New laser-flash method for measuring thermal diffusivity of isotropic and anisotropic thin films

Abstract

A new laser flash method has been developed for determining thermal diffusivity of thin films in the direction parallel and perpendicular to the sample surface. On the measurement of parallel direction of the sample, a pulsed laser beam which has narrow line-shaped cross section is flashed onto the front surface of a film sample and the temperature response at the back surface at a certain distance away from the line-shaped illuminated area of the laser beam is measured by an InSb infrared rays detector. The thermal diffusivity of the sample can be determined from time required for the temperature to reach the half of the maximum temperature rise finally acquired. On the other hand, the measurement of thermal diffusivity in the direction perpendicular to the sample surface, a pulsed laser beam which has the diameter of about 10 mmφ is flashed onto the front surface of a film sample and the temperature response at the back surface is measured by an InSb infrared rays detector. For this purpose, an improved temperature response acquisition system is required to cover the very fast sampling speed of 1 μs interval. The measured temperature response is compared with the theoretical solution coupled with measured laser beam intensity as a function of time for estimating the thermal diffusivity of the sample. The capability of this new method was clearly demonstrated by determining the thermal diffusivities of isotropic ribbon samples for copper(thickness: 18 μm), nickel(thickness: 5 μm) and SUS304 stainless steel(thickness: 50 μm). A new data processing has also been developed for determining thermal diffusivity of anisotropic film samples by considering two-dimensional heat flow. The usefulness of this data processing was demonstrated by measuring thermal diffusivity in the direction parallel to the C-axis of pyrolytic graphite (thickness: 180 μm).