Development of Thermoelectric and Permanent Magnet Nanoparticles for Clean Energy Applications
- Author(s): Nguyen, Phi-Khanh
- et al.
The global trend towards energy efficiency and environmental sustainability has generated a strong demand for clean energy technologies. Among the many energy solutions, the work in this dissertation contributes to two strategic goals: the reduction of fuel consumption in the transportation sector, and the increase of domestic wind power capacity. The key barriers to achieving these goals are materials challenges. Automobiles can be made more efficient by thermoelectric conversion of waste heat from the engine into electricity that can be used to power electrical components in the vehicle. Vehicles can forego petroleum fuel altogether by using electric or hybrid motors. Unfortunately, the conversion efficiency of current thermoelectric technology is too low to be considered economically feasible, and the permanent magnets used in electric vehicle motors and wind turbine generators require critical rare-earth elements that are economically unstable (often referred to as the "rare- earth crisis"). In order to combat these challenges, a "spark erosion" technique was utilized for producing nanoparticles that improve thermoelectric efficiency and contribute to the development of electromotors that do not require rare-earths. In Chapter 2 of this dissertation, I describe the utilization of spark erosion for producing high-quality thermoelectric nanoparticles at a remarkably high rate and with enhanced thermoelectric properties. The technique was employed to synthesize p-type bismuth- antimony telluride (BST) and n-type skutterudite nanoparticles, using a relatively small laboratory apparatus, with low energy consumption. The compacted BST nanocomposite samples made from these nanoparticles exhibit a well-defined, 20 - 50 nm size nanograin microstructure, and show an enhanced Figure of merit, ZT, of 1.36 at 360 K due to a reduction in lattice thermal conductivity. The skutterudite nanocomposites also show reduced thermal conductivity but still require enhancement in the thermoelectric power factor. Such a technique is essential for providing inexpensive, oxidation-free nanoparticles required for fabricating high performance thermoelectric devices for power generation from waste heat, and for refrigeration. We have investigated the spark erosion of MnBi, a promising rare-earth-free permanent magnet, and have determined that spark erosion provides the best approach for producing MnBi particles. The low-temperature phase of MnBi (LTP-MnBi) is an attractive rare-earth free permanent magnet material due it its high uniaxial magnetocrystalline anisotropy, which produces an unusually high coercivity at the elevated temperatures required for motor and generators. However, due to the peritectic Mn-Bi phase diagram and the slow interdiffusion of Mn and Bi below the 350°C phase change temperature, bulk samples of LTP-MnBi with high saturation magnetization (MS) have been difficult to achieve. In Chapter 3, we describe the successful formation of high- purity bulk LTP-MnBi ingots and spark erosion of this material to produce single-domain particles of MnBi at an unprecedented rate. The bulk ingots have MS > 90 wt % of LTP-MnBi, and are formed by chill-casting and by vacuum- annealing of arc-melted ingots. The as-prepared powder then consists of amorphous, crystalline, and superparamagnetic particles, mostly as porous aggregates. The major fraction of the powder consists of 20-30 nm particles. A short anneal crystallizes the amorphous particles producing a high moment, albeit with HC of only a few kOe. If lightly milled, the agglomerates are broken up and yield an HC of 1 T and a maximum energy product of 3.0 MGOe. The particles can be further engineered through milling, annealing, and/or solution processing in order to produce unique properties that hold promise to achieving the first bulk permanent magnet that utilizes the exc