and Conductor Manufacturing R. Flükiger

Transcription

1 The MgB 2 Superconductor: Physical Properties and Conductor Manufacturing R. Flükiger Dept. Solid State Physics (DPMC) & Natl. Center for Competitivity in Research (MANEP) University of Geneva 1211 Geneva, Switzerland R. Flükiger, Summer School, June 08, Pori (Fin) 1

2 Outline Introduction Additives to MgB 2 : Effects on ρ o,b c2 and B irr and the enhancement of J c Examples of applications for MgB 2 wires Powder-in-Tube methods for fabrication of MgB 2 wires and tapes: Ex situ technique, In situ technique J c of MgB 2 wires and tapes vs. B at 4.2 and 20 K Future F t developments, Conclusions Some remarks about the new Fe-As superconductors R. Flükiger, Summer School, June 08, Pori (Fin) 2

3 Introduction R. Flükiger, Summer School, June 08, Pori (Fin) 3

4 Industrial Superconductors Compound T c [K] B c2 (0) [T] NbTi Industrial, Continuous Nb 3 Sn Industrial, continuous Nb 3 Al (industrial), i continuous MgB Industrial, Powder Bi <1 / >100 Industrial, Powder Bi <1 / >100 Industrial, Powder Y / > 100 Industrial, deposition SmFeAsO 1-x F x ????; deposition??? R. Flükiger, Summer School, June 08, Pori (Fin) 4

5 Low T c superconductors Compound T c B c2 (0) ξ (K) (T) (nm) NbTi Nb 3 Sn Nb 3 Al (4) MgB ξ: coherence length R. Flükiger, Summer School, June 08, Pori (Fin) 5

6 The compound MgB 2 Hexagonal lattice a = Å c = Å R. Flükiger, Summer School, June 08, Pori (Fin) 6

7 The binary Mg - B Phase Diagram Gas + solid coexistence gas liquid Mg gb 4 Mg B 7 solid solid MgB 2 Mg at. % Boron B Binary Alloys Phase Diagrams, 2 nd Edition, Ed. T. Massalski (A.S.M.International, 1990) R. Flükiger, Summer School, June 08, Pori (Fin) 7

8 MgB 2 and other industrial superconducting wires T = 4.2 K R. Flükiger, Summer School, June 08, Pori (Fin) 8

9 Physical properties R. Flükiger, Summer School, June 08, Pori (Fin) 9

10 . ρ o = 0.15 μωcm : lowest value among known superconductors perfect ordering of crystal lattice (Clean Nb 3 Sn single crystals: ρ o ~ 1 μωcm) R. Flükiger, Summer School, June 08, Pori (Fin) 10

11 a.c.sus sceptibility (a.u.) MgB 2 Alfa powder T(K) R. Flükiger, Summer School, June 08, Pori (Fin) 11

12 MgB 2, a two gap superconductor Δ σ Δ π R. Flükiger, Summer School, June 08, Pori (Fin) 12

13 Two gap superconductivity in MgB2 3D 2D R. Flükiger, Summer School, June 08, Pori (Fin) 13

14 The enhancement of B c2 in C alloyed MgB 2 R. Flükiger, Summer School, June 08, Pori (Fin) 14

15 Carbon Concentration Dependence of T c C-alloyed HPCVD films C-doped single crystals T c (K) Carbon Concentration (at.%) ρ (50 0K) (μωcm m) Lee et al. Physica C397, 7 (2003) Carbon concentration suppresses T c much more slowly than in carbondoped bulk samples: Only part of carbon is doped into MgB 2 R. Flükiger, Summer School, June 08, Pori (Fin) 15

16 Enhanced B c2 in Carbon alloyed MgB 2 thin films Clean single crystals: B c2 (T) What are the reasons for the strong enhancement of B c2? R. Flükiger, Summer School, June 08, Pori (Fin) 16

17 The enhancement of B c2 in C alloyed MgB 2 Reason 1. Valid for bulk, wires and thin films: Enhancement of ρ o as a consequence of flocal ldisorder d Partial substitution of Boron by Carbon: shortening of the electronic mean free path B C Reason 2. Particular feature of thin films Extra enhancement of B c2 observed in thin film R. Flükiger, Summer School, June 08, Pori (Fin) 17

