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StructureStruct

    Structure

JoURNALoFRAREEARTHs

    ;Vo1.21,No.1,Feb.2003,P.73

    ;StructureAnalysisofOxidationFilmofIgnitionInhibitionAZ91D

    ;MagnesiumAHoyAdded,thCerium

    ;HuangXiaofeng(黄晓锋),ZhouHong(周宏),HeZhenming(何镇明)

    ;(J.SchoolofMaterialsScienceandEngineering,HarbinInstituteofTechnology,Harbin150001,China; ;2.SchoolofMaterialsScienceandEngineering,JilinUniversity,Changchun130025,China) ;Abstract:TheeffectofceriumonignitiontemperatureofAZ91Dmagnesiumalloywasstudied.Bytheadditionofcerium

    ;0fl%.theignitiontemperatureisraisedby180oC,sothemagnesiumalloyaddedwit}Iceriumcanbemeltedinair.1’he

    ;burningtemperatureincreaseswiththeincreasingofcerium.Thestructureandchemicalcompositionsofthesurfaceoxide

    ;filmwereinvestigatedbyXRDandAugerelectronspectrometry(AES).TheresultsofxRDindicatethattheoxidefilmof

    ;thesurfaceofignitioninhibitionmagnesiumalloycanchangefromloosestructureofsimplemagnesiatocompactcomposite

    ;structureconsistingofmagnesia,ceriumoxide,Mgl7A112andaluminumoxide,whichhasexcellentignitioninhibition

    ;effect.AESdepthprofileanalysisshowsthattheoxidefilmcanbedividedintothreelayers.1’heoutsidelayerismainly

    ;madeupofmagnesia.thelniddlelayer.whichconsistsofceriumoxide,magnesia,andaluminumoxide,iscompoundand

    ;compact.Thermodynamicanalysisindicatesthatthestructureofthesurfaceoxidefilmisaccordanttothechangeoffree

    ;energyandhighvaporpressureofmagnesium.

    ;Keywords:metalmaterials;ignition-inhibition;cerium;oxidefilm;magnesiumalloy;earths ;CLCnumber:TG146.2Documentcode:AArticleID:10020721(2003)01007304

    ;Magnesiumalloysaleoneofthelighteststructural

    ;materials,whichhavehighspecificstrengthandgood

    ;combinationofmechanicalproperties,corrosionresis

    ;tanceandcastability….However,commercialappli—

    ;cationsofmagnesiumalloysalelimitedbecauseofin

    ;flammabilityduringdiecasting_2J.Itisverydangemus

    ;toproducemagnesiumalloyintheairduetoignition

    ;duringmeltingandcasting.Thisisacriticalobstacle,

    ;whichhindersindustrialapplicationofmagnesiumal

    ;loys.UseofprotectiongasofSF6,however,isharm

    ;fultoworkerandenvironmentL.So.itisveryimpor.

    ;tanttofindnewmeanstoinhibitignition.Alloyingis ;alleffectivemethodtokeepmagnesiumalloyfrombu. ;rning.Inthepresentwork.ceriumwasaddedto ;AZ91Dmagnesiumalloy.whichisthemostwidely ;useddiecastingmagnesiumalloytostudyitseffecton ;ignitiontemperature.Aconstructingmodelwasestab

    ;lishedtoexplainthermodynamicallytheprocessofthe ;formati0n0foxidefilms.

    ;1Experimental

    ;Thechemicalcompositions(%,massfraction) ;ofAZ91Dalevenasfollows:7.5%9.0%A1,

    ;0.2%0.8%Zn.0.15%0.5%Mn.Theamount

    ;ofceriumis0.5%,0.6%.0.7%,0.8%,0.9%,

    ;1.0%respectivelyandwasaddedwhent}Ietempera. ;tureofthemeltwasabout650oC.11hemasteralloys ;weremeltedincrucibleelectricfumacewit}Iautomatic ;temperaturecontro1.Themeltwasheldat650oCfor ;30mintomakesurethatceriumwascompletelydis. ;solvedunderargonatmosphere,thenitwaspouredin

    ;toarectangularsteelmouldwit}Iallinternaldimen. ;sionsof40mm×40mln×15inln.Themeltwasre. ;finedbeforepouring.Thecompositionofcastingsam

    ;pieremainedunchanged.Gatheringoftemperatures ;wastakenbyEVOCPCIDAS.Thekneepointwas

    ;takenasignitiontemperature.Eachresultingmeasure- ;mentist}Ieaverageofaminimumof10measure. ;ments.Thestructureandchemicalcompositionsoft}Ie ;oxidefilmwereinvestigatedbyXRDandAES. ;2ResultsandAnalysis

    ;2.1Effectofcerinmonburn~temperature

    ;Theburningtemperaturewasdefinedbytheburn

    ;ingofmagnesiumalloyattheambientandthedimen

    ;sionofburningtemperaturedecidedtheeffectofigni

    ;tion-inhibition.AZ91Dwillbumat550oCwit}Iout ;Receiveddate:20010223:reviseddate:20020718

    ;Foundationitem:ProjectsupposedbyJilinProvinceScienceandTechnologyCommi~ion

    ;Biography:HuangXiaofeng(1971),Male,Doctoralcanchidate ;*Correspo~dingauthor

    ;

    ;74

    ;ceriumandveofflotsofenergywhilethereactionof ;magnesiumwithO2Occurs.Effectsofceriumonthe ;ignitiontemperatureofAZ91Dwerestudiedwithout ;theprotectionofargonatmosphere.Effectoftheceri.

