December 18, 2007

Cuttlefish bones template for new superconductors with almost 100 times higher current density

UK chemists have used cuttlefish bones to template the growth of new superconductors that have almost 100 times higher current density. The weight of 1 cm**3 of superconducting cuttlebone replica is 0.06 g, compared to 6.38 g in the case of an equivalent sized monolith of pure Y123. With an overall decrease in mass of two orders in magnitude, these materials could well find application in areas where weight is of critical importance, such as space-based and mobile device technologies. It is envisaged too, that our method could apply equally well in the synthesis of light and porous giant magnetoresistive, piezoelectric and ferromagnetic materials.


Cuttlefish bone templated superconductor
Cuttlefish bone templated superconductor

A very high critical current density using light weight material can enable far better magsails. These lightweight and high critical density superconductors could even enable ground launched magsails. They need to find a good way to reinforce the structural strength (perhaps carbon nanotubes) that do not reduce the critical current too far and they need to produce the superconductors in high volume and a reasonable cost and preferably in the form of wires. Note: they have proved that an ordered, macroporous morphology provides a greatly improved critical current density. Therefore, any other means of creating ordered macroporous morphologies (such as three dimensional nanostructuring) can be used as well and could allow for a better tuning of the critical current and structural strength properties.

The critical current density of the cuttlebone templated Y123 was measured at 1.6 MAcm−2 at 10 K and 1 T field. This is almost two orders of magnitude higher than that observed in a commercially available Y123 powder (Aldrich 99.9% — average particle size 5 µm), for which SQUID magnetometry revealed a Tc of 92 K and a critical current density of 0.02 MAcm−2 at 10 K and 1 T field.

Chart of current density and magnetic field strength

In terms of mechanical strength, the cuttlebone-templated superconductors are self-supporting but very weak (<1.5 kPa). After calcination (with 10% silver nitrate), the monoliths were noticeably more structurally stable. Compressive strength testing confirmed this, with monoliths capable of withstanding a compressive strength of 27.95 kPa, a figure comparable with certain roofing materials used in the construction industry. EDXA revealed the presence of silver in the material, and PXRD showed more pronounced peaks due to Y123 than in the undoped sample, confirmation that the silver is promoting a more pronounced crystalline structure. SEM images show that the silver doped replicas retain the cuttlebone morphology exceedingly well, even to the extent that the fine structure of the S-shaped pillars are reproduced in detail. TEM revealed that the crystallite size was now 3 µm ± 0.2 µm, larger than seen in the undoped samples. This increase in crystallite size has ramifications for the electronic behaviour of these materials, as an increase in crystallite size can often lead to a decrease in critical temperature and current. SQUID magnetometry confirms this, with a pronounced decrease in both Tc (to 73 K) and Jc (to 0.16 MA cm−2) of the silver doped samples. It is apparent therefore, that a balance must be struck between an improvement in compressive strength and the electronic performance of the superconducting monolith.

Simon Hall at the University of Bristol and colleagues soaked the cuttlefish bone - cuttlebone - in a solution of the precursors of the yttrium barium copper oxide superconductor Y123 and then heated the sample to over 900 °C to form the superconductor. Cuttlebone has an open structure, consisting of calcium carbonate layers connected by pillars. This allows the sample to be oxidised efficiently when heated, so unlike other synthesis methods flowing oxygen is not needed to produce high quality Y123.

The researchers found that the complex porous structure of the cuttlebone was retained, giving a lightweight superconducting material. They also discovered that the critical current density of their material was almost two orders of magnitude higher than that of the commercially available Y123 powder.

Magsail analysis for a game using real equations.

Enhancing magsail launches using light weight high volume magnets and a big lightweight tower.

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