Structures of MgB2-based materials manufactured under pressure (up to 2 GPa) by different methods having high superconducting performance and connectivity are multiphase and contain different Mg-B-O compounds. Some oxygen can be incorporated into MgB2 and boron into MgO structures, MgBx (X=4-20) inclusions contain practically no oxygen. Regulating manufacturing temperature, pressure, introducing additions one can influence oxygen and boron distribution in the materials and thus, affect the formation, amount and sizes of Mg-B-O and MgBx inclusions and changing type of pinning, pinning force and so affect critical current density jc. The boron concentration increase in initial Mg and B mixture allows obtaining sample containing 88.5 wt% of MgB12 with Tc of 37.4 K (estimated magnetically)

Basyuk, T.

Borimskiy, A.

Chaud, X.

Dellith, J.

Dittrich, U.

Eisterer, M.

Gawalek, W.

Karau, F.

Karpets, M.

Kovylaev, V.

Kozyrev, A.

Kuznietsov, R.

Litzkendorf, D.

Moshchil, V.

Noudem, J.

Prikhna, T.

Shmidt, C.

Sokolovsky, V.

Tkach, V.

Tomsic, M.

Weber, H.W.

English

AbstractStructures of MgB2-based materials manufactured under pressure (up to 2GPa) by different methods having high superconducting performance and connectivity are multiphase and contain different Mg-B-O compounds. Some oxygen can be incorporated into MgB2 and boron into MgO structures, MgBx (X=4-20) inclusions contain practically no oxygen. Regulating manufacturing temperature, pressure, introducing additions one can influence oxygen and boron distribution in the materials and thus, affect the formation, amount and sizes of Mg-B-O and MgBx inclusions and changing type of pinning, pinning force and so affect critical current density jc. The boron concentration increase in initial Mg and B mixture allows obtaining sample containing 88.5 wt% of MgB12 with TC of 37.4K (estimated magnetically)

Prikhna, Tatiana

Gawalek, Wolfgang

Eisterer, Michael

Weber, Harald W.

Noudem, Jacques

Sokolovsky, Vladimir

Chaud, Xavier

Moshchil, Viktor

Karpets, Myroslav

Kovylaev, Valeriy

Borimskiy, Alexandr

Tkach, Vasiliy

Kozyrev, Artem

Kuznietsov, Roman

Dellith, Jan

Shmidt, Christa

Basyuk, Tetiana

Litzkendorf, Doris

Karau, Friedrich

Dittrich, Ulrich

Tomsic, Michael

Elsevier - Publisher Connector 

 Physics Procedia  36 ( 2012 )  475 – 478 
1875-3892  © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. 
doi: 10.1016/j.phpro.2012.06.220 
 
Superconductivity Centennial Conference 
Superconductivity in multi-phase Mg-B-O compounds 
Tatiana Prikhnaa*, Wolfgang Gawalekb, Michael Eistererc, Harald W. Weberc,                
Jacques Noudemd, Vladimir Sokolovskye,  Xavier Chaudf, Viktor Moshchila,     
Myroslav Karpetsa,g, Valeriy Kovylaevg, Alexandr Borimskiya, Vasiliy Tkacha,                
Artem Kozyreva, Roman Kuznietsova, Jan Dellithb, Christa Shmidtb, Tetiana Basyuka,              
Doris Litzkendorfb, Friedrich Karaui, Ulrich Dittrichi, Michael Tomsicj 
aInstitute for Superhard Materials of the Nat. Ac. of Sci. of Ukraine, 2, Avtozavodskaya Str., Kiev 07074, Ukraine 
bInstitut für Photonische Technologien, Albert-Einstein-Strasse 9, Jena, D-07745,Germany 
cAtominstitut, Vienna University of Technology, Stadionallee 2,  1020 Vienna,  Austria  
d CNRS/CRISMAT/ISMRA, 6, Bd du Maréchal Juin, CNRS UMR 6508, 14050 Caen, France 
eBen-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105 Israel 
f CNRS/CRETA, 25, Avenue des Martyrs BP 166, 38042 Grenoble CEDEX 9, France 
g Institute for Problems in Material Science of the Nat. Ac. of Sci. of Ukraine, 3 Krzhizhanovsky Str., Kiev, 03680, Ukraine  
iH.C. Starck GmbH, Goslar 38642, Germany. 
jHyper Tech Research, Inc. 1275 Kinnear Road Columbus, OH 43212, USA 
 
