Second sound and ballistic heat conduction: NaF experiments revisited

TL;DR

This paper reviews experiments on second sound and ballistic heat conduction in NaF, comparing three theories and concluding that non-equilibrium thermodynamics with internal variables offers the best explanation, though more data and experiments are needed.

Contribution

It provides a comparative analysis of theories explaining second sound and ballistic heat conduction in NaF, highlighting the effectiveness of non-equilibrium thermodynamics with internal variables.

Findings

Non-equilibrium thermodynamics with internal variables best explains the data.

Current experimental data is insufficient for definitive conclusions.

Further experiments are necessary to clarify heat conduction mechanisms.

Abstract

Second sound phenomenon and ballistic heat conduction, the two wave like propagation modes of heat, are the two most prominent, experimentally observed non-Fourier effects of heat conduction. In this paper we compare three related theories by quantitatively analyzing the crucial NaF experiments of Jackson, Walker and McNelly, where these effects were observed together. We conclude that with the available information the best comparison and insight is provided by non-equilibrium thermodynamics with internal variables. However, the available data and information is not the best, and further, new experiments are necessary.