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Densification kinetics and structural evolution during sintering of silica aerogel
Abstract
A silica aerogel was sintered at 1050°C, and the effect of densification on the pore structure was determined by nitrogen sorption. Based on the initial pore size distribution (psd) of the aerogel, the kinetics of densification and the evolution of the psd were calculated using the theory of viscous sintering. If the psd is ignored, and the sintering behavior is predicted using the average pore size, then the agreement with measurements is not very good. However, when the actual psd is used in the calculation, then the predictions are excellent for density versus time, surface area and mean pore size versus density, and psd versus time. The quality of the fit depends on the constitutive laws describing the response of the porous material to stresses; the best results are obtained using the laws originally derived for the cylinder model. The power-laws that describe the density dependence of the elastic modulus of aerogels do not seem to apply to the viscosity; reasons for this dependence are discussed.
... Scherer used a cubic unit cell model, comprised of intersecting, cylinders. 8,11 The cylinders represent strings of oxides (SiO2) with one unit cell consisting of 12 quarter cylinders. This allowed for a construct that was geometrically simple and allotted for porosity. ...
... Scherer's idealized network for the viscous sintering model.11 ...
... While Frenkel's approach provides valuable insight into possible mechanisms of viscous sintering, many questions remain open regarding the sintering in a many particle system. A number of theoretical approaches (Cosgrove 1976, Scherer 1977, Hopper 1984, Scherer 1991, Schön 1992, Scherer 1998, Eggers 1999, Zanotto and Prado 2001, Crowdy 2003, Eggersdorfer 2011) and numerical methods ( Ross 1981, Hiram and Nir 1983, Jagota and Dawson 1990, Kuiken 1990, Martínez-Herrera 1995, Bullard 1997, Van de Vorst 1988, Garabedian 2000, Hooper 2000, Yadha and Helble 2004, Olevsky 2006, Djohari 2009) have, therefore, been proposed to study the densification during the sintering process. So far, most of the simulations reported in the literature on viscous sintering use the finite element method. ...
... In addition, numerous experimental tests have been performed on viscous sintering (Kingery and Berg 1955, Giess 1984, Bellehumeur 1996, Scherer 1998. Prado et al (Prado 2001) present a model which takes into account the particle size distribution as well as the distribution of pores sizes, which successfully captures the experimental results obtained. ...
The viscous flow is one of the important mass transport mechanisms in sintering powder materials such as porous glass. The main underlying driving force here is the tendency of the system to minimize the overall surface free energy. In this work, we first address the fundamental problem of the coalescence of two suspending drops and provide an alternative derivation of Frenkel's formula by explicitly taking account of driving forces which arise from the curvature of the contact zone. The result thus obtained is compared with non-ideal (two phase) fluid lattice Boltzmann simulations. Furthermore, we use this simulation tool to address the more complex case of the sintering of a compact of viscous spherical particles. We observe a significantly faster densification dynamics than in experiments but obtain qualitative agreement upon rescaling the unit of time. We attribute this faster dynamics in simulations to the absence of intergranular friction and discuss a possible improvement of the model. We also propose, for the present LB model, a simple way of imposing shear flow to determine the time dependence of the effective viscosity during the sintering process. The method is benchmarked via a simple analytic case and first simulation results are presented for situations for which no analytic theory exists.
... Confirmation of this assumption would require a chemical analysis of the bubble structure which was not performed in this work. Silica aerogels have been shown to sinter through a viscous sintering mechanism which is controlled by the viscosity of the silica at the sintering temperature, pore size, and pore size distribution (Scherer et al., 1998;Cai et al., 2020). The melting temperature of silica is known to be 1983 K, based on the flame temperature (T f ) over the cooling air mixture temperature (T m ), reported in Table 1, for silica should fall within the range of 0.73 to 1.02; well within the range for sintering to occur. ...
An experimental study was conducted for evaluating the feasibility of using silica aerogel as thermal insulator for combustor liners. Aerogels are a superior material for minimizing heat flux to the metal structure of the combustion liner due to their low thermal conductivity. In this study, a conical natural gas fired swirling-flame combustor was utilized for reproducing the combustion environment. The silica aerogel blanket was attached to the inner side of a perforated combustor liner. Temperature distribution on the outer side of the combustion liner was measured using a calibrated IR camera. To create a protective cooling film over the aerogel surface, cooling air was supplied from the back side of the perforated metal liner and was allowed to penetrate the silica aerogel blanket to be discharged to the combustor. As the combustor was operated at a fixed equivalence ratio of 0.83, cooling air flow rates were varied to evaluate the effectiveness of transpiration cooling on the aerogel blanket as various cooling flow rates. The measured evolution of temperature distribution confirmed thermal equilibriums for every test condition with transpiration cooling. The measured temperature distribution of metal liner demonstrated superior thermal insulation of aerogel blanket under the protection of cooling film with a temperature difference as high as 1580 K between combustion products temperature and the metal liner temperature on the back side. In addition, silica aerogel samples were examined before and after the combustion tests to understand their material degradation exposing to a typical gas turbine combustor environment using high-resolution scanning electron microscope (SEM). Test results suggest multiple degradation mechanisms to the silica aerogel blanket samples from the combustion tests. Improvements can be made to the silica aerogel blankets for a more resilient thermal insulator, for example, by replacing glass fibers in silica aerogels.
... Therefore, increasing the uniformity of Si distributed in alumina matrix and the number of Si-OA-l bonds could improve the thermal stability of the Al 2 O 3 aerogels. Viscous flow is the major dynamic driving force of sintering behavior for oxide aerogels [23,24]. The Al2O3-based aerogels are usually porous materials with 3-dimensional structure composed of interconnected aerogel particles [1][2][3][4][5]. ...
The SiO2-deposited Al2O3-SiO2 aerogel (Si-ASA) with high thermal stability and mechanical properties was synthesized by silica deposition method during aging. Compared with the ASA without silica deposition, the Si-ASA maintains higher specific surface areas after being calcined at 1000°C (384.0), 1100°C (245.5), and 1200°C (124.2 m2/g). When the heating temperature exceeds 1000°C, the mullite phase begins to appear, and the crystallization activation energy of the Si-ASA (1019.24 kJ/mol) is higher than that of the ASA (975.95 kJ/mol). The results show that silica deposition can restrain viscous flow between particles and increase skeleton strength by particles growth and skeleton coarsening, and it can restrain the growth of γ-Al2O3 by forming more Si-O-Al
bonds in the system. This work is of great significance to the design of super thermal insulation materials with high thermal stability.
