An Introduction to High Performance Superconductors

Superconductors are a class of compounds which conduct electricity without resistance and are impermeable to magnetic flux below a critical temperature. As a result of this, they are ubiquitous in applications as diverse as MRI scanners, Earth-orbiting deep space cameras and Maglev trains.

With a worldwide market for superconducting materials predicted to be £20 billion by 2020, there is a huge research effort to discover new families of superconducting materials which may further improve performance and/or range of applications. Although systematic investigation has revealed many new superconducting compounds, some of the most spectacular discoveries owe their origins to chance.

Whilst Science awaits the warm hand of serendipity to uncover the newest superconductor, materials chemists are beginning to explore new synthetic routes to existing superconductors. In particular, it is the improvement of important physical properties such as the critical current density (J_c) which are actively being researched. The critical current density is the point at which the amount of current flowing through the superconductor causes superconductivity to be destroyed, so for practical applications the higher the Jc of the superconductor, the better.

It was found by a number of researchers that J_c was being suppressed in materials with large-angle crystal grain boundaries, leading to the conclusion that grain boundaries act as weak links in limiting the J_c of bulk high-temperature superconductors^1,2. The elimination or minimization of large-angle grain boundaries by homogeneously synthesizing superconductors is therefore being pursued intensively.

Traditionally, metal oxide superconductors have been made by repeatedly grinding together metal salts and/or metal oxide powders, followed by sintering, producing largely uncontrolled crystal growth³. Instead Dr. Hall's work has focussed on the use of biopolymers to sequester metal cations, followed by in situ carbonization, both of which serve to prevent this uncontrolled growth. Once suitable solution-based syntheses had been devised for the high temperature superconductors, it was conceptually effortless for me to invoke the mechanisms of templated crystallization used by biomineralizing organisms as a means to achieving chemical and morphological homogeneity in this class of materials^4-11.

The field of biomimetic materials chemistry is a powerful one, in that researchers are able not only to produce crystals which are morphologically homogeneous and more monodisperse^12,13, but which either concurrently or subsequently can be ordered in a very precise manner^14,15, taking as their cue the procedures and protocols adopted by biomineralizing organisms in order to produce a bewildering array of complex and intricate materials^16,17.

One of the first biopolymers Dr. Hall has applied to templated superconductor growth, owing to its enormous potential to sequester metal cations is dextran. Dextran is a complex branched polysaccharide, which consists of many glucose molecules linked together in long chains of varying molecular weight from 1 kDa to over 650 kDa depending on the source. Very simply, sponges of the superconductor YBa₂Cu₂O_7-ä (YBCO) were prepared by dissolving dextran in a solution of nitrates of yttrium, barium and copper to form a light blue, viscous paste which was allowed to solidify at room temperature. When calcined, the correct crystallinity of the superconductor was achieved; the dextran foaming as it degraded, resulting in an effective mixing and oxygenation of the superconducting phase, leading to a highly porous, monophasic material (Figure 1)¹⁸.

Figure 1. Superconducting YBCO sponge synthesized using dextran. Scale bar is 10 microns.

SQUID magnetometry of the sponges showed that this material was superconducting at a temperature of 90 K and that owing to the reticulated nature of the crystallites, had a critical current density that was two orders of magnitude higher than that observed in a commercially available, high-purity YBCO material. This observed morphological enhancement of Jc is in agreement with that predicted theoretically for YBCO with closely aligned crystallites¹⁹.

Another biopolymer which is showing tremendous promise is chitosan. This is simply derived from chitin, a polysaccharide which is one of the main structural components in fungi, insects and arthropods. Nature uses chitin to form complex architectural structures in many phyla, from tough crab and lobster shells, to strong yet flexible mushroom gills. Chitin molecules adopt a helical conformation, due to the presence of chiral carbon centres formed by glycosidic linkages which can subsequently pack together to form fibres and bundles of fibres. A slight modification of the reactive side groups of chitin produces chitosan.

Chitosan is non-toxic and has therefore found applications in medicine²⁰, cosmetics²¹ and food technology²² due to its low cost and biocompatibility. Of particular relevance to mineralization studies is the fact that is it soluble in acidic aqueous environments and has the ability to preferentially sequester transition and post-transition metal ions from aqueous solutions²³. Chitosan has been shown to be a remarkably stable biomaterial; when subjected to temperatures of 160 °C there is no apparent change of molecular configuration²⁴. This stability makes it ideal as a template in which to perform hydrothermal syntheses without appreciable loss of template structure before nucleation and growth of the inorganic phase is initiated.

For these reasons, chitosan is an ideal templating matrix for the crystallographically controlled syntheses of YBCO superconductors. With a heating rate of 1°C/min, above 400°C nanoparticles of barium carbonate nucleate and grow, well separated in the amorphous remains of the template. Sintering of the nanoparticles was prevented by the amorphous material and thus remained as discrete centres for the subsequent outgrowth of YBCO nanowires. Nanowires were then able to grow from these 'seed' nanoparticles from around 811°C onwards.

Crystal growth proceeded along the crystallographic c-axis, leading to the formation of nanowires, all around 50nm ± 5nm in the short axis and from 600nm to over 1000nm in length (Figure 2). SQUID magnetometry of the nanowires revealed that morphology can be controlled in this manner without significant loss of superconducting performance. With their preferential crystallographic oriented growth, these nanowires should provide a better way of studying and understanding superconductivity in anisotropic systems and perhaps have the potential for incorporation into future computer circuitry.

Figure 2. Superconducting YBCO nanowire synthesized using chitosan. Scale bar is 200 nm.

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