Viktor Moshchil

High-pressure synthesis of MgB2 with addition of Ti

Abstract Magnesium diboride-based material high-pressure synthesized at 2 GPa and 800 °C for 1 h from Mg and B (taken in the stoichiometry ratio of MgB 2 ) with addition of 2–10 wt.% of Ti demonstrated the critical current density ( j c ) higher than 100 kA/cm 2 at 20 K up to 3 T and at 33 K in 0 T field. At 20 K the critical current density higher than 10 kA/cm 2 was observed up to 5 T field. In the magnetic fields up to 2 T high-pressure synthesized MgB 2 (with 10% of Ti) at 20 K has a critical current density comparable to that of Nb 3 Sn at 4.2 K. XRD patterns of magnesium diboride with Ti addition exhibited no evidence of unreacted titanium and only one compound with titanium was identified, namely, titanium dihydride TiH 2 (or more strictly TiH 1.924 ). The sample with the highest critical current density and the irreversibility field in the temperature range of 25–10 K contained some amount of rather homogeneously dispersed pure Mg and high amount of Mg–B inclusions.

The inclusions of Mg?B (MgB12?) as potential pinning centres in high-pressure?high-temperature-synthesized or sintered magnesium diboride

A systematic study of the structure and superconductive characteristics of high-pressure?high-temperature (2?GPa, 700?1000??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.

High-pressure synthesis of a bulk superconductive MgB2-based material

Abstract The addition of Ta (2–10 wt.%) to a starting mixture of Mg and B (taken in the MgB 2 stoichiometry) and application of high pressure (2 GPa) during the synthesis process (800–900 °C for 1 h) allow us to produce bulk MgB 2 -based materials with the critical current densities ( j c ) of: 630 kA/cm 2 at 10 K, 425 kA/cm 2 at 20 K, 165 kA/cm 2 at 30 K in the 0 T field; 570 kA/cm 2 at 10 K, 350 kA/cm 2 at 20 K and 40 kA/cm 2 at 30 K in the 1 T field and 650 A/cm 2 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 Ta 2 H, TaH, TaN 0.1 , etc. The samples with highest superconductive characteristics exhibited a reduced amount or absence of MgH 2 in the Mg–B–O-matrix phase, as well as, the impurity nitrogen and oxygen in MgB 2 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 H v =12.54±0.86 GPa (at 0.496-N load). The nanohardness (at 60 mN load) of MgB 2 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 MgB 2 belongs to superhard materials.

Nanostructural inhomogeneities acting as pinning centers in bulk MgB2 with low and enhanced grain connectivity

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.

Effect of higher borides and inhomogeneity of oxygen distribution on critical current density of undoped and doped magnesium diboride

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).

Superconducting joining of melt-textured Y-Ba-Cu-O bulk material

Abstract The Tm–Ba–Cu–O solder can be successfully used to produce a superconductive joint between melt-textured YBa 2 Cu 3 O 7− δ (MT-YBCO) parts. The peculiarities of solidification, phase formation, structure transformations and electromagnetic properties of MT-YBCO soldered with TmBa 2 Cu 3 O 7− δ are discussed.

Studies of the oxidation stability, mechanical characteristics of materials based on max phases of the Ti-Al-(C, N) systems, and of the possibility of their use as tool bonds and materials for polishing

Thermogravimetry and differential thermal analysis have been used to study the resistance to the air oxidation of high-density samples of Ti3AlC2, Ti2AlC and Ti2Al(C1−xNx) solid solutions. It has been shown that the Ti3AlC2 samples are more stable than Ti2AlC and Ti2Al(C1−xNx) solid solutions and as the nitrogen content of the solid solution increases to x = 0.75, the oxidation resistance decreases. The following characteristics have been exhibited by the material containing 89 wt % Ti3AlC2 (the rest being Al2O3 and TiC) having density 99% of theoretical: bending strength Rbm = 500 MPa, compressive strength Rcm = 700 MPa, fracture toughness KIc = 10.2 MPa·m0.5, hardness HRA = 70 GPa, HV = 4.6 GPa, Young modulus = 149.4 ± 28.7 GPa. After sintering with diamonds or cBN (50 wt %) at 5.5–7.7 GPa and 1350–1960°C for 0.07–1.0 h the Ti3AlC2 MAX phase decomposes to form TiC and TiAl or TiB2 and a thin layer of Al4C3 forms at the interface with diamond. The Al4C3 decomposition in a composite material due to the interaction with the air moisture results in the crack initiation along the diamond perimeter, which brings about the material fracture in 1–2 weeks. It has been found that the Ti3AlC2 powder is efficient for polishing natural and synthetic jewelry crystals and competitive in polishing efficiency and quality with ACM 2/1 grade diamond.

