Atomistic Simulation Studies of the Effects of Defects on Thermal Properties of Ultra High Temperature Ceramics

Due to the harsh environments created by high speeds, significant new demands are placed on materials used for constructing hypersonic vehicles. Ultra high temperature ceramics (UHTCs) like carbides and borides exhibit unique thermal properties, such as very high melting points and good thermal conductivities. These properties make the ceramic materials good candidates for applications like Thermal Protection Systems (TPS) of the hypersonic vehicles. However, thermal properties of UHTCs may be very sensitive to microstructures of the materials. Thus, atomic scale defects may impact certain thermal properties, such as thermal conductivity. The effects of such small defects may be properly studied only through atomistic simulation methods, such as molecular dynamics (MD). Previously, atomistic simulation studies have been performed for the effects of point defects on thermal properties in silicon carbide (SiC). In addition, atomistic simulations have been applied to assess thermal conductivity in zirconium diboride (ZrB2) for perfect crystals and polycrystals. However, to our knowledge, no studies of the effects of point defects have been performed for zirconium diboride. This paper applies atomistic simulations to assess the impact of point defects on thermal conductivity in ZrB2 perfect crystals. Recently derived interatomic potential for ZrB2 along with LAMMPS molecular simulation package and MedeA software environment are employed in this effort. Phonon part of the thermal conductivity is calculated using Green-Kubo method. Calculations for a perfect crystal are conducted first and the results are compared to experimental data available from the literature. Then, several types of point defects are considered (vacancies, substitutions, and interstitials) and their impact on the phonon conductivity is evaluated. It is found that even a small concentration of point defects may have substantial effect and result in a reduction in the thermal conductivity values by almost an order of magnitude. The obtained results are in good qualitative agreement with previous studies conducted for silicon carbide. The methodology which is utilized in this work, the modeling procedure, and the obtained results are discussed in this paper.