Abstract 
Structures of MgB2-based materials manufactured under pressure (up to 2 GPa) by different methods having high 
superconducting performance and connectivity are multiphase and contain different Mg-B-O compounds. Some 
oxygen can be incorporated into MgB2 and boron into MgO structures, MgBx (X=4-20) inclusions contain practically 
no oxygen. Regulating manufacturing temperature, pressure, introducing additions one can influence oxygen and 
boron distribution in the materials and thus, affect the formation, amount and sizes of Mg-B-O and  MgBx inclusions 
and changing type of pinning, pinning force and so affect critical current density jc. The boron concentration increase 
in initial Mg and B mixture allows obtaining sample containing 88.5 wt% of MgB12 with TC of 37.4 K (estimated 
magnetically).  
 
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Horst Rogalla and 
Peter Kes. 
Keywords: magnesium diboride; higher magnesium borides; nanostructure; critical current density, pinning force.  
Available online at www.sciencedirect.com
© 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.
Open access under CC BY-NC-ND license.
Open access under CC BY-NC-ND license.
476   Tatiana Prikhna et al. /  Physics Procedia  36 ( 2012 )  475 – 478 
1. Introduction 
Despite the comparatively simple structure of the MgB2 elementary unit cell, the nanostructure of 
MgB2-based materials is very complicated. A rather high amount of oxygen (4–15 wt.%) are usually 
present in the MgB2 matrix  as well, as inclusions of higher magnesium borides [1-3]. The atomic 
resolution study of oxygen incorporation into bulk MgB2 [4] shows that 20–100 nm sized precipitates of 
Mg(B,O) are formed by ordered substitution of oxygen atoms for boron lattice sites, while the basic bulk 
MgB2 crystal structure and orientation are preserved. The presence of these precipitates correlates well 
with an improved critical current density and superconducting transition behavior, implying that they act 
as pinning centers [4]. 
 Pressure is of great importance in the manufacture of superconducting MgB2-based bulk materials 
because it allows suppressing a volatility of Mg, impeding its oxidation, and promoting the formation of a 
mechanically stable denser structure. A denser material usually exhibits higher superconducting 
properties; it is more stable against degradation during exploitation, less reacts with a moisture, etc. [1]. 
The advantage of high-pressure (HP)-manufactured (2 GPa, 800-1050 oC, 1 h) MgB2 bulk is the 
possibility to get almost theoretically dense (1-2% porosity) material with very high critical current 
densities reaching at 20 K, in 0–1 T jc=1.3–1.0 106 A/cm2 (with 10% SiC) and jc= 9.2–7.3· 105 A/cm2 
(without doping) [1, 5], the highest irreversibility fields (Birr(18.4 K) =15 T and Birr (0 K) = 32.5 T) and 
upper critical fields (BC2 (22 K) =15 T and Bc2(0 K) ~ 42.1 T (in materials synthesized at 2 GPa, 600 oC, 
1h). The spark plasma synthesized (SPS) material (50 MPa, 600-1050 oC 1.3 h, without additions and 3–
10 % porosity), demonstrated at 20 K, in 0–1 T jc= 4.5–4·105 A/cm2. Hot pressed materials (at 30 MPa, 
800–1000 oC, 2 h, with 10 % of Ti additions and 8–15 % porosity) exhibited jc= 3.4–2.3·105 A/cm2. 
Usually the jc of in-situ prepared materials (from MgB2 powder) is higher than of ex-situ prepared ones 
(from Mg and B powders) [1, 5]. 
The paper presents results of SEM, Auger and X-ray structure study of the typical structures of 
materials formed under 2 GPa pressure and high temperature from Mg and B mixtures taken in MgB2, 
MgB7 and MgB12 stoichiometry in connection with their superconducting and mechanical properties. The 
established correlations and regularities of the formation of superconducting Mg-B-O (or MgB2-based) 
materials will be discussed based on the estimation of the type of pinning (using method proposed in [6]) 
in materials manufactured at different pressures and comprehensively studied earlier [1–3, 5]. 
2. Experimental, results, discussions and conclusions 
Samples were prepared by heating of pre-compacted Mg (I or II) and B (I-III) mixtures or MgB2 (98% 
purity) powder with and without additions of Ti, SiC, or C (up to 600–1150 oC: (1) under a high quasi-
hydrostatic pressure of 2 GPa in a recessed-anvil-type high-pressure apparatus [3] (HP), (2) under a 
uniaxial pressure of 16–50 MPa by spark plasma sintering (SPS) [9], (3) under 30 MPa by uniaxial hot 
