Low Temperature Superconductor - Fabrication and Application - BARC

BARC Newsletter

Low Temperature Superconductor Fabrication and Application A.K. Singh, M. M. Hussain, S. P. Singh and V. G. Date Atomic Fuels Division Bhabha Atomic Research Centre

Introduction

T

HE DEVELOPMENT AND fabrication of superconductors for practical application is clearly connected with the physical properties required for the construction of high field magnets. Important parameters which dictate conductor design and fabrication processes are: (a) Critical temperature, Tc and upper critical field Bc2. It depends mainly Sketch showing three critical parameters for a superconductor on alloy composition. (b) Critical current density, Jc. (c) Magnet size, i.e stored energy. It Multi-filamentary conductors are fabricated by will decide the degree of stabilisation i.e extrusion technique. The main advantages of the amount of high conductivity substrate. this technique are:

Properties of Nb-Ti Alloys In addition to the good superconducting properties of Nb-Ti alloys, the excellent combination of mechanical and metallurgical properties make this alloy one of the most important for various magnet applications. Nb-Ti alloy can be co-processed easily with different substrate materials such as Copper and CopperNickel etc. The critical current density Jc, one of the critical parameters shown in the sketch above, is a structure dependent and depends very strongly on the density, size and distribution of imperfection such as dislocations, grain boundaries and precipitates which act as flux pinners.

a) b) c)

Radial stress field during area reduction. Homogeneity of plastic flow. High area reduction in a single step.

Since compressive forces are acting during the process very high packing density of the extrusion billet is required.

Assembly of Extrusion Billet This is the first step of extrusion process. To achieve high packing density different methods are used. Depending on the force of the extrusion press and diameter of the extrusion container, OFHC (Cryogenic grade) copper sleeves are filled with an array of the following: a)

Straight lengths of hexagonally shaped Cu/Nb/Nb-Ti rods.

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BARC Newsletter b)

c)

Straight lengths of hexagon copper rod fillers to arrive at required copper to superconductor area ratio. Straight length of partial hexagons of copper.

Sealing of Billet Residual gases are removed from the billet by evacuation and sealing is done under vacuum to prevent oxidation during preheat. Sealing techniques commonly used are Electron Beam welding and TIG welding.

input material. This rod was drawn to 6mm across flat (A/F) hexagon in several passes. OFHC copper partial hexagons of different shapes were fabricated by cold swaging. 6mm across flat Cu/Nb/Nb-Ti as well as OFHC copper hexagons were manually stacked using a special fixture in hexagonal geometry in 180mm dia OFHC copper sleeve. Partial hexagons were used to get a packing density of more than 99%.

Extrusion Temperature and Preheating The choice of extrusion temperature is very important parameter. The upper limit of applicable temperature is decided by chemical reaction of stabilising copper with Nb-Ti which forms a brittle intermetallic compound (NbTi)2Cu causing wire breakage during drawing. The lower limit is decided by the deformation resistance of the Nb-Ti alloy and precipitation of α-Ti below 600oC. In addition to this, bonding is poor at lower temperature.

Cold Drawing and Intermediate Annealing 1.

2.

Fig. 1 Multi-filamentary billet assembly developed at AFD, BARC

Fig.1, shows the billet cross section. Both the ends were closed by OFHC copper end caps. This billet was sealed by electron beam welding to prevent oxidation. Helium leak testing was done to check the integrity of the weld.

A high degree of cold work provides a structure with a high degree of imperfections. α-Ti nucleates only on small sub-bands of 200 – 500 Å width. Annealing is required for precipitation of αTi.

Twisting The eddy current losses can be reduced by introducing a twist pitch to the filament bundle which reduces the area of loop coupling with the changing magnetic field.

Experiment High homogeneity Cu/Nb/Nb-47 wt %Ti alloy of 7mm dia in as drawn condition was used as

Fig. 2 Cross-section of extruded Cu/Nb/Nb-Ti rod (left) and 1.3 mm dia Nb-Ti superconducting wire containing 492 filaments, each of 40 micron cross-section.

