Structure, Mechanical Properties, and Thermal Transport in Microporous Silicon Nitride Via Parallel Molecular Dynamics
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ABSTRACT Molecular dynamics simulations are performed to investigate structure, mechanical properties, and thermal transport in amorphous silicon nitride under uniform dilation. As the density is lowered, we observe the formation of pores below p = 2.6 g/cc and at 2.0 g/cc the largest pore percolates through the entire system. Effects of porosity on elastic constants, phonons and thermal conductivity are investigated. Thermal conductivity and Young's modulus are found to scale as p 1.5 and p 3 .6 , respectively.
INTRODUCTION 1 Silicon nitride is an excellent material for high-temperature applications. Due to its resistance to wear and corrosion, it has found numerous applications in automotive, aerospace, and cutting-tool industries. Silicon nitride is also used as a dielectric layer in electronics industry. Silicon nitride is synthesized by chemical vapor deposition (CVD), reactive sintering or sintering of
nanosized clusters. Often the samples contain microvoids which have a strong effect on physical properties. 2 Depending on growth conditions, the density of CVD-grown samples can range from 2.6 to 3.0 g/cc. 3 In reaction-bonded Si3N4, the density can be as low as 60% of the theoretical value (3.2 g/cc) for the ce-crystalline phase. We have performed molecular-dynamics (MD) simulations to investigate the effect of porosity on structure, elastic properties, and thermal conductivity of a-Si3N4. In our simulations, pores start to form when the density is decreased to 2.6 g/cc. As the density is lowered below 2.6 g/cc, the number of pores and their sizes increase dramatically and around 2.0 g/cc the largest pore percolates through the entire system. Thermal conductivity and Young's modulus were calculated for densities between 2.0 and 3.2 g/cc. We find that the thermal conductivity scales with density
as pt with t = 1.5. Similar values of the scaling exponent t were observed in experiments on silica and carbon aerogels. 4 ,5 The MD results for the Young's modulus follow the relation E - pt
with c = 3.6, which is close to the experimental result for silica aerogels 6 and the percolation model with vector force constants.
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175 Mat. Res. Soc. Symp. Proc. Vol. 408 0 1996 Materials Research Society
PARALLEL MOLECULAR DYNAMICS In our MD simulations, we have used effective interatomic potentials 8 which include screened Coulomb potential to account for charge-transfer effects, charge-dipole interaction due to large polarizability of nitrogen atoms, and steric repulsion. Additionally, covalent effects are included through three-body terms. The results of MD simulations for ox-crystalline Si 3 N4 using these interatomic potentials are in good agreement with experiments for elastic constants, phonon density-of-states (DOS), and specific heat .8 For amorphous silicon nitride, the calculated static structure factor agrees well with the neutron scattering results of Ref. 3. MD simulations were performed for 36,288-particle systems at various d
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