Synthesis and characterization of nanostructured materials for thermoelectric energy conversion

Haiyu Fang, Purdue University

Abstract

In 2012, more than 58% of the energy produced in the US was rejected in the form of heat. The rapid development of thermoelectric materials in the past decade has raised new hopes for the possibility of directly converting some of this waste thermal energy back to electricity. However, the large scale deployment of thermoelectric devices is still limited by the mediocre conversion efficiency. Nanostructured materials have been proved to be able to significantly improve conversion efficiency. My research is devoted to developing efficient solution phase reactions to synthesize nanostructured thermoelectric materials in an economical and scalable way. We also aim at exploring the unique applications of solution synthesized nanostructured materials, e.g. developing nanocrystal ink to coat on flexible substrates for applications in wearable thermoelectric devices. In this thesis, the fundamentals of thermoelectrics and the benefits of nanostructured materials are first discussed in details. Afterwards, our general method to synthesize a variety of telluride nanowires and binary heterostructures with solution phase reaction is introduced in the following chapters. To demonstrate the scalability of our solution phase synthesis, a 1 liter reactor is used to synthesize tens of grams nanowires at low temperature of 120 °C and within short time of 70 minutes. Meanwhile, we have taken advantage of the flexibility of our method and successfully synthesized different tellurides for applications at different temperature ranges, such as Bi2Te 3 and PbTe nanowires for near room temperature (300 - 500 K) and medium temperature (500 - 800 K) applications. We even synthesized binary phase nanowire heterostructures with two tellurides in a single nanowire, such as PbTe-Bi 2Te3 and Ag2Te-Bi2Te3. To investigate the applications of nanoparticle in flexible thermoelectrics, we also developed a method to synthesize extremely stable nanocrystal ink for coating on various substrates. Furthermore, in order to improve the thermoelectric properties of solution synthesized nanostructured materials and demonstrate their benefits for thermoelectric applications, we applied hot press to consolidate the solution synthesized nanowires and heterostructures into nanocomposites which possess extremely low thermal conductivity, leading to decent ZT. Especially, the binary phase nanocomposites made from heterostructures show much lower thermal conductivity than single phase bulk and even nanocomposite. To further improve the thermoelectric performance, we also applied doping to tune the carrier concentration of our materials to gain more thermoelectric performance enhancement. For example, Se was used to dope Bi2Te3 nanocomposites, which leads to 60% of power factor enhancement. In addition, nanocrystal thin films were fabricated with stable nanocrystal ink on different substrates, even flexible ones. Particularly, the effects of size and iodine doping concentration on the thermoelectric properties on the PbTe nanocrystal thin films are investigated to enhance the understanding of using nanocrystals for thermoelectrics.

Degree

Ph.D.

Advisors

Morgan, Purdue University.

Subject Area

Chemical engineering|Materials science

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