1. 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

2. 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

3. 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)
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)
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)
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)
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

4. 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

5. 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)

6. Fabrication of in-situ PIT and IMD Strands

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

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

9. 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

10. 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

11. 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

12. 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

13. 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

14. “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)

16. 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

17. 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

18. 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)

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

20. 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

21. Potential for further Je improvements to IMD MgB2FF
Thanks from: