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).
I seem to recall that one limit was simply the ceramic nature of most superconductors. If it isn't ductile, you can't use it for wires -- which are kind of important for most superconducting applications. Am I wrong about this?
Superconductors that are immune to interference from magnets would get us further towards Bussard Ramjets. There are other hurdles, like the mechanical strength of the magnetic coils themselves. (So the magnetic forces don't wreck them.) Even if we couldn't make practical ramjets, magnetic sails would also benefit, which would make deceleration of interstellar craft almost "free."
The conventional wisdom on Bussard Ramjets (included in the wikipedia article) is that they reach a terminal velocity due to the drag of collecting the fuel - and asymptotically approach their exhaust velocity. IMHO that's incorrect.
The bug is that the calculation assumes that they must accelerate the collected hydrogen to the velocity of the craft before fusing it, then depend on the fusion energy to re-accelerate it as exhaust.
However, as with the collected air in chemical ramjets, the momentum of the collected material does not need to be discarded. It can be fused on the fly through the ramjet, retaining its original momentum along the flight path (relative to the vessel). Thus the energy of fusion can be applied to accelerating the reaction products toward the rear. None is needed to replace the momentum allegedly lost capturing the fuel.
Now SOME of the axial momentum of the incoming fuel is traded for radial momentum to collect it. But the energy of that "lost" momentum is converted to pressure and temperature, compressing the material like any other gas. There is a drag on the scoop field from this. But when the exhaust expands again after the reaction there is a corresponding thrust against the nozzle field, reconverting the radial expansion of the reaction products to rearward velocity and recovering the "lost" momentum.
If this whole process were lossless there would be no top end to the kenetic energy the ramscoop could accumulate. With less than 100% efficiency in reapplying the compression energy to the mass (both from lost energy and lost mass) there is some drag from collection that is not recovered. (For instance: Mass lost as neutrinos is a non-trivial fraction.) So there may still be a speed limit. But it can be far higher than that calculated by assuming you "stopped" the gas when you "caught" it.
You've reached the wrong conclusion; if it isn't ductile, you can't use it for wires that bend; however, you can certainly use it for wires that follow nonlinear paths.
Right. You won't be running power lines made of ceramics (because of the temperature requirement too) but it's no problem for a fixed installation like a supercomputer.
Well, you won't be running power lines that swing in the air; but power lines in a channel in the ground are possible in regions where seismic activity isn't a threat. Anyway, you can certainly make wires out of ceramic superconductors, is all I was getting at.
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
That's okay for power lines but it's a real pain for anything that involves a coil. Unfortunately (aside from power lines) coils are involved in the majority of applications that might benefit from superconducting: magnets, motors, etc.
Even power lines are a pain with ceramics because you can't easily extrude them to make a wire.
That's a limitation, rather than a limit. Not being ductile makes it less convenient to use. With magnetism, current density and temperature, the superconductivity disappears as each value reaches a critical point (the limit).
Considering they haven't made a superconductor that can retain that property at anything even close to normal earth environment temperatures, I'd say worrying about that is a bit like putting the cart before the horse.
Actually, they're currently working on using a LN-cooled superconductor link in NYC to link some substations in Manhattan. It would replace an oil-cooled copper link. They're expecting to have it running in 2010.
I read an article in the last year that talked about using liquid hydrogen to cool super conducting transmission lines and also being used as an infrastructure to distribute hydrogen for use in cars, fuel cells, etc...
Me too. It was this article [sciam.com] in Scientific American.
The original superconductors were metals for the most part, but only work at liquid helium temperatures. Then a new class of high-temperature superconductors were discovered, some of which work at liquid nitrogen temperatures; this second class is often called "cuprate superconductors" and they could be described as ceramic. The lack of ductility isn't as bad a problem as the low tolerance for magnetic field that still superconducts at 45 tesla (basically the strongest magnetic field the experimenters could produce).
Since flowing current creates a magetic field, you can't use cuprate superconductors to carry large currents.
Evidently a completely new class of materials has been discovered.
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.
Since flowing current creates a magetic field, you can't use cuprate superconductors to carry large currents.
Don't confuse critical fields with critical currents. This paper is talking only about critical fields - it is not trying to describe the amount of current that this material might eventually carry.
You're right that electric current creates a magnetic field. In a type-II superconductor (like the cuprates and these new FeAs materials), this is managed by introducing defects in the material (grain boundaries, inclusions, etc.) that "pin" the quantitized magnetic flux vortices and prevent them from moving through the material and destroying superconductivity. So it's not fair to say that you CAN'T use cuprates to carry large currents - it's just an engineering problem that has to be dealt with by clever manipulation of the structure of the materials.
So here's the short version:
Critical field = intrinsic property of the material.
