Electrochemical Deposition and Optimization of Thermoelectric Nanostructured Bismuth Telluride Thick Films

摘要

Bismuth telluride thick films are suitable for thermoelectric (TE) devices covering large areas and operating at small-to-moderate temperature differences (20 - 200 K). High efficiency and high coefficient of performance (COP) are expected to be achieved by using thick films in some cooling applications. Bismuth telluride thick films fabrication have been achieved with Galvanostatic and Potentionstatic deposition. Stoichiometric bismuth telluride thick film was obtained by Galvanostatic deposition at current density of 3.1 mA·cm~(-2). Bismuth telluride films with average growth rate of 10 μm·h~(-1) and different composition were obtained. Effects of current density and composition of electrolyte in Galvanostatic deposition were studied. The current density affected the film compactness, where films deposited at lower current density were more compact than those deposited at higher current density. The morphology of the films did not depend on the current density, but chemical composition was observed when different composition of electrolyte was used. Effects of distance between electrodes, composition of electrolyte solution, and stirring in Potentionstatic deposition were studied. The shorter the distance between electrodes, the higher the electric field, thus the higher current density was applied and the deposited film was less compact. The current density increased more rapidly with stirring during electrodeposition which leads to less compact film. Through this study, films electrode-posited from solution containing 0.013 M Bi(NO_3)_3·5H_2O, 0.01 M TeO_2 and 1 M HNO_3 at 3.1 mA·cm~(-2) for 6 hours without stirring and with inter-electrode distance of 4.5 cm were free-standing with average film thickness of 60 μm and optimum film composition of Bi_2Te_3. The crystallite size of the later films was found to be around 4.3 nm using Scherrer's equation from XRD patterns. Also, negative Seebeck coefficient for the same samples was revealed with an average value of-82 μV·K~(-1).