Field emission describes the emission of electrons into vacuum under an applied electric field. Current x-ray sources are energy intensive and cumbersome. Clare’s project focusses on using carbon nanotubes and field emission to replace and improve current x-ray technologies.
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Introduction
Field emission has long been studied for the potential in applications such as microwave amplifiers, travelling wave tubes, electron microscopy, and x-ray sources. This project focuses primarily on x-ray applications, with the intention of integrating an optimised chip design into a portable x-ray gun towards the end of the project.
Material comparison
A large meta-analysis of the materials so far used in field emission applications was made. Materials are required to show high maximum current density as well as low turn on electric field to be considered as a promising material in field emission. 1D and 2D materials were shown to have an average turn on field half the size of 3D/bulk materials. The best performing group of materials were the nanocarbons including carbon nanotubes, graphene, carbon nanowalls and carbon nanofibers.
Experiment
A number of different carbon nanotube configurations have been synthesised. The morphology of the emitter plays a large role in emission characteristics. To determine the full extent of morphology influence, field emission from a parallel plate set up is measured. A number of studies are being performed to determine the underlying physics of the emission mechanism. These include studies in pressure, optical excitation, in situ residual gas analysis and stability.
Hysteresis is commonly seen amongst carbon nanotube, and carbon based, field emitters. The aim is to understand the physics of emission and the source of hysteresis. Using data collected from the above studies, we can also examine the nature of the hysteresis and determine a theory about its cause.
Publications
R. J. Parmee, C. M. Collins, W. I. Milne, & M. T. Cole, X-ray generation using carbon nanotubes, Nano Conv., 2, 1, 2015.