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Did We Discover Room-temperature Superconductors?  
User currently offlineDocLightning From United States of America, joined Nov 2005, 18714 posts, RR: 58
Posted (3 years 1 month 1 week 2 days 18 hours ago) and read 3316 times:

Quote:
They chemically dope graphene to significantly higher levels than previously achieved and then probe its band structure with angle-resolved photoemission spectroscopy. The saddle point becomes more extended than localized as the Fermi surface moves across it. The authors calculate that, under these conditions of doping and Fermi surface topology, graphene can achieve superconductivity, in principle due to electron-electron interactions alone. – Sami Mitra

If it turns out that doped graphene can superconduct at ambient temperatures, what will we be able to do that we can't do now?

14 replies: All unread, jump to last
 
User currently onlineBMI727 From United States of America, joined Feb 2009, 15504 posts, RR: 26
Reply 1, posted (3 years 1 month 1 week 2 days 16 hours ago) and read 3280 times:

Quoting DocLightning (Thread starter):
what will we be able to do that we can't do now?

Quite a few things for sure. For one thing, certain pieces of machinery like MRI machines can probably get a fair bit smaller. It depends on the limits of this particular type, but suffice to say, practical superconductors in general have been sought after for quite a while and people should have no shortage of applications.



Why do Aerospace Engineering students have to turn things in on time?
User currently offlineMD11Engineer From Germany, joined Oct 2003, 13815 posts, RR: 63
Reply 2, posted (3 years 1 month 1 week 2 days 12 hours ago) and read 3213 times:

Quoting DocLightning (Thread starter):
Quote:
They chemically dope graphene to significantly higher levels than previously achieved and then probe its band structure with angle-resolved photoemission spectroscopy. The saddle point becomes more extended than localized as the Fermi surface moves across it. The authors calculate that, under these conditions of doping and Fermi surface topology, graphene can achieve superconductivity, in principle due to electron-electron interactions alone. – Sami Mitra

If it turns out that doped graphene can superconduct at ambient temperatures, what will we be able to do that we can't do now?

Depending on how far these superconductors are sensitive to magnetic fields, which can induce a quench (sudden loss of supraconducting properties, essentially the electric resistance suddenly goes from zero to regular while a strong current is flowing through the material. The results are some spectacular fireworks, as has been seen with the large hadron collider last year.), it might make nuclear fusion much cheaper.
One of the biggest problems with commercial application of nuclear fusion is the creation and modelling of the strong magnetic fields which keep the plasma in place in the middle of the reaction chamber.
Currently conventional supraconducting magnets are used, which have to be cooled expensively to the temperatures of liquid helium. The cooling subtracts a lot of energy from the output of the plant and the magnets are very expensive.

Jan


User currently offlinesprout5199 From United States of America, joined Feb 2005, 1833 posts, RR: 2
Reply 3, posted (3 years 1 month 1 week 2 days 11 hours ago) and read 3206 times:

Quoting DocLightning (Thread starter):
If it turns out that doped graphene can superconduct at ambient temperatures, what will we be able to do that we can't do now?

Can you imagine how effecient a power grid would be without voltage drop? there is a lot of loss in the transmission lines of a grid.

Dan in Jupiter


User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 4, posted (3 years 1 month 1 week 2 days 9 hours ago) and read 3158 times:

Here's an interesting blog article (mostly in English) about graphene and superconductivity. If it's possible, that is very significant. http://torcuil.wordpress.com/2008/03...at-room-temperature-with-graphene/

One issue may be thickness -- graphene is a form of carbon arranged in a sheet that is one atom thick -- sort of like an unrolled carbon nanotube. That may limit the total amount of current that can be carried -- for example power lines. But there are probably many applications (for example maybe computing or medical sensing) where the low current flow isn't an issue.