pressing (HotP), and (4) pressureless synthesis at 0.1 MPa of Ar (PL). Several types of amorphous boron 
(type I - grain size <5 μm, 0.66 wt% O, 0.31 wt% C, 0.48 wt.% N, 0.32 wt.% H; type II - grain size 4 μm, 
1.5 wt% O, 0.47 wt% C, 0.40 wt% N, 0.37 wt% H and type III - grain size <1 μm, 3.5 wt% C was 
specially added during boron manufacturing, 1.02 wt.% N, 0.87 wt% H;) were used as starting materials. 
The boron powders and metal magnesium turnings Mg(I) (Technical Specifications of Ukraine 48-10-93-
88) or a fine powder of magnesium Mg (II) <1 μm (-325 mesh) produced by the HyperTech (USA) were 
mixed and milled in a high-speed planetary activator with steel balls for 1-3 min. To investigate the effect 
of additions, 10 wt% Ti (grain size 1-3 m, MaTecK, 99% purity), or SiC (200–800 nm) were added to 
the Mg and B mixture.   
 Tatiana Prikhna et al. /  Physics Procedia  36 ( 2012 )  475 – 478 477
The dominant pinning mechanism has been determined from the volume pinning force jcB, according to 
a scaling procedure proposed in [4]. The field Bpeak, where the maximum of the volume pinning force 
takes place, is normalized by the field Bn, at which the volume pinning force drops to half its maximum 
(on the high field side). The position of the peak, k=Bpeak/Bn, is expected to be at 0.34 and 0.47 for grain 
boundary (GBP) and point pinning (PP), respectively. The microstructure of the materials was 
characterized by X-ray structure (with Rietveld refinement) and SEM with microprobe X-ray and Auger 
(JAMP−9500F) analyses. The critical current density, jc, was estimated from magnetization 
measurements in an Oxford Instruments 3001 vibrating sample magnetometer (VSM) by using the Bean 
model; transition temperature was estimated using a SQUID magnetometer. 
Table 1 gives materials characteristics vs. preparation conditions: in-situ (from Mg and B mixture with 
and without 10 wt.% of SiC, Ti, or 3.5 wt.% of C) or ex-situ  (from MgB2). An increase in pressure (up to 
2 GPa) usually leads to a reduction in porosity (from 47 % to 1%) and enhancement in critical current 
density. In addition, high pressure synthesis or sintering increases the maximal pinning force, Fp(max), in 
materials prepared at high temperatures [5]. Fp(max) also increases by the addition of Ti or SiC both in low- 
and high-temperature-synthesized materials. It is larger in in-situ than in ex-situ prepared samples. The 
position of Fp(max) shifts to higher magnetic fields with (1) manufacturing pressure,  (2) the addition of Ti 
or SiC and (3) in-situ preparation (compared to ex-situ) [5]. Data of Table 1 show that GBP dominates in 
materials prepared at low temperatures (600–800 oC), while high-temperature preparation results mainly 
in PP or mixed pinning (MP) behavior. Exceptions were found in materials produced by SPS (k values 
were too high for the PP mechanism), which needs an additional study. These materials have high variety 
of higher borides MgBx (X=4-20) in their structures [1, 5]. 
Fig.1 shows typical structure of materials prepared from Mg(I) and B(II) taken in Mg:2B and Mg:12B 
ratio and their SC characteristics together with  some other synthesized materials. The Auger study (zone 
of excitation being 10 nm in diameter and two lattice parameters in depth) showed that a structure in Fig. 
1a had composition: MgB0.5-0.8O0.8-0.9 white inclusions, MgB11-13O0.2-0.5 black inclusions and             
MgB2.2-1.7O0.3-0.6 gray matrix. Comparing these results with that of X-ray structure analysis we can 
conclude that boron is incorporated in the MgO structure. Repeated etching in Ar (in the JAMP−9500F 
chamber during study) and quantitative Auger analysis showed practically the same oxygen concentration 
in gray matrix of the structure in Fig.1a after 30 cycles, which points to the oxygen incorporation into the 
MgB2 lattice. The material connectivity estimated by DC measurements turned out to be 80%, shielding 
(or SC) fraction estimated magnetically was 98%. The Auger analysis of the structure in Fig.1b showed 
Mg1.1-1.3OB0.8-1.2 stoichiometry for white areas and MgB6-12O0.6-1.2 stoichiometry for black areas. Line 
profile showed no near MgB2 stoichiometry. Despite the fact that amounts of shielding  fraction in 
samples 4 and 5 (Fig.1c) are rather small 7–10% and 1–1.5%, respectively, they exhibited Tc =37.5 K and 
jc =2-5·102 A/cm2 at 20 K (estimated magnetically) (Fig.1c, d), but they did not conduct the DC current. 
 
