Progress report No. 41 for a program of thermoelectric generator testing and RTG degradation-mechanisms evaluation

Sublimation tests conducted on Si3N4 coated SiGe (78%-22%) legs and SiMo hot shoes produced by G.E. were continued during this reporting period. The present test time for the SiGe legs is 1623 hours, while that for the SiMo hot-shoes is 1028 hours. No unexpected results have been observed. G.E.'s conjecture that the coatings on the SiGe legs which we presently have on test are faulty is confirmed, as this material has displayed coating failure on all n-doped samples at 11500C with failures beginning to show at 11000C. No coating failures have been observed on the hot shoes on test. Thermal conductivity tests conducted on SiGe (78% to 22%) material were extended to over 6000 hours with excellent agreement with MHW results. Testing of the 4 couple module PR-1 has been conducted for over 2,000 hours with similar agreement with past MHW tests. Testing of the Q1-A generator is continuing. The present test time is 33,632 hours. The performance of the generator remains smooth and continuous. Testing of the S/N-1 and S/N-3 eighteen couple modules has been extended to 35,105 hours and 32,403 hours, respectively. As with the Q1-A generator, the performance of these modules remains steady and smooth. Comparisons between computer predictions and the actual performance of the RTGs aboard the Voyager I and II spacecrafts has been extended to approximately four years. The agreement with prediction, in both cases, is within 0.5 percent. The corresponding comparisons for the LES-8 and LES-9 RTGs are for over five years with the same excellent agreement