UBC Theses and Dissertations

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UBC Theses and Dissertations

Micro-electro-discharge machining of carbon nanotube forests for thermionic emission and MEMS applications Hassan, Mohab Osama


Carbon nanotube forests are attracting significant attention due to their unique mechanical and electrical properties. There are research endeavours that explored these properties in which there was a need for patterning the material using conventional microfabrication techniques. These techniques, however, imposed some limitations on the patterning structure, and in turn, on the properties that can be investigated. This thesis work utilizes microplasma-based micro-electro discharge machining in novel patterning processes that enable exploring unique properties through MEMS and vacuum electronic devices. The first process investigates flat and rounded tip cylindrical electrodes in micropatterning of carbon nanotube forests. Both electrodes are used to pattern rectangular slots at different depths from which the discharge gap is measured. The rounded tip electrode shows a smaller discharge gap; however, the variation produced is larger, which affects the structural uniformity of the patterns. Consequently, the flat tip electrode is chosen in the fabrication of the first device in this work, which is a laterally suspended microcantilever made entirely of a carbon nanotube forest, to study the mechanical properties of the material. The microcantilever is electrostatically actuated to characterize its resonance. The measurement result fitted to a free vibrating microcantilever model is used to calculate the in-plane Young’s modulus of the material. The second process is a planarization process of carbon nanotube forests to produce macroscopically flat top surfaces using planar electrodes. This process allows for placing a carbon nanotube forest (emitter) electrode at close proximity down to a few tens of micrometers from a collector electrode in the second device presented in this work, which is a thermionic emission device. This device is used in an interelectrode-gap-variation platform to systematically study the mitigation of the space charge effect in-situ, which impedes the emitted electrons from reaching the collector. A conventional emitter, yttria, is also tested. The yttria emitter is found to follow the expected trend of increasing the output current in the space charge regime with decreasing the interelectrode gap. However, the carbon nanotube forest emitter exhibits an opposite behaviour in which the current decreases instead of increasing. Possible reasons are discussed to explain this unexpected behaviour.

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