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 (LaB
₆
) 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 (La
₂
O
₃
)boron (B), La
₂
O
₃
-B-carbon (C), La
₂
O
₃
-boron carbide (B
₄
C) 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 La
₂
O
₃
-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 LaB
₆
at comparably lower temperatures via all the investigated methods. Pure LaB
₆
is found to be synthesised as low as 1250 °C via the method using high-energy milling. XRD and SEM analyses revealed that LaB
₆
prepared using these methods were nanocrystalline. Thermionic emission measurements show that pure LaB
₆
is found to possess a low Richardson work function of 2.64 eV making it suitable for producing high current density cathodes.