Preparation and degradation behaviour of Bi-cuprate superconductors

The role of preparation conditions and the effect of addition of Pb in Bi-Sr-Ca-Cu-O (BSCCO) superconductor, on theT _c,J _c and grain orientation have been studied. Calcination at a temperature higher than the melting point of Bi₂O₃ after the prereaction at 800°C leads to formation ofc-axis oriented nearly single phase material. The presence of lead yields a nearly highT _c phase (2223) exhibiting a maximum transition temperatureT _c=110 K and ΔT _c=2 K. The addition of lead and grain orientation together result in an increase inJ _c by nearly two orders of magnitude. The degradation behaviour was studied in terms of changes inT _c,J _c and structure on exposure to atmosphere up to 250 days. Samples with 0.6 Pb were found to be superconducting withT _c=97 K even after 250 days. The highT _c phase was stable against degradation. Our studies indicate that the degradation of leaded BSCCO is a surface phenomenon rather than bulk phenomenon.     

Effect of precursor history on synthesis of high-T c BPSCCO superconductor

High-T _c lead-doped BSCCO superconductors (BPSCCO) were prepared from precursors of three different histories. The method, which combined three kinds of calcined powders, decreased the total sintering time for the formation of high-T _c BPSCCO superconductor phases relative to other methods based on one type of powder. The proportion of secondary phase, such as Ca₂2CuO₃ was also reduced. As a result, the transition temperature,T _c, from the current method is higher than for the latter fabrication methods. Thec-axis parameters of BSCCO superconductors from different precursors were compared. The average X-ray particle size of each phase in the bulk sample was calculated from the Scherrer formula. Finally, the surface morphology and composition of BPSCCO were examined by SEM and energy dispersive spectrometry.     

Experimental investigation on constitutive behaviour of superconducting powder BSCCO

As a superconducting material, Bi 2233/Ag tape needs high-critical transport current density J _c, which is influenced by the uniform deformation and density of BSCCO powder in filaments during the forming process. The aim of this paper is to investigate the constitutive behaviour of BSCCO powder. The modified Drucker–Prager/Cap model is introduced to describe the constitutive behaviour of BSCCO powder. A series of cyclic loading experiments for BSCCO powder in a cylinder die were carried out. Based on the experiments, the relationships between the radial stress and the axial stress were obtained, and the parameters in the constitutive model were calculated. By pushing the compact powder from the die, the coefficient of friction between the BSCCO powder and the cylinder die was determined. Finally, the modified Drucker–Prager/Cap model is proposed and used to simulate the confined compression test of BSCCO powder.     

Continuous Deposition of YBCO Films for Coated Conductor Using PLD Method

A reel-to-reel PLD system was set up for studying YBCO coated conductor. YBCO films could be continuously grown on the CeO₂/YSZ/Y₂O₃ buffered Ni-5W tape. Some deposition parameters were investigated. XRD θ–2θ scans were employed to characterize the c-axis orientation and in-plane texture of YBCO films deposited with different laser repetition rates and tape moving speeds. We investigated the dependence of critical current I _c on laser repetition rates and tape moving speeds for YBCO films. It had been found that a-axis oriented grains appeared as YBCO layer thickness increased, which prevented the values of I _c improved.     

Fabrication and properties of BiSrCaCuO superconductors

Fabrication processes for increasing the critical current densityJ _c in BiSrCaCuO superconductor have been studied. Calcining mixtures of raw material in a reduced oxygen partial pressure or in vacuum after in-air calcination is found to be effective. Intermediate cold work (ICW) i.e. uniaxial pressing, cold flat rolling, etc. incorporated in the sintering Agsheathed tape is also effective for raisingJ _c. From a preliminary experiment on Ag-added tape sample, a combination of both processes appeared to be most effective. The highestJ _c obtained is 4.37×10⁴ A/cm² at 4.2 K and 12 Tesla. Effects of additional element Ag etc. onJ _c have also been described.     

