High Critical Current Density YBa2Cu3Ox Tapes Using the RABiTs Approach

Progress in the fabrication of epitaxial, high-J _c, biaxially aligned YBCO thick films on Rolling-assisted biaxially textured substrates (RABiTs) is reported. RABiT substrates comprise a biaxially textured metal substrate with epitaxial oxide buffer layers suitable for growth of superconductors. Oxide buffer layers have been deposited using three techniques: laser ablation, electron-beam evaporation, and sputtering. Epitaxial YBCO films grown using laser ablation on such substrates have critical current densities approaching 3 × 10⁶ A/cm² at 77 K in zero field and have field dependences similar to epitaxial films on single crystal ceramic substrates. Critical current densities in excess of 0.2 MA/cm² have been obtained on stronger, nonmagnetic substrates. In addition, samples with J _e of 12.5 kA/cm² at 77 K have been fabricated. The highest strain tolerence obtained so far is 0.7% in compression and 0.25% in tension. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high-J _c wire capable of carrying high currents in high magnetic fields and at elevated temperatures.     

Charge-transfer in YBa2Cu3O x

The charge-transfer hypothesis is shown to be inconsistent with data for YBa₂Cu₃O_x: (i) The two-step behavior ofT _c(x) (with jumps from zero to ≈60 K and then to ≈90 K) is not reflected as a similar, statistically significant two-step behavior in the bond-valence-sum charge of cuprate-plane Cu ions (as once believed), (ii) as a consequence of the law of conservation of charge, the derivatives of the layer charges with respect to oxygen contentx for both the Ba-O layers and the charge-reservoir Cu-O chains have the opposite signs to those predicted, and (iii) the charge-transfer observed for superconducting YBa₂Cu₃O_x is not sufficient to produce superconductivity, as demonstrated by insulating PrBa₂Cu₃O_x, which has virtually the same layer charges.     

Critical current in YBa2Cu3O7 ceramics

Measurements of critical currents using a four-point direct current (dc) method and an alternating magnetic field method have been performed on several superconducting YBa₂Cu₃O₇ ceramics at 77 K. In the presence of a constant magnetic field, the critical currents obtained with the alternating field method are several orders of magnitude larger than the critical currents measured by the dc method. Also, we observed a minimum in the dc critical current as a function of applied transverse magnetic field. Several authors have suggested that these ceramics behave as individual superconducting grains coupled by Josephson junctions. In this paper, we explain the two observations above using that model.     

Laser surface refinement of YBa2Cu3O x superconductor

A novel laser-processing technique that produces bulk YBa₂Cu₃O _x (1 2 3) plates has been developed. Through the application of a square CO₂ laser beam with uniform energy density distribution to the surface of 1 2 3 powder compact, a single piece of ribbon-like plate is produced. This plate may be separated from the powder compact after laser scanning. The width of the plate is 6 mm, while its thickness is 0.1–0.2 mm. Powder X-ray diffraction indicates that laser-treated samples contain both orthorhombic and tetragonal 1 2 3 phases, as well as Y₂O₃ (2 0 0), Y₂BaCuO₅ (2 1 1), BaCuO₂ (0 1 1), and CuO (0 0 1) phases. Scanning electron microscopy reveals a pattern of phase segregation along the transverse cross-section after solidification of the plate. After oxygen annealing of a single ribbon piece, T _c is found to be 90 K. This technique may be applicable to the mass production of 1 2 3 bulk superconductor by continuous melting of 1 2 3 powders. In addition to its potential for practical applications, the laser technique also helps to explain the complex phases and microstructure formation during melting and solidification of laser-melted 1 2 3 liquid. A model relating the microstructure to the thermal history inside the laser-affected region and to the phase diagram of incongruently melting 1 2 3 material has been developed to analyse phase formation during laser melting and solidification processes. Reasonable correspondence between theoretical analysis and experimental results was obtained.     

Photoemission spectroscopy of YBa2Cu3O6+x

High-resolution angle-integrated photoemission measurements were made on a sample of the compound Y₁Ba₂Cu₃O_6+x, for which it was shown that the onset of superconductivity was as high as 93 K. The density of states associated with the Cu 3d levels appears in an intense band about 4 eV wide beginning about 1.5 eV below the Fermi level. A small wing of this band extends up to the Fermi level, where the density of states is relatively low. These results are in good agreement with the projected density of states of Cu as obtained in recent calculations.     

Compressibility and thermal expansion of YBa2Cu3O x

The linear compressibilities and the thermal expansion of an untwinned, nearly optimal doped YBa ₂ Cu ₃ O _6.94 single crystal were measured along the three crystallographic axes with a high-resolution dilatometer mounted in a high-pressure cell. The measurements were performed in a temperature range from 50 K to 320K under hydrostatic gas pressure of maximal 0.65 GPa. At a temperature of 300 K the measured bulk modulus is reduced and shows a very strong enhancement under increasing pressure due to pressure-induced oxygen ordering. This ordering process can also be related to a glass transition seen in the thermal expansion above 225 K. At lower temperatures, when oxygen ordering is frozen, the bulk modulus is increased and shows a significantly reduced enhancement under pressure. This pressure dependence, however, is still anomalous high, probably caused by bonds of extremely different compressibilities within the unit cell. The widely spread literature data of the bulk modulus can be explained by this high pressure dependence.     

