Specific Heat Capacity of Some Selected Materials with the Laser Flash Method

A comparative method for measuring the specific heat capacity is described. In this method, the laser flash experiment is performed on a reference sample with known thermophysical properties, and on the sample under investigation. The specific heat capacity of the unknown sample is determined from the ratio of the two adiabatic maximal temperature rises. The method is based on the assumption that successive laser shots can be kept constant in power delivery and that samples and references can be made to absorb the pulses identically while kept under identical thermal conditions. Previous attempts in ordinary single sample instruments have failed for not being able to meet most of the above prerequisites. Namely, in the single sample device, the reference is tested first over the entire temperature range, followed by the unknown often days or weeks later. While it is quite possible to keep consecutive laser shots highly repetitive in terms of delivered power, it is almost impossible to do this over a long period of time. This is due to such external factors as atmospheric relative humidity variations that severely vary, high voltage leakage in the laser’s power supply, the charge voltage setting itself, and the temperature of the laser rod and cavity. Added to this is the variation in actual temperature of specimens in a furnace that is controlled to the same setpoint in different and often timewise widely spaced cycles. In the multisample testing system, these factors are reduced to insignificant levels. Successive laser shots have to be constant only over a few minutes time during which neither temperature nor humidity is likely to change. Having the specimens located adjacent to each other in the same furnace cavity and heated together ensures essentially identical thermal environment for both the reference and the unknown. The hypothesis is then confirmed with actual tests on several well defined materials over a range of temperature, resulting in data showing agreement within ±1 to 2°C with published literature values.