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Study on the Preparation of Nanosized-ZnO and Skin-colored Products Through Immediate Precipitation-Microwave Calcining
Author: CaoJun
Tutor: ZhouJiCheng
School: Xiangtan University
Course: Chemical Engineering
Keywords: Zinc precursor basic zinc carbonate direct precipitation method Microwave roasting time roasting Nano microwave field precipitation reaction
CLC: TQ133
Type: Master's thesis
Year: 2004
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Abstract
SummaryStudyonthepreparationofnanosized-ZnOthroughimmediateprecipitation–microwavecalcining1.1Theimmediateprecipitationmethodbelongstothereactiveprecipitationinanhydrousmethods.Atpresent,theenhydrousmethodsareseldomusedinindustrialproductionofnanosized-ZnO,buttheimmediateprecipitationmethodhassamplecraftsandcostsless.TheZnOnanoparticlespreparedbythismethodareverysmallinsizeandevenlydistributed,andskin-coloredproductscanbegainedthroughtreatingforerunnerwithammonia.Incontrastwiththemethodsofadvancedtechniqueandhighcostabroad,theimmediateprecipitationmethodismoresuitabletoourcountry’srealconditions.Asotherreactiveprecipitationmethods,ithasthesameproblemsasfollowed:(1)Becauseofthesmallsizeandstrongsurfacetension,theyareeasilyagglomeratedwhennanoparticlesareprepared.Theagglomerationcanbedividedintotwotypes:oneissoft-agglomeration,andtheotherishard-agglomeration.Thesoft-agglomerationcanberemovedmostlybychemicalormechanicalprocess,whilethehard-agglomerationisreallyhardlyeliminatedbythem.So,thehard-agglomerationmakesthesmallparticleshardtodisperse.(2)Routinecalciningisinefficient.Ifcalciningtemperatureistoolow,theforerunnerwillbenotdecomposedcompletely,iftoohigh,thecrystalparticleswillgrowupandbesintered.1.2Inordertosolvethoseproblemsabove,webringforwardaimprovedcraft.Ithasthosefollowingadvantagesandspecialtiescomparedtothatreported:(1)Advantages:a.ThenewcraftusescheapheptahydratedzincsulfateandAmmoniumhydrogencarbonateasmaterials.wecangetwelldispersedparticlesbywashingitonetimewithethanolaftertheforerunnerwaspreparedandwashed,anditreducedtheagglomeration;b.Thenewmicrowave-calciningtechniquewasusedinthisexperiment.Itmadethesystembeevenlyheatedandthetemperaturerisedsorapidlythattheforerunnerwereswiftlydecomposed.Sowecanrestrainthegrowingandsinteringofcrystalparticlesandpromotethecalciningefficiency.(2)Specialties:Theagglomerationofparticlescanbechangedthroughtreatingforerunnerwithammonia.AsbasicZinccarbonateasconcerned,itisnotseriouslyagglomeratedinliquidevenifwithoutthedisposingbyammonia.Butwiththeaddingofammonia,thefinalproductsareskin-colored.1.3Processofpreparation:TheimmediateprecipitationreactionoccurredwhentheAmmoniumhydrogencarbonatesolutionwasaddedintothezincsulfatesolution.Wecangetforerunnerfromtheprecipitationafterfiltrating,washing,anddrying.Thentheforerunnerwassetintomicrowaveovenandcalcined.Afteritdecomposedcompletely,nanosized-ZnOwasgained.UsingammoniatoadjustthepHvalueoftheliquidaftertheprecipitationreaction,wecangetskin-coloredproductsatlast.1.4Craftsofpreparationofnanosized-ZnOthroughimmediateprecipitationµwave-calciningwasstudiedthroughexperiments,theresultsasfollowed:(1)Theforerunnerwillbeagglomeratedandbeyellowincolor,iftapwaterwithimpuritieswaschosenasreactiveliquid.Distilledwaterwillreducethisdisadvantageousphenomenon.Addingethanolintotheliquidafterimmediateprecipitation,wecanchangetheagglomerationofforerunner.Ifthevolumeratioofethanolinliquidisunder40%(including0%),theagglomerationislight,above60%,thenheavy.Beingwashedwithethanolonceatlasttime,light-agglomeratedforerunnerwasgottenfinally,anditiseasilydispersed.