... Silica aerogels are typical amorphous porous materials with three-dimensional reticular structure formed by the interconnection of aerogel particles, and the sintering behavior of silica aerogels is primarily caused by viscous flow [26][27][28]. At high temperature, the silica aerogel particles will flow viscously under the action of high surface tension, which will lead to the neck formation and fusion of aerogel particles; and the aerogel particles will grow and the pores will collapse with prolonged heating, leading to the decrease of the specific surface area and the macroscopic shrinkage and deformation. ...
Silica aerogels with different particle size were prepared by monodispersed silica sol for the first time. The properties of the silica aerogels under normal and high temperature were systematically studied. Among the four different particle size aerogels, the SA-2 sample consisted of 20.31 ± 1.21 nm particles with narrow size distribution had high temperature resistance and low thermal conductivity. Compared with the traditional acid-base two-step prepared silica aerogels, the monodisperse silica aerogels can significantly increase the temperature resistance while maintaining a low thermal conductivity (0.02723 W·m⁻¹·K⁻¹). The sturdy skeleton structure formed by the interconnection of large particles can effectively inhibit the viscous flow between aerogel particles and avoid pore collapse caused by skeleton failure which can maintain a stable structure until 900 °C and retain a relatively complete structure at 1000 °C with only about 35% volume shrinkage after 2 h heat treatment. At 1100 °C, the viscous flow between aerogel particles and pore collapse cannot be effectively suppressed, and the pure silica aerogels can only be used during a short period at 1100 °C. The results will be meaningful for the design of super thermal insulation materials with high temperature resistance and low thermal conductivity.
... Many studies have employed theoretical calculations and experimental models to investigate the sintering process of aerogels. Scherer [40][41][42] proposed cell models for the viscous sintering of silica aerogels; Rosa-Fox et al. [43] proposed an aggregation model to study the aggregation process of silica aerogels during sintering; Sempéré et al. [44] proposed a simple sintering model for connected fractal aggregate materials; and Phalippou [45] proposed a structural evolution model to explain the sintering and compressing process. In these previous studies, the structure and properties of silica aerogels have been systematically analyzed after the heat treatment process, which has spanned a range of heating temperatures, along with a determination of the maximum operating temperature for silica aerogels and suggestions for improving the temperature resistance of aerogels. ...
The direct in situ TEM imaging method is adopted to investigate the sintering behavior of SiO2 aerogels during the rapid heating process. The structural evolution of SiO2 aerogels and composites at different times during the heat treatment process are further investigated via SEM, FT-IR, BET and XRD. The results indicate that the shrinkage of the SiO2 aerogels and composites primarily occurs during the initial stage of the heating process (within 20 min) with the shrinkage primarily linked to the fusion of small aerogel particles at high temperatures. The aerogel structure then stabilizes with no further shrinkage observed as the heating process continues. The heat treatment process only promotes the space rearrangement and fusion of small aerogel particles with no observed changes to the amorphous structure of the aerogels, and the small-sized particles fusion was the main causes for the structural evolution of SiO2 aerogels and composites under rapid heating condition.
... It indicates that A g SC , increased with increasing porosity . These results were consistent with other experimental data for sintered silica ambigels and aerogels [49,50]. Moreover, predictions by Eq. (21) were in very good agreement with experimental measurements for < 80%. ...
This study aims to computationally generate and fully characterize realistic three-dimensional mesoporous materials. Notably, a new algorithm reproducing gas adsorption porosimetry was developed to calculate the specific surface area and pore size distribution of computer-generated structures. The diffusion-limited cluster-cluster aggregation (DLCCA) method was used to generate point-contact or surface-contact mesoporous structures made of monodisperse or polydisperse spherical particles. The generated structures were characterized in terms of particle overlapping distance, porosity, specific surface area, interfacial area concentration, pore size distribution, and average pore diameter. The different structures generated featured particle radius ranging from 2.5 to 40 nm, porosity between 35 and 95%, specific surface area varying from 35 to 550 m²/g, and average pore diameter between 3.5 and 125 nm. The specific surface area and pore size distribution of computer-generated mesoporous materials were in good agreement with experimental data reported for silica aerogels. Finally, widening the particle size distribution and increasing the particle overlapping were shown to strongly decrease the specific surface area and increase the average pore size of the mesoporous structures. The developed computational tools and methods can accelerate the discovery and optimization of mesoporous materials for a wide range of applications.
... Jean Phalippou and his students performed seminal research on the influence of solution chemistry [1] and processing [2,3] on the structure and properties of aerogels, as well as studying their plasticity [4,5], sintering [6,7], and many other topics. This paper presents a review of poromechanical problems pertinent to the fabrication, application, and characterization of aerogels, most of which was done in collaboration with Jean Phalippou's group or was inspired by their experimental observations. ...
Supercritical drying avoids capillary forces, but damaging stresses can be generated during solvent exchange, pressurization, and depressurization. Each of those steps has been analyzed, so the duration of the process can be minimized without risk to the sample. This work, which was initiated in collaboration with Professor Jean Phalippou’s group, is reviewed. Poromechanics is the study of stresses and strains resulting from interaction of the solid and liquid phases in a porous material. For example, if a gel is immersed in another liquid for solvent exchange, the gel network may undergo a transient contraction if the original pore liquid diffuses out faster than the replacement liquid diffuses in; poromechanical analysis quantifies the resulting stresses and indicates whether there is a risk of cracking. In the preparation of aerogels, when the pressure in the autoclave is decreased, the higher pressure inside the aerogel causes it to expand, which may result in cracking. Poromechanics reveals the rate of depressurization that can be imposed without creating damaging stresses. We also show that the sensitivity of gels to pore pressure can be exploited to measure their permeability to gases and liquids by observing the response to small loads or changes in temperature.
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A dilatometer incorporated into the autoclave measures dilatation of the gel during the supercritical drying cycle, and poromechanical analysis allows extraction of the permeability from the measured strain. Similar analyses are used to minimize the cycle time while avoiding damaging stresses during the solvent exchange, pressurization, and heating, and depressurization steps.
... ZrO 2 aerogels have been proposed as a candidate for applications, such as thermal insulation, catalysis and catalyst support due to its high porosity, high specific surface area, and extremely low density [1][2][3]. However, the porous network structure of the aerogels is inclined to collapse at elevated temperatures due to the condensation among surface hydroxyl groups, nucleation of new oxide crystals, and growth of existing crystals upon heat treatment [4,5]. Thus, the practical applications of ZrO 2 aerogels under high temperature are always hindered by its sintering behavior [6][7][8][9]. ...