User currently offlineMD11Engineer From Germany, joined Oct 2003, 13815 posts, RR: 63
Reply 5, posted (3 years 1 month 1 week 2 days 9 hours ago) and read 3153 times:

Quoting NoWorries (Reply 4):
Here's an interesting blog article (mostly in English) about graphene and superconductivity. If it's possible, that is very significant. http://torcuil.wordpress.com/2008/03...at-room-temperature-with-graphene/

One issue may be thickness -- graphene is a form of carbon arranged in a sheet that is one atom thick -- sort of like an unrolled carbon nanotube. That may limit the total amount of current that can be carried -- for example power lines. But there are probably many applications (for example maybe computing or medical sensing) where the low current flow isn't an issue.

one could probably design sandwich systems of graphene layers and a matrix made out of some other material.

Jan


User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 6, posted (3 years 1 month 1 week 2 days 9 hours ago) and read 3143 times:

Quoting MD11Engineer (Reply 5):
one could probably design sandwich systems of graphene layers and a matrix made out of some other material.


Seems very plausible. Lots of challenges along the way, no doubt. For example, having layers of conductors separated by insulators create a system of conductors that are coupled by capacitance. Shouldn't be a problem for DC current, though. Splicing cables like that would prove very challenging -- but solving difficult problems is often part of innovation. With or without layering, it seems like it would be difficult to maintain the integrity of a single-atom conductor in something that is flexible. But, I'm not saying it couldn't happen, just saying it seems more likely that initial applications -- if this turns out to be true -- will be on a small scale.


User currently offlineDocLightning From United States of America, joined Nov 2005, 18714 posts, RR: 58
Reply 7, posted (3 years 1 month 1 week 2 days 5 hours ago) and read 3094 times:

Quoting NoWorries (Reply 4):

One issue may be thickness -- graphene is a form of carbon arranged in a sheet that is one atom thick -- sort of like an unrolled carbon nanotube. That may limit the total amount of current that can be carried -- for example power lines. But there are probably many applications (for example maybe computing or medical sensing) where the low current flow isn't an issue.

Graphene conducts electricity only within the plane of the sheet. So a stack of graphene would work very well. You could make long graphene ribbons and stack them to form a graphene "wire." In order for them to be superconducting, you would want to ensure that they are as close to one piece as possible with as few joins between wires because as the electrons need to jump from one sheet of graphene to the next, they will lose superconductivity, I'd imagine.


User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 8, posted (3 years 1 month 1 week 2 days 4 hours ago) and read 3066 times:

Quoting DocLightning (Reply 7):
Graphene conducts electricity only within the plane of the sheet. So a stack of graphene would work very well. You could make long graphene ribbons and stack them to form a graphene "wire."


That's kinda neat. The article seemed to indicate that when the sheet interacted with the substrate it could lose it's superconducting properties -- so that would be convenient if it turned out not to be a problem when they are stacked.

Quoting DocLightning (Reply 7):
In order for them to be superconducting, you would want to ensure that they are as close to one piece as possible with as few joins between wires because as the electrons need to jump from one sheet of graphene to the next, they will lose superconductivity, I'd imagine.


Hard to say, I'm assuming the individual pieces would maintain their superconductivity, the jumps might introduce a small regions of resistance, but in the grand scheme of things that probably isn't too bad. But sure, if there are a lot of joins it seems like performance would drop.

There's another article -- http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.104.136803 but I don't feel like paying the $25 to read it. I'm wondering what the the critical current density is -- that will determine how practical it would be for power transmission -- essentially how much of the material is needed to support a given current flow.

The other issue would be flexibility -- do the sheets retain their superconductivity when they flex? With traditional superconductors it's important that the "structure" of the material remain very static -- even slight molecular movements (e.g. molecular vibrations from heat) can destroy superconductivity. So would flexing disrupt the superconductivity. It would be really really neat if this material was robust under a variety of conditions. If it is, and scales up, and can be produced in quantity at a reasonable price, this could change many things.