    ;umontheburningtemperatureofAZ91Disgivenin ;Fig.1.AsindicatedinFig.1.theburningtempera- ;tureincreaseswithincreasingofcerium.Theburning ;temperatureis668oCforAZ91D.0.5Cemagnesiunia1. ;1oy.Theburningtemperatureis724oCforAZ91D.1.0Ce ;magnesiuma?ov.Wheno~dafionfilmisformedonthe ;surfaceofthemelt,burningwillnotoccur. ;2.2Phasecompositionofsurfacefilm

    ;ResultsofXRDanalysis(Fig.2)showthatthe ;mainintermetailiccompoundofMg-A1phaseisMgl7 ;All2,andtheotherphasesaremagnesia,ceriumox- ;ideandaluminumoxide.

    ;2.3Elementdistributioninsurfacefilm ;Itcanbeseenthattheelementdistributionis ;changedfrommeltsurfacetoinner.AESresultis ;shownfromFig.3.AscanbeseeninFig.3,theatom ;densityofmagnesiumisabout46%fromthesurfaceto ;thedepthof110nm,andtheatomdensityofcerium ;andaluminumisabout3%.Furthermore.bigchange ;ofelementdensitytakesplacefrom110to250nm. ;ContentofCe/%

    ;Fig.1Influenceofceriumonburningpoint ;Fig.2XRDanalysisofoxidefilm

    ;G--Ee203;?Mgl7A1l2;?MgO;×Al203

    ;JOURNALOFRAREEARTHS,Vo1.21,No.1,Feb.2003 ;Fig.3Distributionofelementsinoxidefilm ;Densityofceriumgraduallyincreaseswiththedepth ;anddecreasesfrom250Bintoinner.Densityofmag- ;nesiumdecreaseswithincreasingofdepthandincreas

    ;esfrom250Bintoinner.Dentsiyofoxygendecreases ;rapidlywiththedepthfrom110to250Bin. ;3Discussion

    ;3.1Thermodynamicanalyses

    ;Thestructureofoxidefilmcanbeanalyzedby ;thermodynamics.Thereare4reactionsinoxidefilm ;reactionprocess,i.e:L’.

    ;Mg(1)+112O2(g):MsO(s)(1)

    ;2Ce(s)+3/202(g)=Ce2O3(s)(2)

    ;Ce(s)=Ce(1)(3)

    ;3MgO+2[Ce]=Ce2O3+3Mg(1)(4)

    ;rI1lleEq.(4)canbeobtainedbyEqs.(1)(3).rI1lle

    ;relationofstandardGibbsenergiesontemperatureof ;Eqs.(1)(4)canbeshownasfoilOWS:

    ;?G.:6O9570+116.52

;?G.2:178800o+286.6T

    ;?G01=54805.104

    ;?God:4679057.856T

    ;rI1llestandardGibbsenergiesofreaction,AG.4at923 ;Kcanbeobtainedasf0Uows:

    ;AG04:6611.088J?melI1<0

    ;TheGibbsenergiesofreaction,AGO4canbeobtained ;asfollows:

    ;?G4:AGO4+Rln12Ce20312Mg

    ;afi~o12

    ;(5)

    ;whereaMgo,ace,12Ce

    ;2

    ;03andaMgarethedegreeofac

    ;tivity.Solidstateistakenasstandardstate,i.e., ;12MgO:1,12Ce

    ;2

    ;03=1,heremolarfractionisassulned

    ;thesalneasthedegreeofactivity.ThusEq. ;(5)may

    ;berewrittenintheform:

    ;崛一~4+RT(6)

    ;

    ;OxidationFilmofIgnitionInhibitionAZ91DMgAlloyAddedwithCe ;where,?<.,<..

    ;AG~4<0.Soceriumwillreactwithmagnesiaandre

    ;suhintheincreaseofceriumoxide,leadingtothein

    ;creaseofdensityofoxidefilm.

