Temperature limits on the compatibility of insulating ceramics in lithium

CaO and AlN are candidates for electrically insulating coatings in a lithium-cooled fusion reactor. Bulk specimens of AlN+0.04 wt%Y and single-crystal CaO have been exposed to lithium in 1000 h isothermal capsule tests at 500-800 °C to determine the maximum temperature at which acceptable compatibility is likely. A large increase in the amount of mass loss of AlN was observed between 600 and 700 °C. At 700 °C, 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 °C. In this case, changing to a Mo test capsule or adding Ca or O to the lithium did not consistently show a beneficial effect. At 700 °C, 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 °C but that AlN may be able to operate above 600 °C. Because some designs call for operating temperatures of 750 °C, other compositions, such as Er₂O₃ and Y₂O₃, also are being evaluated. Preliminary results show promise for these oxides after exposure at 800 °C.