density to minimize thermal transport. Consequently, this MULTI-FOIL provides an ultrahigh impedance to thermal transport, with minimum mass (Figure 4-24). For the design shown in Figure 4-21, ten layers of 1 mil-thick tungsten foil separated by ZrO2 particles were considered adequate. As can be seen in Figure 4-25, the layers provide substantial insulation capability. The total thickness of such insulation is 1.2 mm (0.03 in.) with a mass of only 0.25 gm per cm2 (0.51 lbm/ft2), or approximately 230 gm per converter. Heat transfer through the 900 K (1160°F) temperature differential between the incident radiation (Ti = 1800 K) (3140°F) and the heat pipe (To = 900 K)(1160°F) is (Ref. 5). where N is the number of MULTI-FOIL layers and T is in degrees Kelvin. This additional heat flow can be radiated out by the heat pipe. Fig. 4-25. Multifoil Thermal Insulation Temperature Profile for Brush-Coated ThO2 on Tungsten 4.6.4 Busbar Design In determining resistive power losses both the converter electrodes and interconnecting leads must be considered. The least losses result if currents are minimized and voltages maximized; i.e., if the thermionic converters are connected in series. At high temperatures, however, electrical insulation of the leads becomes a problem. In the design, shown in Figure 4-21, the temperature of the electrical insulation never need exceed 1000 K, because of a thin metallic (molybdenum) thermally insulating heat choke between the hot emitter and interconverter lead. The design of this choke is determined by optimizing Equation 1 with respect to Sa/la which yields (see Ref. 2) the best ratio for Fig. 4-24. Thermal Conductivity Comparison
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