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

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

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

Different Classes of Doping Sample Name moHirr, T moHc2, T DB Birr/Bc2 Tc, K Tc,on, K Tc,com, K DTc, K MB700 16.0 20.5 4.5 0.78 37.879 38.1 36.4 1.7 MBCSiC700 ~24.5 >33 >8.5 <0.74 35.010 35.4 33.5 1.9 MBCSiC800 ~25.5 >7.5 <0.77 - MBCSiC900 ~28.0 >5 <0.84 MBC700 ~20.0 >13 <0.6 33.233 33.8 32.1 MBC71000 <0.60 32.482 33.9 31.9 2 MBAC800 ~23.0 >10 <0.69 32.952 33.6 31.6 MBZr700 24.0 28.6 4.6 0.84 35.634 33.0 3.4 MBNb700 25.5 5 0.80 36.164 35.1 1.3 MBNb800 18.5 22.8 4.3 0.81 MBNb900 18.0 21.6 3.6 0.83 MBTi700 19.0 22.5 3.5 36.382 36.55 35.7 0.85 MBTi800 22.6 Set A A) Silicon Carbide B) Amorphous Carbon C) Mg + B milled with Acetone Set B D) Metal Diborides ZrB2 NbB2 TiB2 4

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

BC2 and micro with Mg SiC More 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.

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

Boron Powder Origin 95 Boron Powder 99 Boron Powder Origin 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

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

X-ray Fluorescence of MgB2 from 99 and 95 (wt%) 99.9 99.3 Fe .0457 0.201 Al .0151 .0380 Si .0101 .0403 Ca  .0060 .0696 Mn .0215 .0227 K .0085 .120 S  .0044 .0187 P  .0028 .0107 Ni  .0032 .0283 Zr  .0014 .0031 Na  .0182 .0556 Cr   .0207 Cu .0041 .0057 As .0028 Are the significantly larger impurity levels responsible for Tc suppression in 95 B based MgB2?

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

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)

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

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 Nanocrystalline B (“95”) Pre-reaction peak associated with small Mg (OH)2 exothermic induced MgB2 formation Amorphous (99) B Mg melting MgB2 formation

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

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

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

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

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

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

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

SEM on MgB2 (95 Boron) Faceting apparent Texture apparent

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

Resistivity Results using Rowel model vs RR inclusion Sample (100/F) B-G Method* ro (mW-cm) Debye Temperature (K) Single Crystal [4] (100)** 1.04*** 949**** MgB2 Pure (MB700) 23.2 11.29 677 MgB2+TiB2 (MBTi800) 10.6 9.49 764 MgB2+SiC (MBSiC700) 13.4 38.56 593 NRL-HR-10%Mg [2] 81.1 2.12 695 NRL-HR [2] 94.9 1.97 739 Dense wire [3] 58.6 0.31 839 Dirty thin film [1] 17.1 4.05 858 * Measured – after Eltsev ** Obtained by fitting Eltsev’s resistivity data to the B-G function. 24

SMI Powder “Nearly as good as amorphous” 011508

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

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 – Jc increases in SiC due to Bc2 enhancements Some Bc2 enhancements for excess Mg, but also connectivity

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?

Nano Pinning ? 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

Influence of Birr on Fp m = 2.33

TiC additions, milled for 1 and 6 h with B B + TiC, 1 h mill 200 kx, BF Point Pinning? B+TiC, 6 h mill, 260 kx, BF

Comparison of Transport and Magnetic Results

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

Appendix

4.2 K Comparison to thin film

TEM Bright Field Image of MgB2 + ZrB2 Sample And EDX Spectra from the Imaged Area

TEM Bright Field Image of MgB2 + NbB2 Sample and EDX Spectra from the Imaged Area

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)

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