18 Effect of Carbon alloying on electrical resistivity Thin films X. X. Xi, 2004 R. Flükiger, Summer School, June 08, Pori (Fin) 18

19 Anisotropy of ρ o R. Flükiger, Summer School, June 08, Pori (Fin) 19

20 B c2 of a high resistivity MgB 2 film [T] T H c2 Field perpendicular to film surface Field parallel to film surface R. Flükiger, Summer School, June 08, Pori (Fin) 20

21 High H irr in Carbon-Alloyed MgB 2 Films (10% criterion) V (V) K 4 K 0.5 (a) H // ab μ 0 H (T) H irr (T) H // ab V (V) K K 0.5 (b) H ab μ 0 H (T) 10 H ab T (K) (Ferrando, Betts, Mielke) R. Flükiger, Summer School, June 08, Pori (Fin) 21

22 HPCVD Films vs. Bulk: Different Response to C alloying C-alloyed HPCVD films C-doped single crystals c Lattic ce Constan nt (Å) a Lattice Constant t (Å) Lee et al. Physica C397, 7 (2003) Carbon Concentration (at.%) Both c and a increase with C until MgB 2 grains are separated by boron carbide c unchanged, and a decreases with carbon R. Flükiger, Summer School, June 08, Pori (Fin) 22

23 Why is B c2 = 70 T in MgB 2 thin films and not in wires? R. Flükiger, Summer School, June 08, Pori (Fin) 23

24 Paramagnetic limitation of upper critical field H c2 R. Flükiger, Summer School, June 08, Pori (Fin) 24

25 Courtesy: A.Gurevich R. Flükiger, Summer School, June 08, Pori (Fin) 25

26 . Courtesy: A.Gurevich R. Flükiger, Summer School, June 08, Pori (Fin) 26

27 How «dirty» should a superconductor be to reach the paramagnetic limit? Courtesy: A.Gurevich R. Flükiger, Summer School, June 08, Pori (Fin) 27

28 R. Flükiger, Summer School, June 08, Pori (Fin) 28

29 Why B c2 = 70T in MgB 2 thin films and not in wires? * Very high (metastable) C content: Cannot be, since the C content is the same as in bulk samples and Xtals * Small grains: May have an influence, but the grain sizes are similar to bulk MgB 2 with B c2 = T * c axis tilt: Only present in C doped MgB 2 films Other possible reasons: * Special, non equilibrium Carbon sites * Disorder, introduced by amorphous layers, leading to increase of ρ o (40K) - nanograins The question is still under discussion i! R. Flükiger, Summer School, June 08, Pori (Fin) 29

30 Applications for MgB 2 wires R. Flükiger, Summer School, June 08, Pori (Fin) 30

31 R. Flükiger, Summer School, June 08, Pori (Fin) 31

32 Low material cost Industrial manufacturing * Cryogen free, mobile MRI systems *I Intermediate t coils in NMR magnets * Fault current limiters (FCL) * Sensors for Liquid Hydrogen level R. Flükiger, Summer School, June 08, Pori (Fin) 32

33 Conductor Requirement for MRI : a near term application for MgB 2 R. Flükiger, Summer School, June 08, Pori (Fin) 33

34 R. Flükiger, Summer School, June 08, Pori (Fin) 34

35 R. Flükiger, Summer School, June 08, Pori (Fin) 35

36 MgB 2 Wires and Tapes R. Flükiger, Summer School, June 08, Pori (Fin) 36

37 Definitions H c2 : H irr : H H irr H irr Courtesy: Larbalestier R. Flükiger, Summer School, June 08, Pori (Fin) 37

38 B irr : Where is MgB 2 today? R. Flükiger, Summer School, June 08, Pori (Fin) 38

39 Common criteria for stability of s.c. wires and tapes 1 : Chemical stabilityty 2 : Mechanical stability : bulk HF superconductors break at ε <0.05 %! microcomposite (multifilamentary) configuration Filament size: < 5-10 μm (large number) Irreversible strain: ε irr > 0.4 % 3 : Cryogenic stability : presence of Cu or Ag as stabilizer High thermal conductivity of Cu or Ag a minimum quantity of stabilizer is required 4 : Electromagnetic stability : Low AC coupling losses required Twisting of wires ( 25 mm) 5 : Low material costs 6: Length: >1k km R. Flükiger, Summer School, June 08, Pori (Fin) 39

