Thermal Diffusivity by the Laser Flash Technique

Thermal diffusivity is a material transport property characterizing thermal phenomena significant in many engineering applications and fundamental materials studies. It is also directly related to thermal conductivity, another very important thermophysical property. It is frequently easier to measure thermal diffusivity than thermal conductivity. The two other properties involved in the relationship, density and specific heat, are thermodynamic properties and are either known or can be relatively easily measured. Thermal diffusivity experiments are usually short and relatively simple. In most cases they require small samples, disks a few millimeters in diameter and less than 4 mm thick. Another important feature of thermal diffusivity methods is that the temperature variation in the sample during measurement can be quite small, so the measured property is related to an accurately known temperature. This advantage enables studies of phase transitions via thermal diffusivity throughout transition ranges, which is often not feasible with thermal conductivity measurements whose methods involve appreciable temperature gradients. The most popular temperature range for thermal diffusivity measurements is from near room temperature to 2000 K. According to the shape of the temperature disturbance, diffusivity techniques may be categorized into two basic groups: the transient heat flow and the periodic heat flow techniques. Periodic heat flow variants are based on the measurement of the attenuation or the phase shift of temperature waves propagating through the material. Periodic heat flow methods are divided into two groups. The first constitutes temperature wave techniques, which are predominantly devoted to lower and medium temperatures and are frequently called multiproperty, because they can provide data on a number of thermophysical properties within a single experiment. The second group constitutes the high-temperature variants, where the energy input is effected by modulated electron or photon beams bombarding the sample. The laser flash technique is the most popular method. In summarizing advantages and disadvantages of the techniques, the laser flash and the wave techniques may be considered within same category. Both generally require small samples and vacuum conditions, and measurements are made within very narrow temperature intervals. The latter characteristic makes them convenient for studying structural phenomena in the materials to very high temperatures. Contrary to wave methods which need two types of apparatus for the whole temperature range, the laser flash method can cover the entire range with minor modifications in traversing from subzero to elevated temperatures. Basic components of the laser flash equipment as well as complete units are commercially available. In addition, there are well-established research and testing laboratories available for thermophysical property testing, Temperature wave variants cover a very wide materials range and are suitable for operation under high pressures and for multiproperty measurement. Of particular advantage is the possibility of cross checking results by comparing data derived from the amplitude decrement and the phase lag information. The wave techniques have proved particularly convenient for measuring thermal conductivity and thermal diffusivity of very thin films and deposits on the substrates. The laser flash method is still establishing its place in this area. Keywords: laser flash method; thermal diffusivity; method automation; data analysis; specimen modification; problems; thermal conductivity; periodic heat flow; density specific; transient heat flow