V. B. Shields

Lifetimes of AMTEC electrodes: Molybdenum, rhodium-tungsten, and titanium nitride

The lifetime of three types of AMTEC electrodes is predicted from the rate of grain growth in the electrode. Grain size is related to electrode performance, allowing performance to be correlated with grain growth rate. The rate of growth depends on physical characteristics of each material, including the rates of surface self-diffusion and molecule mobility along grain boundaries. Grain growth rates for molybdenum, rhodium-tungsten and titanium nitride electrodes have been determined experimentally and fit to models in order to predict operating lifetimes of AMTEC electrodes. For lifetimes of 10 years or more, RhxW electrodes may be used at any operating temperature supportable by the electrolyte. TiN electrodes may be used in AMTEC cells only at operating temperatures under 1150 K, and Mo may be used only below 1100 K.

Advances in electrode materials for AMTEC

A mixed conducting electrode for the Alkali Metal Thermal to Electric Converter (AMTEC) has been made and tested. The electrode is made from a slurry of metal and TiO2 powders which is applied to the electrolyte and fired to sinter the electrode material. During the first 48–72 hours of operation in a SETC, the electrode takes up Na from low pressure sodium vapor to make a metal-Na-Ti-O compound. This compound is electronically conducting and ionically conducting to sodium; electronic conduction is also provided by the metal in the electrode. With a mixed conducting electrode made from robust, low vapor pressure materials, the promise for improved performance and lifetime is high.

The thermal stability of sodium beta'-Alumina solid electrolyte ceramic in AMTEC cells

A critical component of alkali metal thermal-to electric converter (AMTEC) devices for long duration space missions is the beta″-alumina solid electrolyte ceramic (BASE), for which there exists no substitute. The temperature and environmental conditions under which BASE remains stable control operational parameters of AMTEC devices. We have used mass loss experiments in vacuum to 1573K to characterize the kinetics of BASE decomposition, and conductivity and exchange current measurements in sodium vapor filled exposure cells to 1223K to investigate changes in the BASE which affect its ionic conductivity. There is no clear evidence of direct thermal decomposition of BASE below 1273K, although limited soda loss may occur. Reactive metals such as Mn or Cr can react with BASE at temperatures at least as low as 1223K.

Performance parameters of TiN electrodes for AMTEC cells

In order to model the lifetime of the electrochemical cell in an Alkali Metal Thermal to Electric Converter (AMTEC), studies of TiN electrodes on beta″-alumina solid electrolytes (BASE) have been made to determine the performance parameters over time. Performance parameters include, G, the morphology factor, and B, the temperature independent exchange current. The results of several experiments, both AMTEC cells and Sodium Exposure Test Cells, in which TiN electrodes have been studied at 1120–1200 K are described here.

Metallurgical examination of an AMTEC unit

Two AMTEC units were operated for over 300 hours and then dissected and examined. The converter units were fabricated by Advanced Modular Power Systems, Inc. using a cold end condenser manufactured by Creare. Inc. and were constructed with a stainless steel housing and stainless steel sheathed molybdenum wick. Creare Unit 1 operated for 342 hours with condenser and BASE tube cell temperatures of 663 K and 1003 K respectively: the output power varied from an initial 68 mw to a final 25 mw. Operation of Creare Unit 1 was halted due to a low initial output current and power that remained unchanged. Creare Unit 2 ran for 575 hours with condenser and BASE tube cell temperatures of 543 K and 978 K respectively: the output power varied from an initial 2.6 watts to a final 1.8 watts. Operation of Creare Unit 2 was halted due to a low power output after the heater unit supplying heat failed and was replaced. The expected output power for both units was about 1 to 4 watts. Both units were subsequently disassembled ...

Synthesis and thermoelectric properties of Co/sub 1-x/Ni/sub x/P/sub 3/ and CoAs/sub 3-x/P/sub x/ skutterudites

Two types of promising phosphide skutterudite materials, Co/sub 1-x/Ni/sub x/P/sub 3/ (x= 0.025 to 0.70) and CoAs/sub 3.x/P/sub x/(x=0.5 to 0. 10), have been synthesized and their thermoelectric properties measured. These compounds were prepared using a direct synthesis technique. The samples were hot pressed and analyzed by electron microprobe microscopy. Hall effect measurements were conducted to determine the electrical conductivity, mobility and carrier concentration. In addition, Seebeck coefficient and thermal conductivity measurements were performed. The thermoelectric properties are presented and discussed as a function of temperature up to 1273 K. The thermal stability of the primary CoP/sub 3/ was examined in a static vacuum under isothermal and in-gradient conditions. The effect of the presence of 1 atm of a cover gas on the material loss rate was analyzed.

Challenges facing successful development of long life, high temperature, high efficiency/power AMTECs for space applications

The technical challenges confronting development of Alkali Metal Thermal-to Electric Converters (AMTEC) for up to 15 year life on NASA Mars and outer planetary missions are significant, although approaches to solutions are understood by many investigators. The technical challenges include material issues, specifically involving decomposition of materials or chemical reaction between materials in contact; electrical issues, including electrical shunts that reduce output power and possibly plasma discharges within AMTEC converter units in which potentials in excess of 5.25 Volts exist; and heat and mass transport issues which can have major effects on the converter unit’s power and efficiency.

Reactivity of thin metal films on sodium beta alumina ceramic in high temperature, low pressure sodium vapor

Electrochemical techniques including impedance spectroscopy are routinely used to test the performance of AMTEC electrodes. These experiments may be carried out in an actual AMTEC cell. or in an sodium exposure test cell SETC where the sodium pressure and the temperature are identical at both the anode and cathode. These tests reveal details of electrode material grain growth, electrode decomposition and reaction of electrode materials with the sodium beta″ alumina solid electrolyte. Tests of the same sort may be used to examine the compatibility of metals and sodium beta″ alumina under simulated AMTEC operating conditions. Preliminary tests comparing rhodium/tungsten, molybdenum, titanium and vanadium electrodes are reported.

Mathematical modeling of the impedance of single and multi-cell AMTEC units

AMTEC power systems are designed for use on extended space missions. During the lifetime of such missions the power available for the spacecraft will depend on the degradation of the system performance. Development of a tool that allows monitoring of the system degradation will provide an aid in determining the condition of the power source. Since the power output is a function of the impedance across the BASE tube and electrodes, monitoring its variation can provide an understanding of the internal changes within the AMTEC unit. A lumped element impedance model is being developed for a single electrochemical cell and a six electrochemical cell AMTEC unit. The analysis was performed using the symbolic analysis software program, Mathematica. The model examines the cathode-BASE-anode impedance by allowing the BASE and the electrode-BASE interfaces to be depicted as a combination of resistances. Warburg impedances and pseudo capacitances (i.e. constant phase elements). The model results are compared with ope...

Thermal stability of beta″-alumina solid electrolyte under AMTEC operating conditions

A critical component of alkali metal thermal-to electric converter (AMTEC) devices for long duration space missions is the sodium beta″-alumina solid electrolyte ceramic (BASE), for which there exists no substitute. The major phase in this ceramic, sodium beta″-alumina shows no evidence of thermal decomposition in AMTEC environments including clean liquid sodium and low pressure sodium gas, at temperatures below 1173K, or in vacuum below 1273K. This paper presents additional results of ionic conductivity and exchange current studies in sodium exposure test cells (SETCs) to characterize the changes occurring in BASE below 1273K in low pressure sodium vapor. Also presented are additional annealing studies to characterize the kinetics of processes occurring in the BASE ceramic in the AMTEC operating regime.