Elin Nilsen

Strengthening of silica gels and aerogels by washing and aging processes

Gels were prepared from a polyethoxydisiloxane precursor by using HF as a catalyst. During washing in water solution a significant increase in the permeability of the gels was observed, showing that dissolution-reprecipitation occurs. After washing, the gels were further soaked in a solution of polyethoxydisiloxane precursor to strengthen and stiffen the gel. As expected, a significant enhancement of the mechanical properties of the wet gels was observed. It is also interesting to note, however, that the permeability does not decrease below the value for the as-prepared gels. Hence, a promising process has been developed where both the stiffness and the strength have been increased as well as the permeability. The increase in permeability is of importance to facilitate the supercritical drying process. Reasonably successful scaling up of the supercritical drying of these gels to laboratory scale has been achieved, and monolithic and transparent gels are obtained. Optical properties have been measured on laboratory scale aerogels. The corresponding results have been correlated with structural characteristics measured by small-angle X-ray scattering (SAXS).

Properties of silica gels aged in TEOS

Abstract In previous work the authors have shown that gels can be strengthened and stiffened by providing new monomers to the alcogel after gelation thus giving xerogels with properties similar to aerogels drying at ambient pressure. Low density silica xerogels (ϱ ∼ 0.22 g/cm 3 ) are obtained by changing aging parameters such as time and pH of pore liquid. Linear shrinkage during drying of the gels is considerably lower when dried from n -heptane than from either acetone or ethanol. The differences in shrinkage are attributed to the fact that n -heptane has a lower surface tension at the drying temperature compared to the two other solvents used. In addition, the significance of water in the pore liquid during drying of the gel network is pointed out.

Shrinkage of silica gels aged in TEOS

Abstract A model has been developed for predicting the shrinkage that occurs during drying, given knowledge of the initial pore size and modulus of the gel. The model is in good agreement with experimental results for silica gels given a variety of aging treatments. The initial pore size (used to predict the capillary pressure that drives shrinkage) can be found by making an aerogel and performing nitrogen desorption, or by calculating the effective pore size from the measured permeability; it is shown that those methods give similar results. Thus the drying shrinkage can be predicted using data obtained from the wet gel (viz. permeability and modulus). At the moment that the liquid/vapor meniscus enters a pore, the walls will be covered with a film of adsorbed liquid, which reduces the radius of the meniscus below the radius of the pore, and thereby increases the capillary pressure. The film thickness for the solvents used is not known so one assumes a thickness of 1 nm. If the presence of the layer is ignored, the theory underestimates the drying shrinkage.

Structural development of silica gels aged in TEOS

Abstract It has been shown that wet gels can be strengthened and stiffened by providing new monomers to the alcogel after gelation by aging in a tetraethoxysilane (TEOS) solution. Xerogels with properties similar to aerogels can be obtained by drying at ambient pressure. Structural changes of gels during the aging in TEOS were studied by small angle neutron scattering (SANS). The SANS measurements show only a slight increase in the volume fractal dimension of the porous gel network with increasing aging time indicating no significant change in the geometry and connectivity of the network. However, there is an increase in both primary particle size and cluster size when silica is precipitated from the aging solution. While the primary particle size stays relatively constant after only 6 h aging cluster size shows a steady increase. In addition, to obtain further information about the structural development, skeletal density of corresponding aerogels has been measured both as a function of aging time and heat treatment time. The skeletal density is higher if the wet gel is aged; however, it is independent of the aging time. A skeletal density close to the theoretical value is reached at about 350°C for both aged and unaged samples.

