Showing papers in "Superconductor Science and Technology in 1994"

Progress in melt texturing of YBa2Cu3Ox superconductor

Abstract: Melt processing is the most prominent method utilized in the fabrication of bulk YBa2Cu3Ox(123) because of the superior transport and magnetic properties of this material. Due to the Y-diffusion-controlled growth mechanism of 123, the maximum allowable growth rate is sensitive to the size and distribution of the Y2BaCuO5 precipitates. In addition, a slow cooling rate and a slow growth rate have to be employed to maintain the stability of the 123 planar growth front. Among the many melt-texturing methods and modifications, seeded directional solidification has the potential to produce long single-domained samples of large cross-sectional area with the strongly superconducting a-b planes aligned along the sample axis. Steady progress has also been achieved in flux-pinning enhancement through defect engineering. This includes the introduction and modification of defects such as twins, irradiation damage and second-phase inclusions. In particular, high-temperature mechanical deformation has been shown to be effective in increasing the dislocation density and results in improved Jc as well as reduced Jc anisotropy.

Substrates for high-Tc superconductor microwave integrated circuits

Abstract: This review paper presents a discussion on dielectric substrate materials suitable for the preparation of YBa2Cu3O7-x thin-film based microwave integrated circuits. The requirements on the properties of the substrate materials are specified. They cover the properties crucial both for the preparation of high-quality YBa2Cu3O7-x films and for the design of microwave elements. The former includes mainly the lattice match, the match of thermal expansivities, chemical stability, and absence of twinning. The latter includes the relative dielectric permittivity ( epsilon ) and the related tolerances, the microwave loss tangent, and the substrate area required for the accommodation of a microwave circuit. The properties of the currently available substrates suitable for YBCO film epitaxy are discussed in view of these requirements. The main attention is paid to the microwave properties. Current achievements and potential difficulties of the crystal growth technology in the preparation of the substrates are taken into account as well.

A second-order SQUID gradiometer operating at 77 K

Abstract: We describe a portable second-order electronic SQUID gradiometer cooled with liquid nitrogen and capable of operating in unshielded space. The measured magnetic-field resolution is <300 fT Hz-1/2 and <170 fT Hz-1/2 When operating in the first-order mode. The gradiometer was successfully used for the recording of screening magnetocardiograms of over 200 human subjects at four different geographic locations.

Lorentz-force-independent dissipation in high-temperature superconductors

Abstract: The evolution of resistivity in the mixed state of high-Tc superconductors has been a source of debate in the fundamental problem of whether or not energy dissipation originates from the Lorentz-force-driven flux motion. The conventional flux-creep and flux-flow models are essentially based on this mechanism and have frequently been used to explain the broad resistive transition in magnetic fields in high-Tc superconductors. However, recent intensive experimental studies of high-quality single crystals, such as (La1-xSrx)2CuO4, YBa2Cu3O4, Bi2Sr2CaCu2O8+ delta , etc, have revealed clear evidence that excludes the contribution of the effect of flux motion to the resistivity broadening, indicating that the conventional flux-motion mechanism does not play a major role in dissipation in high-Tc superconductors. Instead, mechanisms that take into account the superconducting fluctuation effect in the superconducting order parameter have recently been proposed and have successfully been applied to analyse experimental results. Possible similarities between the fundamental concepts of superconducting fluctuation models and the Kosterlitz-Thouless transition have also been discussed.

The electric field within high-temperature superconductors: mapping the E-J-B surface

Abstract: At a single temperature, the macroscopic irreversible magnetic behaviour of a superconductor can be described fully by the relationship between the electric field E, the current density J and the magnetic induction B We show that this relationship can be readily visualized as a three-dimensional E-J-B surface The utility of this surface is that it is model and parameter independent, and so obviates the need to define the 'critical current density', and the 'irreversibility' field by arbitrarily imposed criteria Furthermore, the flux creep rate corresponds directly to a gradient of this surface Data from magnetic studies can be used to generate the surface, but some care with experimentation is needed to avoid distortion by artefacts, such as self-field effects and anisotropy Transport studies map the surface too, but in a rather different (and limited) region of E-J-B space The shape of the surface and its dependence on temperature reflect the underlying physics of vortex behaviour, and we discuss briefly the limitations on theoretical models imposed by its form

Magnetic flux decoration of type-II superconductors

Abstract: Recent experimental studies of the flux line lattices in type-II superconductors are reviewed. All the experiments were performed using the Bitter decoration technique which provides means of direct observation of static vortex structures at the surface of the superconductor. Various aspects of vortex behaviour that can be studied in decoration experiments are considered, in particular vortex lattice ordering and the effects of pinning and anisotropy on the equilibrium vortex arrangement. New vortex phases, such as vortex chains and oval vortices, which were recently discovered in high-temperature superconductors, are also discussed.

