Low-temperature thermal properties of a sol-gel glass

The thermal conductivity and specific heat of an uncompacted sol-gel glass have been measured over the temperature range T{}0.1--10 K. Relative to the compacted product, vitreous silica, the specific heat attributed to two-level states at T{}1 K is larger by a factor of 15, but the thermal conductivity is unchanged even though the phonon velocities are smaller by a factor of 2. At T>1 K, phonon transport appears to be limited by scattering from pores intrinsic to the sol-gel product.