I suppose that depends on your definition of practical. We have three things that might qualify right now:
1. Low temperature superconductors
Benefits:
- Easy to make, with well-known metals.
- Malleable (flexible)
- Ductile (so they can be formed into wires)
- Relatively insensitive to magnetic fields (so they can be used in maglev trains, MRIs, etc.)
Problems:
- They only operate at liquid-helium temperatures. (Some operate in the range of liquid hydrogen, so if we run out of helium, we might be alright.)
2. "High-temperature" superconductors
Benefits:
- Can use liquid nitrogen cooling, which is much cheaper than liquid helium.
Problems:
- Require unusual, sometimes costly materials.
- Are ceramics, so they aren't very malleable or ductile.
- Relatively sensitive to magnetic fields (so they aren't completely superconducting when carrying large currents, or when formed into (useful) electromagnets.)
3. Carbon nanotube wires
"...A perfect metallic nanotube should be a ballistic conductor.... Although a ballistic conductor does have some resistance, this resistance is independent of its length.... Indeed, only a superconductor (which has no electrical resistance whatsoever) is a better conductor." (
1)
Benefits:
- Works at room temperature! Wikipedia says they're stable up to 750 C in air.
- Cheap materials (e.g. carbon)
- Probably malleable, if not ductile
- Probably not sensitive to magnetic fields
Problems:
- Not a superconductor; just a very low-resistance conductor. "...nanotubes are predicted to have a minimum resistance of about 6500 Ohms, independent of their length." That's quite large, but that's per-nanotube; and "it would take several million lying side by side to cover an inch..." (
2)
- Can't store current in a loop (probably, although I wonder what would happen if a single nanotube were made into a ring).
- Hard to manufacture and manipulate (so far).
So to be "practical":
- How much does it have to be cooled, if at all? Dry-ice temperature? Regular ice temperature?
- How cheap must the materials be?
- How easy must the manufacturing process (for the material and/or for wires) be?
- How much magnetic field tolerance is required?