气凝胶制备、表征、应用

0040-6090/97/$17.00q1997Published by Elsevier Science S.A. PII S0040-6090(96)09441-2/97/$17.00q1997Elsevier Science S.A.All rights reserved applicationsD-97218Wurzburg,Germany1930’s,today’s researchreview of aerogels whileOptical and infrared properties;and liquid phase whichother words,if the liquidnon-destructive manner,with approximately thegel.difficult to achieve.Evap-in large capillary pres-tension caused by thebest,a greatly reducedcracking of the fragileremove the liquid by plac-the pressure and thencritical point.In the super-behaves like a liquid but molecules have enough energytensions cease,and menisciJ.Fricke,T.Tillotson /Thin Solid Films 297(1997)212–223213Fig.1.Development of an electromagnetic shock wave caused by the impact of relativistic particles (Cerenkov effect).true to this day as the high cost of aerogel production up to now has limited its wide-spread commercial exploitation.2.2.The alkoxide sol–gel methodRenewed interest in aerogels occurred in 1962when the French government approached Stanislas Teichner at the Uni-versity of Lyons with the idea of using the highly porous solids for rocket fuel storage.Kistler’s ‘‘water glass’’gel preparation required a labori-ous and time consuming solvent exchange step.He discov-ered this early on when supercritical water peptized his water-filled gels,‘‘aquagels’’,made from hydrolyzing sodium silicate in water.Consequently,prior to SCE,the ‘‘aquagels’’were first rinsed with methanol to prepare ‘‘alco-gels’’(alcohol-filled gels).So,after weeks of effort,Teichner’s student,who was completing his thesis work on this topic,had prepared just two aerogels!Remorseful,he temporarily left the University and is said to have been quite despondent over the seemingly monumental task that lay before him.On his return,he suc-cessfully attempted a new synthetic procedure for the gel preparation using organosilanes and the modern day sol–gel method was introduced [3].In the French group’s work,tetramethoxysilane (TMOS)was dissolved in methanol and hydrolyzed by a molar excess of water in the presence of either an acid or base catalyst according to the following overall net reaction:Si(OCH )q H O ™SiO q 4CH OH34223The TMOS first hydrolyses producing silicic acid which then condenses to SiO 2.Small nanometer size silica particles form a ‘‘sol’’.The viscosity of the sol rises as the particles become linked until a highly cross-linked three-dimensional silica network,an alcogel results.This sol–gel method produced methanol as the liquid phase of the gel,eliminating the need for a solvent exchange step,and drastically sped up the aero-gel production process.2.3.Cerenkov detectorsAlthough aerogels as rocket fuel storage devices were never used,scientists studying high-energy physics became very interested in aerogels as Cerenkov detectors.Cerenkov radiation occurs if a charged particle penetrates a medium,having an index of refraction,h ,with a speed,v ,which is higher than the velocity of light,c /h in that medium.Under the action of the fast moving electric field which the particle carries along,the medium is polarized and gives off light in a cone-shaped pattern around the flight path (Fig.1).For the detection of fast pions,koans,or protons,a medium with h (1is needed.It just so happened that aerogels covered a range of h -values unattainable by either compressed gases or liquids.The index of refraction for aerogels spans from h (1.006to 1.1,corresponding to densities from 3to 500kg m y 3.The subsequent demand for this material prompted two groups to begin large scale aerogel production in the 1970’s.D.Poelz produced 1.5m 3of silica aerogel tiles (measuring 15cm =15cm =2.5cm)for the Deutches Elektronen Syn-chrotron (DESY)located in Hamburg,Germany [4].While,S.V.Henning and G.V.Dardel,at the University of Lund,provided the European laboratory for particle physics,CERN,with 1.0m 3of aerogel [5].The latter Swedish group went on to form a private company,Airglas AB,which is still a commercial source of silica aerogels with the capability of making large 60cm =60cm tiles.2.4.Aerogels todayWith these developments,aerogel research flourished in the 1980’s.Scientists studied the unusual physical properties,probed the microstructure,and developed new processing schemes and synthetic routes.Industry explored their use in solar panels and as spacers in double-pane windows.The first International Symposia on Aerogels (ISA)was held in Wuerzburg,Germany in 1985and has been held with increas-ing participants every three years since.Fittingly,an excellent review book,Sol–Gel Science:The Physics and