(2)Theagglomerationcanbechangedbyusingammoniatoadjusttheliquid’spHvalueaftertheprecipitationreaction.BetweenpH=7.8~8.1,hard-agglomerationcomesintobeing.Soagglomerationislightevenifnoammoniawasbeenaddingintotheliquidafterthereaction.Skin-coloredproductscanbegainedultimatelyifwetreatsforerunnerwithammonia,andmoreammoniaisbeenadding,deeperthecoloris.WhenthepHisabove8.5,itisdistinctlyskin-colored,buttheforerunnerislargelydissolved.(3)Calcinedbymicrowaveat900W&2450MHz,theforerunnerwasswiftlyandcompletelydecomposed.Itisefficient.Thetimeofcalciningisinterrelatedwiththechangeofrateofweightlessnessofforerunnerandintensityofelectricfield、distributingofradiationofmicrowave、accumulatingofparticles、contentofcombo-waterinforerunnerandetc.1.5Throughanalysisofproduct’sAFM,TEMimagesandXRDspectrum,itshowsthatnanosized-ZnOpreparedbyimmediateprecipitation-microwavecalciningispurifiedhexagonalstructuredandapproximatlyspherical,under100nmoreven10~20nminsize,evenlydistributed,welldispersedandsoft-agglomerated.2.mechanismofmicrowave-calciningandcomparewithroutinecalciningBasedonthestudyoncraftsofprepatrationofnanosized-ZnOthroughimmediateprecipitation-microwavecalcining,westudiedthemechanismofmicrowave-calciningandcomparewithroutinecalciningthroughexperimentsandtheoreticaldeduction.2.1Experimentalmeasures:Setthedriedforerunnerintothemicrowaveoventocalcine,at900Wandhighfire.Controlthecalciningtimeandpayattentiontothechangeofweightoftheforerunner.Useaccurateelectronicbalancetomeasureits’weight.AnnexathermocoupletothemicrowaveoveninordertomeasuretherisingtemperatureofZnOunderthemicrowaveradiation.2.2Theoryofmicrowave-heating:Microwave-heatingchangesmicrowave-electromagneticenergyintoheatenergy.Theenergytransmitsaselectromagneticwavethroughspaceandintermedium,whathasaimportantrelationtotheheatingprocessofsubstanceandpolarizationofinnermoleculesofsubstance.Theheatingofsubstanceinmicrowavefielddependsontwotypesofpolarization:polarizationofdipolaronandinterface.Thepolarizationofsubstanceappearsasullageofdensityofcurrentinelectricfield.Ullageofconductanceofsubstancepossessedcharactersofsemi-conductordevotesmuchtotheefficientullage.Dipolarpoisecausedbylacunasinthecrystalparticlesplaysalargeroleinthehighullageofsubstance.Besides,becauseofthelacunasinboundaryfloorofsubstance,Maxwell-Wangerpoiseoccurstorespondtothepolarization,anditmakesthetemperatureofsystemsharplyraisedtoo.Correspondingtothevelocityofrisingoftemperatureinmicrowavefield:(T-To)/t=C·ε”eff·f·E2/Cp·ρ(Cisaconstant,ρisthedensityofsubstance,Cpisthespecificheat,fisthefrequencyofmicrowave,Eistheintensityofelectricfield)thepowerconsumedbyelectricfieldistheenergyabsorbedbythesubstance,anditwasincreasedasthetimeofradiationraised,sodidthetemperatureofthesystem.2.3Conclusionsandresultsofexperimentsasfollowed:(1)ThetemperatureofforerunnerbasicZinccarbonateinthemicrowavefieldisnotsharplyrising,onthecontrary,risinggently.ButtheZnOhasasharprisingoftemperatureinthemicrowavefieldasitcanreach600℃in10sseconds.(2)Underthemicrowaveradiation,theforerunner’stemperaturerisesgradually,thenitwasdecomposed.ThetemperatureisexplosivlyrisingwhenZnOisgenerated,ifdidn’tputitundercontrol,thetemperaturerisedtoohightoburntheproducts.Sobeforethathappened,wemustcontrolthetimeofcalcining,interrupttheradiationandchurnuptheforerunnertolettheheatgetputinordertoavoidtheparticles’beingsintered.Infact,microwave-calciningisaintermittentprocess,andthetemperatureisfluctuatingaroundacertainvalue.Thetimeofcalciningiscorrelativetothecharactersofthesubstance,sizeofcrystalparticles,fillingvolumeanddensity,andintensityofelectricfieldandetc.Itistoocomplextoexplain.