Fabrication of Bi2Sr2CaCu2O x /Ag superconducting tape using a screen-printing method

High- T _c superconducting Bi₂Sr₂CaCu₂O _x /Ag tape was successfully fabricated using a screen-printing method and a partial-melting process. A highly oriented layer structure was achieved and confirmed by X-ray diffraction (XRD), pole-figure measurement and scanning electron microscopy (SEM). The critical current density, J _c, of the tape at 77 K, 0 T, was about 20000 A cm^–2. High-temperature XRD was used to clarify the mechanism of the grain alignment. An aligned structure of the 2212 phase was originated from Bi-free compounds, such as (Sr,Ca)CuO₂ and (Sr,Ca)₂CuO₃ developed during a holding stage at 875°C.     

Theory of the Phase-Sensitive Bi2Sr2CaCu2O8+δc-Axis Twist Josephson Junction Experiments

We studied theoretically the phase-sensitive c-axis Bi₂Sr₂CaCu₂O₈₊ (BSCCO) bicrystal twist Josephson junction experiments of Li et al. (Phys. Rev. Lett. 83, 4160 (1999)), calculating the critical current as a function of the twist angle in a wide variety of models. The data provide strong evidence that the c-axis tunneling in BSCCO is incoherent, and that the order parameter is s-wave for T T _c.     

Microwave Properties of In Situ Grown MgB2 Superconducting Thin Films

The electrodynamic response at 20 GHz of c-axis oriented MgB₂ superconducting thin films is reported. Mg-rich Mg-B precursor samples were grown on MgO and Al₂O₃ single crystal substrates by a d.c. planar magnetron sputtering technique, and subsequently annealed in situ at 800°C for 10 min in a In-sealed Nb box in the presence of saturated Mg vapor. The films were characterized by a variety of structural and electronic techniques including XRD, EDS, STM-AFM analyses, and transport measurements. The dependence of the surface impedance from temperature and radiofrequency (r.f.) field amplitude was measured via a dielectric resonator technique. Temperature data clearly confirm the s-wave nature of the newly discovered superconductor, even if the value of the energy gap is smaller than BCS prediction. An effective two-band model can be applied to quantitatively explain the experimental results. In spite of previous reports claiming the absence of weak link behavior in MgB₂, the power dependence show that granularity governs the performance of these films in the microwave region.     

Effect of Various Intermediate Deformation Techniques on Jc and Grain Texture for Ag/Bi-2223 Tapes

Three Ag sheathed Bi-2223 multifilamentary tapes were produced by a processing method that consists of two sintering treatments with an intermediate deformation, involving sandwich rolling (SR), pressing (P), or normal rolling (NR). The magnetic field dependence of the critical current density J _c was measured with the magnetic field H applied parallel to both the ab plane (H ab) and the c-axis (H c) of the Bi-2223 grains. Experimental results show that J _c of the pressed (P) tape (J _cP) for both H ab and H c is about 1.5–1.8 times higher than that for the NR tape (J _cNR) and the SR tape, although J _cSR is always larger than J _cP. The ratio of J _cSR/J _cNR for H c increases rapidly with the applied magnetic field and reaches a maximum of about 12 at ₀ H 900 T. The calculated density of the pinning force F as a function of magnetic field shows that curves of F for SR, NR, and P tapes all have their maximum F _max at different magnetic fields and the magnitudes of F _max are also different from each other. The SR tape has the largest value of F _max, while NR has the smallest. XRD analysis shows that an intermediate deformation can destroy the grain alignment, and the larger the deformation, the worse the grain texture will be. Our experimental results, however, clearly show that J _c for Bi-2223 multifilamentary tapes is independent of grain alignment. The significant differences in J _c for tapes processed using the three different intermediate deformation procedures are dependent on the density of the pinning force and cannot be attributed to the grain alignment. Our experimental results support the view that SR processing is the best method for fabricating Ag/Bi-2223 tapes of high quality.     

Theory of the Phase-Sensitive Bi2Sr2CaCu2O8+δ c-Axis Twist Josephson Junction Experiments

We studied theoretically the phase-sensitive c-axis Bi₂Sr₂CaCu₂O_8+δ (BSCCO) bicrystal twist Josephson junction experiments of Li et al. (Phys. Rev. Lett. 83, 4160 (1999)), calculating the critical current as a function of the twist angle in a wide variety of models. The data provide strong evidence that the c-axis tunneling in BSCCO is incoherent, and that the order parameter is s-wave for T ≤ T _c.     

