Twenty years ago in a series of amazing discoveries it was found that a large family of ceramic cuprate materials exhibited superconductivity at temperatures above, and in some cases well above, that of liquid nitrogen. Imaginations were energized by the thought of applications for zero-resistance conductors cooled with an inexpensive and readily available cryogen. Early optimism, however, was soon tempered by the hard realities of these new materials: brittle ceramics are not easily formed into long flexible conductors; high current levels require near-perfect crystallinity; and — the downside of high transition temperature — performance drops rapidly in a magnetic field. Despite these formidable obstacles, thousands of kilometres of high-temperature superconducting wire have now been manufactured for demonstrations of transmission cables, motors and other electrical power components. The question is whether the advantages of superconducting wire, such as efficiency and compactness, can outweigh the disadvantage: cost. The remaining task for materials scientists is to return to the fundamentals and squeeze as much performance as possible from these wonderful and difficult materials.

Materials science challenges for high-temperature superconducting wire

Strong isotropic flux pinning in solution-derived YBa 2 Cu 3 O 7− x nanocomposite superconductor films

Strong isotropic flux pinning in solution-derived YBa2Cu3O7-x nanocomposite superconductor films.

Unlike Y123 which forms only a stoichiometric compound, the light rare earth elements (LREs: La, Nd, Sm, Eu, Gd) form a solid solution . The presence of such solid solution caused a depression in the superconducting transition temperatures , particularly for La123, Nd123 and Sm123 when they are melt processed in air. Recently, we have found that the of these LRE123 superconductors can greatly be enhanced when they are melt processed in a reduced oxygen atmosphere. Furthermore, values of these superconductors were larger than that of a good quality Y123 superconductor in high magnetic fields at 77 K. In this article, on the basis of our study over the last several years, the melt processes for LRE - Ba - Cu - O are described, the microstructural and superconducting properties of the superconductors are reviewed and the flux pinning mechanism is also discussed.

https://iopscience.iop.org/article/10.1088/0953-2048/9/12/001/pdf

Melt-processed light rare earth element - Ba - Cu - O

Retaining a dissipation-free state while carrying large electrical currents is a challenge that needs to be solved to enable commercial applications of high-temperature superconductivity. Here, we show that the controlled combination of two effective pinning centres (randomly distributed nanoparticles and self-assembled columnar defects) is possible and effective. By simply changing the temperature or growth rate during pulsed-laser deposition of BaZrO(3)-doped YBa(2)Cu(3)O(7) films, we can vary the ratio of these defects, tuning the field and angular critical-current (Ic) performance to maximize Ic. We show that the defects' microstructure is governed by the growth kinetics and that the best results are obtained with a mixture of splayed columnar defects and random nanoparticles. The very high Ic arises from a complex vortex pinning landscape where columnar defects provide large pinning energy, while splay and nanoparticles inhibit flux creep. This knowledge is used to produce thick films with remarkable Ic(H) and nearly isotropic angle dependence.

Synergetic combination of different types of defect to optimize pinning landscape using BaZrO 3 -doped YBa 2 Cu 3 O 7

The development of biaxially textured, second-generation, high-temperature superconducting (HTS) wires is expected to enable most large-scale applications of HTS materials, in particular electric-power applications. For many potential applications, high critical currents in applied magnetic fields are required. It is well known that columnar defects generated by irradiating high-temperature superconducting materials with heavy ions significantly enhance the in-field critical current density. Hence, for over a decade scientists world-wide have sought means to produce such columnar defects in HTS materials without the expense and complexity of ionizing radiation. Using a simple and practically scalable technique, we have succeeded in producing long, nearly continuous vortex pins along the c-axis in YBa2Cu3O7?? (YBCO), in the form of self-assembled stacks of BaZrO3 (BZO) nanodots and nanorods. The nanodots and nanorods have a diameter of ~2?3?nm and an areal density ('matching field') of 8?10?T for 2?vol.% incorporation of BaZrO3. In addition, four misfit dislocations around each nanodot or nanorod are aligned and act as extended columnar defects. YBCO films with such defects exhibit significantly enhanced pinning with less sensitivity to magnetic fields H. In particular, at intermediate field values, the current density, Jc, varies as Jc~H??, with ?~0.3 rather than the usual values 0.5?0.65. Similar results were also obtained for CaZrO3 (CZO) and YSZ incorporation in the form of nanodots and nanorods within YBCO, indicating the broad applicability of the developed process. The process could also be used to incorporate self-assembled nanodots and nanorods within matrices of other materials for different applications, such as magnetic materials.

https://iopscience.iop.org/article/10.1088/0953-2048/18/11/021/pdf

Irradiation-free, columnar defects comprised of self-assembled nanodots and nanorods resulting in strongly enhanced flux-pinning in YBa2Cu3O7−δ films

This paper describes the microstructure to improve the magnetic field dependence of the critical current density, Jc, of Y(RE)Ba2Cu3O7−x [Y(RE)123, RE: Gd and Sm] coated conductors. A columnar microstructure 10nm in diameter has been obtained by using Y123 targets including yttrium-stabilized zirconium for the pulsed-laser deposition. This columnar structure, composed of BaZrO3 and Y123, continued from the substrate to the surface of the film 0.25μm in thickness. We have named it “the bamboo structure” from its morphology. The bamboo structure was effective for increasing Jc in a magnetic field especially parallel to the c-axis. We have also found stacking faults in RE123 effective to improve Jc.