Extrusion was utilised to transform the assembled array of components into a composite structure. Hot extrusion was carried


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BARC Newsletter out at 6000C at slow ram speed followed by water quenching. The fig. 2, shows 50mm dia extruded rod cross section containing 492 Nb-Ti elements. Short samples were fabricated using three different thermomechanical treatments. Ageing treatment temperature was between 3700C-4200C with a soaking time of 6hrs-80hrs at different stages of cold reduction. 1.39mm dia wire was twisted and cold drawn to 1.37mm dia with a twist pitch of 12.4mm. The fig. 2, also shows 1.37mm dia wire containing 492 NbTi filaments each of 40 micron size with Cu:SC ratio of 1.15:1.

Results and Discussion

Fig.3 Comparison of critical current vs magnetic induction plots for Nb-Ti LTSC obtained from different sources. Sc-1, Sc-2 & Sc-3 represents the Nb-Ti LTSC Multifilamentary wires developed by Atomic Fuels Division, BARC

Short samples were tested for Ic (critical current) at the reputed Russian laboratory as well as at TPPED BARC. A critical transport current of 1294, 1330 and 1390 Amps was measured through sample-1, sample2 and sample-3 respectively at 5.5 Tesla field and 4.2 K temperature (refer table 3). ‘n’ value i.e resistive transition index was observed to be 58. The maximum critical current density achieved in the wire is of the order of 2500Amps/mm2 at 5.5 Tesla field and 4.2 K temperature. No filament breakage was

Test Results

1. Metallography Sample Sc. I

Filament number 492

observed in all the three samples. Metallography results have been shown in table 1. Residual Resistivity Ratio (ratio of resistance at 293oK and resistance at 10oK) for all the three samples have been shown in table 2 and mechanical properties are listed in table 4. Fig. 3, shows the comparison of critical current v/s magnetic field induction plot for 1.37mm dia Nb-Ti wire obtained from different sources.

Table 1 Filament diameter, µm 40.6

Spacing, µm 6.3

Cu:SC ratio 1.2:1

Sc. II

492

40.6

6.3

1.17:1

Sc. III

492

35.5 (in central nondistorted zone)

7.6

1.18:1

Note Geometry of cross section is slightly distorted Right geometry of cross section Strong distortion of cross section geometry


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BARC Newsletter 2. Residual Resistivity Ratio Table 2 Sample

RRR293/10 Annealed at 275oC for one hour 150 155 138

As drawn 35 43 41

Sc. I Sc. II Sc. III

3. Critical Current versus B on untwisted wires, n value, critical current density Table 3 Ic (0.1µV/cm), A/Ic (1.0µV/cm), A/n Induction, T 4

5

5.5

6

7

8

9

Sc. I

1850/q*

1540/q

1390/q

1201/q

866/902/61

603/633/47

302/330/26

Sc. II

1710/q

1405/q

1294/q

1163/q

877/900/89

626/646/73

330/360/26

Sc. III

1790/q

1470/q

1330/q

1194/q

884/926/49

606/638/45

316/345/26

Jc (0.1 µV/cm), A/mm2 Sc. I

3025

2518

2273

1964

1416

986

494

Sc. II

2796

2297

2116

1902

1434

1024

540

Sc. 2901 III Note: q* - quench

2383

2156

1935

1433

982

512

δ, %

Note

1.2

Out of guage length Out of guage length

4. Mechanical Properties at Room Temperature Table 4 UTS, YS, Kg/mm2 Kg/mm2 95 58.0

No.

Sample

1.

Sample I

2. 3.

Sample II

95 95

48.0 54.0

2.3 1.5

4. 5.

Sample III

95 94

47.5 45.0

2.3 2.4

96

46.0

2.4

6. 5. Filament breakage No filaments broken were revealed.


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Application Areas Physical research

2.

Measuring techniques

3.

Energy technology

4.

Processing technology

5.

Transportation

6.

Telecommunication and computing Medicine

7.

