Characteristic Parameters and Selection Principles of Substrates and Gases for Graphene Thermophones

Purpose A graphene thermophone requires a substrate to support its extremely thin structure. It is essential to determine the characteristic parameters and selection principles of substrates and gases in both low- and high-frequency ranges for improving the sound pressure of graphene thermophones. Methods First, the theoretical models for thermoacoustic emission from a graphene on a substrate are established. Analytical expressions of the sound pressure are derived based on analysis of the thermal piston sound source. Then, experimental results of graphene on various transparent substrates are presented to verify the theoretical models. Finally, performance parameters of substrates and gases are summarized and analyzed including the thermal effusivity of substrates, the heat capacity per unit area of substrates, the thermal effusivity of gases and the heat capacity of gases. Results The theoretical predictions agree well with the experimental results. At high frequencies, a lower thermal effusivity of substrates leads to a higher sound pressure. While at low frequencies, the heat capacity per unit area can be selected as the evaluation parameter for substrates. Substrates with a lower heat capacity show better acoustic performances. The performance evaluation parameters of ambient gases can be concluded as the thermal effusivity and the heat capacity for thermoacoustic emission at both high and low frequencies. For gases, the higher thermal effusivity combined with the lower heat capacity is better to achieve a higher sound pressure. Conclusions The theoretical models are applicable to on-substrate thermophones and the results are useful to the optimization of the gas/graphene/substrate multilayer structures.