气凝胶综述 Aerogels--Airy Materials Chemistry, Structur

VortitelVA208eCMYKTwoofthefascinatingcharacteristicsofaerogels––highlyporousmaterialsmade,forexample,fromSiO2andair––aredemonstratedbythesepictures:Intheirhydrophobicformtheaerogelsfloatonwaterformonths,andundersuitableconditionsoneobtainsplatesthataretranslucentevenwithathicknessofabout2cm.1.IntroductionWhippedeggwhitesorsolidbubblebathareclosestinappearancetoaerogels.Inprinciple,thenamesaysevery-thing:themaincomponentofthismaterialisairwhichissurroundedbyaveryfiligrane,solidnetwork.Thisnetworkcanbeconstructedinawaythattheresultingaerogelshaveadensitythatisonlythreetimesthatofair.Ifanaerogelisgiventoachemist,aphysicist,oramaterialsscientist,eachofthemwoulddiscoverinterestingpropertieswhichindividuallyarealsoobservedinothermaterials.Forexample,SiO2aerogelshaveahightransparencythatisclosetothatofglass,athermalconductivitycorrespondingtothatofpolystyreneorpolyurethanefoams,orveryhighspecificsurfaceareas,whicharealsofoundincharcoal.Theuniquefeatureofaerogelsisthecombinationofthesephysicalpropertiesinonematerialwhichcanhavedifferentchemicalcompositions.Importantaspectsforvariousapplicationsarethuscombined,forexample,forheatinsulation(Figure1),opticalapplications,sensors,catalysts,orcatalystcarriers.Whatexactlyisanaerogel?Thetermaerogelisnotuniformlyusedintheliterature,andtherearetwodifferentdefinitions:1)First,allmaterialspreparedfromwetgels(seebelow)byaspecialdryingprocess,thesupercriticaldryingtechnique,werecalledaerogelsirrespectiveoftheirstructuralproper-ties.Withthedevelopmentofnewdryingtechniques(e.g.,thepossibilityofdryingunderconventionalconditions,seeSection3.3),thisdefinitionnolongerappearsappropriate.2)Accordingtothealternativedefinition,materialsinwhichthetypicalstructureoftheporesandthenetworkislargelymaintained(whichisnotalwaysthecaseforsupercriticaldrying)whiletheporeliquidofagelisreplacedbyairarecalledaerogels.However,itisnotalwayscleartowhatextentthestructurewasmaintained.Rearrangementandshrinkageinthegelbodyduringdryingarenormalphenomena!Inmanypublicationsattentionisnotpaidtothisissue.Inthisreviewwewillmainlyfollowtheseconddefinition,whichbetterappliestothespecialpropertiesofthisclassofmaterials.However,wedonotgenerallyexcludematerialswhichcorrespondonlytothefirstdefinition.Therearealreadysomeexcellentreviewsonaerogels,[1]mostofwhichfocusonphysicalpropertiesorapplications.Here,wewanttopresentthesefascinatingmaterialsfromamorechemicalpointofview.Aerogelsareanexcellentexampleforhowimportantchemistryisforthedevelopmentofnewmaterials.Wewanttouseaerogelsasanexampletodemonstratethattheintimateknowledgeandunderstandingofthechemistryofprecursorsandthechemistryoftheformationofthesolidareindispensibleforthedeliberateoptimizationandtailoringofmaterialsproperties.AerogelsÐAiryMaterials:Chemistry,Structure,andPropertiesNicolaHüsing*andUlrichSchubert*Aerogelsareuniqueamongsolidma-terials.Theyhaveextremelylowden-sities(upto95%oftheirvolumeisair),largeopenpores,andahighinnersurfacearea.Thisresultsininterestingphysicalproperties,forexampleex-tremelylowthermalconductivityandlowsoundvelocitycombinedwithhighopticaltransparency.Theyarepre-paredfrommolecularprecursorsbysol±gelprocessing.Theinitiallyformedgelshaveaspongelikesolidnetworkfilledwithliquid.Specialdryingtech-niquesmustbeappliedtoexchangetheporeliquidwithairwhilemaintainingthefiligranesolidnetwork.Supercrit-icaldryingismostcommon.However,recentlydevelopedmethodsallowre-movaloftheliquidinaclassicalwayafterchemicalmodificationofthein-nersurfaceofthegels.Thestructureofthegelnetwork,andthusthephysicalpropertiesofaerogels,decisivelyde-pendonthechoiceoftheprecursorsandthechemicalreactionparametersforpreparingthegels.Therefore,thelatermaterialpropertiesarepredeter-minedatthebeginningoftheprepa-rationprocess.Inthisreview,connec-tionsbetweentheunderlyingsol±gelchemistry,thestructureofthegels,andtheresultingpropertiesofthemateri-alswillbeshown,whicharethebasisforachemicalªdesignºofaerogelproperties.Keywords:aerogels´materialssci-ence´mesoporosity´nanostructures´sol±gelprocesses[*]Dr.N.Hüsing,Prof.Dr.U.SchubertInstitutfürAnorganischeChemiederTechnischenUniversitätGetreidemarkt9,A-1060Wien(Austria)Fax:Int.code‡(43)1-5816668e-mail:[e-mail]@fbch.tuwien.ac.atREVIEWSAngew.Chem.Int.Ed.1998,37,22±45WILEY-VCHVerlagGmbH,D-69451Weinheim,19981433-7851/98/3701-0023$17.50+.50/023REVIEWSN.Hüsing,U.SchubertFigure1.Aerogelshaveexcellentinsulatingproperties,asshownhereforasilicaaerogeltile.Untilrecently,thechemicalapproachfordevelopingmaterialswithnovelpropertiesmainlyconsistedofpreparingsubstanceswithanewchemicalcomposition,thatis,inanattempttocorrelatethemacroscopicpropertiesandthechemicalbuildingblocks.Theimportanceofthecontrolledsynthesisofporestructureswasonlyrecentlyrealized.Atopicalexampleisthefamilyofmesoporousmolecularsieveswithregularchannelsystems(theM41Sfamily).[2]Severalemergingtechnologiesdependonthedevelopmentofengi-neeredporousmaterials.[3]Nevertheless,deliberatetailoringofporesisstillamajorchallengeinmaterialschemistry.Materialsareporousif