Structure and properties of melt-textured YBa2Cu3O7-δ, high pressure-high temperature treated and oxygenated under evaluated oxygen pressure

MT-YBCO samples oxygenated under controlled oxygen pressure exhibited at 77 K a critical current density jc = 85 kA cm−2 in zero field and more than 10 kA cm−2 up to 5 T field when the external magnetic field was perpendicular to the ab-plane of Y123, and a jc = 23 kA cm−2 in zero field and jc close to 1 kA cm−2 in 10 T field when the magnetic field was perpendicular to the c-axis of Y123. The microstructure of these samples contained an unusually high density of twins (about 30 twins µm−1) as well as a lot of stacking faults around Y211 inclusions. Using quasi-hydrostatic high pressure–high temperature (HP–HT) treatment we may vary the twin and dislocation densities in the material by changing the sample orientation in high pressure apparatus, while the oxygen content of Y123 phase as well as the lattice parameters remain unchanged. The microstructure of the material in the case where the highest pressure has been applied in the direction perpendicular to the ab-plane of Y123 is characterized by a very low twin density, perfect dislocations stepped along directions and small faulted loops corresponding to CuO intercalating in the matrix. For this material jc = 10 and 8 kA cm−2 in zero field were observed (when the external magnetic field was perpendicular to the ab-plane and perpendicular to the c-axis of Y123, respectively). High pressure–high temperature treatment causes an increase in the material density (up to near the theoretical one), microhardness and fracture toughness.

High-pressure synthesis of MgB2 with and without tantalum additions

Abstract The high-pressure (HP) synthesis seems to be very promising for manufacturing of the bulk MgB 2 superconductive material. The positive influence of tantalum in the form of a foil that covered the sample and as an addition of Ta powder in the starting mixture of B and Mg on critical current density in the magnetic field and on the field of irreversibility of MgB 2 has been observed during MgB 2 synthesis. At 20 K HP-synthesized MgB 2 has a critical current density of 68 kA/cm 2 in the 1 T field and of 160 kA/cm 2 in the zero field, its field of irreversibility at 20 K being 5.7 T. Vickers microhardness of the samples is 12.79±1.14 GPa at a 4.96-N load, fracture toughness is 4.1 MN/m 3/2 at the 147.2-N load (while at the 4.96-N load there are no cracks from the corners of the indent). The density of HP-synthesized MgB 2 samples is 95–97% the theoretical one.

Formation of Higher Borides During High-Pressure Synthesis and Sintering of Magnesium Diboride and Their Positive Effect on Pinning and Critical Current Density

Critical current density (j_c) of high-pressure (2 GPa) manufactured MgB₂-based superconductors depends on the amount and distribution of higher borides (MgB₁₂) in MgB₂ matrix, which in turn are determined by the nature of the initial components first of all B or MgB₂ and the temperature of sintering or synthesis. Ti and Ta additions can improve j_c by promoting the higher boride formation via impurity hydrogen absorption, thus preventing MgH₂ detrimental for j_c being formed, which possibly increases the MgB₁₂ nucleation barrier. SiC (0.2-0.8 mum) addition increases j_c of MgB₂, allowing us to get j_c = 10⁶ A/cm² at 20 K in the 1 T field: pinning is increased by SiC and higher boride grains and there is no notable interaction between SiC and MgB₂ . As the synthesis temperature increases from 800 to 1050degC, Ti and SiC additions may affect the oxygen segregation and formation of Mg-B-O inclusions enriched with oxygen as compared to the amount of oxygen in the MgB₂ matrix, which can also promote an increase in pinning. Materials high-pressure synthesized from Mg and B taken in 1:4, 1:6, 1:7, 1:8, 1:10, 1:12, 1:20 ratios were superconductive with T_c of about 37 K. High j_c (7middot10⁴ - 2middot10⁴ A/cm² in zero field at 10-30 K, respectively) showed materials with the matrix composition near MgB₁₂ stoichiometry, they have doubled microhardness of MgB_2.