Strengthening of water glass and colloidal sol based silica gels by aging in TEOS

Abstract Previous work demonstrated that gels can be strengthened and stiffened by providing new monomers to the alcogel thus giving xerogels ambient pressure dried with properties similar to aerogels with porosity up to 90%. That work is extended to increase the strength and stiffness of wet gels obtained from cheaper water soluble sodium silicate (water glass) and colloidal silica sols (3 and 6 nm). During aging the water glass based gels in a TEOS solution, a more than 10-fold increase in shear G modulus was obtained within 27 h, showing a very high reactivity of this system compared to alkoxide based gels. Water glass based gels generally show a lower xerogel density compared to the gels prepared from colloidal sols and a xerogel density of 0.22 g/cm3 was obtained. Starting from colloidal sols, a smaller increase in G was observed for the same increase in initial density, however, the larger particle size and hence pore size of these gels reduces the capillary pressure during drying, giving low density xerogels. A lower density was obtained for the xerogels prepared from 6 nm sol compared to 3 nm sol for the gels aged in TEOS. A large decrease in xerogel density was observed when the colloidal sol based gels were aged in TEOS.

Subcritical drying of silica gels

Silica aerogel is a sol-gel prepared material characterized by high porosity and large inner surface area. Aerogels can be prepared with a high transparency and low thermal conductivity, giving a material excellent for application as transparent thermal insulator. The traditional route to prepare silica aerogels is by formation of an alcogel by hydrolysis and condensation of silicon alkoxides followed by supercritical drying in an autoclave at high pressure (–100 atm). Unfortunately, this process is expensive and might be dangerous, so drying methods have been developed that operate under ambient conditions. In previous work, we have shown that gels can be strengthened and stiffened by providing new monomers to the alcogel giving xerogels with similar properties as aerogels by drying at ambient pressure (porosity up to 90%). This method of obtaining ambient pressure dried aerogels will be described and special emphasis will be given on the effect of the initial gel structure on the preparation of the xerogels.

Thermal and temporal aging of two step acid-base catalyzed silica gels in water/ethanol solutions

Dissolution and reprecipitation of silica during aging in water improve the wet gels’ mechanical stiffness and strength, and hence shrinkage during supercritical drying is reduced. We have investigated how the strength and stiffness of a 2-step TEOS acid-base catalyzed wet gel can be improved by aging in a solution of water/ethanol (20–40 vol%) at various temperatures (20–70°C) and time (2 h and 24 h) and how this influences the aerogels’ properties. The linear shrinkage during supercritical drying was reduced from 29% to 2% by introducing the aging step in the 20 vol% water/ethanol solution for 24 h at 60°C.We have also in previous works introduced the idea of preparing ambient pressure dried silica aerogels by increasing the wet gels’ stiffness by aging in a TEOS solution until shrinkage during drying is almost eliminated. The gels aged in the water/ethanol solutions were further aged in a TEOS/ethanol solution and the effect of the increasing water content in the pore liquid was studied. A xerogel density of 0.20 g/cm3 is reported for gels with a shear modulus (G) of 30 MPa.

Effect of precursor and hydrolysis conditions on drying shrinkage

Abstract Using a theory developed by Smith et al. [J. Non-Cryst. Solids 188 (1995) 191], the drying shrinkage of a gel can be predicted, given the density dependence of the bulk modulus and the characteristic pore size. The theory was tested for two types of silica gel made from tetramethoxysilane, and the agreement between measured and calculated densities was satisfactory.

Sol-Gel Derived Binder for Inorganic Composites

A new low cost inorganic binder system for large volume products like fiber insulation, building materials, etc. has been developed based on sol-gel technology. The precursor for the binder system is an amorphous mineral raw material containing silica as the major component. The sol was prepared by dissolving the amorphous mineral material in formic acid and the mineral was dissolved in a few hours dependent on the molarity of the formic acid. The sol stability was dependent on the solids content and the pH. The gel formation was studied using light scattering and NMR. The results show a growing particle size of particles mainly consisting of silica while the other cations were dissolved in the pore liquid. During the drying of the wet gels, salts of these cations were crystallized in the pores and further decomposed during heating. The derived binder shows good wetting properties to mineral fiber surfaces and a good strength of paper-binder composites. The new binder system applicable to approximately 800°C has a great potential as a substitute for some traditional organic systems.