Carbon nanotube growth depends on the catalytic activity of metal nanoparticles on alumina or silica supports. The control on catalytic activity is generally achieved by variations in water concentration, carbon feed, and sample placement on a few types of alumina or silica catalyst supports obtained via thin film deposition. We have recently expanded the choice of catalyst supports by engineering inactive substrates like c-cut sapphire via ion beam bombardment. The deterministic control on the structure and chemistry of catalyst supports obtained by tuning the degree of beam-induced damage have enabled better regulation of the activity of Fe catalysts only in the ion beam bombarded areas and hence enabled controllable super growth of carbon nanotubes. A wide range of surface characterization techniques were used to monitor the catalytically active surface engineered via ion beam bombardment. The proposed method offers a versatile way to control carbon nanotube growth in patterned areas and also enhances the current understanding of the growth process. With the right choice of water concentration, carbon feed and sample placement, engineered catalyst supports may extend the carbon nanotube growth yield to a level that is even higher than the ones reported here, and thus offers promising applications of carbon nanotubes in electronics, heat exchanger, and energy storage.
Thermal instability is an important concern for practical use of high-current field emitters in display, X-ray generation,
Hall thruster, and microplasma generation. Carbon nanotubes (CNTs) and their bundles have high thermal conductivity
and offers great promise in this aspect. A wide-range of experiments has recently been performed with CNT-based
emitters containing single or a bundle of nanotubes. Analysis of these experiments is executed using the classical
Fowler-Nordheim (FN) equation and the heat equation with no self-consistency. The space-charge effect – one of the
most important aspect of high-current field emission – is often ignored in these theoretical analyses. In this work, we use
a numerical framework to study thermal instability in the CNT-based emitters by solving electrostatics, space-charge
effect, quantum-mechanical tunneling (with FN equation as the limiting case), thermionic emission and heat flow in a
self-consistent manner. Simulation compares well with the experimental results and allows study of temperature rise –
the root cause of thermal instability – for the emitter in a wide range of conditions. Our analysis suggests that higher
thermal conductivity and/or electrical conductivity and their reduced temperature dependence are beneficial for the field
emitters, as these improve the thermal stability of the emitter by reducing temperature rise.
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