Sinclair- Defects, Problems and Production Methods in SCs

Question 1

Vortices in cuprates and problem in 2223

Answer 1

Ideally type 2nwill carry non-dissipatrice current density smaller than Jc. Vortex lines continuous in isotropic SC but form weakly coupled pancake vortices whose circulating screening currents are mostly confined within the CuO2 layers. Flux pinning improves Jc but for wires of 2223 the cross section of current carrying regions is <100% due to porosity, cracks and secondary (non SC) phases.

Question 2

What affects flux pinning interactions?

Answer 2

Atomic point defects, variation in O content, nanoscale defects, grain boundaries.

Question 3

Why do you need to create texture in oxide-based conductors?

Answer 3

Good crystal orientation so minimise influence of ‘weak links’ linked grain boundaries and grain-grain misorientation.

Question 4

Grain boundaries in BiSCCO and YBCO

Answer 4

Large devices need km of polycrystalline conductors and so GBs play important role. They are strong barriers to current flow in high Tc SCs especially YBCO. Jc across GBs decreases exponentially compared to in the grains as a function of misorientation angle between crystallites. Need to texture the ceramics to minimise effect. Rolling 2223 induces marked uniaxial c-axis texture but YBCO requires epitaxial growth to get GB angle <5° as required

Question 5

Grain boundaries in low Tc alloys

Answer 5

Contrast high Tc ones as these GBs are transparent to current and also contribute to flux pinning and increase Jc as grain size decreases. This is because of the strong dependence of Tc on the hole concentration and low carrier density.

Question 6

How does misorientation angle affect high Tc SCs?

Answer 6

As it increases the spacing between the insulating dislocation cores decreases and becomes of the order of the coherence length for angles of 5-7°. The GBs posses extra ionic charge whose magnitude increases with misorientation angle and causes hole depletion. They are underdoped regions and become weak links

Question 7

Way of improving Jc at GBs in YBCO

Answer 7

Locally dole them with Ca for Y

Y3+ -> Ca2+ + h+

Question 8

Current percolation in polycrystalline material

Answer 8

All conducting materials except NbTi are brittle so cracks are endemic. Oxides prône to porosity and BiSCCO can also have secondary phases. Current percolates via many instructions, some are partial blocks and others total blocks. Local Jc can exceed average Jc by factor of 5. Get better regions at Ag/BiSCCO interface. Current percolates in 3D manner in BiSCCO wires/tapes

Question 9

Fabrication problems with BiSCCO

Answer 9

Require long length of cable of brittle ceramic that is difficult to draw as wires. Require high Jc but polycrystalline samples have poor grain alignment, have GBs and porosity and conduction highly anisotropic. Best Bi-material is Pb-doped Bi-2223. Ag sheath is expensive (partly overcome by replacing some Ag content with Ni.

Question 10

Why is Pb doping a problem in BiSCCO?

Answer 10

Difficult to make phase pure and any secondary phases (non SC) will block the current. Difficult to control the cation composition due to volatility of Pn and Bi at sintering temperatures (880-890C). Oxygen content also varies with processing conditions and needs to be optimised to obtain Tc,max.

Question 11

Steps in PIT process for making 1G BiSCCO conductors

Answer 11

Powder in tube process. Make BiSCCO 2212 powder:
Bi2O3 + 2 SrCO3 + CaCO3 + 2CuO -> Bi2Sr2CaCu2O8+δ + 3CO2(g)
Pack powder in Ag sheath, deform, stack, deform, heat treatment:
1. Ramp up at 10C/min to 890C.
2. Hold at 890C for 2-6mins
3. Cool to 865C and hold for 4-8hours
4. Slow cool to room temperature

Question 12

Important information from phase diagram for BiSCCO

Answer 12

Bi-2212 undergoes a peritectic reaction at 880C.
2212 goes to liquid phase and Bi2(Sr,Ca)4O7 and (Sr,Ca)14Cu24O38 (latter two not SC)
These are 2:4 and 14:24 phases

Question 13

Why won’t solid phase sintering work for BiSCCO?

