KINETICALLY CONTROLLED LITHIATION - A VARIANT OF PHYSICAL VAPOR-DEPOSITION WITH APPLICATION TO LIGHTWEIGHT ALLOYS AND LITHIUM BATTERIES

Employing a distinct variation of the physical vapour deposition techn ique, lithium vapour has been used to form Mg-Li alloy films whose phy sical structure can be modified through substrate temperature control over a considerable composition range. In addition to these Mg-Li allo y films, alloys of aluminium and copper can be prepared and modified p rimarily by controlling the interaction of lithium vapour with the pre cursor metal in the form of a cast sheet ranging in thickness from 0.0 2 to 0.07 in. A lithium-mediated process is found to produce a signifi cant vaporization enhancement from the surface of the magnesium sheet at temperatures close to 200 degrees C below that required for vaporiz ation in the absence of lithium. The interaction process not only prom otes the vaporization of the magnesium but also leads to an intimate m ixing of magnesium and lithium vapours. The lithium and magnesium cont ents of the formed vapours have been varied to produce alloy films of between 0.08 and 30wt%Li. As the vapour mixture is subsequently deposi ted onto a temperature-controlled substrate, the physical make-up of t he films produced is modified through temperature variation. With the lowering of the substrate temperature, the microstructure of the depos ited film transforms from the cubic crystalline structure characterist ic of a phase-equilibrated Mg-Li alloy with greater than 26wt%Li to a densely packed fibrous columnar microstructure,and, on further cooling , to a tapered columnar microstructure with extensive voids. This latt er structure may prove useful in the development of higher-efficiency lithium batteries. A cast aluminium sheet can be modified to an Al-Li alloy as an impinging lithium vapour creates an excess lithium content at the surface. The excess lithium can be removed or passed further i nto the aluminium employing a solid-state diffusion process, as Al-Li alloys whose lithium content ranges from 0.2 to 5 wt% are prepared. Th e deposition process, which requires the stringent control of the alum inium temperature over an approximately 20 degrees C range, is distinc t in that it can permit the introduction of the reactive element, lith ium, into an alloy near the final stage in the production of a wrought product and might also be used to replace the surface lithium lost fr om an alloy during heat treatment. The techniques described also appea r applicable to alloy formation with additional elements soluble in li thium including copper, zinc and silver.