Devices Detection and beam handling in high energy physics, RF cavities for particle accelerators, High field for solid state physics Superconductinelectron microscope, Josephson junction as low field detectors and measuring oscillators Superconducting eletricgenerators (a.c. and d.c.), Superconducting magnetic energy storage system (SMES), Cryogenic power transmission, Fault limiter Magnetic ore separation, Particle accelerator for materials investigation Levitation trains (MAGLEV), Electric motors for ship propulsion RF telecommunication cable, Josephson junction for computer logic, Ultra-high-performance filters, E-bomb Particle accelerators for cancer therapy, MRI system and SQUID

Conclusion Though no filament breakage was observed in all the three short samples but wire breakage was observed during cold drawing in route followed for sample 1. The thermomechanical treatment followed for sample 2 was better from fabrication point of view. This route was followed for fabrication of longer lengths of 1.37 mm dia wire containing 492 number of Nb-Ti filament each of 40 micron size with Cu:SC ratio of 1.15:1. This wire was soldered onto Ugrooved rectangular OFHC copper channel which will be used for fabrication of magnets to be used in Superconducting cyclotron coming up at VECC, Kolkata. This wire is capable of carrying a critical current of 1330 Amps against VECC requirement of 1030 Amps at 5.5 Tesla field and 4.2 K Temperature.

References 1. 2. 3. 4. 5.

6.

7.

H. Hillmann and I. Pfeiffer, A. f. Metallkde 58, 129 (1967). I. Pfeiffer and H. Hillmann, Acta Met. 16, 1429 (1968). D. C. Larbalestier, IEEE Trans. On Magnetics MAG-15, 209 (1972). A. D. McInturff and G. Chase, JAP 44, 2378 (1973). T. Luhman and D. Dew Hughes,”Treaties on Materials Science and Technology,” 14 Academic Press (1979). “Development and fabrication of superconductor for cyclotron magnet”, by S.K. Gupta, M.M.Hussain, M.K.Malik and DSC Purushotham, DAE solid State Physics Symposium Calcutta, Dec. 27-31, 1995. “Development and fabrication of Nb-Ti superconductor for cyclotron magnet”, M. K. Malik, M. M. Hussain & A. K. Singh, ENSC – 2001, Nov. 21-23, NPL, New Delhi.

This paper won the Dr G.S. Tendulkar prize for the overall Best Oral Presentation at the 56th Annual Technical Meeting of Indian Institute of Metals held at Vadodara during November 14-17, 2002


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BARC Newsletter About the authors …

A. K. Singh, B. E. in Metallurgical Engineering, joined Atomic Fuels Division, BARC, in 1998 after graduating from the 41st batch of Training School. He has been actively involved in development & fabrication of Low Temperature Superconducting wires and cables for various applications. He is also involved in extrusion of uranium metal and Al-alloy components for Dhruva fuel assembly.

M. M. Hussain joined Atomic Fuels Division, BARC, in 1985 through 28th batch of Training School. He completed his B. E. in 1983 from Ranchi University and M. Tech. (Met. Engg.) in 1994 from IIT, Bombay. His fields of expertise includes melting & casting, rolling, extrusion and heat treatment of uranium. He has been actively involved in development and fabrication of Low Temperature Superconducting wires and cables for various applications. He is life member of several professional bodies.

S. P. Singh is B. E. and M. Tech. in Metallurgical Engineering. He joined Atomic Fuels Division, BARC in 1970 through 13th batch of Training School. Initially, he worked on production of thorium metal and its fabrication into different shapes. He was deputed to KFA, Jurich, Germany, for a period of one and half years for carrying out metallurgical compatibility studies of thorium metal with different cladding materials under Indo-German joint collaboration program. Later on, he worked on development of powder metallurgical products for nuclear and non-nuclear applications. At present, he is Head, Metallic Section, AFD, mainly looking after melting and casting of uranium metal and alloys, mechanical working, heat treatment of various metals and alloys, and fabrication of Nb-Ti multifilamentary low temperature superconducting wires/cables for various applications. He is life member of various professional bodies.

V. G. Date is B. E. in Metallurgical Engineering from Pune College of Engineering. He joined Atomic Fuels Division, BARC, in 1964 after graduating from 7th batch of Training School. He worked on powder metallurgy of nuclear fuels, reactor control rods and welding of nuclear materials. He became Head, AFD, in April 2001 and retired on superannuation on 31st August 2002. He is life member of several professional bodies.


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