CVD processing of carbon fiber superconductors

NbC1-yNysuperconductors are possible candidates for high magnetic field applications because of the high Bc2> 25 T. The deposition of NbC1-yNyon carbon fiber bundles with more than 1000 fibers using a two stage chemical vapor deposition process: NbCl5+ 5/2 H2350 - 1000 ° C, N2Nb + 5 HCl; Nb + (1-y) CH4+ yNH31100 °C NbC1-yNyhas been investigated. The deposition process is carried out at total pressures 0.1 mbar < Ptot< 1000 mbar and is supported by a rf-gas discharge and an ultrasonic field. The influence of the deposition parameters on the grain structure and on the superconducting properties are investigated. It is found that at low pressures the grain size decreases with increasing Nb deposition temperature in contrast to earlier experiments at atmospheric pressure. The following high temperature carbonitriding process causes a slight coarsening of the grains (< \sim 10%). The main parameters, which influence the microstructure are discussed. The superconducting properties of NbC1-yNydeposited without the gas discharge are measured. The critical temperature Tcwas in the range of 15 K < Tc< 17 K. The following maximum critical currents were measured at 4.2 K: jc(O T) = 10⁶Acm^-2jc(13 T) = 10⁴Acm^-2. The extrapolated maximum critical magnetic field was Bc2= 21 T at 4.2 K.