Nanoscale Thermal Transport in SiC: Effects of Inter-Grain Misorientation, Grain Size and Crystal Anisotropy

Detailed understanding of the thermal transport across grain boundaries (GBs) and nano-size grains in nanocrystalline thermal functional materials is essential for optimizing their performance across variety of advanced energy applications [1,2]. This study aims at elucidating the nature of inter-grain heat transport as well as spatially-resolved thermal conductivity κ inside small grains by controlling the misorientation tilt angles θ, grain sizes D and crystal anisotropy to advance our fundamental insight into thermal transfer in nanocrystalline materials.<br/><br/>For a detailed atomistic treatment of interfacial heat transport across GBs in SiC bi-crystals, non-equilibrium molecular dynamics (NEMD) simulations are employed. We focus on the (hk0)[001] family of symmetrical tilt GBs, where [001] represents the tilt axis, (hk0) signifies the GB plane with tilt angle θ. The primary interest in structural analysis of GBs revolves around uncovering the atomistic configuration that represents the lowest energy state for a specific θ, commonly referred to as the ground-state structure. To discern this equilibrated state for each GB, we employ an optimization procedure involving the conjugate gradient (CG) method [2,3].<br/><br/>To examine the heat propagation behavior across these GBs, a temperature gradient is imposed on both ends of the SiC structure. It is anticipated that κ of nanocrystalline material will be significantly lower than that of the bulk single crystal due to the presence of thermal resistance at the GB interfaces, defined as the Kapitza resistance R. The variation of R with θ across individual GBs is studied with minimized GB energy. Observed asymmetry in R with respect to θ = 45 degrees is due to two dissimilar atoms (Si and C) present in the lattice.<br/><br/>Moreover, we employ perturb MD [3] to compute spatially-resolved anisotropic local thermal conductivities in SiC by varying D and θ. It was explored that the intra-grain κ decreases with the reduction of D and also varies with the rise of θ in the cross-GB and along-GB directions. The κ remains relatively uniform within the grain interiors but decreases near the GB plane. It should be noted that this reduction is more dominant in larger grains than in smaller ones. This observed phenomenon is discussed in terms of the enhanced phonon scattering in the nanoscale vicinity from GBs, phonon mean free paths, interatomic interaction within a diatomic SiC crystalline lattice across the GBs connecting grains misoriented from each other at various tilt angles directly affecting inter- and intra- grain phonon mediated heat transport.<br/><br/><b>References:</b><br/>[1] J. P. Crocombette and L. Gelebart “Multiscale modeling of the thermal conductivity of polycrystalline silicon carbide” J. Appl. Phys. 106, 083520 (2009)<br/>[2] M. Wojdyr, S. Khalil, Y. Liu and I. Szlufarska “Energetics and structure of &lt;0 0 1&gt; tilt grain boundaries in SiC”, Modelling Simul. Mater. Sci. Eng. 18 075009 (2010)<br/>[3] S. Fujii, K. Funai, T. Yokoi and M. Yoshiya “Grain-size dependence and anisotropy of nanoscale thermal transport in MgO” Appl. Phys. Lett. 119, 231604 (2021)