A systematic study of the structure and superconductive characteristics of high-pressure-high-temperature (2 GPa, 700-1000 deg C)-synthesized and sintered MgB2 without additions from different ...initial powders was performed. Among various secondary phases Mg-B inclusions with a stoichiometry close to MgB12 were identified. With an increasing amount of these inclusions the critical current density increased. So these inclusions can be feasible pinning centres in MgB2. The highest jc values in zero field were 1300 kA cm-2 at 10 K, 780 kA cm-2 at 20 K and 62 kA cm-2 at 35 K and in 1 T field were 1200 kA cm-2 at 10 K, 515 kA cm-2 at 20 K and 0.1 kA cm-2 at 35 K for high-pressure-synthesized magnesium diboride and the field of irreversibility at 20 K reached 8 T. The average grain sizes calculated from x-ray examinations in materials having high jc were 15-37 nm.
The addition of Ta (2-10 wt.%) to a starting mixture of Mg and B (taken in the MgB2 stoichiometry) and application of high pressure (2 GPa) during the synthesis process (800-900DGC for 1 h) allow us ...to produce bulk MgB2-based materials with the critical current densities (jc) of: 630 kA/cm2 at 10 K, 425 kA/cm2 at 20 K, 165 kA/cm2 at 30 K in the O T field; 570 kA/cm2 at 10 K, 350 kA/cm2 at 20 K and 40 kA/cm2 at 30 K in the 1 T field and 650 A/cm2 at 10 K in the 10 T field. X-ray and SEM studies have shown that Ta did not react with B or Mg, but absorbed the impurity gases to form Ta2H, TaH, TaN0.1, etc. The samples with highest superconductive characteristics exhibited a reduced amount or absence of MgH2 in the Mg-B-O-matrix phase, as well as, the impurity nitrogen and oxygen in MgB2 single crystals distributed over the matrix. Samples with a higher level of critical currents included some amount of unreacted Mg. The Vickers microhardness of the matrix material was Hv = 12.54 #+ 0.86 GPa (at 0.496-N load). The nanohardness (at 60 mN load) of MgB2 single crystals located in the matrix was 35.6#+0.9 GPa, i.e. higher than the nanohardness of sapphire (31.1 #+ 2.0 GPa), and that means that MgB2 belongs to superhard materials.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Peculiarities of High-Pressure Oxygenation of MT-YBCO Prikhna, T.A.; Chaud, X.; Gawalek, W. ...
IEEE transactions on applied superconductivity,
06/2007, Volume:
17, Issue:
2
Journal Article, Conference Proceeding
Peer reviewed
The oxygenation under the high 16 MPa oxygen pressure allows reducing the duration of oxygenation and in the case of thin-walled MT-YBCO ceramics (with drilled holes) leads to a decrease of the ...amount of macrocracks in the structure and to an increase in critical current density in the ab-planes. An increase in the twin density enhances the critical current density in the ab-planes of the Y123 textured structure of MT-YBCO. After oxygenation under different conditions the MT-YBCO ceramics produced from a mixture of Y123 with Y211 demonstrated a critical current density in magnetic fields above 2 T at 77 K higher in the ab-planes and lower in the perpendicular c-direction than the MT-YBCO produced from Y123 and Y 2 O 3 . Mechanical properties (hardness, fracture toughness, Young modulus) have been estimated for various types of MT-YBCO oxygenated under different conditions. Both traditional and thin-walled ceramics have a higher microhardness after oxygenation under high 16 MPa oxygen pressure.
The effect of doping with Ti, Ta, SiC in complex with synthesis temperature on the amount and distribution of structural inhomogeneities in MgB2 matrix of high-pressure-synthesized-materials (2 GPa) ...which can influence pinning: higher borides (MgB12) and oxygen-enriched Mg-B-O inclusions, was established and a mechanism of doping effect on jc increase different from the generally accepted was proposed. Near theoretically dense SiC-doped material exhibited jc= 106 A/cm2 in 1T field and Hirr =8.5 T at 20 K. The highest jc in fields above 9, 6, and 4 T at 10, 20, and 25 K, respectively, was demonstrated by materials synthesized at 2 GPa, 600 °C from Mg and B without additions (at 20 K jc= 102 A/cm2 in 10 T field). Materials synthesized from Mg and B taken up to 1:20 ratio were superconductive. The highest jc (6×104 A/cm2 at 20 K in zero field, Hirr= 5 T) and the amount of SC phase (95.3% of shielding fraction), Tc being 37 K were demonstrated by materials having near MgB12 composition of the matrix. The materials with MgB12 matrix had a doubled microhardness of that with MgB2 matrix (25±1.1 GPa and 13.08±1.07 GPa, at a load of 4.9 N, respectively).
Regularly distributed structural inhomogeneities in the MgB sub(2) matrix, such as nano-areas with a high concentration of boron (MgB sub(x)) 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 degreesC), and in materials with different connectivity (18-98%) and density (55-99%). Such inhomogeneities can act as pinning centers in MgB sub(2) because the variation of their size and distribution are well correlated with variations of the critical current density, j sub(c). The decrease in size of MgB sub(x) inclusions, the transformation of 15-20 mn 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.
PDF
