Ya. M. Savchuk

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.

Chemical interactions in Ti doped MgB2 superconducting bulk samples and wires

Magnesium diboride superconducting wires and bulk samples synthesized at high pressure have been produced with a variety of reactive metal additions. All these samples showed high critical current densities, but here we are most interested in the striking result that reactive metal additions such as titanium substantially improved the critical current values, but had little detrimental effect on Tc values. For instance, the critical current values in the wires could be increased by more than a factor of 3.5 at 4 K and 10 T by the inclusion of up to 10 wt% of Ti. In bulk materials the Jc values can be increased by even larger values, and the irreversibility field at 20 K increased to above 7 T. In a detailed study of the microstructure of these materials we have shown that PIT ex situ wires with 10 wt% titanium additions have a complicated layered microstructure around the Ti-rich particles, possibly forming titanium–boron phases, but also incorporating impurity elements including hydrogen. X-ray diffraction data, TEM and elemental mapping in the NanoSIMS confirmed the surprising suggestion that samples synthesized at high pressure from Mg and B with Ti additions contain a titanium hydride. These new compositional data support the idea that reactive metal additions are extremely beneficial in MgB2 wires and bulk materials because they preferentially adsorb deleterious impurities from the superconducting matrix.

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).

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.

Improvement of superconductive and mechanical properties of bulk and thin-wall MT-YBCO ceramics in oxygenation

The possibility to oxygenate the YBa2Cu3O7−δ(Y123) structure to 7−δ ≈ 6.9 atoms (which ensures the highest temperature of transition into the superconductive state of this compound) at high temperature (800°C) and relatively low pressure (16 MPa) of oxygen has been first shown. This fact differs from the generally accepted notions of the equilibrium in the given system. It has been found that the MT-YBCO oxygenation at enhanced pressures and high temperatures decreases the oxygenation time and the amount of cracks and increases the twin density in the material, which positively affects the critical current density and mechanical characteristics of the ceramics. The experiments have shown that twins are largely responsible for high density of the critical currents jc and irreversibility fields in the MT-YBCO ceramics. In the case of high dislocation density (1012 cm−2) and low twin density (0–1 μm−1) in the Y123 structure, the critical current turned out to be an order of magnitude lower than in the case of high twin density (22 μm−1) and absence of dislocations and stacking faults. The density of twins and microcracks (parallel to the ab plane) in the structure of the YBa2Cu3O7−δ phase has been found to depend on the size of the Y2BaCuO5 inclusions and the pattern of their distribution, which in turn is defined by the initial materials. A process has been developed of the oxygenation of the thin-wall (cellular) MT-YBCO ceramics under the conditions of the controllable oxygen pressure from 0.5 kPa to 16 MPa and temperatures from 900 to 800°C. The process allows one to attain record high jc values and double the trapped magnetic field as compared to both the bulk MT-YBCO ceramics oxygenated under the same conditions and thin-wall MT-YBCO ceramics oxygenated at atmospheric pressure and optimal temperature.

Oxygenation of bulk and thin-walled MT-YBCO under controllable oxygen pressure

The possibility to saturate MT-YBCO with oxygen up to about 7 oxygen atoms per one unit cell of YBa2Cu3O7-δ at 800 °C under controllable oxygen pressure (from 0.5 kPa to 16 MPa) has been established. On condition that the material heating to high temperatures occurs in the nitrogen atmosphere with gradual substitution of it for oxygen and further increase of the oxygen pressure, the saturation of MT-YBCO by oxygen takes place practically without the macrocrack formation and results in the decrease of microcrack density parallel to ab-planes, that leads to the increase of critical current density and mechanical characteristics. It has been shown that in the case of a higher twin density, the material demonstrates higher critical currents, jc. Bulk and thin-walled MT-YBCO oxygenated at high controllable pressure and 800 °C contained a large amount of twins (4-11 and 20-22 μm-1, respectively) and were practically free from dislocations and stacking faults, but demonstrated very high values of jc at 77 K: jc higher than 10 kA/cm2 was observed in the fields up to 5T in ab-planes and up to 2 T in the direction of the c-axis. Specially detwinned MT-YBCO containing a large amount of dislocations (up to 1012 cm-2) demonstrated critical currents lower by about one order of magnitude, which gives us ground to conclude that the presence of twins is extremely important for attaining high critical currents in MT-YBCO. The high-pressure oxygenation allows one to essentially reduce the duration of the oxygenation process as compared to that under atmospheric pressure.

HIGH‐PRESSURE OXYGENATION OF MT‐YBCO: THE WAY TO REDUCE THE OXYGENATION TIME, TO PREVENT MACROCRACKING, AND TO OBTAIN MATERIALS WITH HIGH CRITICAL CURRENTS.

The oxygenation of MT‐YBCO under isostatic oxygen pressure (up to 16 MPa) at 900‐800 °C allowed reduced process time, lower macrocracking, and reduced microcracks. Additionally higher critical currents, trapped fields and mechanical characteristics can be attained. At 77 K thin‐walled MT‐YBCO had a jc in the ab plane of 85 kA/cm2 at 0 T and higher than 10 kA/cm2 in fields up to 5 T and the irreversibility field was 9.8 T. In the c‐direction jc was 34 kA/cm2 in 0 T and higher than 2.5 kA/cm2 in a 10 T field. At 4.9 N‐load the micohardness, Hv, was 8.7±0.3 GPa in the ab‐plane and 7.6±0.3 GPa in the c‐direction. The fracture toughness, K1C, was 2.5±0.1 MPa⋅m0.5 (ab‐plane) and 2.8±0.24 MPa⋅m0.5 (c‐direction). The samples with a higher twin density demonstrated a higher jc, especially in applied magnetic field. The twin density correlates with the sizes and distribution of Y211 grains in Y123. The thin‐walled ceramics that demonstrated the highest jc contained about 22 twins in 1 μm and were practically free f...