Manufacturing strategy for high current cold field emission cathodes: Floating catalyst chemical vapour deposition grown carbon nanotube fibres and films enhanced by laser patterning and laser purification process
The aim of this work is to produce a manufacturing strategy for high current (>10 mA) field emission (FE) devices for military (microwave generation) and civilian (particle accelerator electron beam) applications using carbon nanotubes (CNT) as base material. With a particular focus on the relationship of the laser time pulse duration used for cutting CNTs and how this affects the field emission performance. Material selection for this work was narrowed to CNT as they possess unique characteristics such as: high aspect ratio; high thermal conductivity; high chemical stability and high current carrying capacities up to a theoretical limit of 1,200 MA∙cm-1 making them an ideal material for FE. The CNT material studied in this work is produced in two distinct forms, fibres (∅~7-10 µm) and films (h~30 µm), using a floating catalyst chemical vapour deposition process which produces high quantities of CNT material with mixed mechanical and electrical properties. The material is difficult to handle because of its dimensions and is susceptible to environmental changes i.e. electrostatic forces. In order to reduce the variability in electrical properties, a laser purification process was developed. The process consists of locally irradiating an infra-red (IR) laser several microseconds directly at the material. A percentage is vaporised (mainly non-conductive or defective material) and the remaining CNT material shows very high crystallinity with an increase of up to ten times (G/D ratio > 100) compared to the original material and electron mean free path is increased by an order of magnitude. The production strategy is based on directly coating the CNT material with copper using an electroplating process. This allowed for CNT fibre and film to be easily handled and improved the overall electrical contact. Emitter geometry was customised by a laser cutting process to achieve increased enhancement factor geometries, in this case, triangles with 29 tips whilst reducing FE variability. FE performance was quantified by testing the devices in a continuous DC mode with a sweep up to 1,000 V until the material suffered catastrophic failure. The gap distance between the tip of the triangles and the anode was varied to increase the electric field until failure. FE results using the production strategy improved more than 400% compared to untreated material. Applications for these devices are intended to be in the creation of high energy electron beam lines and generation of high powered directed microwaves.