Phase change modulated thermal switch and enhanced performance enabled by graphene coating

Abstract

A novel concept thermal switch that modulates heat transport between the terminals via the phase change of the liquid medium to a vapor medium is investigated. The thermal conductivity of the vapor medium is one order of magnitude smaller than that of the liquid medium, resulting in a significant reduction in heat transport between the terminals. In the on state, the heated terminal is at a lower temperature; the gap between the heated and unheated terminal is filled with liquid, allowing an efficient heat transfer between the terminals. In the transition state, the temperature of the heated terminal is above the nucleate boiling temperature; thus vapor bubbles begin to form adjacent to the heated terminal, and their motions induce thermal mixing within the liquid, resulting in a higher heat transfer efficiency. In the off state, the temperature of the heated terminal is beyond the cut-off temperature, resulting in a continuous vapor layer developed around the surface of the heated terminal, and heat transfer efficiency across the terminals reduces due to the lower thermal conductivity of the vapor layer. Interestingly, by applying a thin layer of graphene on the surface of the heated terminal, the switch becomes more responsive (85% reduction in thermal cut-off time) and energy-efficient (85% reduction in thermal cut-off energy) due to the rapid formation of vapor bubbles adjacent to the heated terminal. This rapid formation of vapor bubbles is attributed to the unique surface texture of graphene coating, which facilitates the abundance of active nucleation sites. Furthermore, with the graphene coating, the continuous vapor layer can be developed at lower temperatures, a characteristic that is favorable to many engineering applications. Additionally, the thermal switch can also be used to cut-off electricity. By applying an electric potential between the terminals, in the off state, the electrical resistance increases substantially upon the formation of a continuous vapor adjacent to the heated terminal.