Chemistry of Sol–Gel Processing ,by Brinker and Sherer,was published in 1990.Let us now review some of the major breakthroughs in aerogel research.A new drying scheme was developed when a research group at the Lawrence Berkeley Laboratory succeeded in making silica aerogels using supercritical CO 2[6].A com-mon practice used by microbiologists to remove water from biological tissues,Hunt and Tewari extended its use to the drying of gels.The alcogel is placed in a pressure vessel where liquid CO 2is exchanged for the alcohol.Then,the temperature and pressure is raised above the critical point of the CO 2(T c s 31.18C,p c s 73.9bar)and the supercritical fluid is vented off.The use of non-flammable carbon dioxide with its low critical temperature significantly reduced the hazard associated with the high temperature and pressure alcohol process.Several groups began to study the microstructure,physical properties,and the influence of synthetic and processing con-J.Fricke,T.Tillotson/Thin Solid Films297(1997)212–223 214ditions.Among these,Woignier and Phalippou determined the scaling law behavior of the mechanical properties of silica aerogels using a three-point bending technique[7].They found that the elastic modulus,Y,is proportional to the den-sity,r,by the relation Y a r2–4depending upon solution chemistry and processing conditions.This result was later confirmed by LeMay and co-workers at Lawrence Livermore National Laboratory using a uniaxial compression method [8].Woignier also showed that alcogels undergo physio–chemical changes during the high temperature and pressure SCE process[9].Theses changes result in lower density aerogels with higher moduli than similar gels prepared by the CO2process.In the same group at the University of Montpellier,Vacher began using small angle X-ray scattering(SAXS)and small angle neutron scattering(SANS)measurements to probe the microstructure of aerogels[10].Concurrently using the same techniques,Schaefer and his colleagues at the Sandia National Laboratory in New Mexico made systematic inves-tigations of the differing fractal nature of aerogels resulting from variations in the solution chemistry[11].Also at the Sandia National Laboratory,Brinker and co-workers prepared thefirst borate aerogels[12].More signif-icantly,they went on to study in detail the hydrolysis and condensation reactions of silicon alkoxides[13].This work culminated in kinetic growth models which scientists cannow use to control aerogel microstructure and predict their phys-ical properties.The aerogel research group at the University of Wuerzburg pioneered the study of the thermal properties of aerogels[14–17].Although the thermal conductivity was known to be extremely low,a detailed understanding of the thermal trans-port in aerogels was lacking.Thermal transport in aerogel was shown to be governed by gaseous conduction,solid con-duction,and infrared radiative transfer.From thesefindings, a model was developed,which was based on the aerogel composition,nano-structure and porosity.Also,in Wuerz-burg,the sound propagation in aerogels was investigated. Depending on density,the sound velocity in aerogels can be as low as100m s y1[18].How the sound propagation can be used to determine the aerogel modulus and a more detailed description of the thermal transport in aerogels will be pre-sented later.Towards the end of the nineteen eighties,two synthetic advances came out of the Lawrence Livermore National lab-oratory.Pekala,working on inertial confinement fusion tar-gets for the NOVA laser,prepared thefirst organic aerogels by the polycondensation of resorcinol with formaldehyde [19].Measurements at Wuerzburg have shown them to have the lowest thermal conductivity,0.012W m y1K y1,of any solid ever tested.Further pyrolysis of these organic aerogels, in an inert environment,produced an electrically conductive carbon aerogel[20].The other synthetic advance occurred when Tillotson and Hrubesh modified a two-step sol–gel method,first described by Brinker[21],for the preparation of ultralow density silica aerogels.By distilling off the reac-tion-generated alcohol and replacing it with an aprotic solvent (to prevent reverse equilibrium reactions),monolithic aero-gels with densities as low as3kg m y3(99.8%porous)were produced[22].These aerogels are now being used on NASA space shuttleflights[23].3.Aerogel productionWhile the applications for aerogels are virtually unlimited, they will have to be cheaply produced in order to have an impact in the commercial marketplace.As such,researchers have now turned their attention to achieving these goals. Some