"Energy consumption and utilization influence the economic prosperity, technological innovation, public health, and environmental impact of modern communities. Even with scientific advancements affording vast improvements in the efficiency with which energy is utilized, much more than half of the total energy content consumed globally is ultimately lost as rejected energy. This energy rejection manifests in the form of waste heat which is pervasive in numerous sectors of society such as transportation (e.g., warm exhaust from automobiles) and industry (e.g., hot steam from industrial cooling towers). Accordingly, if there existed a means to harvest waste heat and exploit it for a useful purpose, we could significantly ameliorate the efficiency of modern energy systems. Thermoelectric technology may be a viable approach for generating power via waste heat recovery. These solid-state energy conversion systems comprise semiconducting materials that can transform thermal energy to electrical power by a phenomenon known as the Seebeck effect. The thermoelectric conversion efficiency is dependent on properties like the electrical conductivity, thermopower, and thermal conductivity of the materials. These properties combine to provide a figure of merit, ZT, which characterizes the thermoelectric performance of a material (i.e., higher figure of merit yields higher efficiency). Overall, solid-state energy conversion technology boasts benefits of long lifetimes and no carbon emissions. Unfortunately, some of the best thermoelectrics to date are compounds that contain toxic elements, like lead, or low-abundance elements, like tellurium. These elemental constraints hinder the widespread application of thermoelectric technology. In turn, there has been increased motivation to explore earth-abundant compounds for thermoelectric applications. One such class of materials is tetrahedrites, which are extremely common minerals of copper and sulfur found all over the world. Tetrahedrites demonstrate relatively good ZT values, approximately equal to unity at a temperature of 700 K, while also consisting of low toxicity and earth-abundant elements. These ZT values rival those of other state-of-the-art thermoelectrics (e.g., lead telluride). However, one impediment to commercially employing tetrahedrite materials is the time-consuming and inconsistent synthetic method used to generate these compounds. The conventional reaction is a furnace melting procedure which requires roughly two to three weeks to fabricate a single 2 gram sample. Therefore, exploring alternative techniques for producing tetrahedrite thermoelectrics is a necessary step before these materials become feasible for large-scale applications. This report will detail investigations of three novel approaches for synthesizing tetrahedrite materials. First, the solution-phase modified polyol process has shown success in producing nanostructured tetrahedrites with exceptionally good ZT values. Next, the mechanical alloying procedure is capable of consistently yielding high-purity tetrahedrite in about 48 hours of total reaction time. Lastly, a novel reactive spark plasma sintering method may be used to generate a broad compositional range of tetrahedrites in an expedient process requiring less than 2 hours from start to finish. The thermoelectric performance of samples fabricated by these methods is on par or better than that reported for tetrahedrites made by the traditional method. Ultimately, this work serves as a foundation for expediting tetrahedrite research and rendering these technologically important materials more amenable to commercial thermoelectric applications."--Pages ii-iii.
Read
