Characteristics of Ceramic Electrode for AMTEC 929006
We have shown that TiN, NbN and TiC electrodes prepared from metal or respective compound powders by a screen printing method work well as AMTEC electrode (26th IECEC). TiC electrode (Thickness:2-3μm, 800°C, 0.07Pa) shows the highest power density (0.40W/cm2) in our work. This is almost same power density of others' electrodes under same temperature. This shows that AMTEC electrode can be prepared by screen printing method from metal or ceramic powders. The maximum power densities of TiN, NbN and TiC depend markedly on the degree of vacuum of low temperature room of AMTEC. In the measurements of NbN electrode (thickness:2-3μm), the power density of AMTEC increased from 0.12W/cm2 (at 130Pa) to 0.27W/cm2 (at 0.037Pa) with improvement of vacuum. TiC electrode shows same tendency. This indicates that the transfers of sodium vapor through electrode give an effect on the power density. Electrode characteristics under AMTEC operation were measured by interruption method and a.c. impedance analysis. These measurements (NbN electrode) show that the AMTEC cell has impedances due to interface of Na/BASE or BASE/electrode, in addition to an ohmic resistance. The overvoltage (η) resulted from polarization was linearly depending on the logarithm of current density (J) at higher overvoltage region. However, the deviation from this linear slope between η vs log J was observed at lower overvoltage region. This shows that an overvoltage obeyed Butler-Volmer equation exists in the AMTEC cell with NbN electrode. It is thought that these overvoltages depend on Na temperature, and material, thickness, porosity and microstructure of electrode. The effects of these parameters on the polarization at interface of BASE/electrode and transfer resistance of Na vapor through electrode will be discussed with emphasis on increase in power density.