Direct measurement of solids thermal effusivity by cw photoacoustic technique

Abstract

A new approach based on a variant of differential photoacoustic (PA) cells, and a first-order approximation of RG model, is used to measure directly the thermal effusivity in solids is proposed. The configuration consists of two identical conventional photoacoustic cells, in both, the system is gas-sample-backing. In one of them, the backing works as reference material (RM) and in the other one as study material (SM). The solid sample thickness and the work frequency range are chosen to guarantee the thermally thin regime. The solid sample (SS) is selected to be a strong optical absorber and then it just works as the medium that converts electromagnetic energy into heat. The thermal paste (TP) effect on the photoacoustic signals through the equivalent thermal properties of SS–TP system was considered. The ratio of photoacoustic pressures graph, as a function of frequency, showed an increasing behaviour with concavity downwards when the thermal effusivity of the RM is less than the effusivity of SM, and a decreases behaviour with concavity upwards in the opposite case. The thermal effusivity of two well characterized SM (aluminium and copper) was determined as a concept proof with maximum errors of 0.54 and 2.17%, respectively. And for further experiments this concept proof can be use as the cell calibration PA protocol.

Keywords

• Thermal effusivity of solids
• Photoacoustic
• Equivalent thermal properties
• Thermal coupling