Heat is the natural by-product of energy conversion processes. Of the 4.25 × 1020J of energy the United States consumes every year, more than 60% is wasted in the form of heat. Therefore, waste heat recovery is a crucial step to improve the energy generation and utilization efficiency. Thermoelectric (TE) materials, which can directly convert rejected or waste heat into usable electric power, has been extensively developed for this issue. Thick-film-based devices have advantages over conventional TE module because of its compact size. By shrinking the size of thermoelectric devices, it not only allows the device to operate under smaller temperature gradients, but by so doing it expands its capability to handle a wider range of thermal and power management microelectronic systems. The combination of electrochemical deposition of compound semiconductors (metal tellurides) with standard integrated circuit technique makes fabrication of thermoelectric microdevices possible. However, the commonly used baths for electrochemical deposition of metal telluride are acidic baths, in which the deposition rate is limited by the solubility of Te (+IV), making the process much less efficient. For instance, it can take more than 24 hours to deposit a 100 µm thick, compact film.
In our work, we demonstrated a rapid deposition process (100 µm/h) to synthesize dense thick PbTe film (>50 µm) for thermoelectric applications. The electrodeposition rate can be limited by mass transfer and reaction kinetics, which results in powdery and compact films, respectively. To achieve high deposition rate and avoid deposition of a powdery film, a high Te (+IV) concentration is necessary. Alkaline baths were developed for this purpose. In acidic solutions, the solubility of Te (+IV) is low (1.6 g/L at pH of zero). In contrast, Te (+IV) dissolves readily in an alkaline bath with a solubility of 87.8 g/L at pH of 10.5, which is 55 folds greater than that at pH of zero in an acidic solution. Control over the composition of the thick PbTe film was achieved by tuning the electrodeposition conditions, such as the ratio of [TeO32-]/[Pb2+] and [EDTA4-]/[Pb2+] in the electrolyte, pH, as well as the applied potential, agitation and temperature. The electrodeposition mechanism of PbTe was examined. The electrical and thermoelectric properties of thick PbTe films were investigated.
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