The metal Al is lighter in weight than other substrate materials for MgB2 films such as Si and Ni. This property inspires MgB2 fabrication on a large-scale Al substrate as a new route to MgB2 tapes. Here we report microstructural factors influencing critical current density, Jc, in MgB2/Al films. MgB2/Al films were prepared by the following steps: deposit a boron layer of 3 nm in thickness on an Al substrate heated at 280°C ; deposit Mg and boron on the boron layer (sample A). For comparison, Mg and boron were deposited directly on an Al substrate heated at 265°C (sample B). The microstructure was observed by transmission electron microscopy (TEM) and scanning TEM. Jc values at 20 K in the self-field were 4.9 × 106 A cm-2 for sample A and 2.7 × 106 A cm-2 for sample B. Both the samples form an oxygen-rich layer of 10 nm in thickness at the substrate surface. This oxygen-rich layer may suppress Al diffusion into MgB2 lattices. The  texture of columnar MgB2 crystals grown on the substrate is stronger in sample A than in sample B. This indicates that the boron layer deposition on the Al substrate is effective for fabricating well-textured MgB2 polycrystals, resulting in the higher J c enhancement for sample A.
- Electron beam evaporation method
- MgB film
- transmission electron microscopy