Silica aerogel; synthesis, properties and characterization

This review examines the stages of drying, with the emphasis on the constant rate period (CRP), when the pores are full of liquid. It is during the CRP that most of the shrinkage occurs and the drying stresses rise to a maximum. We examine the forces that produce shrinkage and the mechanisms responsible for transport of liquid. By analyzing the interplay of fluid flow and shrinkage of the solid network, it is possible to calculate the pressure distribution in the liquid in the pores. The tension in the liquid is found to be greatest near the drying surface, resulting in greater compressive stresses on the network in that region. This produces differential shrinkage of the solid, which is the cause of cracking during drying. The probability of fracture is related to the size of the body, the rate of evaporation, and the strength of the network. A variety of strategies for avoiding fracture during drying are discussed.

Theory of Drying

Aerogel insulation for building applications: A state-of-the-art review

Silica aerogels have drawn a lot of interest both in science and technology because of their low bulk density (up to 95% of their volume is air), hydrophobicity, low thermal conductivity, high surface area, and optical transparency. Aerogels are synthesized from molecular precursors by sol-gel processing. Special drying techniques must be applied to replace the pore liquid with air while maintaining the solid network. Supercritical drying is most common; however, recently developed methods allow removal of the liquid at atmospheric pressure after chemical modification of the inner surface of the gels, leaving only a porous silica network filled with air. Therefore, by considering the surprising properties of aerogels, the present review addresses synthesis of silica aerogels by the sol-gel method, as well as drying techniques and applications in current industrial development and scientific research.

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Silica aerogel: synthesis and applications

Aerogel is a kind of synthetic porous material, in which the liquid component of the gel is replaced with a gas. Aerogel has specific acoustic properties and remarkably lower thermal conductivity (≈0.013 W/m K) than the other commercial insulating materials. It also has superior physical and chemical characteristics like the translucent structure. Therefore, it is considered as one of the most promising thermal insulating materials for building applications. Besides its applications in residential and industrial buildings, aerogel has a great deal of application areas such as spacecrafts, skyscrapers, automobiles, electronic devices, clothing etc. Although current cost of aerogel still remains higher compared to the conventional insulation materials, intensive efforts are made to reduce its manufacturing cost and hence enable it to become widespread all over the world. In this study, a comprehensive review on aerogel and its utilization in buildings are presented. Thermal insulation materials based on aerogel are illustrated with various applications. Economic analysis and future potential of aerogel are also considered in the study.

Toward aerogel based thermal superinsulation in buildings: A comprehensive review

Ambient-temperature supercritical drying of transparent silica aerogels☆

In this paper, three periodic structures are used to derive the apparent conductivity of a porous medium and a specific application is made to aerogel. The characteristics of the structure are determined using results for the porosity and specific surface area obtained from nitrogen adsorption-desorption measurement. 10 refs., 2 figs., 2 tabs.

Geometric Structure and Thermal Conductivity of Porous Medium Silica Aerogel

The motion of gas molecules in silica aerogel is restricted by the solid silica matrix. As a result, the mean free path, velocity distribution function, diffusivity, viscosity and thermal conductivity are changed. In this work, relations for these quantities are derived for a gas in silica aerogel using an approach similar to that used for a gas in free space. Results for the mean free path predict that, for p ≥ 10 bar, the mean free path of the gas molecules in aerogel will be almost the same as in free space. However, as the pressure is reduced, the mean free path reaches a constant finite value instead of increasing as in free space. The thermal conductivity of a gas in aerogel starts to decrease at p = 10 bar and is almost negligible at 0.01 bar, while the thermal conductivity of a gas between parallel walls 1 cm apart starts to decrease at p = 10 −4 bar and is almost negligible at p = 10 −7 bar. The predicted thermal conductivity of a gas in aerogel is in good agreement with experimental results.

Transport properties of gas in silica aerogel

A new method to introduce carbon into aerogels to block the infrared component of radiant heat transfer within the aerogel was developed to improve the thermal properties of silica aerogel. Chemical vapor infiltration and heat treatment were used to deposit and grow nanophase carbon deposits inside the aerogel. Infrared measurements were performed on the doped material to determine the absorption. A vacuum thermal tester was used to measure the thermal conductivity as a function of air pressure and specimen temperature on flat plate aerogel samples.

Thermal characterization of carbon-opacified silica aerogels☆

An improved supercritical drying process for forming transparent silica aerogel arrays is described. The process is of the type utilizing the steps of hydrolyzing and condensing aloxides to form alcogels. A subsequent step removes the alcohol to form aerogels. The improvement includes the additional step, after alcogels are formed, of substituting a solvent, such as CO2, for the alcohol in the alcogels, the solvent having a critical temperature less than the critical temperature of the alcohol. The resulting gels are dried at a supercritical temperature for the selected solvent, such as CO2, to thereby provide a transparent aerogel array within a substantially reduced (days-to-hours) time period. The supercritical drying occurs at about 40° C. instead of at about 270° C. The improved process provides increased yields of large scale, structurally sound arrays. The transparent aerogel array, formed in sheets or slabs, as made in accordance with the improved process, can replace the air gap within a double glazed window, for example, to provide a substantial reduction in heat transfer. The thus formed transparent aerogel arrays may also be utilized, for example, in windows of refrigerators and ovens, or in the walls and doors thereof or as the active material in detectors for analyzing high energy elementry particles or cosmic rays.

Arlon J. Hunt

Papers

Ambient-temperature supercritical drying of transparent silica aerogels☆

Geometric Structure and Thermal Conductivity of Porous Medium Silica Aerogel

Transport properties of gas in silica aerogel

Thermal characterization of carbon-opacified silica aerogels☆

Process for forming transparent aerogel insulating arrays