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 (Jc)
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 Jc 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
Jc of bulk high-temperature superconductors1,2. 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 materials4-11.
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 monodisperse12,13, but which either concurrently or subsequently can be
ordered in a very precise manner14,15, taking as their cue the procedures and protocols
adopted by biomineralizing organisms in order to produce a bewildering array of
complex and intricate materials16,17.
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 YBa2Cu2O7-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.
Superconducting YBCO sponge synthesized using dextran. Scale bar is 10
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
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 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.
Superconducting YBCO nanowire synthesized using chitosan. Scale bar is 200
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