While the possibility of
room-temperature superconductivity has largely dominated public interest,
the main problem with current superconductors is the fabrication cost rather
than the cooling requirements. This is true even when liquid helium is
needed, despite that being much more expensive than liquid nitrogen cooling.
NbTi systems can be produced for ~$1.5/kA-m, so economically viable proposals
for high-temperature superconductors should aim for lower cost than that.
Much of the cost difference between NbTi and high-temperature superconductors is due to the latter being ceramics, which are harder to process into wire than metal alloys. The essentially 2-dimensional nature of most superconductivity allows for the possibility of solution spinning of soluble superconductor into wires. Below, I'll describe 2 new proposals for accomplishing that.
1) wide conjugated polymers
In 1964, William Little published a paper suggesting that polyacetylene with pendant chromophore groups could be a high-temperature superconductor. This led to interest in organic superconductors, but Little's proposal was not just difficult to synthesize but also fundamentally flawed. Extrapolation from studies of superconductivity in metallic nanowires indicates to me that organic superconductivity in polymers requires a polymer width of 3nm to 8nm, which is much wider than typical polymers.
Lower width should lead only to weaker superconductivity that can be found in organic crystals, such as K-doped polycyclic hydrocarbons with Tc ~= 33 K.
However, wide polymers can be synthesized using Diels-Alder polymerization of tetraphenylcyclopentadienone acetylide, as was recently demonstrated by people at the Max Planck Institute for Polymer Research. The dibenzyl ketone used to make tetraphenylcyclopentadienone could be substituted to allow attachment of oligomers of some conductive polymer, eg poly(p-phenylene) made by Ni(0) catalyzed coupling of aryl halides.
However, graphite is not a good
superconductor despite being wide, so we know that something else is
required. The advantage of this system over graphite could come from:
1) precise side chain lengths (synthetically difficult) creating a resonance frequency
2) use of heterocycles instead of phenylene groups
3) electron-rich functional groups on the side chains, such as dimethylamine
While I think this topic deserves some further consideration, I also think the approach (2) below is more promising, so I'll move on to that.
2) CuO2 with organic ligands
YBCO and BSCCO are
superconductive due to hole pairs in doped CuO2 planes, which are stabilized
by the other ions. If the yttrium and barium in YBCO could be replaced by
organic ligands, those ligands could have side chains to make the complex
CuO2 does not have enough space for one side to have an organic ligand for each Cu atom. So, either each side must have ligands for only half the Cu atoms, or each ligand must bind to 2 Cu atoms. Either way, there is 1 ligand per Cu atom, so each ligand must be able to exist in both +1 and +2 charge states. Because the CuO2 planes must be doped, the ligands must be oxidizing with an appropriate potential.
Some examples of possible ligands (not including solubilizing side chains) are pictured below. Weakly coordinating ions such as PF6 and tetrabutylammonium can be added to adjust the doping level.
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