Abstract
Rare-earth/alkaline-earth hexaborides with high chemical stability and low work function have recently attracted much interest as promising thermionic cathodes. Their synthesis at low temperatures without post-synthesis purification treatments is still a significant challenge. The present work demonstrates a simple synthesis technique to produce pure barium, cerium and gadolinium hexaborides (BaB6, CeB6, and GdB6). For each hexaboride, a batch of samples was created by first milling highly pure precursor powders and then firing at various temperatures in a low-vacuum furnace. The synthesised products were characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron dispersive spectroscopy (EDS). Their thermionic properties were investigated utilising a Schottky device. Pure BaB6, CeB6, and GdB6 were successfully synthesised at lower temperatures (<= 1400 degrees C). Nanocrystalline CeB6 was found to be produced at the lowest temperature of 1200 degrees C. The mean particle size of pure CeB6 and GdB6 synthesised is seen to be in the sub-micron range, with the exception of large BaB6 particles. The estimated work functions of the BaB6, CeB6, and GdB6 cathodes are 1.61 +/- 0.18 eV, 2.50 +/- 0.09 eV, and 2.42 +/- 0.09 eV, respectively. These findings imply that hexaborides, as promising thermionic emitters, can be easily produced at low temperatures. (C) 2022 Elsevier B.V. All rights reserved.
NSTL
Elsevier