    ;3.2Compositionofoxidefilm

    ;BytheinvestigationofFigs.2and3,theoxide ;filmcanbedividedintothreelayers.Thedepthof ;outerlayeris110mminwhichthecontentofalumi

    ;numandceriamislowandseldomchangeswiththe ;depth.Theouterlayerismainlymadeupofmagne

    ;sia.Themiddlelayers.whichconsistofceriumox. ;ide,magnesia,andaluminumoxide,arecompound ;andcompact.Thereasonthatcomponentchangesin ;thesurfaceoxidefilmmaybeduetolargequantitiesof ;magnesiumvaporarisingfromhighvaporpressureof ;magnesiumduringthemeltingprocess(‘.Themag

    ;nesiumvaporcanreactwithoxidetoformmagnesia, ;coveringatthesurfaceofmagnesiumalloymelt.Mag

    ;nesiahasloosestructureandcannotpreventoxygen ;frompassingthroughthemagnesialayer.Thuscerium

    ;willreactwithoxygenandmagnesiatoformceriumox. ;ide,littlealuminumreactingwithoxygentoformalu

    ;minumoxide.AccordingtoHume:Rotheryempirical ;rules,whentherelativeatomicradiusdifierenceis ;largerthan15%.1ittlesolidsolutionwillgenerateL9_. ;Becausetheatomicradiusofceriumis15%larger ;thanthatofMg,ceriumhaslittlesolubilityinMg ;meh.Sothereisatendencyforceriumtoconcert. ;trateonthesurfaceofthemelt.Themiddlelayerhas ;themainfunctiontopreventtheburningofmagne

    ;slum.Themainreasonistheincreaseofthetitox.

    ;idefilmunity.ThetightnessisalwaysexpressedbyP

    ;Bratio….WhentheP.Bratioislargerthan1.the

    ;oxidefilmhasthefunctiontopreventtheburningof ;magnesium.WhenthePBislessthanunity.theox.

    ;idefilmwillbetheloosestructureandcannotprevent ;theburningofmagnesium.ThePBratioofmagnesia

    ;is0.78,sothefilmofmagnesiaisloose.TheP.Bra

    ;tioofaluminumoxideislargerthanunityandthepore ;spaceofmagnesiawillbefilledbyceriumoxide,and ;thecapacityofaluminumoxideandceriumoxidewill ;expandduringtheprocessofformationincreasingthe ;densityofoxidefilmandpreventingthediffusionof ;oxygenintoinnerlayer.Thepropertyofignitioninhi

    ;bitionwiIIincrease.

    ;3.3Modelofoxidation

    ;Themodelofoxidationcanbebuiltasshownin ;Fig.4.Therearelargequantitiesofmagnesiumvapor ;becauseofitshighvaporpressureduringmelting.The ;75

    ;magnesiumvaporwillreactwithoxygentoformmag

    ;nesia.Magnesiaisakindofloosestructure,which ;cannotpreventtheoxygenpassingthroughthemagne- ;sialayer.Accordingtothermodynamiccomputation, ;ceriumwillreactwithmagnesiaandO2toformcerium ;oxideandMgwillreactwith02toformmagnesia.A?

    ;theprocesseswillstopuntilthedensityofoxidefilm ;increasestosomevalue.

    ;Fig.4Oxidefilmmode1

    ;..?.———

    ;Mg~Ah+

    ;CeaO,7All2

    ;..——

    ;Oxide+melt

    ;Bytheadditionofceriumof1%.theignition ;temperatureisraisedby180?.Thesurfaceofmelt

    ;alloyformsoxidationfilmwhichisaprotedingform, ;butitvailbemeltedinairwhenburning.Insurface ;film,thecontentofceriumishigherthanthatinouter ;layerandinnerlayer.rI’hecontentofoxygeninouter

    ;lateriShigherthanthatinmiddlelayerandinner ;layer.TheoxidefilmCallbedividedintothreelayers ;i.e.,outerlayer,middlelayerandmatrix.Theouter ;layerismainlymadeofmagnesia,whilethemiddle ;layer,whichconsistsofceriumoxide,magnesia, ;Mg,7AI,2andaluminumoxide,iscompoundandcorn. ;pact.

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    ;RareEarth-BasedAB5TypeElectrodeHydrogenStorage

    ;AlloywithLowContentCobalt

    ;LiPing,WangXinlin,LinYufang,WuJianmin,LiRong(CentralIron&SteelResearchlnsti. ;tute,Beijing100081,China)

    ;Abstract:Inordertoreducecostofrareearth-basedandactivepropertyisCu>Cr>Si.Onlyasmall ;AB5-typehydrogenstoragealloy,thereplacementofquantityofCu,Cr,Siisfavorable.Thedischargeca- ;Cr,Si,Cuf0rCoandcontrolofthestoichiometricpro-pacity,activationandratedischargeoftheselowCo ;portionofalloywereusedtoprepareAB5-typehydro-contentalloyshavereachedrequirementintheNi/MH

    ;genstoragealloywithlowCocontent.Theresultsbatteryapplicationthroughcontrolofstoichiomotry, ;showthatthesubstituenteffectofCu.Cr,SiforC0inbutthecyclelifeisnotimprovedbythisproeedure. ;circlelifeisSi>Cr>Cu.andindischargecapacity

    ;Keywords:energystoragetechnolgy;electrodehydrogenstoragealloys;11011-stoichiometric;electrochemicalprope~y;lowCocontentalloy;rareearths

    ;(SeeJ.Chin.RE.Soc.(inChin.).2003,21(1):65forfulltext)

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