Materials Science Forum Vols. 745-746

Abstract: t is important to simulate the levitation force and its relaxation with time for the designers in the process of designing a HTS (high temperature superconductor)-magnet levitation bearing. In this paper, FEM (finite element method) based on HTSs critical-state model using H formulation was introduced. In the zero field condition, the process of magnetization of a cylindrical HTS bulk was simulated. The result showed that, current would be induced when HTS bulk was put in the external magnetic field, and the induced current tried to prevent the change of the magnetic field in the HTS bulk. When the external field disappeared, HTS bulk trapped some of the field and the bulk was magnetized. HTS-magnet levitation models were built in both the axisymmetric field and the plane field. Forces and their relaxations with time were calculated. In the HTS-magnet levitation system, force faded with time and faded slower as time went on. The force was bigger when the HTS bulk was put closer to the magnet and relaxed a little more.

Abstract: Effect of sintering conditions in the first heat treatment on the phase and microstructure evolution of Bi-2223/Ag/Ni composite-sheathed tapes fabricated by powder-in-tube method was studied. Samples were sintered at different temperatures for different time in an atmosphere of 14.5% O₂. The results showed that this higher O₂ atmosphere improved the content of Bi-2223 phase. XRD and SEM results showed that Bi-2223 content increased with the extension of the dwelling time. Meanwhile, the sintering temperature had significant influence on the Bi-2223 content and secondary phase dimensions as well. With the increase of the sintering temperature, the Bi-2223 content reached to a maximum at 834. With further increase of sintering temperature, the Bi-2223 phase was decomposed to Bi-2212 and others secondary phases.

Abstract: Using a low-fluorine solution, YBa₂Cu₃O_7-x (YBCO) superconducting films were prepared on LaAlO₃ (LAO) substrates by dip-coating method. YBCO films were fired at different temperatures with same oxygen pressure of 1.3 vol% and water vapor pressure of 7.4 vol%. Effect of firing temperature on film microstructure and superconductivity was investigated. The results indicated that YBCO films with high-degree c-axis orientation (the degree of c-axis orientation reached 96%) can be obtained when heat treated at 800 °C. At this temperature YBCO films were grown on LAO with cubic-on-cubic mode, resulting in a high critical transition temperature (T_c) of 91.5 K, and critical current density (J_c) of lager than 1 MA/cm². However, when fired at lower temperatures, YBCO films trended to form a-axis grains, which degraded J_c of films. And a higher heat treatment temperature also depressed superconductivity of YBCO films.

Abstract: Ultrathin MgB₂ film is essential for the fabrication of MgB₂ superconducting single photon detectors (SSPDs). In this paper, we prepared 20nm and 10nm MgB₂ film using ex-situ annealing of Mg-B multilayer method. The precursor films were prepared by electron beam evaporation. A flowing Mg vapour and H₂ was introduced in the annealing process to keep MgB₂ thin film thermodynamically stable. The annealing temperature was between 680 and 740 and annealing time was 1-10min. 20nm MgB₂ films on MgO(111) substrates had the critical temperature (T_c) of 32.2K. The films grew along c-axes direction. As the substrate changed to SiC(001) and Al₂O₃(001), T_c decreased to 30.3K and 10.2K respectively. For 10nm MgB₂ film on SiC(001) substrate, T_c was 24.2K. The self-field critical current density for 10nm and 20nm film on SiC(001) substrate was 2.1×10⁶A/cm² and 2.3×10⁶A/cm², respectively. AFM image showed that the film had a flat surface with mean roughness of 0.899nm for 10nm MgB₂ film.

Abstract: A novel method to fabricate Magnesium diboride ( MgB₂) film by electron beam annealing was presented. The MgB₂ thin films could be prepared in a second or sub-second without any toxic diborane gas, extra Mg vapor or argon gas. The method has the advantages of short formation time and high efficiency. Based on this, the relationship between MgB₂ conductivity and electron beam annealing duration was investigated experimentally with an accelerating voltage of 40 × 10³ V, a beam current of 3×10³ A and different annealing durations of 0.1796s, 1860s, 0.2108s, 0.2200s and 0.2332s. The experimental results showed the MgB₂ film with 0.2200s has the highest T_c^onset , the most dense structure and the strongest diamagnetic signal. Its zero-field Jc at 5 K was 5.0 × 10⁶ A/cm². Also the variation of the film superconducting properties with the annealing duration was found, it will provide an important reference for the preparation of high-quality MgB₂ thin films.

