Superconductivity in multi-phase Mg-B-O compounds

Structures of MgB2-based materials manufactured under pressure (up to 2 GPa) by different methods having high superconducting performance and connectivity are multiphase and contain different Mg-B-O compounds. Some oxygen can be incorporated into MgB2 and boron into MgO structures, MgBx (X=4-20) inclusions contain practically no oxygen. Regulating manufacturing temperature, pressure, introducing additions one can influence oxygen and boron distribution in the materials and thus, affect the formation, amount and sizes of Mg-B-O and MgBx inclusions and changing type of pinning, pinning force and so affect critical current density jc. The boron concentration increase in initial Mg and B mixture allows obtaining sample containing 88.5 wt% of MgB12 with Tc of 37.4 K (estimated magnetically)

Presence of Oxygen in Ti-Al-C MAX Phases-Based Materials and their Stability in Oxidizing Environment at Elevated Temperatures

The Ti₃AlC₂-, (Ti,Nb)₃AlC₂- and Ti₂AlC-based materials turned out to be more resistant than Crofer JDA steel in oxidizing atmosphere as 1000 h long tests at 600°C have shown. But the amounts of oxygen absorbed by the 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-based material. Scanning electron microscopy and the Auger study witnessed that amounts of oxygen in the MAX phases before the exposure in air were as well different: the approximate stoichiometries of the matrix phases of materials were Ti_{3.1-3.2}AlC_{2-2.2}, Ti_{1.9-4}Nb_{0.06-0.1}AlC_{1.6-2.2}O_{0.1-1.2} and Ti_{2.3-3.6}AlC_{1-1.9}O_{0.2-0.6}, respectively. The higher amount of oxygen present in the MAX phase structures may be the reason for higher resistance to oxidation during long-term heating in air at elevated temperature. The studied materials demonstrated high stabilities in hydrogen atmosphere as well. The bending strength of the Ti₃AlC₂- and (Ti,Nb)₃AlC₂-based materials after keeping at 600°C in air and hydrogen increased by 10-15%, but the highest absolute value of bending strength before and after being kept in air and hydrogen demonstrated the Ti₂AlC-based material (about 590 MPa)

Synthesis Pressure–Temperature Effect on Pinning in MgB2-Based Superconductors

International audienceThe 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 MgB2 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 % MgB12, 2.5 wt % MgB2, 9 wt % MgO or 53 wt % MgB12, 31 wt % MgB20 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 2–5×102 A/cm2, they were found to be insulating on the macroscopic level

Presence of Oxygen in Ti-Al-C MAX Phases-Based Materials and their Stability in Oxidizing Environment at Elevated Temperatures

International audienceThe 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)

Synthesis Pressure–Temperature Effect on Pinning in MgB2-Based Superconductors

International audienc

Superconductivity in multi-phase Mg-B-O compounds

Structures of MgB2-based materials manufactured under pressure (up to 2 GPa) by different methods having high superconducting performance and connectivity are multiphase and contain different Mg-B-O compounds. Some oxygen can be incorporated into MgB2 and boron into MgO structures, MgBx (X=4-20) inclusions contain practically no oxygen. Regulating manufacturing temperature, pressure, introducing additions one can influence oxygen and boron distribution in the materials and thus, affect the formation, amount and sizes of Mg-B-O and MgBx inclusions and changing type of pinning, pinning force and so affect critical current density jc. The boron concentration increase in initial Mg and B mixture allows obtaining sample containing 88.5 wt% of MgB12 with Tc of 37.4 K (estimated magnetically)

Pinning in MgB2\hbox{MgB}_{2}- and YBaCuO-Based Superconductors: Effect of Manufacturing Pressure and Temperature

International audienceBulk MgB 2 - and YBaCuO-based materials are competitive candidates for applications. The properties of both compounds can be significantly improved by high temperature-high pressure preparation methods. The transformation of grain boundary pinning to point pinning in MgB 2 -based materials with increasing manufacturing temperature from 800 to 1050 ° C under pressures from 0.1 MPa to 2 GPa correlates well with an increase in critical current density in low and intermediate magnetic fields and with the redistribution of boron and oxygen in the material structure. As the manufacturing temperature increases (to 2 GPa), the discontinuous oxygen-enriched layers transform into distinct Mg-B-O inclusions, and the size and amount of inclusions of higher borides MgB X (X>;2) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC addition. The oxygenation of melt-textured YBa 2 Cu 3 O 7 - δ (MT-YBaCuO) under oxygen pressure (16 MPa) allows one to increase the oxygenation temperature from 440°C to 700-800°C, which leads to an increase of the twin density in the Y123 matrix and to a decrease of dislocations, stacking faults, and the density of microcracks, and as a result, to an increase of the critical current density, J c , and the trapped magnetic field. In MT-YBaCuO, practically free form dislocations and stacking faults and with a twin density of 22-35 μm -1 , J c of 100 kA/cm 2 (at 77 K, 0 T) has been achieved, and the importance of twins in Y123 for pinning was demonstrated experimentally

Pinning in MgB2\hbox{MgB}_{2}- and YBaCuO-Based Superconductors: Effect of Manufacturing Pressure and Temperature

International audienc