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New Superconductor Found "Immune To Magnetism"
Lisandro sends in news that testing of the new class of superconductors we discussed a while back (compounds of iron, lanthanum, and rare earths) has turned up a major surprise: magnetism doesn't shut off the superconducting state. Magnetic fields represent one of three factors that limit expanded applications for superconductors (the others are current density and temperature dependence.) The research will appear in Nature; here's a preprint (PDF).
Resistance is ductile.
No folly is more costly than the folly of intolerant idealism. - Winston Churchill
Read that preprint, or at least look at the pictures -- specifically Fig. 6. It's a measurement of the upper critical field (i.e. the magnetic field that destroys the superconducting state) versus temperature. The 90% line (where the resistivity is 90% of its normal-state value) does indeed go off the graph at low temperatures; it extrapolates to about 60 T for 5 K.
There's a big difference between "This material has a very high critical field" (which is what the article said) and "This material has no critical field" (which is what the summary said).
Bugrit! Millenium hand and shrimp!
optical fibres are amorphous, and definitely not ductile. However they are used for miles of cable. You can bend them a few degrees, which is all you really need. I suppose a superconducting ceramic would be worse, but you could still get a significant bend over a kilometre. I think the main barrier is still temperature, I think I read the best we have so far is just above the boiling point of Nitrogen, ~80K
Nothing to see here.
When will people use standard units? I'm sorry it's a particular gripe of mine; kelvin is the universal scale. The sooner we wipe out imperial units the better (unless anyone else wants to convert to a base 12 system?).
Nothing to see here.
That's really neat and all, but please let me know when they find something that's immune to gravity, as it's essential to a project I'm working on. (I have a deadline.)
Ack - looks like caffeinated_bunsen beat me to the punch. But it bears repeating - this paper certainly says nothing like "this superconductor is immune to magnetism". This material has a very high critical magnetic field, and if they figure out how to improve the connectivity then it might even someday be able to carry a current density of engineering significance. But it certainly is not "immune" to magnetism in any qualitatively different way than any other type-II superconductor out there. Still...it's nice to see that high-temperature superconductivity can be observed outside the cuprate family, and this paper (showing that it also has a high critical magnetic field) should spur some serious R&D work outside the theoretical physics community.
Reasoning is never, like poetry, judged from the outside at all.
I am a condensed-matter physicist but not a superconductor specialist.
The article does not say that the material is immune to magnetism.
The data relevant to this discussion is presented in Fig. 6 in the paper, which is a plot of the upper critical field (the maximum field the material can support and still be superconducting) versus temperature. Look at the traces marked with square markers.
Notice that these curves do not diverge to infinity as the summary would have you believe.
Granted, values in the 50's of Tesla seem pretty big, considering that the ambient magnetic field on Earth is about 0.5 Tesla. But note that other superconductors have critical fields in this same range. The famous high-Tc superconductor YBCO has a critical field of 135 Tesla (ref: http://www.springerlink.com/content/j0128jt30843362u/)
Compared to elemental superconductors, whose critical fields are around 1 Tesla or less, this material does indeed support a lot more magnetic field. But it certainly isn't "immune to magnetism"
As I understand it, they embed the superconducting material in a soft, non-superconducting metal like silver. There's a proximity effect at boundaries between superconductors and normal metals which allows the superconducting state to extent a short distance into the normal metal -- think of it as the Cooper pairs leaving the superconductor and taking a bit of time to notice that they're in a normal metal and split into single electrons. If the layers of normal metal between the superconducting grains are thin enough, then the supercurrent can run from one grain to the next, through the normal metal, without experiencing resistance.
The ductility of the metal allows some flexibility and tolerance for thermal expansion, as well as providing a low resistance at high temperatures. That's useful because the ceramic materials have rather high resistance when they're not superconducting, which means that if a small segment of wire warmed up above the transition temperature, its suddenly high resistance and the large current flowing through it would cause it to heat up extremely rapidly. The silver provides a secondary current path, so the wire's likely to heat up slowly enough to turn the power off before the wire melts.
Bugrit! Millenium hand and shrimp!
There are a variety of techniques (depending on the application) that manufacturers use to overcome the inherent brittle nature of most superconductors.
For magnet windings, the preferred technique is to fabricate the wire from ductile precursors, draw to final size, wind the coil, and then perform a heat treatment to react the precursors and form the brittle, superconducting phase. This, for example, will be the technique used when brittle Nb3Sn is used in the magnets for the ITER project.
A related solution is to grind the brittle superconductor into powder, insert it into a tube, and use the natural rolling and sliding action of the particles to draw the material into a fine wire that can be subsequently wound into a magnet, with a heat treatment employed to sinter the powder particles back together to form a continuous superconducting path. This is a common technique for MgB2 superconductors.
For non-magnet applications (like power transmission), the preferred technique is to make a tape (e.g. YBCO) that has only a very thin layer of brittle superconductor. Just like a glass fiber, this very thin layer has a very small bending moment in one direction, and so can be spooled (and unspooled) in this direction, allowing you to manage long lengths.
Reasoning is never, like poetry, judged from the outside at all.
This isn't a matter of opinion, it's an international standard. There is a reason decimalisation took place; we have a base 10 number system. If everyone uses their own defined set of units then people waste time when we try and cooperate.
Nothing to see here.