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The thermodynamic cycle appropriate to an AMTEC (alkali metal thermal-to-electric converter) cell is discussed for both liquid- and vapor-fed modes of operation, under the assumption that all processes can be performed reversibly. In the liquid-fed mode, the reversible efficiency is greater than 89.6% of Carnot efficiency for heat input and rejection temperatures (900-1300 K and 400-800 K, respectively) typical of practical devices. Vapor-fed cells can approach the efficiency of liquid-fed cells. Quantitative estimates confirm that the efficiency is insensitive to either the work required to pressurize the sodium liquid or the details of the state changes associated with cooling the low pressure sodium gas to the heat rejection temperature.

Ruthenium silicide (Ru2Si3) is currently under development as a promising thermoelectric material suitable for space power applications. Key to realizing the potentially high figure of merit values of this material is the development of appropriate doping techniques. In this study, manganese and iridium have been identified as useful p- and n-type dopants, respectively. Resistivity values have been reduced by more than 3 orders of magnitude. Anomalous Hall effect results, however, complicate interpretation of some of the results and further effort is required to achieve optimum doping levels.

The iridium-silicon phase diagram on the silicon-rich side was investigated by means of X-ray powder diffraction, density, differential thermal analysis, metalography, microprobe analysis, and electrical resistivity. Attempts were made to prepare eight previously reported silicon-rich iridium silicide compounds by arc melting and Bridgman-like growth. However, microprobe analysis identified only four distinct compositions: IrSi, Ir3Si4, Ir3Si5 and IrSi∼3. The existence of Ir4Si5 could not be confirmed in this study, even though the crystal structure has been previously reported. Differential thermal analysis (DTA) in conjunction with X-ray powder diffraction confirm polymorphism in IrSi∼3, determined to have orthorhombic and monoclinic unit cells in the high and low temperature forms. A eutectic composition alloy of 83±1 atomic % silicon was observed between IrSi∼3 and silicon. Ir3Si4 exhibits distinct metallic behavior while Ir3Si5 is semiconducting.

An integrated lightweight structure and thermal insulation has been developed for the Mars Rover Warm Electronics Box (WEB). It combines a low thermal conductivity fiber reinforced composite structure with co-cured thermal control surfaces and an ultralightweight hydrophobic solid silica aerogel which eliminates convection and minimizes conduction and radiation. It provides excellent thermal insulation both at low gas pressures and in vacuum; and meets the structural requirements for spacecraft launch loads and for a 60 g impact landing at Mars without damage to the insulation or structure. The integrated design provides for variable conductance to meet the different thermal requirements for the different phases of operation. The system incorporates heat conduction paths to provide thermal control on the ground and during interplanetary cruise, which are then disconnected for Mars operation.

The Alkali Metal Thermal to Electric Converter (AMTEC) is a static energy conversion technology that is expected to provide low mass thermal to electric conversion with efficiencies between 20 and 35%. The US program to develop this technology for space power applications has grown substantially over the past 3 years. This expanding program has brought together several laboratories and technical consultants, in separately sponsored projects, to develop the key elements of the technology. An assessment of this multi-party program indicates that, in general, the effort has focused on the high priority technical elements with only moderate overlap between individual projects. There are, however, several areas where additional coordination is needed between major participants in the existing projects, and other areas where new projects should be started, in order to provide reliable space power systems without unnecessary delays.