UC Berkeley

The advancement of energy and sensing technologies hinges on the development of materials with novel physical and chemical properties and the rational design of new configurations that leverage synergistic interaction between multiple materials. In the context of devices that generate or utilize energy or sense the environment, new materials or material configurations can offer unique or improved properties that can help resolve technological barriers introduced by stringent design parameters, necessary performance requirements, or harsh and extreme operating environments. This work discusses the existing material limitations of two separate technologies, thermionic energy converters and chemiresistive gas sensors, and details the investigation of new materials or material configurations aimed toward addressing device-related issues.

The first part of this work centers on microfabricated thermionic energy converters (TECs). TECs leverage the thermionic emission of electrons from materials at high temperature to directly convert thermal energy to electrical energy. For optimal efficiency and reliable performance, the emitter electrode, an essential component of the TEC, must demonstrate good mechanical, chemical, and surface stability during high-temperature operation. This work first demonstrates the protection of a tungsten (W) emitter from oxidation using a silicon carbide (SiC) passivation layer. The addition of functional layers such as titanium nitride (TiN) and tantalum carbide (TaC) diffusion barriers is investigated. A W/TiN/SiC multilayered emitter that exhibits stability at 1100 °C for up to 24 h under representative operating conditions is realized. The effects of TiN thickness and post-deposition annealing are examined, and a relationship between thermal stress and observed cracking patterns in the TiN is established. An alternative configuration involving W/TaC/SiC is also investigated, showing enhanced stability.

The second part of this work focuses on chemiresistive gas sensors. These devices detect gases by monitoring changes in the resistance of a material as a result of gas adsorption and desorption and are key components in health, environmental, and industrial process monitoring. While several classes of materials have been investigated for chemiresistive gas sensing, the transition metal diborides have been largely unexplored. The fabrication of sensors made from hafnium diboride (HfB2) and zirconium diboride (ZrB2) aerogels is presented and their sensing behavior probed. Of particular note is the sensitivity and selectivity of HfB2 toward NO2 and the relative insensitivity of ZrB2 to NO2. The disparity in their gas sensing behavior is discussed. Computations based on density functional theory indicate charge transfer between HfB2 and NO2. The requirements and advantages of printable gas sensors are detailed, and the printability of HfB2 is examined.

The studies presented in this dissertation illustrate the impact novel materials and material configurations can have in improving and expanding the capabilities of energy and sensing technologies.