Parallel Tight-Binding Simulations of Nanophase Ceramics: Atomic and Electronic Transport at Grain Boundaries
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Parallel Tight-Binding Simulations of Nanophase Ceramics: Atomic and Electronic Transport at Grain Boundaries Kenji Tsuruta, Hiroo Totsuji, and Chieko Totsuji Department of Electrical and Electronic Engineering, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, JAPAN Email: [email protected], URL: http://www.mat.elec.okayama-u.ac.jp ABSTRACT We report on tight-binding molecular dynamics (TBMD) of neck formation processes and atomistic and electronic diffusivity at grain boundaries of nanocrystalline silicon carbide. The TBMD simulations are based on an O(N) algorithm (the Fermi-operator expansion method) for calculating electronic contributions to energy and forces. The code has been fully parallelized on our PC-based parallel machines. The TBMD simulations of collision of SiC nanospheres show that the processes of neck formation depend strongly on contact angles between the two grains. Atomic diffusions are quite different in the necks formed with different angles. Also, the electronic transport property at grain boundary is investigated via a TB representation of an electronic diffusivity. A preliminary result on the diffusivity at a Σ=9 grain boundary of SiC indicates significant enhancement of electron mobility along the grain boundary. INTRODUCTION Nanophase ceramics [1] have gained rapidly a great deal of attention due to their unique properties such as larger fracture toughness and higher sinterability than conventional ceramics. Also some of them have shown superplastic behavior at elevated temperatures [2]. High chemical reactivity and enhanced catalytic behavior, presumably due to large surface-to-volume ratio in the system, have also been observed in some nanophase ceramics such as n-TiO2 [1]. Silicon carbide (SiC) is a well-known high-temperature ceramic, and SiC-based nanophase materials (n-SiC) are very promising for both structural and electronic applications under extreme conditions, including radar/microwave applications and gas/irradiation sensors [3]. Understanding the atomic organizations and the electronic properties of grain boundaries in nSiC is very important for designing and optimizing material performances in these industrial applications. A theoretical investigation of these material properties requires methods for multilevel descriptions of the physical hierarchy in nanophase systems−−−from local electronic/atomic structures to mesoscopic grain dynamics. Recent efforts in developing efficient order-N methods for semi-empirical tight-binding molecular dynamics [4] have made a realistic quantum/atomistic simulation of nanophase materials possible. In this paper, we report on tight-binding molecular-dynamics (TBMD) simulations of neck formation of SiC nanospheres (diameter ~ 27 Å), and atomic and electronic diffusivity at grain-boundary regions. The Fermi-operator expansion method (FOEM) [4, 5] has been employed to calculate efficiently the electronic part of the energy and forces in the TBMD simulations, and it has been run on our sixteen-node parallel PC cluster. Using
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