Development of refractory metal oxide collector materials and their thermionic converter performance

Refactory metal oxides of NbO x, WO x, TaO x, and Silver oxide (AgO x) have been studied for a high performance collector. The metal oxide materials were deposited on metal substrates by RF sputtering in the Ar/O 2 gas mixture, in which the partial pressure of O 2 was deliberately set at the lower values in order to sputter in the stoichiometrically oxygen gas short conditions. Work function of the metal oxides was measured by cesium plasma immersion technique. As results, minimum work function values of each oxide materials were obtained as follows; AgO x = 1.25eV, NbO x =1.38eV, WO x = 1.42eV, TaO x = 1.43eV. NbO x and AgO x are considered most promising for a collector. A thermionic converter with a plane parallel type of a polycrystalline W emitter and an AgO x collector, an interelectrode spacing at room temperature 0.1mm, was set up and the power generation experiments were conducted. The maximum power, 3.9W/cm(2), 0.6V, 6.5A/cm(2), was obtained under the unignited mode operation at T E = 1583K. The barrier index was V B = 1.5V at T E = 1578K. Based on the experimental results, a new type of a FGM collector was proposed for a micro-gap thermionic converter.