Early Stages of Sintering of Si 3 N 4 Nanoclusters Via Parallel Molecular Dynamics
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Kenji Tsuruta, Andrey Omeltchenko, Rajiv K. Kalia, and Priya Vashishta Concurrent Computing Laboratory for Material Simulations Department of Physics & Astronomy, Department of Computer Science Louisiana State University, Baton Rouge, LA 70803-4001 E-mail: [email protected] World Wide Web: http://www.cclms.lsu.edu ABSTRACT We investigate early stages of sintering of silicon nitride (Si3N4) nanoclusters by molecular-dynamics (MD) simulations on parallel computers. Within 100 pico seconds, an asymmetric neck is formed between nanocrystals at 2,000K. In the neck region, there are more four-fold than three-fold coordinated Si atoms. In contrast, amorphous nanoclusters develop a symmetric neck, which has nearly the same number of three-fold and four-fold coordinated Si atoms. In the case of sintering among three nanoclusters, a chain-like structure forms in 200 pico seconds. The present study shows that sintering is driven by rapid diffusion of surface atoms and cluster rearrangement. INTRODUCTION Sintering 1 of fine grains is of great importance in the formation of high-density ceramics. In recent years, a great deal of effort has been made to synthesize less brittle ceramics by consolidation of nanometer size clusters. 2 ,3 Many different experimental probes 4 have been used behavior of nanosize to determine the properties of so-called nanophasematerials. The sintering 6 particles 4 ,5 is found to be different from that of macroscopic particles. In this paper, we present a large-scale molecular-dynamics study of sintering of Si3N4 nanoclusters. An atomistic picture of neck formation and underlying dynamical processes during the initial stages of sintering 1 are obtained through the simulation. Crystalline nanoclusters at 2,000K initially undergo considerable relative rotation and then a few Si and N atoms join the two nanocrystals. After 100 ps, the relative motion subsides and an asymmetric neckl,5 is formed between the two nanocrystals. We have also investigated sintering of a pair of amorphous silicon nitride nanoclusters at 2,000K. The neck between amorphous nanoclusters is much more symmetric than the neck between thermally rough nanocrystals. In the neck region between amorphous nanoclusters, the populations of three-fold and four-fold coordinated Si atoms are almost equal; in the neck between nanocrystals, there are more four-fold than three-fold coordinated Si atoms. 7 For both crystalline and amorphous nanoclusters, three nanoclusters form chain-like structures. 1 ,5 In all cases, we find that sintering is driven by diffusion of surface atoms. 6 ,8 The atomic diffusion in the neck region of amorphous nanoclusters is four times faster than in the neck between nanocrystals. PARALLEL MOLECULAR DYNAMICS OF Si3N4 We carried out molecular-dynamics calculations for Si3N4 with effective interatomic potentials that combine two-body and three-body terms. 9 The two-body potentials include: charge-transfer effects through screened Coulomb potentials; charge-dipole interaction to take into 181
Mat. Res. Soc. Symp.
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