Thermal_Analysis_Techniques.pdf

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1、UniversityofCambridge,MaterialsScience&MetallurgyH.K.D.H.BhadeshiaThermalAnalysisTechniquesThermalanalysiscomprisesagroupoftechniquesinwhichaphysicalpropertyofasubstanceismeasuredasafunctionoftemperature,whilethesubstanceissubjectedtoacontrolledtemperatureprogramme.Indi®erent

2、ialthermalanalysis,thetemperaturedi®erencethatde-velopsbetweenasampleandaninertreferencematerialismeasured,whenbotharesubjectedtoidenticalheat{treatments.Therelatedtechniqueofdi®erentialscanningcalorimetryre-liesondi®erencesinenergyrequiredtomaintainthesampleandreferenceatani

3、denticaltemperature.Lengthorvolumechangesthatoccuronsubjectingmaterialstoheattreatmentaredetectedindilatometry;X{rayorneutrondi®ractioncanalsobeusedtomeasuredimensionalchanges.Boththermogravimetryandevolved{gasanalysisaretechniqueswhichrelyonsampleswhichdecomposeatelevatedtem

4、peratures.Theformermonitorschangesinthemassofthespecimenonheating,whereasthelatterisbasedonthegasesevolvedonheatingthesample.Electricalconductivitymeasurementscanberelatedtochangesinthedefectdensityofmaterialsortostudyphasetransitions.DifferentialThermalAnalysis(DTA)Introduct

5、ionDTAinvolvesheatingorcoolingatestsampleandaninertreferenceunderidenticalconditions,whilerecordinganytemperaturedi®erencebetweenthesampleandreference.Thisdi®erentialtemperatureisthenplottedagainsttime,oragainsttemperature.Changesinthesamplewhichleadtotheabsorptionorevolution

6、ofheatcanbedetectedrelativetotheinertreference.Di®erentialtemperaturescanalsoarisebetweentwoinertsampleswhentheirresponsetotheappliedheat{treatmentisnotidentical.DTAcanthereforebeusedtostudythermalpropertiesandphasechangeswhichdonotleadtoachangeinenthalpy.ThebaselineoftheDTAc

7、urveshouldthenexhibitdiscontinuitiesatthetransitiontemperaturesandtheslopeofthecurveatanypointwilldependonthemicrostructuralconstitutionatthattemperature.ADTAcurvecanbeusedasa¯ngerprintforidenti¯cationpurposes,forexample,inthestudyofclayswherethestructuralsimilarityofdi®erent

8、formsrendersdi®ractionexperimentsdi±culttointerpret.TheareaunderaDTA