Processing and characterization of LaB6-coated hairpin cathodes

Abstract

Lanthanum hexaboride (LaB6) was cataphoretically deposited on a filamentary hairpin support of tantalum. Such filaments were further processed to form cathodes. Various procedures involved during the processing of cathodes, such as carburization of the core tantalum wire in hairpin shape, deposition of LaB6 onto it, and sintering of such cathodes are described. These cathodes were characterized for thermionic work function measurements, life tests, and thermal shocks in a diode configuration. The ultrahigh-vacuum (UHV) sintering gives better emission stability, compact coating, low porosity, etc., as compared to high-vacuum sintering. The work function values were found to be in the range of 2.6–2.8 eV for various cathodes whereas, the pre-exponential factor seemed to vary in the range of 30–200 A cm−2K−2. These cathodes showed life in excess of 55 h of continuous operation. It was found that the life of such cathodes depends mainly on the adhesion of the LaB6 coating to the core wire instead of the evaporation rate of LaB6 at operating temperatures. The cathodes sustained a larger number of thermal shocks in UHV than in a high-vacuum environment.