Thermal properties of organic and inorganic aerogels
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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/m 3 , 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.
I. INTRODUCTION Aerogels are highly porous, open-cell foam materials produced by sol-gel processes and dried by supercritical extraction.1'2 The nanosized cells/pores and particles which make up aerogels are primarily responsible for their very low thermal conduction. The thermal conduction through the solid in aerogels is limited by the extremely small connections between particles making up the conduction path. Similarly, the gaseous conduction is suppressed because the cells/pores are only the size of the mean-free-path for molecular collisions. Therefore, molecules collide with the solid network as frequently as they collide with each other. Also, the radiative conductivity through aerogels is low because the aerogels have such low mass fractions and very large surface areas, although the conductivity increases dramatically with rising temperature. As an example of their low thermal conductivity, values as low as 0.020 W/m • K (i.e., an R per inch value =7) have been measured for air-filled silica aerogels at 300 K and 1 atm pressure.3 Recently, organic aerogels and carbon particleloaded (opacified) silica aerogels have been developed.4 The organic aerogels exhibit a lower intrinsic conductivity and a higher infrared (IR) extinction coefficient iian for silica aerogels, resulting in lower overall thermal conductivity. The carbon particles in the silica aerogel increase the IR extinction coefficient by almost a factor of four over unloaded silica. The lowest
thermal conductivities measured at 1 atm of air and at 300 K for organic and opacified aerogels are 0.012 and 0.013 W/m • K (i.e., a thermal resistance R-factor of 12 and 11 per inch), respectively.5'6 Here, the thermal resistance
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