Fig. 1. (a, b) Structures obtained by SEM (SEI) of HP-synthesized at 2 GPa, 1200 oC, 1 h materials from Mg(I) and 
B(II) taken in Mg:2B and Mg:12B ratio, respectively, which according to X-ray structure analysis contained: 76 wt % 
MgB2, 24 wt % MgO (a) and 88.5 wt % MgB12, 2.5 wt % MgB2, 9  wt % MgO (b); (c) jc vs. magnetic field, oH, of 
HP-synthesized at 2 GPa for 1h: 1 –  from Mg(II):2B(I) at 1050 oC, 2 -  from Mg(I):2B(II) at 1200 oC, 3 – from 
Mg(II):2B(III) at 600 oC, 4– from Mg(I):12B(II) at 1200 oC, 5– from Mg(I):7B(II) at 1400 oC (according to X-ray 
structure analysis, material contained 53 wt % MgB12, 31 wt %MgB20, 16 wt % MgO); (d) - Tc of sample 4.   
478   Tatiana Prikhna et al. /  Physics Procedia  36 ( 2012 )  475 – 478 
Table 1. Characteristics of MgB2-based materials prepared under pressure, P, at temperature, t, from Mg 
and B taken in MgB2 stoichiometry without and with additions of SiC, Ti (10 wt%) or C (3.5 wt%) or 
from MgB2 powder. PP, GBP, MP- point, grain boundary, mixed type of pinning, respectively.  
 
Preparation, 
addition  
Type 
 of B 
P, MPa t, o C Fp(max)/ 
109(N/m3) 
k= Bpeak/Bn pinning 
in-situ, SiC (HP) I 2000 1050  10.9 0.51 PP 
in-situ, SiC (HP) I 2000 800 1.9 0.31 GBP 
in-situ, (HP) I 2000 1050  7.6 0.53 PP 
in-situ, (HP) I 2000 800 1.6 0.36 GBP 
in-situ, Ti, (HP) II 2000 1050 4.8 0.42 MP 
in-situ, Ti, (HP) II 2000 800 1.9 0.24 GBP 
in-situ, (HP) II 2000 1050 2.3 0.43 MP 
in-situ, (HP) II 2000 800 0.8 0.30 GBP 
ex-situ (HP) - 2000 1050 3.1 0.30 GBP 
in-situ, (SPS) II 50 1050 4.6 0.63 >PP* 
ex-situ (SPS) - 50 1050 3.3 0.58 >PP* 
in-situ, (SPS) II 50 800 2.7 0.56 >PP* 
in-situ,  C (HP) III 2000 600 0.6 0.31 GBP 
in-situ, Ti (HP) II 30 1000 2.7 0.42 MP 
ex-situ (SPS) - 16 1150 1.5 0.45 PP 
in-situ, (PL) II 0.1 800 1.9 0.35 GBP 
* Note: type of pinning is impossible to characterize because of high k ratio; ** all “in-situ” materials 
were prepared from Mg(I) and only C added from Mg(II). 
 
