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• Standard-compliant element analysis of silicon carbide and mixtures containing silicon carbide with

  Elementalanalyzersareimportanttoolsforqualitycontrolofawiderangeofproducts.Avarietyofmatrices,suchasceramics,coal,steelorsoil,canbeanalyzedfortheirelementconcentrationswithdifferenttypesofinstruments.TheproductrangeofEltraGmbH,locatednearDüsseldorf,Germany,comprisesanalyzersforC,H,N,S,Oandthermogravimetrywhichallowforthestandard-compliantdeterminationofcarboninvariouschemicalbondings,aswellasoxygenandnitrogeninSiCandinmaterialscontainingSiC.Therequirementswhichhavetobefulfilledforastandard-compliantanalysismay,however,varygreatly,dependingonthedesiredparameters.1IntroductionSiliconcarbidehasahighmeltingpointof2,700°Candisthereforeanimportantrawmaterialforrefractoryandceramicproducts.AnothercharacteristicofSiCisitsresistanceagainstchlorineandstrongacids,alsoathightemperatures.Thankstoahardnessof9.6Mohs,itisalsousedinthemetallurgicalindustryfortheproductionofabrasivesandpolish.TheEuropeanstandardseriesENISO21068(2008)regulatethechemicalanalysisofsiliconcarbideandrawmaterialscontainingsiliconcarbide.Part1dealswithsamp領(lǐng),part2withthechemicalanalysisofcarbon,siliconandlossofignitionandpart3coversmetalanalysisanddeterminationofoxygenandnitrogenconcentrations.Eltracombustionanalyzersarewellsuitedforthequalitycontrolofrefractoryandceramicproductswhichcontainsiliconcarbide.Thisarticleoutlinesthepossibilitiesandlimitsofelementalanalyzerswhendea領(lǐng)withthesematerials.2ThermogravimetricparametersThedeterminationofthermogravimetricparameterssuchas,forexample,lossofignition,isdescribedinthesecondpartofISO21068.Thermogravimetryisbasedonthecontinuousrecordingofmasschangesasafunctionofacombinationoftime,temperatureandatmosphere.Thestandardclearlydefinestheuseofmufflefurnacesandbalancesforthisprocess.Allmethodsdescribedinthestandarduseadefinedsamplecontainermadeofsteel,ceramicmaterialorplatinumwhichispreheatedattheprescribedtemperaturebetween250°Cand1050°C.Thesampleweightisnotalwaysdefined(e.g.lossondryingLOD250)orrangesfrom25gto1kg(changeofmassinairat200°Cand400°C).Afterweighingthesampleandapplyingthedefinedtemperatureprogram(e.g.forLOI850,heatingupto850°Candmaintainingfor3h),thehotcruciblesneedtocooldowninthedesiccatorandarethenweighed.Thermogravimetricanalyzerswhichareequippedwithacombinationoffurnaceandbalanceconsiderablysimplifythemanualprocedure.Usually,theseanalyzershaveaninteriorchamberwhichcanbeheatedupto1,000°Candaseparateweighingcellintheanalysischamberwhichisconnectedwiththefurnacebyaceramicpedestal.Arotatingcarouselplacesupto19differentsamples,oneaftertheother,onthepedestaltobeweighed.Themarketoffersthermogravimetricanalyzersforbothsmallsamplequantities(e.g.20mg)andquantitiesof5gormore(suchasEltra’sThermostep,fig.1).Byusinganemptyreferencecrucible,thermalbuoyancyiscompensatedandmeasurementscanbecarriedoutreliablyevenathightemperatures.Theceramiccruciblesusuallyhaveavolumeof12ml,allowingforsampleweightsofupto5g.This,however,isnotverypracticalduetothehighfil領(lǐng)level.Sofar,theDINENISO21068-2standarddoesnotmentionautomatedthermogravimetricanalysis.IfathermogravimetricanalyzerisusedforthedeterminationofLOI850(lossonignitionat850°C),thespecificationsofthestandardcanbemettoahighdegree,ifnot1**%.ForaseriesofteststhestandardEuronormCRMNo.781-1wasweighedintocrucibleswhichhadbeenpreheatedto850°CinEltra’sThermostep.Figure2showsatypicalmeasurementcurve,table1containsthecorrespondingresults.Thestandardwasfirstdriedat105°Covernightandthen3crucibleswerefilledwith1geach.TheLOI850valuemeasuredwiththeThermostepcorrelateswellwiththevalueoffreecarbonstipulatedinthestandard.Anadvantageofthethermogravimetricmethodisthefactthatthesamples,afterhavingbeenheated,arenotsubmittedtoambientair;however,duetotheselectedweighingandmeasurementprocessitonlyrepresentsanapproximationtotheISOstandard.Currentlythemethodcannotbeadaptedtofurtherparameterswhichispartlyduetotherequiredhightemperature(LOI1050)resp.tothelargesampleweights(LOI200with1kgsamplevolume).Duetothemissingreferencematerial,validationoftheprocessremainsadifficulttask.3Determinationofthecarboncontent3.1DeterminationoftotalcarbonPart2oftheISO21068seriesalsodescribestheanalysisofsiliconcarbideanditscarboncontent.AcarefuldifferentiationhastobemadebetweenthetotalcarboncontentandtheSiC-boundcarboncontent.Dependingontherelationofthetwobondings,differentanalysismethodsarestipulatedinthestandard.Therearevariouswaystodeterminethetotalcarboncontentinsiliconcarbidesamples,whichdifferinthecombustionmethod(resistanceorinductionfurnace)andinthedetectionmethod.Coulometric,gravimetricandconductometricmethodsdon’trequirecalibration.Thecarboncontentcanbedeterminedbymeasuringtheweight,theelectricchargeortheconductivity.Forthesedetectionmethodstherequirementofchemicalsishigh;moreover,themarketdoesn’tofferadequateinstruments.Alternativeprocedurespermittedbythestandardincludetheuseofelementalanalyzerswithinductionfurnace(EltraCS-800)orresistancefurnace(EltraCS-580)aswellasdetectionwithinfraredcells.AcombinationofthetwocombustiontechnologiesandacombinedusageofthedetectorshavebeenrealizedinEltra’sCS-2000analyzer(fig.3).Fig.3:ELTRA’sCS-2000featurestheuniqueDualFurnaceTechnologyResistancefurnaceswithceramictubeoperateatlowertemperaturesthaninductionfurnaces.Theceramiccombustiontubesallowforamaximumtemperatureof1,500°C.TosafelyoxidatethetotalcarboninthesiliconcarbidetoCO2,resistancefurnacesrequiretheadditionofacceleratorssuchasleadborateortinpowder.Duetoatemperatureofapprox.2,500°Ctheoxidationofsiliconcarbidetendstotakeplacemorequicklyandmorereliablyinaninductionfurnace;moreover,themeasurementresultsshowfewervariations(seetable2).Whereastheacceleratorsfortheresistancefurnacesareusedforoxidation(leadborate)orlocaltemperaturerise(tinpowder),themetalacceleratorsusedininductionfurnacesarerequiredtoensurecombustioninthefirstplace,assiliconcarbidehasnoproperelectricconductivity.Aclearerdefinitionofthecalibrationmethodsforthevariousprocedureswouldbeawelcomeextensionofthestandard.Though[1-2]mentionsuitablecalibrationmaterials,thecalibrationwithgraphiteorcalciumcarbonateisonlydescribedindetailforthedetectionmethodwiththermalconductivitycells(chapter5.4.5).Theresultsshownintable2wereobtainedbycalibrationwithgraphite.3.2DeterminationoffreecarbonThisparametercanbeeasilydeterminedwithelementalanalyzers.Thestandardstipulatestheuseofananalyzerwithresistancefurnaceandinfrareddetection;howeverthedirectdeterminationislimited(chapter6.4.1).Astheupperlimitforthefreecarboncontentisgivenwith2%,analysissolelybycombustion,forexampleoftheEuronormstandardCRM781-1withaninformativevalueof37.22%isnotpermitted.Thus,characterizationoffreecarbonbydirectcombustionispracticallylimitedtopuresiliconcarbide.Tomeasureverylowconcentrations,thestandardspecifiestheuseofaquartztube(asisused,forexample,inEltra’sCW-800analyzer).Theanalyzerdescribedinchapter3.1ofthisarticleisnotsuitableforthisapplication;duetotheusageofdifferentfurnaces,ceramictubescannotsimplybeexchangedforquartztubes.Duetotheirpermeablestructureceramictubesarenotsuitedforthedeterminationofverylowconcentrations.Table3showstheresultsofastandardcompliantanalysisofthereferencematerialBAM-S003andadditionallyoftheEuronorm781-1standard,carriedoutwithEltra’sCW-800.Thestandardwasalsomeasuredwithananalyzerwithceramiccombustiontube(EltraCS-580).Theresultsintable3clearlysupporttheusageofafurnacewithquartztubewhenanalyzinglowcarbonconcentrations.Iftheconcentrationsaresufficientlyhigh,analyzerswithceramictubescanalsoprovidemeaningfulmeasurementresults.Inthiscase,thecalibrationoftheelementalanalyzerisofgreatimportance.Whereashigherconcentrationscanbeeasilycalibratedwithgraphite(1**%carbon)orpurecalciumcarbonate(12%carbon),lowconcentrationscanonlybecalibratedeitherwithanexpensivereferencematerialorasyntheticcarbonstandard.3.3DeterminationofthesiliconcarbidecontentTheSiCcontentcanbedeterminedfromthedifferencebetweentotalcarbonandfreecarboncontent;however,thisisonlypermissiblewhenthefreecarboncontentis50%orlessofthetotalcarboncontent.Thisapplies,forexample,totheBAMS-003standardbutnottoCRM781-1.Iftherelationbetweenfreeandboundcarbonismorethan25%,adirectanalysisispossible.IftheCRM781-1sampleisfreedofsurfacecarbon(table3),itcanbeanalyzeddirectlywithaninductionorresistancefurnace(table4).4AnalysisofoxygenandnitrogenThethirdpartoftheDINEN21068seriesdealswiththedeterminationofoxygenandnitrogen.Incontrasttothedeterminationofcarbonspecies,thesampleneedstobeheatedinaninertgasflow(helium)tomeasureitsoxygenandnitrogencontent.Theinertgasfusionmethodhasbeenestablishedformanyyearsandisalsousedforanalyzingmetals.Thestipulatedtemperatureofapprox.2,800°Ccanonlyberealizedinanelectrode/impulsefurnace.Anupperandalowerelectrodeapplyvoltagetothegraphitecruciblecontainingthesamplewhichhaspreviouslybeenpurgedwithinertgas.Theanalysisqualityisstronglyinfluencedbythepowerlineavailable(approx.5kWrecommended)andthepurityofthefluxused(nickelbasketsorcapsules).Thesematerialsarebasicallyusedformeltingpointreductiontoreleasethegassescontainedinthesiliconcarbidefromthemoltensample.Table5showstheresultsfortheBAMS-003standardobtainedwithEltra’sONH-2000analyzer.Theanalysiswascarriedoutwithpre-edgednickelbasketsanda5kWpowerline.Aftercalibrationwithacertifiedsteelstandard,goodcompliancewiththecertified/informativevalueswasachieved.Primarysubstances(suchasKNO3)canalsobeusedforcalibration;however,thesesubstancesneedtobedissolvedanddilutedfirst,andarethendriedinnickelcapsules.Themeasuringprincipledoesnotallowfordirectanalysisofliquidstandards.Theinertgasfusionprocedurecanbeeasilyappliedtotheanalysisofotherceramicswithhighnitrogen(e.g.Si3N4)oroxygen(e.g.SiO2,ZrO2)content.5ConclusionAcomprehensiveanalysisofsiliconcarbideandmixturescontainingsiliconcarbideinaccordancewiththeDINENISO21068standardseriesinvolvessophisticatedtechnicalequipment.Ifthedeterminationofmetals,notelaboratedoninthisarticle,istakenintoaccountaswell,theadditionaluseofspectrometers(ICP,OES,XRF)iscalledfor.Themarketoffersanalyzersforstandard-compliantanalysisofcarbonconcentrations;however,differentanalysisspecificationsrequiredifferentconfigurations(inductionfurnace,combustioninceramicorquartzglasstube).Oxygenandnitrogenareeasilyandreliablydeterminedwithanalyzersusinginertgasfusion.Forafutureupdateofthestandardamoreextensivetreatmentofthermogravimetricmethodswouldbehighlyrecommendable.ModernTGAanalyzersnotonlyreducehumanerrorsthroughautomizationbut,thankstoflexibleuseofcarriergasandtemperatures,providemanifoldpossibilitiestocharacterizeSiCandrelatedproducts.Author:Dr.AndreKlostermeierProductManagerELTRAGmbHPhone:+49(0)2104/2333-300E-Mail:a.klostermeier@eltra.comLiterature[1]RmpChemieLexikon,10thedition(2002)[2]ENISO21068(2008),Parts1,2,3[詳細(xì)]

  2018-08-16 10:00

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