Microstructure and internal strain control in NbN on carbon fibers

Magnetron sputtering technique has been used for the NbN formation on moving carbon fiber bundles. For an NbN film thickness of about 0.5 μm, the mean crystallite size could be controlled from 25 nm to 10 nm by an applied bias. Without bias, the NbN coatings reveal tensile strains due to the lower thermal expansion of the carbon fibers used. This strain can be reversed to a compressive strain as function of bias. The superconducting Tcvalues are about 16 K, the critical current density values are about 10⁵A/cm²at 4.2 K and 13 tesla. Bc2values were extrapolated to 35 tesla. Some conclusions for this alternative conductor concept are given.