Tight-Binding Molecular Dynamics of Ceramic Nanocrystals Using Pc-Based Parallel Machines
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ABSTRACT Evolution of atomic and electronic structures of silicon-carbide (SiC) nanocrystals during sintering is investigated by a tight-binding molecular dynamics (TBMD) method. An O(N) algorithm (the Fermi-operator expansion method) is employed for calculating electronic contributions in the energy and forces. Simulations are performed on our eight-node parallel PC cluster. In a sintering simulation of aligned (no tilt or twist) SiC nanocrystals at T=I00OK, we find that a neck is formed promptly without formation of defects. Analyses of local electronic density-of-states (DOS) and effective charges reveal that unsaturated bonds exist only in grain surfaces accompanying the gap states. In the case of tilted () nanocrystals, surface structures formed before sintering affect significantly the grainboundary formation.
INTRODUCTION Silicon-carbide (SIC) ceramic has drawn great deal of interest as a promising material for new electronic applications including high-power, high-frequency, and high-temperature electronic systems, in which silicon or gallium-arsenic devices cannot effectively work. Applications of this material to gas sensors and irradiation detectors in harsh environment have also been investigated extensively [1]. Such wide variety of industrial applications is made possible by the excellent properties of this material such as high barrier for electric breakdown, chemical stability, and high thermal conductivity. However, the brittleness of this hard ceramic is a major drawback in reliability and in tailoring the materials for specific applications. Nanocrystalline SiC (nc-SiC) synthesized by consolidating nano-particles may improve mechanical stability of SiC-based devices: It has been known [2] that ceramics with ultrafine microstructure tend to have enhanced ductility against the coarse-grained counterpart. In order to optimize the material properties and fabrication processes for electronic applications, it is essential to understand the relationship between microstructures and electrical properties of the nanocrystalline ceramic. Semiempirical tight-binding molecular dynamics method [3] is powerful tool to investigate theoretically both atomistic and electronic processes in synthesizing nanocrystalline materials. In this paper, we report on tight;binding molecular-dynamics simulations for sintering of SiC nanocrystals (diameter - 24 A) at a temperature of 1000K. The Fermi-operator expansion method (FOEM) [3, 4] has been employed to calculate efficiently the electronic part of the energy and forces, and it has been run on our eight-node parallel PC cluster. Using the parallel TBMD, we investigate the processes of neck formation (a) between aligned nanocrystals, and (b) between tilted nanocrystals. In the case (a), we find that effect of surface relaxation prior to the neck formation is small, and the neck is formed quickly with no defect trapped in the grain boundary. In the case (b), on the other hand, the surface structures relaxed at T=1000K before sintering affect significantly processes in the
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