Investigation of Bismuth Telluride Nanowire Composites for Enhanced Flexible Thermoelectric Films

Thermoelectric generators can convert waste heat into electrical power offering means of energy recovery, however the materials used in the creation of these devices are far from being optimized. New geometries and device structures will require new materials with enhanced thermoelectric performance and increased flexibility and durability to assist in harnessing the energy losses seen in modern engines and power plants which cannot efficiently convert or transfer the heat energy. The human body is a great example of a power plant in which the heat generated by our bodies is lost to the environment. This lost waste heat can be converted into usable electricity, helping to bring wearable thermoelectric generators closer to reality. Thermoelectric nanomaterials and nanocomposites can pave the way for these wearable devices, powering body sensors and other electronic devices humans carry today. The scope of this dissertation is to thoughtfully engineer nanomaterials and nanocomposites with enhanced thermoelectric properties, that when used to make devices, can provide higher conversion efficiencies. Engineering materials which are flexible, easily applicable, and have higher levels of thermoelectric property retention when bent and/or damaged will allow for new uses of these materials and devices to harness the waste heat that can be found in all energy conversion processes. In this dissertation, thermoelectric nanomaterials were designed and synthesized with higher conversion efficiencies by two methods. First it will be shown that nanocomposites which contain only a small volume percent of carbon nanotubes along with the main thermoelectric component (bismuth telluride nanowires in these works) can be optimized, providing better thermoelectric properties and improved flexibility and durability, by means of oxidizing the carbon nanotubes first with concentrated acids. This process will be shown to not only increase the power factor of the composite material films, but also to allow for much better retention of thermoelectric properties after the films are bent repeatedly in a cyclical fashion, similar to the bending that would occur by a human user wearing a thermoelectric generator. Different methods of oxidation are studied, and the improvements observed are reported. By oxidizing the carbon nanotubes in HNO3 for 15-minutes prior to ex-situ mixing, electrical conductivity, Seebeck value, and power factor were seen to increase by 39.9%, 0.97% and 42.8%, respectively. These increases are due to purified, debundled, and more defect free carbon nanotubes, and also from the improved interfacial bonding seen between the composite’s components. Additionally, higher thermoelectric property retention due to increased flexibility and improved interfacial bonding was seen for the samples containing HNO3 treated carbon nanotubes. This was caused by preferential alignment of the bismuth telluride nanowires with the carbon nanotubes due to the debundling during the HNO3 oxidation process. A second method to improve thermoelectric nanomaterial performance was to modify the dimensions of the individual bismuth telluride nanowires by means of altering certain aspects of the synthesis conditions in which they were grown. Reduced dimensions have been theorized and shown to enhance thermoelectric properties of the films created from the nanomaterials. The diameters of the bismuth telluride nanowires in this study were reduced by means of increasing the amount of directional growth aid, polyvinylpyrrolidone, which is used during the synthesis process. Diameter reductions of up to 5.30% were seen. Maximum TE properties were observed when an additional 10 wt.% of polyvinylpyrrolidone was added showing an impressive 64.9%, - 0.47% and 64.4% increase in electrical conductivity, Seebeck value, and power factor, respectively. The lengths of the nanowires were increased by modifying the synthesis time in which the tubular structure of the nanowires grow. A 21.0% length increase was seen when an additional 40-minutes was added to the synthesis time showing 38.1%, 6.1%, and 56.7% increases in electrical conductivity, Seebeck value, and power factor, respectively when compared to bismuth telluride nanowires grown with the standard synthesis conditions. The methods discovered in this dissertation are easily adoptable by thermoelectric nanomaterials other than the bismuth telluride nanowires used in this study and should assist in the creation of efficient wearable thermoelectric generators