In matrices of MgB2-based materials synthesized at low temperatures the oxygen admixture is 
distributed more homogeneously than in those synthesized at higher temperatures and materials prepared 
under higher temperatures usually demonstrate higher jc in low and medium magnetic fields, while 
prepared under lower temperature have higher jc in high magnetic fields [1–5]. The amount and size of 
inclusions of higher borides decrease as the synthesis temperature increases. Additions of Ti, Ta, Zr, SiC 
and pressure can affect the distribution of oxygen and boron in the materials. In materials with Ti added a 
decrease of oxygen in the MgB2 matrices has been fixed. So, temperature, pressure and additions are the 
factors, which can influence oxygen and boron distribution in MgB2–based materials, thus affecting the 
pinning and critical current density.  
References 
[1] T.A. Prikhna et al. High-pressure synthesized nanostructural MgB2 materials with high performance of 
superconductivity, suitable for fault current limitation and other applications, IEEE Transactions on              
Applied Superconductivity, 21, No 3 (2011)2694. 
[2] T.A. Prikhna et al., Higher borides and oxygen-enriched Mg–B–O inclusions as possible pinning centers in 
nanostructural magnesium diboride and the influence of additives on their formation,  Physica C 470 (2010) 935. 
[3] T.A. Prikhna, et al., The inclusions of Mg-B (MgB12?) as potential pinning centers in high pressure-high 
temperature synthesized or sintered magnesium diboride, Supercond. Sci. Technology 20 (2007) S257.  
[4] R.F. Klie, J.C. Idrobo, N.D.Browning, A. Serquis, Y.T. Zhu, X.Z.Liao; F.M. Mueller, Observation of coherent 
oxide precipitates in polycrystalline MgB2, Applied Physics Letters. 80, n 21 (2002) 3970-3972.  
[5] T.A. Prikhna et al., The effect of high-pressure synthesis on flux pinning in MgB2-based superconductors 
(Physica C 2012, in press). 
[6]  M. Eisterer, Calculation of the volume pinning force in MgB2 superconductors, Phys. Rev. B 77 (2008) 144524. 


Superconductivity in Multi-Phase Mg-B-O Compounds 

Weber, Harald

HAL: Hyper Article en Ligne

Superconductivity in Multi-Phase Mg-B-O Compounds

HAL - Normandie Université

Hal - Université Grenoble Alpes

Tatiana Prikhna

Wolfgang Gawalek

Michael Eisterer

Harald W. Weber

Jacques Noudem

Vladimir Sokolovsky

Xavier Chaud

Viktor Moshchil

Myroslav Karpets

Valeriy Kovylaev

Alexandr Borimskiy

Vasiliy Tkach

Artem Kozyrev

Roman Kuznietsov

Jan Dellith

Christa Shmidt

Tetiana Basyuk

Doris Litzkendorf

Friedrich Karau

Ulrich Dittrich

Michael Tomsic

Crossref

Physics Procedia

Repositorium für Naturwissenschaften und Technik  (TIB Hannover)