40 Inductive and transport measurement of J c Standard 4 point method V ~ I n M(H) hysteresis loops 1.0x10 MgB /Ni annealed ltage (V) Vol 5.0x10-6 4T3.5T 3T 2.5T 2 T μ 0 H = 1.5T E c =1μVcm Current (A) Transport measurements: 45mm length with sheath materials Contact distance: 10mm Field parallel to sample surface and perpendicular to the current 1µv/cm criterion 300A m (e e.m.u.) MgB 2 /Ni annealed μ o H (T) Inductive measurements: M(H) loops from 5K to 35K Field parallel to flat sample Bean critical state model R. Flükiger, Summer School, June 08, Pori (Fin) 40

41 Ex situ PIT(Powder-In-Tube) Tape processing Sheath materials: Ni, Fe, Ti,. Ball-milled MgB 2 powder Operating in Glove box under Ar Measurements Heat treatment: after deformation: Annealing at 950 C, 980 C for 0.5h in pure Ar atmosphere Metallic Plug Powder in tube Swaging Drawing Rolling R. Flükiger, Summer School, June 08, Pori (Fin) 41

42 Industrial fabrication of «in situ» MgB 2 tapes Columbus Superconductors, srl, Genova (Italy) R. Flükiger, Summer School, June 08, Pori (Fin) 42

43 Binary MgB 2 Effect of particle size on B c2 and B irr of MgB 2 /Fe μ 0 H (Te esla) μ 0 H c2 ball milled powder μ 0 H irr ball milled powder μ 0 H c2 commercial powder μ 0 H irr commercial powder H c2 : unchanged μ H commercial powder H irr : improved in tapes prepared by ball milled powders Temperature (K) At 4.2 K: from 9 to 13 T R. Flükiger, Summer School, June 08, Pori (Fin) 43

44 High exponential n factor in Fe/MgB2 tapes R. Flükiger, Summer School, June 08, Pori (Fin) 44

45 Effect of Carbon on Jc R. Flükiger, Summer School, June 08, Pori (Fin) 45

46 Effect of Carbon content on T c (in at.%) R. Flükiger, Summer School, June 08, Pori (Fin) 46

47 Carbon Alloying: reduces J c Anisotropy J c anisotropy decreases as Carbon concentration increases. R. Flükiger, Summer School, June 08, Pori (Fin) 47

48 Limitations of Carbon doping: ρ o increases, while T c decreases compromise at ~ 10 wt.% C R. Flükiger, Summer School, June 08, Pori (Fin) 48

49 In Situ PIT(Powder-In-Tube) Wire processing Mg + Boron powder Metallic Plug Sheath materials: Ni, Fe, Ti,. Operating in Glove box under Ar Measurements (XRD, SEM, Jc) Heat treatment at C for h in pure Ar Powder in tube Swaging Drawing R. Flükiger, Summer School, June 08, Pori (Fin) 49

50 Y.W. Ma, ISTNM Beijing 2007 Inst. Electrical Engineering, Beijing,

51 T. Collings, M. Sumption, 2007 R. Flükiger, Summer School, June 08, Pori (Fin) 51

52 C doping of MgB 2 wires: a pinning effect? J c has been improved by substituting B by Carbon Carbon doping leads to enhanced ρ o values Dopants: C, SiC, organic salts,. Question: Effect on pinning? Inst. Electrical Engineering, Beijing,

53 Monofilamentary MgB2 tape with C-doped precursor ,2K j A/cm -2 C / K 600 C/3h 625 C/3h 650 C/3h H / T W. Hässler, M. Herrmann and A. Kario, 2008 Leibniz i Institute t for Solid State t Materials Research Dresden IFW)

