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This is a viewpoint on the letter by Majkic et al (2019 Supercond. Sci. Technol. 33 07LT03).
High temperature superconductors (HTS) from the time of their discovery have been a centre of attention due to their promising potential in several high-field applications, including but not limited to trapped field magnets, NMR/MRI coils, superconducting rotating machines and high-field magnets. HTS materials have been explored in different geometries and configurations including bulks [1–3], thick films [4, 5] and thin films [6–8]. Coated conductor technology soon emerged as the best option compared to the traditional powder-in-tube approaches for realising practical wires/tapes employing these materials. Improvements in coated conductor technology can readily realise several applications in the domain of high energy physics and nuclear fusion reactors where a large magnetic energy density is the key figure of merit [9]. Characteristics of HTS such as anisotropy and weak links and its brittle nature motivated the development of coated conductor technology via ion-beam assisted deposition achieved on rolling-assisted biaxially textured substrates. In 1995, the second generation (2G) flexible tape was first developed at Los Alamos National Laboratory [10] and subsequently extensive studies [11, 12] have been carried out in this direction which enabled coated conductors with a higher critical current density Jc (∼106 A cm−2 at 75 K), better magnetic field dependence of Jc i.e. Jc(B) and improved strain tolerance compared to their traditional counterparts such as Nb3Sn.
For (RE)BCO coated conductors, their angular anisotropy needs careful attention to facilitate the development of large field magnets, since (RE)BCO materials exhibit a strong anisotropic angular dependence of Jc. Typically Jc is maximised when the applied magnetic field is parallel to the a–b plane, and minimised when the field is parallel to c-axis [13]. Further, with increasing applied field, the magnitude of Jc reduces drastically and the only way this aspect can be addressed is through introduction of suitable artificial flux pinning centres (APCs) such as BaZrO3 (BZO) or BaHfO3 (BHO) in the REBa2Cu3O7−x (RE-123) matrix. In 2018, BZO nano-rods were successfully introduced in (RE)BCO thick film tape which enabled the achievement of an engineering current density Je in excess of 5 kA mm−2 at 4.2 K in an applied field of 14 T [14]. APCs of dimensions comparable to the coherence length of the superconducting phase enables the material to effectively pin the magnetic flux vortices, resulting in improved Jc(B) maintained to large applied fields.
In this context, the recent work by Goran Majkic et al [15] is immensely valuable as it provides pathways for realising 4 µm thick (Gd-Y)BCO films with 15% Hf addition (a source for APCs) significantly increasing the critical current density. BHO nano-rods of ∼5.4 nm in diameter and with average spacing of ∼18 nm were successfully introduced and integrated in the superconducting matrix. The in-field critical current Ic performance of these films, fabricated via advanced metal–organic chemical vapour deposition (referred as A-MOCVD) approach, were studied up to a field of 31.2 T. At 4.2 K and with B∥a − b, the Ic in these films exhibited a self-field value of 7700 A/4 mm, which reduced only to 5812 A/4 mm in a 30 T field—thanks to the effective APCs employed. Especially from the view point of the toroidal design of a fusion tokamak, these results are greatly appealing. Even in the B∥c-axis configuration, the engineering current density of the conductor at liquid helium temperature (4.2 K) and in an applied field of 14 T was better than 5 kA mm−2 (similar to that for BZO addition) which is much better than the performance of conventional Nb3Sn material. Some of the salient advancements achieved in this work are provided in table 1. This work which clearly signifies an achievement of five-fold increment in Ic with the configuration of B∥a − b and with a 16-fold increment in the engineering critical current density at 15 T field when compared to the conventional Nb3Sn is indeed a promising step forward for the development of high-field magnets.
Table 1. Comparison of superconducting properties obtained in (GdY)BCO coated conductors with 15% Hf addition. Measurements were carried out at 4.2 K.
| B∥a − b | B∥c | ||
|---|---|---|---|
| Performance @ 4.2 K and14 T | Critical current Ic @ 14 T | 6300 A/4 mm-width | 2114 A/4 mm-width |
| Critical current density Jc | ∼40.5 MA cm−2 | 13.6 MA cm−2 | |
| Engineering current density Je | 16.2 kA mm−2 | 5.4 kA mm−2 | |
| Pinning force Fp | ∼5.7 TN mm−2 | 1.7 TN mm−2 | |
| Performance @ 4.2 K and 30 T | Critical current Ic @ 30 T | 5812 A/4 mm-width | 1013 A/4 mm-width |
| Critical current density Jc | ∼36.3 MA cm−2 | 6.5 MA cm−2 | |
| Engineering current density Je | 14.9 kA mm−2 | 2.6 kA mm−2 | |
| Pinning force Fp | ∼11 TN mm−2 | 1.7 TN mm−2 |
The new achieved world record Je is twice as high as the best values reported so far in CCs and five times higher than the commercial values for CCs. The key advancement of this CC arises from an improved MOCVD processing approach allowing the increase of the film thickness up to 4.6 μm while the BZO nano-rods keep the structural coherence across the whole film. This achievement will certainly stimulate further advancements in other competing CCs and wire developments and so the future availability of magnets in the range of 25–30 T appears closer on the horizon.
The primary aspect of improving critical current performance even in large applied magnetic fields is now addressed to a satisfactory level through deployment of suitable APCs in coated conductor technology. Now, on a parallel track, efforts need be made to fabricate wider tapes retaining the superconducting performance over long lengths, and simultaneously reduce the processing and overall cost of such coated conductors to further stimulate the development across a wide range of engineering and technological applications.