Liquid Li is a strong reducing agent, and thus a coating layer of many common oxides is not stable on the wall material. Therefore, intentional coating with insulator ceramics, which are stable in liquid Li, is necessary. Figure 2 shows free energy for oxidation and nitridation for various elements. From the

thermodynamic viewpoint, CaO, Y2O3 and Er2O3, and AlN are expected to be stable. (Note that the resistivity of TiN is not sufficient for the coating.) The instability, however, is a function of the impurity level of Li.10

Early development efforts focused on CaO and AlN by in situ formation during exposure to chemi­cally controlled lithium.