Lanthanum (oxy)boride thin films for thermionic emission applications

The chemical-physical properties of thin lanthanum (oxy)boride films deposited on polycrystalline tantalum substrates by different deposition techniques (Pulsed Laser Deposition with either nanosecond or femtosecond laser source, and electron beam evaporation) were compared in order to investigate the effect of chemical composition, crystallinity, surface morphology of the thin films on the thermionic electron emission capability. The emission performance of the films was analyzed in the 700-1600 °C temperature range, allowing the determination of work function values ranging from 2.59 to 2.85 eV and an effective Richardson constant at least one order of magnitude lower than the ideal value for all the films. The highest measured thermionic current density value of 1.68 A/cm2 at 1600 °C was provided by the samples grown by femtosecond PLD, indicating a promising performance for practical application in high-temperature thermal-to-electric energy conversion.