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 , NASA NNC05CA04C and, NIH grant No. 2R44EB A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No. DMR 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
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

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

6. 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.

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 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 ( 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%)
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?

11. 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

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
95 (nano-crystalline, high density)
99 (amorphous, low density)

14. 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

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 + 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

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/Connectivitygb p
Measured resistance/unit length 23

24. 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

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

26. Predominantly GB in Nature, but not good scaling
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 – Jc increases in SiC due to Bc2 enhancements
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. Nano Pinning ?
TiC and Si3N4: Fp,max, respective values of and 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: nm. Fluxon spacing: nm (5-10 T). Next Step is clearly to increase quantity

30. Influence of Birr on Fp
m = 2.33

31. 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

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 Sample And EDX Spectra from the Imaged Area

37. TEM Bright Field Image of MgB2 + NbB2 Sample and 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) MgO+H2O (weak endo)
Mg+H2O MgO+H2 (strong exo)

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