Sintering of Amorphous Si 3 N 4 Nanoclusters: A Molecular Dynamics Study of Stress Analysis
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ABSTRACT We investigate the onset and growth of the neck between amorphous Si 3N4 nanoclusters at zero and finite pressures. Local stress fluctuations and atomic self-diffusion in the interface region are found to be responsible for neck formation. External pressure has a dramatic influence on the rate of sintering.
INTRODUCTION Silicon nitride, due to strong covalent bonding and low thermal expansion coefficient, is one of the best thermal shock and chemical corrosion resistant ceramic materials. It also displays excellent mechanical properties at high temperatures.[1] Like all ceramics, the one undesirable characteristic of silicon nitride is its brittle behavior. Recent experiments reveal that nanostructured ceramics are less brittle than coarse-grained ceramics.[3-5] One of the important issues concerning the synthesis of nanophase ceramics is their sintering behavior. Many theoretical models have been proposed [6] to account for different mechanisms operating at various stages of sintering. Many of the models consider only large spherical particles with an isotropic surface energy and the capillarity to be the driving force. Recent experimental results [7] indicate that these assumptions may break down in the case of sintering of nanosize particles. Theoretical models have not yet addressed issues concerning the structure and mechanical behavior of nanoparticle sintering at the atomistic level. Microscopics of the neck formation during the early stages of sintering is also difficult to investigate experimentally. In this paper we report a molecular-dynamics (MD) simulation study of initial stage of sintering of a pair of amorphous Si 3N4 nanoclusters at T = 2000K. The size of nanoclusters is 80 A. At zero pressure, the clusters form a symmetric neck of size - 20 A within the first 700 ps of the MD simulation. The neck formation is preceded by an incubation period which lasts for approximately 160 ps. During incubation the clusters are joined by a few Si-N intercluster bonds, and the local stresses in the neck region are small. After the incubation period, non-uniform stresses on the order of a few GPa develop in the neck region. Concomitantly there is a significant increase in the number of Si-N bonds joining the two nanoclusters. The neck region has nearly the same number of four-fold and three-fold coordinated Si atoms. The atomic self-diffusion in the neck region is five times larger than in the interior of the clusters. External pressure has a dramatic effect on the rate of sintering.
RESULTS Molecular-dynamics simulations were performed with effective interatomic potentials consisting of two-body and three-body terms.[8] The two-body terms include screened Coulomb potential, charge-dipole interaction, and steric repulsion between atoms. The first two terms take into account charge-transfer and electronic polarizibility effects in the system. Three-body bond bending and bond stretching terms take into account covalent effects in silicon nitride.
573 Mat. Res. Soc. Symp. Proc. Vol. 408 © 1996 Materials R
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