T. A. Prikhna

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.

Batch-processed melt-textured YBCO with improved quality for motor and bearing applications

Results on an established batch process preparing melt-textured YBCO of high quality and in large quantities are reported. We used a standard composition Y1.5Ba2Cu3O7?x+1?wt %?CeO2 without further doping to fabricate single domain YBCO monoliths in different sizes and shapes (cylindrical, quadratic) as well as rectangular multi-seeded YBCO monoliths. Up to 2?3?kg of melt-textured YBCO blocks were grown, reproducible in one box furnace run. Top seeding by a self-made SmBCO was improved and rationalized. Optimization of an oxygen annealing treatment led to macro-crack free YBCO monoliths. Each YBCO monolith was characterized by integral levitation force and field mapping. In a single domain, a quadratic monolith with a edge length of 38?mm, a maximum induction of 1.44?T at 77?K and a distance of 0.5?mm was frozen. The reproducibility of the batch process is guaranteed. Mean maximum induction from 1.1 to 1.2?T at 77?K per batch was reached. A trapped magnetic field of 2.5?T was achieved between two single domain monoliths in a gap of 1.5?mm at 77?K. Depending on the application, function elements with different sizes, designs and more or less complex geometry are constructed in several working steps by cutting, machining, bonding and passivation. Selected function elements were checked with field mapping at 77?K. The results of our function elements in HTSC reluctance motors with an output power of up to 200?kW using single domain material are shown. We report on a fly wheel system DYNASTORE and a system to levitate people.

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.

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).

Mechanical properties of materials based on MAX phases of the Ti-Al-C system

By studies of materials based on the Ti3AlC2 MAX phase containing inclusions of titanium carbide it has been shown that as a titanium carbide content increases from 2 to 99%, the nanohardness and Young modulus of the material increase from 2.0 ± 0.4 to 23.6 ± 1.2 GPa and from 137 ± 21 to 447 ± 11 GPa, respectively. The exponent in the equation of creep for these samples has been found to vary from 104 to 140, which indicates that mechanical properties of the material and, hence, of the Ti3AlC2 MAX phase depend on the strain rate only slightly. The formation of broad hysteresis loops has been observed in the cyclic loading/unloading of the indenter for samples consisting mainly of the Ti3AlC2 MAX phase. This points to serious losses in elastic energy of the MAX phase in strain cycling and, hence, the prospects of the MAX phase application as a damping material. It has been found that the microhardness of a sample consisting of 98% Ti3AlC2 produced by sintering under a load of 4.9 N was 2.1 GPa and its fracture toughness was high (no cracks from the indent corners were observed even under a load of 149 N). Microhardness and fracture toughness of the material consisting of 71% Ti3AlC, 6% Ti2AlC, and 23% TiC were 3.0 ± 0.6 GPa and 4.3 ± 1.4 MPa·m1/2, respectively.

Polishing of optoelectronic components made of monocrystalline silicon carbide

The authors of this paper have substantiated the appropriateness of application of polishing suspensions of diamond micron powders, boron nitride and Ti3AlC2 MAX-phase powders in the study of the mechanism of polishing monocrystalline silicon carbide. They have proved suitability of using the following parameters as criteria for efficiency of the machining process: the specific value of the energy of transfer, the density of the energy of transfer, and the density of vibrational energy spent for the formation of debris particles. Computer modeling of the polished surface micro- and nanoprofile produced the profile charts from which the surface roughness parameters have been determined and the characteristic relations between the parameters and the most probable debris particle size have been defined.

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.