Mechanical Engineering ETDs

Publication Date

7-1-2016

Abstract

Thermoelectric energy conversion represents a solid-state technology based on the "Seebeck phenomenon", where a temperature gradient generates an electrical voltage difference across semiconductors. Conversely, cooling (refrigeration) can be achieved by applying an electric voltage across the material. One can imagine countless opportunities where thermoelectrics could be used for cooling or harvesting heat to produce energy. Recently, thermoelectric energy conversion has received a great deal of attention as a promising technology to generate electricity from waste heat. Much effort has been put into the improvement and/or development of thermoelectric materials, both inorganic and organic, with higher power conversion efficiency. Organic materials and specifically carbon nanotube (CNT) based thermoelectrics have recently shown great promise for thermoelectric applications. The most efficient organic thermoelectric materials reported to date have efficiencies that are comparable to that of bismuth telluride at room temperature, which has the highest reported ZT for a bulk inorganic material at room temperature (ZT~1). Although the potential of organic thermoelectrics is clear, there is insufficient fundamental information to provide a clear path to the optimization of their performance. Thermoelectric conversion efficiency is accompanied by a high electrical conductivity, high Seebeck coefficient and a low thermal conductivity. Organic thermoelectric materials have an inherent low thermal conductivity. Researchers have therefore focused on the improvement of the Seebeck coefficient and electrical conductivity of these materials. On the experimental front, it is crucial to establish processing-thermoelectric properties-structure relationships for organic materials. There are no set standards, methods or setups for measuring the characteristic properties of thermoelectrics, i.e., Seebeck coefficient, electrical conductivity and thermal conductivity. In this thesis, the design and development of a novel apparatus for the simultaneous measurement of electrical resistivity and Seebeck coefficient of films is reported. Sample mount, where the sample is placed with all connections for measurement and data acquisition, is integrated inside a cryostat chamber enabling measurements over the 10-400 K temperature range. This temperature range is suitable for organic thermoelectrics in that it captures their performance in their intended application environment (i.e., 200-400 K) and provides insight on their structure and transport mechanisms (10-300 K). The whole setup is automated and computer controlled via LabVIEW, for measurement and data acquisition. The program executes all the steps to run the experiment, acquires the measured values, and executes calculations to provide Seebeck coefficient and electrical conductivity as a function of temperature. The sample holder is plug and play type that can be easily mounted or dismounted from the sample stage or sample mount inside the cryostat chamber. Finite element method was used to analyze the thermo-mechanical response of the sample holder in the 10-400 K range. The apparatus was calibrated against high purity Nickel film and a very good agreement was found. Lastly, spray coated polymer and carbon nanotube-based films were characterized using this device. The analysis of these results revealed the different transport mechanisms in these systems.

Keywords

Thermoelectric, Seebeck coefficient, Electrical resistivity, Power factor, Organic material

Degree Name

Mechanical Engineering

Level of Degree

Masters

Department Name

Mechanical Engineering

First Committee Member (Chair)

Shen, Yu-Lin

Second Committee Member

Leseman, Zayd

Third Committee Member

None

Document Type

Thesis

Language

English

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