Synthesis of nanostructured lanthanum hexaboride via simple borothermal routes at low temperatures

Abstract

Rare-earth hexaborides, especially lanthanum hexaboride (LaB6) are popular materials as thermionic cathodes. It is a great challenge to synthesise these hexaborides at a low temperature without any post-synthesis purification treatments. In this research work, we demonstrate a simple synthesis technique, namely borothermal method for preparing pure lanthanum hexaboride by reacting lanthanum oxide (La2O3) with boron (B). In this investigation, different pressed pellets were made using the starting powders mixed either by milling mildly for 30 min or high-energy milling (HEM) for 8/16 h. Later, the prepared pellets were put into a low-vacuum furnace at different temperatures. The synthesised products were characterised using X-ray diffraction (XRD), scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) and a Schottky device for their structural, chemical, morphological and thermionic properties. Both synthesis methods are found to produce LaB6 with a purity above 99% at a relatively low temperature of 1250 °C. High-energy ball milling does not result in reducing synthesis temperatures, rather introduces some iron impurities. From XRD and SEM investigations, the average crystallite size of pure LaB6 prepared by borothermal methods is observed to be in the range of 225–250 nm. From thermionic emission data, partially sintered LaB6 pellets prepared from powders via the simple borothermal and the borothermal method with HEM exhibit work functions of 2.86 ± 0.05 eV and 2.64 ± 0.08 eV, respectively. The results suggest that nanostructured LaB6 with a low work function can be easily prepared at lower temperatures for applications in thermionic energy converters, high-power electron devices and photonics.