Abstract: Epitaxial and textured EuBa₂Cu₃O_7-x(EuBCO) superconductive films were prepared via a F-free polymer-assisted chemical solution deposition (PACSD) method on LaAlO₃ (LAO) single crystal substrate in nitrogen atmosphere. Because of the fluorine-free metal organic system, no harmful gas was produced during processing by the PA-CSD method. The film morphorlogy, phase composition, texture and the superconducting critical temperature (Tc) of EuBCO films were investigated by x-ray diffraction (XRD),φ-scan, ω-scan rocking curves, an environmental scanning electron microscope (ESEM) and four-probe method. The J_c value of the EuBCO film in self-field at 77K was measured by using J_c-scan Leipzig system. The results shoed that a denser microstructure and better biaxial texture EuBCO film after a high temperature treatment with a superconducting critical temperature (Tc) of 91K was obtained . The critical current density (Jc) reached 2.4MA/cm² at 77K in the self-field.

Abstract: In the present work, a fast pyrolysis process is developed for YBa₂Cu₃O_7-δ (YBCO) films preparation with trifluroacetates metalorganic deposition technique (TFA-MOD). The decomposition of TFA salts as well as the surface morphology of pyrolyzed films is investigated with respect to the pyrolysis temperature and its heating rate. Two typical surface morphologies for the pyrolyzed films, being smooth or buckled, are revealed subjecting to the heating rate and film thickness, which related to the subsequent YBCO films property. Smooth and wrinkle-free pyrolyzed films are achieved by using this fast pyrolysis process with a heating rate as high as 5°C/min. The final crystallized YBCO films exhibit good performances with the superconducting transition temperature of 91 K and critical current density of 2.0 MA/cm² at 77 K, suggesting a rapid, promising MOD route available for scale-up preparation of YBCO coated conductors.

Abstract: Template-directed electrosynthesis has been employed widely to prepare solids of defined dimension. It offers controllable routes to create nanostructures. In this study, one electrochemical method to fabricate one-dimensional metal oxide nanostructures was developed. The electrochemistry strategy was employed to manipulate the pH value within the pores of a template and the growth of continuous one-dimensional metal oxide nanostructures was controlled. The strategy was exemplified by the growth of tin oxide nanotubes. At room temperature, the reduction of nitrate within pores was employed to electrogenerate hydroxide ions and drive local precipitation of stannic oxide nanotubes. The nanotube walls displayed nearly uniform thickness along their entire length which were obtained within commercial track-etched polycarbonate membranes. One-dimensional Zn/SnO₂ core-shell nanostructures were achieved by the further electrodeposition of Zn within SnO₂ nanotubes.

Abstract: Al₈₆Ni₇Y_4.5Co₁La_1.5 (at.%) alloy powder was produced by argon gas atomization process. After high-energy ball milling, the powder was consolidated and extruded by using vacuum hot press sintering under different process conditions, sintering temperature, extrusion pressure, sintering time, etc.. The microstructure and morphology of the powder and consolidated bulk alloy were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The phase transformation of the powder was investigated by differential scanning calorimetry (DSC). Mechanical properties of the consolidated bulk alloy were examined. The results showed that as the milling time increase, the volume fraction of amorphous materials and the hardness and yield strength of the bulk alloy were obvious improved.

Abstract: Nanocrystalline (NC) Al alloy powder was fabricated by milling 2024 Al alloy powder and Fe-based metallic glass (FMG) particles. The NC Al alloy powder was consolidated into bulk sample by adding a part of atomized coarse-grained (CG) 2024 alloy powder. The microstructure and mechanical properties of powder and consolidated bulk materials were examined by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and mechanical test. It revealed that the FMG particles were uniformly distributed in the NC aluminum alloy powder. In the consolidation process, the grain size increased, and Al2CuMg phase precipitated. The multi-modal Al alloy by consolidation of FMG particles, NC and CG powder, exhibited higher yield strength up to 517 MPa and better plasticity in comparison to the samples without CG powder.