Processing and Properties of Si-Based Bulk Nanocomposites and Hybrid Devices for Thermoelectric Applications

Thermoelectric (TE) materials can directly convert waste heat into electrical energy. The conversion efficiency is an increasing function of a dimensionless figure of merit, ZT=S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, S2σ is the power factor, κ is the thermal conductivity, and T is the temperature. One of the main challenges in the design of thermoelectric materials with better performance is to minimize the thermal conductivity while preserving the electrical conductivity. This requires mechanisms that scatter heat-carrying phonons without perturbing the electrons. Nanostructuring, combined with electronic structure optimization, can be an effective approach to enhance thermoelectric performance. This strategy can potentially decouple the thermal and electrical properties, as electrons and phonons possess different characteristic lengthscales of wavelength and mean free path.
Both Si-based bulk nanocomposite (work 1) and nano-meshed thin-film (work 2) materials are studied in this dissertation. Both works aim to understand the coupling between processing/fabrication, the resultant nanostructuring, and thermal/electronic transport. In work 1, synthesis and transport properties of the bulk Fe-Si-Ge system is studied. Synergistic approaches including hierarchical structuring, phase percolation, and selective doping are implemented using a novel powder process scheme, successfully improving the thermoelectric performance of eco-friendly β-FeSi2 – SiGe nanocomposites, which are promising for industrial-scale waste heat recovery applications. In work 2, a holey Si thin-film device is fabricated and the in-plane electrical/thermal transport properties are studied. Subsequently, a hybrid F¬4TCNQ – Si device utilizing organic – inorganic charge transfer as doping mechanism is investigated. We anticipate that this study will open a door for applying the concept of hybrid holey Si/organics towards efficient thermoelectric materials in device applications.