 Physics Procedia  36 ( 2012 )  475 – 478 1875-3892  © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. doi: 10.1016/j.phpro.2012.06.220  Superconductivity Centennial Conference Superconductivity in multi-phase Mg-B-O compounds Tatiana Prikhnaa*, Wolfgang Gawalekb, Michael Eistererc, Harald W. Weberc,                Jacques Noudemd, Vladimir Sokolovskye,  Xavier Chaudf, Viktor Moshchila,     Myroslav Karpetsa,g, Valeriy Kovylaevg, Alexandr Borimskiya, Vasiliy Tkacha,                Artem Kozyreva, Roman Kuznietsova, Jan Dellithb, Christa Shmidtb, Tetiana Basyuka,              Doris Litzkendorfb, Friedrich Karaui, Ulrich Dittrichi, Michael Tomsicj aInstitute for Superhard Materials of the Nat. Ac. of Sci. of Ukraine, 2, Avtozavodskaya Str., Kiev 07074, Ukraine bInstitut für Photonische Technologien, Albert-Einstein-Strasse 9, Jena, D-07745,Germany cAtominstitut, Vienna University of Technology, Stadionallee 2,  1020 Vienna,  Austria  d CNRS/CRISMAT/ISMRA, 6, Bd du Maréchal Juin, CNRS UMR 6508, 14050 Caen, France eBen-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105 Israel f CNRS/CRETA, 25, Avenue des Martyrs BP 166, 38042 Grenoble CEDEX 9, France g Institute for Problems in Material Science of the Nat. Ac. of Sci. of Ukraine, 3 Krzhizhanovsky Str., Kiev, 03680, Ukraine  iH.C. Starck GmbH, Goslar 38642, Germany. jHyper Tech Research, Inc. 1275 Kinnear Road Columbus, OH 43212, USA  Abstract Structures of MgB2-based materials manufactured under pressure (up to 2 GPa) by different methods having high superconducting performance and connectivity are multiphase and contain different Mg-B-O compounds. Some oxygen can be incorporated into MgB2 and boron into MgO structures, MgBx (X=4-20) inclusions contain practically no oxygen. Regulating manufacturing temperature, pressure, introducing additions one can influence oxygen and boron distribution in the materials and thus, affect the formation, amount and sizes of Mg-B-O and  MgBx inclusions and changing type of pinning, pinning force and so affect critical current density jc. The boron concentration increase in initial Mg and B mixture allows obtaining sample containing 88.5 wt% of MgB12 with TC of 37.4 K (estimated magnetically).   © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Horst Rogalla and Peter Kes. Keywords: magnesium diboride; higher magnesium borides; nanostructure; critical current density, pinning force.  Available online at www.sciencedirect.com© 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.Open access under CC BY-NC-ND license.Open access under CC BY-NC-ND license.476   Tatiana Prikhna et al. /  Physics Procedia  36 ( 2012 )  475 – 478 1. Introduction Despite the comparatively simple structure of the MgB2 elementary unit cell, the nanostructure of MgB2-based materials is very complicated. A rather high amount of oxygen (4–15 wt.%) are usually present in the MgB2 matrix  as well, as inclusions of higher magnesium borides [1-3]. The atomic resolution study of oxygen incorporation into bulk MgB2 [4] shows that 20–100 nm sized precipitates of Mg(B,O) are formed by ordered substitution of oxygen atoms for boron lattice sites, while the basic bulk MgB2 crystal structure and orientation are preserved. The presence of these precipitates correlates well with an improved critical current density and superconducting transition behavior, implying that they act as pinning centers [4].  Pressure is of great importance in the manufacture of superconducting MgB2-based bulk materials because it allows suppressing a volatility of Mg, impeding its oxidation, and promoting the formation of a mechanically stable denser structure. A denser material usually exhibits higher superconducting properties; it is more stable against degradation during exploitation, less reacts with a moisture, etc. [1]. The advantage of high-pressure (HP)-manufactured (2 GPa, 800-1050 oC, 1 h) MgB2 bulk is the possibility to get almost theoretically dense (1-2% porosity) material with very high critical current densities reaching at 20 K, in 0–1 T jc=1.3–1.0 106 A/cm2 (with 10% SiC) and jc= 9.2–7.3· 105 A/cm2 (without doping) [1, 5], the highest irreversibility fields (Birr(18.4 K) =15 T and Birr (0 K) = 32.5 T) and upper critical fields (BC2 (22 K) =15 T and Bc2(0 K) ~ 42.1 T (in materials synthesized at 2 GPa, 600 oC, 1h). The spark plasma synthesized (SPS) material (50 MPa, 600-1050 oC 1.3 h, without additions and 3–10 % porosity), demonstrated at 20 K, in 0–1 T jc= 4.5–4·105 A/cm2. Hot pressed materials (at 30 MPa, 800–1000 oC, 2 h, with 10 % of Ti additions and 8–15 % porosity) exhibited jc= 3.4–2.3·105 A/cm2. Usually the jc of in-situ prepared materials (from MgB2 powder) is higher than of ex-situ prepared ones (from Mg and B powders) [1, 5]. The paper presents results of SEM, Auger and X-ray structure study of the typical structures of materials formed under 2 GPa pressure and high temperature from Mg and B mixtures taken in MgB2, MgB7 and MgB12 stoichiometry in connection with their superconducting and mechanical properties. The established correlations and regularities of the formation of superconducting Mg-B-O (or MgB2-based) materials will be discussed based on the estimation of the type of pinning (using method proposed in [6]) in materials manufactured at different pressures and comprehensively studied earlier [1–3, 5]. 