This is a very difficult question to give a general answer. It is possible to apply to these materials the general formula of the thermal conductivity equal to one third of the product of the phonon specific heat per unit volume, the velicity of averaged phonons and the mean free path for the phonons, taking into account that we have phonon-phonon coupling besides phonon-electron and phonon-atom. At difference of what happens in crystalline structures, the phonons here varying several orders of magnitude. This makes strongly frequency dependent the Debye specific heat function besides the mean free path in the general form.

Given the degrees of freedom involved this means that there are very different behaviours in function of the atoms or dimensions, which tell us that it is necessary to select ranges of temperatures or frequencies and for finding different analytical expressions.

This is a question not solved and only certain approaches are possible using this extension of the phonons, which is the most generally used.

1.Ziman J M., Electrons and Phonons, Oxford University Press, U.K (1960)

2. Berman R. Thermal Conduction in Solids, Oxford University Press,U.K(1976)

3. Bonnet J.P., On the thermally activated structural relaxation in glasses, J.Non-Crystalline Solids 127, 227 (1991)

Thermal conduction in crystalline as well as in non- crystalline insulators is through lattice vibrations, i. e, phonons. In glasses, scattering of phonons takes place due to defects present in these material which causes thermal resistance.