Thermoelectric properties of antimony doped bismuth telluride alloys

Thermoelectric energy conversion is a unique alternative green-energy technology that can be used for both electrical energy generation and refrigeration applications. Current state of the art research is focused on developing novel thermoelectric materials and exploring quantum confinement effects in nanostructures to enhance the efficiency, characterized by the figure of merit ( ZT), of thermoelectric materials. Bi alloys are the most efficient thermoelectric materials at room temperatures, with a ZT of 1 while a ZT of 2 to 3 is required for commercial feasibility. The dissertation presents development of novel BiSbTe composite nanowires, through electrochemical routes, to obtain higher Seebeck values ( S), which lead to enhanced power factors S 2σ and ZT values. N-type BiSbTe thermoelectric nanowires were developed by electrodeposition from tartaric-nitric acid based electrolytes. Different electrolyte compositions were prepared with varying Te concentration to optimize the electrolyte bath for obtaining better thermoelectric properties. The Te induced Sb codeposition at lower potentials was analyzed from the polarization and composition studies. XRD analysis showed a preferential (1 1 0) orientation for Sb rich BiSbTe alloy nanowires. The highest room temperature Seebeck coefficient of -630 µV/K was measured for Bi2Sb0.6Te3 nanowires. Thermoelectric calculations predicted a ZT\u3e3 for Bi2Sb0.6 Te3 nanowires deposited from sol-C (2.5 mM Bi3+, 10 mM HTeO2+, and 5 mM SbO+) at -150 mV. Dependence of thermoelectric properties on dimensionality of materials was studied by analyzing the Seebeck coefficient and electrical conductivity in 15 nm, 30 nm, 50 nm, and 100 nm nanowires, 200 nm nanotubes, and thin films. All the nanostructures and thin films were electrodeposited from the same sol-C that resulted in the highest Seebeck coefficients. Variations of composition among the different diameter nanostructures and thin films were analyzed. A Seebeck coefficient of -698 μV/K, the highest ever reported in literature among the Bi alloys, was obtained for 15 nm nanowires. Fabrication of a MEMS type micropost thermoelectric generator, using SU8 photoresist to obtain higher aspect ratios, is presented. The six-step fabrication process involved fabricating a gold pattern on Si wafer, lithography of SU8 layer to deposit thermoelectric legs in microposts, using positive photoresist for selective electrodeposition of n-type and p-type materials, and depositing alumina contacts to get series and parallel arrangement of thermoelectric legs. Thermoelectric models for optimization of micropost dimensions to maximize device efficiency and to estimate the temperature difference across the micropost are presented