Thermal diffusivity recovery and defect annealing kinetics of self-ion implanted tungsten probed by insitu Transient Grating Spectroscopy

TL;DR

This study investigates how ion-implanted tungsten's thermal diffusivity degrades and recovers with temperature, revealing defect dynamics and the impact of irradiation dose on thermal properties relevant for fusion reactor materials.

Contribution

It introduces insitu Transient Grating Spectroscopy to analyze defect annealing kinetics and thermal diffusivity recovery in tungsten after ion irradiation.

Findings

Thermal diffusivity recovers between 450 K and 650 K due to vacancy mobility.

Samples with lower damage levels nearly recover pristine thermal diffusivity after annealing at 1073 K.

Extended dislocation structures cause residual diffusivity reduction at high irradiation doses.

Abstract

Tungsten is a promising candidate material for plasma-facing armour components in future fusion reactors. A key concern is irradiation-induced degradation of its normally excellent thermal transport properties. In this comprehensive study, thermal diffusivity degradation in ion-implanted tungsten and its evolution from room temperature (RT) to 1073 K is considered. Five samples were exposed to 20 MeV self-ions at RT to achieve damage levels ranging from 3.2 x 10-4 to 3.2 displacements per atom (dpa). Transient grating spectroscopy with insitu heating was then used to study thermal diffusivity evolution as a function of temperature. Using a kinetic theory model, an equivalent point defect density is estimated from the measured thermal diffusivity. The results showed a prominent recovery of thermal diffusivity between 450 K and 650 K, which coincides with the onset of mono-vacancy mobility. After 1073 K annealing samples with initial damage of 3.2 x 10-3 dpa or less recover close to the pristine value of thermal diffusivity. For doses of 3.2 x 10-2 dpa or higher, on the other hand, a residual reduction in thermal diffusivity remains even after 1073 K annealing. Transmission electron microscopy reveals that this is associated with extended, irradiation-induced dislocation structures that are retained after annealing. A sensitivity analysis shows that thermal diffusivity provides an efficient tool for assessing total defect content in tungsten up to 1000 K.