Anisotropic thermal-diffusivity measurements by a new laser-spot-heating technique

Abstract

A new technique to measure thermal diffusivities of solid materials, including their anisotropic behaviours, has been developed. The technique is based on periodic heating: an intensity-modulated laser beam is focused to make a small heat spot on the front side of a thin-plate specimen and the excited temperature waves are detected by a thin thermocouple attached onto its rear side. The phase lag of temperature waves is monitored as a function of the distance between the heated spot and the sensing point. The accuracy and the applicability of the present technique were well verified by using two kinds of isotropic reference samples, an austenitic stainless steel and pure copper. The typical uncertainty is estimated to be 5% at room temperature. This technique was applied to evaluate the highly anisotropic thermal diffusivity of highly oriented pyrolytic graphite (HOPG). Its anisotropy, Dab/Dc (the ratio of the in-plane thermal diffusivity to the out-of-plane one), was observed to be about 220.

Thermal diffusivity measurements of thin plates and filaments using lock-in thermography

“…The amplitude and phase profiles correspond to the shortest dimension of the sample. The effect of the sample boundaries has been accounted for by using the image method, 18 which requires adiabatic boundary conditions at the sample side surfaces. This assumption is plausible since the sample sides have a very small area ͑sample thickness is 100 m͒ for the heat to be efficiently transferred from the sample contour.…”

“…For instance, a thermocouple attached at the rear surface has been used to measure the thermal diffusivity. 18,23,24 However, noncontact techniques such as infrared thermography are preferable when dealing with thin samples.…”

“…Slabs have been covered by a 200 nm thick graphite layer. The error is 5%.References 15,17,18,25,and 26. …”

Micro–Macro Analysis of Thermal Diffusion Anisotropy and Particle Orientation in a Composite Material

“…3, we show the directional dependence of the inverse resistivity ρ −1 (Θ) , divided by c p ρ d , of our sample. The measurement was carried out by using a thermo-wave analyzer (TWA) [9] which uses a intensity-modulation laser heated point source (∼150 µm in diameter) and an infrared detector to measure a phase lag with respect to either distance (∼3 mm) from the point heat source at a sample, or modulation frequency of the incident laser. TWA allows us, by measuring the phase lag along radial heat flow spreading out of the heat point source, to measure in-plane inverse resistivity with typically better than 2 % reproducibility and better than 5 % accuracy.…”

Temperature distributions in the laser-heated diamond anvil cell from 3-D numerical modeling

“…The thermal conductivity of graphite is highly anisotropic, with the thermal conduction parallel to the graphite sheets being around 220 times faster than thermal conduction along the c-axis perpendicular to the sheets. 16 Using the thermal diffusivities from Kato et al, 16 the thermal conductivity values in the directions parallel and perpendicular to the graphite sheets are approximately 1500 W/m K and 6.8 W/m K, respectively. We assumed a graphite crystal of thickness 80 lm and radius 60 lm, oriented such that the fast and slow thermal conductivity directions both lay in the radial plane.…”