Preparation of lanthanum hexaboride cathodes for thermionic energy generation

Abstract

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.