Molecular dynamics simulations of boron-nitride nanotubes embedded in amorphous Si-B-N

Abstract

In this article, we examine the elastic properties of boron-nitride nanotubes, which are embedded in amorphous silicon-boron-nitride ceramics. We employ molecular dynamics simulations using the Parrinello-Rahman approach. To this end, all systems are modeled with a reactive many-body bond order potential due to Tersoff, which is able to describe covalent bonding accurately. We apply external stress and derive stress-strain curves for various tensile and compressive load cases at given temperature and pressure. In addition to Young moduli and Possion ratios, we compare radial distribution functions, average coordination numbers, ring statistics and self-diffusion coefficients to characterize the short-range, medium-range and long-range order of Si₃BN₅, Si₃B₂N₆ and Si₃B₃N₇ matrices, respectively. Here, our results show that Si₃B₃N₇ exhibits the highest Young modulus and the largest elastic range. Then, we study the properties of a ceramics composite material made from Si₃B₃N₇ matrix and BN nanotubes. We calculate stress-strain curves for the composite to predict the rates of reinforcement of the matrix due to the BN nanotubes. Here, also the influence of the nanotube/matrix-ratio on the elastic modulus of the composite is examined. Finally, we compare the Young moduli derived from our numerical simulations to predictions given by both, a simple macroscopic rule-of-mixtures, which depends on the volume fraction only, and an extended rule-of-mixtures, which also takes the geometry of the BN nanotube into account. Our numerical results show that the extended rule-of-mixtures predicts the Young modulus of the composite with a relative error of 5 % or less.