Qingrong Feng

The process of forming 2223 phase from 2212 phase in Bi(Pb)-Sr-Ca-Cu-O System

The process of forming (2223) phase was studied by x-ray diffraction, thermal analysis, as well as electric and magnetic measurements. It was discovered that the formation of (2223) from (2212) is a phase transition-like process. Some possible mechanisms of (2212) to (2223) phase transition-like are discussed. Our results not only explain the difficulty for getting pure (2223) phase sample, but also give a way to prepare high quality (2223) phase sample.

Phase formation of polycrystalline MgB2 at low temperature using nanometer Mg powder

Abstract The MgB2 superconductor synthesized in a flowing argon atmosphere using nanometer magnesium powder as the raw materials, denoted as Nano-MgB2, has been studied by the technique of in situ high temperature resistance measurement (HT-RT measurement). The MgB2 phase is identified to form within the temperature range of 430–490 °C, which is much lower than that with the MgB2 sample fabricated in the same gas environment using the micron size magnesium powder, denoted as Micro-MgB2 and reported previously. The sample density of the Nano-MgB2 reaches 1.7 g/cm3 with a crystal porosity structure less than a μm, as determined by the scanning electron microscope (SEM) images, while the Micro-MgB2 has a much more porous structure with corresponding density of 1.0 g/cm3. This indicates that the Mg raw particle size, beside the sintering temperature, is a crucial factor for the formation of high density MgB2 sample, even at the temperature much lower than that of the Mg melting, ∼650 °C. The X-ray diffraction (XRD) pattern shows a good MgB2 phase with small amount of MgO and Mg and the transition temperature, TC, of the Nano-MgB2 was determined to be 39 K by temperature dependent magnetization measurements (M–T), indicating good superconducting properties.

Ultrathin MgB2 films fabricated on Al2O3 substrate by hybrid physical?chemical vapor deposition with high Tc and Jc

Ultrathin MgB2 superconducting films with a thickness down to 7.5xa0nm are epitaxially grown on (0001) Al2O3 substrate by a hybrid physical–chemical vapor deposition method. The films are phase-pure, oxidation-free and continuous. The 7.5xa0nm thin film shows a Tc(0) of 34xa0K, which is so far the highest Tc(0) reported in MgB2 with the same thickness. The critical current density of ultrathin MgB2 films below 10xa0nm is demonstrated for the first time as Jc ~ 106xa0Axa0cm − 2 for the above 7.5xa0nm sample at 16xa0K. Our results reveal the excellent superconducting properties of ultrathin MgB2 films with thicknesses between 7.5 and 40xa0nm on Al2O3 substrate.

Significant improvements of the high-field properties of carbon-doped MgB2 films by hot-filament-assisted hybrid physical–chemical vapor deposition using methane as the doping source

We report a significant enhancement in upper critical field Hc2 and irreversibility field Hirr in carbon-doped MgB2 films fabricated by hot-filament-assisted hybrid physical–chemical vapor deposition (HFA-HPCVD) using methane as the doping source. For the parallel field, a very large temperature derivative value of 3xa0Txa0K−1 near Tc was achieved in a heavily doped film with Tc near 28xa0K. Carbon doping also enhanced flux pinning, resulting in a much higher critical current density in a magnetic field Jc(H) than in undoped samples. The result suggests a more effective doping of carbon into the MgB2 structure and a better connectivity between the carbon-doped MgB2 grains than in previous reports. A clear correlation between the decrease in grain size and the enhancement of Hc2 was observed.

Thermoelectric power in normal state of superconductor MgB2

Abstract Experimental results of thermoelectric power (TEP) measurements in MgB 2 from 80 to 320xa0K are presented. The TEP is positive and about 7.6xa0μV/K at room temperature. It varies with temperature linearly in temperature regions below 190xa0K and above 240xa0K, but with different slope. The results imply a multiple band contribution to the transport properties.

