Low temperature carbothermal and boron carbide reduction synthesis of LaB6

Rare-earth hexaborides are widely used as thermionic emitters however their economic production for large scale applications such as solar thermionic electricity generation is hampered by a need to synthesise them at lower temperature without post-synthesis cleaning treatments. Two simple synthesis techniques for producing pure lanthanum hexaboride (LaB6), carbothermal reduction using La2O3-boron-carbon and boron carbide reduction using La2O3-B4C blends respectively were studied. Using fine grained starting materials and a mild pre-milling treatment, the carbothermal method was found to produce high-purity LaB6 at a temperature 1400 °C or below. The B4C method also appeared from XRD and SEM analyses of abraded surfaces to produce high-purity LaB6 at 1400 °C however EDS maps of polished cross-sections revealed the presence of unreacted B4C necessitating a temperature of 1450 °C to complete the reaction. XRD and SEM analyses indicate that the mean particle sizes of LaB6 using the boron carbide method (220 nm) is smaller than that for the carbothermal reduction method (600 nm). The finer grains of samples prepared via the boron carbide method result in a partially sintered powder. LaB6 prepared in this way is shown to have a Richardson work function of 2.64 eV.