Pinning force in superconducting NbAlGe systems

Results of direct mechanical measurements of the pinning force in NbAlGe superconducting bulk samples are given. The dependence of average measured pinning force on the external magnetic field up to 5 T at 4.2 K was determined experimentally. The results obtained are compared with those determined by critical current density measurements previously performed electrically on the same samples.     

High Tc multifilamentary NbAl wires

The values of the superconducting transition temperature, T_c of multifilamentary Nb8wt%AI wires fabricated by powder metallurgy were measured and found to be in the range 19.2–20.3 K. Samples were heated for a short time at > 2000°C and annealed for 47–185 h at 700–750°C. Their upper critical field, H_c2(T), was also measured resistively. On these curves, an upward curvature below 1 T near T_c is observed. The measured high values of T_c are believed to be due to a high degree of atomic ordering attained by annealing, rather than to a stoichiometric A15 phase resulting from rapid quenching.     

Design and fabrication of SnNbCuTaC composites for multifilamentary superconducting Nb3Sn wires by using the modified tube technique

The factors determining the design and fabrication of Nb₃Sn multifilamentary wires by the tube technique are discussed. New improved methods of obtaining multifilamentary Nb₃Sn wires on the basis of both external diffusion and internal diffusion processes, by using the tube technique in a simpler and less expensive way, are presented.     

Emergence of NbTi as supermagnet material

The discovery and emergence of NbTi as a high field superconductor is reviewed. The prehistory and setting for its discovery are described, and an anecdotal history follows its development up to the first successful large scale applications.     

Phase evolution and gas-phase particle size distributions during spray pyrolysis of (Bi,Pb)SrCaCuO and Ag(Bi,Pb)SrCaCuO powders

Phase evolution, gas-phase particle size distributions and lead loss were studied during formation of (Bi,Pb)SrCaCuO powders and their composites with silver by spray pyrolysis starting from nitrate solutions. The 10 wt% Ag/90 wt% Bi_1.8Pb_0.44Sr₂Ca_2.2Cu₃O_x composite powders made at 700°C consisted of 20–60 nm grains of silver and mixed-oxide phases with a fine dispersion of Ag grains within the particles. At 700°C, the primary phases present in (Bi,Pb)SrCaCuO powders were (Bi,Pb)₂Sr₂CuO_x (2201), Ca₂PbO₄ (plumbate), (Bi,Pb)₂Sr₂CaCu₂O_x(2212), and (Bi,Pb)₃Sr₂Ca₂Cu₁O_x(3221). For T≥800°C, the powders were considerably depleted in lead, and the plumbate and 3221 phases were absent. For T = 900°C, a large number of ultrafine particles (<30 nm) were formed, probably from the PbO vapor released from the reactor walls. Using spray pyrolysis, it is easy to control stoichiometry and limit the phase segregation at the nanometer-scale so that homogeneous and phase-pure materials can be obtained rapidly during subsequent processing.     

Spiral superconducting NbTi filaments: electric field and critical current

As a first step to a better understanding of the voltage—current characteristics of fine filament multifilamentary composite superconductors, the electric field and critical current along a single spiral-shaped filament were studied. When the spiral superconducting filament was carrying constant current and placed in a magnetic field, the local critical current and electric field vary due to the change of the angle between the magnetic field and filament axis. A theory is presented which enables us to calculate the mean and local values of the electric field and critical current of spiral samples for arbitrary orientation between the magnetic field and spiral axis. The presented experimental results are in good agreement with this theory.     

Irregularity in NbTi filament area and electric field versus current characteristics

There is a correlation between irregularity in filament area (‘sausaging’) and the shape of a superconductor's electric field (E) versus current (I) relationship. The shape of the E−I characteristic is quantified in terms of the resistive transition parameter, n, defined by E∝Iⁿ. Low values of n less than about 20 correlate with a wide filament diameter distribution, while n values over 50 correspond to a distribution more than 2.5 times smaller. It is proposed that the low-field (constant) value of n be used as an index of filament quality in evaluating different superconductors for practical applications. A model is also suggested to explain this effect in terms of a locally depressed filament critical current, which forces current to transfer across the normal matrix material into neighbouring filaments. The relationship between n and the statistical distribution of filament diameters may be useful as an easy method of estimating the extent of sausaging in practical multifilamentary NbTi superconductors from measurements of n.     

Influence of CuNi matrix on a.c. losses and voltage-current characteristics of fine filament NbTi superconductors

The results of a.c. loss measurements at 4.2 K in two types of fine filament NbTi superconductors with a CuNi matrix are presented. The conductors were manufactured by Alsthom Atlantique (France) and Showa Electric Wire & Cable (SEWC, Japan). A.c. losses per cycle were determined from magnetization measurements on samples without transport current, subjected to the external magnetic field perpendicular to the conductor axis. Measurements were carried out at two frequencies: 0.1 and 50 Hz. The range of magnetic field amplitudes for Alsthom Atlantique and SEWC samples was 2.5 × 10⁻³ − 3T and 5 × 10⁻² − 2 T, respectively. By measurements at T ≈ T_c, 77 and t~300 K, we proved the existence of hysteretic loss in the CuNi matrix of these conductors. Current-voltage characteristics were also measured. Critical current densities determined from magnetization and current-voltage characteristics were compared. The observed difference is discussed.