54 Are additions effective on pinning i? Magnetic Relaxation & Relaxation Rates According to the Kim-Anderson model the relaxation rate is 0 ln U 0 k BT t 1 k kb M M 1 B lnt M = M0 0 1 U U0 0 t0 = = 0 ln t t 0 1 dm kb T S = = M0 dlnt U0 t t0 Pinning i energy does not change with doping Enhancement of ρ o dominant for the increase of J c axation Rat te (%/decad de) Rel J c (A/cm 2 ) MgB 2 MgB wt% SiC MgB 1.9 C 0.1 B dc = 3T T = 10 K t (s) MgB 2 MgB 2 +10wt%SiC MgB 1.9 C K 10 K 5K B dc (T) Inst. Electrical Engineering, Beijing,

55 Mechanisms for higher h J c values in MgB 2 wires * Substitution of Boron by Carbon : OK Enhancement of B c2 and B irr * Pinning effects at grain boundaries: not yet achieved * Connectivity, percolation:? Quantity and shape of foreign phases: DensificationD HIP Idea of simultaneous doping (codoping) : MgB 2 + X + Y +.. Results so far not conclusive Examples: Inst. Electrical Engineering, Beijing,

56 7.5 wt.% B 4 C wt.% SiC 11.3 T Flükiger et al., IEEE Trans. Appl. Supercond. 17(2007)2846 Inst. Electrical Engineering, Beijing,

57 Improvement? Density, percolation, Inst. Electrical Engineering, Beijing,

58 Internal stress in MgB 2 as detected by specific heat measurements Measurement of the T c distribution in binary and C alloyed MgB 2 bulk samples and in Fe sheathed tapes Inst. Electrical Engineering, Beijing,

59 Electronic specific heat of bulk, binary MgB 2 Quantitative determination of reacted MgB 2 phase (c -c J/K 2 s n )/ T [J g] MB MgB C 725 C C C 650 C 800C C C H H: quantity of reacted sample T[K] 800 C: Highest amount of formed MgB 2 phase C: Lower amount of MgB 2 phase: Mg losses (Karpinski et al, 2008) 650 C, 725 C: Lower amount of reacted MgB 2 phase: slower kinetics Inst. Electrical Engineering, Beijing,

60 Distribution of T c in binary MgB 2 for various reaction temperatures 0.7 T c Distribut tion C C 800C 1000 C C 800 C C C MgB T c [K] Variation of T c with Mg content : negligible ; broadening of T c distribution attributed to internal stress. Higher T: narrower curves, decreasing stress Inst. Electrical Engineering, Beijing,

61 Distribution of T c in alloyed MgB 2 : Effect of reaction temperature 0.6 MgB wt% SiC Distribut tion T c C 1000 C 800C C C C T c [K] Very large distribution for 650 C: cannot be assigned to a higher C content Observation of large stress effects. Potential for improvement? Inst. Electrical Engineering, Beijing,

62 NMR Magnets: persistent mode operation All wire types (Bronze Route, Internal Sn, PIT) exhibit sufficiently high J c values allow to fabricate NMR magnets with fields > 22 T Criteria: Thermal Stability Mechanical Stability Persistent Mode Operation Longitudinal homogeneity of cross section over >> 1 km Persistent mode: value of the exponential n coefficient over 30 under operation conditions, i.e. low magnetic relaxation rates Inst. Electrical Engineering, Beijing,

63 Relaxation Rates: MgB 2, Nb 3 Sn, YBCO and Bi-2223 Relax xation Rate (% / decade) K YBCO Bi-2223 // MgB 2 Nb 3 Sn // ade) Relaxation Rate (% / dec Bi-2223 Nb 3 Sn MgB 2 // 10 K // YBCO B (Tesla) Relaxation Rate (% / deca ade) Bi-2223 // // MgB 2 20 K YBCO B (Tesla) C. Senatore, P. Lezza and R. Flükiger, B (Tesla) Inst. Electrical Engineering, Beijing,

