Anisotropic thermoelectric energy converters based on single-crystal Bi microwires and films

Abstract

We present a demonstration of a novel approach to thermoelectric energy conversion using a single element composed of an anisotropic material. In such materials, a heat flow induces a transverse thermoelectric field that is oriented perpendicular to the direction of the heat flow. A distinctive characteristic of anisotropic thermoelectrics is that the generated voltage is directly proportional to the anisotropy of the thermopower and the element’s length, and inversely proportional to its effective thickness. We fabricated an experimental sample of a heat flux sensor using a 10-m-long, glass-insulated, single-crystal tin-doped bismuth microwire (outer diameter - 20 μm; core diameter - 4 μm). A key factor in this process was the ability to grow the microwire as a single crystal, achieved through a laser-assisted recrystallization technique performed under a strong electric field. The microwire was coiled into a flat spiral and mounted onto a thin copper disk. This sample demonstrated high sensitivity to heat flow, reaching up to 10−2 V/W, with a time constant of approximately 0.2 s. Polycrystalline bismuth films with thicknesses ranging from 2 to 5 μm were deposited onto mica substrates using a vacuum thermal evaporation technique. Experimental samples of heat flux sensors were then fabricated by recrystallizing these films under laser heating in a strong electric field. The observed voltage dynamics at the output of all sensors, in response to modulated heat fluxes, align well with theoretical predictions for anisotropic thermoelectric elements.