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The total thermal conductivity lambda of resorcinol-formaldehyde aerogel monoliths has been measured as a function of density rho in the range from rho = 80 to 300 kilograms per cubic meter. A record-low conductivity value in air at 300 K of lambda approximately 0.012 watt per meter per kelvin was found for rho approximately 157 kilograms per cubic meter. Caloric measurements under variation of gas pressure as well as spectral infrared transmission measurements allowed the determination of solid conductivity, gaseous conductivity, and radiative conductivity as a function of density. The development of such low conductivity materials is of great interest with respect to the substitution of environmentally harmful insulating foams made from chlorofluorocarbons.

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InfroductionA process for the capacitive deionization (CDI) of water with a stack of carbon aerogel electrodes has been developed by Lawrence Livermore National Laboratory.Aqueous solutions of NaC1 or NaNO3 are passed through a stack of carbon aerogel electrodes, each having a very high specific surface area (400 to 1100 m2/g). After polarization, nonreducibie and nonoxidizable ions are removed from the electrolyte by the imposed electric field and held in electric double layers formed at the surfaces of electrodes, as shown in Fig. la. As desired, the effluent from the cell is purified water. This process is also capable of simultaneously removing a variety of other impurities. For example, dissolved heavy metals and suspended colloids can be removed by electrodeposition and electrophoresis, respectively. CDI has several potential advantages over other more conventional technologies. Unlike ion exchange, no acids, bases, or salt solutions are required for regeneration of the system. Regeneration is accomplished by electrically discharging the cell. Therefore, no secondary waste is generated. In contrast to thermal processes such as evaporation, CDI is much more energy efficient. Since no membranes or high pressure pumps are required, CDI offer operational advantages over electrodialysis and reverse osmosis (RO).

Aerogels are open-cell foams that have already been shown to be among the best thermal insulating solid materials known. This paper examines the three major contributions to thermal transport through porous materials, solid, gaseous, and radiative, to identify how to reduce the thermal conductivity of air-filled aerogels. We found that significant improvements in the thermal insulation property of aerogels are possible by (i) employing materials with a low intrinsic solid conductivity, (ii) reducing the average pore size within aerogels, and (iii) affecting an increase of the infrared extinction in aerogels. Theoretically, polystyrene is the best of the organic materials and zirconia is the best inorganic material to use for the lowest achievable conductivity. Significant reduction of the thermal conductivity for all aerogel varieties is predicted with only a modest decrease of the average pore size. This might be achieved by modifying the sol-gel chemistry leading to aerogels. For example, a thermal resistance value of R = 20 per inch would be possible for an air-filled resorcinol-formaldehyde aerogel at a density of 156 kg/m3, if the average pore size was less than 35 nm. An equation is included which facilitates the calculation of the optimum density for the minimum total thermal conductivity, for all varieties of aerogels.