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.

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

We demonstrated short segments of a superconducting wire that meets or exceeds performance requirements for many large-scale applications of high-temperature superconducting materials, especially those requiring a high supercurrent and/or a high engineering critical current density in applied magnetic fields. The performance requirements for these varied applications were met in 3-micrometer-thick YBa 2 Cu 3 O 7-δ films epitaxially grown via pulsed laser ablation on rolling assisted biaxially textured substrates. Enhancements of the critical current in self-field as well as excellent retention of this current in high applied magnetic fields were achieved in the thick films via incorporation of a periodic array of extended columnar defects, composed of self-aligned nanodots of nonsuperconducting material extending through the entire thickness of the film. These columnar defects are highly effective in pinning the superconducting vortices or flux lines, thereby resulting in the substantially enhanced performance of this wire.

https://science.sciencemag.org/content/sci/311/5769/1911.full.pdf

High-performance high-TC superconducting wires.

Crystalline defects on the nano-scale, which are called artificial pinning centers (APCs), were successfully introduced into high-temperature superconductors (HTS) by nanotechnology, in order to strongly pin the quantized vortices. The critical current densities, Jc, of the HTS films were dramatically improved by APCs. It is possible to form APCs in high-quality epitaxial films, keeping the desired dimensionality, volume fraction, spatial distribution and so on. The in-field Jc of HTS films at 77 K was improved by one order of magnitude compared with previous values using APCs. This technology can be applied to the coated conductor technology in progress, and a high Jc has already been reported. A current outline of the research is described in this review.

https://iopscience.iop.org/article/10.1088/0953-2048/23/1/014001/pdf

Artificial pinning center technology to enhance vortex pinning in YBCO coated conductors

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

A study has been conducted on the magnetic properties of a series of biaxially textured Ni1� xWx materials with compositions x ¼ 0; 3; 5; 6, and 9 at.% W. These materials are important as substrates for ‘‘RABiTS’’-type coated conductors that incorporate high temperature superconductors for current transport. The quasi-static dc and ac hysteretic loss W was determined to support estimates of the ferromagnetic contribution to the overall ac loss in potential ac applications. The alloys were prepared by either vacuum casting or powder metallurgy methods, and the hysteretic loss tended to be lower in materials that were recrystallized at higher temperatures. Some samples were progressively deformed (0.4% bending strain) to simulate winding operations; this increased the hysteretic loss, as did sample cutting operations that create localized damage. In ac magnetization measurements, the effects of ac frequency and dc bias field on the ferromagnetic loss were determined. � 2003 Elsevier B.V. All rights reserved.

Magnetism and ferromagnetic loss in Ni–W textured substrates for coated conductors

The critical current density ${J}_{c}$ flowing in thin $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) films of various thicknesses $d$ has been studied magnetometrically, both as a function of applied field $H$ and temperature $T$, with a central objective to determine the dominant source of vortex pinning in these materials. The films, grown by a $\mathrm{Ba}{\mathrm{F}}_{2}$ ex situ process and deposited on buffered rolling assisted biaxially textured substrates (``RABiTS'') substrates of $\mathrm{Ni}\text{\ensuremath{-}}5%\phantom{\rule{0.3em}{0ex}}\mathrm{W}$, have thicknesses $d$ ranging from $28\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ to $1.5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Isothermal magnetization loops $M(H;T)$ and remanent magnetization ${M}_{\mathit{rem}}(T)$ in $H=0$ were measured with $H\ensuremath{\Vert}c$-axis (i.e., normal to film plane). The resulting ${J}_{c}(d)$ values (obtained from a modified critical state model) increase with thickness $d$, peak near $d\ensuremath{\sim}120\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, and thereafter decrease as the films get thicker. For a wide range of temperatures and intermediate fields, we find ${J}_{c}\ensuremath{\propto}{H}^{\ensuremath{-}\ensuremath{\alpha}}$ with $\ensuremath{\alpha}\ensuremath{\sim}(0.56--0.69)$ for all materials. This feature can be attributed to pinning by large random defects, which theoretically has power-law exponent $\ensuremath{\alpha}=5∕8$. Calculated values for the size and density of defects are comparable with those observed by TEM in the films. As a function of temperature, we find ${J}_{c}(T,sf)\ensuremath{\sim}{[1\ensuremath{-}{(T∕{T}_{c})}^{2}]}^{n}$ with $n\ensuremath{\sim}1.2--1.4$. This points to ``$\ensuremath{\delta}{T}_{c}$ pinning'' (pinning that suppresses ${T}_{c}$ locally) in these YBCO materials.

Critical currents of ex situ YBa2Cu3O7- δ thin films on rolling assisted biaxially textured substrates : Thickness, field, and temperature dependencies

Interfacial engineering via nanoparticle substrate surface decoration has been extended to coated conductor templates. Preformed BaTiO3 and BaZrO3 nanoparticles were applied to substrate surfaces, prior to YBa2Cu3O7−δ (YBCO) deposition, by using a scalable and inexpensive technique of solution-based suspension. Compared to untreated reference samples, nanodecoration yields improved in-field critical current density (Jc) as well as strong correlated pinning along the c-axis direction of the YBCO film. Accordingly, a much reduced falloff of Jc with magnetic field strength was observed in all of the modified samples. In addition, scaling behavior of the normalized volume pinning force density (Fp) with respect to temperature provided insight as to the differences in flux pinning mechanisms dependent on the decoration technique. Finally, with these results our earlier proof-of-concept demonstrations on nanoparticle modified single crystal substrates were replicated on technological substrates, pointing to the...

A.O. Ijaduola

Papers

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

Magnetism and ferromagnetic loss in Ni–W textured substrates for coated conductors

Critical currents of ex situ YBa2Cu3O7- δ thin films on rolling assisted biaxially textured substrates : Thickness, field, and temperature dependencies

Enhanced flux pinning and critical currents in YBa2Cu3O7−δ films by nanoparticle surface decoration: Extension to coated conductor templates