Abstract
A new approach to measure the cross-plane thermal diffusivity of a microscale slab sample, which can be fabricated by the focused ion beam and attached to a substrate, is proposed. An intensity-modulated pump laser is applied to heat the front surface of the sample uniformly, and the thermoreflectance signal is observed at the rear surface to evaluate thermal wave transport in the material. The thermal diffusivity can be obtained by fitting the phase lags of the experimental data with a theoretical model. The model was developed for the sample with thin-film coatings and heat transfer to the substrate. Although the absorbed heat can cause a significant DC temperature increase in the microscale sample, a thin-film coating with high thermal conductivity can effectively reduce the DC temperature increase within low thermal conductivity samples. To validate the method, we conducted measurements of a fused silica sample of 2.16 µm thickness, coated with 95 nm Ti film on the front surface and 120 nm Au film on the rear surface. The measured thermal diffusivity is in good agreement with the literature value. The uncertainty analysis shows that the measurement uncertainty is within 6%. This proposed approach, designed for microscale samples, offers a unique option for thermal property measurements of special materials, such as irradiated nuclear fuel or other irradiated materials, to enable microscale property determination while minimizing sample radioactivity.
Highlights
Knowledge of local thermal transport properties of irradiated fuels is essential for prediction and control of fuel performance in a nuclear reactor
We develop an approach to measure the crossplane of a microscale sample by the thermoreflectance technique
We developed an approach to measure the cross-plane thermal diffusivity of microscale materials by the thermoreflectance technique
Summary
Knowledge of local thermal transport properties of irradiated fuels is essential for prediction and control of fuel performance in a nuclear reactor. The spatial-scanning thermoreflectance technique has been proposed to measure the thermal diffusivity of a focused ion beam (FIB) fabricated plate-shaped SiC sample.. We develop an approach to measure the crossplane of a microscale sample by the thermoreflectance technique. The approach has other advantages over the spatial-scanning thermoreflectance technique for measuring a microscale sample. The spatialscanning method requires precise focus of the laser beams on such a small surface and high-precision control and knowledge of the accurate distance between the beam spots. Because FIB-fabricated plate samples can be micrometers thick, requiring a thermal wave of high frequency for detection at the rear surface, the thermoreflectance technique becomes a better choice than other measurement options. The paper aims to develop an approach to measure the crossplane thermal diffusivity of a microscale plate sample by the thermoreflectance technique. III, an experimental system was built to validate the approach with a fused silica sample of a dimension of 2.16 × 11.2 × 18.3 μm
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