Plenary
THERMODYNAMIC PROPERTIES OF ULTRA-HIGH-TEMPERATURE CERAMICS

The thermodynamic properties of many materials at high-temperature are hard to predict with quantitative accuracy, and often even harder to measure. With recent advances in computational statistical mechanics, it is becoming feasible to use DFT to calculate thermal expansion and heat capacity up to temperatures close to the melting point of a crystal. The approach of quasiharmonic lattice dynamics works well up to about half-the melting point, above which true anharmonic lattice dynamics, besides electronic excitations must be considered. Building on previous work by Grabowski and co-workers[1] (the UP-TILD method), we have combined quasiharmonic lattice dynamcs with thermodynamic integration, using empirical interatomic potentials as an auxilliary function to smooth the path of integration (called the TU-TILD method)[2, 3], which made it possible for us to calculate the aforementioned properties of UHTCs with DFT accuracy up to close to the melting point. The method and its results will be reviewed with reference to the carbides and diborides of Zr and Hf, including explanation of the surprising temperature-dependence of the c/a ratio in the hexagonal diborides. The energies of point defects have also been studied in the quasiharmonic approximation. This is a necessary step to providing data of DFT quality to improve the accuracy of phase diagram calculations in such systems.

[1] Grabowski B, Ismer L, Hickel T, Neugebauer J, Phys. Rev. B 2009;79:134106.

[2] Duff AI, Davey T, Korbmacher D, Glensk A, Grabowski B, Neugebauer J, Finnis MW. Phys. Rev. B 2015;91:214311.

[3] Duff AI, Finnis MW, Maugis P, Thijsse BJ, Sluiter MHF, Comp. Phys. Commun. 2015;196, 439-445.