Molecular dynamics study of geometric stability, melting, and sintering of cubic boron nitride nanoparticles

A Molecular Dynamics (MD) study of cubic boron nitride (c-BN) nanoparticles of varying shapes and sizes is performed. Four geometric shapes of c-BN nanoparticles are investigated: (1) cube, (2) octahedron, (3) cuboctahedron, and (4) truncated octahedron, where facets have either boron or nitrogen termination. Using a Stillinger-Weber potential, the stability of the nanoparticles is determined to possess a strong crystal geometry and surface dependence, with the {111} facet having the lowest surface energy. Surface reconstruction is observed to occur via two mechanisms: (i) transformation from less stable {100} facets to more stable {111} facets, and (ii) dimerization of nitrogen and boron atoms into rows on the nanoparticle surfaces. Geometric stability based on ground state energy is analyzed, with the octahedron being the most stable, followed in order by the truncated octahedron, the cuboctahedron, and the cube (least stable). However, MD simulations for dynamic melting as a function of temperature reveal that the truncated octahedron may actually be more stable than the octahedron. For detailed examination of the melting mechanism, the octahedral c-BN nanoparticle, which consists solely of {111} facets, is focused on. Interestingly, phase separation occurs during melting of c-BN nanoparticles, resulting in the formation of segregated boron clusters inside the c-BN nanoparticles, along with the vaporization of surface nitrogen atoms. Four different sized octahedral c-BN nanoparticles are examined, i.e. 2.04 nm (969 atoms), 2.55 nm (1771 atoms), 3.57 nm (4495 atoms), and 4.59 nm (9139 atoms), in order to study the dependence of the melting point temperature on size. The Lindemann index of different concentric shells that comprise the nanoparticle, as well as the average Lindemann index for the entire nanoparticle, is utilized to assess melting of the nanoparticle. This assessment is compared with models considering coordination number, cohesive energy, and geometric factors, which all consistently show the large drop in melting point temperature for smaller nanoparticles. Finally, particle-particle collision of two equal-sized octahedral c-BN nanoparticles at various initial temperatures, 2500 K to 3100 K with an increment of 200 K, is investigated in isolated environments using MD simulations to evaluate the optimal temperature range for sintering. The results show that the alignment of the {111} orientation of the two-aggregated nanoparticles occurs at a temperature slightly above the melting point, and rapid grain growth is observed when the temperature is about a few hundred degrees higher than the melting point. However, phase separation also takes place at the corners away from the plane of collision of the aggregated nanoparticles. During sintering of the two 2.55 nm octahedron c-BN nanoparticles, rapid grain growth with a nice crystallographic {111} facet occurs between 3100 K and 3250 K; however, above 3300 K, phase separation dominates and drives melting of the entire sintered nanocluster.Ph.D.Includes bibliographical referencesby Hsiao-Fang Le