Anisotropic thermal conductivity tensor of β-Y2Si2O7 for orientational control of heat flow on micrometer scales

The ability to relate the spatially-varying anisotropy of thermal conductivity to crystal structure would provide a foundational advance in our understanding of length-scale dependent heat flow. The challenge, however, is in determining the thermal conductivity tensor in polycrystalline systems with anisotropic crystal structures. Apparent isotropic thermal conductivity in randomly oriented polycrystals with anisotropic properties break down at length scales approaching the grain size. As a result, a measured isotropic thermal conductivity from the macroscale perspective may differ greatly from that measured at the nano or microscale, and thus microscale anisotropic heat flow could underlie a seemingly isotropic thermal conductivity. We experimentally investigate the anisotropic thermal conductivity in polycrystalline beta-phase yttrium disilicate (beta-Y2Si2O7). This is achieved through micrometer-resolution thermal conductivity mapping correlated to the phase and orientation of individual grains, allowing for the determination of the thermal conductivity tensor of beta-Y2Si2O7. Our results are the first to reproduce the thermal conductivity tensor of a polycrystalline system with anisotropic crystal structure based on the spatial distribution of thermal conductivity. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.