No abstract

Low current x-ray tubes operating at 25-40 kV have been developed using monolithic carbon nanotube ͑CNT͒ cold cathodes as electron sources. The authors have tested CNT cathodes from various sources. They were systematically evaluated and conditioned in a vacuum chamber and then went through high temperature baking and high voltage processing of standard tube production processes. Acceptance criteria were developed for each step in order to ensure that the final tube will meet the performance requirement of a commercial product. The tubes were subsequently operated continuously for an extended amount of time for life and reliability measurements. It was found that it is possible to use individually selected and preconditioned CNT cathodes in a commercial x-ray tube product. However, to find wide application and, particularly, to compete with existing hot filament thermionic cathodes, CNT cathodes need dramatic improvement in reproducibility and robustness. In addition, an empirical mathematical model for monolithic CNT cathodes has been developed for simulating the electron optics required in x-ray tubes. The model led to a successful design of a magnetically focused x-ray tube with a spot size of about 80 m.

The addition of a small amount of oxygen to a hot filament assisted chemical vapor deposition reactor allows diamond to be deposited at significantly lower filament and substrate temperatures. Scanning electron microscopy and Raman spectroscopy are used to compare films grown with and without oxygen addition as a function of substrate temperature at high and low filament temperatures. Oxygen addition is found to favor growth of high quality diamond at low substrate temperatures (<600 °C). The amount of nondiamond carbon is reduced and the clarity and smoothness of facets improves dramatically under these conditions. Equilibrium calculations and residual gas analysis indicate there is H2O in the gas above the substrate during these depositions. The correlation between the dramatic reduction in the nondiamond carbon content of the films and the increased H2O levels near the substrate at low temperatures leads to the conclusion that H2O plays an important role in facilitating deposition at lower temperatures. Potential roles for H2O include terminating carbon dangling bonds by dissociative adsorption and enhancing selective etching of nondiamond carbon by O2. The gas and surface chemistry of diamond deposition at low temperatures with oxygen addition is also discussed.

Addition of a small amount of oxygen to the CH4 and H2 feed gas permits hot filament assisted chemical vapor deposition (HFCVD) of diamond at significantly lower filament and substrate temperatures. The former can be reduced to as low as 1400 °C and the latter to 450 °C. The amount of oxygen required is much lower than what has been used in most studies of the oxygen effect. For each CH4%, there is a narrow window in the O/C ratio, where diamond can be deposited at low temperature. This window shifts to higher O/C ratios as the CH4% increases and expands with increases in filament temperature. The effect of changing substrate and filament temperatures on growth rate and film quality are often not consistent with previous experiences with HFCVD of diamond. Increasing the filament temperature does not always improve the growth rate and film quality, and the non-diamond carbon content in the film is dramatically reduced at lower substrate temperatures. Optimum conditions were found that gave reasonable growth rates (∼0.5 μm/h) with high film quality at filament temperatures below 1750 °C and substrate temperatures below 600 °C. With these reductions in operating temperatures, power consumption can be significantly reduced and the filament lifetime extended indefinitely.

Tests were performed on FEPET, Inc. carbon thin film cathodes to characterize lifetime, resistance to environmental conditions, and performance stability. The cathodes were found to exhibit half lifetimes on the order of 7000 h in sealed glass envelopes. Operation in oxygen and water were found to degrade the cathode performance severely. Xenon had no effect on the cathode lifetime or stability, demonstrating its resistance to ion sputtering. During characterization in vacuum systems, initial turn on was quick and stable. Carbon thin films are suitable cathode materials for display, microelectronic, and ion propulsion uses.