Silicon nitride

History

HenryEdinSt.ClairDevilleandFriedrichWeilerfirstreportedthesynthesisofsiliconnitridein1857.Inthesynthesismethodtheyreported,anothercruciblefilledwithsiliconwasburiedinacruciblefilledwithcarbonandheatedinordertoreducetheinfiltrationofoxygen.Theyreportedaproductthattheycalledsiliconnitride,buttheycouldnotfigureoutitschemicalcomposition.In1879,PaulSchuetzenbergermixedsiliconwithaliningmaterial(apastethatcanbeusedasacruciblelining,obtainedbymixingcharcoal,coalorcokeandclay)andheateditinablastfurnace,andreporteditasacomponentItisacompoundofSi₃N₄.In1910,LudwigWeissandTeodorEngelhardtheatedthesiliconelementunderpurenitrogentoobtainSi3N4.In1925,FriederichandSittigusedthecarbothermalreductionmethodtocombinesilicondioxideandcarboninanitrogenatmosphere.Heatto1250-1300℃tosynthesizesiliconnitride.

Inthefollowingdecades,untilthecommercialuseofsiliconnitrideappeared,siliconnitridewasnotpaidattentiontoandstudied.From1948to1952,AchesonfoundedtheEmeryCompanynearNiagaraFalls,NewYork,andregisteredseveralpatentsforthemanufactureanduseofsiliconnitride.In1958,thesiliconnitrideproducedbyUnionCarbidewasusedtomakethermocoupletubes,rocketnozzlesandcruciblesformoltenmetal.Britishresearchonsiliconnitridebeganin1953,withthegoalofmanufacturinghigh-temperaturepartsforgasturbines.Thisledtothedevelopmentofbondedsiliconnitrideandhot-pressedsiliconnitride.In1971,theDefenseAdvancedResearchProjectsAgencyundertheUSDepartmentofDefensesigneda$17millioncontractwithFordandWestinghousetodeveloptwoceramicgasturbines.

Althoughthepropertiesofsiliconnitridehavelongbeenwellknown,thesiliconnitride(approximately2×5µminsize)thatexistsinthenaturalworldoftheearthwasonlydiscoveredinmeteoritesinthe1990s.Tocommemoratethepioneerofmassspectrometryresearch,AlfredOttoCarlNealnamedthistypeofsiliconnitrideorefoundinnature"nierite".However,thereisevidencethatthistypeofsiliconnitrideoreinmeteoritesmayhavebeenfoundinAzerbaijanintheformerSovietUnionearlier.MeteoritescontainingsiliconnitridemineralshavealsobeenfoundinGuizhouProvince,China.InadditiontothemeteoritesthatexistonEarth,siliconnitrideisalsodistributedincosmicdustinouterspace.

Crystalstructureandcharacteristics

Thebluespheresarenitrogenatomsandthegrayspheresaresiliconatoms

SiliconNitride(Si₃N₄)Therearethreecrystalstructures,namelyα,βandγthreephases.ThetwophasesofαandβarethemostfrequentlyoccurringformsofSi₃N₄andcanbepreparedundernormalpressure.Theγphasecanonlybesynthesizedunderhighpressureandhightemperature,anditshardnesscanreach35GPa.

Synthesismethod

Hexagonalβ-Si₃N₄

Canbeat1300-1400℃Siliconnitrideisobtainedbydirectchemicalreactionwithelementalsiliconandnitrogenundertheconditions:

• 3Si(s)+2N₂(g)→Si₃N₄(s)

Itcanalsobesynthesizedwithdiimine

• SiCl₄(l)+6NH₃(g)→Si(NH)₂(s)+4NH₄Cl(s)at0℃

• 3Si(NH)₂(s)→Si₃N₄(s)+N₂(g)+3H₂(g)at1000℃Undertheconditionsof

Orusecarbothermalreductionreactiontosynthesizeinanitrogenatmosphereat1400-1450℃:

• 3SiO₂(s)+6C(s)+2N₂(g)→Si₃N₄(s)+6CO(g)

Thesynthesismethodofnitridingelementalsiliconpowderwasdevelopedwiththedevelopmentofsiliconnitrideinthe1950s.Rediscoveredanddeveloped.Itisalsothefirstmethodformassproductionofsiliconnitridepowder.However,ifthepurityofthesiliconrawmaterialusedislow,theproducedsiliconnitridewillcontainimpuritysilicateandiron.Thesiliconnitridesynthesizedbythediaminedecompositionmethodisamorphous,anditneedstobefurtherannealedundernitrogenat1400-1500℃toconvertitintoacrystallinepowder.ThediaminedecompositionmethodissecondonlytotheimportanceofNitridingisacommercialproductionmethodofsiliconnitride.Carbothermalreductionreactionistheeasiestwaytomanufacturesiliconnitrideanditisalsothemostcost-effectivewaytomanufacturesiliconnitridepowderinindustry.

