Systematics and Optimisation of Hole Doping in Bi-2223

We report systematic changes in thermopower, T _c , Raman modes and magnetic irreversibility H _irr as a function of carrier hole concentration p in Bi-2223. We have established that Raman spectroscopy can provide an indicator of doping level: the 630cm ^–1 mode is found to soften with increasing p. A departure of the thermopower from the universal thermopower-doping relationship displayed by other HTS compounds is explained by a non-uniform charge distribution over the three CuO ₂ planes. Scaling of H _irr shows the optimal hole concentration for maximum J _c lies beyond currently achieved levels.     

Instabilities above critical current region in Bi-2223/Ag superconducting coils cooled by liquid nitrogen

A sudden drop of the coil voltage and a hysteresis of I-V curve were observed in measurement of one-layer Bi-2223/Ag coils cooled by liquid nitrogen at currents well above critical current region. Their temporal behavior indicates, that the improvement of the cooling and corresponding decrease of temperature after the jump takes place. To study this phenomenon we measured I-V curves of two Bi-2223/Ag coils made from tapes with various degree of critical current homogeneity and analogical curves of two non-superconducting coils made from thin Cu tapes having various widths. In Cu coils we really observed a sudden drop of the temperature, measured in parallel with Cu resistance drop, after reaching heat flux of about 0.4 W cm⁻² during current ramping up. In spite of non-superconducting character of the tape, the hysteresis, i.e. difference between increasing branch and decreasing branch of I-V curves, was observed too! Approximately the same value of heat flux, at current corresponding to the jump, was found also in superconducting coil on segment with least value of local critical current. We conclude that observed voltage drop of the Bi-2223/Ag does not bear upon superconducting nature of the coil and, as that for Cu coil, can be explained by dynamics of heat transfer to liquid nitrogen and its history.     

Magnetic field hysteresis of critical currents in Bi(2223)/Ag tapes and a way to overcome it

Experiments showing hysteresis of critical currents versus the external magnetic field I_c(B_e) were performed with two multifilamentary Bi(2223)/Ag tapes. The I_c(B_e) hysteresis is observable in the transversal as well as in the longitudinal orientation of the long axis of the tape with respect to the magnetic field. Based on the idea that the hysteresis is the effect of trapped flux in a network of well-connected current paths, a way to overcome this effect has been proposed and experimentally verified. The induced frozen-in screening currents are split into several parallel current patterns by cycling the external magnetic field around the adjusted value. Using the proposed method, the ‘neutral’ I_c(B_e) characteristics have been found. Approximate calculations of the penetration depth of the trapped flux show that the network of well-connected current paths could be formed by several disk-shaped grains (≈ Φ8 × 0.4 μ) stacked into more or less axially ordered (quasi cylindrical) colonies of average dimensions estimated to ≈ Φ8 × 4 μm.     

The giant dispersion of critical currents in a superconductor with fractal clusters of a normal phase

The influence of fractal clusters of a normal phase on the dynamics of a magnetic flux trapped in a percolation superconductor is considered. The critical depinning current distribution and the current-voltage characteristics of fractal superconducting structures in the resistive state are determined for an arbitrary fractal dimension of the cluster boundaries. The interval of fractal dimensions is found in which the dispersion of critical currents exhibits unlimited growth. It is established that the fractality of clusters favors a decrease of the electric field arising during the magnetic flux motion, thus increasing the critical current value. The region featuring the giant dispersion of critical currents can be expected to provide for the maximum current-carrying capacity of a superconductor.     

Acid-base equilibrium—A thermodynamic study of formation and stability of the Bi-2223 phase

A general acid-base equilibrium theory was proposed to explain the formation and stability of the Bi-2223 phase based on the Lewis acid-base theory, and principle of metallurgical physical chemistry. The acid-base nature of oxide was defined according to the electrostatic force between cation and oxygen anion. A series of experimental facts were systematically explained based on the theory: substitution of Bi for Ca in. the Pb-free 2223 phase, and the effect of substitution of the high-valent cation for Bi³⁺; oxygen-pressure atmosphere, and the heat-schocking technique on the formation and stability of the 2223 phase.     