The thermal stability of ZrO2–SiO2 aerogels was significantly improved by inorganic–organic synergetic surface modifications: inorganic ions [Fe(III)] surface modification and hexamethyldisilazane gas phase modification. The replacement of Hs from surface hydroxyl groups on the aerogel by Fe(III) ions and silyl groups played a critical role in isolating the hydrous particles of ZrO2–SiO2 aerogels. So the particle growth caused by the condensation of hydroxyl groups upon firing was inhibited. Meanwhile, the decomposition of the silyl groups upon heat treatment produced SiO2 particles, which could serve as pining particle to inhibit the crystallization of ZrO2. Hence, the porous microstructure of the modified aerogels was still well preserved up to 1000 °C, with a high specific surface area of 203.5 m2/g, and a considerable pore volume of 0.721 cc/g. These characteristics of the modified aerogels suggest that it has great potential on ultrahigh-temperature applications in the fields of thermal insulation, catalysis, and catalyst support, etc.
... The viscous Poisson's ratio was expressed as function of the relative density (Scherer et al., 1998) as follows: ...
... [24], Brinker and Scherer [23], Scherer [30] and Yoldas [31] have shown that with thermal treatment, the microstructure is densified. This densification leads to the formation of new siloxane bonds at the interface of the particles by the condensation of the surface silanol groups. ...
Transparent heat insulation materials (TIMs) exhibit great potential for different solar applications. Despite this fact, they are not used widely, since they are either very expensive (aerogel) or made from organic materials and, thus, are not resistant to higher temperatures. Xerogel, obtained from silica gel by drying at ambient pressure, is a promising material for the production of TIMs. This study investigates different treatments applied during the production of silica gel and their influence on its optical and structural properties. To this end, silica sol was gelled and the resulting silica gel was aged before it was either hydrothermally treated or fired. Subsequently, radiation transmission measurements from 400 to 2700 nm as well as porosity, specific surface area, and scanning electron microscopic/transmissions electron microscopic measurements were conducted. It was found that under certain conditions, the transmittance can be improved by firing as well as by hydrothermal treatment. Firing at 600 °C with 10-min dwell time and hydrothermal treatment at 120 °C with 5-h dwell time resulted in the silica gels with the highest transmittance of 63 up to 66 %. The porosity (24–76 %), the pore radii (3–26 nm), and the specific light absorption by embedded water and SiOH molecules could be adjusted over a wide range.
Graphical abstract
... Silica aerogels can be densified by thermal sintering at ~1050°C, isostatic compression at room temperature, or combination of both methods. The kinetics of thermal sintering is affected by the initial pore size distribution and the rate of densification can be accurately predicted from the theory of viscous sintering by Scherer et al(1998). The isostatic compression method uses mercury that cannot enter the small interconnected pores of the aerogel to partially densify the porous network of aerogel materials (Alaoui et al. 1998;Duffours, Woignier, and Phalippou 1995;Pirard et al. 1995;Scherer et al. 1995). ...
This is a white paper covering the results of a literature search and preliminary experiments on materials and methods to remove and immobilize gaseous radionuclided that come from the reprocessing of spent nuclear fuel.
... Thermal sintering is a way to produce dense glass from aerogel (e.g., Phalippou et al. 2004), but it is a kinetically dependant process. For instance, it requires several hours at 1050 °C to complete (Scherer et al. 1998; Perin et al. 2003). The melting temperature of aerogel corresponds to its glass transition temperature since aerogel is an amorphous material. ...
We report the results of high-resolution, analytical and scanning transmission electron microscopy (STEM), including intensive element mapping, of severely thermally modified dust from comet 81P/Wild 2 caught in the silica aerogel capture cells of the Stardust mission. Thermal interactions during capture caused widespread melting of cometary silicates, Fe-Ni-S phases, and the aerogel. The characteristic assemblage of thermally modified material consists of a vesicular, silica-rich glass matrix with abundant Fe-Ni-S droplets, the latter of which exhibit a distinct core-mantle structure with a metallic Fe,Ni core and a iron-sulfide rim. Within the glassy matrix, the elemental distribution is highly heterogeneous. Localized amorphous "dust-rich" patches contain Mg, Al, and Ca in higher abundances and suggest incomplete mixing of silicate progenitors with molten aerogel. In some cases, the element distribution within these patches seems to depict the outlines of ghost mineral assemblages, allowing the reconstruction of the original mineralogy. A few crystalline silicates survived with alteration limited to the grain rims. The Fe- and CI-normalized bulk composition derived from several sections show CI-chondrite relative abundances for Mg, Al, S, Ca, Cr, Mn, Fe, and Ni. The data indicate a 5 to 15% admixture of fine-grained chondritic comet dust with the silica glass matrix. These strongly thermally modified samples could have originated from a finegrained primitive material, loosely bound Wild 2 dust aggregates, which were heated and melted more efficiently than the relatively coarse-grained material of the crystalline particles found elsewhere in many of the same Stardust aerogel tracks (Zolensky et al. 2006).
... He compares the results with the material properties (like Young's modulus and thermal conductivity) of the porous bodies of sintered glass powders and with foamed glasses. More recently Scherer [19] developed the cylindrical model incorporating the influence of the pore size distribution, because Emmerling et al. [20] showed that the change in surface area with density is inconsistent with the cylindrical model when the pore size distribution is not taken into account. The main features of the cylinder model are that the cylinders are not straight and that they are not fully connected. ...
This paper concentrates on sintering characteristics of nano-sized ceramic SiO2 particles. The sintering process is studied as a function of temperature using a conventional furnace and using a laser beam. The underlying idea is to combine the nanoceramic sol-gel concept with inkjet technology and laser treatment. A solution containing nano-sized ceramic particles is fed to an inkjet nozzle that generates a software-controlled pattern on a surface. Afterwards the drops are exposed to an intense laser beam that gives rise to drying and densification of the drops, thereby forming a sintered ceramic layer. The question addressed in the paper is whether the laser treatment leads to the same densified layers as obtained with conventional heat treatments. It turns out that the morphologies are similar but that laser treatment provides much faster the same results.
