Anisotropic Phonon Scattering at Grain Boundaries in Nanocrystalline β-Ga₂O₃ Under High-Flux Neutron Irradiation: A Defect-Mediated Thermal Transport Analysis

Abstract

This study investigates anisotropic phonon scattering mechanisms at grain boundaries in nanocrystalline β-phase gallium oxide (β-Ga₂O₃) subjected to high-flux neutron irradiation at fluences ranging from 10¹⁶ to 10¹⁹ n/cm². Using time-domain thermoreflectance (TDTR) spectroscopy combined with molecular dynamics (MD) simulations employing embedded-atom method potentials, we quantify defect-mediated suppression of thermal conductivity along the [010] and [001] crystallographic axes. Irradiation-induced Frenkel pair accumulation and interstitial oxygen clustering were identified as dominant scattering centers, reducing anisotropic thermal conductivity by up to 67% at peak fluence. Phonon lifetime analysis via Green–Kubo autocorrelation functions reveals preferential umklapp scattering along monoclinic off-axis directions. These findings provide a quantitative framework for predicting thermal degradation in β-Ga₂O₃-based power electronics and neutron-environment semiconductor devices.