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.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/25582 |
| Date | January 2020 |
| Creators | Wagner, Devan |
| Contributors | LaPierre, Ray, Novog, Dave, Engineering Physics |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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