Regularly distributed structural inhomogeneities in the MgB2 matrix, such as nano-areas with a high concentration of boron (MgBx) and impurity oxygen (Mg-B-O nano-layers or inclusions), are observed ...in all materials independently of the preparation method, pressure (0.1 MPa-2 GPa) and temperature (600-1100 °C), and in materials with different connectivity (18-98%) and density (55-99%). Such inhomogeneities can act as pinning centers in MgB2 because the variation of their size and distribution are well correlated with variations of the critical current density, jc. The decrease in size of MgBx inclusions, the transformation of 15-20 nm thick Mg-B-O nano-layers into separated inclusions, and the localization of impurity oxygen are accompanied by an increase in critical current density in low and medium magnetic fields. The efficiency of these defects is evidenced by a shift from grain-boundary pinning to point pinning.
Preparation and Properties of MgB2 Thin Films Prikhna, Tatiana A.; Eisterer, Michael; Shaternik, Anton V. ...
IEEE transactions on applied superconductivity,
10/2018, Volume:
28, Issue:
7
Journal Article
Peer reviewed
The superconducting transition temperature of 140 nm ± 10 nm thin films on sapphire substrates, which were deposited by magnetron sputtering, was around 36 K. Using a magnetization technique, the ...film's critical current density was estimated as J C = 1.8 × 10 7 A/cm 2 at 10 K, J C = 8 × 10 6 A/m 2 at 20 K in a zero magnetic field B, and J C = 3 × 10 6 A/m 2 at 10 K and B = 5 T. The values of the upper critical magnetic field B c2 and the irreversibility field B irr estimated using the four-probe technique were B C2 (22 K) = 15 T when H||film surface, 11 T when H⊥film surface, and B irr (22 K) = 11 T when H||film surface. The X-ray study showed that the microstructure of the film contains only MgB 2 and MgO (in minority). The SEM and EPXMA study and quantitative Auger spectroscopy analysis revealed periodical variations of the film composition on the nanolevel and the presence of (mainly) two intercalated Mg-B-O-C phases of slightly different, especially in oxygen content, and thus with different conductivity and, possibly, with different T C . The characteristics of superconducting magnesium diboride films make them promising for application in electronic devices, e.g., as high-pass filters.
The volume pinning force,
F
p(max)
, increases with increasing synthesis or sintering pressure (0.1 MPa–2 GPa) in materials prepared at high temperature (1050 °C) while it stays practically unchanged ...in those prepared at low temperature (800 °C). The position of
F
p(max)
can be shifted to higher magnetic fields by: (1) increasing the manufacturing pressure or decreasing the temperature (2) additions (Ti, SiC, or C, for example), and (3) in-situ preparation. Grain boundary pinning (GBP) dominates in materials prepared at low temperatures (600–800 °C), while high-temperature preparation induces strong point pinning (PP) or mixed pinning (MP) leading to outstanding properties. In materials produced by spark plasma sintering (SPS), the position of
F
p(max)
is higher than expected for both grain boundary and point pinning. The distribution of boron and oxygen in MgB
2
based material, which can changed by additions or the preparation conditions, significantly affects the type and strength of pining.
Materials prepared under a pressure of 2 GPa with a nominal composition of Mg:7B or Mg:12B consist of 88.5 wt % MgB
12
, 2.5 wt % MgB
2
, 9 wt % MgO or 53 wt % MgB
12
, 31 wt % MgB
20
16 wt % MgO, respectively. Their magnetic shielding fractions at low temperatures are 10 % and 1.5 %, with a transition temperature,
T
c
of 37.4–37.6 K. Although their magnetic critical current density at zero field and 20 K was 2–5×10
2
A/cm
2
, they were found to be insulating on the macroscopic level.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
This paper describes the established correlations between the characteristics of MgB 2 -based superconducting materials and their structural features, in particular the oxygen distribution and ...content. As indicated by SEM and Auger analysis, a sizeable amount of oxygen is usually present in superconducting MgB 2 -based materials (bulks, thin films, and wires). The matrix phase of bulk MgB 2 contains rather small amounts of oxygen, but contains a high amount of dispersed inclusions or areas with compositions close to MgBO. X-ray phase analysis with Rietveld refinement (using the FullProf Suite program package) of several highly dense MgB 2 -based bulk samples (with high critical current densities) showed that the superconducting phase had a composition within the range MgB 1.68 O 0.26 and MgB 1.74 O 0.32 instead of pure MgB 2 . Besides, a small amount of a phase with MgO structure was observed in the materials by X-rays as well. The calculation of the enthalpy of formation confirms the possibility of oxygen solubility in MgB 2 and shows that the formation of MgB 1.75 B 0.25 is most favorable. The results of ab initio calculations of the electronic structure and stability of the MgB 2 compounds with partial oxygen substitution for boron show that it is energetically preferable for oxygen atoms to replace boron pairwise.
The distribution of nanostructural inhomogeneities, such as areas with high concentration of B and impurity O, plays one of the key roles on the variation of the critical current density,
j
c
, of ...MgB
2
materials. The effect of O and B redistribution can be enhanced by Ti addition and thus leads to the
j
c
increase. The effect of Ti addition and the mechanism of this effect are discussed based on experiments of MgB
2
synthesis under 2 GPa pressure at 800 and 1,050 °C.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
8.
Preparation and Properties of MgB 2 Thin Films Prikhna, Tatiana A.; Eisterer, Michael; Shaternik, Anton V. ...
IEEE transactions on applied superconductivity,
10/2018, Volume:
28, Issue:
7
Journal Article
The Ti3AlC2-, (Ti,Nb)3AlC2- and Ti2AlC-based materials turned out to be more resistant than Crofer JDAsteel in oxidizing atmosphere as 1000 h long tests at 600◦C have shown. But the amounts of oxygen ...absorbed bythe materials during testing were different. The Ti2AlC-based material demonstrated the lowest oxygen uptake,(Ti,Nb)3AlC2-based absorbed a somewhat higher amount and the highest amount was absorbed by Ti3AlC2-basedmaterial. Scanning electron microscopy and the Auger study witnessed that amounts of oxygen in the MAXphases before the exposure in air were as well different: the approximate stoichiometries of the matrix phases ofmaterials were Ti3.1−3.2AlC2−2.2, Ti1.9−4Nb0.06−0.1AlC1.6−2.2O0.1−1.2and Ti2.3−3.6AlC1−1.9O0.2−0.6, respectively.The higher amount of oxygen present in the MAX phase structures may be the reason for higher resistanceto oxidation during long-term heating in air at elevated temperature. The studied materials demonstrated highstabilities in hydrogen atmosphere as well. The bending strength of the Ti3AlC2- and (Ti,Nb)3AlC2-based materialsafter keeping at 600◦C in air and hydrogen increased by 10–15%, but the highest absolute value of bending strengthbefore and after being kept in air and hydrogen demonstrated the Ti2AlC-based material (about 590 MPa).
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