a result of these tests which saves 1.7 kilograms per pump and reduces the GFS mass by 20.4 Kg. The thermoelectric power source for the pump has been designed using the analytical codes that have been developed to predict thermoeletric performance. This thermoelectric power source will use the same components and materials that are being developed for the power converter subsystem. The thermoelectric components and materials will be configured in the pump to produce low DC voltage and high current which is required to pump the lithium. The gas separator conceptual design is complete and was accepted at the system design review. The same separator concept was designed and fabricated in accordance with design analyses codes which predicted the separation of air from water. The separator design was tested with air and water and the experimental results verified the analytical predictions. These same analysis codes with the properties of helium and lithium are used to predict and determine the separator design parameters for the GFS. The mass of the flight separator is then calculated based on the flight component design. The lithium 7 that is specified as the flight reactor coolant in the GFS was irradiated in a fast test reactor and the irradiation results verified the analytically predicted helium generation rate. Power Conversion Subsystem The conversion subsystem design for the GFS is complete and was approved for development at the May 1988 system design review. The 100 kWe space reactor power systems consist of 12 power converter assemblies (PCAs). One PCA is shown is Fig. 22. Each PCA contains six thermoelectric converter assemblies (TCAs) which each produce 1.5 kWe at 34.8 volts DC. There are 120 thermoelectric (TE) cells in each TCA and 32 thermoelectric (TE) couples in each TE cell. The 100 kWe power conversion subsystem consists of 12 PCAs, 72 TCAs, 8640 TE cells and 276 480 TE couples, which are electrically connected in a series paralled array and are capable of producing a gross output of 115.2 kWe at 200 V DC at the beginning of the power system lifetime. The heart of the power system is the TE cell which is 2.64 X 2.64 X 1.3 cm thick. An exploded view of the TE cell and its materials are shown in Fig. 23. The standard 80% silicon (Si) and 20% germanium (Ge) thermoelectric material that has been used on the Voyager spacecraft and will be used on the Galileo spacecraft is being modified to increase the material figure-of-merit from 0.7 X 10-3 to 0.85 X 10-3. The 80:20 SiGe with 3-5% gallium phosphide (GaP) has been under development, and some but not all of the material has resulted in an n-leg figure-of-merit of 1.0 X 10-3. The present effort is focused on understanding the w-leg SiGe (GaP) material characteristics that cause the increased figure-of-merit and then consistently reproduce this high figure-of-merit w-leg SiGe (GaP). The SiGe (GaP) p-leg does not perform any better than the standard SiGe. Therefore the />-leg development is investigating fine-grain SiGe and other material additives to reduce the material thermal conductivity without changing the Seeback voltage or the electrical conductivity and increase the figure-of- merit. The electrode graphite to SiGe bond needs to be made so that the electrical contact resistivity of that joint is less than 25 microhm-cm2. A Mo/Ge braze process demonstrated initial contact resistivities below 25 microhm-cm2 but the resistivity goes above 25 microhm-cm2 after 70 h at 1273 K. Using a Ti/Ge braze gives very similar results. Metallographic analysis of the Mo/Ge brazed bonds shows a substantial loss of
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