Epitaxial nanostructure and defects effective for pinning in Y(RE)Ba2Cu3O7−x coated conductors

The liquidus compositions of Y, Sm, Gd, Dy and Yb for a melt with a Ba to Cu ratio of 3 to 5 has been investigated by a) dipping thermally equilibrated (held in the furnace for at least 10 min) MgO single crystals into the melt and analyzing the melt stuck to the MgO by ICP; b) dipping a relatively cool not thermally equilibrated Al2O3 polycrystalline rod into the melt (i.e. effective quenching) and analyzing the melt stuck to the rod by ICP. The measurements have been made in the temperature range from approximately 950° C up to about 1300° C. Following thermodynamic calculations, expressions for the liquidus lines are derived and the enthalpy of dissolution for the various RE123 and RE211 are calculated to be nearly the same for the same melt composition. Nevertheless a small increase of the enthalpy of dissolution with increasing peritetic temperature could be detected.

Solubility of RE elements into Ba−Cu−O melts and the enthalpy of dissolution

To investigate whether for NdBa2Cu3O7−δ (Nd123) an anomalous peak effect in the magnetization hysteresis (M-H) loops at 77 K is an intrinsic property or not, different series of heat treatments were performed in a pure oxygen gas flow using Nd123 single crystal, and critical current density (Jc-H curves at 77 K were determined from the M-H loops. The Jc-H curves with either H∥α-axis or H∥c-axis for a constant heat treatment at 340°C for 200 h showed no anomalous peak effect. On the other hand, when two step heat treatments, where the first step heat treatments was set to be at 500°C for 100 h and the second one was set to be at 340°C for 200 h, were carried out, the Jc-H curve at 77 K with only H∥ c-axis exhibited an anomalous peak effect. These results indicated that for Nd123 the anomalous peak effect in the Jc-H curve at 77 K (in other words, the M-H loop) with H∥c-axis was considered to be not due to an intrinsic property. Further it was thought that the solid reaction occurred at 500°C and this solid reaction resulted in the formation of an anisotropic structure, causing an anomalous peak effect in the Jc-H curve at 77 K with only H∥c-axis. Further the anomalous peak effect could be eliminated by the heat treatment. Therefore this solid reaction was found to be a reversible reaction. Accordingly the existence of the anomalous peak in the Jc-H curve at 77 K could be considered to be controlled by the heat treatment. We propose that the phase separation which is derived from the spinodal decomposition of an unstable solid solution takes place and the nano-scale microstructure resulting from phase separation leads to the anomalous peak effect and anisotropic property in the Jc-H curve at 77 K.

Heat treatment and anomalous peak effect in Jc−H curve at 77 K for NdBa2Cu3O7−δ single-crystal superconductor

We developed a very simple and new joint technique for coated conductors that could achieve a very low resistance less than 0.02 μΩ across the joint with an area of 2 × 20 mm2 without any degradation of Ic. It is a diffusion joint using the Ag stabilizing layer of YBCO coated conductors. Two tapes were stuck with Ag stabilizing layer in contact in a face to face manner and were pressed uniaxially during a heat treatment from 500 °C to room temperature in a pure oxygen atmosphere. The relationship between the joint pressure, the joint structure and Ic were studied.

Diffusion joint of YBCO coated conductors using stabilizing silver layers

Critical thickness for crack formation has been determined for the YBCO crystalline film deposited on the SrTiO 3 (STO) substrate by the liquid phase epitaxial (LPE) method. As-prepared films, even more than 4 μm in thickness, have no visible cracks, while oxygen-annealed films show apparent crack propagation depending on the film thickness, leading to the estimation of critical thickness of about 1.35 μm. Films formed on MgO buffered STO substrates have almost the same critical thickness. The effective thermal expansion coefficient of YBCO crystalline film has been deduced to be 13.4×10 −6 /K by considering thermal expansion of the substrate and the YBCO film, which causes internal tensile stress and crack formation in the film.

Yasuji Yamada

Papers

Epitaxial nanostructure and defects effective for pinning in Y(RE)Ba2Cu3O7−x coated conductors

Solubility of RE elements into Ba−Cu−O melts and the enthalpy of dissolution

Heat treatment and anomalous peak effect in Jc−H curve at 77 K for NdBa2Cu3O7−δ single-crystal superconductor

Diffusion joint of YBCO coated conductors using stabilizing silver layers

Critical thickness and effective thermal expansion coefficient of YBCO crystalline film