UC San Diego

Climate change is mainly caused by carbon dioxide emission, at least 90% of which results from the fossil fuel consumption to meet the world's growing energy need. Hence, development and deployment of renewable energy is of paramount importance and becoming increasingly urgent for human society. Recently, the utilization of thermal energy conversion technologies is gradually prevailing for clean energy production due to their unique functions, such as dispatchability. In this thesis, advanced nanostructured materials for applications in two thermal energy conversion technologies, thermoelectrics and concentrating solar power, are discussed. The approaches to improve materials' functionalities pertaining to these two technologies are proposed and experimentally demonstrated. Semiconductor nanowires have been attractive as efficient thermoelectric materials, since small diameter of nanowires induces phonon-boundary scattering to reduce thermal conductivity. Also low-dimensional quantum confinement effect has been predicted to drastically increase the power factor. In Chapter 2 of the thesis, thermoelectric power factor of intrinsic Ge-Si core-shell nanowires is investigated. The nanowires consist of intrinsic Ge core and Si shell, however, free holes are accumulated inside the Ge core due to the Fermi level pinning effect at the epitaxial core-shell interface.. This unique doping mechanism avoids the introduction of ionized dopants while obtaining a sufficiently high electrical conductivity. As a result, it provides an opportunity to enhance the carrier mobility with reduced ionized impurity scattering. The carrier concentration in core/shell nanowires has also been readily controlled by a gate voltage in a field effect transistor configuration. The field effect modulation allows one to conveniently probe the thermoelectric properties within a wide range of carrier concentrations without the use of chemical doping. The Seebeck coefficient of nanowires follows the behavior of bulk Ge as the Ge core diameter reduces down to 11nm. Based on these results, the power factor is strongly related to carrier mobility, which is enhanced in the Ge- Si nanowires compared to the bulk Ge. In Chapter 3 and 4, solar absorbing materials for Concentrating Solar Power (CSP) are discussed. CSP is becoming a significant renewable energy technology around the world, and yet the cost reduction is still needed in order to compete with traditional power plants. As higher operating temperature leads to higher energy conversion efficiency and cost reduction, the development of high temperature CSP system has been actively pursued. With regards to CSP receivers, light absorbing coatings and spectrally selective coatings (SSCs) of solar receivers are a critical component enabling high-temperature and high-efficiency operation of CSP systems. In Chapter 3, a multi-scaled approach is employed to efficiently improve the light absorption. To demonstrate the feasibility of the fractal nanostructures, semiconductor powders with particle sizes ranging from ̃10 nm to ̃10 um are fabricated using a spark erosion process, which is a scalable power production method. Optical measurement results on the multi-scaled structure show high solar absorptivity (̃90-95%) and <30% infrared emissivity near the peak of 500°C black body radiation. In Chapter 4, black oxide cobalt oxide nanoparticles are synthesized via hydrothermal process and employed as a light absorbing material for the high-temperature CSP receivers. The black oxide nanopowders are embedded in SiO₂ dielectric matrix. The surface texturing is created by utilizing sacrificial polymer stamps and beads. All of the coating samples are fabricated using a scalable spray coating method. The thermal absorption efficiency of the cobalt oxide layers is ̃ 88.2% and ̃85.4% for coatings with and without the textured surface respectively. Furthermore, the performance of coating shows no degradation after a long-term(1000-hour) aging test at 750°C in air. These results point out that the cobalt oxide light absorbing coatings with surface texturing via a scalable process can be readily applicable for future high temperature CSP systems