Low-Temperature Study of a Sputtered Metal Film Gas-Supporting Electrode on Beta-Alumina Solid Electrolyte in the Sodium Heat Engine.
Low temperature (250(DEGREES)C-400(DEGREES)C) studies of sputter-deposited inert-metal film (Molybdenum) electrode processes in the Sodium Heat Engine, which employs a beta''-alumina solid electrolyte, showed that the cell current-voltage relationship could be resolved in terms of the cell polarizations. These include the polarizations due to charge transport and exchange, mass transport (diffusion) in the electrode, in-plane electronic conduction, and in -plane thermal gradients. The experimental techniques for studying these polarizations included current interruption, sodium vapor flux detection in vacuo, back-biasing, and in-plane voltage measurements. Two regimes of differing electrode behavior were studied. These were associated with the submonolayer sodium containing electrode, and the supermonolayer (quasi-liquid) sodium containing electrode. The condensation of quasi -liquid sodium permitted the charge exchange polarization to be distinguished from the overall polarization due to charge transport and exchange. The Butler-Volmer model of charge exchange was critically reviewed, but the data neither supported or denied the appropriateness of the Butler-Volmer model's application to solid electrolyte systems. Other electrode processes had different behavior in the submonolayer and supermonolayer regimes. The relationship between activity (vapor pressure) and concentration (coverage) in the electrode was found to be described by the Temkin isotherm. A novel method for measuring the in-plane sheet resistance was used. Ordinary methods are shown to be inaccurate due to the coupling between the electronic and ionic charge carriers. A perturbation method for the analysis of the variation in current density is presented. The adsorption of sodium in the electrode was studied using absorption and flow measurements along with back-bias measurements. From the analysis of this study emerged a physical model of the electrode. An intergranular phase may account for the strong adsorption and immobilization of several micromoles of sodium in the electrode. The ordered crystalline phase adsorbs about a tenth as much as the intergranular phase. Sodium transport through the electrode is modeled as taking place on the surface of the crystalline phase within the electrode.
- Pub Date:
- Physics: Condensed Matter