groups have sought ways to eliminate the SCE process and the high capital cost associated with it,while others have sought to make it more efficient.Studies have shown that the high cost of the raw materials contributes significantly to the final cost of the aerogel[24].Consequently,lower cost pre-cursors such as Kistler’s water glass are being looked at once again.Here,we briefly highlight some of the recent developments.3.1.Ambient pressure dryingUsing the alkoxide sol–gel method described above,the solid phase is composed of linked nanometer sized spherical particles with unreacted silanol groups covering their surface. By modifying this gel surface,two groups have successfully produced silica aerogels at ambient pressure. Einarsrud and her colleagues at the Norwegian Institute of Technology aged their wet gels infirst water/alcohol and then alkoxide/alcohol solutions.This treatment effectively fills in the necks between the linked particles,reducing the surface area while strengthening and stiffening the gel microstructure[25].With this increase in Young’s modulus, the wet gels are now strong enough to withstand the capillary pressures arising from surface tension as the receding liquid meniscus penetrates the gel body.Aerogels with densities as low as240kg m y3have been prepared.At the University of New Mexico,Smith and Brinker took another approach.As a gel shrinks from the evaporation of pore liquid,sterically favored condensation reactions may occur between two surface silanol groups leading to irrevers-ible syneresis.Recognizing this,they methylated the gel sur-face by proton abstraction of the surface hydroxyl groups according to the following reaction:s Si y OH q Cl y Si y(CH)33™s Si y O y Si y(CH)q HCL33As can be seen in Fig.2,the methylated gel shrinks but springs back to96.9%of its original volume while the unsi-lylated gel when completely dry occupies only30.1%of its original volume[2].J.Fricke,T.Tillotson /Thin Solid Films 297(1997)212–223215Fig.2.Photographs showing silica gels with either a hydroxylated or methylated surface at various stages of ambient pressure drying.(Reprinted from D.M.Smith et al.,JNCS,186(1995)104.).Fig.3.Schematic diagram of apparatus for the rapid supercritical extraction (RSCE)process.(Reprinted from Poco et al.,Mater.Res.Soc.Proc .,in press.).3.2.Rapid supercritical extraction process (RSCE)During the heat cycle of the high temperature and pressure SCE process,Scherer has shown that stresses arise within the gel from the different thermal expansion coefficients of the solid and liquid phases.In other words,rapidly heating a non-constrained gel causes the liquid to expand faster than the solid network.If the resulting differential stresses exceed the basic strength of the gel,i.e.the modulus of rupture,cracking occurs [26].Simply stated,for uncracked aerogel monoliths,the heating rate must be slow.This leads to batch processing times on the order of 30h.Poco and his colleagues at Lawrence Livermore National Laboratory have recently developed a RSCE process which is similar to injection molding,a common process used to manufacture certain types of plastics [27].In their process,the strains and associated stresses during heating are mini-mized by gel containment within a high pressure stainless steel mold (Fig.3).In this case,the positive strain due to the expanding liquid is essentially zero.In practice,the sol is injected into a two-piece sealed mold which is rapidly heated.During this rapid heat cycle,the sol gels while the expanding liquid creates a hydrostatic pressure.Once the liquid phase becomes a supercritical fluid it is rapidly decompressed and the heaters de-energized.The whole process typically takes 3h but has been done in a little less than 1h.The resulting aerogels prepared by this RSCE process have three times higher elastic modulus than similarly prepared gels by the SCE process.They estimate an eight-fold reduction in pro-duction cost.3.3.Aerogel beadsFor more than 10years,at B.A.S.F.in Ludwigshafen,Germany,aerogel beads,Basogel w ,have been produced by spraying an acid and water–glass solution from a mixing jet into a flask,where the beads are solvent exchanged and then supercritically dried with respect to CO 2[28]The resulting beads have diameters 2–6millimeters and densities of some 150to 200kg m y 3.This process is amenable for mass pro-duction and uses sodium silicate,a cheaper source of silica.4.New classes of aerogelsAlthough,in theory,any multifunctional monomer that can be cross-linked to form a gel could be converted into an aerogel via SCE,very little work has been done outside of the silica and RF organic systems.Some groups have pre-pared aerogels from the transition metal alkoxides but theirJ.Fricke,T.Tillotson /Thin Solid Films 297(1997)212–223216Fig.4.Linear shrinkage results showing improved temperature stability of alumina silicate over pure silica aerogels.fast hydrolysis rates and low cross-linking densities have made monolithic aerogels hard to achieve.For catalytic appli-cations,however,monolithicity is not important and work on zirconia and titania aerogels is being pursued [29,30].For an excellent review article on transition metal gels the reader is referred to work published by Livage et al.