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Transport of alkali metal atoms through porous cathodes of alkali metal thermal-to-electric converter (AMTEC) cells is responsible for significant, reducible losses in the electrical performance of these cells. Experimental evidence for activated transport of metal atoms at grain surfaces and boundaries within some AMTEC electrodes has been derived from temperature dependent studies as well as from analysis of the detailed frequency dependence of ac impedance results for other electrodes, including thin, mature molybdenum electrodes which exhibit transport dominated by free molecular flow of sodium gas at low frequencies or dc conditions. Activated surface transport will almost always exist in parallel with free molecular flow transport, and the process of alkali atom adsorption/desorption from the electrode surface will invariably be part of the transport process, and possibly a dominant part in some cases.

Electrode materials for the Alkali Metal Thermal to Electric Converter (AMTEC) play a significant role in the efficiency of the device. RhW and PtW alloys have been studied to determine the best performing material. While RhW electrodes typically have power densities somewhat lower than PtW electrodes, PtW performance is strongly influenced by the Pt/W ratio. The best performing Pt/W ratio is ∼3.4. RhW electrodes sinter more slowly than PtW and are predicted to have operating lifetimes up to 40 years; PtW electrodes are predicted to have lifetimes up to 7 years. Interaction with the current collection network can significantly decrease lifetime by inducing metal migration and segregation and by accelerating the sintering rate.

JPL is entering a new era of spacecraft (s/c) mission operations. The number of s/c tracked is steadily increasing. For many missions, the mission durations are getting longer and mission operations requirements are becoming more complex. For other missions, the emphasis will be on low cost and therefore a less elaborate mission operations undertaking. S/c engineering analysis is conducted to verify s/c engineering performance, characterize the s/c, determine s/c capability, track consumables, and support mission engineering analysis. Engineering analysis at JPL can be characterized by too much done manually, a lack of sufficient analysis tools, the uneven distribution of these tools among subsystems, the difficulty in generating predicts, and the lack of tight integration and therefore cumbersome interaction among subsystems. This paper discusses the concept of an integrated environment for engineering analysis which will enable increased productivity.

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% thermal to electric conversion efficiencies, and one experiment has demonstrated 19% 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% early in cell life and 9.7% 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.

Future missions to Mars for the 1998 launch opportunity and beyond will require advanced thermal control for electronics to minimize enclosure mass, power and volume. An additional requirement is that radioactive heating units (RHU) will not be available for future Mars missions. These strict requirements can be accomplished by integrating phase change material (PCM) panels with aerogel insulation in a structural/thermal enclosure for electronics and instruments. The aerogel insulation has extremely low thermal conductivity, and the PCM panels provide thermal capacitance. The advanced PCM panels consist of a sandwich panel design with an interlocking carbon fiber core which is filled with a suitable phase change material. The fibers provide structural stiffness, and prevent the PCM from forming voids or migration of voids by capillary action. With this design, a PCM mass fraction of 70% has been achieved.

A theoretical model has been developed (Klemens 1987) that predicts that the addition of ultra-fine, inert, phonon-scattering centers to SiGe thermoelectric material will reduce its thermal conductivity and improve its figure-of-merit. To investigate this prediction, ultra-fine particulates (20Å to 200Å) of boron nitride have been added to boron doped, p-type, 80/20 SiGe. All previous SiGe samples produced from ultra-fine SiGe powder without additions had lower thermal conductivities than standard SiGe, but high temperature (1525K) heat treatment increased their thermal conductivity back to the value for standard SiGe. However, the SiGe samples with inert boron nitride or silicon nitride, phonon-scattering centers retained the lower thermal conductivity after multiple heat treatments at 1525K.

Several interesting planetary missions are either enabled or significantly enhanced by nuclear electric propulsion (NEP) in the 50 to 100 kW power range. These missions include a Pluto Orbiter/Probe with an 11-year flight time and several years of operational life in orbit versus a ballistic very fast (13 km/s) flyby which would take longer to get to Pluto and would have a very short time to observe the planet. (A ballistic orbiter would take about 40 years to get to Pluto.) Other missions include a Neptune Orbiter/Probe, a Jupiter Grand Tour orbiting each of the major moons in order, a Uranus Orbiter/Probe, a Multiple Mainbelt Asteroid Rendezvous orbiting six selected asteroids, and a Comet Nucleus Sample Return. This paper discusses potential missions and compares the nuclear electric propulsion option to the conventional ballistic approach on a parametric basis.