Development of electrically insulating coatings for service in a lithium environment

Several experiments were conducted to develop electrically insulating CaO coatings on a V-4Cr-4Ti alloy for application in a Li environment. The coatings were developed by vapor-phase transport external to Li, and also in situ in a Li-Ca environment at elevated temperature. In the vapor-phase study, several geometrical arrangements were examined to obtain a uniform coating of Ca on the specimens, which were typically coupons measuring 5 to 10 x 5 x 1 mm3. After Ca deposition from the vapor phase, the specimens were oxidized in a high-purity argon environment at 600 deg. C to convert the deposited metal into oxide. The specimens exhibited insulating characteristics after this oxidation step. Several promising coated specimens were then exposed to high-purity Li at 500 deg. C for 48-68 h to determine coating integrity. Microstructural characteristics of the coatings were evaluated by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis. Electrical resistances of the coatings were measured by a two-probe method between room temperature and 700 deg. C before and after exposure to Li