D. O'Connor

Lifetime Studies Of High Power Rhodium/tungsten And Molybdenum Electrodes For Application To Amtec (alkali Metal Thermal-to-electric Converter)

A detailed and fundamental model for the electrochemical behavior of AMTEC electrodes is developed which can aid in interpreting the processes which occur during prolonged operation of these electrodes. Because the sintering and grain growth of metal particles is also a well-understood phenomenon, the changes in electrode performance which accompany its morphological evolution may be anticipated and modeled. The grain growth rate observed for porous Mo AMTEC electrodes is significantly higher than that predicted from surface diffusion data obtained at higher temperatures and incorporated into the grain growth model. The grain growth observed under AMTEC conditions is also somewhat higher than that measured for Mo films on BASE (beta-alumina solid electrolyte) substrates in vacuum or at similar temperatures. Results of modeling indicate that thin Mo electrodes may show significant performance degradation for extended operation (greater than 10,000 h) at higher operating temperatures (greater than 1150 K), whereas W/Rh and W/Pt electrodes are expected to show adequate performance at 1200 K for lifetimes greater than 10,000 h. It is pointed out that current collection grids and leads must consist of refractory metals such as Mo and W which do not accelerate sintering or metal migration.

Developments in Amtec Devices, Components and Performance

Improvement of the performance of an AMTEC device requires improvement and development of components as well as of device geometry and construction. The research and development effort at JPL includes studies which address both overall device construction and studies of components. This paper discusses recent studies on components and devices which have been carried out at JPL. Components investigated include the electrode materials titanium nitride (TiN) and rhodium‐tungsten (RhW) and the electrolyte materials sodium β“‐alumina and potassium β” ‐alumina. We have studied the mechanical characteristics of sodium and potassium β“‐alumina ceramic and conditions for fabrication of potassium β”‐alumina. Device studies include fabrication and operation of a wick fed cell using a graded, sintered wick, a higher voltage vapor‐vapor multicell which includes three “ subcells” which are internally series connected, and an AMTEC which uses potassium as the working fluid.

Efficiency of an AMTEC Recirculating Test Cell, Experiments and Projections

The alkali metal thermal to electric converter (AMTEC) is an electrochemical device for the direct conversion of heat to electrical energy with efficiencies potentially near Carnot. The future usefulness of AMTEC for space power conversion depends on the efficiency of the devices. Systems studies have projected from 15 to 35 percent thermal to electric conversion efficiencies, and one experiment has demonstrated 19 percent efficiency for a short period of time. Recent experiments in a recirculating test cell (RTC) have demonstrated sustained conversion efficiencies as high as 10.2 percent early in cell life and 9.7 percent after maturity. Extensive thermal and electrochemical analysis of the cell during several experiments demonstrated that the efficiency could be improved in two ways. First, the electrode performance could be improved. The electrode for these tests operated at about one third the power density of state of the art electrodes. The low power density was caused by a combination of high series resistance and high mass flow resistance. Reducing these resistances could improve the efficiency to greater than 10 percent. Second, the cell thermal performance could be improved. Efficiencies greater than 14 percent could be realized through reducing the radiative thermal loss. Further improvements to the efficiency range predicted by systems studies can be accomplished through the development and use of an advanced condenser with improved reflectivity, close to that of a smooth sodium film, and the series connecting of individual cells to further reduce thermal losses.

High temperature conductivity of potassium-β''-alumina

Abstract Potassium β″-alumina single crystals have been reported by several groups to have higher ionic conductivity than sodium β″-alumina crystals at room temperature, and similar conductivities are obtained at temperatures up to 600–700 K. Potassium β″-alumina ceramics have been reported to have significantly poorer consuctivities than those of sodium β″-alumina ceramics, but conductivity measurements at temperatures above 625 K have not been reported. In this study, K + -β″-alumina ceramics were prepared from Na + -β″-alumina ceramic using a modified version of the exchange reaction with KCl vapor reported by Crosbie and Tennenhouse, and the conductivity has been measured in K vapor at temperatures up to 1223 K, using the method of Cole, Weber and Hunt. The results indicate reasonable agreement with earlier data on K + -β″-alumina ceramic measured in a liquid K cell, but show that the K + -β″-alumina ceramics conductivity approaches that of Na + -β″-alumina ceramic at higher temperatures, being within a factor of four at 700 K and 60% of the conductivity of Na + -β″-alumina at T > 1000 K. Both four-probe dc conductivity and four probe ac impedance measurements were used to characterize the conductivity. A rather abrupt change in the grain boundary resistance suggesting a possible phase change in the intergranular material, potassium aluminate, is seen in the ac impedance behavior.