An integrated DC SQUID magnetometer with variable additional positive feedback

Abstract: An easily adjustable direct-coupled magnetometer is presented. A typical usable voltage swing of 30 mu V and noise levels of down to 1.13 fT Hz-1/2 at 1 kHz have been achieved with additional positive feedback (APF). A compensation circuit integrated into the magnetometer chip allows the reduction of APF gain in three steps within a factor of 1.8, by simply adding one or two bond connections on the magnetometer chip. In this way the tolerance of the critical current I0 is enlarged from +or-10% to +or-27% at the traditional SQUID working point. If additionally the shunt resistors are trimmed by plasma etching the tolerance of I0 can be increased further to about +or-55%.

The growth and superconducting properties of epitaxial Tl2Ba2CaCu2O8 thin films

Abstract: Epitaxial Tl2Ba2CaCu2O8 thin films have been prepared on (001) LaAlO3 substrates by DC magnetron sputtering and a post-annealing process. X-ray diffraction (XRD) theta -2 theta scans, phi scans and rocking curves proved that the thin films are strongly textured with the c axis parallel to the c axis of the substrate and epitaxially grown on the substrate. A critical current density of 7.6*106 A cm-2 at 77 K has been obtained in zero magnetic field, and the films have strong flux pinning force in high magnetic fields. When magnetic fields were applied perpendicular to the Cu-O plane of the film, the critical current density dropped only by a factor of seven in a 4 T magnetic field and a factor of 15 in a 6.5 T magnetic field.

The effect of Ag on the superconductivity of Bi2−xPbxSr2Ca2Cu3Oy superconductors prepared by an optimum thermal procedure

Abstract: The effect of Ag addition (0-100 wt%) to Bi2-xPbxSr2Ca2Cu3Oy (x=0.1-1.0) was systematically studied using differential thermal analysis (DTA), electrical resistivity, change in inductance, scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies. The sample was prepared by the optimum thermal procedure in which the sintering was done at the endothermic peak temperature judging from the DTA curve. It was found that the 110 K phase was produced with good reproducibility by the lowering of the melting temperature due to the addition of Ag. It was also found that high-Tc (end) corresponded well to the high-Tc fraction m= Sigma H/( Sigma H+ Sigma L+ Sigma l) (H: 2223, L: 2212, l: Ca2PbO4, Ca2CuO3) defined from the result of XRD measurements.

The effect of Er211 additions on the microstructure and magnetic properties of zone-melt textured Y123

Abstract: YBa2Cu3O6+x(Y123) wires with Er2BaCuO5 (Er211) and Y2BaCuO5 (Y211) additions were textured by a zone-melting process. The microstructure and magnetic Jc of wire segments taken from different sections of textured wires were analysed and compared. Microstructural analysis on the Er211-doped wires confirms that Er211 dissolves in the melt and results in the formation of a YzEr(1-z)Ba2Cu3O6+x superconducting phase upon solidification which shows improved magnetic properties over Y211 doped wires.

The crystal structure and twinning of neodymium gallium perovskite single crystals

Abstract: By means of X-ray structure analysis, the crystal structure of neodymium gallium perovskite (NGP) single crystals (NdGaO3) being used as a substrate for HTSC film epitaxy has been refined and the position of atoms has been determined. The possibility of YBa2Cu3O7-x film epitaxy on the plane (110) of NGP crystal as well as its advantages and pitfalls are analysed from structural data. The twinning types in the NGP crystal were established. The twinning structure of NGP substrates is found to be stable up to a temperature of 1173 K, as differentiated from the LaGaO3 and LaAlO3 substrates. It is intimated that the twinning in the NGP substrates oriented as (001) can result in creation of 90 degrees twin bonds in a film, and in the case of (110)-oriented plates it is possible to ignore the twinning presence in substrate completely.

