粉末和散装材料的处理.docx
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粉末和散装材料的处理.docx
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粉末和散装材料的处理
ProcessingofPowdersandBulkMaterials
INTRODUCTION
Thepresentsol–geltechnologystartedaround1970withthepurposetomakebulkglassesfrombulkgelsatlowtemperatures(Distich,1971;Roy,1969).Afewyearslater,silica-basedopticalfibersforopticaltelecommunicationemerged.Suchopticalfibersweredrawnfromtransparentsilicaglassperforms,whichwerepreparedbychemicalvapordeposition.Thisstimulatedmanysol–gelpeopletopreparesilicaglassperformsbysol–gelmethodandagreatmanyeffortsweredirectedtothepreparationofbulksilicaandsilica-basedglasses(Sake,1994).
Ontheotherhand,manymulti-componentglasseshavebeenmadebysol–gelmethod.Meanwhile,theproblemonthedifferenceamonggels,heatedgelsandglassesattractedattentionofthesol–gelresearchers.Especially,glassformationandthenatureofso-calledsol–gelglassesandcomparisonbetweensol–gelglassandmelt-quenchedglasswereimportantproblemstobediscussed(Mackenzie,1982).Whenorganic–inorganichybridsbecameaverypopulartargetinthefieldofsol–gelscience,oxycarbideglasseswereprepared.Thoseglassescanbepreparedefficientlybysol–gelmethod(Pentaneetal.,1999).
Thischapterisdevotedtothefollowingtopics:
(1)Sol–gelprocessingofbulksilicaglasses.
(2)Sol–gelprocessingofbulk,multi-componentglasses.
Inthischaptermainemphasiswillbelaidoninorganicglasses,althoughorganic–inorganicglassesaredealtwithwhennecessary.Abulkglassisdefinedhereasaglasspieceofwhichthesmallestsizeisintheorderofseveralmillimetersorlarger.
Directpreparationoffibers,coatingfilmsandparticlesandpowdersisnotincluded.
SOL–GELPREPARATIONOFBULKSILICAGLASSES
HistoricalBackground
In1971,Dislichpreparedsmallpiecesoftransparentborosilicateglassbysol–gelprocessusingastartingsolutioncontainingethanol,pentadiol,Si(OCH3)4,Al(O-secC4H9)3,NaOCH3andKOCH3(Dislich,1971).Gelpiecesobtainedfromthesolutionbecamehardpiecesuponheatingat530°C.Aglassplatewasobtainedbyhot-pressingthepiecesat630°C.These“glasses”hadpropertiesverysimilartothoseofthesamecompositionproducedbytheconventionalmelt-quenchingtechnique.
AfterDistichshowedtheglassformationatlowtemperatures,manyglassresearcherstriedtomakebulksilicaglassesbysol–gelmethod.Inthosedayspeoplestartedtopayattentiontotheformationofsilicafibersforopticalcommunication,whicharedrawnfromalargesilicaglassrod.
However,itwasthoughtthatpreparationofbulkbodiessuchasrodslargerthan10mmindiameterandplateslargerthan5–20mmwasnoteasy,untilYamanereportedtheformationoflarge-sizesilicagels(Yamaneetal.,1978)andsilicaglasses(Yamaneetal.,1979).
Themainreasonfordifficultyinmakingbulkglassconsistsinthefactthatcracksandfracturesareoftengeneratedwhenabulkwetgelbodycontainingsolventsisdried.Themethodforavoidingoccurrenceofcracksandfractureduringdryingofgelsandsubsequentheatingforgeltoglassconversionisdiscussedinthefollowings.
PreventionofCracksandFractureduringDrying
Sol–gelprocessingofabulkglassconsistsofprocessesofwetgelformation,dryingofthegelandsubsequentsinteringofthegeltoglassbyheating(Rabinovich,1985).Amongtheseprocesses,dryingofawetgelbodyisoneofthecriticalprocesseswhenonedesirestomakeabulkglasswithoutcracksandfracture.
Driedgelscontain40–75vol.%poresdependingontheprocessingcondition.Thisindicatesthatawetgelisfullofsolvents.Wetgelsareassumedtoconsistofporesfilledwithsolventsandporewallsofconnectedparticlesornetworks.Initially,thewetgelbodyiscoveredwithsolvents,andthenthesolventfrontentersintothesurfacelayerofthegelbody.Zarzyckietal.(1982)consideredthatthecracksorfractureoccurindryinggelswhenthecapillaryforceexertedbytheporeliquidtotheporewallexceedsthestrengthofthegelnetwork.IfthecontinuousfineporesareregardedascapillariesofdiameterD,thesolventwithsurfacetensionγcausesthecapillaryforceΔPatthemenisci:
(6-1)
whereθiscontactanglebetweenthecapillarywallandthesolvent.Accordingly,thesurfacedriedlayerofthegelbodytendstoshrink,inducingthetensilestressinthesurfacelayer,becausetheinnerlayerdoesnotshrink.Thisresultsinthecrackformationorfracturewhenthestressexceedsthestrengthofthegel(Zarzyckietal.,1982).
