REVIEW OF RECENT DEVELOPMENTS IN LITHIUM METAL-BEARING ALLOYS AND COMPOSITES FOR IN-VESSEL FUSION APPLICATIONS

An extensive program has been underway over the last several years to develop lithium metal-bearing alloys and composites as a means of reducing plasma impurities and sputter erosion of fusion first wall and limiter/divertor surfaces. One major issue that has slowed the introduction of the lithium metal bearing alloys/composites into the experimental tokamak environment is the result of the deleterious oxidation effects upon these materials. The uptake of oxygen may be responsible for the observed increases in the lithium vapor pressure, the reduction of the secondary-ion fraction, and the inability to sustain a lithium overlayer for long-term sputter-erosion periods. However, these problems have been overcome by optimizing the process parameters of the starting lithium metal-bearing alloys and/or by the alloying addition of dilute concentrations of a getter and/or barrier metal in the host metal matrix. Incomplete Cu-Li alloying, second-phase alloys and/or oxides, and the inclusion of lithium interstitials may be responsible for the oxygen uptake in lithium metal-bearing Cu alloys. Beryllium and titanium oxygen getters have been added to Cu-Li alloys and to W-(LiAl) composites, respectively, resulting in lithium residing in the metal-state as a uniform and continuous overlayer when subjected to elevated temperature and/or sputter-erosion conditions. Without the getter metal additions, lithium tends to reside as an oxide or a mixed metal/metal-oxide on the respective Cu-Li and W-(LiAl) surfaces. A small alloying addition of Ag (< 1 at%) into Cu-Li has been shown apparently either to reduce the oxygen uptake at the surface or to act as a barrier to bulk oxygen diffusing to the surface, thus allowing the formation of lithium-metal overlayers without any detectable oxygen.