Microstructure and electrical properties of YBCO thin films

Microstructures of c-axis oriented YBCO thin films made by high-pressure d.c. sputtering on LaAlO₃ and MgO substrates were examined by TEM. The a-axis oriented grains, second phases and micro-twins were frequently observed in the film. The a-axis oriented grains expanded along their c-axis directions during film growth. The a- and b-axis misorientations were observed in the film on MgO due to serious lattice mis-match between YBCO and MgO. The second phases were often accompanied with a-axis oriented grains suggesting they act as nuclei. These observed results were correlated with the measured T _c and J _c of the films.     

Morphology of growth of Bi2Sr2CaCu2O8 single crystals

A good correlation of twins on the basal surface of flux-grown Bi₂Sr₂CaCu₂O_x (BSCCO) single crystals with surface. growth steps is observed, the b-axis being perpendicular to the steps and, thus, parallel to the growth direction. It is found that mono-twin BSCCO single crystals produced by the travelling solvent floating zone method also grow preferentially along b, i.e. nearly perpendicularly to the boule axis, contrary to the common belief. This new understanding of the morphology of growth explains the nature of major defects in these crystals, which considerably change their measured superconducting properties, in a different way.     

The Angular Dependence of the Critical Current in UPt3 and the Symmetry of the Order Parameter

The superconducting critical current along the c-axis of the unconventional superconductor UPt₃ has been determined as a function of magnetic field direction in the basal (a-a*) plane in the three (A, B and C) phases of the mixed state. The critical current has maxima for field directions parallel to both the a- and a*-axis in the superconducting C phase, but has maxima only parallel to the a-axis in the A and B phases. Our experiments thus show that the anisotropy of the superconducting critical current in UPt₃ changes at the same field at which the symmetry of the order parameter changes. This demonstrates for the first time that the critical current is sensitive to the symmetry of the superconducting order parameter.     

Grain alignment and magnetization anisotropy of Nd1.85Ce0.15CuO4

Procedures have been developed to make Nd_1.85Ce_0.15CuO₄ powders which have thec-axis of all the grains aligned in one direction. By rotating a powder sample mixed with liquid epoxy about an axis perpendicular to a strong applied magnetic field, thec-axis of the grains aligns with the axis of rotation. The procedure works for any material having the maximum room-temperature susceptibility perpendicular to thec-axis. Using this technique, we measured the magnetic anisotropy of this electron-carrier high-temperature superconductor.     

Intrinsic critical current of Ag-clad (Bi,Pb)2Sr2Ca2Cu3Oz tapes

True zero-field critical current densityJ _c of a well-characterized BPSCCO/Ag tape has been determined by means of high-resolution ac susceptibility in the temperature range 77–110 K. The resultant values (30,000 A/cm² at 77 K) agree well with the transportJ _c of the same tape. Because of a very thin BPSCCO, the coreJ _c determined from the imaginary part of the ac susceptibility is nearly the same as the zero field one. AllJ _c 's follow the same (1-T/T _c )ⁿ withn=1.45 dependence.J _c shows an approximateH ^–0.5 field dependence over the explored temperature range. Accordingly, the variations ofJ _c withT andH seem to be determined by the flux creep.     

Model for the a.c. hall effect of high-Tcsuperconductors

The sign of the Cooper pairs in the superconducting state of the currently found high-T _c superconductors can be best measured by utilizing the so-called a.c. Hall effect. In this experiment a d.c. magnetic field is applied normal to the surface of the superconductor and a planar electromagnetic wave perpendicularly polarized incident upon the sample surface. By measuring the reflected electric field polarized in the incident plane, one could determine the sign as well as the magnitude of the a.c. Hall coefficient. The measurement provides a direct means to determine the sign and density of the Cooper pairs in the superconducting state of the high-T _c superconducting sample.     