[31].Here,we describe some other recent work and advances in aerogel synthesis.4.1.Mixed oxide and doped aerogelsThe most widely studied mixed oxide aerogel systems are the alumina silicates including the mullite composition [32–35].Yoldas pioneered the use of aluminum alkoxides for the preparation of aluminum xerogels (50%porosity)in 1975[36,37].He hydrolyzed aluminum sec-butoxide under acidic conditions in a 100molar excess of water to prepare a boehm-ite sol.Today,chemists are mixing this sol with TMOS to prepare alumina silicate aerogels with varying composition.The interest in alumina silicates arises from the improved temperature stability of these materials as compared to pure silica aerogels (Fig.4).Mixing two metal alkoxides together to form binary com-positions is becoming common practice.In our laboratories,we are now making zirconia–silicate aerogels using a two-step sol–gel process.In the first step,a sub-stoichiometric amount of water is added to a TMOS/methanol solution which is refluxed under acidic conditions.Hydrolysis of the TMOS and subsequent condensation reactions result in the growth of oligomeric silica species with some uncondensed Si–OH groups still present [38–40].Zirconium alkox-ide,once added,reacts with the uncondensed silanol groups forming –Zr–O–Si–linkages and alcohol as a by-product.In the second step,water to complete the hydrolysis/conden-sation reactions,an acid or base catalyst,and an alcohol for density control is added to the partially hydrolyzed–con-densed sol to induce gellation.SCE completes the process.Another example of mixing different alkoxides is the work of Schwertfeger and Schubert [41].They mixed TMOS with methyl-trimethoxysilans (MeTMOS).By carefully control-ling the reaction conditions they were able to preferentially hydrolyze and condense first the TMOS then the MeTMOS.Aerogels prepared by this process demonstrated a permanent hydrophobicity owing to the methyl substituents covering most of the surface.Doping of aerogels is another research area where consid-erable work has been done.For alkoxide systems,any alcohol soluble compound (such as the acetates,nitrates,chlorides of Cu,Ni,Co etc.)can be added to the sol prior to gellation.Opacified silica aerogels have been made by doping the sol with carbon black just prior to gellation [42].Despite a loss in transparency,the carbon black,lowers the thermal con-ductivity for reasons we describe below.Fluorescent dyes have been added to low density silica aerogels for the enhancement of Cerenkov light detection [43].Phosphors have also been doped into silica aerogels to improve the output of radio luminescent light sources [44].In our labo-ratories,we are also preparing PbTiO 3aerogels by mixing lead acetate trihydrate with titanium alkoxides.These aero-gels could potentially be used as acoustic impedance layers in ultrasonic transducers operating in air.4.2.New organic aerogelsPekala and his colleagues have gone to develop two more organic aerogel synthetic routes.In one synthesis,transparent aerogels are produced from the polycondensation of mela-mine with formaldehyde [45].These aerogels have lower thermal conductivity’s and comparable transparencies as their inorganic counterparts.Unlike previous organic sol–gel reactions,the second syn-thesis could be carried out with an alcohol instead of water [46].This eliminated the need for solvent exchanging the aqueous gels prior to SCE.A commercially availablepolymer solution (FurCab UP520,QO Chemicals)was diluted in propanol,and a mixture of aromatic acid chlorides (Q2001,QO Chemicals)was added as a catalyst.Gelatin is carried out at 858C.These,like the RF aerogels,can be further pyr-olized to vitreous carbon.5.Aerogel characterizationSeveral methods are currently being used to probe the structure and properties of aerogels with varying degrees of success.Here,we describe the most common laboratory prac-tices employed by sol–gel scientists noting the limitations