In order to increase the efficiency of betavoltaic devices, an architecture utilizing nanowires has been developed. In this architecture, a radioisotope is deposited between a nanowire array in order to increase the fraction of beta particles captured by the semiconductor converter and minimize the energy lost to self-shielding. Previous work has prototyped such a design; however, performance was limited to an efficiency of 0.5%. This thesis outlines the design and optimization of the nanowire-based betavoltaic generator. Both the nanowire array geometry and the nanowire p-i-n diode design are optimized for maximum radiation capture and conversion efficiency, respectively. First, a model was developed in the GEANT4 Monte Carlo toolkit in order to investigate the radiation capture of various array geometries. Radioisotope sources of elemental 3H, 63Ni, and 147Pm, as well as compounds of each were examined with gallium phosphide nanowires. Overall, it was found that nanowires should be grown as long as possible to accommodate the most source material while the ratio of the diameter to array pitch can be optimized for maximum power capture. Optimized arrays presented an improvement in energy capture of approximately 6 and 15 times for 63Ni and 3H devices, respectively, while 147Pm devices indicated no improvement. Optimized array geometry was extended to both silicon and gallium arsenide and the radiation capture simulations were coupled to drift-diffusion calculations in COMSOL Multiphysics for axial junction nanowires. Following the junction optimization, devices were predicted to be between 4 and 10% efficient with power outputs ranging from 2 to 6 μW cm^-2. Despite the large improvement compared to experimental results, surface recombination was found to limit the performance of long gallium phosphide nanowires. Therefore, core-shell junctions were then investigated and found to improve upon all axial designs. Overall, it has been determined that the nanowire device design is advantageous over planar betavoltaics due to the mitigation of self-shielding effects. Devices utilizing 10 μm long gallium phosphide core-shell nanowires with a 3H source are predicted to achieve the top performance of 12% effciency and a power density of 7 μW cm^-2. In addition, gallium phosphide and gallium arsenide devices with 63Ni are able to achieve an energy density in excess of 1 Wh cm^-2 due to the long half-life. / Thesis / Master of Applied Science (MASc) / Widely used batteries, such as lithium-polymer cells, are bulky and suffer from short discharge times or temperature sensitivity. Betavoltaics - also known as "nuclear batteries" - offer an opportunity to surpass these issues. Beta particles, or energetic electrons, are the result of certain nuclear decay reactions. Betavoltaic batteries create electricity from these particles, can remain active for hundreds of years, and are insensitive to environmental conditions. In addition, these particles are easy to shield, rendering them safe for users. This work focuses on a new type of betavoltaic which uses nanowires to capture more beta particles and ultimately improve performance. These devices have been designed through a simulation-based approach that has maximized the total power output as well as effciency by fine-tuning different parameters. The designs described in this work exhibit huge improvements over conventional devices and will allow nanowire-based betavoltaics to compete with the top performing devices developed to date.

There is an intense interest in the availability of radioactive isotopes that could be developed into nuclear batteries. Promethium-147 is one of the isotopes of interest for use in nuclear batteries as well as in other compact low power applications. Pm-147 is a pure beta (β-) emitter with a half-life of 2.62 years. For this research, Pm-147 was produced from enriched Nd-146 via the neutron capture method in the Hydraulic Tube facility of HFIR at the Oak Ridge National Laboratory. Radioisotopic impurities produced via the neutron capture method have significant effects on its potential final use for nuclear battery applications. This research provides information on the co-production levels of the radioisotopic impurities in the samples containing Pm-147 and their effects on the required shielding. Gamma spectroscopy analysis served as the primary method in the evaluation of the impurities. Previous research had identified the presence of these impurities but it had not studied them in detail.

There is an intense interest in the availability of radioactive isotopes that could be developed into nuclear batteries. Promethium-147 is one of the isotopes of interest for use in nuclear batteries as well as in other compact low power applications. Pm-147 is a pure beta (β-) emitter with a half-life of 2.62 years. For this research, Pm-147 was produced from enriched Nd-146 via the neutron capture method in the Hydraulic Tube facility of HFIR at the Oak Ridge National Laboratory. Radioisotopic impurities produced via the neutron capture method have significant effects on its potential final use for nuclear battery applications. This research provides information on the co-production levels of the radioisotopic impurities in the samples containing Pm-147 and their effects on the required shielding. Gamma spectroscopy analysis served as the primary method in the evaluation of the impurities. Previous research had identified the presence of these impurities but it had not studied them in detail.

<p>  </p> <p>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.</p> <p>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. </p> <p>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.</p> <p>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.</p> <p>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.</p>

The betavoltaic battery is a type of micro nuclear battery that harvests beta emitting radioactive decay energy using semiconductors. The literature results suggest that a better model is needed to design a betavoltaic battery. This dissertation creates a comprehensive model that includes all of the important factors that impact betavoltaic battery output and efficiency. Recent advancements in micro electro mechanical systems (MEMS) necessitate an onboard miniaturized power source. As these devices are highly functional, longevity of the power source is also preferred. Betavoltaic batteries are a very promising power source that can fulfill these requirements. They can be miniaturized to the size of a human hair. On the other hand, miniaturization of chemical batteries is restricted by low energy density. That is why betavoltaics are a viable option as a power source for sophisticated MEMS devices. They can also be used for implantable medical devices such as pacemakers; for remote applications such as spacecraft, undersea exploration, polar regions, mountains; military equipment; for sensor networks for environmental monitoring; and for sensors embedded in bridges due to their high energy density and long lifetime (up to 100 years). A betavoltaic battery simulation model was developed using Monte Carlo particle transport codes such as MCNP and PENELOPE whereas many researchers used simple empirical equations. These particle transport codes consider the comprehensive physics theory for electron transport in materials. They are used to estimate the energy deposition and the penetration depth of beta particles in the semiconductors. A full energy spectrum was used in the model to take into account the actual radioactive decay energy of the beta particles. These results were compared to the traditional betavoltaic battery design method of estimating energy deposition and penetration depth using monoenergetic beta average energy. Significant differences in results were observed that have a major impact on betavoltaic battery design. Furthermore, the angular distribution of the beta particles was incorporated in the model in order to take into account the effect of isotropic emission of beta decay. The backscattering of beta particles and loss of energy with angular dependence were analyzed. Then, the drift-diffusion semiconductor model was applied in order to estimate the power outputs for the battery, whereas many researchers used the simple collection probability model neglecting many design parameters. The results showed that an optimum junction depth can maximize the power output. The short circuit current and open circuit voltage of the battery varied with the semiconductor junction depth, angular distribution, and different activities. However, the analysis showed that the analytical results overpredicted the experimental results when self-absorption was not considered. Therefore, the percentage of self-absorption and the source thickness were estimated using a radioisotope source model. It was then validated with the thickness calculated from the specific activity of the radioisotope. As a result, the battery model was improved significantly. Furthermore, different tritiated metal sources were analyzed and the beta fluxes were compared. The optimum source thicknesses were designed to increase the source efficiencies. Both narrow and wide band gap semiconductors for beryllium tritide were analyzed. / PHD