A method to improve grain boundary current-carrying capability in melt-textured YBa2Cu3O7- delta

Abstract: A major impediment for many applications of bulk high-Tc superconductors is the extremely low current-carrying capability due to grain boundary weak links. A method has been developed to eliminate secondary phases and reduce microcracks at the grain boundaries in YBCO superconductor by removing a fraction of the liquid phase formed during melt texturing. Samples fabricated using this method consist of large domains containing several grains of oriented platelets with grain boundaries free of secondary phases and microcracks. The processing conditions have been tailored to obtain domains extending over a length of 15 to 20 mm, a width of 8 to 10 mm and a thickness of up to 5 mm. The grain boundary misorientation is characterized by optical microscopy as well as X-ray pole figure analysis. These grain boundaries, though having large misorientation angles up to 27.80 degrees , are found to carry high currents at 77 K, both in self-field and in an applied magnetic field of 1.5 T. These results give promise of the manufacture of polycrystalline YBCO superconductors with high current-carrying capability.

Infra- to ultra-sound studies of lattice instability and carriers in high-Tc cuprate superconductors

Abstract: Acoustic measurements from 10-2 to 107 Hz show that phase-like transitions (PLTs) characterized by a small jump of lattice parameters in the range 100-300 K always exist in high-Tc (near 100 K) superconducting Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O. Ferroelastic loops associated with the elastic softening of C'=((C11+C22)/2-C12)/2 and C66 invariably occur at the PLT temperatures, and in some cases the shape memory effect may be observed. For various samples of Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O with different Tc, the acoustic attenuation (Q-1, alpha ) from the kilohertz to megahertz range reveals a plateau (Qp-1, alpha p) above Tc and drops obviously below Tc with the turning point at Tc. This kind of anomaly has not been observed for non-superconducting samples. Qp-1 is found to be closely related to the carrier density for YBa2Cu3O7- delta , Y1-xPrxBa2Cu3O7 and Gd1-xPrxBa2Cu3O7, and the drop of Q-1 and a below Tc is considered to be caused by superconducting condensation. The energy dissipation (Qp-1 and alpha p) related to carriers can be explained reasonably by using a coupling model of carriers with local dynamic distortion. Furthermore, taking account of the smearing of the superconducting gap structure and by using the modified Bardeen-Cooper-Schrieffer (BCS) relative jump rate, S(E, E', Gamma )=Re(1- Delta 2/((E-i Gamma )(E'-i Gamma ))), the calculated results of acoustic attenuation below Tc are in fairly good agreement with the experimental data. The superconducting gap Delta and the damping rate Gamma for both Bi-Sr-Ca-Cu-O and TI-Ba-Ca-Cu-O have also been obtained.

A comparison of the effects of PtO2 and BaSnO3 additions on the refinement of Y2BaCuO5 and magnetization of textured YBa2Cu3O6+x

Abstract: Y2BaCuO5 (211) and liquid (L=3 BaCuO2:2CuO) were used as the precursor materials to study the effect of additions of BaSnO3 and PtO2 on the coarsening behaviour of 211 particles in the liquid above the peritectic temperature of YBa2Cu3O6+x (123), and the magnetic properties after melt texturing. Additions of BaSnO3 and PtO2 both reduced the coarsening of 211, but PtO2 did so much more effectively. The PtO2 additions were found to refine 211 by altering the 211/liquid interfacial energy and/or Y diffusivity, and not by acting as heterogeneous nucleation sites. BaSnO3 additions were found to not be inert in the melt. They reduce 211 coarsening by acting as a Y sink. Magnetization measurements at 20, 40 and 60 K on crushed melt-textured 123 samples showed enhanced intragrain flux pinning effects in samples prepared from 211+L+PtO2 as compared with 211+L+BaSnO3 or 211+L precursor samples.

The increase of pinning in (Bi,Pb)2Sr2Ca2Cu3O10+d bulk ceramics

Abstract: Considering the phase equilibrium diagram of the Bi2O3-PbO-SrO-CaO-CuO system, ceramics of the 2223 phase with the composition Bi1.8Pb0.4Sr2Ca2Cu3O10+d have been transformed by a simple annealing procedure into multi-phase samples. The transformation results in a four-fold increase of the critical current density at 2 T and 30 K (three-fold at 1 T and 50 K), which is believed to express an enhanced pinning capacity of the material. The nature of the pinning centres produced is not yet clarified.

What do we learn from vibrating high-temperature superconductors?

Abstract: This work reviews the theory and experiments of vibrating high-temperature superconductors (period 1988-94). In particular we discuss results on the elastic coupling between the flux line lattice and pinning centres, magnetization measurements, flux diffusion, possible phase transitions and matching effects of the flux line lattice. Special emphasis has been devoted to the comparison of these results with results obtained by more conventional methods such as AC susceptibility, low-frequency torsional oscillators, I-V curves and surface impedance measurements.

The effect of Ag diffusion on the crystal structure and electrical properties of Bi(Pb)SrCaCuO superconductors

Abstract: X-ray diffraction of Bi(Pb)SrCaCuO compounds, prepared by a conventional solid-state technique, were measured from 20 degrees C to 500 degrees C. It was found that diffusion of Ag (600-825 degrees C) in Bi ceramics stimulates a high degree of grain orientation along the c-axis. In Ag-doped samples the lattice parameters a and c are decreased (in comparison with the undoped samples) and their dependences on temperature reveal a non-linear character. The critical temperature, determined from the resistivity measurements, for Ag-doped samples increased to 118 K. Ag diffusion at 790 degrees C increases the critical current density more than twice. The influence of low annealing temperature for Ag-doped samples on critical current density was also investigated. These results are discussed on the basis of precipitation of Ag solid solution in Bi(Pb)SrCaCuO superconductors.

Superconducting properties of multilayered Bi(Pb)-2223/Ag tapes

Abstract: The preparation of silver sheathed Bi(Pb)-2223 multilayered tapes, fabricated by a new route-called the 'accordion folding method'-and through intermediate rolling and sintering processes, is reported. The electrical properties of these tapes were determined by transport Jc measurements in applied fields up to 23 T and up to 0.5 T, at 4.2 K and 77 K respectively. Multilayered tapes, having up to ten high-temperature superconducting layers, exhibited large transport current capabilities Ic up to 300 A and critical current densities as high as 8.8*103 A cm-2 at 77 K in self-field. Magnetic Jc values, calculated from magnetization loops in a wide range of temperature, are in accordance with transport measurements below 30 K. Electrical and magnetic properties were also correlated with the processing parameters and compared with those of monofilamentary tapes.

Spray deposited Tl-Ba-Ca-Cu-O films and bulks subjected to a common anneal

Abstract: Superconducting TI-Ba-Ca-Cu-O (TBCCO) 2212 films have been prepared by annealing the spray deposited precursor Ba-Ca-Cu-O/MgO layers with crude TBCCO pellets. Properties of both films and bulks are described. To deposit the precursor films, an aerosol generated ultrasonically from aqueous nitrate solution was used. The prepared 2212 films were 23 mu m thick; the highest Tc=102 K was obtained in both film and pellet annealed at 850 degrees C for 30 min. The Jc values measured on some films were in the interval 103-104 A cm-2; the XRPD data show rather low grain orientation both in films and in bulks. The X-ray patterns revealed that the film samples contain more than 95 vol.% of 2212 phase; the bulks contain more than 90 vol.% of 2212. The AES depth profiles in the surface layers of films show a slight drop in the Tl content towards the film-substrate interface. From the Raman spectra, XRPD data, structural investigation and physical properties of obtained 2212 phases, correlations among cationic disorder, c axis parameters, Tc values and frequency of the Raman modes were deduced. Results of detailed thermogravimetric TG, DTG and DTA measurements of homogenized precursor Ba-Ca-Cu nitrate and oxide mixtures as well as those of the crude Tl-Ba-Ca-Cu oxide mixture are presented.

Formation and stability of the 2223 phase in high-Jc Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3O10+ delta tapes

Abstract: Powder XRD, electron microscopy and critical current measurements at 77 K were used to study phase composition, microstructure and transport properties on selected Ag-sheathed (Bi,Pb)Sr-Ca-Cu 2223 phase-rich tapes. The selected composite tapes were put through one or several mechanical plastic deformation processes, followed by varying thermal annealing treatment cycles. The formation of 2223 and 2212 phases and the development of texture were studied after each processing cycle, concentrating on the effects arising from the sintering conditions. These included temperature, duration, the number of cycles and the cooling rate.

