Date of Award

5-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Physics

Advisor

Tritt, Terry M

Committee Member

He , Jian

Committee Member

Marinescu , Catalina

Committee Member

Kang , Hye-Jung

Abstract

In the past several decades there has been an intensive study in the field of thermoelectric study that is basically materials driven. As the simplest technology applicable in direct heat-electricity energy conversion, thermoelectricity utilizes the Seebeck effect to generate electricity from heat or conversely achieve the solid-state cooling via the Peltier effect. With many technical merits, thermoelectric devices can be used as spot-size generators or distributed refrigerators, however, their applications are restricted by the energy conversion efficiency, which is mainly determined by the figure of merit ZT of the thermoelectric materials that these devices are made of.
A higher ZT (ZT=α2*σ/κ) entails a larger Seebeck coefficient (α), a higher electrical conductivity (σ) and a lower thermal conductivity (κ). However, it is challenging to simultaneously optimize these three material parameters because they are adversely correlated. To this end, a promising approach to answer this challenge is nano-compositing or microstructuring at multiple length scales. The numerous grain boundaries in nano-composite allow for significant reduction of lattice thermal conductivity via strong phonon scattering and as well an enhanced Seebeck coefficient via, carrier energy filtering effect.
As the same grain boundaries also scatter carriers, a coherent interface between grains is needed to minimize the degradation of carrier mobilities. To this end, in-situ, instead of ex-situ, formation of nano-composite is preferred. It is noteworthy that electrical conductivity can be further enhanced by the injection of high-mobility carriers introduced by the secondary nano-phase.
In view of the prevalent use of Antimony (Sb) in thermoelectric materials, Indium Antimonide (InSb) naturally becomes one of the most promising nano-inclusions since it possesses one of the largest carrier mobilities (~7.8 m2/V-s) in any semiconductors, while at the same time possesses a reasonably narrow band gap (~0.17 eV at 300 K).
In this dissertation, I experimentally investigate whether InSb could be a 'good' nano-secondary phase in two thermoelectric bulk matrix materials, FeSb2 and half-Heusler compounds. In these in situ formed nano-composites, three mechanisms are utilized to decouple the otherwise adversely correlated Seebeck coefficient (α), electrical conductivity (σ), and thermal conductivity (κ). First, low energy carriers will be filtered out via the carrier energy filtering effect, enhancing the Seebeck coefficient without degrading the power factor (PF= α2σ). Second, high mobility carriers from the InSb nano-inclusions will be injected to the system to increase the electrical conductivity. Last, the numerous grain boundaries present in nano-composites allow for strong phonon scattering so as to reduce the thermal conductivity.
After the initial in situ synthesis of nano-composites with the optimized composition, further nano-structuring processes are applied in the samples of FeSb2 with 0.5% atomic ratio of InSb. The results indicate that not all nano-structures are thermoelectrically favorable, multi-scale microstructures with the length scale comparable with the phonon mean free path are needed to effectively scatter phonons over a wide range of wavelength.
In summary, the successful combination of the carrier energy filtering effect, high mobility carrier injection effect, and strong phonon scattering effect in the in situ synthesized FeSb2-InSb and half-Heusler-InSb nano-composites leads to a significantly enhanced ZT. This approach of in situ formation of nano-composites based on InSb secondary nano-phase may also be applied to other thermoelectric materials.

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