(3)TheMicrowave-calciningofbasicZinccarbonateconsistsofthreeparts:Losingofcombo-water,warming-upofforerunner-risingoftemperatureofforerunner-decomposingofforerunner.Intheprocessofcalcinging,thesubstanceintenselyabsorbedtheenergyofmicrowaveandchangeditintoheatenergy.Theforerunnerwasrapidlydecomposedandsmallparticleswasgeneratedeasilyundertheconditionofevenheating-upandtemperature’ssharprising.3.Studyonpreparationofskin-coloredflake-shapedZnOandmechanismofpigmentationSomesampleswastreatedwithammoniaaftertheforerunnerwaspreparedbyimmediateprecipitationreaction,sosomeproductsappearedasskin-colored.Itshewusanewmethodtoprepareskin-colorednanosizedZnO.Atthesametime,thecollectingratedescendedastheconcentrationofammoniaisraised.Becausetheforerunnerisamphotericandcanbeeasilydissolvedbyammonia,apartofZincionsreturnedtotheliquidandflewawaywiththefiltrate.IflettingoffthewastewithZincions,itwillwastethematerialsandpollutetheenvironment.Sotakingthisintoaccount,wereclaimedthepartofZincionsfromthefiltratebyvaporizingandre-crystaling,andgotskin-coloredflake-shapedZnOatlast.3.1Experimentalmeasures:afterthepHvalueoftheliquidwasadjustedto8.5byammonia,apartoftheforerunnerwasdissolved.Thefiltrateandtheprecipitationwerecollectedrespectivelyafterfiltrating,andthetreatmentsofthemisasfollowing:(1)Washingtheprecipitationwithdiluentammoniaandethanolbeforesetttingitintotheovenunder80℃,andcalciningitinamicrowaveovenafteritwasdried,thentheskin-colorednanosized-ZnOwasgotten.(2)Filtratingthefiltrateseveraltimestillclearliquidwasgained.Vaporizingtheclearliquidinthemicrowaveoventillitturnsintodope.Then,washingandfiltratingthedopewithdistilledwaterandethanol.Dryingthesub-forerunnerinovenunder80℃andcalciningit,thentheflake-shapedskin-coloredproductswasgottenfinally.ObservetheappearanceofthecrystalwithX-650ScanElectronicMicroscopeandPicoscanAtomicForceMicroscoperespectively,analysethestructureoftheproductswithD/max-3cX-radiationdiffracter.3.2Theoryofexperiment:BecausetheforerunnerbasicZinccarbonate:ZnCO3?2Zn(OH)2?H2Oisamphoteric,soitcanbedissolvedbyammoniaandtheZincionsreturnstothefiltrate.Byvaporizingthefiltrate,theZincionswasprecipitatedassub-forerunnerZn(OH)2.Then,beingcalcined,theZn(OH)2wasdecomposedintoZnO.3.3Tokenofskin-coloredZnO:(1)TheSEMimagesofflake-shapedandskin-coloredZnOpreparedbyfiltrate-vaporizingandcalciningisirregularflake-shapedstructure.Thosesmallandbigfragmentspilesuptogetherorlapsover.Thosefragmentsareseveraltenstoseveralhundredsmicronsinsize.TheXRDspectrumshowsthatafewmaindiffractiveapicesarethediagnosticdiffractiveapicesofhexagonalZnOcrystal.Itprovedthattheflake-shapedproductiscomposedbynumeroushexagonalZnOcrystals.Butseveralsecondarydiffractiveapicesemergingneartothemainonesindicatedthattheflake-shapedstructureisdevotedtothispeculiarXRDspectrum.(2)TheXRDspectrumoftheskin-colorednanosizedZnOshowedthattheparticlesispurifiedhexagonalZnOcrystals.TheAFMimagesindicatedthatthesizeoftheparticlesisunder100nm,andseveralparticlespilesuptogether.3.4Studyonmechanismofpigmentation:calcinethesub-forerunnerpreparedbyfiltrate-vaporizingunderdifferenttemperaturetoseeaboutthetemperatureinfluenceonskin-coloredproductscanbegainedthroughtreatingforerunnerwithammonia.ThecoloroftheZnOiscausedbythedissolvingofnitrogenatomsintothesolidphaseofZnO.Above350~400℃,thenitrogenatomscancompletelypenetratetheZnOcrystallatticeandmakestheproductshowskincolor.4.conclusionsandexpectationsNanosized-ZnOcanbepreparedthroughimmediateprecipitation–microwavecalciningandskin-coloredproductscanbegainedthroughtreatingforerunnerwithammonia.Vaporizeingthefiltrateafterfiltratinganddisposingitproperly,wecangetflake-shapedskin-coloredZnOultimately.Buttherearelotsofworkscanbedoneontheinfluenceofammoniaontheparticles,thepigmentationofZnOandthemicrowave-calciningoftheforerunnerandsoon.……
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CLC: > Industrial Technology > Chemical Industry > Inorganic compounds, the chemical industry of the metal elements > Section III group metal elements of inorganic compounds
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