Critical current = extrinsic property that depends on critical field, grain structure, presence of second phases, etc.
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 [wikipedia.org] is used in the magnets for the ITER [iter.org] 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 [wikipedia.org] superconductors.
For non-magnet applications (like power transmission), the preferred technique is to make a tape (e.g. YBCO [wikipedia.org]) 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.
That's truly fascinating if they can tolerate such a large magnetic field. While we may rarely need to tolerate 45 tesla magnetic fields in practice, the physics behind this must be new to our experience. Unexplained experimental results always spark interesting theoretical work, possibly leading to more practical materials.
Scanning the paper, it seemed to have little bearing on this magnetic field tolerance, but rather talked about the effects of grain boundaries. Did anyone understand how the paper related to the press release?
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).
There's a 60T pulsed magnet at LANL. "Power comes from a pulsed power infrastructure which includes a 1.43 gigawatt motor generator and five 64 megawatt power supplies. The 1200-ton motor generator sits on a 4800-short ton (4350 t) inertia block which rests on 60 springs to minimize earth tremors."
When the generators which power those things lie still, their steel shafts warp under their own weight. Even when the generators aren't being spun to create the electricity, they have to spin (albeit more slowly) to make sure they don't bend. Wicked cool stuff.
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.)
It might not be as plentiful as you think. I've heard an anecdote that the Guinness Book of World Records lists vacuum as the most expensive substance known to man...
Most expensive by weight, that is. Additionally, higher quality vacuums are exponentially more expensive!
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.
by Anonymous Coward
on Tuesday June 03 2008, @03:28PM (#23643241)
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"
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?).
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.
Another limit? (Score:3, Informative)
Re: (Score:2)
Bussard Ramjets! (Score:5, Insightful)
http://en.wikipedia.org/wiki/Bussard_ramjet [wikipedia.org]
Parent
Re: Ramscoops: I think the analysis has a bug. (Score:5, Interesting)
The conventional wisdom on Bussard Ramjets (included in the wikipedia article) is that they reach a terminal velocity due to the drag of collecting the fuel - and asymptotically approach their exhaust velocity. IMHO that's incorrect.
The bug is that the calculation assumes that they must accelerate the collected hydrogen to the velocity of the craft before fusing it, then depend on the fusion energy to re-accelerate it as exhaust.
However, as with the collected air in chemical ramjets, the momentum of the collected material does not need to be discarded. It can be fused on the fly through the ramjet, retaining its original momentum along the flight path (relative to the vessel). Thus the energy of fusion can be applied to accelerating the reaction products toward the rear. None is needed to replace the momentum allegedly lost capturing the fuel.
Now SOME of the axial momentum of the incoming fuel is traded for radial momentum to collect it. But the energy of that "lost" momentum is converted to pressure and temperature, compressing the material like any other gas. There is a drag on the scoop field from this. But when the exhaust expands again after the reaction there is a corresponding thrust against the nozzle field, reconverting the radial expansion of the reaction products to rearward velocity and recovering the "lost" momentum.
If this whole process were lossless there would be no top end to the kenetic energy the ramscoop could accumulate. With less than 100% efficiency in reapplying the compression energy to the mass (both from lost energy and lost mass) there is some drag from collection that is not recovered. (For instance: Mass lost as neutrinos is a non-trivial fraction.) So there may still be a speed limit. But it can be far higher than that calculated by assuming you "stopped" the gas when you "caught" it.
Parent
Bussard Polywell Instead! (Score:3, Interesting)
A high temperature superconductor that is resistant to high magnetic fields would allow significant efficiency gains and eventually miniaturization.
Who knows in 40 years every new home might have it's own fusion reactor in the basement because of this material.
Re:Another limit? (Score:5, Informative)
You've reached the wrong conclusion; if it isn't ductile, you can't use it for wires that bend; however, you can certainly use it for wires that follow nonlinear paths.
Parent
Re: (Score:3, Informative)
Re:Another limit? (Score:4, Insightful)
Well, you won't be running power lines that swing in the air; but power lines in a channel in the ground are possible in regions where seismic activity isn't a threat. Anyway, you can certainly make wires out of ceramic superconductors, is all I was getting at.
Parent
Re: (Score:3, Interesting)
Re: (Score:3, Informative)
Northeastern Montana, for one. Right where I live.
But for more details, go here [usgs.gov].
Re:Another limit? (Score:4, Insightful)
Parent
Re: (Score:3, Informative)
There's always a chance that seismic activity could break stuff. But that hasn't stopped people from _rebuilding_ stuff in earthquake zones.
Re:Another limit? (Score:5, Funny)
Parent
Re: (Score:3, Funny)
Re: (Score:2)
Re:Another limit? (Score:5, Informative)
Parent
Re: (Score:3, Insightful)
Even power lines are a pain with ceramics because you can't easily extrude them to make a wire.
