Sinclair- Fabrication of Superconducting Tapes and Cables Flashcards
Example of 1st and 2nd generation SCs
1st is BiSCCO
2nd is YBCO
How are today’s 2G YBCO tapes made?
Based on very long and 4cm wide strips of SC material. Produced in a high speed continuous reel-to-reel deposition process (similar to the low cost production of motion picture film in which celluloid strips are coated with a liquid emulsion). Wires are laminated on both sides with copper, stainless steel or brass metals to provide strength, durability and certain electrical characteristics needed in applications
What happens in the vortex state of type 2 SC?
There is flux penetration but still 0 resistivity providing the current is not too high
What type of SCs are needed for magnets and transmission cables and how were they originally made?
Type 2. Early methods used alloying to convert elemental metals (type 1) into type 2 SCs. Led to the development of low temperature superconductors (LTSC)
Applications of LTSC
Superconducting magnets. Large hadron collider has 1200 tonnes of NbTi cable cooled to 2K to work safely up to fields of about 8.3T
Why is it surprising that alloying produces superconductors from elemental metals?
Normally resistivity is lower in pure metals as the mean free path λ is larger.
Describe the Meissner effect
At T less than Tc, a surface current is set up on the surface of the SC that is sufficient to generate an internal magnetic field that exactly cancels the applied magnetic field. This screens the inside of the SC from the magnetic field. The surface current is confined to a thin layer called the penetration depth, usually 10-100nm
What is the penetration depth?
Symbol λsubL (London penetration depth). The distance in which the magnetic field decreases by a factor of 1/e within the surface of the superconductor. Used in the formula:
B(x)=B0exp(-x/λL)
What is the coherence length?
Symbol ξ. A measure of the distance within which the SC electron concentration (in the form of Cooper pairs) cannot change drastically in a spatially varying magnetic field. The intrinsic coherence length ξ0 is characteristic of a pure SC. In impure materials and alloys ξ is shorter than ξ0.
What do the coherence length and penetration depth depend on and how?
The mean free path l of the electrons in the normal state. Graph of ξ/ξ0 vs l/ξ0 is constantly high on the right then curves down at increasing rate for small l. Graph for λ/ξ0 starts constantly low then curves up at increasing rate for small l. A small l favours type 2 superconductivity
How do coherence length and penetration depth depend on Tc?
They both diverge at Tc but their ratio (Ginzburg-Landau parameter κ) is independent of temperature near Tc.
This is κ=λ/ξ
How is the Ginzburg-Landau parameter κ used?
κ>1/rt(2) is type 2 behaviour κ<1/rt(2) is type 1 behaviour When SC very impure like an alloy then l is very small and κ=λ/l This is the dirty superconductor limit
Describe the vortex state for type 2 SCs
Above Hc1 and below Hc2. Have normal cores surrounded by SC current vortices. Normal cores about 300nm is diameter called vortices or fluxoids. The flux lines from the currents form an ordered structure called a flux or vortex lattice.
How does the vortex state work?
As there is normal material present, magnetic field penetration can occur in the normal cores but is excluded from the SC current vortices (SC regions). The currents act so as to repel each other. If the vortices are stationary (pinned) the magnetic field can penetrate while still maintaining zero electrical resistivity through the material.
How does temperature or external magnetic field affect the vortex state?
As either are increased the normal cores are more closely packed and eventually overlap as the SC state is lost to the normal state. The vortices feel a force when current flows and if they move the SC state is lost.