2. Experimental, results, discussions and conclusions Samples were prepared by heating of pre-compacted Mg (I or II) and B (I-III) mixtures or MgB2 (98% purity) powder with and without additions of Ti, SiC, or C (up to 600–1150 oC: (1) under a high quasi-hydrostatic pressure of 2 GPa in a recessed-anvil-type high-pressure apparatus [3] (HP), (2) under a uniaxial pressure of 16–50 MPa by spark plasma sintering (SPS) [9], (3) under 30 MPa by uniaxial hot pressing (HotP), and (4) pressureless synthesis at 0.1 MPa of Ar (PL). Several types of amorphous boron (type I - grain size <5 μm, 0.66 wt% O, 0.31 wt% C, 0.48 wt.% N, 0.32 wt.% H; type II - grain size 4 μm, 1.5 wt% O, 0.47 wt% C, 0.40 wt% N, 0.37 wt% H and type III - grain size <1 μm, 3.5 wt% C was specially added during boron manufacturing, 1.02 wt.% N, 0.87 wt% H;) were used as starting materials. The boron powders and metal magnesium turnings Mg(I) (Technical Specifications of Ukraine 48-10-93-88) or a fine powder of magnesium Mg (II) <1 μm (-325 mesh) produced by the HyperTech (USA) were mixed and milled in a high-speed planetary activator with steel balls for 1-3 min. To investigate the effect of additions, 10 wt% Ti (grain size 1-3 m, MaTecK, 99% purity), or SiC (200–800 nm) were added to the Mg and B mixture.    Tatiana Prikhna et al. /  Physics Procedia  36 ( 2012 )  475 – 478 477The dominant pinning mechanism has been determined from the volume pinning force jcB, according to a scaling procedure proposed in [4]. The field Bpeak, where the maximum of the volume pinning force takes place, is normalized by the field Bn, at which the volume pinning force drops to half its maximum (on the high field side). The position of the peak, k=Bpeak/Bn, is expected to be at 0.34 and 0.47 for grain boundary (GBP) and point pinning (PP), respectively. The microstructure of the materials was characterized by X-ray structure (with Rietveld refinement) and SEM with microprobe X-ray and Auger (JAMP−9500F) analyses. The critical current density, jc, was estimated from magnetization measurements in an Oxford Instruments 3001 vibrating sample magnetometer (VSM) by using the Bean model; transition temperature was estimated using a SQUID magnetometer. Table 1 gives materials characteristics vs. preparation conditions: in-situ (from Mg and B mixture with and without 10 wt.% of SiC, Ti, or 3.5 wt.% of C) or ex-situ  (from MgB2). An increase in pressure (up to 2 GPa) usually leads to a reduction in porosity (from 47 % to 1%) and enhancement in critical current density. In addition, high pressure synthesis or sintering increases the maximal pinning force, Fp(max), in materials prepared at high temperatures [5]. Fp(max) also increases by the addition of Ti or SiC both in low- and high-temperature-synthesized materials. It is larger in in-situ than in ex-situ prepared samples. The position of Fp(max) shifts to higher magnetic fields with (1) manufacturing pressure,  (2) the addition of Ti or SiC and (3) in-situ preparation (compared to ex-situ) [5]. Data of Table 1 show that GBP dominates in materials prepared at low temperatures (600–800 oC), while high-temperature preparation results mainly in PP or mixed pinning (MP) behavior. Exceptions were found in materials produced by SPS (k values were too high for the PP mechanism), which needs an additional study. These materials have high variety of higher borides MgBx (X=4-20) in their structures [1, 5]. Fig.1 shows typical structure of materials prepared from Mg(I) and B(II) taken in Mg:2B and Mg:12B ratio and their SC characteristics together with  some other synthesized materials. The Auger study (zone of excitation being 10 nm in diameter and two lattice parameters in depth) showed that a structure in Fig. 1a had composition: MgB0.5-0.8O0.8-0.9 white inclusions, MgB11-13O0.2-0.5 black inclusions and             MgB2.2-1.7O0.3-0.6 gray matrix. Comparing these results with that of X-ray structure analysis we can conclude that boron is incorporated in the MgO structure. Repeated etching in Ar (in the JAMP−9500F chamber during study) and quantitative Auger analysis showed practically the same oxygen concentration in gray matrix of the structure in Fig.1a after 30 cycles, which points to the oxygen incorporation into the MgB2 lattice. The material connectivity estimated by DC measurements turned out to be 80%, shielding (or SC) fraction estimated magnetically was 98%. The Auger analysis of the structure in Fig.1b showed Mg1.1-1.3OB0.8-1.2 stoichiometry for white areas and MgB6-12O0.6-1.2 stoichiometry for black areas. Line profile showed no near MgB2 stoichiometry. Despite the fact that amounts of shielding  fraction in samples 4 and 5 (Fig.1c) are rather small 7–10% and 1–1.5%, respectively, they exhibited Tc =37.5 K and jc =2-5·102 A/cm2 at 20 K (estimated magnetically) (Fig.1c, d), but they did not conduct the DC current.  