Accounting for the length-scale dependence of thermal diffusivity in 3C-SiC measured with transient thermal gratings

TL;DR

This study combines transient thermal gratings, ion irradiation, and theoretical modeling to understand and quantify the length-scale dependence of thermal diffusivity in 3C-SiC, revealing how defects and grain boundaries affect thermal transport.

Contribution

It introduces a suppression factor to reconcile TGS and LFA measurements, enabling accurate in situ assessment of irradiation-induced thermal transport degradation in ceramics.

Findings

Discrepancy between TGS and LFA varies with defect density and grain boundaries.

A suppression factor accounts for length-scale effects in thermal diffusivity measurements.

The framework links irradiation damage to thermal transport degradation quantitatively.

Abstract

Pump-probe optical methods like transient grating spectroscopy (TGS) enable rapid, nondestructive thermoelastic property measurements. But, in phonon-dominated ceramics, they can underpredict bulk thermal diffusivity when long mean free path (MFP) phonons do not equilibrate over experimental length scales. We combine in situ TGS with Si4+ ion irradiation of CVD 3C-SiC (300 and 550C, 0.5-1 dpa) and density functional theory informed Boltzmann transport equation solutions to understand the origins of this offset. We show how the discrepancy between laser flash analysis (LFA) and TGS-measured thermal diffusivity varies with grain-boundary density, temperature, and defect concentration. We introduce a dimensionless suppression factor that accounts for this discrepancy and demonstrate its utility by using it to show an agreement between the thermal defect resistance of neutron irradiated 3C-SiC (measured using LFA) and ion irradiated 3C-SiC (measured using TGS). This theory-informed experimental framework enables quantitative, in situ tracking of ion irradiation damage induced thermal transport degradation in ceramics.