Bi2212 Conductor and YBCO Cable Programs Supercon, NCSU, MIT Bill Nachtrab, Justin Schwartz, Amir Kajbafvala, Golsa Naderi, Makoto Takayasu, Joe Minervini, Mark Rudziak

HTS TOPICS High Strength Ag-Al 2 O 3 Alloys Bi2212 Precursor Powder Leakage Bi2212/Ag Saw-Tooth Heat Treatments YBCO Tape Cables YBCO Cable Joints

High Strength Ag-Al 2 O 3 Alloy Only Current Bi2212 Program Expected Benefits  Improved strain tolerance  Better filament uniformity  Increased resistance to leakage

High Strength Ag-Al 2 O 3 Powder Processing Fluidized Bed Heat Treating Completed Furnace Design and Construction Completed Process Development for Annealing and Internal Oxidation – Anneal and Oxidize Powder without Sintering – More Uniform Oxidation Reaction – Improved Metallurgical Structure

Alloy and Thermomechanical Processing Evaluating four alloys – 0.5, 0.75, 1.0, 1.25 Al – Oxidation kinetics – Hardness vs. oxidation time and temperature – 0.5 and 0.75 Al exhibit sufficient strength and stability up to 900 °C Processing internally oxidized powder – Drawing and annealing studies Conductor fabrication – Delayed for precursor

Bi2212 Precursors Granulate and Powder Quality is inconsistent Not optimized for wire drawing – Low packing density – Hard particles Greatly affects filament structure – Sausaging and continuity – Reduces J c Most experience with granulate Chemically produced precursor generally have higher carbon, poor packing density and poor drawing quality Nexans Granulate Nexans Powder SCI Powder

Granulate Precursor Powder Good chemical composition Low carbon < 30 ppm High phase purity < 3% non 2212 phases Not optimized for wire drawing Low tap density Contains large hard agglomerates – Causes sausaging and discontinuities in filaments New lot has particularly high content of large, hard particles. 40% lower I c than first lot.

Powder Processing to Improve Precursor Grinding and milling to remove large particles, increase density and improve filament uniformity Objective – Andreasen type size distribution – F(D) = A (d/d L ) 0.6 – D 90 < 2µm – D max < 3 µm – Low aspect ratio x ≈ y ≈ z E = x/y - elongation F = y/z - flatness – tap density > 4 g/cm 3 – Hall flow rate < 40 sec

Leakage Our view of leakage Ag absorbed into alumina-silica ceramic through direct reaction or evaporation – seems to displace Si Reduces effective thickness and weakens sheath Sheath ruptures at weak point – grain boundaries, oxide particles Gas pressure likely contributes to rupture as most leaks seem to be pin-hole type

Test to Evaluate Leakage Resistance Working on test to evaluate leakage resistance Semi-quantitative Correlate time to leak at 885 °C to leakage in coils Use this test to evaluate – Pretreatments – Atmosphere – Sheath thickness

Saw-Tooth Heat Treatments Heat treatment has large effect on J c Cyclical heat treatment can be effective in reducing 2201 and non- SC phases to increase J c Work by Golsa Naderi at NCSU on “Saw-Tooth” Heat Treatments Temperature cycling accelerates transformation in peritectic systems where transformation rate is controlled by solid state diffusion. T l1 Hot Saw Tooth Processing (HSTP) TpTp T1T1 T r1 T2T2 T r2 T3T3 T4T4 T l2

Saw-Tooth Heat Treatment 11B conductor – Original lot of Nexans granulate precursor (best lot) – 0.8 mm diameter – Single restack – 45% SC fill – 20 µm filaments Improvement of 1.5X in J e compared to standard PMP Reduction in 2201 and AEC phases

MIT-Supercon YBCO Cable Program (Phase II) Test Conditions: 77 K, SF I c = E c =100 micro-V/m, n= tape conductor stack 200 mm twist pitch 7.8 mm overall diameter MIT

Alternate Cable Designs With these configurations, tapes are wound into twisted U- channel or Y-channel copper guide, and the assembly is inserted into a copper tube or wrapped with copper tape. Three Slot Y-Channel Single Slot U-Channel MIT

Cable Joints Current Phase I Program STTR with MIT Mechanical design and fabrication of connectors and terminations for YBCO cable MIT based design concept Supercon to develop design for manufacture and fabricate joints MIT to perform electrical and mechanical evaluation MIT

Status and Issues for Bi2212 Conductor Currently there is no market for Bi2212 wire Not producing standardized commercial product No incentive to invest in technology due to lack of market driven application Main Developmental Issues – Better understanding of heat treating – Better precursor – Leakage control without reducing J c