The effect of ionic radius of metal element (M) on (Pb,M)-1212 superconductors (M= Sr, Ca, Mg, Hg, Cd, Cu)

A (Pb 0.5 Sr 0.5 )Sr 2 (Y 0.5 Ca 0.5 )Cu 2 O y sample was prepared and the obtained T c(onset) and T c(zero) were 109 K and 51 K, respectively. A comparison of the M 2+ ionic radius, lattice constants a and c , and the interatomic distance sum of the Cu-O(2) and (Pb,M)-O(2) samples in the (Pb 0.5 M 0.5 )Sr 2 (Y 0.5 Ca 0.5 )Cu 2 O y system was made, where M = Sr, Ca, Mg, Hg, Cd or Cu. It was found that if the atomic radius of M 2+ , which is in the (Pb,M)-O plane of the (Pb,M)-1212 specimen, is larger, then the sample will have a higher T c(onset) and the major effect of the M 2+ on the T c(onset) of (Pb 0.5 M 0.5 )Sr 2 (Y 0.5 Ca 0.5 )Cu 2 O y samples was to change the interatomic distance (bond distance) sum of Cu-O(2) and (Pb,M)-O(2) when the Cu 2+ ion was exchanged for an M 2+ ion.

In situ resistance measurement of superconducting MgB2 in flowing argon atmosphere

Abstract In situ resistance measurement of MgB2 sample was performed in flowing argon atmosphere. This set of measuring curve was compared with the in situ resistance measurement result of MgB2 sample performed in vacuum. This measurement result presents a resistance decreasing taken as a whole as increasing temperature from room temperature to 800 °C, and it indicated that the Mg powder began to melt at about 530 °C. At this temperature the resistance values of MgB2 sample presented a sharp down, and it is lower than MgB2 sample prepared in vacuum conditions that appeared at 638 °C. This indicated that the phase formation of MgB2 in argon atmosphere was at lower temperature than in vacuum.

A sol–gel method for growing superconducting MgB2 films

In this paper we report a new sol–gel method for the fabrication of MgB2 films. Polycrystalline MgB2 films were prepared by spin-coating a precursor solution of Mg(BH4)2 diethyl ether on (001)Al2O3 substrates followed with annealing in Mg vapor. In comparison with the MgB2 films grown by other techniques, our films show medium qualities including a superconducting transition temperature of TC ~ 37xa0K, a critical current density of JC(5xa0K, 0xa0T) ~ 5 × 106xa0Axa0cm − 2, and a critical field of HC2(0) ~ 19xa0T. Such a sol–gel technique shows potential in the commercial fabrication of practically used MgB2 films as well as MgB2 wires and tapes.

Fabrication of superconducting magnesium diboride thin films by electron beam annealing

The technique of electron beam annealing for fabricating MgB2 thin films has been explored. MgB2 thin films were prepared by e-beam evaporation of M?B precursor films on 6H?SiC (0001) substrates followed by ex?situ electron beam annealing without any extra Mg vapor or argon gas protection. The very short annealing duration, about 1?s, effectively prevented volatilization and oxidation of Mg and decomposition of MgB2. In comparison with the MgB2 thin films grown by other techniques, our films show medium qualities including a superconducting transition temperature of Tconset ~ 35.3?K, a transition width ?Tc ~ 0.2?K, and a critical current density of Jc(5?K, 0?T) ~ 3.2 ? 106?A?cm ? 2. Such an electron beam annealing technique has potential for the fabrication of large-scale MgB2 thin films as well as MgB2 wires and tapes.

Nanoscale disorder in pure and doped MgB2 thin films

MgB2 thin films have superior superconducting properties compared to bulk MgB2 and demonstrate the potential for further improving the performances of MgB2 wires and tapes. Using transmission electron microscopy, we have characterized the microstructure of pure and C-doped MgB2 using various carbon sources grown by hybrid physical?chemical vapor deposition (HPCVD), and cold-grown?annealed film deposited by molecular beam epitaxy (MBE). The MgB2 HPCVD films increase in crystal quality in the order (MeCp)2Mg-sourced films, CH4-sourced films, B(CH3)3-sourced films, pure films, while the Hc2 values of these films follow the opposite order. The cold-grown?annealed MgB2 MBE film contains non-epitaxial ? 10?nm MgB2 grains and MgO nanoparticles. The microstructural origins of electron scattering and flux pinning in both films are discussed. We also show the structure and chemistry of the degraded phase in HPCVD films and its effects on superconducting properties.