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.