Year

2018

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronics Engineering

Abstract

Thermoelectric (TE) materials are capable of directly converting heat into electricity and vice versa. Potential areas of application for TE materials are processes with large quantities of wasted heat, such as industrial furnaces, power plants, or even in the exhaust pipe of automobiles. The recovery and transformation of heat into electricity is done without moving parts in the integration of TE materials in a thermoelectric generator, which comprises TE couple/s and heat exchangers for the hot and cold sides of application.

The conversion efficiencies (figure-of-merit) of TE materials and TE multicouples strongly depend on temperature. The former, figure-of-merit of materials, which often begins with an upward trend proportional to temperature can also plateau or decrease its value when higher temperatures are reached. Complementary, the conversion efficiency of a TE multicouple, which relates to the applied temperature gradient, follows a similar pattern to the figure-of-merit of materials. This multicouple efficiency increases with the temperature difference, though some concerns have been raised about the accuracy of TE performance measurements under large gradients. Nonetheless, the industry and research communities have the common aim of achieving thermoelectric applications comprising TE multicouples that function under high temperatures with larger gradients.

Alternatively, this push towards higher temperatures in thermoelectric applications raises important engineering problems that are seen in the fabrication process for TE multicouple and during operation. Since the components in a multicouple comprise not just TE materials but also metallic electrodes, diffusion and undesired chemical reactions between the elements on both parts could become problematic during the bonding process as the operation temperature increases. Thus, suitable contacts at the interfaces of the TE materials and electrode components are critical for the future of thermoelectric applications. The fabrication of contacts is often approached via a direct or indirect joining process. A direct process involves the use of either temperature, or temperature and pressure for joining the parts, whereas an indirect process usually entails the deposition of a diffusion barrier layer before joining the electrode to the TE material. In addition, diffusion barrier layers are critical components for medium to high temperature TE multicouples because diffusion rises with temperature.

The work in this thesis investigates the joining of a mid-temperature TE material, lead telluride (PbTe), to a nickel metallic electrode, in order to obtain an effective contact between the parts. Following the characterization of the obtained joints, p- and n-type PbTe thermoelements were integrated in a PbTe thermoelectric multicouple with Ni interconnectors and alumina plates.

This thesis is unavailable until Thursday, September 05, 2019

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