Metallic materials for superconductor stabilization with very high specific heat and good thermal conductivity

In a search for materials useful for enthalpy stabilization of technical superconductors we have performed specific-heat measurements on PrCu₂, PrB₆, NdSn₃ and Nd_0.9Pr_0.1Sn₃ between 3 and 10 K and in magnetic fields up to 7 T. Due to different types of phase transitions these compounds exhibit large peaks in the heat-capacity curves. In these materials the high specific heat values are not substantially reduced the the external magnetic fields, which makes them attractive for technical application.     

Thermal conductivity of a Kondo superconductor in the gapless state

The thermal conductivity coefficient for a Kondo superconductor in the gapless state is obtained within the framework of the Matsuura, Ichinose, and Nagaoka theory. By expanding the heat current correlation function appearing in the Kubo formula for the thermal conductivity in powers of the order parameter, analytical expressions for the thermal conductivity to second power in the order parameter are obtained. The thermal conductivities for bothT _KT _c 0 andT _KT _c 0 superconductors are obtained.     

Observation of magnetic flux frozen in high conductivity metal during flux jumping in a type II superconductor

In the concentric arrangement of a superconducting tube and a high-conductivity aluminium cylinder, the magnetic flux variation arising from flux jumps occurred only within the superconducting tube itself, whereas the magnetic flux in the normal metal was almost completely frozen during the flux jumps. Provided that the magnetic flux was completely frozen in the normal metal, the energy released by a flux jump was calculated to be equal to a quarter that released by a complete flux jump. It was found that the magnetic diffusion time-constant was much larger than that given by $\text{L}{}^2\text{π}{}^2\text{D}{}_{\mathrm{m}}$.     

Effect of magnetic field, thermal cycling and bending strain on critical current density of multifilamentary Bi-2223/Ag tape and fabrication of a pancake coil

Multifilamentary HTSC tapes are important for their applications in various electrical devices. Powder-in-tube technique with improved optimized synthesis parameters is regarded as one of the most promising ways to prepare long-length multifilamentary Bi-2223/Ag tapes. Nevertheless, usefulness of such tapes depends on their electrical and mechanical properties. Critical current density of a Bi-2223/Ag tape with 37 filaments has been studied at 77 K with field, field orientation, thermal cycling and bending strain as parameters. Results have been discussed in light of various mechanisms and models. A small pancake coil has been fabricated out of the same tape and the test results presented.     

The influence of electroplastic rolling on the mechanical deformation and phase evolution of Bi-2223/Ag tapes

Electroplastic rolling (EPR) of Bi-2223/Ag superconducting wires was performed, where pulse currents were applied during rolling to introduce an electroplastic effect. It was found that the rolling force decreased significantly compared with the traditional rolling process. Furthermore, EPR favorably minimized the sausage effect. It is revealed that the electroplastic effect can facilitate the mechanical deformation of Bi-2223/Ag composites. Segments of the Bi-2223/Ag tapes were heat treated at 830 °C for different time periods. The phase assemblies of these samples suggest that current pulses contribute to faster transformation kinetics from the Bi-2212 phase to the Bi-2223 phase. In addition, a preliminary improvement of 28% of critical current density has been achieved in a fully processed tape with EPR.     

Transverse thermal conductivity of a dirty superconductor in the mixed state

We consider a dirty superconductor in the mixed state with a temperature gradient applied transverse to the magnetic flux. Using microscopic time-dependent superconductivity theory, we obtain formulas for the thermal conductivity of the electrons. The results agree with earlier phenomenological calculations at low temperature. At higher temperatures the theories do not coincide.This work was supported by the Science Research Council (U.K.) and the Kernforschungsanlage Jülich (Germany).On leave from Research Institute of Electrical Communication, Tohoku University, Sendai, Japan.     

