The microstructural evolution of a crystalline MgB2 layer was examined during ex situ annealing in evaporated amorphous and Mg vapor. It was found that a polycrystalline MgB2 layer started to form at the surface, via a nucleation and growth process. It appeared that there existed distinct growth fronts, on each side of the MgB2 layer. Consequently, further growth of the MgB2 layer proceeded by both out-diffusion of B through the MgB2 layer and by in-diffusion of Mg through the layer. The microstructural evolution of this layer exhibited significant differences; depending upon the location of these 2 growth fronts. In addition, microstructural variation of ex situ annealed MgB2 layers was also observed as a function of the composition of the as-deposited film. When the composition of the as-deposited film was close to MgB2, the samples exhibited the least surface roughness and best superconducting properties.

Microstructural Evolution of MgB2 Layers Prepared by Post Annealing of Evaporated Boron and Co-Evaporated Films. K.B.Kim, H.M.Kim, S.S.Yim, S.H.Moon, Y.W.Kim: Materials Science Forum, 2005, 475-479, 3765-70