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M.D. Sumption,, S. Bhonenstiel, M. Bhatia, M. Susner, E.W. Collings, LASM, MSE, OSU

MgB 2 Superconducting Strands: B c2 , B irr , Doping, Connectivity, Phase Formation, Flux Pinning. M.D. Sumption,, S. Bhonenstiel, M. Bhatia, M. Susner, E.W. Collings, LASM, MSE, OSU.

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M.D. Sumption,, S. Bhonenstiel, M. Bhatia, M. Susner, E.W. Collings, LASM, MSE, OSU

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  1. MgB2 Superconducting Strands: Bc2, Birr, Doping, Connectivity, Phase Formation, Flux Pinning M.D. Sumption,, S. Bhonenstiel, M. Bhatia,M. Susner, E.W. Collings, LASM, MSE, OSU This work was funded by the U.S. Dept. of Energy, Division of High Energy Physics, under Grant No. DE-FG02-95ER84363A State of Ohio grant no. TECH 02-071, NASA NNC05CA04C and, NIH grant No. 2R44EB003752-02. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No. DMR-0084173 and by the State of Florida. HTR, M. Tomsic, M. Rindfleisch

  2. Achieved critical current densities in selected MgB2 conductors -- collected by Goldacker and presented at EUCAS 07 C-bearing dopants well known to be effective EUCAS-Poster-0631 105A/cm2 In-situ/ex-situ SiC doped USHP and CIP treated Morawski et al. 2007 c-doped Braccini et al. EUCAS-Poster-0724 Mech.all. C-doped tape Hässler et. al. 2007 104 Acm-2 at 14 T EUCAS-Poster-0282

  3. Bc2 enhancement with SiC dependent on SiC size 10% SiC amounts to more doping than 5%

  4. Different Classes of Doping Set A A) Silicon Carbide B) Amorphous Carbon C) Mg + B milled with Acetone Set B D) Metal Diborides ZrB2 NbB2 TiB2

  5. Birr Variation with sensing current (Kramer, 100 A/mm2, R 10%-30nm SiC 625oC-180min

  6. More Mg SiC BC2 and micro with Mg Scanning Electron Microscopy images of samples- 10 m scale. (a) Mg0.85B2-1A, (b) Mg0.90B2-1A, (c) Mg1.00B2-1A, (d) Mg1.10B2-1A, (e) Mg1.15B2-1A, (f) Mg1.15B2+SiC.

  7. Effect of B sources on Transport Jc It is well known that different B gives different properties, But are the differences due to: • Particle size and morphology • Impurities • Crystalline vs amorphous • Excess Oxygen • Something else ? “99” cluster “95” cluster

  8. Boron Powder Origin • 95 Boron Powder • Produced by reacting Mg and B2O3 powder to form B and MgO followed by acid wash to remove MgO • A thermitic reaction that reaches adiabatic temperatures (>1800 °C) • Boron crystallizes to b-rhombohedral form above ~1250 °C • Significant impurity content • Particles much larger than 1 micron • Slow MgB2 formation with partial non-random orientation of MgB2 grains • 99 Boron Powder Origin • Produced from boron hydrides by high temperature decomposition into boron and hydrogen gas • Resulted in high purity and very high surface area due to fine particle size (100-800 nm) • High surface area results in fast formation of MgB2 even below Mg melting with random orientation of MgB2 grains

  9. Particle Size of “99 (amorphous) B” versus “95 (nanocrystalline) B” 95 B has a bimodal distribution with most of the mass above 20 μm 99 B is essentially submicron

  10. X-ray Fluorescence of MgB2 from 99 and 95 (wt%) Are the significantly larger impurity levels responsible for Tc suppression in 95 B based MgB2?

  11. B2O3 XRD shows distinct crstallined nature for 95 Amorphous boron shows two broad amorphous peaks with a B2O3 crystalline peak 95 Boron definitely has some β-rhombohedral crystalline particles along with some B2O3 Red lines are  rhombohedral

  12. Forms of Boron • Amorphous (purity is very dependent on source). • α-rhombohedral (only available by crystallization of amorphous boron or by CVD via hot wire technique). Meta-stable structure: nearly regular icosahedra in slightly deformed fcc structure, 12 atom unit cell, Primitive structural element: B12 (one icosahedra) • β-rhombohedral (most stable form of boron, least reactive, only B form stable above ~1300 C). 105 atom unit cell • α-tetragonal (least studied monotrope of boron, forms when boron fibers are doped with >2% carbon in Specialty Materials boron fiber production)

  13. Transmission Electron Microscopy of Boron Powder 500 nm 500 nm 95 (nano-crystalline, high density) 99 (amorphous, low density)