Beforediscussingthecrackformationbasedonformula(6.1),anevidencethatthecracksandfracturearecausedbycapillaryforcewillbepresented(SakkaandAdachi,1990).Forthispurposeadriedsilicagelcylinderwithmicroporesofabout16nmaveragediameterwaspreparedfromaSi(OCH3)4–(CH3)2NCHO–CH3OH–H2O–NH3solution.Thedriedgelcylinderwas120mmlongandof32mminlength.Discs8mmthickand32mmdiameterwerecutfromthedriedgelcylinder.Themechanicalstability(crackingornocracking)ofthegeldiscswasexaminedbyexposingtothevaporofasolventandsubsequentlyimmersingintheliquidsolvent.Thesurfacetensionofthetestsolventrangedfrom17.1dyn·cm–1fordiethyletherto72.8dyn·m–1forwater.Onlythebottomhalfpart(about4mmthickpart)ofthediscisimmersedintheliquidsolvent.Uponimmersing,thesolvententersintothedisc,climbinguptowardthetopofthedisc,andsomeofthediscsarecracked.TheresultsareshowninTable6-1.
Table6-1showsthatthereisadistinctcriticalsurfacetensionofthesolventincausingfractureandcracksinthegel.Itisassumedthatthecrackformationofthedriedgelcylindermaybecausedbythecapillaryforceofthesolventpresentinporesonporewallsconsistingofsilicaparticlesornetworks.Theseindicatethatthecapillaryforceobtainedfromformula(6.1)determineswhetherawetgelfraturesornotduringdrying(Kirkbiretal.,1996;Grandietal.,2002).
MethodofPreparingBulkGelWithoutCracking
ExamplesoftechniquesformakingcracklessdriedbulksilicagelsanddensesilicaglasssofarproposedarelistedinTable6-2.ThecommonprinciplesunderlyingthetechniqueslistedinTable6-2istosuppressthecapillaryforceactingonthesilicawallsoftheporeinawetgelbody.Thiscanbeachievedwhenthedryingiscarriedoutundersupercriticalcondition,thesolparticlesaregranularandlargeandthesolventremainingintheporeduringdryinghasalowsurfacetension.ThemethodslistedinTable6-2willbediscussedinthefollowings.
TABLE6-1.SURFACETENSIONANDBOILINGPOINTSOFVARIOUSSOLVENTSUSEDANDCRACKFORMATIONINTHEGELDISC
Solvent
Surfacetension(dyncm–1)
Boilingpoint(°C)
Exposuretosolventvapor
Immersioninaliquidsolvent
Diethylether
17.1
34.6
Nocrack
Nocrack
Di-iso-propylomine
20.0
83.4
Nocrack
Nocrack
Acetone
23.7
56.2
Nocrack
Nocrack
Benzene
28.9
80.1
Nocrack
Nocrack
Dichloroethane
32.2
83.5
Nocrack
Nocrack
Dioxane
34.5
101.3
Nocrack
Nocrack
Dimethylformamide
36.8
153
Nocrack
Nocrack
Phenol
40.9
181.8
Nocrack
Nocrack
Dimethylsulfoxide
43
189
Nocrack
Nocrack
Aniline
44.5
184.7
Nocrack
Nocrack
Ethyleneglycol
46.5
197.9
Afewcracks
Manycrocks
Diethyleneglycol
48.5
244.3
Manycracks
Tinypieces
Monoethonolomine
48.9
180
Afewcracks
Manycracks
Water
72.8
100
Afewcracks
Manycracks
TABLE6-2.METHODSFORMAKINGLARGE-SIZECRACKLESSBULKSILICAGELSANDGLASSES
Method
Reasonfornon-occurrenceoffractureondryingandheating
Reference
(1)Alkoxidemethod
(a)Controlofgelationcondition
Largeporesize
Yamaneetal.,1979;Susoetal.,1982
(b)Useofspecialsolvent
Lowsurfacetensionofthesolventandlargeporesize
WallaceandHench,1984;AdachiandSakka,1987a;Adachietal.,1987b;Kirkbiretal.,1996
(2)Colloidmethod
(a)Colloiddispersionmethod
Verylargeporesizeduetolargeparticles
SchererandLuong,1984;Rabinovichetal.,1984;MacChesneyetal.,1998
(b)Alkalisilicatemethod
Verylargeporesizeduetolargeparticles
Shoup,1992
(3)Colloid–alkoxidemixturemethod
Verylargeporesizeduetolargeparticles
Tokietal.,1988;Morietal.,1988
(4)Aerogelmethod
Absenceofcapillaryforceindryingandlargeporesize
Zarzyckietal.,1982;Grandietal.,2002
AlkoxideMethod.Inthispaper“alkoxidemethod”meansthatsiliconalkoxidessuchasSi(OCH3)4andSi(OC2H5)4areemployedassourcematerials.Thesesiliconalkoxidesdissolvedinwaterandalcoholsolutionsaresubjectedtohydrolysisandpolycondensation,inordertomakebulkgels.Dryingiscarriedoutunderatmosphericpressure.Whenthewetgelisdriedundersupercriticalpressuresandtemperatures,themethodiscalled“aerogelmethod”here,evenifthesiliconalkoxidesareemployedassourcematerials.
ControlofGelationCondition.Yamaneetal.madebulksilicarodswithoutcracksfromSi(OCH3)4–CH3OH–H2OsolutionscatalyzedwithNH3(Yamaneetal.,1978,1979).Theydiscussedthecompositionsrequiredforpreparingcracklessbulkgelsandglasses.Inordertostrengthenthesilicanetworksorbondsbetweenparticles,thewatercontentofthestartingsolutionmustbehigh.Basiccatalysts,suchasammoniaNH3aresuitableforthisprocess,becauseammoniaretainsthelargeporevolumeandlarge-sizeporesduringdryingbyacceleratinggellingreactionandbondingcoagulatedparticles.Astothegellingtemperature,highertemperaturesarebetterinordertomaketheporevolumelarge.Itwasshownthatthebulkdensityofadriedgelwas50%smallerforgellingat70°Cthanforgellingat54°C.Itiswidelyknownthattheslowdrying(evaporat
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