Temperature dependence of ac-losses in multifilamentary Bi-2223 tapes between 4.2 K and Tc

The ac-losses of twisted multifilamentary Bi-2223/Ag tapes were measured in the temperature range between 4.2 K and the critical temperature T_c∼110 K. Stacks of tapes in perpendicular and in parallel field were investigated. The loss measurement is performed by the standard magnetisation technique, the absolute loss value is obtained by a calorimetric calibration measurement. For a fixed ac-field amplitude the energy loss per cycle Q is determined, at a given temperature, as a function of frequency f. The extrapolation to f→0 gives the hysteresis loss. Coupling losses are obtained from the slope of the loss curve Q(f) in the low frequency range ωτ?1. The measured total losses as a function of temperature show, at small field amplitudes, a minimum around 50 K. This is explained by the different temperature dependence of hysteresis and coupling losses. While coupling losses decrease with increasing temperature, hysteresis losses increase for field amplitudes below the penetration field. Coupling losses show a much weaker temperature dependence than the silver conductivity, which is explained by the existence of an interfacial resistance between filaments and silver matrix. Measured hysteresis losses were compared with available theoretical models and good agreement was found in parallel field.     

Anisotropy of the thermal conductivity of a dirty superconductor in the mixed state

We present numerical calculations of the anisotropy in the electronic thermal conductivity in the mixed state, and compare the calculations with experiment by estimating the conductivity due to phonons.Work supported by Science Research Council grant No. B/RG/68265.     

Thermal conductivity of ethane in the critical region

A coaxial cylinder method was used to measure the thermal conductivity of ethane in the pressure range from 10 up to 280 bar and in the temperature range from 308 up to 365 K.     

Thermal conductivity of oxygen in the critical region

The thermal conductivity of oxygen has been measured in a broad region around the critical point by means of Rayleigh light scattering. Measurements were made on two isochores and on the saturation boundary. The results are compared with current methods of predicting the anomalous thermal conductivity in the critical region.     

Thermal conductivity of difluoromonochloromethane in the critical region

The thermal conductivity of difluoromonochloromethane (refrigerant R22) has been measured along six near-critical isotherms at reduced temperatures varying from =1.005 to =1.112 and at pressures ranging from 2.0 to 9.5 MPa. An anomalous enhancement of the thermal conductivity has been observed in the critical region. This anomalous behavior is consistent with theoretical predictions and equations for the thermal conductivity as a function of density and temperature are presented.     

Preparation and characterization of silver clad (Bi,Pb)-Sr-Ca-Cu-O 2223 superconducting tapes with high critical current density

Silver clad Bi-2223 tapes with consistently high critical current densities of over 30,000 A/cm² at 77 K and zero field were prepared by powder-in-tube (PIT) technique. Powder XRD, electron microscopy, a.c. susceptibility and critical current measurements were used to study the phase assemblage, microstructure and transport properties of these tapes at various stages of processing. The precursor powder for PIT process was prepared by a sol-gel route by acrylate method using freshly prepared nitrates of Bi, Pb, Sr, Ca and Cu. The carbon content in the powder was minimized by subjecting it under dynamic vacuum calcination followed by heating in free flow of oxygen for long durations with intermittent grindings. The choice of initial stoichiometry, high reactivity of the precursor, effective removal of carbon, choice of phase assemblage at the filling stage and the multistage thermomechanical processing at optimized conditions were found to be responsible for the high critical current density. The work was done under the National Superconductivity Programme funded by the Department of Science and Technology (DST), New Delhi.     

The frequency-dependent conductivity of a dirty superconductor and the role of inelastic scattering

The frequency-dependent conductivity of a dirty superconductor is calculated as a function of temperature by solving a kinetic equation that describes the effect of electron-phonon scattering at frequencies well below the gap frequency. The numerical solutions of the kinetic equation are compared with results based on a relaxation time approximation.