Answer 13

The Ag expands more than it and won’t be in good thermal contact with the ceramic. Liquid also useful for surface tension.

Question 14

Why 890C for 2mins for BiSCCO?

Answer 14

Partial melting of BiSCCO produces some liquid phase which assists with sintering (liquid phase sintering) and it produces wetting of the Ag sheaths. Holding for longer periods produces too much liquid and a lot of non-SC 2:4 and 14:24 phases and BiSCCO will break down.

Question 15

Why 865C for 8hours for BiSCCO?

Answer 15

Reformation of 2212 from liquid + 2:4 + 14:24 secondary phases and allow for some grain alignment (texturing). This takes time

Question 16

Why slow cool to 25C for BiSCCO?

Answer 16

To allow pick-up of O2 to adjust oxygen content (δ) for 2212 to optimise Tc based on Cu2.15+. This requires diffusion of oxygen through the Ag sheath and into the ceramic core

Question 17

Why is YBCO better than BiSCCO?

Answer 17

Higher intrinsic Jc
Higher H* at 77K
Lower anisotropy
Reduced metal costs (no Ag)
Can eliminate weak links associated with GBs
Strong flux pinning to enhance grain Jc and H*

Question 18

Solution to GB problem in YBCO

Answer 18

Biaxial textured growth. Techniques that deposit YBCO onto tapes of Ni, steel, etc so that the SC displays near perfect crystalline orientation (single-crystal like). Can use:
RABiTS (rolling assisted biaxial oh textured substrate) or
IBAD (ion-beam-assisted deposition)

Question 19

Components of RABiTS/MOD method for YBCO

Answer 19

Ni-based substrate with aligned crystalline texture (3wt% W). Well oriented in two crystallographic directions (biaxially textured). Buffer layers Y2O3, YSZ, CeO2 upwards from Ni to transfer texture and adjust lattice structure suitable for deposition of nearly single crystalline YBCO. YBCO film deposited by metal-organic deposition MOD, nano-dots of BaZrO3 incorporated. Ag on top.

Question 20

Why is BaZrO3 incorporated in YBCO film?

Answer 20

To produce extended defects with enhanced flux pinning characteristics to significantly increase intra-grain Jc. It is perovskite and doesn’t react with YBCO and lattice mismatch of about 7% results in extended defects. Zr won’t replace the Cu as is too big but Ni would.

Question 21

MOD process

Answer 21

Like sol-gel synthesis relies on hydrolysis and condensation of metal alkoxides. Starting precursors. Coating solution. Spin/dip/spray coating. Wet precursor film. Amorphous or nanocrystalline film. Heat treatment to crystallisation

Question 22

Why are artificial pinning centres APC important?

Answer 22

For optimising Jc. They create non superconducting regions to generate pinning forces that oppose vortex movement.

Question 23

Issue for APCs

Answer 23

Not one ideal microstructure involving APCs. Their effectiveness changes with T, B and so APC design based on the HTSC application.

Question 24

The four types of APCs and the volume of vortex they trap

Answer 24

L is length of linear vortices.
1D- dislocations (ξ^2 L)
2D- stacking faults, GBs (ξ^2 L)
3D- pores, secondary phases (ξ^2 L)
0D- vacancies/dopants in HTSC (ξ^3)

Question 25

How does temperature affect APCs?

Answer 25

At higher temperatures (65-77K) there is enough thermal energy for vortices to escape any weak pinning potentials (0D) making them inactive so 1-3D dominate. At lower T (<30K) 0D are extremely effective as low thermal energy and high density of defects.

Question 26

Compromise with APC volume fraction

Answer 26

Compromise between increasing Jc due to more pinning and decreasing Jc due to decreased HTSC volume fraction. Best obtained when APC volume fraction about 10%