Electronicgradesiliconnitridefilmismanufacturedbychemicalvapordepositionorplasmaenhancedchemicalvapordepositiontechnology:

• 3SiH₄(g)+4NH₃(g)→Si₃N₄(s)+12H₂(g)

• 3SiCl₄(g)+4NH₃(g)→Si₃N₄(s)+12HCl(g)

• 3SiCl₂H₂(g)+4NH₃(g)→Si₃N₄(s)+6HCl(g)+6H₂(g)

Ifyouwanttodepositsiliconnitrideonasemiconductorsubstrate,therearetwomethodsavailableUse:

1. Usinglow-pressurechemicalvapordepositiontechnologyatrelativelyhightemperatureusingverticalorhorizontaltubefurnace.

2. Plasma-enhancedchemicalvapordepositiontechnologyiscarriedoutunderrelativelylowtemperaturevacuumconditions.

Theunitcellparametersofsiliconnitridearedifferentfromthatofelementalsilicon.Therefore,dependingonthedepositionmethod,thegeneratedsiliconnitridefilmmaygeneratetensionorstress.Especiallywhenusingplasma-enhancedchemicalvapordepositiontechnology,thetensioncanbereducedbyadjustingthedepositionparameters.

Silicaisfirstpreparedbythesol-gelmethod,andthenthesilicagelcontainingultra-finecarbonparticlesisprocessedbythecarbothermalreductionmethodandnitridationatthesametimetoobtainsiliconnitridenanowires.Theultra-finecarbonparticlesinsilicagelareproducedbythedecompositionofglucoseat1200-1350°C.Thereactioninvolvedinthesynthesisprocessmaybe:

• SiO₂(s)+C(s)→SiO(g)+CO(g)

• 3SiO(g)+2N₂(g)+3CO(g)→Si₃N₄(s)+3CO₂(g)or

• 3SiO(g)+2N₂(g)+3C(s)→Si₃N₄(s)+3CO(g)

Features

Exceptforhydrofluoricacidandhotphosphoricacid,itdoesnotreactwithotherinorganicacidsandhasstrongcorrosionresistance.

Application

[Applicationofsiliconnitride]

Siliconnitrideisusedasadvancedrefractorymaterial,suchasincooperationwithsicSI3N4-SICrefractoriesareusedinblastfurnaceshaftsandotherparts;forexample,incooperationwithBN,SI3N4-BNmaterialsareusedforhorizontalcontinuouscastingseparationrings.SI3N4-BNserieshorizontalcontinuouscastingseparationringisafine-structureceramicmaterialwithuniformstructureandhighmechanicalstrength.Ithasgoodthermalshockresistanceandwillnotbewettedbymoltensteel,whichmeetstheprocessrequirementsofcontinuouscasting.Seethetablebelow

Moreinformation

Physicalproperties

Relativemolecularweight140.28.Gray,whiteoroff-white.Itisahigh-temperatureinsolublecompound,nomeltingpoint,strongresistancetohigh-temperaturecreep,andthereactionsinteredsiliconnitridewithoutabinderhasaloadsofteningpointabove1800°C;hexagonalcrystalsystem.Thecrystalisahexahedron.ThedensityofSi3N4preparedbyreactionsinteringmethodis1.8~2.7g/cm3,andthedensityofSi3N4preparedbyhotpressingmethodis3.12~3.22g/cm3.TheMohshardnessis9~9.5,theVickershardnessisabout2200,andthemicrohardnessis32630MPa.Meltingpointis1900°C(underpressure).Itusuallydecomposesatabout1900°Cundernormalpressure.Thespecificheatcapacityis0.71J/(g·K).Theheatofformationis-751.57kJ/mol.Thethermalconductivityis(2-155)W/(m·K).Thecoefficientoflinearexpansionis2.8~3.2×10-6/℃(20~1000℃).Insolubleinwater.Solubleinhydrofluoricacid.Thetemperatureatwhichoxidationstartsintheairis1300~1400°C.Thespecificvolumeresistanceis1.4×105·mat20°Cand4×108·mat500°C.Themodulusofelasticityis28420~46060MPa.Thecompressivestrengthis490MPa(reactionsintered).Itreactswithdicalciumdinitrideat1285°Ctoformcalciumdinitrogensilicide,andat600°Citreducesthetransitionmetalandreleasesnitrogenoxides.Thebendingstrengthis147MPa.Itcanbepreparedbyheatingsiliconpowderinnitrogenorreactingsiliconhalidewithammonia.Theresistivityis10^15-10^16Ω.cm.Itcanbeusedasahigh-temperatureceramicrawmaterial.

