarepreferred.slowinertatmosphereMaintainingacertainflowratethesamethermalmeasurementisaffectedbytheinternalenergyofsamples,andtheinternalenergycanbechangedbythermalandmechanicalprocesses.ReproducibleresultsthemostimportantparameterAfastheatingathermallag62.DifferentialThermalAnalysis(DTA)and

DifferentialScanningCalorimetry(DSC)2.1WorkingPrinciples(DTA)Differentialthermalanalysis(DTA)instrumentation.VTsandVTrarethethermocouplevoltagesformeasuringsampleandreferencetemperatures,respectively.itdoesnotundergothermaleventsovertheoperationtemperaturerange,doesnotreactwithanycomponentintheinstrument,andhassimilarthermalconductivityandheatcapacitytothesamplebeingexamined.analysisisbasedondifferencesbetweensamplematerialandareferencematerialinwhichtheexaminedthermaleventsdonotoccur.temperaturedifferencesolidphasetransformation,glasstransition,crystallizationandmelting7DTAcurveforapolymersampleunderaconstantheatingrate.∆T<0,endothermic(absorbingheat)∆T>0,exothermic(releasingheat).∆T=0,Nothermalevent8(DSC)Differentialscanningcalorimetry(DSC)instrumentationdesign:(a)heatfluxDSC;and(b)powercompensationDSC.A,furnace;B,separateheaters;andC,sampleandreferenceholders.measuretheheatflowdifferencebetweensampleandreference.endothermicpowerheatflowdifferencequantitativeDTATemperaturedifferencepowerchange=heatflowtocompensateanalgorithmincomputersoftwareexothermicpower9SchematicDSCcurvesforapolymericsample.Tg,glasstransitiontemperature.aunitofenergyperunittimeperunitmass,usuallyinunitsofWg−1.Commonly,heatflowintoasampleisindicatedasanupwardfeatureoftheDSCcurve.ThisdiffersfromaDTAcurveinwhichanendothermiceventisindicatedasadownwardfeature.10DTAqualitativetechniqueMeasure:temperaturedifferenceswidertemperaturerange(upto1500oC)DSCquantitativetechniqueMeasure:enthalpychangesmaximumtemperatureofabout750oC.112.2ExperimentalAspectsSampleRequirementsdensepowderorsmalldiscsLargeshearforcesduringcuttingsamplesshouldbeavoidedLow-masssamplesarepreferredPlatinumandgoldcruciblesDTA(hightemperature)aluminumpansDSC(lowtemperature)BaselineDeterminationinstrumentbaseline(alsocalledthezeroline)thesamplebaselinebesealedavoidsamplemasschangeduetoevaporationThepansrecordedwithoutthesample12Examplesofsamplebaselineinterpolationfordifferenttypesofdifferentialthermalanalysis(DTA)andDSCcurves.Thesamplebaselineofanexperimentalcurve,isthelinethatconnectsthecurvebeforeandafterapeak,asifnopeakhaddeveloped.Accuracyofenthalpychangemeasurementreliesoncorrectlyconstructingthesamplebaselinefromthecurve.13EffectsofScanningRateScanningrateeffectsonDSCcurvesofapoly(ethyleneterephthalate)(PET)sample:(a)heating;and(b)cooling.I,10Kmin−1;II,20Kmin−1;III,40Kmin−1.Commonlyvariesfrom0.1to40◦Cmin−1.scanningrateshouldbereportedcoolingheatingthescanningrateeffectsareevenmoreobviousincoolingthaninheating.142.3MeasurementofTemperatureandEnthalpyChangeTransitionTemperaturesDTAcurveofglass–ceramicsample(Li2O–Al2O3–6SiO2).Temperaturesofglasstransition(Tg),crystallization(Tc)andmelting(Tm)aredeterminedfromthecurve.15ComparisonofrealandidealmeltingpeaksinDTAcurveataconstantheatingrate.TcMeltingcompletes16MeasurementofEnthalpyChangecalibrationfactor,fromstandardsamplepeakareaCalibrationofTemperatureandEnthalpyChangewithstandardmaterials联苯苯偶酰Forthepower-compensatedDSC,Kcisvirtuallyindependentoftemperature.FortheheatfluxDSC,Kcbecomestemperaturedependentandhastobedeterminedatatemperatureclosetothatofthepeaktobemeasured.172.4ApplicationsDeterminationofHeatCapacityHeatcapacitymeasurement:(a)proportionalityofheatcapacityandDSCresponse;(b)twodisplacementsrequiredforheatcapacitycalculation;and(c)heatcapacityasafunctionoftemperature.heatingratecalibrationfactorTemperaturedependent18DeterminationofPhaseTransformationandPhaseDiagramsTheDSCcurveofcarbontetrachlorideexhibitsthreesolid-statephasetransformationsbeforemelting.四氯化碳19DeterminingabinaryphasediagramcontainingaeutecticreactionbyDSCorDTA.20DSCcurvesofplasticwastecontainingseveralpolymers:lowdensitypolyethylene(LDPE),highdensitypolyethylene(HDPE),polypropylene(PP),Nylon-6TM,Nylon-66TMandpolytetrafluoroethylene(PTFE).DSCcurvescanalsobeusedtoidentifyindividualpolymersinapolymermixture.聚乙烯低密度聚乙烯聚丙烯聚四氟乙烯individualmeltingpeaks21IsothermalDSCcurveofpolyethylenedemonstratingtheeffectivenessofstabilizertoimproveresistancetooxidation.聚乙烯oxidationanddegradation22ThermogravimetriccurvesthatexhibitdecompositionstartingtemperatureTiandfinishtemperatureTf.Theonsetofmasschangethemasschangeiscompleted3.Thermogravimetrymeasuringmasschangeofasamplewithtemperature.thermobalance233.1InstrumentationStructureofinstrumentation.keycomponent24StructureofaCahnmicrobalancewithcapabilitytomeasurethemasslossofasamplewithoutthesamplemovingdownward.25SamplepositioninfurnaceinaTGsystemwithprotectivegasflowdownward.263.2ExperimentalAspectsSamplessmallsamplemass(severalmilligrams)EffectsofsampleformonTGcurveofapolymethylmethacrylate(PMMA)sample.Experimentalconditions:5mgPMMAandaheatingrateof5Kmin−1.AblocksampleshouldbegroundorslicedtoobtainasuitableformforTGexamination.27AtmosphereBuoyancyeffectsofupwardprotectivegasflow.inertgaslittlewatervapor15–25mlmin−1isrecommendedabout2–10mgforThermalshieldingBuoyancyeffectsCarefulcalibrationandgoodinstrumentdesign28TemperatureCalibrationCuriepointmethodfusiblelinemethodferromagneticmaterialswiththeCuriepointranging163–1000◦C铝镍合金fusiblewirewithknownmeltingtemperaturethethermocouplejunctioncannottoucheitherthesampleorsampleholder.29HeatingRateEffectsofheatingratesonTGcurvesofpowderedpolyvinylchloride(PVC).Experimentalconditions:5mgindryN2gaswithaflowrateof20mlmin−1.highheatingrateismorelikelytogenerateatemperature