The thermal and mechanical properties of hafnium orthosilicate: Experiments and first-principles calculations

Hafnium orthosilicate (HfSiO4 : hafnon) has been proposed as an environmental barrier coating (EBC) material to protect silicon and silicon-based ceramic materials at high temperatures and as a candidate dielectric material in microelectronic devices. It can naturally form at the interface between silicon dioxide (SiO2) and hafnia (HfO2). When used as an EBC its coefficient of thermal expansion (CTE) should match that of the protecting layer (e.g. silicon and SiC composites) to reduce the stored elastic strain energy, and thus the risk of failure of these systems. In this work, the physical, mechanical, thermodynamic and thermal transport properties of hafnon have been investigated using a combination of density functional theory (DFT) calculations and experimental assessments. The average linear coefficient of thermal expansion (CTE) calculated using the quasi-harmonic approximation increase from 3.06 x 10(-6) K-1 to 6.36 x 10(-6) K-1, as the temperature increases from 300 to 1500 K, in agreement with both X-ray diffraction lattice parameter and dilatometry measurements. The predicted thermal conductivity from Boltzmann transport theory is approximately 16.1 W/m.K at 300 K. The thermal conductivity of our samples using both hot disk and laser flash measurements gave a value of 13.3 W/m.K. This slightly lower value is expected and is indicative of residual disorder in the experimental samples, which is absent in the theoretical analysis. First-principles calculations and nanoindentation techniques are used to assess the ambient temperature elastic constants and bulk modulus respectively. The elastic properties obtained by both approaches agreed to within 5%, validating the computational approach and its future use for the study of the thermomechanical properties of other oxides or silicates.