An experimental study was conducted for evaluating the feasibility of using silica aerogel as thermal insulator for combustor liners. Aerogels are a superior material for minimizing heat flux to the metal structure of the combustion liner due to their low thermal conductivity. In this study, a conical natural gas fired swirling-flame combustor was utilized for reproducing the combustion environment that's typical to a gas turbine combustor. The silica aerogel blanket was attached to the inner side of a perforated combustor liner. Temperature distribution on the outer side of the combustion liner was measured using a calibrated thermal infrared (IR) camera. To minimize the uncertainty in steel surface emissivity due to surface oxidation at elevated temperatures, designated areas on the metal surface were coated with a ceramic cement whose emissivity was carefully determined. To create a protective cooling film over the aerogel surface, cooling air was supplied from the back side of the perforated metal liner and was allowed to penetrate the silica aerogel blanket to be discharged to the combustor. As the combustor was operated at a fixed equivalence ratio of 0.83, cooling air flow rates were varied to evaluate the effectiveness of transpiration cooling on the aerogel blanket as various cooling flow rates.
The measured evolutions of temperature distribution confirmed thermal equilibriums for every test condition with transpiration cooling. The measured temperature distribution of metal liner demonstrated superior thermal insulation of aerogel blanket under the protection of cooling film with a temperature difference as high as 1580 K between combustion products temperature and the metal liner temperature on the back side. In addition, silica aerogel samples were examined before and after the combustion tests to understand their material degradation exposing to a typical gas turbine combustor environment using high-resolution scanning electron microscope (SEM). Test results suggest multiple degradation mechanisms to the silica aerogel blanket samples from the combustion tests. The material analysis suggests that improvements can be made to the silica aerogel blankets for a more resilient thermal insulator, for example, by replacing glass fibers in silica aerogels.
The ZrO2–SiO2 aerogels (ZSA) with varied particle size were synthesized by solvent-thermal aging (STA) method. The effects of STA temperature (60, 110, 170 and 210 °C) on microstructure and thermal stability of the ZSA were investigated by SEM, TEM, FT-IR and XRD. The ZSA-210 presents the minimum shrinkage (9.3%), and it has the largest retention values of specific surface area (59.2%) and pore volume (90.8%) after 1000 °C heat treatment for 1 h. Compared with the aerogels prepared by traditional aging, the ZSA prepared by STA can maintain a low thermal conductivity (0.0285–0.0328 W m⁻¹ K⁻¹) while improving the compression strength (∼1.28 MPa, 10% strain). STA can restrain viscous flow between ZSA particles by particles growth and skeleton coarsening; and STA can restrain the growth of t-ZrO2 by forming more Si–O–Zr bonds in the system. The work will be meaningful for the design of super thermal insulation materials with high thermal stability and low thermal conductivity.
Silica aerogel is a typical nanoporous material exhibiting excellent thermal insulation property, and its nanostructure change at high temperature has attracted extensive attention. However, there is a lack of real-time research on the nanostructure evolution of silica aerogel under rapid heating. Here, silica aerogel was rapidly heated and imaged via an in-situ heating TEM to characterize the nanostructure evolution of from 600 °C to 1300 °C. After rapid heating at different temperatures, the silica aerogels were characterized by SEM, BET, TG-DSC, XRD and FT-IR. The nanostructure evolution of the aerogels can be divided into three temperature stages and experimental strategies for inhibiting aerogel shrinkage at high temperature can be provided. In stage Ⅰ (600 °C–1000 °C), the shrinkage of the silica aerogels occurred during the initial time of the heating process (within 2 h), and then the structure of silica aerogel became stable. The shrinkage of silica aerogels is primarily caused by the fusion of secondary particles, and increasing the secondary silica aerogel particle size may contribute to reducing the aerogel shrinkage. In stage Ⅱ (1100 °C), the aerogel shrinkage can be observed all the time with the heating process continues, and finally closed to the dense state. The shrinkage of silica aerogels is caused by the fusion of secondary particles and the macropore collapse, increasing the particle size and reducing the macropore content may slow down aerogel shrinkage. In stage Ⅲ (1200 °C–1300 °C), the aerogel will shrink rapidly and reach a dense state in a very short time (within 0.5 h). The shrinkage of silica aerogels is caused by the rapid pore collapse and the rapid fusion and shrinkage of the secondary particles of the aerogel, and pure silica aerogels cannot be used at this temperature.
Materials are being developed in the U.S. for the removal and immobilization of radioiodine from the gas streams in a nuclear fuel reprocessing. Silver-functionalized silica aerogel is being investigated because of its high selectivity and sorption capacity for iodine and its possible conversion to a durable silica-based waste form. In the present study, we investigated the effect of pressureless isothermal sintering at temperatures of 900-1400°C for 2.5-90 min or isothermal hot-pressing at 1200°C for 2.5 min on densification of raw and silver-functionalized silica aerogel granules with nitrogen sorption and helium pycnometry. Rapid sintering was observed at 1050 and 1200°C. Pressureless sintering for only 15 min at 1200°C resulted in almost complete densification: the macropores disappeared, the specific surface area decreased from 1114 m2/g to 25 m2/g, pore volume from 7.41 × 10-3 m3/kg to 9 × 10-5 m3/kg, and adsorption pore size from 18.7 to 7 nm. The skeletal density of sintered granules was similar to the bulk density of amorphous silica (2.2 × 103 kg/m3). Low-pressure hot-pressing accelerated the sintering process, decreasing significantly the pore size and pore volume.
The multilayer materials processed by powder metallurgy allow reducing the manufacturing cost. However, the multilayers assembly often leads to cracks or even debonding, which decreases the piece properties.The main purpose of this work is to better understand which important parameters have to be taken into account and their influences on the multimaterials processing. In order to fulfil this objective, the cosintering of three different couples of bimaterials in thick layer has been studied. To identify the damaging reasons in the bilayers, an in situ monitoring device has been developed to observe deformations, cracks and debonding. Moreover, the cosintering simulation with a Binghamien law model was performed to evaluate the internal stresses field : the experimentally observed cracks were correlated with this internal stresses field.For a first couple of materials with no interaction (Al2O3/ZrO2), it appears the necessity to have shrinkages which begin at similar temperatures, similar final shrinkages and an incomplete densification to avoid damaging. The study concerning the presence of a liquid phase at the interface (Al2O3/WC-Co) underlined that this liquid phase increases the bonding during the sintering, but decreases it during the cooling. With a reaction at the interface (ZrO2/Ti), a strong chemical bonding allows to avoid the bad effects of an important differential final shrinkage.