Productionmethod

Therearetwoproductionmethodsforsiliconnitrideceramicproducts,namelyreactionsinteringmethodandhotpressingsinteringmethod.Thereactionsinteringmethodistoshapethesiliconpowderorthemixtureofsiliconpowderandsiliconnitridepowderaccordingtothegeneralproductionmethodofceramicproducts.Theninthenitridingfurnace,pre-nitridingat1150~1200℃,afterobtainingacertainstrength,itcanbemachinedonthemachinetool,andthenfurthernitridingat1350~1450℃for18~36h,untilallbecomessiliconnitride.Theproductthusobtainedhasaccuratedimensionsandstablevolume.Thehotpressingsinteringmethodistopressandheatthesiliconnitridepowderandasmallamountofadditives(suchasMgO,Al2O3,MgF2,AlF3orFe2O3,etc.)underthepressureof19.6MPaand1600~1700℃.Generally,productsmadebyhotpressingsinteringhavehigherdensityandbetterperformancethanproductsmadebyreactionsintering.Table1liststhepropertiesofsiliconnitrideceramicsproducedbythesetwomethods.

Otherapplications

Siliconnitrideceramicmaterialshaveexcellentpropertiessuchashighthermalstability,strongoxidationresistanceandhighproductdimensionalaccuracy.Sincesiliconnitrideisacovalentcompoundwithhighbondstrengthandcanformanoxideprotectivefilmintheair,italsohasgoodchemicalstability.Itwillnotbeoxidizedbelow1200°C.Theformationofaprotectivefilmat1200~1600°CcanpreventfurtherItisoxidizedandisnotinfiltratedorcorrodedbymanymoltenmetalsoralloyssuchasaluminum,lead,tin,silver,brass,nickel,etc.,butcanbecorrodedbymoltenmagnesium,nickel-chromiumalloy,stainlesssteelandothermoltenmetals.

Siliconnitrideceramicmaterialscanbeusedforhigh-temperatureengineeringcomponents,advancedrefractoriesinthemetallurgicalindustry,corrosion-resistantcomponentsandsealingcomponentsinthechemicalindustry,toolsandcuttingtoolsinthemachiningindustry,etc.

Sincesiliconnitridecanformastrongbondwithsiliconcarbide,aluminumoxide,thoriumdioxide,boronnitride,etc.,itcanbeusedasabondingmaterialandmodifiedindifferentproportions.

Inaddition,siliconnitridecanalsobeappliedtosolarcells.AfterthesiliconnitridefilmisplatedbythePECVDmethod,itcannotonlybeusedasananti-reflectionfilmtoreducethereflectionofincidentlight,butalso,inthedepositionprocessofthesiliconnitridefilm,thehydrogenatomsofthereactionproductenterthesiliconnitridefilmandthesiliconwafer,Playedaroleinpassivatingdefects.Theratioofthenumberofsiliconnitridetosiliconatomsisnotstrictly4:3,butfluctuateswithinacertainrangeaccordingtodifferentprocessconditions.Thephysicalpropertiesofthefilmcorrespondingtodifferentatomicratiosaredifferent.

Usedforultra-hightemperaturegasturbines,aircraftengines,electricfurnaces,etc.

Structure

ThetwotopsoftheregularoctahedronareSi,andthefourNsarethe4pointsofthemiddleplaneoftheoctahedron,andthenthecenteroftheplanegeneratedbythesefourNs,ThisisthethirdSiattheend.BesuretoconfirmthateachSiisconnectedtofourNs,eachNisconnectedto3silicons,andthereisnoconnectionbetweenNNs.

