Thermal and elastic characterization of glassy carbon thin films by photoacoustic measurements

A portable photoacoustic device is designed and applied to measure thermal diffusivity and linear thermal expansion coefficient of glassy carbon by means of the standard photoacoustic model involving both the thermal diffusion and thermoelastic contributions. This is done by measuring the evolution of the open-cell photoacoustic signal within the modulation frequency interval of 20 Hz-10 kHz, for four samples with thicknesses of 180μm, 140μm, 100μm, and 60μm. A proper fitting procedure of the theoretical amplitude and phase to their corresponding experimental counterparts yielded an average thermal diffusivity of 0.68mm^2·s^-1 and expansion coefficient of \(4.3\times 10^{-6}\) K-1 which are in good agreement with their values reported in the literature for glassy carbon. Furthermore, we demonstrate that the theoretical amplitude does not properly describe the thermoelastic behavior of the samples thinner than \(l \le 100\) μm, due to their strong bending and vibrations driven by the highly disordered fullerene microstructure of glassy carbon followed by the increasing non-homogeneity effects violating 1D heat conduction.