Scandate Cathode for High-Power, Long-Life Electric Space Propulsion

Applications include electron beam-stimulated lasers for high-power video projection systems and new-generation CT-scan x-ray imaging systems.

John H. Glenn Research Center, Cleveland, Ohio

Scandate cathodes are able to produce about 20 times the emission of conventional all-tungsten cathodes at the same temperature. Conversely, they operate at about 200 °C lower temperature for the same emission. Scandate cathodes have been studied since at least the 1960s. Between then and 2002, numerous approaches were investigated. All cathodes either did not work or degraded within a few thousand hours. The current nanoparticle approach appears to have overcome previous shortcomings.

Scandate cathodes employ a semiconductor layer at the emitting surface to enhance emission. This allows field penetration into the emitting region, which, in turn, lowers the work function. In practical terms, the semiconductor forms a kind of resistive layer at the surface of the cathode.

The essential innovation of this scandate cathode is the method of distributing nanoparticles of scandium oxide throughout the matrix. The liquid-solid (L-S) approach involves dissolving scandium oxide in a solution. Then, one-micron tungsten particles are suspended in that solution. After this, the scandium oxide is precipitated out. If the tungsten particles are numerous enough and suspended uniformly enough, the scandia will nucleate onto the tungsten and grow distinct 50-nm particles intimately attached to the tungsten.

A technique for evaluating scandium-doped tungsten powders using SEM and EDS tools was also developed, as was a process for pressing the powders isostatically into billets and sintering without fracturing and with good porosity and strength. The research consisted of fabricating tungsten powders doped 5 to 10% by weight with nanocrystalline scandium oxide. Based on the emission levels achieved and other factors, the powders were ranked.

A hollow scandate cathode was fabricated that produced 5 A/cm2 of emission at over 100 °C below comparable all-tungsten cathodes. Moreover, this cathode continued to produce unprecedented emission at low temperatures in a 1-torr xenon environment. It produced over 50 A of emission at 880 °C.

This work was done by Bernard Vancil of E-Beam Inc. for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steven Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-19152-1.

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