Preparation of lanthanum hexaboride cathodes for thermionic energy generation

Due to low work function, high melting point and superior chemical stability at high temperatures, lanthanum hexaboride (LaB₆) has great potential as a thermionic emitter in renewable energy applications. The main challenge is to synthesise these hexaborides at lower temperatures without any post-synthesis cleaning treatments. In this present work, we investigated several techniques to prepare pure lanthanum hexaboride using different blends of lanthanum oxide (La₂O₃)boron (B), La₂O₃-B-carbon (C), La₂O₃-boron carbide (B₄C) blends, respectively. The starting powders for each blend were mixed by milling for 20 to 30 min and pressed as pellets. Additionally, several samples of La₂O₃-boron blends were prepared by high-energy milling (HEM) for 16 hours. Subsequently, the pellets were placed in a tube furnace at different temperatures under a moderate vacuum to undergo solid state reactions. The synthesised products were investigated for the structural, morphological and thermionic properties using X-ray diffraction (XRD), scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) and a Schottky apparatus. We are able to synthesis pure LaB₆ at comparably lower temperatures via all the investigated methods. Pure LaB₆ is found to be synthesised as low as 1250 °C via the method using high-energy milling. XRD and SEM analyses revealed that LaB₆ prepared using these methods were nanocrystalline. Thermionic emission measurements show that pure LaB₆ is found to possess a low Richardson work function of 2.64 eV making it suitable for producing high current density cathodes.