Thermoelectric generator based on carbon nanotubes

Abstract

Thermoelectric (TE) materials are used within devices that can be used to convert heat energy directly into electrical energy. When a temperature gradient is applied across a TE device, it is observed that an electrical potential is established. An efficient TE device requires a high figure of merit (ZT) which means a high power factor and a low thermal conductivity are necessary. In this project, Carbon Nanotubes (CNTs) were selected for investigation as an alternative to commercial TE devices made from Bismuth Telluride, mainly due to their availability, low carbon foot print, high design capability, mechanical flexibility, low manufacturing cost and potential for better device performance. This work includes the fabrication process of CNT films which has been explored as well as doping them to n-type and p-type semiconductors. It also compares the effect of seven surfactants: Sodium dodecylbenzenesulfonates (SDBS), Sodium dodecyl sulfate (SDS), Pluronic F-127, Brij 58, Tween 80, Triton X-405 and Benzalkonium chloride (ADBAC). These surfactants are categorised depending on their hydrophilic group polarity (anionic, non-ionic and cationic). Samples exposed to ambient oxygen were found to exhibit p-type behaviour, while the inclusion of Polyethylenimine (PEI) results in n-type behaviour. The highest output power from the TE devices made of a single pair of p and n-type elements was measured to be as high as 1.5 nW/K (67 nW for a 45K temperature gradient), which is one of the highest obtained. This was achieved with Triton X-405. In addition, the electrical data obtained revealed that Triton X-405 has the highest Seebeck coefficient with 81 µV/K and a conductivity of 3.7E+03 S/m due to its short hydrophobic end and non-polar hydrophilic tail which constitutes one of the novelties of this PhD. On the other hand, the anionic surfactant SDBS with its positive end showed a 55 µV/K but a significantly higher electrical conductivity at around 2.6E+4 S/m which is believed to be due to the contribution of additional carriers (sodium ions) from the surfactant. Thermogravimetric analysis (TGA) conducted on the surfactants confirm the maximum operating temperature of each surfactant by showing their thermal degradation points. With this, it was observed that Triton X-405 and Tween 80 indicated a thermal degradation point around 364 ˚C and very low residue left of around 0.12% compared to 33% and 25% for SDBS and SDS respectively. In regard to the thermal behaviour of the CNT samples, it was revealed that CNT films with lengths above 1 cm showed heat losses due to emissivity, therefore, making longer films was deemed inefficient. Finally, a TE device is made from the best performing surfactant (Triton X-405) because of its optimum power factor, with 6 pairs of p and n type semiconducting CNT films. This device was used for a motorcycle exhaust in order to simulate heat waste harvesting which resulted in a ~ 42 mV output voltage at ~ 87 ⁰C temperature difference. This means that many alternating pairs of p-n devices are required to achieve a high output power.