A nonclassical model is developed to describe the deformation of nanostructured ceramics. The model takes into account the scale effects and can be used to determine the effective elastic moduli of such ceramics with allowance for the grain size, the damage level (porosity), and the adhesion parameters characterizing the intergrain contact quality. This model is based on a particular version of the gradient theory of defective media, in which the free strain tensor is determined by the free volume change strains and the continuum model of adhesion interactions is consistent with a chosen kinematic model. The effective characteristics of ceramics are determined with the gradient cohesion-adhesion model using the modified three-phase Eshelby method. Examples of simulating the effective elastic moduli are given, and they demonstrate the efficiency of the model and its adequacy to the well-known experimental data.
Copper-silica composites prepared by the sol-gel method were thermally
treated at 110, 500, 900 and 1100 °C and studied by nitrogen
adsorption, powder X-ray diffraction and temperature programmed
reduction. Samples treated at 500 °C showed the highest surface area
and copper exposed on the surface of the silica matrix resulting in a
high catalytic activity for carbon monoxide oxidation. Treatment at 900
and 1100 °C produced the crystallization/densification of the
samples with a strong decrease in sample area. As a result of these
processes, the copper species were entrapped inside the vitreous silica
matrix and the catalytic activity for carbon monoxide oxidation was
strongly reduced.
The model developed in Part I is applied to nitrogen adsorption isotherms obtained for a series of silica aerogels whose densities are varied by partial sintering. The isotherms are adequately described by a cubic network model, with all of the pores falling in the mesopore range; the adsorption and desorption branches are fit by the same pore size distribution. For the least dense gels, a substantial portion of the pore volume is not detected by condensation. The model attributes this effect to the shape of the adsorbate/adsorptive interface, which can adopt zero curvature even in mesopores, because of the shape of the network.
Pore volume of aerogels is often seriously underestimated by nitrogen adsorption measurements. Undetected pore volume has sometimes been attributed to macropores (i.e., with diameters >50 nm), but permeability measurements indicate that the volume of macropores in gels with solids contents exceeding 5–10 vol.% is below the percolation threshold (∼20 vol.%). The discrepancy may be attributable to the use of the conventional adsorption model, which assumes that the pores are cylindrical, to interpret adsorption isotherms on networks. The adsorbate/vapor interface in a sparse network can adopt a surface of zero curvature while much of the pore space remains empty. The extent of this phenomenon depends on the density of the network, not its pore size, so adsorption can be quite limited in a sparse network, even if the pores are small. A model is presented that accounts for the adsorption/desorption isotherm of a silica aerogel containing ∼13% solids, in which almost half of the pore volume is undetected, even though the largest pore diameter is less than 35 nm.
Micron-sized internal cracks were introduced into pure iron via plastic-strain-controlled low cycle fatigue (LCF) loadings. The cracks on grain boundaries were found to have a regular shape of penny-like. The length of the cracks is less than 30μm, and the thickness less than 3μm. The aspect ratio of diameter-to-thickness is within a range of 5–45. The as-fatigued specimens were annealed for 7h in vacuum at 1173K. Scanning electron microscopy (SEM) observations show that the intergranular microcrack first changes as an unduloid-like shape at 2h of annealing time, then evolves into an array of “circular voids” at 5h, with some isolated voids scarcely observed at 7h. Finite element method (FEM) simulations predict that the intergranular penny-shaped microcrack would evolve into a spherical void when its aspect ratio is less than 19.5, and the evolution time is also quantitatively given. The shrinkage time of the spherical voids evolved from the parent microcrack is predicted by a modified Speight–Beere model. The shrinkage rate of the intergranular spherical voids is faster not only than that of a single intergranular spherical void, but also than that of the intragranular spherical voids.
A model was developed to simulate macroscopic material properties of polycrystalline ceramics from the material properties of the constituting phases and the microstructure. Cubic and random structures were included. The model allows a variation of volume fractions of the phases, grain size and grain boundary areas. Representative for a large number of material properties, elastic tensor, thermal conductivity, coefficient of thermal expansion and thermal stress are calculated for individual microstructures using finite element methods (FEM). Simulations focus on two types of bi-continuous ceramic composites: zirconia toughened alumina (ZTA) and a porous zirconia ceramic which was infiltrated by a spinel-glass. Microstructure of experimental samples is represented by two different model structures: a Voronoi type structure for the ZTA ceramic and a cubic structure of cubes interconnected by cylinders for the infiltrated zirconia system. A substantial impact of microstructure on macroscopic material properties and internal stress distribution is obtained. A good agreement between measured and simulated material properties was found.
SiC retains high mechanical strength and oxidation stability at temperatures above 1500 °C, representing a viable alternative to silica, alumina, and carbon, which have been in use as catalyst supports for more than 60 years. Preparation of monolithic porous SiC is usually elaborate and porosities around 30% v/v are typically considered high. This report describes the synthesis of monolithic highly porous (70% v/v) SiC by carbothermal reduction (1200−1600 °C) of 3D sol−gel silica nanostructures (aerogels) conformally coated and cross-linked with polyacrylonitrile (PAN). Synthesis of PAN-cross-linked silica aerogels is carried out in one pot by simple mixing of the monomers, whereas conversion to SiC is carried out in a tube reactor by programmed heating. Intermediates after aromatization (225 °C in air) and carbonization (800 °C under Ar) were isolated and characterized for their chemical composition and materials properties. Data are interpreted mechanistically and were used iteratively for process optimization. Solids 29Si NMR validates use of skeletal densities (by He pycnometry) for the quantification of the conversion of silica to SiC. Consistent with the topology of the carbothermal process, data support complete conversion of SiO2 to SiC requiring a C:SiO2 ratio higher than the stoichiometric one (=3). The morphology of the SiC network is invariant of the processing temperature between 1300 and 1600 °C, and hence it is advantageous to carry out the carbothemal process at higher temperatures where reactions run faster. Those samples are macroporous and consist of pure polycrystalline β-SiC (skeletal density: 3.20 g cm−3) with surface areas in the range reported previously for biomorphic SiC (20 m2 g−1). Although the micromorphology remains constant, the crystallite size of SiC increases with processing temperature (from 7.1 nm at 1300 °C to 16.5 nm at 1600 °C). Samples processed at 1200 °C are mesoporous and amorphous (by XRD), even though they consist of 75% mol/mol SiC. The change in the morphology of SiC in the 1200−1300 °C range has been explained by a melting mechanism. This comprises the first report of using a polymer cross-linked aerogel for the synthesis of another porous material.
Small-angle X-ray scattering (SAXS) and nitrogen adsorption techniques were used to study the temperature and time structural evolution of the nanoporosity in silica xerogels prepared from acid- and ultrasound-catalyzed hydrolysis of tetraethoxysilane (TEOS). Silica xerogels present a structure of nanopores of fully random shape, size, and distribution, which can be described by an exponential correlation function γ(r) = exp (–r/a), where a is the correlation distance, as predicted by the Debye, Anderson, and Brumberger (DAB) model. The mean pore size was evaluated as about 1.25 nm from SAXS and about 1.9 nm from nitrogen adsorption. The nanopore elimination in TEOS sonohydrolysis-derived silica xerogels is readily accelerated at temperatures around 900 °C probably by the action of a viscous flow mechanism. The nanopore elimination process takes place in such a way that the pore volume fraction and the specific surface are reduced while the mean pore size remains constant. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
This work pointed out the preparation of glass-ceramic based on fluoramphibole using different alkalis. Phases were crystallized using heat-treatment at 850 °C/2.5 h and were identified using XRD. Fluorophlogopite, fluorrichterite, enstatite and cristobalite were found in the heat-treated glasses. Crystallization of fluorophlogopite or fluorrichterite was detected in samples containing high K and Na, respectively, accompanied with crystallization of enstatite in the last sample. Cristobalite was crystallized only in equal alkali-containing glass beside enstatite and richterite. Nanoparticles of silver have grown within a silicate glass via modification of ion exchange process. The metal particle diameters were detected using XRD and TEM. The particles size ranges from 4 to 10 nm. The amount of silver ions exchanged was varied according to type and amount of alkali on fluoramphibole compositions. The composites show low values of dielectric permittivity (10–30) due to the formation of interconnected metal nanoparticle chains. The resistivity of the specimen dropped from 108 to 1010 Ω cm2 to 104 to 108 Ω cm2 after ion exchange process which shows metallic and semiconducting behavior according to the reduction temperature.
Silver nanowires of diameter ;40 nm and length ;0.3 mm have been grown by electrodeposition
within the pores of silica gels which were heat treated in the temperature range 523 to 823 K and,
subsequently, soaked in a silver nitrate solution. A staircase current–voltage characteristic was
observed in the direction of electrodeposition after nanowires were disrupted by the application of
a dc voltage pulse. Such gels containing interrupted nanowires of silver showed a dielectric constant
value ;104 both in directions parallel and perpendicular to that of electrodeposition.
Aerogels are a class of lightweight materials with intriguing properties, resulting from the combination of their customizable porosity and pore size, as well as their backbone morphology. Since synthesis starts on a molecular level, bottom‐up design can be used to tailor their chemical backbone composition and surface properties. In addition, the aerogel synthesis via the sol–gel process opens up seemingly endless options for making composites combining different functionalities and materials that can easily be shaped into binderless monolithic solids or applied as thin layers.
This article starts with a short overview on the history of aerogels, before guiding the reader through the key synthesis and processing steps. Suitable characterization techniques for aerogels are presented and discussed and information on typical properties of aerogels is provided.
The final section is dedicated to aerogel applications, as well as current commercial and research activities.
Plastic-strain-controlled fatigue was performed on pure iron specimens with uniaxial symmetric tension-compression loadings
at room temperature. The as-fatigued specimens were then annealed in vacuum at 1173 K from 1 to 7 hours. The morphologies
of internal fatigue microcracks were observed by scanning electron microscopy (SEM) in the as-fatigued and as-annealed specimens.
The density of the specimens was measured with an electronic analytical balance. The density of the as-fatigued specimens
decreased continuously as the fractional fatigue life increased, and was nearly constant when the specimens were annealed
up to 2 hours at 1173 K, but increased gradually after 2 hours of annealing time. The density of some specimens eventually
approximates to the value of ρ
0, the initial density, at 7 hours of annealing time. This suggests that the initial decrease in density is due to crack initiation
and propagation in the as-fatigued specimens. At the early stage of annealing, the specimen density is nearly constant because
the crack morphological change is controlled by surface diffusion. At the later stages, the density increases and finally
returns to the initial density because the spherical voids evolved from the parent crack are reduced by volume diffusion coupled
with grain-boundary diffusion. A combined model is presented to predict the shrinkage of the spherical voids within the specimens,
and is in broad agreement with the experimental data.
Base catalysed silica aerogels have been densified using both an isostatic pressure and a thermal treatment. The density range investigated corresponds to 0.186–0.65. Textural properties such as the specific surface area and the pore size distribution are analysed as a function of the sample''s bulk density, using the N2 adsorption-desorption technique. A comparison between specific surface areas obtained previously by small angle X-ray scattering is done. Experiments show that the isostatic pressure leads to materials having a narrow pore size distribution while the specific surface area remains unchanged.
This letter reports a joint experimental and numerical investigation of high temperature morphological healing of micron-sized intragranular microcracks in pure iron. Irregular penny-shaped microcracks were first created by low-cycle fatigue and then subjected to annealing in vaccum at 1173 K . It is shown theoretically that, depending on its initial aspect ratio, a penny-shaped microcrack may evolve via surface diffusion into an isolated spherical void, or a doughnut-shaped channel pore with or without a central spherical void. Subsequent evolution causes the doughnut-shaped channel pore to break up into a ring of spherical voids via Rayleigh’s instabilities. These results were confirmed with experimental observations of typical configurations of voids that result from the crack healing process. The experimentally observed evolution time is also in good agreement with the predictions of finite element simulations of the evolution process.
Thermoelastic properties of various bi-continuous porous ceramics are simulated by a new finite element model. The model considers various particle shapes which allow for an independent variation of pore volume and particle contact area. Phenomena like neck formation, agglomeration, particle size distribution and coordination are included in the model geometry. Particle arrangement is modelled using cubic super cells as well as random particle positions. Young's moduli, Poisson's ratios and stress concentration factors are simulated and thermal shock resistance is estimated from these data. A close correlation between thermal conductivity and Young's modulus is found for all types of microstructure. Stress concentration is strongly affected by the particle shapes in the contact region.
