Carbon nanotube pillar arrays (CPAs) for cold field emission applications were grown directly on polished 70∕30at.% NiCr alloy surfaces patterned by photolithography. A carbon nanotube (CNT) pillar is a localized, vertically aligned, and well-ordered group of multiwalled CNTs resulting from van der Waals forces within high-density CNT growth. The edge effect, in which the applied electric field is enhanced along the edge of each pillar, is primarily responsible for the excellent emission properties of CPAs. We achieved efficient emission with turn-on fields as low as 0.9V∕μm and stable current densities as high as 10mA∕cm2 at an applied macroscopic field of 5.7V∕μm. We investigated the effects of pillar aspect ratio, density, and spacing on CPA field emission and quantified the edge effect with respect to pillar aspect ratio through modeling. We also investigated the field emission stability and found substantial improvement with CPAs compared to continuous and patterned CNT films.
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.
This paper reports on the use of high-power traveling wave tubes (TWTs) as a source of microwave energy for materials processing applications. Recent work by Oak Ridge National Laboratories and Microwave Laboratories personnel has demonstrated the usefulness of sweeping the microwave processing frequency over substantial (>20%) bandwidths in order to achieve uniformity of heating over volumes unattainable using conventional microwave sources ∼ e.g., magnetrons. Properly constructed high-power TWTs are a logical choice of microwave source in such systems. After briefly reviewing the basic operating principles of the TWT, the required characteristics of a TWT for materials processing applications and how those requirements affect the TWT's design are discussed. Comments on the present product lines and areas of development for all of the major TWT manufacturers are also presented. Finally, the issue of the ultimate potential cost of TWTs designed for microwave processing applications is addressed.
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