of the technique,if any.5.1.Structural investigationsThe specific surface area of accessible sites in the aerogel microstructure can be derived from nitrogen adsorptionmeas-urements.Aerogel BET surface areas range from 200–1000m 2g y 1.The extracted pore-size distribution and the total pore volume may not be reliable,however,because onlyJ.Fricke,T.Tillotson /Thin Solid Films 297(1997)212–223217Fig.5.Longitudinal sound velocity (c l )for silica aerogels of varying density (r )produced at LLNL,Airglass AB,BASF,and DESY,the small dots are for sintered aerogels and the open triangles for evacuated specimens.pores up to about 30nm are detectable by this method [47].In mercury porosimetry,because of the high surface tension mercury,only compression of the tenuous skeleton occurs instead of penetration [48,49].As a result,pores much larger than are actually present are found.Helium pycnometry is used to determine the skeletal volume (solid-phase).Skeletal densities of silica aerogels have been found in the range of 1700to 2100kg m y 3[28,50].Full density silica has a value of 2200kg m y 3and the difference is attributed to micropo-rosity in the primary particles comprising the aerogel backbone.The use of microscopy has been particularly frustrating.Kistler wrote of the difficulties of optical techniques and concluded by saying:‘‘perhaps,some interesting observa-tions could be made with the aid of the new Spierer lens’’[51].Today,even with the use of electron microscopy good,reliable results are hard to attain.The problem arises from the interaction between the impinging electron beam and the tenuous aerogel skeleton.Even for small electron-beam cur-rents,the absorbed energy may cause sintering resulting in distorted images.Evaporated metal coatings to eliminate this charging effect also have been shown to distort the true aero-gel microstructure.However,with this disclaimer,a beautiful a high resolution TEM study of silica and organic aerogels,including stereo-imaging,has been done by Rueben using a Pt–C replication technique [52].Nuclear magnetic resonance (NMR)is used to non intru-sively explore the structure on the molecular level.The extent and kinetics of hydrolysis and condensation has been studied from the initial stages of polymerization until gelation [13,38].Doping of aerogel with paramagnetic relaxation agents also allows the fractal appearance of the gel to be probed [53].The most powerful and reliable methods to probe aerogel structures are scattering techniques.The specimen is irradi-ated by monoenergentic photons (light,X-rays)or particles (neutrons),with part of the incoming beam being scattered by the inhomogenieties of the sample.The amount of scat-tered intensity depends on the type and energy used,the concentration of the scattering units,and the contrast between the solid phase and the voids.The characteristics of the scat-tering pattern can be related to the different structural features on the respective length scale,for example,the (fractal)morphology of network and particles;mean cross-section lengths of pores,clusters,and particles;and the specific sur-face area [54].The reader may refer to one of our previously published papers for a more detailed explanation of how such experiments are performed and what structural information can be obtained [55].5.2.Elastomechanical propertiesElastomechanical properties are measured by three-point bending [7]uniaxial compression [8],or ultrasonic tech-niques [56].We’ll describe aerogel elastomechanical prop-erties from the perspective of the ultrasonic work done in our laboratories.As a result of the aerogel’s high porosity,the longitudinal sound velocity (v l )can be as low as 100m s -1(Fig.5).Generally,a scaling behavior of v l (and thus also of Young’s modulus (Y ))with aerogel density (r )is observed:v l a r a (where a (1.3)and Y ar b (where b (3.6).Such scaling laws can be either explained in terms of per-colation theory [57,58]or as a result of a heirarchical struc-ture [59].As is shown in Fig.5,deviations up to a factor of 2in elastic properties at a given density can be observed.This is owing to the degree of interparticle connectivity resulting from different synthetic conditions.Aerogels prepared under neutral or acidic conditions are about twice as stiff as base catalyzed aerogels.At ultralow aerogel densities,the elastic-ity of the air within the pores of the aerogel makes a significant contribution to the sound propagation.Evacuation allows to reveal the elastic properties of the skeleton alone.5.3.Optical and IR propertiesThe optical and IR properties of aerogels have