Thermal conduction in disordered materials

Abstract

We use a combination of molecular dynamics simulations and vibrational mode analysis to study the thermal transport in disordered materials. We describe how the compositional and structural disorder affects the phononic thermal conductivity in the semiconductor alloy, and the increase of thermal conductivity in the permanently densified oxide glasses. The size dependence of thermal conductivity in various amorphous materials is also discussed.

The examination of the size dependence of thermal conductivity is conducted on several types of amorphous materials: silica glass, amorphous Si and metallic glass. It is demonstrated that the phonon-phonon coupling charactering the anharmonicity of the system and the structure factor determining the degree of order cannot explain the different thermal transport behavior in semiconducting vs. metallic or oxide glasses.

We studied the effect of pressure on the thermal conductivities of various oxide glasses with experiments and molecular dynamics simulations. The pressure quenched oxide glasses retain permanent densification after pressure release at room temperature and exhibit an increased thermal conductivity in both simulations and experiments. The enhancement of thermal conductivity in densified silica glass is attributed to the increased density and elastic moduli as predicted by the minimum thermal conductivity model. In sodium aluminosilicate and sodium aluminoborate glasses, the pressure induced increase in the average coordination number of the network-forming cations strengthens the network connectivity and thus further promotes the thermal transport.

We investigate the relative role of compositional and structural disorder in a phononic thermal conductivity reduction by studying three 50-50 SiGe alloy structures: ordered alloys, disordered alloys, and amorphous alloys, as well as pure amorphous Si and Ge structures for reference. While both types of disorder significantly reduce thermal conductivity, structural disorder is much more effective to this aim. The examination of phonon lifetimes in disordered alloys shows high values in a low frequency regime governed by Umklapp scattering that are reduced rapidly with increasing frequency following Rayleigh scattering behavior. The local properties analysis reveals that the structural disorder leads to elastic heterogeneities that are significantly larger than density heterogeneities, which is likely the key reason for amorphous semiconductor alloys having lower thermal conductivity than disordered alloys. Temperature dependence of thermal conductivity indicates the importance of propagating phonons and associated Umklapp scattering in SiGe alloy structures. Interestingly, longitudinal modes in amorphous and disordered alloys exhibit similar lifetimes, while transverse modes lifetimes show significant differences and are more temperature dependent.