Measurements of the Effect of Pressure on the Thermal Conductivity of Silica Aerogel Particles

Abstract Aerogels are highly porous solids that are made out of a variety of materials such as silica, alumina, or zirconia. Silica aerogel, under a modest vacuum, has the lowest thermal conductivity of any known solid. Values as low as 0.008 W/mK have been reported. While monolithic aerogel is fragile, expensive, and difficult to work with, silica aerogel particles may pose a viable, cost-effective alternative as a thermal insulator. This study details thermal conductivity tests on silica aerogel particles at various pressures. An apparatus that enables cost effective and simple measurements of the total thermal conductivity of aerogel particles at a range of pressures is described. The apparatus employs a platinum resistor placed in the center of a glass spherical bomb filled with aerogel particles. The resistor is the heat source, whose direction of heat flow can easily be traced. Drawing on basic principles of heat transfer, the thermal conductivity of the aerogel particles is found by developing a heat balance to model the data gathered from the apparatus. Furthermore, to test the effects of pressure, a vacuum is used to reduce the pressure and to thus decrease the effective thermal conductivity. Crushed aerogel particles with diameters no greater than 300 μm were found to have thermal conductivities which fell within the same order of magnitude as bulk silica aerogel and the overall thermal conductivity decreased with decreasing pressure, as has been reported with monolithic aerogel.