Ronald E. Enstrom

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

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 Jc 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 Jc. We have now established that the less stable C2H6 and C3H8 are as effective as CO2, and that BCl3 increases Jc to a lesser extent. However, H2S, NH3, and NO decrease Jc compared with an undoped Nb3Sn sample. By optimizing the CO2 concentration in the gas phase, the Jc 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 ...

Preparation and high‐field superconducting properties of vapor‐deposited Nb3Sn alloys

Nb3Sn alloyed with Si, Ti, V, Ta, Bi, or Mo has been grown from the vapor phase on stainless‐steel (Hastelloy) ribbon substrates. Compared to a value of 185 kOe observed for unalloyed Nb3Sn, 1.3% Si increased Hc2 to 225 kOe, while 0.002% Bi or 0.05% V raised Hc2 to about 215 kOe. Whereas 0.0006% Ti or 3.7% Ta did not affect Hc2 appreciably, 0.4% Mo decreased Hc2 to 140 kOe. Diffusion of Ni from the stainless‐steel substrate into the Nb3Sn layer appears to be responsible for the observed Hc2 value of 196 kOe for the unalloyed (but carbon doped) ribbon since the same Nb3Sn layer on a Nb substrate has an Hc2 value of 225 kOe. The effect of the Ni outdiffusion on Hc2 can be reduced by reducing the Nb3Sn growth temperature or by increasing the Nb3Sn layer thickness. Hc2 does not appear to correlate directly with Tc since Tc is about 15.5°K for both the Si‐ and Mo‐alloyed Nb3Sn, as well as for the unalloyed Nb3Sn deposited on a Nb substrate, while Hc2 ranges from 140 to 225 kOe. The intermediate phase region be...

The effect of lattice parameter mismatch in NEA GaAs photocathodes grown on GaP/InxGa1−xP substrates

Transmission‐mode negative electron affinity (NEA) GaAs photocathodes were investigated. In order to assess quantitatively the effect of lattice‐parameter mismatch between the cathode and substrate, several 1‐μm‐thick GaAs photocathode layers were vapor deposited onto GaP/InxGa1−xP composite substrates with x ranging from O to about 0.5; the corresponding lattice mismatch varied from 0.2 A to approximately zero. Photoluminescence and ir free‐carrier absorption measurements of the Zn doping concentration in the 1‐μm‐thick GaAs layers revealed that the doping concentration was about 3–6 times greater than that obtained on 15‐μm‐thick GaAs layers grown under identical Zn partial pressure conditions. Photoemission measurements showed that the transmission‐mode quantum efficiency at a wavelength of 0.7 μm increased from about 0.045 electron/photon for GaAs on GaP to 0.13 electron/photon for GaAs on In0.50Ga0.50P as the lattice‐parameter mismatch decreased to approximately zero. Correspondingly, transmission‐mo...

Superconducting Properties of Nb6Sn5 and of Multiphase Nb–Sn Alloys

The critical temperature of Nb6Sn5 is less than 2.8°K and the maximum critical field is less than 600 G at 2.1°K. Since NbSn2 also has a transition temperature below 4.2°K, Nb3Sn is the only compound in the Nb—Sn system that is superconducting at 4.2°K. Nb3Sn can be formed at temperatures as low as 600°C, and accounts for the observed high‐field superconductivity in multiphase alloys where Nb6Sn5 or NbSn2 may be the predominant phases present.