been well studied.Despite their high porosity,silica aerogels are highly transparent in the visible spectral region [6].At 630nm the specific extinction coefficient (e vis )is on the order of 0.1m 2kg y 1[60].Thus,the photon mean free path at that wave-length for an aerogel density of 100kg m y 3would be:1s 1/(e =r )(0.1mphoton vis Towards the blue and ultraviolet spectral region strong Rayleigh scattering is observed for aerogels,which is expected for a material with pore structures in the 1to 100nm size range [6,10].Accordingly the angular scattering distribution is nearly isotropic [60].The variation of the extinction coefficient with wavelength in this spectral region cannot be described satisfactorily by a 1/l 4dependence,valid for a medium containing isolated point scatterers.Rea-sons may be the occurrence of absorption or anisotropic scat-tering.Due to the strong scattering power at smallerJ.Fricke,T.Tillotson /Thin Solid Films 297(1997)212–223218Fig.7.The effect of gas pressure on the thermal conductivity of base-catalyzed silicaaerogels.Fig.8.Variation of thermal conductivity (l )with density (r )showing the scaling law dependency of the solid conductivity (l s).Fig.6.Specific extinction coefficient of silica aerogels in the infrared,the dotted curve is for opacified aerogels.wavelengths,extraction of data from transmission measure-ments is non-trivial as multiple scattering has to be taken into account.Particle diameters,which can be derived from assuming Rayleigh scattering,are in the order of 10nm,whereas comparative TEM’s show 5nm sized particles [61,62].In the infrared,silica aerogels show the strong adsorption bands typical for SiO 2(Fig.6).Additional adsorption is caused by water,chemisorbed and physisorbed on the huge internal aerogel surface,and organic substituents resulting from unreacted alkoxy groups in the sol–gel process.Typical is a specific extinction of F 10m 2kg y 1below 7m m and above 30m m and a specific extinction of G 100m 2kg y 1between 8and 25m m.If one compares these data with the extinction in the visible a ratio:e IR /e vis )100is recognized.As a result,silica aerogels can be considered transparent insulation materials,which effectively transmit solar light but block thermal infrared radiation.Infrared spectroscopy has also been used to characterize other aerogels including alumina [63],boron–phosphorous–silicon mixed oxide [64],zirconia [65],cordierite [66],and organic aerogels [19].The index of refraction for low density aerogels is very close to 1.Experimentally,one finds h y 1(2.1=10y 4=r (kg m y 3)in agreement with a simple theoretical estimation [5].Thus,a silica aerogel with a density of 3kg m y 3would have an index of refraction of h (1.006.As a consequence,light enters a piece of aerogel nearly without reflective losses.5.4.Thermal propertiesThe thermal conductivity of aerogels is about 100times smaller than that of full density silica glass [51]Although Kistler made initial thermal conductivity measurements,a detailed understanding of the thermal transport in aerogel resulted from investigations carried out in our laboratories [14–17].We will briefly summarize our findings using silica aerogels as a case study.Thermal transport in aerogel occurs via gaseous conduc-tion,solid conduction,and infrared radiative transfer.Oneway to lower the thermal conductivity of an aerogel is to suppress the gaseous conduction.As can be seen in Fig.7,evacuating an aerogel tile down to 0.01bar of air pressure reduces the thermal conductivity at T s 300K from about 0.02W (m =K)y 1to below 0.01W (m =K)y 1.The latter value is owing to solid conduction and to radiative transport.The solid conductivity of low density silica aerogel is a factor of several hundred smaller than the conductivity of non-porous vitreous silica.This is expected because of the large amount of missing mass which restricts the propagation of local excitations to the chains in the tenuous structure.An important question was how the solid conduction l s changes with skeletal density r .A series of measurements (Fig.8)revealed a scaling law where l s ar a ,with a (1.6.The radiative transport in aerogels is governed by infrared adsorption.Characteristic are the high absorption region above 10m m and the low absorption region between 3and 5m m (Fig.6).While the former provides effective attenuation of thermal radiation at ambient temperatures,the latter is responsible for a strong increase in thermal conductivity at elevated temperatures.We ought to state that solid conduction via the skeleton and radiative transport cannot be lineary superimposed in。