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Carbon Nanotube Field Emitters
 

We are developing Carbon Nanotube (CNT) field emitters to improve their efficiency and durability. Current densities of ~1A/cm2 have been measured from these emitters. X-ray tubes, built with these CNT bases cathodes, have shown improved efficiency and robustness. Increase in electrical conservation and thermal management can be realized from CNT field emitters.

Benefit

Nanotechnology allows for the production of advanced materials that are critical for the development of many instrument components. These components, designed with advanced concepts, provide energy conservation due to those advanced materials and concepts, and will make feasible many instruments that would not otherwise have been possible. Two applications include the use of the CNT field emitter as the ionization source for a mass spectrometer, and the CNT field emitter as part of the miniature X-ray tube used in a XRD/XRF instrument. Both of these instruments are being developed for flight missions. Additionally, habitat purification can be achieved with low levels of field emission to remove the organic and biological contamination from the air.

Research Overview

At NASA Ames, we have the ability to produce and evaluate the performance of CNT field emitters. We have a patented (pending) process that is used for the deposition of the CNT catalyst, which can then be grown in either of two reactors, one thermal and the other an inductively coupled plasma reactor. We have the experience to design and tailor CNT growth to fit with the applications required.

Field emission current densityFigure 1. Field emission current density emitted from a 2mm diameter CNT cathode as it is progressively ramped to higher and higher fields.

The growth process includes the deposition of an aluminum/iron catalyst onto the substrate material. Molybdenum and silicon are the most common substrate materials used. Then the CNTs are grown in either the thermal or plasma reactor. The design of the catalyst system provides for CNTs that are well attached to the substrate and will not detach during the field emission process, a common failure point for CNT emitters that are created from pre-grown CNTs that are later fabricated into emitters.

Variation of the electric fieldFigure 2. Variation of the electric field required to extract 100mm from a 2mm diameter CNT cathode after conditioning at 500mm.

These cathodes can produce current densities as high as 1A/cm2. Figure 1 shows the field emission from a 2 mm diameter cathode as it is conditioned to higher fields. A current density of 0.3A/cm2 can be obtained with a field as low as 2V/mm as shown in Figure 2. A CNT field emitter built from this process sees no degradation of the emission current due to repeated on-off cycling. Further, lifetime tests show that these CNT field emitters will have a useable life of many years.

Finally, with the flexibility of the deposition and growth process, CNT field emitters can be fabricated with various sizes, patterns and substrates.

Background

A low-power, low-temperature, high-current electron emitter is crucial to the development of many instruments. Several technologies are available, but often fall short in the final analysis.

Thermionic emitters, while able to produce high currents, are very inefficient in their use of power. Most of the power goes to heating the filament, which in turn causes heating of the instrument that normally has to be cooled, also requiring additional power usage.

An alternate means of generating electrons is field emission, which presents many advantages over thermionic emission.

As field emitters do not require heating to generate electrons, they are more energy efficient. The cold nature of the emission prevents thermal drift of the cathode, allowing better and more stable electron focusing. Cold cathode field emitters can be rapidly switched on and off, eliminating the need for a mechanical shutter. Most attempts to use field emission in X-ray tubes have been unsuccessful because the emitters were rapidly destroyed by the arcing and cation sputtering that inevitably occurs in an X-ray tube. Carbon nanotubes have been shown to be extremely good field emitters and are among the most robust materials in terms of their mechanical, thermal and chemical properties (in non-oxidizing environments). Additionally, field emitters are less likely to produce outgassing that could potentially deteriorate the device and contaminate the system.