Materialproperties

Thestrengthofsiliconnitrideisveryhigh.High,especiallyhot-pressedsiliconnitride,isoneofthehardestmaterialsintheworld.Itisextremelyresistanttohightemperatures,anditsstrengthcanbemaintainedtoahightemperatureof1200°Cwithoutfalling.Itwillnotmeltintoameltafterbeingheated,andwillnotdecomposeuntil1900°C.Ithasamazingchemicalcorrosionresistanceandcanwithstandalmostallinorganicacids.Andcausticsodasolutionbelow30%,itcanalsowithstandthecorrosionofmanyorganicacids;atthesametime,itisahigh-performanceelectricalinsulatingmaterial.

Siliconnitride-propertieschemicalformulaSi3N4.Whitepowderycrystal;meltingpoint1900℃,density3.44g/cm(20℃);Therearetwovariants:αtypeishexagonalclose-packedstructure;βtypeisspar-likestructure.Siliconnitrideappearsgraywhenithasimpuritiesorexcesssilicon.

Siliconnitridehasalmostnoeffectwithwater;itisslowlyhydrolyzedinastrongacidsolutiontoformammoniumsaltandsilicondioxide;itiseasilysolubleinhydrofluoricacid,anddoesnotworkwithdiluteacid.Thestrongalkalisolutioncanslowlycorrodesiliconnitride,andthemoltenalkalicanquicklytransformsiliconnitrideintosilicateandammonia.Siliconnitridecanreducetransitionmetal(seetransitionelement)oxides,leadoxide,zincoxide,andtindioxideattemperaturesabove600°C,andreleasenitrogenoxideandnitrogendioxide.ThefollowingreactionoccursbetweensiliconnitrideandcalciumdinitrideCa3N2at1285℃:

Ca₃N₂+Si₃

Ca₃N₂+Si₃N₄─→3CaSiN₂

Thereareseveralmethodsforpreparingsiliconnitride:Reactwithnitrogen;reactpuresiliconwithammoniaat1500℃;burnamixtureofsilicondioxideandcarboninnitrogencontainingasmallamountofhydrogen;theammonolysisproductofSiCl4Si(NH2)4Completethermaldecomposition.Siliconnitridecanbeusedascatalystcarrier,hightemperatureresistantmaterial,coatingandabrasive.

Siliconnitrideceramicshavethecharacteristicsofhighstrengthandhightemperatureresistance.Amongtheceramicmaterials,theyhavethebestcomprehensivemechanicalproperties,suchasthermalshockresistance,oxidationresistance,abrasionresistance,andcorrosionresistance.Itisthefirstcandidatematerialforceramicsforheatenginecomponents.Inthemachineryindustry,siliconnitrideceramicsareusedasbearingballs,rollers,ballraces,molds,newceramiccuttingtools,pumpplungers,andmandrelsealingmaterials.

Inthechemicalindustry,siliconnitrideceramicsareusedaswear-resistantandcorrosion-resistantparts.Suchasballvalves,pumpbodies,combustionvaporizers,filters,etc.

Inthemetallurgicalindustry,siliconnitrideceramicsareresistanttohightemperatures,havelowfrictioncoefficientsandareself-lubricating.Itisstabletomostmetalandalloysolutions.Therefore,itcanbeusedtoproducemetalmaterialprocessingtoolsanddies,suchasdialmandrels,extrusionandwiredrawingdies,rollers,transferrollers,heatingelementfixtures,thermocouplesets,metalheattreatmentsupports,Crucible,aluminumliquidguidecamp,aluminumcladdinglining,etc.

Siliconnitrideceramicmaterialsarealsowidelyusedinelectronics,militaryandnuclearindustries.

1.Thephysicalandchemicalpropertiesofsiliconnitrideceramicpowderandthetechnicalindicatorsoftheproduct

Siliconnitrideceramicisawhite-graypowderwiththemolecularformula:SI3N4;

Molecularweight:140.3,density3.2g/cm³

Thechemicalcomposition:N>38-39;0<1-1.5;C<0.1;Fe<0.2.

Thegranularityisdeterminedbyuserrequirements.