Silica glasses are obtained by the densification of aerogels. The transformation of the material into a glass is followed by differential thermal analyis, thermo-gravimetric analysis, dilatometry and by the evolution of the structural, textural and mechanical properties of the material. The organic species and the hydroxyl groups are removed by oxidation and chlorination heat treatments in such a way as to avoid bloating and crystallization phenomena during sintering. Densification is obtained by heat treatment at a low temperature (1100 to 1300 ° C). The densified aerogel shows physical properties identical to those of molten silica. Moreover, this material is very pure and its water content is low. The same process can be extrapolated to multicomponent glasses and composite materials.
Silica aerogels were subjected to isostatic pressure using a Hg porosimeter. Nitrogen adsorption experiments were performed to estimate specific surface area, porous volume and pore size distribution. In a same way, gas permeability measurements have been carried out. During the first stages of compression, the specific surface area seems to be constant. The difference between the total pore volume and that measured by the BJH method (using the desorption part of the isotherm) is less important in compressed aerogels than in sintered ones. Permeability decrease with densification is faster in partially compressed aerogels. This behavior is correlated to disappearance of largest pores and has been associated to the evolution of the pore size distribution which evolves toward smaller pore diameters.
The model developed in Part I is applied to nitrogen adsorption isotherms obtained for a series of silica aerogels whose densities are varied by partial sintering. The isotherms are adequately described by a cubic network model, with all of the pores falling in the mesopore range; the adsorption and desorption branches are fit by the same pore size distribution. For the least dense gels, a substantial portion of the pore volume is not detected by condensation. The model attributes this effect to the shape of the adsorbate/adsorptive interface, which can adopt zero curvature even in mesopores, because of the shape of the network.
Materials with very low density, such as aerogels, are networks with polymers or chains of particles joined at nodes, where the spacing of the nodes is large compared to the thickness of the chains. In such a material, most of the solid surface has positive curvature, so condensation of an adsorbate is more difficult than condensation in a body containing cavities whose surfaces have negative curvature. A model is presented in which the network is represented by straight cylinders joined at nodes with coordination numbers 4, 6, or 12. The shape of the adsorbate/adsorptive interface is obtained for each network by minimizing its surface area. The adsorption behavior is found to depend on the ratio of the node separation,l, to the radius of the cylinders,a: ifl/aexceeds a critical value (which depends on the coordination of the node), then the curvature of the adsorbate/adsorptive interface approaches zero while the adsorbate occupies a small fraction of the pore volume; ifl/ais less than the critical value, then condensation occurs. Even in the latter case, interpretation of the adsorption isotherm in terms of cylindrical pores (as in the BJH model) yields apparent pore sizes much greater than the actual spacing of the nodes. In a companion paper (J. Colloid Interface Sci.202,412 (1998).) this model is applied to silica aerogels and found to give a good fit to both the adsorption and desorption curves with a single distribution of node spacings.
The model developed for sintering of a bimodal pore-size distribution is generalized to describe an arbitrary distribution. The model is further extended to allow for the presence of nonsintering (i.e., rigid) inclusions. This analysis uses the self-consistent approach that takes account of the local stresses created by differential sintering rates.
Structural changes during isothermal sintering were studied for two base-catalyzed SiO2 aerogels with initial densities ρ1 = 122 and 256 kg m−3 by means of small-angle X-ray scattering (SAXS). The relevant structural parameters are the specific surface area, which is determined by the size of the compact primary particles, the mean diameter of the secondary particles, which represent the building blocks of the gel network, and finally the size of the macropores within the tenuous tertiary structure. The major finding of our investigations is the observation of two sintering processes with different time scales: a fast reduction of the size of the macropores and a slow increase in the diameter of the secondary particles as well as a relatively small decrease in the specific surface area. For the two samples investigated the changes of these parameters can be unequivocally correlated with the achieved density upon sintering – independent of the initial density.
Resorcinol-formaldehyde (RF) gels with different monomer concentrations were characterized before and after supercritical drying from carbon dioxide in order to determine the influence of the drying process on elastic moduli and structure. Wet gel shear modulus, Young's modulus and Poisson ratio were determined using the beam-bending method. In addition, acoustic shear waves were used to measure the shear modulus of wet gels. The two methods are shown to agree well. During supercritical extraction of the pore liquid the elastic moduli typically increase by a factor of 1.5 to 5 depending on density. Although in general the elastic moduli do not exhibit power-law dependence on density, in the limited density range covered by our experiments scaling exponents of 6.2 for the wet gels and 4.8 for the aerogels are derived. Aging in acetic acid is shown to have no significant impact on gel elastic properties. Wet and dry gels were analyzed for their structural efficiency and the fraction of elastically effective mass increases with density from 8 to 70%.
Aerogels have such low moduli that they can exhibit large strains under very small loads, and this leads to errors in measurements of porosity and pore size. The use of porosimetry, thermoporometry and nitrogen sorption on aerogels is examined and it is shown that all of these techniques underestimate the pore volume and pore size of typical aerogels. Even at the highest pressures there may be no intrusion of mercury at all, so porosimetry serves only to measure the bulk modulus of the aerogel network, not its pore size distribution; an analysis of the conditions for intrusion as a function of modulus and pore size of the gel is presented. Direct experimental observation of a silica aerogel indicates that its volume shrinks by ⋍ 50% as liquid nitrogen condenses in its pores. Consequently, it is likely that the quantity of macropores in aerogels has been seriously overestimated in the past.
Isothermal sintering has been studied for two base-catalyzed SiO2-aerogels with initial densities ρ0 = 122 kg/m3 and 256 kg/m3, covering a temperature range from 750 to 950 °C and time periods up to 200 h; the highest density achieved was 339 kg/m3. The change in density with time at a given temperature is compared with predictions of sintering theories. The elastic constants determined by ultrasonic measurements reveal scaling with the density achieved upon sintering. Microscopic structural changes during sintering have been studied by means of small angle X-ray scattering (SAXS). Characteristic structural parameters are the mean diameter of the particles, i.e. the building blocks of the gel network, the mean size of the macropores within the tenuous structure and the specific surface area. The major finding of the investigations is the observation of two processes with different time scales: a fast reduction of the size of the macropores accompanied by a slow decrease in specific surface area as well as a slight increase in particle diameter. For both samples investigated all parameters can be unequivocally correlated with the achieved density upon sintering - independent of the initial density.
