Synthesis and thermionic properties of oxide-tungsten composites and boride materials for solar thermionic energy conversion

Energy conversion of solar energy through a thermionic energy converter (TEC) is one of the least explored areas of research to date due to pre-assumed massive difficulties and challenges inherent to the physical phenomena associated with thermionic materials and the energy conversion processes. Though a range of thermionic materials have been widely investigated by researchers to study pulse mode electron emission capabilities particularly on vacuum devices, for the first time, this work presents investigations on a wide range of potential materials as emitters or collectors for application in a high-temperature direct-current (DC) TEC indicating their suitability for utilising solar power. Promising rare-earth hexaborides (lanthanum hexaboride LaB6, ceriumhexaboride CeB₆, gadolinium hexaboride GdB6, barium hexaboride BaB₆and a mixed boride (Ba_xLa1-_x)B₆) were synthesised using an optimised simple method tailored to be suitable for mass-scale production method. The synthesis temperatures to produce different pure hexaborides were found to be significantly lower than previously reported and it was as low as 1200 °C for Ce₆. A variety of microstructures were observed with many nano-size crystals for most samples. The raw powders synthesised were sintered to moderate densities and were tested for thermionic properties. Stable DC thermionic emission data was collected using a Schottky device in the temperature range 1000-1300 °C. Low work function values were observed in the range 1.03-3.32 eV. Similar to dispenser or oxide cathodes, several novel compositions of metal-oxide composite cathodes; tungsten (W)-barium titanate or barium hafnate or a triple oxide mix and a nickel-triple oxide mix, were also investigated to observe high temperature behavior through structural and morphological modifications. With necessary surface modifications, thermionic properties were found to be suitable for collectors in particular, such as nickel-triple oxide mix with ϕ_R=1.22 eV. Comparing emission constants, high current density was observed from LaB6 via the borothermal and carbothermal methods, though the work functions were not as low as expected (2.86-3.32 eV). The lowest work function was found 1.03 eV for the (Ba_xLa1-_x)B6 cathodes. Other materials returned intermediate work function values. One selected pair, the LaB6 cathode (via the borothermal method ~2.9 eV) as an emitter and the LaB₆-20Ba%HfO₃ cathode (~2.3 eV) as a collector, was tested in an in-house TEC suitable for solar applications. This initial test indicates that the selected materials were capable of emitting/collecting thermally excited electrons with stable and albeit low power output. From the findings of the thesis, it was concluded that LaB6 having a work function of 2.92 eV (via the simple borothermal method) and Ni-triple oxide mix (1.22eV) are the most promising pair for future runs of the TEC device. Several solid-state routes with reduced reaction temperatures (as low as 1200 °C) have been identified for the production of different hexaborides and one boride solid-solution. These techniques yield very high purity materials (<99%) which do not require any subsequent purification process. The lower synthesis temperature for LaB₆ in particular resulted in finer grain size, increased sinterability and was found ideal for the production of emitters suitable for energy conversion applications.