Temperature limits on the compatibility of insulating ceramics in lithium

CaO and AIN are candidates for electrically insulating coatings in a lithium-cooled fusion reactor. Bulk specimens of AIN + 0.04 wt%Y and single-crystal CaO have been exposed to lithium in 1000 h isothermal capsule tests at 500-800 degreesC to determine the maximum temperature at which acceptable compatibility is likely. A large increase in the amount of mass loss of AIN was observed between. 600 and 700 degreesC. At 700 degreesC the amount of dissolution was reduced when a Mo capsule was used instead of a V alloy test capsule. High mass losses for single-crystal specimens of CaO were observed after exposure at 600 degreesC. In this case, changing to a Mo test capsule or adding Ca or 0 to the lithium did not consistently show a beneficial effect. At 700 degreesC, neither doping the Li with Ca or O significantly altered the high mass losses. These results suggest that CaO may be limited to exposure temperatures of less than 600 degreesC but that AIN may be able to operate above 600 degreesC. Because some designs call for operating temperatures of 750 degreesC other compositions, such as Er2O3 and Y2O3, also are being evaluated. Preliminary results show promise for these oxides after exposure at 800 degreesC. (C) 2002 Elsevier Science B.V. All rights reserved.