Transverse thermoelectric effect in glass-coated Bi-Sn microwires and Bi films for practical application

Abstract

There is currently growing interest in developing efficient thermoelectric materials for use in energy conversion devices. The most commonly used elements today are thermocouples, which operate based on the Seebeck and Peltier effects. While these devices offer clear advantages, they also come with notable drawbacks. For example, to achieve the required output voltage, multiple thermocouples must be connected in series. Additionally, manufacturing reliable low-resistance contacts between the n-type and p-type legs poses technological challenges, which can compromise the overall reliability of the device. A promising alternative that avoids these limitations is the anisotropic thermoelectric element (AT). In crystals exhibiting anisotropy in thermal conductivity, electrical conductivity, and thermopower, an electric voltage can be generated perpendicular to the direction of heat flow - provided that the heat flux is oriented off-axis relative to the main crystallographic directions. This voltage is directly proportional to the temperature difference across the isothermal faces, the degree of thermopower anisotropy, and the length of the crystal, and inversely proportional to its thickness. An AT can be fabricated from a single crystal of appropriate size, without requiring thermoelectric junctions. Unlike conventional thermocouples, it eliminates the need for complex electrical interconnections.

We fabricated a sample using a 10-meter-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 spiral, mounted onto a copper disk, and used in a series of experiments. The sample demonstrated a high sensitivity to heat flow, reaching up to 10⁻² V/W, with a time constant of approximately 0.2 seconds. Also, polycrystalline bismuth films with thicknesses from 2 to 5 μm were deposited on mica substrates by vacuum thermal evaporation. Prototypes of heat flux sensors were fabricated from these films after recrystallization under the influence of laser heating and a strong electric field. The observed voltage dynamics at the output of these sensors in response to modulated heat fluxes is in good agreement with theoretical predictions for anisotropic thermoelectric elements.