Measurement of Nb3Sn conductor dimension changes during heat treatment D. Bocian, G. Ambrosio, M. Whitson   Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510-0500, USA The main author would like to thank E. Barzi, T. Beale, R. Bossert, D. R. Dietderich, M.Field, A. Ghosh, A. Nobrega, A. McInturff, D. Turrioni for their help and support during the course of this work.

OUTLINE Motivation Simulation plans Experimental setup Results Conclusions and future plans

MOTIVATION Nb3Sn demonstrate better performance than NbTi  possible use for LHC upgrade Nb3Sn is a brittle material  sensitive for strain state of the conductor During the reaction of LARP Long Nb3Sn Quadrupole coil some issues were found the inner layer had ~5.3 kN of longitudinal tension, the total length of the outer and inner layers pole parts changed by different amounts Experimental data needed to model and understand this problem Material properties E, ρ, CTE are required Strand Model Strand Subelement Model Nb barier Sn Nb+Cu Cu

HEAT TREATMENT SIMULATION Temperature 400 → 640°C Nb3Sn formation start (Sn diffusion into Nb) Need Nb3Sn properties → tuning of the model Temperature 210 → 400°C Continuation of Sn diffusion to Cu (Sn melts at 238°C) Temperature 20 → 210°C Linear/non-linear elongation Temperature 640 → 20°C → strand/cable shrinks I II III IV V VI VII VIII T [°C] 48 h Preload on Nb (z-axis) (Nb under tension, Cu under compression) 640 50°C/h 48 h 400 25°C/h ~48 h 72 h Strand/cable annealing ( 2 h at 200 °C) Linear elongation, Plastic deformation. 210 25°C/h Cu6Sn5 Cu3Sn Nb3Sn 20 7h36’ 79h36’ 87h12’ 135h12’ 140h 188h Ch. Scheuerlein et al., IEEE Trans. Appl. Supercond. VOL.18, NO. 4, 2008, Temperature=210°C Sn diffusion to Cu → Cu6Sn5 formation Strand/cable shrinkage Need Cu6Sn5 properties → tuning of the model Temperature=400°C Further Sn diffusion to Cu → Cu3Sn formation, → strand/cable shrinks, → all Sn reacted Need Cu3Sn properties → tuning of the model Temperature = 640°C → Strand/cable expand, → Plastic deformation? → Larger strand cross-section? E, ρ and CTE needed for each step

WORK PLAN Bibliography study and discussion with experts Collection of the data and material properties for simulation (limited data available, poor statistics ) Experimental data collection strand/cable sample measurement before/after heat treatment dilatometer measurement  material properties estimation FEM simulation of the strand/cable/coil behavior during reaction parameter tuning based on dilatometer measurement results Feedback to the coil fabrication technology Strand Model Strand Subelement Model Nb barier Sn Nb+Cu Cu

EXPERIMENTAL DATA COLLECTION reference data from off-line strand measurements prepare samples for dilatometer measurements basing on off-line measurements perform strand dilatometer measurements with dilatometer Dilatometer sample preparation: Sample length < 50 mm. sample ends fused and filled Linear Variable Differential Transformers (LVDT) Continous ΔL/L measurement during Heat Treatment measurement tip furnace measured sample reference material sample holder D. R. Dietderich et al., “Dimensional changes of Nb 3Sn, Nb3Al, and Bi2Sr2CaCu2O8 conductors during heat treatment and their implication for coil design.” Adv. Cryo. Eng., vol. 44b, 1013–1020,

EXPERIMENTAL SETUP Strand samples ends fused  avoid Sn leak Strand samples filled  measurements precision, dilatometer Strand sample inserted into quarts tube  keep straight, reduce environment impact Measured samples OST RRP 54/61 and 108/128, diameter 0.7 mm and 0.8 mm Strand samples twisted and non-twisted Sample length: 50, 100, 200, 300, 560 and 1000 mm Different sample ends preparation: crimped, fused, as cut Samples painted along with temperature resistant marker Measurements tools Video Measuring System (5 μm precision) – sample length Micrometer – sample diameter (5 μm precision) Twist-meter – a dedicated tool build to measure twist change

First measurements results After a few first measurements, following questions needed to be answered: Why strand sample elongate? in literature strand shrinks! (heat treatment duration?) strand twist change contribution? Why ΔL/L depend on sample length? is there end effect? Does ΔD/D depends on sample length? Any other effects? conductor type, Cu content? Before dilatometer measurements (lsample<50 mm): short sample behavior during heat treatment single strand vs. bunch of strands STRANDS ENDS: C – crimp, F – fused, NF – not fused (as cut)

Strands diameter change measurement ΔD/D remains constant with sample length. Show conformance with previous measurements (Andreev N.et al., Adv. in Cryogenic Eng.,  vol. 48B,  p.941 , 2001) STRANDS ENDS: C – crimp, F – fused, NF – not fused (as cut)

Strands length change measurement summary The length changes were measured from scratch to scratch starting from centre strand In the ends ΔL/L „end effect” observed. STRANDS ENDS: C – crimp, F – fused, NF – not fused (as cut)

End preparation impact on ΔL/L The length changes were measured from scratch to scratch ΔL/L remains constant in the straight part of strands In the ends ΔL/L show large end effect.

Strands twist change measurement

Twist change impact on ΔL/L OST RRP 108/127 strand (#12521) Twisted samples elongate during the heat treatment nt- non twisted tw - twisted

STRAND measurement summary 4/13/2018 Measured strand samples: Strand OST RRP 54/61 and OST RRP 108/127 strand diameter 0.7 mm – twisted and 0.84 mm – „non-twisted”, Sample length: 700-1000 mm, sample ends: fused Measured strand samples demonstrate: OBSERVATIONS: Twisted strands are longer after reaction because they „untwist” Correlation strand diameter change and Cu/sc ratio (54/61 – 0.85, 108/127 -1.155) SAMPLE Length change Diameter change Twist change Strand 108/127 - twisted Strand 108/127 - „non-twisted” Strand 54/61 – twisted Cable 54/61 – not constrained -

Some conclusions Dilatometer measurements - on going Strand dimension changes before/after reaction completed ΔL/L and twist change dependence confirmed by testing „non-twisted” strands Strand ends can introduce „noise” in measurements Important for measurements of short sample (< 50 mm) with dilatometer Welded and filed samples are currently the best solution „non-twisted” strands length change is the same order as measured cable length change BUT comparison done with „54/61” cables It should be compare with „108/127” cable measurements (on going) „non-twisted” strands are preferred in dilatometer measurements twist change effect eliminated very likely behaves as confined strand in cable Dilatometer measurements - on going

FUTURE PLANS PENDING NEEDED (for all HT phases) IN FUTURE? Dilatometer strand measurement NEEDED (for all HT phases) Young modulus Material density CTE (coefficient of thermal expansion) IN FUTURE? Full image of strand/cable/coil behavior during HT FEM modeling of strand/cable/coil behavior during reaction Feedback to the coil fabrication technology ← Data for simulations, (possible estimations from dilatometer measurements) Dariusz Bocian 15