A better approach to assess quality and performance of coated conductors

This is a viewpoint on the fast track communication by Xu A et al (Supercond. Sci. Technol. 28 082001).

Critical current, temperature, and magnetic field define the superconducting domain in which the superconducting electronic states can be created and maintained while obeying other physical laws. One of the important criteria to characterize a superconductor is by the level of the critical current at a given temperature and in a given magnetic field (either self or orientated externally with respect to the direction of supercurrent). With everything else being equal in this three-dimensional space, the higher the critical current density (denoted as J_c(T, H)), the bigger the potential of the technological usefulness of the corresponding superconductor. The great challenge to superconductivity-based technology development, though, largely lies in the intrinsically complex relationship among the higher current, stronger magnetic field, and cryogenic temperature that are all well beyond the border of conventional non-superconducting electronic technologies.

From a technology development perspective, in order to further mature superconductivity for application in the magnet [1] and electric power [2] areas, a more detailed and thorough understanding of the characteristics and behaviors of the superconducting electronic states in their respective material host is nothing but absolutely essential. In other words, there is a need to establish a comprehensive, broad, and in-depth knowledge base to cover most of, if not the entire, three-dimensional space mentioned previously. Each type of superconductor may have its own specific characteristics within this space, especially for the high temperature superconductors that are more technologically promising for broader use but also more challenging in elucidation of the complicated mechanisms within their relative space, in comparison to their low temperature counterparts.

The recent paper by Xu et al [3] reported a J_c(T, H) characterization of more than 50 REBCO coated conductors at temperature from 20 to 77 K and in external magnetic field from zero to 9 Tesla in parallel and perpendicular orientations of the filed to the surface of the tape. Interestingly, J_c(77 K, 3 T $\parallel c)$ was identified as a general indicator of the overall in-field performance of the tape. This indicator may be more capable of representing the conductor's quality and performance than that of J_c(77 K, self field). As such, it represents an important contribution to the desired knowledge base on which applications of this type of coated conductors rely; this new finding is also expected to provide a useful tool to coated conductor manufacturers [4–6], for example, and others in manufacturing to evaluate product quality and improve production processes. Even more intriguing, it was found that only below 40 K does the parameter of J_c(77 K, 3 T $\parallel c)$ correlate linearly with that of J_c(T, 3 T $\parallel ab),$ whereas at other temperatures above 40 K, it is the parameter of J_c(77 K, self field) that correlates linearly with J_c(T, 3 T $\parallel ab).$ This is likely tied directly to the nature of the particular pinning incorporated into the sample. In the case cited, BZO nanorods were used. The extensive data from this research, which was based on more than 50 REBCO samples of different pinning compositions, illustrate both the commonality and complexity of the superconducting state at different points in that three-dimensional space of superconductivity.

To move superconductivity-based technology to a level beyond that of the matured NMR and MRI magnet application, a key enabling factor will be the enhancement of the current-carrying capacity of the coated conductor wires. This improvement in current must be for a variety of given temperatures and magnetic fields, which may be estimated by key indicators. The larger J_c(T, H) will provide an enlarged envelope in device, component, and system designs to ensure that the necessary operational conditions can be met with superconductor technology. Significant progress in increasing J_c(T, H) through flux pinning enhancement has been continuously made over time by nanoparticle engineering [7, 8] and metal oxide doping in nano-scale [3, 9], to name a couple of the more extensively investigated means. Nevertheless, to fully understand and comprehend the nature of the high temperature superconductor and the interplay among material properties and engineering principles, there is still an enormous amount of knowledge to be learned. This critical knowledge foundation will enable the science and technology community to develop an ability not only to engineer the superconducting materials with a precision down to the nano-scale, but also to maintain this nano-scale precision at a consistency that extends to hundreds, or even thousands, of meters in length of the coated conductors. No matter how challenging it seems, along the way, 'a holistic multidisciplinary enlightened empirical approach' [10] may well be helpful to the thinking, learning, and doing in our journey to advance the science and technology of superconductivity.