Q.R. Feng

Preparation of superconducting HgBa2CaCu2Ox films with a zero-resistance transition temperature of 121 K

Abstract Hg-1212 films were prepared by means of annealing of mercury-free precursor films with bulk Hg-1212 pellets in an evacuated quartz tube. The mercury free precursor films were deposited by laser ablation on (100) SrTiO 3 substrates. The annealed films exhibited the (00 n ) oriented X-ray diffraction pattern of Hg-1212. The superconducting transition temperature, T c ( ϱ = 0), was 121 K.

Superconductivity at 77K in (Pb, Cu)Sr2(Ca, Y)Cu2O7 by post-annealing

Abstract The Pb-based layered copper oxide of (Pb 0.7 Cu 0.3 )Sr 2 (Y 0.6 Ca 0.4 )Cu 2 O z was prepared by solid state reaction. X-ray diffraction analysis showed that the sample had a structure isomorphous to the Tl-1212 phase. By a post-annealing, without any high pressure treatment, the sample showed superconductivity at 77K, which is higher than the reported values to our knowledge. Our experiments not only simplify the process for preparing Pb-based superconductors, but also give a quite new view for this superconductor.

Determination of flux pinning potentials from field-induced anisotropy in oxide superconductors

Abstract The angular dependence of magnetization of YBa2Cu3O7 and YBa2Cu2.85 Ag0.15O7 was measured at 77K by rotating the sample with respect to the cooling field. The distribution of the depth of pinning potential wells for flux lines can be determined. It is unsymmetrical and can be well described by the Rayleigh distribution. The average depth for a single pinning potential well is about 0.2 eV for YBa2Cu3O7 at 77K.

Change in crystal structure related with two different blocks in Bi1.7Pb0.3Sr2Ca2Cu3Ox

Abstract Some structural changes are observed in the sample of Bi 1.7 Pb 0.3 Sr 2 Ca 2 Cu 3 O x . Temperature dependebce x-ray diffraction demonstrates that there are two lattice softness in low temperature range, one at about 220 K, another at about 120 K, and two lattive expansions at high temperature range, one at 300 °C and another at 750 °C. Thermal analyses support the results of x-ray diffractio. we suggest that these changes in crytal structure related with the two different blocks in the titled compound.

Preparation of superconducting HgBa2CaCu2Ox and Hg0.8Bi0.2Ba2CaCu2Ox films by means of annealing of mercury-free precursor films

Abstract HgBa2CaCu2Ox films and Hg0.8Bi0.2Ba2CaCu2Ox films were prepared by means of annealing of mercury-free precursor films with the Hg-1212 bulks in an evacuated quartz tube. The mercury-free precursor films were deposited by laser ablation on (100) SrTiO3 substrates. The annealed films exhibited the (00n) oriented X-ray diffraction pattern of the Hg-1212. The superconducting transition temperatures, Tc(ϱ=0), are 121 K and 100 K respectively.

CARRIER CHARACTER OF BI CONTAINING 2223-PHASE DOPED BY MO AND W

Abstract Single-phase samples of the Mo, W doped BiSrCaCuO 2223 phase were synthesized. No impure phases appeared in the X-ray diffraction patterns. The samples, as-prepared and after annealing in O 2 and N 2 atmospheres, were characterized by using thermal analysis, Hall effect and R-T relationship measurement, as well as the X-ray diffraction technique. The experiments showed that there is a close relationship between T c and carrier concentration in these doped samples. To our knowledge, this relationship is obtained for the first time. The effects of Mo and W on the superconductivity were discussed. We suggest that the effect of W and Mo is to change the overall content of oxygen in the samples, hence influencing the superconductivity. The metallic ions in the Bi sites do not directly dominate the superconductivity.

The Effect of Interstitial Ag-Doping on Flux Pinning in the YBa2Cu3O7 Superconductor

The angular dependence of magnetization of three interstitial Ag-doped YBa2Cu3O7 was measured at 77 K by rotating the sample with respect to the cooling field. It was found that Ag-doping strengthens the flux pinning and the energy of a pinning centre is about 0.3 eV. The couplings between the superconducting grains were also notably improved.

A tetragonal to orthorhombic phase transition in Bi2Sr2CaCu2Ox

Abstract Bi 2 Sr 2 CaCu 2 O x superconductor was studied in situ by high temperature X-ray diffraction, differential scanning calorimetry (DSC), and thermogravimetry (TG). It is discovered that there is a tetragonal to orthorhombic phase transition at about 600°C in the sample. Careful high temperature X-ray diffraction and thermal analysis reveal that the phase transition in a sample with a higher value of T c (89 K) is very obvious, but in a sample with a lower value of T c (65 K) is hardly observed. By comparing this phase transition and the orthorhombic to tetragonal transition in YBa 2 Cu 3 O y system, some difference and similarity among Bi-system and Y-system superconductors are obtained.

An unknown phase transition in and cuprates

By means of differential scanning calorimetry, thermogravimetry, and high-temperature x-ray diffraction in different atmospheres, an opposite thermal dynamic change in superconducting and semiconducting cuprates of (Y-123) and (Pr-123) is observed. At about the superconducting Y-123 polycrystals show an endothermal anomaly and a weight loss, while the semiconducting Pr-123 polycrystals show an exothermal anomaly and weight gain. Similar phenomena are also observed in the superconducting and semiconducting Y-123 single crystals at about . High-temperature x-ray diffraction reveals that the lattice parameters of the Y-123 increase significantly at this temperature. We argue that this can be attributed to an unknown phase transition and a thermal instability, which is important for understanding the difference in properties between superconducting and semiconducting cuprates.

Quality of Inward Curved MgB 2 Films Prepared by HPCVD

Arising from the superconductivity at ~39 K, magnesium diboride (MgB2) films have great prospect in the field of superconducting radio-frequency (SRF) cavities. To match the geometrical shape of the inner surface of the SRF cavities, the deposition techniques for inward curved MgB2 superconducting films are explored. By using pre-bent metallic substrates, curved MgB2 films were deposited in situ. These curved films show properties comparable with those of the films deposited on a plane surface. Even when the curvature radius (CR) of the films is as small as ~2 mm, the superconducting transition temperature TC(R = 0) of the films reaches ~36 K. The in situ-grown films were also compared with the films prepared by postdeposition bending. In the bending of CR ~ 2 mm, TC(R = 0) of the films falls off to 25 K, much lower than that of the in situ-grown films with the same CR. The scanning results of electron microscope indicate that the wrinkles and cracks appearing in the bending are the main reason for the degradation of transition temperature.