Recent Advances in Internal Magnesium Diffusion (IMD) Strand Design and Processing G.Z. Li Y. Yang M.A. Susner M.D. Sumption E.W. Collings Center for Superconducting and Magnetic Materials (CSMM) Dept. of Materials Science & Engineering (MSE) The Ohio State University (OSU) Prepared for the: 2012 IEEE Low Temperature High Field Superconductivity Workshop (LTHFSW) Nov. 5-7, 2012, Napa, CA, USA

Made possible by the processing expertise of: HyperTech Research Inc Columbus OH M. A. Rindfleisch M. J. Tomsic C. J. Thong J. Yue and by numerous grants from: The U.S. Department of Energy, and in particular The Office of Science, Division of High Energy Physics

Recent Activity (2012) In-Situ PIT Strands Y. Yang, M. A. Susner, M. D. Sumption, M. A. Rindfleisch, M. J. Tomsic and E. W. Collings, Influence of strand design, boron type, and carbon doping method on the transport properties of powder-in-tube MgB2-xCx strands, IEEE Trans. Appl. Supercon. 22 (2012) 6200110. M. A. Susner, T W Daniels, M D Sumption, M A Rindfleisch, C J Thong and E W Collings, Drawing induced texture and the evolution of superconductive properties with heat treatment time in powder-in-tube in situ processed MgB2 strands, Supercond. Sci. Technol. 25 (2012) 065002. Transfer of Focus to IMD Strands and PIT vs IMD Comparison G. Z. Li, Y. Yang, M. A. Susner, M. D. Sumption and E. W. Collings, Critical current densities and n-values of MgB2 strands over a wide range of temperatures and fields, Supercond. Sci. Technol. 25 (2012) 025001. G. Z. Li, M. D. Sumption, M. A. Susner, Y. Yang, K. M. Reddy, M. J. Tomsic, C. J. Thong and E. W. Collings, The critical current density of advanced internal-Mg-diffusion-processed MgB2 wires, Supercond. Sci. Technol. 25 (2012) 115023. G Z Li, M D Sumption, J Zwayer, M A Rindfleisch, M J Tomsic, and E W Collings Critical Current Density of Multifilamentary Internal Mg Diffusion MgB2 Strands: One Meter Segments and Short Samples, -- in preparation

Layout of the Presentation Reactive diffusion principle underlying both in-situ-PIT and IMD strands Fabrication of in-situ-PIT and IMD strands In-situ-PIT layer shrinkage; IMD layer expansion Porous in-situ-PIT reaction zone and dense IMD reaction layer Comparison of in-situ-PIT and IMD properties including connectivity, K % Present status of Je and steps towards further improvement Summary of present results

The “reactive liquid infiltration” or “internal magnesium diffusion” routes for MgB2 formation The reactive-diffusion principle underlying both in-situ-PIT and IMD strand processing 1 kbar before sealing 3h/900oC stainless steel cylinder with welded lid ~ 99% dense crystalline B <100 μm ~ 54% dense -- after Giunchi et al. Physica C 401 (2004) 310-315

Fabrication of in-situ PIT and IMD Strands

PIT IMD Mg 13.74 cm3/mol 2B 9.26 cm3/(2mol) Mg + 2B 23.0 cm3/(3mol)

Layer Microstructures of in-situ-PIT strands and IMD Strands BSE SE

Comparison of the layer critical current densities 2%C-doped monocore IMD 2%C-doped monocore PIT Comparison of the layer critical current densities at 4.2 K, 5 T --- PIT, 105 A/cm2; IMD, 106 A/cm2

Bulk pinning force density, Fp = Jc.B, GN/m3, -- employed as a gauge of connectivity PIT strand x 10.7 Responses of Fp,max to postulated increases in connectivity of x 4.4 and x 10.7 based on a the Fp(B) of a representative PIT strand x 4.4

Comparison of the layer critical current densities 2%C-doped monocore IMD 2%C-doped monocore PIT Comparison of the layer critical current densities at 4.2 K, 5 T --- PIT, 105 A/cm2; IMD, 106 A/cm2

10K, 0 T transport CCDs, JctO(10T), the Fp,max values, and hence the percent connectivities, K% Strand 2%C Process PIT IMD Jct0(10K), 105A/cm2 8.1 25.6* 59.6 Fp,max(10K), GN/m3 5.5 41.5 Estimated K ** 7.8% 58.7% Layer properties * Overall “within-Nb” CCD at 10K, 0T, Je,10K,0T ** Based on a previous set of strand/pellet measurements in which an Fp,max(10K) of 4.53 GN/m3 corresponded to a K-value of 6.41% Conclusion: The high connectivity of the IMD layer results in an engineering Je,10K,0T that is 25.6/8.1 = 3 times greater than that of the PIT strand

The Important Parameters The Engineering Je the Ic/(strand cross sectional area) --- important to the magnet designer The Layer Jc the “intrinsic Jc of the MgB2 core of the strand: depends on “materials science quantities”: B powder selection, dopant type, method of insertion, dopant concentration, connectivity, anisotropy γ = Bc2ab/Bc2c The Fill Factor, FF the MgB2 content (%) of the strand; a strand design and processing issue The three quantities are connected by Je = Jc x FF and can be displayed for discussion in plots of Jc vs FF -- a set of “iso-Je” rectangular hyperbolae

“Li diagram” * based on the general relationship Je = Jc. FF. Particularly useful in discussing the fabrication and optimization of IMD MgB2 Jc, layer CCD Je, engineering CCD FF, fill factor * G. Z. Li et al. Supercond. Sci. Technol. 25 (2012) 115023

Essential Parameters Fill factor Layer Jc Anisotropy Strand engineering and processing Maximize Fill factor Optimize Mg/B ratio HT to completely react the B layer Materials selection/design Use Nanosize B Maintain connectivity Introduce C without reducing connectivity (SMI-C) Optimize C doping level Reduce anisotropy

Liquid Mg Infiltration Demonstration: MgB2 Layer Growth Mechanism --- after Guangzhe Li (CSMM) Melting of Mg Dense MgB2 Layer Liquid Mg Infiltration & Reaction B Particles Mg Dense MgB2 Formation & Mg Atoms Diffusion

isotropic grains, γ = 1 Influence of intrinsic granular anisotropy on Jc(B) in terms of Eisterer’s anisotropy parameter γ = Bc2ab/Bc2c . Plot is from a detailed percolation calculation by Susner [1] based on e.g. [2] [1] M.A. Susner, Ph.D. Thesis OSU 2012 [2] M. Eisterer, Supercond. Sci. Technolog. 20 R47-R73 (2007)

Estimated effect of anisotropy reduction (e.g. via C-doping) on the Je of an IMD strand

2G MgB2 2G MgB2 γ effect Layer Jc = Ic / (MgB2 area) Engineering Je = Ic / (whole area) NbTi NbTi 2G MgB2 2212 2G MgB2 Nb3Sn 2223 γ effect MgB2 MgB2 4.2 K 4.2 K

Potential for further Je improvements to IMD MgB2 FF Thanks from: