Table of contents

The physics and technology of superconducting bearings is reviewed. Particular attention is given to the use of high-temperature superconductors (HTSs) in rotating bearings. The basic phenomenology of levitational forces is presented, followed by a brief discussion of the theoretical models that can be used for conceptual understanding and calculations. The merits of various HTS bearing designs are presented, and the behaviour of HTS bearings in typical situations is discussed. The article concludes with a brief survey of various proposed applications for HTS bearings.

We report on measurements of the evolution of the magnetization Mupon warming from T₀(4.2 or 77 K) to T_cfor type II superconductors with the locus of Minitially situated along bridges between the envelopes of the major hysteresis curves. Strongly hysteretic and semi-reversible specimens of low and high T_ctype II superconductors have been studied (Nb₃ Zr, VTi, YBCO, Nb and BiSCCO). Concentric, oppositely directed magnetic moments coexist in the specimens when Mlies along the bridges. We focused on the cases where M= 0, hence, before warming, the constituent moments cancel each other exactly. Since they diminish in magnitude at different rates as Tis increased to T_c, their sequential `release' leads to a rich variety of phenomena. A simple model exploiting the critical state, an equilibrium Meissner current and conservation of flux accounts for all the intricate behaviour encountered.

Characteristic frequencies describing the rates of current decay in N -strand flat two-layer superconducting cables are studied. It is shown that these frequencies when assembled in ascending order form N- 1 simple subspectra. To each of these subspectra correspond characteristic currents possessing a certain type of symmetry. For a uniform case, comparison of the characteristic currents with those generated by a time dependent magnetic field has been performed.

Phase assemblage was investigated in the Bi-Sr-Ca-Cu-Sn-O system at temperatures of Bi-2212 phase crystallization. It is found that the Sr_1-x Ca_x SnO₃phase (x= 0.1-0.2) is compatible with Bi-2212 as well as with Bi-2201, CuO, alkaline-earth metal cuprates and bismuthates. The solubility of Sn in Bi-2212 is limited to low values. The superconducting composites Bi-2212-(Sr, Ca)SnO₃were prepared by melt crystallization using single precursor and mixed precursor routes. The materials consisted of Bi-2212 matrix with submicron (Sr, Ca)SnO₃grains included in Bi-2212 lamellas and agglomerated in between. The composites exhibited enhanced flux pinning at T= 60 K, while at 5 K the effect of (Sr, Ca)SnO₃addition was not revealed.

Samples of the Hg_1-x Re_x Ba₂ Ca₂ Cu₃ O₈₊superconductor (Hg, Re-1223), with low rhenium (Re) content (x= 0.05), have been produced with the help of a novel thermobaric analysis technique which could monitor the total pressure inside the quartz tube during synthesis at high temperature. The sample quality was verified by means of Rietveld analysis of the x-ray diffraction data, and by ac susceptibility measurements. The formation and stability of the Hg, Re-1223 phase has also been investigated by varying the oxygen content of the Re_0.05 Ba₂ Ca₂ Cu₃ O₈₊precursor, and the mercury filling factor (ff_Hg= 0.00 to 0.014 g cm^-3). It was observed that the reduction of oxygen content in the precursor (Re_0.05 Ba₂ Ca₂ Cu₃ O₈₊) and the increase of the mercury partial pressure inside the quartz tube enhanced the yield of the superconductor phase.

Resistance measurements as a function of temperature under hydrostatic pressure (0-1.0 GPa) show an increase in the superconducting transition temperature which can be fitted with a parabolic curve. The reduction of the lattice volume induced by external hydrostatic pressure is similar to the one induced by chemical pressure due to rhenium (Re) doping (0.00 < x < 0.10); however T_Cdoes not depend on the chemical pressure. This behaviour can be understood on the basis of the pressure-induced charge transfer model (PICTM) modified by Almasan et al.

At PTB, a fabrication process has been developed in SNS Nb/PdAu/Nb technology for the verification of small Josephson junctions (JJs) in the deep sub-micron range to enable the implementation of JJs as active elements in highly integrated superconducting circuits. Two steps of this technological development are described with regard to appropriately designed circuit layouts of JJ series arrays (JJAs), the first one in a conventional window type junction (WTJ) configuration and the second one in a ramp type junction (RTJ) configuration. Test circuits of JJAs containing up to 10 000 JJs have been fabricated and experimentally tested. In WTJ configuration, the circuits proved to be sensitive to external perturbing effects affecting the stability of circuit operation. In contrast to that, in RTJ configuration, the circuits realized showed correct function and a high grade of reliability of operation. To produce RTJ circuits, the technology parameters have been set to realize JJs with contact areas of A= 0.25 µm × 1.3 µm. At a thickness of the PdAu normal metal layer of d= 40 nm, the values achieved for the critical current density and for the product of critical current and normal state resistance are about j_c= 200 kA cm^-2and about I_c R_N= 21 µV.

We present the transport and magnetization properties of NBCO single crystals grown by the travelling solvent floating zone (TSFZ) and the self-flux (SF) methods under low O₂partial pressure. The TSFZ-grown crystal shows a continuous transition exhibiting a vortex-glass behaviour in V -Icharacteristics for H= 1 T. In contrast, a first-order melting transition was observed in the crystal grown by the SF method under 0.03% O₂partial pressure displaying a kink and hysteresis in resistance. The temperature dependent magnetization curves show a change in slope at the melting transition. The V -Icharacteristics in field for the SF-grown crystal exhibited a characteristic S-shape behaviour at the melting transition. The different behaviour in transport properties between two set of crystals suggests that the presence of twin planes and the substitutional defects play a major role in the observed vortex dynamics.

The voltage-current (V -I ) characteristics, critical temperatures and the critical currents of Tl₂ Ba₂ CaCu₂ O₈(Tl-2212) and Tl₂ Ba₂ Ca₂ Cu₃ O₁₀(Tl-2223) ceramic superconductors were studied in magnetic fields up to 80 mT after several irradiations up to 100 MR. The effect of irradiation on the structure of these materials has also been studied. The normal state resistance increases by 20-30% with irradiation, most of the changes taking place in low doses up to 10 MR. The critical temperatures of Tl-2212 and Tl-2223 decrease by about 8 K up to 30 MR and approach the saturation values of 101 K and 113 K, respectively, with further increase of dose. For Tl-2212 in the 96-98 K temperature range the V -Ibehaviours become nearly ohmic under a dose of about 30 MR or in an external magnetic field of 20-30 mT. In the same temperature range the critical current decreases rapidly in low doses up to 10-25 MR; above that it decreases only slightly. For both materials, the positions and widths of the x-ray diffraction peaks almost remain the same but the peak intensities decrease by 10-40% with irradiation, the rate of decrease being higher in the 0-30 MR range. These observations indicate that drastic changes in the lattice do not take place with irradiation but substantial electronic excitations and point defects may be generated, at higher rate in low doses, to affect the concentration and arrangements of the carriers and the links between the grains.

Bi₂Sr₂CaCu₂O₈₊single crystals embedded with MgO particles were successfully synthesized. Preliminary results indicate that MgO particles introduced during the single crystal growth process have little effect on T_cof the Bi2212 superconductor. The critical current density of the MgO-added single crystal has been increased by over a factor of three at low temperatures below 20 K with high magnetic fields parallel to the c -axis as compared with samples without MgO added. The underlying flux pinning mechanism is discussed in detail.

Superconducting thick films of (Hg, Sb)Ba₂Ca₂Cu₃O₈₊have been fabricated on polycrystalline yttria-stabilized-zirconia substrates utilizing an Hg-free precursor film reacted with Hg vapour, released from a solid Hg source, in a sealed quartz tube. The resulting films have been studied by x-ray diffraction, scanning electron microscopy, ac susceptibility and resistance measurement techniques. A high quality Hg(Sb)-1223 superconducting thick film on YSZ can be fabricated by using a pre-melted Hg-free precursor film. The zero resistance superconducting transition temperature in the post-growth oxygenated thick film is in excess of 130 K and the transport critical current density for the film is 510 A cm^-2at 77 K.

Growth of epitaxial Cr-doped (Tl,Bi)-1212 films on LaAlO₃substrates by pulsed laser deposition (PLD) and post-deposition annealing in static air has been achieved. The films exhibit a high transport J_cof over 1.5 × 10⁶A cm^-2at 77 K and self-field. T_cvalues measured by the four-probe method are in the range 94-100 K. ICP emission spectroscopy measurement shows a thallium deficiency, with about 0.15 atom bismuth per formula in the film. XRD -2 , and scans show that the dominant phase is 1212, with excellent epitaxy. The film surface has typical PLD morphology as analysed by SEM. TEM analysis found some inclusions in the film, and HREM found that three 1201 layers exist between the interface of 1212 phase and substrate.

The development and characterization of SmBa₂NbO₆, which is a new ceramic substrate material for the YBa₂Cu₃O_7-superconductor, are reported. SmBa₂NbO₆has a complex cubic perovskite structure with lattice constant a= 8.524 Å. The dielectric properties of SmBa₂NbO₆are in a range suitable for its use as a substrate for microwave applications. SmBa₂NbO₆ was found to have a thermal conductivity of 77 W m^-1K^-1and a thermal expansion coefficient of 7.8 × 10^-6°C^-1at room temperature. Superconducting YBa₂ Cu₃O_7-and YBa₂Cu₃O_7- -Ag thin films have been grown in situon polycrystalline SmBa₂NbO₆by the pulsed laser ablation technique. The films exhibited (00l) orientation of an orthorhombic YBa₂ Cu₃O_7-phase and gave a zero resistivity superconducting transition (T_C(0)) at 90 K with a transition width of ~1.5 K. The critical current density of YBCO-Ag thin films grown on polycrystalline SmBa₂NbO₆ substrate was ~3 × 10⁵A cm^-2at 77 K. A comparative study of YBCO and YBCO-Ag thin films developed on polycrystalline SmBa₂NbO₆substrate by PLD based on the crystallinity, orientation and critical current density of the YBCO film is discussed in detail.

Coupling through random superconducting bridges between filaments in a multifilamentary Ag-sheathed Bi₂Sr₂Ca₂Cu₃O₁₀₊tape has been investigated by magneto-optical imaging at temperatures from 20 K up to T_c. Magnetic flux distributions have been measured on the surface of an intact tape in the remanent state on applying a strong perpendicular magnetic field. The flux distributions observed at low temperatures reflect the arrangement of individual filaments. At high temperatures, the distribution becomes more similar to that for a uniform monocore tape, indicating that superconducting connections appear between the filaments. To discuss the relative contributions of the intra- and inter-filament currents, a simple model based on the Bean critical state was proposed and applied to analyse the temperature dependent behaviour. The inter-filament coupling, increasing with temperature, reaches at 77 K a point where the currents flowing in large inter-filament loops are roughly equal to the intra-filament currents.

The powder-in-tube technique, which includes wire drawing and rolling, has been widely used to fabricate superconducting tapes for possible electric power applications. In the present study the starting billet was reduced in size by groove rolling instead of by wire drawing. To optimize the deformation and thermomechanical treatment processes, BSCCO wires of various dimensions were fabricated. The wires were flat rolled to a final thickness of 250 µm. Short-length tapes were subjected to a series of thermal and deformation steps. Phase development and microstructural development during the process were monitored by XRD, SEM and TEM. Current transport properties of the tapes were determined in the temperature range of 20-77 K in self-field. Thermomechanical treatment resulted in tapes with average critical current densities of 18 000 A cm^-2at 77 K. High-resolution electron microscopy revealed better texturing and grain structure at the superconductor-silver interface. Superconducting lap- and butt-joints have been formed by a chemical etching technique. Detailed microstructural and current transport properties of the joints have also been measured.

Y-Ba-Cu-O superconductors were prepared by the top-seeded melt growth process with multiseeding technique. By using several seeds at the same time, large samples could be fabricated in a short time with simple heat treatment. However, the samples fabricated by the normal multiseeding technique show a rapid decrease of trapped magnetic field value across the grain boundaries because of the residual liquid layer. To remove the residual liquid layer, various types of multiseeding were newly suggested. The optimum result was obtained when the seeds were arranged without spacing. The individual grains were combined as a single domain and did not show any deterioration of magnetic properties at the boundary. The formation mechanism of a well-combined single domain by the multiseeding technique is discussed.

We investigated the current-voltage (E - J) characteristics in melt-textured YBa₂Cu₃O_7-strips by measuring the magnetic-field sweep rate dependence of magnetization. We took account of the current density Jdistribution in the specimen using a previously developed method (Mawatari Y et al1997 Appl. Phys. Lett.702300). For a wide temperature and magnetic-field range (60-80 K, 0.2-5.0 T), the E - Jcurves in the electric-field window E= 10^-10 -10^-5V m^-1exhibited power-law behaviour EJⁿ, and the power index ngenerally became smaller at higher magnetic fields and temperatures. In low magnetic fields (µ₀H_a 0.5 T) the nvalues were large ( 20), and thus the Bean model becomes a good approximation. The E - Jcharacteristics in the lower Ewindow were also derived from the relaxation of magnetization, the flux creep, and we found that the wide-range E - Jcharacteristics exhibit near-power-law behaviour but that there exist slight downward curvatures in the log Eversus log Jplots. This downward curvature reveals that the dissipation approaches zero when the current is substantially reduced. The drastic decrease of the flux creep, which was observed when the sample temperature was decreased in a fixed magnetic field, is consistent with the observed E - Jcharacteristics.

The critical current density of YBa₂Cu₃O_7-films dc magnetron sputtered onto buffered (CeO₂) sapphire substrates is measured. The peculiarities of these dependences are interpreted by taking into account the crystal defect (edge dislocations) film structure which in turn is determined by the film growth mode.

The relationships between the crystal structure modification, due to the doping element of Sn, and the critical temperature T_cof Sn-doped YBa₂Cu_3-x Sn_xO_7- , with x= 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, have been investigated by means of the peak shift and the peak broadening of the powder x-ray diffraction pattern. The profiles of x-ray diffraction peaks, due to crystal planes (006), (020), (021) and (200), were studied using the multi-peak fitting technique which is based on the Gaussian distribution function. It is interesting to note that the Sn-doped atoms effectively occupy the Cu2 sites in the CuO₂layer. The critical temperature of the Y-123 phase depends more sensitively on the lattice parameters aand b, rather than on the c-axis parameter c. The samples used for this investigation underwent the same treatment to ensure the requirement for maintaining the same oxygen content, particle size and residual stress. In this way, we can make sure that the Sn doping basically causes the peak shift and the peak broadening. Based on our results, we conclude that Sn doping in YBCO leads to an increase of the lattice constants aand b. The greater the expansion in aand b, the lower the critical temperature that results.

Epitaxial c -axis-orientated HgBa₂ CaCu₂ O₆(Hg-1212) superconducting thin films with their dimensions as large as ½ inch × ½ inch have been recently achieved using a cation-exchange process. Uniformity of the film was studied by slicing samples into eight pieces, on which uniform zero-resistance T_cvalues above 120 K were observed. In self-field, J_cvalues of the film range from 14.6 × 10⁶to 23.7 × 10⁶A cm^-2at 5 K, 1.77 × 10⁶to 3.85 × 10⁶A cm^-2at 77 K and 0.54 × 10⁶to 1.38 × 10⁶A cm^-2at 100 K. The scattering of J_cis found to be within 44% across the film and could be smaller with improvement of processing.

A mathematical model of the critical state based on averaged fluxon motion has been implemented to solve for the current and field distributions inside a high temperature superconducting hysteresis machine. The machine consists of a rotor made from a solid cylindrical single domain HTS placed in a perpendicular rotating field. The solution technique uses the finite difference approximation for a two-dimensional domain, discretized in cylindrical polar co-ordinates. The torque generated or equivalently the hysteresis loss in such a machine has been investigated using the model. It was found that to maximize the efficiency, the field needs to penetrate the rotor such that B₀ /µ₀J_c R= 0.56, where B₀is the applied field amplitude, J_cis the critical current density and Ris the rotor radius. This corresponds to a penetration that is 27% greater than that which reaches the centre of the rotor. An examination of the torque density distributions across the rotor reveal that for situations where the field is less than optimal, a significant increase in the performance can be achieved by removing an inner cylinder from the rotor.

We evaluated the effects of the joining process on the electrical and mechanical properties of Bi-2223 superconductor tape fabricated by the powder-in-tube technique. The joining of tapes was carried out by a lap-joint method. In the process, tapes were masked and etched to expose the superconductor cores in windows located near or at the end of the tape. The exposed cores of the two tapes were brought into contact, uniaxially pressed in the range 140-4000 MPa and sintered. The current carrying capacity of the jointed tape was evaluated as a function of uniaxial pressure and the shape of windows. It was observed that the current carrying capacity was reduced in the transition region of the jointed tape because of the non-uniform microstructure during the pressing. In addition, the current carrying capacity was significantly dependent on the uniaxial pressure. The highest current carrying capacity was obtained to be ~90% for the jointed tape to the tape itself by optimizing the window shape and pressure. It is believed that the highest value of current carrying capacity results from improvements in interface uniformity, core density, contacting area and grain alignment. The strain tolerance of the jointed tape was also evaluated, and the irreversible strain was measured to be 0.1%, which is lower than that of the unjointed tape. The decrease in the strain tolerance for jointed tape is believed to be due to the irregular interface and Ag intrusion in the transition region which acts as a stress concentration.

This paper reports on the evaluation of the effective suppression parameter _effin double-barrier Josephson junctions realized in Nb-Al/Al_x O_y /Al/Al_x O_y /Al-Nb SINIS technology. This parameter describes the overall resistance of current flow across the interfaces within the whole structure configuration and characterizes its I_CR_Nproduct attainable. By comparison with theoretical descriptions of tunnelling structures in the dirty-limit approximation, the effective suppression parameter _effhas been extracted from the measured temperature dependence of the I_CR_Nproduct. For SINIS junctions of a series of different wafer productions with various critical currents of the fabricated junctions, the evaluated values of _effrange between 80 and 250. With reference to the proximity effect and to the possibility of removing the Al layers at the Nb electrodes, a value of _effhas been deduced, which ranges between 50 and 120. This is very close to the assigned value between 10 and 20 ideally suitable for integrated circuit applications, e.g. Josephson voltage standards and rapid single flux quantum circuits.