Sub-grain boundaries were removed from crystals, prepared by using the floating zone method, by adding the rare earth hexaborides CeB6, PrB6 and NdB6. The thermionic emission properties of cathodes made from the boundary-free crystals were examined and compared with those of a LaB6 crystal cathode. Addition of rare earth borides did not influence the emission properties (emission current density, brightness or life) under practical working conditions.

Thermionic Emission Properties of Rare-Earth-Added LaB6 Crystal Cathodes. Otani, S., Hiraoka, H., Ide, M., Ishizawa, Y.: Journal of Alloys and Compounds, 1992, 189[1], L1-3