Optimization and Evaluation of Tritium Storage Mediums for Betavoltaic Devices
Betavoltaics are self-contained radioisotope power sources where radioisotopes irradiate a semiconductor and generate electricity similar to a photovoltaic cell. Betavoltaics differ from other power sources as it is ideal for long-lasting (>20 years), low, continuous power applications where battery replacement is not feasible. Ideal functions for betavoltaics include sensors in hard to reach places such as underwater and deep space applications, as well as cardiac pacemakers where power source replacement is undesirable or impossible. However, betavoltaics are limited in application by its power output since it only produces power in the nanowatt range. Betavoltaic performance can be improved by two methods: Increasing the amount of activity of the radioisotope or increasing the performance of the semiconductor. Currently, commercial betavoltaics utilize a titanium tritide film to irradiate a gallium arsenide semiconductor. The objective of this dissertation is to identify a tritium storage medium that can produce more power in the betavoltaic than the currently used titanium tritide. This was done in three steps: First, metal film options were simulated in MCNP to evaluate tritium substrate self-shielding, semiconductor beta irradiation and determine ideal thicknesses. Second, metal film options at ideal thicknesses were manufactured and evaluated during the hydrogen loading process to determine the viability of materials fully absorbing hydrogen. Lastly, the loading kinetics would be evaluated to further investigate hydride/tritide formation in the storage medium if full loading is not realized to determine the ideal thickness required, or if other factors during the loading process need to be considered.
Metallic films were evaluated to maximize tritium packing and optimized for minimizing self-shielding to improve performance for betavoltaic cells beyond the titanium tritide films currently used. Ideal, fully loaded tritium metallic films, such as lithium, aluminum, titanium, magnesium and palladium tritides, were simulated in MCNP6 (Monte Carlo N-Particle 6) to evaluate power deposition into a gallium arsenide semiconductor by varying the thickness of the films. Lithium was identified as the best storage option with an optimal thickness of 4 μm and a theoretical betavoltaic current output of 644 nA for a gallium arsenide semiconductor, tripling the current output emitted by an ideal titanium-loaded film.
The viability of lithium and aluminum film loading were evaluated in the hydrogen loading system while comparing to titanium as a benchmark. Unlike titanium and aluminum films where films were in a solid state through the loading process, lithium has to be melted into a liquid state to be loaded. The uptake of hydrogen by the films was determined by Sievert's method, where the pressure drop recorded by the Hydrogen Loading System was the measured pressure of hydrogen absorbed by the film. All film loadings showed a pressure drop that corresponded to the expected pressure drop from loading. The films were characterized after loading to confirm hydrogen absorption and formation of hydride. Both lithium and titanium demonstrated hydride formation while the aluminum did not.
The pressure drops during loading were compared to the Mintz-Bloch model. For some loadings in all materials, there was good correlation between experimental loadings and Mintz-Bloch models, primarily due to the hydride formation happening quickly. Differences can be explained from the speed of the hydride reaction and thermal decomposition of the hydride during loading. The Mintz-Bloch model further confirmed that the aluminum did not form a hydride during loading.
Lithium was demonstrated to be a viable hydrogen loading substrate. The film was characterized to be lithium hydride after hydrogen loading and its loading kinetics matched very well with the Mintz-Bloch model. Aluminum was demonstrated to not be viable as a hydrogen loading substrate as it requires significantly higher pressures, beyond the allowed limits for tritium handling, to form a hydride and permanently hold when exposed to atmosphere.
- Doctor of Philosophy
- Nuclear Engineering
- West Lafayette