Buckytube Cold Field Emitter Array Cathode Experiments
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B. H. FISHBINEt a, C. J. MIGLIONICO a, K. E. HACKETTa, K. J. HENDRICKS a, X. K. WANG b R. P. H. CHANG b j. D. SHOVLIN c AND M. E. KORDESCH c aPhillips Laboratory, Kirtland AFB NM 87117 bNorthwestern University, Evanston IL 60208 cOhio University, Athens OH 45701
ABSTRACT We discuss experiments and theory directed towards the development of a buckytube cold field emitter array electron cathode. INTRODUCTION Our interest is to generate intense electron beams for high-power microwave devices using cold field electron emitters. Cold field emission provides electrons without generating plasma. Thus diode gap closure problems typically encountered in long-pulse (> 1 kts) applications are avoided. There are also commercial applications for cold field emitters, particularly in flat screen displays. The potential of carbon nanotubes for cold field emission is due to several exceptional measured or predicted properties: 1) Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have shown that nanotubes may have high aspect ratios (length/radius), permitting high field enhancement which reduces the anode-cathode potential difference required for field emission. This is particularly attractive for flat screen displays, and may be a consideration for intense electron beam cathodes as well. 2) The rigidity of some nanotubes is predicted to be ten times that of iridium of comparable dimensions [1]. Also predicted is the inhibited introduction of impurities and defects into the nanotubes, offering the potential of great tensile strength [2]. This would reduce the possibility of emitter breakage due to tensile failure at high electrostatic pressures, or shock in rugged applications such as in fieldable microwave generators. 3) The conductivity of some nanotubes is predicted to be more than ten times that of copper [3]. 4) Single layer nanotubes in vacuum have survived for a day at temperatures higher than the melting point of copper [4]. 5) Buckybundle measurements have shown positive temperature coefficient of conductivity near room temperature and below (down to -10 K) [5]. These last three properties bode well for high emission current densities without emitter destruction by Joule heating [6]. 6) Finally, the one-dimensional electronic properties of nanotubes will result in emission behavior substantially different than that predicted by the Fowler-Nordheim equation for a three-dimensional conductor. Theoretical studies of 2-D quantum well field emitters have shown a lowering of work function which reduces required emission fields [-7]. Discussions in the literature suggest using buckytubes as forms to fill with metals that would be quantum wires because buckytube diameters can be comparable to Fermi wavelengths. In fact, it has been predicted that some buckytubes will be one-dimensional metals without filling them [8]. Further, scanning tunneling spectroscopy of unfilled buckytubes has shown peaks in conductance associated with peaks in the densities of states resulting from onedimensional electronic st
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