AbstractsEngineering

Thermoelectric effects can be used for fast response temperature sensing, active solid state cooling, and solid state electricity generation from heat. It is favorable to have a material with large thermopower alpha, high electrical conductivity sigma, and low thermal conductivity kappa. The criterion for good thermoelectric materials that have a high energy conversion efficiency is the dimensionless thermoelectric figure of merit, ZT=(alpha^2 sigma)/kappa T. A ZT value of 1 is the current commercialization criterion. Typically, good thermoelectric materials are semiconductors with large electron effective masses. Doping is a common way to manipulate carrier concentration within a semiconductor class to optimize thermoelectric properties. Bi2Te3 is the paradigm for the class of (Bi1-xSbx)2(Te1-ySey)3 thermoelectric alloys that have been successfully commercialized. The ZT values for this class are of order 1. Recently, theorists have predicted that p-type Bi2Se3 could have comparable ZT to the Bi2Te3, but without the expensive element tellurium. In this dissertation, p-type Bi2Se3 was prepared by Bi substitution doping with Ca, Mn, and Pb. Then two independent experimental methods, the transport properties and the Shubnikov–de Hass measurements, were used to establish the effective mass of Bi2Se3. The results show that the effective mass of the Bi2Se3 valence band is lower than theoreticians have calculated. This work represents the first experimental study of the valence band of Bi2Se3, and concludes that p-type Bi2Se3 does not have a large enough effective mass to have good thermoelectric properties.In the second part of this dissertation, thermoelectric metallic alloys are discussed. Metals are the materials used in temperature-sensing, thermocouple applications. An accurate thermocouple requires metal alloys with large thermopower. Copper-nickel alloys have been widely used as the n-type leg of thermocouples under the name “constantan” due to their large thermopower. For this dissertation, a nonmetal element, sulfur, is introduced in the copper-nickel alloy system to improve thermoelectric properties. Preliminary results showing promising improvement of the thermoelectric properties in the copper-nickel alloy system is presented.This dissertation consists of three chapters and two appendices. Chapter 1 is a review of the fundamentals of thermoelectricity and its application. A literature review is presented on Bi2Se3-related semiconductor materials and thermoelectric metallic metals. Experimental methods used in later chapters are also introduced. Chapter 2 is an experimental study of the valence band of Bi2Se3. This is the first detailed valence band mapping of Bi2Se3. Chapter 3 reports preliminary data of sulfur as a dopant in copper-nickel alloys that improves the thermoelectric properties of the alloy. Appendix A describes a novel thermoelectric combustion chamber design, this design combines the functionalities of combustion chamber, heat exchanger, and thermoelectric generator. Appendix B is a theoretical…