User currently offlineDocLightning From United States of America, joined Nov 2005, 18714 posts, RR: 58
Reply 9, posted (3 years 1 month 1 week 2 days 1 hour ago) and read 3035 times:

Quoting NoWorries (Reply 8):

The other issue would be flexibility -- do the sheets retain their superconductivity when they flex?

I believe that to within certain tolerances, yes.

The superconductivity of graphene is hardly a surprise. Graphene is basically a bunch of benzene rings stuck together edge-to-edge.

In a benzene ring, there are six carbon atoms. Each has three sp2 orbitals projecting out of it in a planar configuration with even spacing of 120°. Two of these sp2 orbitals contact sp2 orbitals on adjacent carbon atoms and the third contacts a hydrogen atom on the outside of the benzene ring.

In the above image, you can see how those sp2 orbitals are arranged. But the thing is that each of these orbitals is discrete. Electrons cannot move from one sp2 orbital to another.

But there is a 4th orbital, called a p orbital that extends at 90° angle to the plane of the sp2 orbitals. One lobe on each face of the ring. Typically, each of these orbitals is occupied by one electron. But when in the form of a benzene ring, the orbitals merge together to form a single, large "delocalized" orbital that exists in a ring on each facet of the benzene molecule. Unlike the sp2 orbitals, electrons can move freely through the entire delocalized orbital created in the benzene ring.

This diagram shows the basic idea:


OK, now consider this: in a graphene sheet, it's basically a bunch of benzene molecules but instead of hydrogen atoms on the outside, each carbon atom is bonded to another carbon. Thus:



So you can see how now the p orbitals could be delocalized across the entire sheet of graphene. In other words, the orbital of any given electron is the entire sheet! This means that electrons can travel freely across the sheet and, indeed, behave as if they were massless photons. It is also obvious, with this explanation, as to why graphene conducts electricity in one direction only.

A strong enough bend in the sheet might disrupt the orbitals enough to suppress this phenomenon, but I don't think that the sorts of bends that you would see in a typical power line would be enough to cause a disruption of the common orbital. I know that buckytubes are strongly conductive and that they are also being worked on as potential superconductors, but they are most certainly bent at far higher angles than you would ever see in a graphene wire, so clearly some bend is tolerable.


User currently offlineMD11Engineer From Germany, joined Oct 2003, 13815 posts, RR: 63
Reply 10, posted (3 years 1 month 1 week 2 days 1 hour ago) and read 3022 times:

Traditional high temperature supraconductors are manufactured like this:
E.g. a copper pipe of about an inch diameter and 10 feet length is being filled with the ceramic supraconductor in finely powdered form, then it´s ends are welded shut. This pipe is then first narrowed down in a pilgrim step rolling mill and then, when it reached a diameter of maybe 3/8 of an inch (with a corresponding increase in length to maybe 40 feet), drawn through a wiremaking die until it´s diameter is about 3/16 of an inch. The last step uses a hexagonal die to give the wire a hexagonal shape. After heat treating this wire in an inert gas filled furnace to soften it, it is cut into 30 feet sections, which are being bundled, so that the bundle diameter reaches about an inch again. Then the ends of the bundle are welded together and the whole bundle again pushed through the rolling mill and the wire drawing dies (with steps of heat treatment included), until the final wire has a diameter of 1/32 of an inch. The result is a copper wire with embedded channels of supraconducting material, which can be bent and used like any other wire.

Jan


User currently offlineDocLightning From United States of America, joined Nov 2005, 18714 posts, RR: 58
Reply 11, posted (3 years 1 month 1 week 2 days ago) and read 3004 times:

Quoting MD11Engineer (Reply 10):
Traditional high temperature supraconductors are manufactured like this:
E.g. a copper pipe of about an inch diameter and 10 feet length is being filled with the ceramic supraconductor in finely powdered form, then it´s ends are welded shut. This pipe is then first narrowed down in a pilgrim step rolling mill and then, when it reached a diameter of maybe 3/8 of an inch (with a corresponding increase in length to maybe 40 feet), drawn through a wiremaking die until it´s diameter is about 3/16 of an inch. The last step uses a hexagonal die to give the wire a hexagonal shape. After heat treating this wire in an inert gas filled furnace to soften it, it is cut into 30 feet sections, which are being bundled, so that the bundle diameter reaches about an inch again. Then the ends of the bundle are welded together and the whole bundle again pushed through the rolling mill and the wire drawing dies (with steps of heat treatment included), until the final wire has a diameter of 1/32 of an inch. The result is a copper wire with embedded channels of supraconducting material, which can be bent and used like any other wire.

Yes, but they only superconduct at very cold temperatures. Warmer than classical superconductors, but still very cold. Cold enough that even in Siberia they couldn't be used for power transmission in January.


User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 12, posted (3 years 1 month 1 week 1 day 23 hours ago) and read 2976 times:

Quoting DocLightning (Reply 9):
The superconductivity of graphene is hardly a surprise. Graphene is basically a bunch of benzene rings stuck together edge-to-edge.

That suggests that it should be no suprise that Graphene is a semiconductor, but not a superconductor. It has extremely high electron mobility, but high mobility does not make it a superconductor -- silver for instance has the highest electron mobility of any conventional conductor and it cannot be coerced into superconducting under any situation. Graphene's high mobility only makes it a good semiconductor. In the quoted article above, the graphene is doped, in other words certain impurities are added. It may seem like a bit of a nitpick, but doped graphene is very different from graphene in terms of its properties.

Quoting DocLightning (Reply 9):
So you can see how now the p orbitals could be delocalized across the entire sheet of graphene. In other words, the orbital of any given electron is the entire sheet!

Superconductors don't depend on high electron mobility -- they depend on the formation of cooper pairs (pairs of electrons that counterintuitively experience weak attraction) -- which in turn is due to some unique structure of the material. In the case of graphene, it's the doping imperfections in the structure that enable superconductivity.

Quoting NoWorries (Reply 8):
A strong enough bend in the sheet might disrupt the orbitals enough to suppress this phenomenon, but I don't think that the sorts of bends that you would see in a typical power line would be enough to cause a disruption of the common orbital

I was thinking more along the lines of very slight bends -- since superconductors are so sensitive to even slight structural variations, I was thinking that twists and turns might upsets things. On balance though, I'm thinking probably not -- it's really the localized translations due to heat that break superconductivity, and graphene seems especially resistant to that type of movement.

Quoting DocLightning (Reply 9):
This means that electrons can travel freely across the sheet and, indeed, behave as if they were massless photons.

This is a very interesting phenomenon in graphene that mimics relativistic particle pairs (e.g. electron and positron pairs traveling together at the speed of light) as first predicted by Paul Dirac -- sometimes called Dirac fermions. This is not the mechanism of superconductivity. The mechanism of superconductivity is cooper pairs.


User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 13, posted (3 years 1 month 1 week 1 day 23 hours ago) and read 2973 times:

Quoting MD11Engineer (Reply 10):
Traditional high temperature supraconductors are manufactured like this:

Yep, for example the niobium-titanium superconductors used in the LHC. My earlier post about being susceptible to bending problems doesn't really have any basis -- it's really thermal motion that disrupts superconductivity.


User currently offlineMD11Engineer From Germany, joined Oct 2003, 13815 posts, RR: 63
Reply 14, posted (3 years 1 month 1 week 1 day 19 hours ago) and read 2939 times:

Quoting DocLightning (Reply 11):

Yes, but they only superconduct at very cold temperatures. Warmer than classical superconductors, but still very cold. Cold enough that even in Siberia they couldn't be used for power transmission in January.

I just gave it as an example how a normally hard and brittle ceramic oxide superconductor can be brought into a form suitable for technical application. I know that "high" in this context refers to temperatures of liquid nitrogen (-196´C or 77k)

Jan

[Edited 2011-03-17 21:59:27]

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