Tunneling and Phase-Sensitive Studies of the Pairing Symmetry in Sr2RuO4

Results obtained from several tunneling experiments on the pairing symmetry in Sr₂RuO₄ are reported. A normal surface layer was found to be present on the ab face of single-crystalline Sr₂RuO₄ in the c-axis tunnel junctions. Measurements in the in-plane junctions, however, revealed features suggesting a suppressed, but non-zero superconducting gap and Andreev bound states (ABSs) associated only with an unconventional superconductor. Josephson coupling between a conventional s-wave superconductor and Sr₂RuO₄ has been used to probe the pairing symmetry in the latter, through both the selection rule and the phase sensitive experiments. All results indicate that Sr₂RuO₄ is unconventional, most likely spin-triplet superconductor. Finally, some recent studies of several newly-characterized ruthenates are reviewed.     

Microwave Properties of In Situ Grown MgB2 Superconducting Thin Films

The electrodynamic response at 20 GHz of c-axis oriented MgB₂ superconducting thin films is reported. Mg-rich Mg-B precursor samples were grown on MgO and Al₂O₃ single crystal substrates by a d.c. planar magnetron sputtering technique, and subsequently annealed in situ at 800°C for 10 min in a In-sealed Nb box in the presence of saturated Mg vapor. The films were characterized by a variety of structural and electronic techniques including XRD, EDS, STM-AFM analyses, and transport measurements. The dependence of the surface impedance from temperature and radiofrequency (r.f.) field amplitude was measured via a dielectric resonator technique. Temperature data clearly confirm the s-wave nature of the newly discovered superconductor, even if the value of the energy gap is smaller than BCS prediction. An effective two-band model can be applied to quantitatively explain the experimental results. In spite of previous reports claiming the absence of weak link behavior in MgB₂, the power dependence show that granularity governs the performance of these films in the microwave region.     

Phase and Texture Evolution of Ag/Ni Composite-Sheathed Bi-2223 Tapes with Different Thickness

The influence of green tape thickness on the Bi-2223 phase formation and texture evolution in Ag/Ni composite-sheathed tapes fabricated by the “powder-in-tube” technique has been studied. Microstructural observations by SEM as well as critical current density (J _c) measurements at 77 K, 0 T have been performed to analyze the performance of the tapes. The results show an important influence of the green tape thickness on the critical current depending on the content and texture of Bi-2223 phase. The J _c increases with decreasing thickness. Moreover, texture measured by omega scans shows that the texture of the Bi-2223 phase is significantly influenced by the thickness of the green tape after the first and final sintering processes. Alignment of Bi-2223 grains in the thin tapes is much better. Higher performance of Ag/Ni composite-sheathed Bi-2223 tapes can be obtained by controlling the thickness of the green tapes.     

Effect of Different Nanosized NiO Addition on Ag-Sheathed (Bi1.6Pb0.4)Sr2Ca2Cu3O10 Superconductor Tapes

(Bi_1.6Pb_0.4)Sr₂Ca₂Cu₃O₁₀–(NiO) _x superconductor with x=0, 0.01, 0.02, 0.03, 0.04, and 0.05 wt.% was prepared using the coprecipitation technique. The average size of NiO used was 8 nm and 16 nm. The highest value of the transport critical current density, J _c in the bulk form was observed in the x=0.01 wt.% samples for both sizes. Based on this result, Ag sheathed superconductor tapes with starting composition (Bi_1.6Pb_0.4)Sr₂Ca₂Cu₃O₁₀–(NiO)_0.01 (with NiO particle size 8 and 16 nm) were fabricated using the powder-in-tube method. The effect of these nanoparticles addition on the microstructure, phase formation, and transport critical current density were studied. The J _c of the tape at 77 K in zero fields with NiO (8 nm) addition (4500 A/cm²) was slightly higher than that of NiO (16 nm) added tape (4120 A/cm²). The nonadded tape showed a lower J _c (1080 A/cm² at 77 K). These results indicated that magnetic nanoparticles such as NiO could act as an effective flux pinning centers leading to the enhancement of J _c in the bulk and tape form. Moreover, NiO with size closer to the coherence length, ξ was more effective in enhancing J _c.