Panas Henrik

Thermodynamic Properties of Ultra High Temperature Ceramics (UHTCs) Michael W. Finnis1,, T. Davey1*, A.I. Duff 1,2, T.A. Mellan 1 1Imperial College London, Thomas Young Centre, Dept. of Materials and Dept. of Physics, London, UK 2Hartree Centre, STFC Daresbury Laboratory, Scientific Computing Department, Warrington, UK *Present address: Department of Finemechanics, Fracture and Reliability Research Institute, School of Engineering, Tohoku University Yin, 6‐6‐01 Aramakiaoba, Aoba‐ku, Sendai, 980‐8579, Japan Email: [email protected] 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. Acknowledgements: We acknowledge the EPSRC for support within the Program Grant [grant number: EP/K008749/1‐2] Material Systems for Extreme Environments (XMat) [grant: EP/K008749/1‐2] . The calculations were performed on the Imperial College High Performance Computing facilities as well as ARCHER, the UK’s national high‐performance computing service, which is provided by UoE HPCx Ltd at the University of Edinburgh, Cray Inc and NAG Ltd, and funded by the Office of Science and Technology through EPSRC’s High End Computing Programme. We acknowledge support of the Thomas Young Centre under grant TYC101. References: [1] Grabowski B, Ismer L, Hickel T, Neugebauer J. Ab initio up to the melting point: Anharmonicity and vacancies in aluminum. Physical Review B 2009;79:134106. [2] Duff AI, Davey T, Korbmacher D, Glensk A, Grabowski B, Neugebauer J, Finnis MW. Improved method of calculating ab initio high‐temperature thermodynamic properties with application to ZrC. Phys. Rev. B 2015;91:214311. [3] Duff AI, Finnis MW, Maugis P, Thijsse BJ, Sluiter MHF. MEAMfit: A reference‐free modified embedded atom method (RF‐MEAM) energy and force‐fitting code. Comp. Phys. Commun. 2015;196 439. O1