High Tc superconductor polymer composites based on YBa2Cu3O7−x

This work reports on the study of the microstructure and electrical properties (conduction and superconduction) of YBa₂Cu₃O_7–x (Y) and polypropylene (PP) composites. For the purpose of enhancing and improving their conductive properties, different amounts of carbon black (N) and copper (Cu) were incorporated into these composites, and the effects of N, Cu and Y were determined, both on the microstructure of PP and on the electrical properties of the resulting composites. The complex systems on the basis of PP/Cu/Y and PP/N/Y were sintered in order to study their mechanical characteristics, their morphology and the superconductor properties of the end products.     

Reaction kinetics in the superconducting YBa2Cu3O7-x and SmBa2Cu3O7x

The compounds under investigation were prepared with nominal composition MBa₂ Cu₃ O_7-x, M being Y or Sm, through solid state reaction starting from Y₂O₃, Sm₂O₃, BaCO₃ and CuO powders. Reactions were investigated by thermo-gravimetric and differential thermal analysis in the temperature range 900–980°C. Reaction kinetics of MBa₂Cu₃O_7-x solid state synthesis were evaluated according to the mathematical relationships theoretically derived for phenomenological models. In the Y-based system the phase boundary model provides a reaction kinetics suitable to describe the process, while a three dimensional diffusion model fits the data in the Sm-based system. The influence of the mixing method on the reaction kinetics was evaluated. Microstructural features of superconducting samples were analyzed.     

YBa2Cu3O7 − x 240 K Phase Transition

YBa₂Cu₃O_{7 ? x} ceramic samples have been investigated simultaneously by the thermal expansion and the acoustic emission methods during thermocycling through 200–300 K into liquid nitrogen steam. No jumps in the dilatation curves but change in the slope of one has been found at 228 and 234 K on heating and cooling respectively. These points are accompanied by the acoustic emission signals, a small value testifying for oxygen atoms displacement into the crystal lattice. On the basis of these measured experimental data, a thermal expansion coefficient α and a specific capacity c _m have been calculated. By this c _m value an oxygen atom activation energy ΔE = 0.15 eV has been calculated. It is shown that an isostructural oxygen-displacement second-order phase transition takes place between certain orthorhombic phases in YBa₂Cu₃O_{7 ? x} near 240 K.     

Fluctuation conductivity in Li-doped YBa2Cu3O7−x

The paraconductivity of Li-doped YBa₂Cu₃O_7?x was measured. We have found that the character of the fluctuation changes as lithium content increases: instead of a crossover from 2D to 3D behavior (D is the dimensionality), as we have observed for a low doping level, a double crossover to the 2D percolative and 3D percolative regimes becomes manifest by increasing the lithium content.     

Superheating of YBa2Cu3O7−x thin-film microbridges

It is found that the hysteresis that develops with decreasing temperature in the current-voltage characteristics of thin-film YBa₂Cu₃O_7−x microbridges is due to superheating of the bridges by the transport current. The heat transport coefficient is determined to be α=5500–7500 V/(cm²·K) and the thermal resistance of the YBa₂Cu₃O_7−x /MgO film-substrate interface is R _if=(1.3–1.8)×10₋₄ (cm²·K)/W. Pis’ma Zh. Tekh. Fiz. 23, 56–62 (June 26, 1997)     

Zinc diffusion in YBa2Cu3O7−x ceramics

The process of zinc diffusion in YBa₂Cu₃O_7?x ceramics with a porosity of 20–30% was studied in the temperature interval from 110 to 450°C using a ⁶⁵Zn radioactive tracer. The temperature dependence of the tracer diffusion coefficient is described by the relation D=5×10^?9 exp(?0.25 eV/kT) cm²/s. It is concluded that zinc migrates predominantly via pores and intergranular layers in the ceramics.     

Density dependence of the ultrasonic properties of high-T c sintered YBa2Cu3O7−x superconductors

Journal of Materials Science Letters -     

Understanding High Critical Currents in YBa2Cu3O7 Thin Films and Coated Conductors

The main challenges for the success of high temperature superconducting wires, the YBa₂Cu₃O₇ (YBCO) coated conductors (CC), are to avoid the the weak-link problem through the production of biaxially textured films, and to increase the critical current density (J _c) through the introduction of large densities of appropriate defects. To that end, it is essential to understand the pinning mechanisms and their correlation with the microstructure of the CC. We first present a brief overview of the main methods currently used to produce YBCO CC, and we describe the architecture of the YBCO on IBAD fabricated at Los Alamos, summarizing the recent improvements of their structural and superconducting properties. Then, we analyze some aspects of the J _c dependence on temperature and magnetic field (orientation and intensity) for the best CC available, and we compare and contrast the results with those of YBCO thin films on single crystal substrates, in order to determine if the defects controlling the pinning mechanisms are the same in both cases. Our results indicate that over large field and angular ranges J _c on CC is higher than J _c in thin films on SCS.