  14. Nanocrystalline B (“95”) Amorphous (99) B Amorphous B reacts below Mg’s melting point, while nanocrystalline reaction needs Mg melting --DSC of Mg+amorphous (99) Boron vs Mg + nanocrystalline (95) B Pre-reaction peak associated with small Mg (OH)2 exothermic induced MgB2 formation Mg melting MgB2 formation

  15. Amorphous B + Mg ~450°C is Mg(OH)2 decomposition which initiates some reaction ~600°C is the beginning of MgB2 formation which is essentially complete before Mg melting

  16. Amorphous B + MgH2 ~450°C is MgH2 decomposition ~600°C is the beginning of MgB2 formation which is complete before Mg melting

  17. Crystalline B () + MgH2 ~450°C is MgH2 decomposition 650°C is Mg melting ~900°C is MgB2 formation

  18. Crystalline B + Mg95 B + Mg Crystalline B (blue) and 95 B (red) show very similar reaction steps and temperatures Mg(H2O) decomp Mg melting ? MgB2 formation

  19. SEM on MgB2 (99 Boron) Heat treated at 900C Whitish regions are MgO coated, and suffer from charging during SEM MgB2 platelets, thin and randomly oriented About 12-15% by Vol is MgO

  20. SEM on MgB2 (99 Boron) Whitish regions are MgO coated, and suffer from charging during SEM MgB2 platelets, thin and randomly oriented Very little obvious faceting

  21. SEM on MgB2 (95 Boron) Some texture apparent in MgB2 platelets Considerable MgO debri present XRD analysis give about 17 % MgO for “95”

  22. SEM on MgB2 (95 Boron) Faceting apparent Texture apparent

  23. Basis for Resistive Estimations of Porosity/Connectivity p gb gb A a Measured resistance/unit length

  24. Resistivity Results using Rowel model vs RR inclusion * Measured – after Eltsev ** Obtained by fitting Eltsev’s resistivity data to the B-G function.

  25. SMI Powder“Nearly as good as amorphous” 011508

  26. Pinning in Binary MgB2 Predominantly GB in Nature, but not good scaling

  27. Excess Mg results seem to be GB pinning as well Both SiC and excess Mg are dominated by GB pinning at low fields and temps – Jcincreases in SiC due to Bc2enhancements Some Bc2 enhancements for excess Mg, but also connectivity

  28. Fp for Doping with smaller SiC particles Many measurements on various SiC give similar results MB30-Si5 Some small amount of point pinning at higher T A drop to vol pinning at higher field, or a mixture of Birr?

  29. TiC and Si3N4: Fp,max, respective values of 11.89 and 11.70 GN/m3 compared to the undoped value of 8.35 GN/m3 at 5K. Size of nanoparticulates predicates either point or volume pinning, depending on the field-deconvolution difficult Nanoparticulate spacing: 250-1100 nm. Fluxon spacing: 22-15 nm (5-10 T). Next Step is clearly to increase quantity Nano Pinning ?

  30. Influence of Birron Fp m = 2.33

  31. B + TiC, 1 h mill 200 kx, BF B+TiC, 6 h mill, 260 kx, BF TiC additions, milled for 1 and 6 h with B PointPinning?

  32. Comparison of Transport and Magnetic Results

  33. Summary • Various dopants can increase Bc2/Birr – • SiC dopants are more effective when introduced as smaller powders (10%) • Character of high field transport and resistivity results suggest both low total connectivity, and inhomogenous dopant distribution (combined with known anisotropy) • For 99 B (amorphous) Mg-rich stoichiometries better Leads to importance in understanding the basic MgB2 reaction – • Amorphous B + Mg reacts below Mg melting • Many forms of B exist, many in use have significant crystalline components, degrade Jc. • A number of crystalline forms react at higher temperatures than amorphous • 99 amorphous B has less impurities than 95 • All in-situ wires have a lot of MgO, maybe 15% usually • 99 growth random platelets, 95 faceted and some orientation • Resistivity extraction with B-G gives sensible interpretations of boundary phases • Pinning mostly GB, but perhaps high milling can give point pins • Variance between transport and magnetic Results

  34. Appendix

  35. 4.2 K Comparison to thin film

  36. TEM Bright Field Image of MgB2 + ZrB2 SampleAnd EDX Spectra from the Imaged Area

  37. TEM Bright Field Image of MgB2 + NbB2 Sampleand EDX Spectra from the Imaged Area

  38. DSC to determine the formation of MgB2 Magnesium powder and amorphous B (99.9%) were analyzed by DSC study. (stoichiometric mixture) Endo Mg(OH)2 MgO+H2O (weak endo) Mg+H2O MgO+H2 (strong exo)

  39. dBc2/dTm = 15/25 = 0.6 T/K

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