Assessment of thermal conductivity in rolled pure molybdenum after ultra-high temperature exposure via frequency-domain thermoreflectance

Abstract

Mo-UO₂ cermet dispersed fuel stands out as one of the highly promising solid-state fuels for nuclear thermal propulsion (NTP) systems. As the matrix material, the thermal conductivity of Mo is a critical parameter that not only dictates the temperature distribution within the fuel elements and dissipates heat rapidly but also plays a pivotal role in preserving the structural integrity and stability of the reactor core. Therefore, this study presents the pioneering implementation of Frequency-Domain Thermoreflectance (FDTR) technique to investigate the evolution of thermal conductivity in rolled pure Mo subjected to annealing from room temperature (RT) to 2300 °C. The results revealed a gradual decline in thermal conductivity with increasing annealing temperature, decreasing from 176.00 W/(m·K) at RT to 143.33 W/(m·K) at 2100 °C. A notable abrupt drop occurred at 2300 °C, where the thermal conductivity plummeted to 71.47 W/(m·K), corresponding to a 60 % decrease of its RT baseline. Microstructural characterization revealed a pronounced transition in grain morphology, evolving from initially elongated to equiaxed configurations upon post-annealing treatment, concomitant with substantial grain coarsening. Remarkably, annealing at 2300 °C facilitated the nucleation and growth of polyhedral gas bubbles with sizes spanning the nano-to-micrometer range within the Mo matrix. These bubbles acted as thermal resistance barriers by creating localized gas-filled regions that impeded lattice heat conduction, thereby accounting for the sharp decline in thermal conductivity at 2300 °C. This study systematically elucidates the thermal conductivity evolution of rolled pure Mo under ultra-high-temperature atmosphere, thus providing critical feedback for optimizing the fabrication conditions of matrix materials.