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2.4.TypicalMethodsforEvaluationofQualityofOxideCrystals
43
Thefundamentalmethodsforverificationofthequalityofacrystalob-
tainedarethermalanalyses.Changesinthephysicalandchemicalproper-
tiesofagivencrystalasafunctionoftemperatureshouldbestudiedtaking
intoaccountthethermalhistoryofthecrystal,ensuringstablepressureand
usingproperlychosenworkinggas.Belowashortdescriptionofthemost
oftenmethodsusedisgiven.
DTA-DiferentialThermalAnalysis.Inthismethodadirectdiference
intemperaturebetweenasamplestudiedandareferencesampleismeas-
uredintheso-calledBlock-typeMeasuringSystem,thetemperaturesen-
sorsareinthesamples.Themethodofershighsensitivityandimmediate
responsetotheprocessestakingplaceinthesamplestudied.Inanother,
moreuniversalandmoreoftenusedsolution,theMeasuringSystemwith
Free-standingCrucibles,thethermocouplejunctionisinthermalcontact
withthebottomofthecrucible.Itsadvantagesarethelackofpossibility
ofchemicalreactionwiththesamplestudied,thepossibilityofusingdif-
ferentsamplecontainers,highsensitivity[72].
DSC-DiferentialScanningCalorimetry.Inthismethodastablemeas-
uringsystemwithanexactlydefinedheatflowpathwayisemployed.The
DSCideacanberealizedintwoapproaches,theheatfluxDSCandthe
powercompensationDSC.Inmoredetail,threemethodsofmeasurements
canbeapplied:heat-compensatingcalorimetersinwhichthemeasured
quantityistheenergyrequiredforcompensatingtheheattobemeasured,
heat-accumulatingcalorimetersinwhichthemeasuredquantityisthe
temperaturechangeinthesubstanceduetotheheattobemeasuredand
heat-exchangingcalorimetersinwhichthemeasuredquantityistheheat
flowratebetweenthesamplestudiedanditssurroundingscausedbythe
heattobemeasured[72].
TGA-ThermogravimetricAnalysisisatechniqueinwhichthemass
ofasubstanceismonitoredasafunctionoftemperature.Themasslossis
measuredbyaprecisionbalance[72].
Thermalanalysispermitsaccuratedeterminationofthetemperatures
ofphasetransitionsinthecrystalstudiedandbesidestheX-rayanalysis,it
isappliedatthefirststageofcharacterizationofthecrystalobtained.Be-
causeofalargenumberofmethodsforobtainingperovskitefamilycrystals
theinformationontheactualmethodusedisoftenprovidedinliterature
reports.
Theabovemethodsofstructuralandthermalanalysesareapplied
alsoforsinglecrystalsofverysmallsize.Inthestudiesofnanocrystalline
samples,thequestionsappearaboutthephasetransitiontemperaturede-
pendenceonthecrystalsizeandaboutthecriticalsizeinwhichitisstill