The fabrication of Bi-based superconductor tape by electrophoretic deposition and melt-texturing techniques

Abstract: A processing technique has been developed for the fabrication of high-current Bi-2212 superconductor tapes. The technique comprises three steps: (i) treating the particulate ceramic superconductor with trichloroacetic acid in 2-butanol to generate a negative charge on the particle surface; (ii) electrophoretic deposition of the particulate ceramic onto an Ag-based substrate and (iii) melt-texturing of the deposit to form a superconductor coating on the substrate. It has been found that the 2212 superconductor can only be melt textured on an Ag substrate in a narrow temperature window around 885 degrees C. Slow cooling from the molten state not only allows the development of superconductor phase texture, but also suffers from the growth of Ca-Cu-O oxides. These oxides can be transformed into 2212 phase by annealing at 850 degrees C. However, the 2212 grains formed from the Ca-Cu-O precipitates are invariably oriented at right angles to the 2212 phase formed in the course of solidification. The presence of such large-angle grain boundaries is detrimental to the superconductor properties. An optimal processing procedure has been proposed and some important processing parameters discussed. A pilot plant has been set up and semi-continuous tape of Bi-2212 superconductor on Ag has been produced. Critical currents as high as 155 A have been measured over a length of 1 m of multilayer coil at 4.2 K. The performance corresponds to a critical current density of about 70000 A cm-2 in the superconductor layer in a self-field up to about 64 mT. The uniformity of Jc measured on adjacent 10 cm lengths in the multilayer coil is much better than that in a single layer coil, about 30% variation in the former compared with 80% in the latter. The results show that electrophoretic tapes have a promising future for integration into small magnet windings.

Formation and growth mechanisms of Bi2Sr2CaCu2Oy on an Ag substrate in the partial melting method

Abstract: We investigated the phase changes of a Bi-Sr-Ca-Cu-O/Ag composite tape during cooling from a partially molten state using the high-temperature X-ray-diffraction and oil-quenching techniques. In the cooling process, (SrCa)CuO2 and Bi(SrCa)2Oy appeared at the areas where the (SrCa)2CuO3 phase disappeared. At around 860 degrees C, these solid phases melted into the liquid phase, and then Bi2Sr2CaCu2Oy (Bi-2212) grew rapidly at the surface. First, nucleation of Bi-2212 occurred at the surface of the liquid phase. With decreasing temperature the Bi-2212 crystal growth proceeded slowly from the surface to the inner area with the a-b plane almost parallel to the surface. During the growth of Bi-2212, excess Bi was ejected from Bi-2212 and a thin Bi-enriched layer was formed at the growth front of Bi-2212.

Flux-pinning and flux-creep in melt-textured (y1-xprx)ba2cu3o7-delta

Abstract: Flux pinning and flux creep behaviour for the H//c axis have been investigated on a set of MTG (Y1-xPrx)Ba2Cu3O7- delta (x

Magnetic field penetration through a superconducting film

Abstract: The mutual inductance of two coils, with a superconducting film between them, is expressed in closed form, suitable for computation. In addition it is shown that the coupling due to leakage of magnetic field round the edges of an otherwise impermeable film falls off as the fifth power of the dimensions of the film, and should not cause serious trouble when this arrangement is used to study penetration in superconductors.

An experimental implementation of high-Tc-based RSFQ set-reset trigger at 4.2 K

Abstract: A rapid single-flux quantum (RSFQ) set-reset trigger circuit has been implemented using YBaCuO grain boundary junctions on an asymmetric 32 degrees Y-ZrO2 bicrystal. The circuit contains 18 Josephson junctions aligned along the grain boundary and required six film layers: a YBaCuO base electrode, two amorphous SrTiO3 insulating layers, a superconducting Pb/Au/Cr ground plane and Au/Cr wiring. It consists of two DC-to-SFQ converters, two Josephson transmission lines and an SFQ-to-DC converter. Two pulse trains, 'signal' and 'clock', were applied to change the state of the trigger from '0' to '1' and vice versa, and registered at the SFQ-to-DC: output. All experiments were performed at 4.2 K.

