Thermionic emission from CVD poly- and nano-crystalline diamond films grown on different substrates

CVD diamond represents a very attractive material for the fabrication of devices based on thermionic emission, due to the possibility to achieve a very low or even negative electron affinity (NEA) by a hydrogen surface termination. We present here a preliminary study on thermionic conversion from diamond thin films grown on different substrates, ranging from silicon to engineered ceramic materials. In particular, among them, HfC-based ceramics have been selected for its integration with diamond in concentrated solar systems. CVD polycrystalline and nanocrystalline diamond films were deposited by MW-CVD (MicroWave-Chemical Vapour Deposition) by varying also their doping. Thermionic performance was evaluated in an ultra-high-vacuum (pressure <10-8 Torr) characterization setup able to accurately control the emitter and collector temperatures. Experimental activity has been carried out to integrate the HfC-diamond system in a more complex conversion module that was tested in a concentrating solar system to verify the conversion performance under operating conditions. These results are finally reported.