Comparisontableofmeshnumberandparticlesize(μm)

Technologydevelopment

(1)Technicaloverviewofsiliconnitrideindustry

Siliconnitrideisacompoundsynthesizedunderartificialconditions.Althoughsiliconnitridewasdirectlysynthesizedmorethan140yearsago,itwasonlyastable"refractory"nitridethatremainedinpeople'smemory.AftertheSecondWorldWar,withtherapiddevelopmentofscienceandtechnology,materialswithhightemperatureresistance,highhardness,highstrengthandcorrosionresistanceareurgentlyneeded.Afterlong-termefforts,siliconnitridewasnotvalueduntil1955,anditwasonlyinthemid-1970sthathigh-quality,low-costsiliconnitrideceramicproductswithawiderangeofimportantuseswereproduced.

mycountryhasbeenstudyingsiliconnitridetechnologysincethemid-1980s.Mainlytostudythestructuralsiliconnitridematerialwiththehighestweightreductionefficiency-poroussiliconnitridematerial.Theresearchonsiliconnitridecompositematerialshasjuststarted,andtherearefewresearchesonthetheoreticaldesignandexperimentaldesignoftheporoussiliconnitridecompositematerialcompositionsystem.,Isstillintheexploratorystage,affectedbythelackofrelevantresearchmaterialsathomeandabroad,mycountry’sresearchhasalwaysbeenrelativelybackwardinthisregard.Isblank.Researchinthisareaneedstobefurtherstrengthened.Thepredictionofthedielectricconstantofporoussiliconnitrideceramicsandtheinfluenceofitspropertiesarenotfullyunderstood,andtherearefewresearchesonitstheoreticalworkandexperimentalwork.

(2)Theproductionprocessofsiliconnitrideproducts:

Siliconnitrideproductscanbedividedintoreactionsinteredproducts,hotpressedproducts,atmosphericsinteredproducts,isostaticpressingaccordingtotheprocessSinteredproductsandreactionre-firedproducts,etc.Amongthem,reactionsinteringisacommonlyusedmethodforproducingsiliconnitriderefractoryproducts.

Thereactionsinteringmethodtoproducesiliconnitrideproductsistousegroundsiliconpowder(generallylessthan80μminparticlesize)tobemoldedbymachineorisostaticpressing.Afterthegreenbodyisdried,itisheatedto1350～Itismadebynitridingat1400℃atthesametimeduringthefiringprocess.Withthisproductionmethod,therawmaterialconditions,firingprocessandatmosphereconditionshaveagreatinfluenceontheperformanceoftheproduct.

Siliconpowdercontainsmanyimpurities,suchasFe,Ca,Aì,Tiandsoon.Feisconsideredtobethecatalystinthereactionprocess.Itcanpromotethediffusionofsilicon,butatthesametime,itwillalsocausedefectssuchaspores.ThemainfunctionofFeasanadditive:itcanbeusedasacatalystinthereactionprocesstopromotetheformationofSiO2oxidefilmonthesurfaceoftheproduct;theformationofiron-siliconmeltsystem,nitrogendissolvesinliquidFeSi2,andpromotestheformationofβ-Si3N4.However,iftheironparticlesaretoolargeorthecontentistoohigh,defectssuchasporeswillalsoappearintheproduct,whichwillreducetheperformance.Generally,theamountofironaddedis0-5%.ImpuritiessuchasAl,CaandTicaneasilyformeutecticwithsilicon.Appropriateadditioncanpromotesinteringandimprovetheperformanceofproducts.

Thefinertheparticlesizeofthesiliconpowderandthelargerthespecificsurfacearea,thesinteringtemperaturecanbelowered.Comparedwithsilicafumewithcoarserparticlesize,thecontentofα-Si3N4intheproductishigherforthefinerparticlesizesiliconpowder.Reducingtheparticlesizeofsiliconpowdercanreducethemicroporesizeoftheproduct.Properparticlesizeratiocanincreaseproductdensity.

Temperaturehasagreatinfluenceonthenitridingrate.Thenitridationreactionstartsat970-1000°C,andthereactionrateincreasesataround1250°C.Inthehightemperaturestage,duetotheexothermicreaction,ifthetemperaturequicklyexceedsthemeltingpointofsilicon(1420°C),theflowofsiliconwilleasilyoccur,whichwillcausethesiliconpowderbodytomeltandcollapse.