New Findings on Powder Synthesis for Scandate Cathode Matrices

The weight percentage of scandia nanoparticles in scandate cathodes was varied and found to have a critical impact on emission performance and predictability. The conventional concentration of 4-5 wt% scandia was found to be too high and is discussed. Large numbers of cathodes were characterized via Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy cross sections, then emission tested in close-spaced diodes. The balance or ratio of barium-calcium-aluminate to scandium oxide was found to be the most important factor in predicting a cathode's operating temperature. Excessive scandia inhibited the flow of molten impregnant into matrix pores. Furthermore, excessive scandia was found to lower matrix mechanical strength. This work also revealed the importance of lowering the oxygen content in tungsten particles prior to sintering. Large numbers of dry-hydrogen-fired powders were compared to wet-hydrogen-fired ones and were found to have much higher emission performance. Four different powder types were studied, including those from a novel two-stage liquid-liquid (L-L) precipitation technique, ones from single L-L precipitation, and ones from the liquid-solid (L-S) precipitation technique. This paper also reports life test results on selected cathodes.