Carbon nanotube field emitter array and its application to novel X-ray systems
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With the emergence of ever more demanding X-ray imaging applications, including radiotherapy, radiography, computed tomography and tomosynthesis, there is a tendency to use field emission (FE) to take the place of thermionic emission in X-ray generation. Carbon nanotube (CNT) field emitter array (FEA) is able to provide enhanced current and low energy dispersion for X-ray generation. The low operating temperature, instantaneous response, and small size of CNT FEA enable efficient and controllable FE performance for high spatial and temporal resolution X-ray imaging and instantaneous switching among multiple X-ray sources. Challenges are still remaining for CNT FEA study and development. The first is the insufficient understanding of FE uniformity of CNT FEA and CNT emitter failure mechanism, which limits the development of stable and reliable CNT FEA. The second is the lack of applications of CNT FEA for the emergence of novel X-ray imaging applications that require dose adjustments, fluence field modulation, and shape variation. In this thesis, recent advances in CNT FE and field emitters are comprehensively reviewed. Issues related to the principles, characteristics, and applications of CNT based FE are discussed. The research starts with a fundamental study of understanding the FE uniformity and emitter failure mechanism of CNT FEA by designing and demonstrating a simple and reliable field emission microscopy method using PMMA (Poly (methyl methacrylate)) thin film with microscopic camera. This novel approach is able to resolve the challenges of observing FE uniformity of CNT FEA and CNT field emitter failure behaviour. The unique phenomenon of light emission and Coulomb explosion of CNT field emitter failure induced by FE is represented and systematically characterized, which is a combined effect of Joule heating and excessive charging. Then, individual ballast of vertically aligned CNT FEA using silicon current limiters is designed and fabricated, which exhibits improved reliability and stability. The CNT FEA is applied to a low-cost self-contained and self-focusing compact X-ray source integrated with resonant transformer. The X-ray source can be powered by low voltage to generate pulsed X-ray and achieve sub-millimetre resolution. Using finite element analysis and Monte Carlo simulation, CNT FEA based two dimensional multi-pixel X-ray source is designed and experimentally verified, which is feasible for X-ray fluence field modulation in next generation X-ray imaging systems.
Cite this version of the work
Yunhan Li (2018). Carbon nanotube field emitter array and its application to novel X-ray systems. UWSpace. http://hdl.handle.net/10012/13408