Thermal conductivity decomposition and analysis using molecular dynamics simulations - Part II. Complex silica structures

Using molecular dynamics simulations, the thermal conductivity of silica-based crystals is found to be a result of two independent thermal transport mechanisms associated with atomic structure. The first mechanism is temperature independent, produces a thermal conductivity on the order of 1 W/m K, and is related to short length scale behavior. It is governed by the silicon coordination, which is unique to a given structure. The second mechanism is temperature dependent and is related to long length scale behavior. At a temperature of 300 K, the associated thermal conductivity ranges from 9 W/m K for the c-direction of quartz to 0.4 W/m K for zeolite-A. This mechanism is controlled by the atomic bond lengths and angles. Complex unit cells, notably cage structures, can distort the SiO4 tetrahedra, leading to a shortening of the phonon mean free path and a spatial localization of energy. The results suggest that an alternative to the available minimum thermal conductivity model for amorphous materials is needed for the crystalline state. (C) 2003 Elsevier Ltd. All rights reserved.