Effects of sodium chloride inclusions and micro-scale porosity on the figure of merit of bismuth telluride
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Near stoichiometric bismuth telluride (Bi2Te3) powder was mixed with various concentrations of sodium chloride (NaCl) ranging from 0% to 50% by volume, in 10% increments. The sodium chloride was mechanically ground to achieve micro-scale particles before mixing. Cylindrical pellets were made from these mixtures using a cold pressing and pressureless sintering technique. The sodium chloride was leached from the samples in hot water, resulting in porous samples with varying levels of porosity. The electrical conductivity, Seebeck coefficient, and thermal conductivity were measured at room temperature using a testing apparatus designed and built by the researcher, both before and after the leaching of sodium chloride. From this data, the figure of merit was calculated. Samples of pure bismuth telluride (0% NaCl) served as the baseline samples for comparison. Both the presence of NaCl and pores were efficient at increasing the dimensionless figure of merit. Porous samples initially containing 20% NaCl had a 37.55% higher figure of merit compared to baseline samples, and an 89.07% increase in the figure of merit was seen from the solid samples with NaCl inclusions at a concentration of 30% by volume. The electrical conductivity was negatively affected by both inclusions and porosity, but significant increases in Seebeck coefficient, and reduced thermal conductivity were substantial enough for an overall increase in dimensionless figure of merit. The figure of merit for the baseline sample was approximately .18, and the highest values observed for the NaCl inclusion and porous samples were .34 and .25 respectively. These values are far less than that of what is considered a state of the art thermoelectric material, however the materials and methods used were simple, inexpensive, and scalable, showing great potential for applicability for use with optimized thermoelectric materials in hopes of further increasing their figure of merit.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering