Thermal Diffusivity and the Laser Flash Method

Overview

Source: Elise S.D. Buki, Danielle N. Beatty, and Taylor D. Sparks, Department of Materials Science and Engineering, The University of Utah, Salt Lake City, UT

The laser flash method (LFA) is a technique used to measure thermal diffusivity, a material specific property. Thermal diffusivity (α) is the ratio of how much heat is conducted relative to how much heat is stored in a material. It is related to thermal conductivity ( Equation 1 ), how much heat is transferred through a material due to a temperature gradient, by the following relationship:

Equation 2 (Equation 1)

where ⍴ is the density of the material and C_pis the specific heat capacity of the material at the given temperature of interest. Both thermal diffusivity and thermal conductivity are important material properties used to assess how materials transfer heat (thermal energy) and react to changes in temperature. Thermal diffusivity measurements are obtained most commonly by the thermal or laser flash method. In this technique a sample is heated by pulsing it with a laser or xenon flash on one side but not the other, thus inducing a temperature gradient. This temperature gradient results in heat propagating through the sample towards the opposite side, heating the sample as it goes. On the opposite side an infrared detector reads and reports the temperature change with respect to time in the form of a thermogram. An estimate of the thermal diffusivity is obtained after these results are compared and fit to theoretical predictions using a least squares model.

The laser flash method is the only method that is supported by multiple standards (ASTM, BS, JIS R) and is the most widely used method for determining thermal diffusivity.

Procedure

Turn on the machine and wait for the warm-up process to end (approximately 2 hours).
Fill up the detector compartment with liquid nitrogen using a small funnel until the nitrogen vapor can be seen coming from the detector. Let the liquid settle until there is no more vapor coming out and close the detector.
Measure the thickness of your sample with a micrometer over several spots and calculate the average thickness and the standard deviation. The edges of the sample should be between 6mm and 25.4mm, with a flat geo

Results

Figures 1, 2, and 3 show the data from an LFA run of an iron standard sample. Figures 1 and 2 show laser pulse vs time plots for two temperatures (48.2°C and 600°C); the blue trace shows the collected laser pulse from the iron sample and the thin red line shows the calculated pulse from the Cowan model. Both temperature pulses fit well to the model because this is a well-defined standard material. Generally, experimentally calculated values match the Cowan model best at high tem

Application and Summary

The laser flash method is a widely used technique for determination of thermal diffusivity which consists of radiating one side of a sample with thermal energy (from a laser source) and placing an IR detector on the other side to pick up the pulse. The wide range in temperature of different models enables measurement on various types of samples. The LFA requires relatively small samples. Other tools that measure thermal conductivity directly, rather than thermal diffusivity, include the Guarded Hot Plate, Heat Flow Meter