Re:Another limit? (Score:4, Informative)
Parent
Re: (Score:3, Insightful)
Re:Another limit? (Score:5, Informative)
link [newscientist.com]
Parent
Re: (Score:3, Informative)
Me too. It was this article [sciam.com] in Scientific American.
Re:Another limit? (Score:4, Informative)
Parent
Re:Another limit? (Score:5, Informative)
Since flowing current creates a magetic field, you can't use cuprate superconductors to carry large currents. Evidently a completely new class of materials has been discovered.
Parent
Re:Another limit? (Score:5, Informative)
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.
Parent
Re:Another limit? (Score:4, Interesting)
You're right that electric current creates a magnetic field. In a type-II superconductor (like the cuprates and these new FeAs materials), this is managed by introducing defects in the material (grain boundaries, inclusions, etc.) that "pin" the quantitized magnetic flux vortices and prevent them from moving through the material and destroying superconductivity. So it's not fair to say that you CAN'T use cuprates to carry large currents - it's just an engineering problem that has to be dealt with by clever manipulation of the structure of the materials.
So here's the short version:
Critical field = intrinsic property of the material.
Critical current = extrinsic property that depends on critical field, grain structure, presence of second phases, etc.
Parent
Re:Another limit? (Score:5, Informative)
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 [wikipedia.org] is used in the magnets for the ITER [iter.org] 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 [wikipedia.org] superconductors.
For non-magnet applications (like power transmission), the preferred technique is to make a tape (e.g. YBCO [wikipedia.org]) 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.
Parent
Re:Another limit? (Score:4, Insightful)
Parent
Interesting... (Score:4, Insightful)
Scanning the paper, it seemed to have little bearing on this magnetic field tolerance, but rather talked about the effects of grain boundaries. Did anyone understand how the paper related to the press release?
Summary is flat-out wrong. (Score:5, Informative)
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).
Re:Summary is flat-out wrong. (Score:5, Insightful)
If you have a 60T magnet laying around, please get in touch. I have an evil plan that needs hatching.
Parent
Re:Summary is flat-out wrong. (Score:5, Informative)
There's a 60T pulsed magnet at LANL. "Power comes from a pulsed power infrastructure which includes a 1.43 gigawatt motor generator and five 64 megawatt power supplies. The 1200-ton motor generator sits on a 4800-short ton (4350 t) inertia block which rests on 60 springs to minimize earth tremors."
And they're building a 100T edition.
Parent
Re:Summary is flat-out wrong. (Score:5, Interesting)
Parent
60 T is pretty strong (Score:5, Interesting)
- 1.5 T is what's used in MRIs (people have died when metallic objects fly around in these fields)
- 16 T will levitate a frog
- 45 T is the strongest magetic field continuously produced in a laboratory.
- 10,000 T is instantly lethal to organic life
Basically we're fine levitating frogs, but probably won't be able to use it as part of an instant-death ray.Parent
Re: (Score:3, Funny)
If only they'd expressed it in powers of two (e.g. 2^16).
"Immune to Gravity" coming soon? (Score:5, Funny)
Re:"Immune to Gravity" coming soon? (Score:5, Funny)
Parent
Re:"Immune to Gravity" coming soon? (Score:4, Funny)
Parent
Re: (Score:3, Funny)
Most expensive by weight, that is. Additionally, higher quality vacuums are exponentially more expensive!
Re: (Score:3, Funny)
Did you ever heard about something involving two cats?
The only detected problem was the noise...
Worst. Summary. Ever. (Score:5, Interesting)
reality check (Score:5, Informative)
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"
Re:reality check (Score:5, Informative)
--
Just another condensed matter physicist.
Parent
Re: (Score:3, Insightful)
But having a new class of super conductors opens up further research into new high temp ones.
Re:Internal Resistance (Score:5, Insightful)
Parent
Re: (Score:3, Funny)
Just don't tell today's kids that the number of cubic feet in a gallon is
Re:Internal Resistance (Score:5, Insightful)
Parent
Re:Internal Resistance (Score:4, Informative)
Parent
Re:Internal Resistance (Score:4, Informative)
Use Google.
e.g. http://www.google.com/search?hl=en&q=0F+in+C [google.com]
It does other unit conversions kph to mph, US gallon to UK gallons, currency conversion.
And also stuff like how long it takes to transfer 700MB over a 512Kbps link:
http://www.google.com/search?hl=en&q=700+MB%2F+512kbps [google.com]
http://www.google.com/search?hl=en&q=700+MB%2F+512Kbps+in+seconds [google.com]
Parent
Re:Internal Resistance (Score:4, Funny)
But useful for McGuyver!
Parent
Re: (Score:3, Funny)
~Wolverine