ABSTRACTSILICON-BASED SEMIMETALS AND SEMICONDUCTORS FOR THERMOELECTRIC APPLICATIONSByHui SunThe direct conversion between heat and electricity can be achieved by thermoelectric devices. Thus, thermoelectricity is considered as not only an environmentally friendly substitute for compressor-based refrigerators but also a promising energy solution to harvest waste heat. State-of-the-art thermoelectric materials are often comprised of expensive tellurium or germanium elements and hence are hardly suitable for mass production. The silicon-based thermoelectrics, e.g. semimetallic CoSi and semiconducting β-FeSi2 materials we study here, are composed of abundant elements in nature. They are also chemically stable, non-toxic, and mechanically robust. Despite the above benefits, they exhibit relatively lower efficiencies compared to state-of-the-art materials. In this dissertation, we have intended to understand the thermal and electrical transport in these materials and enhance their thermoelectric performance.CoSi possesses one of the highest power factors among thermoelectrics due to the sharp features around the Fermi level in its electronic density of states. In order to improve the performance, the effects of p-type dopants, isoelectronic substitutions, n-type dopants, and double doping were systematically studied for arc-melted CoSi samples. The results show that p-type dopants like iron and gallium and n-type dopants like nickel and palladium deteriorate the electrical properties due to the introduction of excess holes and electrons, respectively. Boron and platinum have very limited solubility in CoSi and the segregated impurity phases at grain boundaries are helpful to improve the electrical properties. The isoelectronic substitutions influence the power factor slightly; however, they result in a drastic decrease in the lattice thermal conductivity and hence an enhancement in the figure of merit. In addition, CoSi samples prepared by powder processing were investigated to further reduce the lattice thermal conductivity. Unfortunately, all the consolidated samples show worse performance than the arc-melted CoSi.β-FeSi2 is one of the most cost-efficient thermoelectric materials. Its thermoelectric performance can be tuned by cobalt doping and the highest figure of merit is close to 0.4 at 1000 K. Grain size reduction was also used to reduce the lattice thermal conductivity. However, no improvement has been obtained yet. SiC nanoparticles were dispersed into β-FeSi2 matrix to form a composite structure. With addition of the nanoparticles, the thermal conductivity is slightly decreased associated with a decrease in the power factor. Although the figure of merit is less than that of some Te-based materials, β-FeSi2-based thermoelectrics may be suitable in large-scale applications where material abundance and cost are concerns.
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