5. Superconductors: Conductors and Applications Flashcards
What are the basic material requirements for a superconducting wire?
- Ductile/easy to form into a wire
- Large Hc2 (field at which superconductivity disappears)
- Large Jc (Critical current)
What force interaction occurs at the critical current?
At the critical current point, the Lorentz force on the vortex lines cause them to start moving (it exceeds the pinning force)
Why is the transition to higher resistances not a sudden change with straight lines at the critical current?
Because of thermal fluctuations, the random vibrations can cause breakdown of superconductivity at a range of currents
According to the bean critical state model, what happens in a constant magnetic field to a superconductor?
A constant current density Jc is established up to some depth in the superconductor
What happens to Jc if Temp increases, and thus the penetration depth
It decreases, so thus the penetration depth must increase
What is ‘quenching’ in superconductors?
It is the cascade process where an initial temperature increase in a superconductor causes Jc to fall, the penetration depth to increase to make up for this, the moving of flux lines to do this is dissipative and releases heat, causing a knock on further temperature rise and so on.
What are the methods to improve the stability of superconductors and reduce ‘quenching’?
- Add conductive metal eg copper (improves cooling of superconductor, called cryogenic stabilisation)
- Adiabatic stabilisation (find a critical thickness from the critical state model)
Is superconductivity resistance-less for both AC and DC?
No, just DC as AC moves flux lines when it reverses direction
Why is there a limit to the amount of copper you can put around a superconductive cable?
As if it is too conductive you’ll get eddy current losses
What does the AC losses in a type 2 superconductor depend on?
The frequency and the thickness of the conductor
Why is it beneficilal to use a phase diagram to process a material such that it grows a precipitate?
THe precipitation phase will act as pinning centres for magnetic flux lines
What differentiates intermetallic materials from metals?
They have covelent bonds rather than metalic, which makes them brittle and hard to deal with
What are the 3 practical methods for producing Nb3Sn wires (intermetallic)?
Bronze process
Internal Tin
Powder in tube (PIT)
What are the two main issues processing the ceramic High temperature superconductors into practical wires?
- Theyre too brittle
- Crystaline misorientation drastically reduces Jc (therfore they need to processed in a way to ensure crystalign allignment)
What industries are high temperature superconducting wires aimed at?
Industries willing to pay alot for small increases in performance eg Military, aviation, medicine
