Room temperature rectification in tapered-channel thermal diodes through nanoscale confinement-induced liquid–solid phase change

Designing thermal diodes is attracting a considerable amount of interest recently due to the wide range of applications and potentially high impact in the transportation and energy industries. Advances in nanoscale synthesis and characterization are opening new avenues for design using atomic-level tools to take advantage of materials properties in confined volumes. In this paper, we demonstrate using advanced modeling and simulation the rectification properties of tapered-channel thermal diodes relying on asymmetric heat flow brought about by thermal conductivity differences between the liquid and solid phases of suitably selected phase-change materials (PCM). Our prototypical design considers Ga as PCM and anodized alumina as the structural material. First, we use a thresholding scheme to solve a Stefan problem in the device channel to study the interface shape and the hysteresis of the phase transformation when the temperature gradient is switched. We then carry out finite-element simulations to study the effect of several geometric parameters on diode efficiency, such as channel length as aspect ratio. Our analysis establishes physical limits on rectification efficiencies and point to design improvements using several materials to assess the potential of these devices as viable thermal diodes. Finally, we demonstrate the viability of proof-of-concept device fabrication by using a non-conformal atomic layer deposition process in anodic alumina membranes infiltrated with Ga metal.