Investigations of barium beta -diketonate complexes used in chemical vapour deposition of high-Tc oxide films

Abstract: This paper discusses barium complexes containing the 2,2,6,6-tetramethyl-3,5-heptanedionate (thd) ligand, describing properties relevant to their use as precursors in MOCVD. We describe the chemical characterization of commercial 'Ba(thd)2', similar oligomeric material we prepared from aqueous Ba(OH)2 solution, and the novel complex Ba(thd)2(tetraglyme). Of these three materials, only Ba(thd)2(tetraglyme) has a reproducible chemical composition. We compared the volatility and thermal stability of the barium complexes by means of MOCVD experiments involving film deposition under oxidizing conditions onto sapphire substrates at 500 degrees C. The resulting BaCO3 films were characterized by stylus profilometry, SEM, XRD and XPS. Both types of 'Ba(thd)2' initially showed a useful degree of volatility at 250 degrees C, although material we prepared from aqueous Ba(OH)2 solution decomposed on a timescale of hours at this temperature. We observed no systematic reduction in the volatility of 'Ba(thd)2' samples stored at room temperature for periods of up to 26 months. We also investigated the effect of triethylamine vapour on the volatilization of 'Ba(thd)2'. In contrast to an earlier report of dramatic volatility enhancement, highly sensitive XPS measurements indicated complete inhibition of barium transport under some conditions. Finally, our experiments with Ba(thd)2(tetraglyme) demonstrated the possibility of using this complex as a precursor in certain types of MOCVD process.

High-sensitivity microwave RF SQUID operating at 77 K

Abstract: High-Tc RF SQUIDS with bias frequencies in the microwave range exhibit low noise and high energy resolution compared with conventional 20 MHz RF SQUIDS. Especially interesting for applications is the absence of any low-frequency excess noise above about 1 Hz. Due to a relatively low flux focusing factor of the SQUID body, the field sensitivity of bare SQUIDS is only of the order of 1 pT Hz-1/2. In this work, several modifications of the SQUID geometry were studied in some detail to decrease the flux noise further and to enhance the field sensitivity.

(001) MgO single crystal as a reaction-barrier material for melt processing of Y-Ba-Cu-O superconductor

Abstract: The usefulness of a (001) MgO single crystal as a reaction-barrier material for melt processing was examined by investigating the microstructure near the interface between the MgO and melt-textured Y-Ba-Cu-O superconductors. It is found that there is no reaction among a (001) MgO single crystal and phases produced during peritectic decomposition/reaction, and no wetting of a Ba-Cu-O liquid phase on the (001) surface. After melt processing, the Y-Ba-Cu-O/MgO interface was clean except for the presence of a semi-continuous CuO-based liquid layer at the interface. Furthermore, the used MgO plate was easily separated from the melt-textured sample because of no chemical/physical bonding between two materials. Therefore, it can be used repeatedly. A (001) MgO single crystal is recommended as a good supporting plate or a crucible material for fabrication of Y-Ba-Cu-O single crystal and bulk superconductor using melting technique.

The behaviour of the flux flow resistance in YBCO/(Ag2O)x

Abstract: Measurements of the flux flow resistivity ( rho f) were performed on YBCO/(Ag2O)x high-Tc superconducting samples near their transition temperatures. Results revealed that log( rho f) changes linearly with log(1-T/Ton) with two different exponents below and above a certain temperature (T*) separating two different behaviours of the flux flow resistivity. Both exponents were found to be approximately independent of Ag content up to 10% Ag.

Hot pressing of Bi(Pb)-Sr-Ca-Cu-O superconducting pellets

Abstract: Dense and 2223 single-phase bulk bars of approximately 0.3 mm2 cross-section were fabricated by uniaxial hot pressing. Effects of preliminary annealing and intermediate grinding on the properties of hot pressed bars are discussed. Optimal structure and physical properties were obtained for preliminary annealing of duration 100-120 h and subsequent hot pressing under a pressure of approximately=25 MPa, and 80 h at a temperature of 850 degrees C for both processes, that resulted in highest contact measured value of Jc=11500 A cm-2 at 77 K and 0 T.