Preparation, Microstructure, and High-Field Superconducting Properties of Nb3 Sn Doped with Group-III, -IV, -V, and -VI Elements

Abstract

The influence of incorporating elements from groups III, IV, V, and VI into vapor‐deposited Nb₃Sn superconducting ribbon on the critical current density in magnetic fields up to 190 kOe has been examined, and the mechanism of the enhancement of J_c by carbon‐containing gases has been investigated. It had been found previously that CO₂, CO, and N₂, but not O₂ or CH₄ are effective in increasing J_c. We have now established that the less stable C₂H₆ and C₃H₈ are as effective as CO₂, and that BCl₃ increases J_c to a lesser extent. However, H₂S, NH₃, and NO decrease J_c compared with an undoped Nb₃Sn sample. By optimizing the CO₂ concentration in the gas phase, the J_c increase can be as much as a factor of 5; this optimum concentration corresponds to about 2000 ppm carbon in the Nb₃Sn vapor‐grown layer, as determined by radioactive tracer and mass spectrographic analyses. Since the solid solubility limit of carbon in Nb₃Sn is less than 200 ppm, carbon or carbide in excess of this amount is distributed as a second phase probably along subgrain or grain boundaries where it can effectively pin flux lines, and thereby increase J_c. From extrapolations of $J_c^{\text{1/2}}\hspace{0.17em}\mathrm{vs}\mathrm{\hspace{0.17em}H}$ , the upper critical fields at 4.2 °K for undoped and BCl₃‐ and C₂H₆‐doped Nb₃Sn on stainless‐steel substrates are 184, 194, and 196 kOe, respectively, while for CO₂‐doped Nb₃Sn on a Nb substrate it is 225 kOe. α = J_cH values ranging from 22 to 32×10⁶ kOeA/cm² were attained for C₃H₈‐, C₂H₆‐, and CO₂‐doped Nb₃Sn on stainless‐steel samples and as high as 42×10⁶ kOeA/cm² for CO₂‐doped Nb₃Sn on Nb samples. For high‐ and medium‐α materials, α is a function of the applied magnetic field; α/α_max vs H/H_c2 is an approximately parabolic function with a maximum value at H/H_c2 = 0.4 for high‐α materials and H/H_c2 = 0.6 for low‐α materials. The latter value is in general agreement with theoretical predictions.