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Self-consistent Vlasov-Poisson analysis of carrier transport in vacuum-based thermionic/thermoelectronic devices Khoshaman, Amir Hossein

Abstract

Thermionic conversion involves the direct conversion of heat, including light-induced heat, from a source such as solar energy, to electricity. The progress of thermionic converters has been limited by issues such as the space charge effect and lack of materials with desirable mechanical, electrical and thermal properties. Nanotechnology could help address some of the main challenges that thermionic converters encounter. However, existing models, which were developed for macroscopic converters, are not adequate for many aspects of nanostructured devices. The work presented in this thesis primarily advances a new model to partially address this void and study emergent thermionic devices. We demonstrate a self-consistent and iterative approach to the Vlasov-Poisson system that overcomes the inherent limitations of the traditional methods. This approach serves as the foundation for more advanced and yet crucial cases of the operation of thermionic converters in the presence of back-emission, grids in the inter-electrode region and low-pressure plasmas. We develop the physics of the device in the presence of grids and demonstrate that momentum gaps could arise in the phase space of the electrons; taking into account these gaps, which had not been noticed in the past, is key to designing efficient thermionic converters and we predict improvements of 3 orders of magnitude in current density using a properly designed grid. We also develop the physics of the device in the presence of low-pressure plasmas, which are prime candidates for reducing space charge. We show that the output power density of a thermionic converter can improve by a factor of ~ 10 using a modest plasma pressure of 500 Pa. On a different front, we have also improved the traditional analytical model and developed an approach to extract the internal device parameters such as emission area and workfunction based on a limited set of experimental output characteristics. These parameters are highly dependent on the operating conditions and ex-situ measurements are not applicable. Therefore, our approach allows for a more systematic study of the device and material properties, which is key to further the development of thermionic converters, in particular based on novel materials and nanostructures.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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