Interpretation of the Thermal Conductivity of Glasses

The thermal conductivity of glasses decreases with decreasing temperature, while the conductivity of crystalline substances increases with decreasing temperature. The behavior of glasses is interpreted in terms of an approximately constant free path for the lattice phonons, so that the conductivity decreases roughly with the specific heat. The value of the phonon mean free path at room temperature is of the order of magnitude of the scale of the disorder in the structure of glasses as determined from x-ray evidence---that is, of the order of 7A. This is about the size of the unit cell of the crystalline forms of silica.