64 Conclusions MgB 2 has reached an industrial level Critical current density has still to be optimized; further progress requires improved pinning behavior Several applications are seriously envisaged: MRI at 20 K Intermediate coils at 9 12 T in high field NMR magnets Sensors for measuring LH2 levels Fault Current limiters R. Flükiger, Summer School, June 08, Pori (Fin) 64

65 Upper critical fields well above 100 T for the superconductor Sm(O 0.85 F 0.15 )FeAs C. Senatore, R Flükiger, G. Wu*, R. H. Liu* and X. H. Chen* Submitted to Cond-mat, arxiv: * Sample preparation Hefei National Laboratory for Physical Science at Microscale and Department of Physics, University of Science and Technology of China Inst. Electrical Engineering, Beijing,

66 Crystal structure of LaOFeAs (a) Layered structure. Distorted FeAs 4 tetrahedrons are connected in an edge sharing manner to form the FeAs layer. (b) Top view (c-plane) Yellow squares: unit cell (tetragonal, P4/nmm) Blue square: orthorhombic phase, Cmma. Y.Kumihara, T.Watanabe, M.Hirano, H.Hosono, Hosono J. Am. Chem. Soc. 130,2008, Inst. Electrical Engineering, Beijing,

67 SmO 0.85 F 0.15 FeAs polycrystalline sample Specific heat measurements up to 20 T Determination of Bc2( (T ) Sample homogeneity and distribution of T c Granularity and inductive J c (B ) Peak effect in magnetization Inst. Electrical Engineering, Beijing,

68 XRD pattern and lattice parameters SmO 0.85 F 0.15 FeAs a = nm c = nm Intensity [a.u.] * * Inst. Electrical Engineering, Beijing, Θ [deg.] 68

69 Superconducting Superconducting contribution to specific jump of heat specific heat K 2 ] / T [J/g T 6 T 10 T 14 T 20 T C s / T max T mid T [K] CB,T ) was determined by subtracting the background (phonons + normal electrons) from the measured curves Background fit for T >50K K: C n (T ) = a + bt + ct 2 Inst. Electrical Engineering, Beijing,

70 Determination of the T c distribution Determination of the T c distribution B = 0 T B = 10 T n distributio T c Dist. = d dt ns ( T ) c ( T ) ( n 1) γt T c T c [K] Broadening T c distribution of width the distribution at B = 0 ~ at 5 B K 0 B c2 distribution, due to the intrinsic anisotropy and the random orientation of the grains Inst. Electrical Engineering, Beijing,

71 R. Flükiger, Summer School, June 08, Pori (Fin) 71

72 Magnetization measurements T = 5 K T = 55 K M [em mu] B [T] Ferromagnetic background, nearly temperature independent, probably due to unreacted Fe or Fe 2 O 3 Comparable intensity of the superconducting and ferromagnetic signals

73 intragrain superconductivity χ' [a.u.] AC susceptibility curves exhibit a double transition crossover from intragrain to intergrain superconductivity intergrain superconductivity T [K] Irreversible magnetization ΔM does not depend on the sample size AC susceptibility curves exhibit a double transition crossover from intragrain to intergrain superconductivity R. Flükiger, Summer School, June 08, Pori (Fin) 73

74 Critical current density J c vs B 10 μm 0.1 μm J A/cm 2 c [A ] 5 K 10 K 15 K 20 K K 35 K 25 K B [T] J c [A/cm 2 ] J c ΔMM Δmm = 3 = 3, calculated for R = 0.1 and 10 μm R V R Inst. Electrical Engineering, Beijing,

75 Marco Cantoni, EPFL Journée GAP,

76

77 B T phase diagram of SmO 0.85 F 0.15 FeAs MgB 2 T 1 Tc 2 B peak = Φ l 0 2, l being the shorter length between λ ab and λ J =Γd Inst. Electrical Engineering, Beijing,

78 Conclusions The high value of the slope db c2 /dt Tc 5 T/K suggests an extrapolated B c2 (T = 0) around 150 T, using the WHH formula A first estimation of the field dependence of J c is given The presence of a peak effect in magnetization is reported, suggesting a crossover in the vortex dynamics The samples show granularity, in a similar way to HTS superconductors s Inst. Electrical Engineering, Beijing,