Fig. 1. (a, b) Structures obtained by SEM (SEI) of HP-synthesized at 2 GPa, 1200 oC, 1 h materials from Mg(I) and B(II) taken in Mg:2B and Mg:12B ratio, respectively, which according to X-ray structure analysis contained: 76 wt % MgB2, 24 wt % MgO (a) and 88.5 wt % MgB12, 2.5 wt % MgB2, 9  wt % MgO (b); (c) jc vs. magnetic field, oH, of HP-synthesized at 2 GPa for 1h: 1 –  from Mg(II):2B(I) at 1050 oC, 2 -  from Mg(I):2B(II) at 1200 oC, 3 – from Mg(II):2B(III) at 600 oC, 4– from Mg(I):12B(II) at 1200 oC, 5– from Mg(I):7B(II) at 1400 oC (according to X-ray structure analysis, material contained 53 wt % MgB12, 31 wt %MgB20, 16 wt % MgO); (d) - Tc of sample 4.   478   Tatiana Prikhna et al. /  Physics Procedia  36 ( 2012 )  475 – 478 Table 1. Characteristics of MgB2-based materials prepared under pressure, P, at temperature, t, from Mg and B taken in MgB2 stoichiometry without and with additions of SiC, Ti (10 wt%) or C (3.5 wt%) or from MgB2 powder. PP, GBP, MP- point, grain boundary, mixed type of pinning, respectively.   Preparation, addition  Type  of B P, MPa t, o C Fp(max)/ 109(N/m3) k= Bpeak/Bn pinning in-situ, SiC (HP) I 2000 1050  10.9 0.51 PP in-situ, SiC (HP) I 2000 800 1.9 0.31 GBP in-situ, (HP) I 2000 1050  7.6 0.53 PP in-situ, (HP) I 2000 800 1.6 0.36 GBP in-situ, Ti, (HP) II 2000 1050 4.8 0.42 MP in-situ, Ti, (HP) II 2000 800 1.9 0.24 GBP in-situ, (HP) II 2000 1050 2.3 0.43 MP in-situ, (HP) II 2000 800 0.8 0.30 GBP ex-situ (HP) - 2000 1050 3.1 0.30 GBP in-situ, (SPS) II 50 1050 4.6 0.63 >PP* ex-situ (SPS) - 50 1050 3.3 0.58 >PP* in-situ, (SPS) II 50 800 2.7 0.56 >PP* in-situ,  C (HP) III 2000 600 0.6 0.31 GBP in-situ, Ti (HP) II 30 1000 2.7 0.42 MP ex-situ (SPS) - 16 1150 1.5 0.45 PP in-situ, (PL) II 0.1 800 1.9 0.35 GBP * Note: type of pinning is impossible to characterize because of high k ratio; ** all “in-situ” materials were prepared from Mg(I) and only C added from Mg(II).  In matrices of MgB2-based materials synthesized at low temperatures the oxygen admixture is distributed more homogeneously than in those synthesized at higher temperatures and materials prepared under higher temperatures usually demonstrate higher jc in low and medium magnetic fields, while prepared under lower temperature have higher jc in high magnetic fields [1–5]. The amount and size of inclusions of higher borides decrease as the synthesis temperature increases. Additions of Ti, Ta, Zr, SiC and pressure can affect the distribution of oxygen and boron in the materials. In materials with Ti added a decrease of oxygen in the MgB2 matrices has been fixed. So, temperature, pressure and additions are the factors, which can influence oxygen and boron distribution in MgB2–based materials, thus affecting the pinning and critical current density.  References [1] T.A. Prikhna et al. High-pressure synthesized nanostructural MgB2 materials with high performance of superconductivity, suitable for fault current limitation and other applications, IEEE Transactions on              Applied Superconductivity, 21, No 3 (2011)2694. [2] T.A. Prikhna et al., Higher borides and oxygen-enriched Mg–B–O inclusions as possible pinning centers in nanostructural magnesium diboride and the influence of additives on their formation,  Physica C 470 (2010) 935. [3] T.A. Prikhna, et al., The inclusions of Mg-B (MgB12?) as potential pinning centers in high pressure-high temperature synthesized or sintered magnesium diboride, Supercond. Sci. Technology 20 (2007) S257.  [4] R.F. Klie, J.C. Idrobo, N.D.Browning, A. Serquis, Y.T. Zhu, X.Z.Liao; F.M. Mueller, Observation of coherent oxide precipitates in polycrystalline MgB2, Applied Physics Letters. 80, n 21 (2002) 3970-3972.  [5] T.A. Prikhna et al., The effect of high-pressure synthesis on flux pinning in MgB2-based superconductors (Physica C 2012, in press). [6]  M. Eisterer, Calculation of the volume pinning force in MgB2 superconductors, Phys. Rev. B 77 (2008) 144524. 

Superconductivity in multi-phase Mg-B-O compounds

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Superconductivity in multi-phase Mg-B-O compounds

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