Isothermal sintering has been studied for a homologous series of supercritically dried base-catalyzed silica aerogels with different initial densities. The change in density and macroscopic viscosity has been recorded in situ using a dilatometer and a beam-bending viscosimeter. Changes of the nanostructural features of the aerogel network have been monitored by means of small angle X-ray scattering. The cylinder model introduced by Scherer is generally accepted to describe the sintering behaviour of bodies with moderate porosity, such as xerogels. In case of highly porous aerogels, however, preferential densification of large pores and little loss of specific surface area is observed, at least during the initial stages. For the samples under investigtion the increase in density and the corresponding variation of specific inner surface and viscosity are compared to the analytical predictions by Scherer, the scaling approach introduced by Sempr et al. and the results obtained from numerical simulation of viscous flow.
In this series of papers, the constitutive equations used to analyze concurrent shear and densification are critically evaluated. In Part I, it is shown that the sintering materials are not linearly viscoelastic, so Laplace transform techniques cannot be applied. This is not a serious limitation, however, because the relevant deformation of the matrix can be treated as purely viscous flow. Assuming that the strains produced by sintering and applied stresses are linearly additive, a simple constitutive equation is obtained. This will be compared with other constitutive equations from the literature in Part II.
A heuristic analysis is given for the determination of the elastic moduli of a composite material, the several constituents of which are each isotropic and elastic. The results are intended to apply to heterogeneous materials composed of contiguous, more-or-less spherical grains of each of the phases.
Several authors have empirically shown that, in aerogels, a power law exists between the mechanical moduli and the bulk density. An exponent value can be determined from mercury porosimetry curves. The shrinkage of aerogels under mercury pressure follows a buckling mechanism which links the pore size to the exerted pressure. The present study relates the exponent to the pore volume distribution which can be described by a hierarchical model valid in a large range of pore size, so a physical meaning is given to the exponent.
This review is concerned with the properties and structure of silica glass. The following topics are treated: Types of silica glasses; The vitreous state of silica glasses: thermodynamical approach, atomistic approach; Optical properties; absorption and fluorescence, refractive index and homogeneity; Mechanical and thermal properties: specific volume, volume relaxation, volume and pressure, elastic and internal friction behaviour, heat capacity and heat conduction, strength, crystallization.
Viscoelastic properties and permeability of a silica gel have been followed during the course of drying. The modulus and viscosity of the network rise by several orders of magnitude, and exhibit power-law dependence on the density of the gel. An aerogel was prepared from an undried gel, and its modulus was found by hydrostatic compression in a mercury porosimeter. Pore size distribution of the aerogel was determined by nitrogen sorption before and after compression. The bulk modulus of the aerogel was very close to that of the wet gel at low densities, but at higher densities the wet gels were more rigid that the compressed aerogels. This difference is attributed to aging of the wet gels in the pore liquid (water). The final shrinkage of the wet gels was greater than expected, based on the viscoelastic properties and the calculated capillary pressure, and the difference is attributed to accelerated viscoelastic relaxation of the network under high capillary stresses. The permeability decreases by almost four orders of magnitude as the gel contracts and pore size inferred from the permeability (rw) decreases in direct proportion to the pore volume. The pore size measured by nitrogen desorption (rBT) agrees closely with rw, as expected on theoretical grounds. For compliant aerogels, the nitrogen desorption process causes substantial compression of the gel, because of the capillary pressure exerted by liquid nitrogen. In such cases, the true pore size and pore volume can be estimated, given knowledge of the elastic modulus of the network; the corrected value of rBT agrees well with rw. At high densities rw was about 0.5 nm smaller than rBT, and this discrepancy is probably caused by neglect of the existence of a relatively immobile layer of water of that thickness on the surface of the network.
High resolution transmission electron microscopy (HRTEM) was used to study the morphology of ultralow-density transparent condensed-silica (CS) aerogels. Silica aerogels were synthesized by two slightly different two-step polymerization processes and they were supercritically dried in a high temperature autoclave. Aerogel CS1 had a density of 9 kg/m3 and a BET surface area of 574 m2/g; aerogel CS2 had a density of 30 kg/m3 and a specific surface area of 630 m2/g. Both samples were fractured, vertically replicated with 0.95 nm Pt-C and backed with approximately 12 nm of rotary evaporated carbon. The silica aerogel was then removed from the replica with dilute HF acid and the replicas were studied by HRTEM. The stereoscopic HRTEM images reveal that connectors in both CS aerogels are extended filaments which resemble bottlebrushes, having microporosity. This morphology results from side-chain formation on a nearly linear CS stem. The slightly different chemistry leads to different morphologies for the two aerogels. For CS1, the connectors between stems have diameters ranging from 1.7 to 14.2 nm with an average of 6.4 ± 0.5 nm and connector lengths averaged 62 ± 21 nm with some as long as 132 nm. Pore sizes ranged from 13 to 240 nm with an average of 74 ± 43 nm. The pores were slightly larger than those in CS2 which ranged from 12 to 277 nm and averaged 61 ± 56 nm. For CS2, the connectors had diameters ranging from 1.5 to 16.5 nm and averaging 9.7 ± 0.5 nm. The connector lengths in the CS2 aerogel averaged 58 ± 27 nm with some as long as 127 nm. The connector diameters in CS2 (9.7 ± 0.5 nm) were the only important aerogel feature significantly different and greater than those in CS1 (6.4 ± 0.5 nm). The CS1 and CS2 connectors had side-chain diameters of 2 ± 0.7 and 0.95 ± 0.5 nm, respectively, and a similar microporosity of approximately 2.0 ± 1 nm.
Variations in viscosity or pore size within a glass body cause uneven contraction during sintering. Consequently, stresses develop which alter the local sintering rate and, in some cases, produce bulk flow. This paper illustrates how these stresses can be analyzed by analogy to thermal stress. As a particular example, sintering of optical waveguide preforms made by the OVPO process is examined in detail. The magnitude of the self-stresses, and the conditions required for bulk flow are determined.
Le comportement mécanique des aérogels de silice a été étudié par des méthodes ultrasonores et par compression statique. Dans cet article nous présentons les valeurs de la vitesse du son en fonction de la densité, de la charge externe et de la pression de gaz interne. Nous démontrons que la vitesse du son varie localement sur des distances de quelques millimètres. De plus nous avons étudié le frittage des aérogels en fonction de la température et de la durée du traitement et nous en déduisons ainsi des lois d'échelle pour les constantes élastiques. The mechanical behavior of SiO2 - aerogels was investigated in ultrasonic and static compression experiments. In our paper we present data on the sound velocity as a function of density, of external load and internal gas pressure. We also demonstrate local variation of sound velocity over distances of a few mm. Furthermore we investigated the sintering behavior of aerogels with respect to temperature and duration of treatment and derived scaling laws for the elastic constants.