Accessing temperature waves: a dispersion relation perspective

TL;DR

This paper analyzes the dispersion relation of the Dual-Phase-Lag model to identify conditions under which temperature waves can be observed, aiding the design of nanoscale thermal devices on ultra-fast scales.

Contribution

It provides an analytical method to determine optimal conditions for observing temperature wave oscillations in the DPL model, considering a wide range of space and time scales.

Findings

Material acts as a bandpass filter for temperature waves.

Optimal parameters for wave observation depend on delay times in the DPL model.

Applicable to quantum materials and graphite at nanoscale.

Abstract

In order to account for non-Fourier heat transport, occurring on short time and length scales, the often-praised Dual-Phase-Lag (DPL) model was conceived, introducing a causality relation between the onset of heat flux and the temperature gradient. The most prominent aspect of the first-order DPL model is the prediction of wave-like temperature propagation, the detection of which still remains elusive. Among the challenges to make further progress is the capability to disentangle the intertwining of the parameters affecting wave-like behaviour. This work contributes to the quest, providing a straightforward, easy-to-adopt, analytical mean to inspect the optimal conditions to observe temperature wave oscillations. The complex-valued dispersion relation for the temperature scalar field is investigated for the case of a localised temperature pulse in space, and for the case of a forced temperature oscillation in time. A modal quality factor is introduced showing that, for the case of the temperature gradient preceding the heat flux, the material acts as a bandpass filter for the temperature wave. The bandpass filter characteristics are accessed in terms of the relevant delay times entering the DPL model. The optimal region in parameters space is discussed in a variety of systems, covering nine and twelve decades in space and time-scale respectively. The here presented approach is of interest for the design of nanoscale thermal devices operating on ultra-fast and ultra-short time scales, a scenario here addressed for the case of quantum materials and graphite.