Papers by Keyword: Superconductor

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: 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: This work is devoted to development of synthesis methods of nano- and macrofunctional materials for industry, electronics, and modern science and technology. The authors have developed advanced methods of obtaining various carbon, silicate and chalcogenide materials by melt and hydrothermal methods with using of optimized chemical and thermodynamic parameters of synthesis. As a result a new super hard material - carbonate-synthetic diamond polycrystals were obtained. Nanodiamonds were synthesized by direct conversion of graphite into diamond. New species of macro- and fine crystalline piezoelectric tourmaline containing 3d-elements were produced by hydrothermal method. Using a flux technique the single crystals of different chalcogenide phase with unique superconducting properties were grown.

Abstract: The samples with nominal composition of Bi_1.6Pb_0.4Sr₂Ca _2-xDy_x Cu₃O_y where x = 0.000, 0.025, 0.050, 0.100 and 0.200 were prepared by the co-precipitation (COP) method. The samples were characterized by x-ray diffraction, electrical resistivity measurement and critical current density. The critical current density (J_C) and superconductivity transition temperature (T_C) of Dy substituted were found to be lower than the Dy-free sample. The T_C values vary between 100 K and 75 K toward Dy concentration due to a small change of carrier concentration. The highest TC in Dy-doped sample was found at 96 K in x = 0.025. The J_C decreased towards Dy substitution, and it was measured to be 5751.2 mA/cm2 in Dy-free and 3769.8 mA/cm² in x = 0.025 at 77 K. XRD analysis showed the substitutions of Dy reduced the volume fraction of the 2223 phase and increased the volume fraction of the 2212 phase. The proportion of Bi-2223/Bi-2212 (%) were estimated from 76.74/23.26 in Dy free to 18.90/81.10 in x = 0.200.

Abstract: Superconductor samples Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) are synthesized from nano SiC, Mg and amorphous boron powders by microwave direct synthesis in a short time. Powder X-ray diffraction (XRD) analysis indicates that the phases of the synthesis sample are MgB₂ (major phase) and a small amount of MgO and Mg₂Si. The main peaks of MgB₂, (100), (101), (002) and (110) are shift to the higher diffraction angle position and the width of half height of the diffraction plane is broaden for the SiC doping Mg(B_1-2x(SiC)_x)₂, which show that the B positions of MgB₂ are partly substituted and the grains of MgB₂ are fine. Scanning electron microscope (SEM) observation shows that the MgB₂ grain size is very small and the sample is tightness (compact). The onset superconducting transition temperature of the Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) samples measured by magnetization measurement are about 37.6 K, 37.0 K, 36.8 K respectively. The critical current density J_c are calculated according to the Bean model from the magnetization hysteresis loop of the slab Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) samples. The critical current density Jc of nano SiC doping Mg(B_1-2x(SiC)_x)₂ samples are greatly enhanced. In higher external magnetic field, the J_c of 10% SiC doped sample is the highest; in lower external magnetic field, the Jc of 5% SiC doped sample is the highest; while in the whole external magnetic field, the J_c of undoped sample is the lowest.

Abstract: The influence of calcium substitution at the barium site of porous Y(Ba1-xCax)2Cu3Oδ (x= 0.00, 0.10, 0.20 and 0.30) samples prepared via solid-state reaction method have been investigated. The structure, morphology, critical temperature and critical current were determined by x-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and four-point probe method, respectively. Generally, the curves of normalized resistance for all samples displayed normal metallic behavior above Tc onset. The Tc zero was between 84 K and 71 K with increase in Ca concentration corresponding to a small change of carrier concentration. The critical current density, Jc decreases with increase in Ca concentration. The highest Jc of 2.657 A/cm2 at 50 K was obtained in Ca-free porous YBCO which is higher than that of Ca-free non porous YBCO. Further substitution of Ca at Ba site decreased Jc monotonously. The increase of Ca concentration decreased the volume of unit cell but the crystallographic structure remains in the orthorhombic form where a≠b≠c. The grains are highly compacted and randomly distributed while the grain size decreased as the Ca concentration increased.

Abstract: The effect of Ca substitution in Y1-xCaxBa2Cu3O7 superconductor prepared via co-precipitation method has been investigated. The concentration of Ca substitution was varied from x = 0.05 to x = 0.20. The samples were characterized using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (SEM), resistivity measurement and current density measurement (without magnetic field). The critical temperature of the samples decreased as the concentration of Ca be increased whereby the highest value of critical temperature (Tc zero) was 83 K at x = 0.05. The crystallographic structure of pure sample was evidenced to be orthorhombic form where a ≠ b ≠ c.. At x = 0.10, the microstructure showed an improvement in grain alignment compared to other Ca concentration. The critical current density (Jc) increased with Ca concentration. The value of Jc for x = 0.05 at 50 K and 60 K was 1.51 A/cm2 and 1.00 A/cm2, respectively.

Abstract: The effect of Ca substitution by Na, Mg and Yb on the structural and transport properties of Bi1.6Pb0.4Sr2Ca2-xMxCu3Oy (M = Na, Mg and Yb) (x = 0.0 and 0.2) superconducting samples have been investigated. The samples were prepared by the coprecipitation (COP) method. The samples were characterized by x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), electrical resistivity measurement and critical current density. The critical current density (Jc) and transition temperature (Tc zero) of Na, Mg and Yb substituted with x = 0.2 were found to be lower than the pure sample. Tc zero varies between 100 K and 63 K. Mg concentration was found to give the highest Tc zero of 93 K. Tc zero gradually decreased from Mg, Yb to Na corresponding to a small change in the carrier concentration. Jc decreased with Mg, Yb and Na substitution, and it was measured to be 7.4611 A/cm2, 0.0667 A/cm2, 1.4579 A/cm2 and 1.2479 A/cm2 for pure, Na, Mg and Yb substitution, respectively at 60 K. XRD analysis showed that the decrease of the volume fraction for the 2223 phase and increase of the volume fraction for the 2212 phase with substitution of Na, Mg and Yb. The proportion of Bi-2223/Bi-2212 (%) was estimated from 78.13/21.87 for pure to 51.71/48.29 for Na substitution.

Abstract: MgB₂ has the higher critical temperature of superconducting transition (T_C : 39K) among the intermetallic compound superconductive materials, however, MgB₂ is hard for practical use because of its unworkable and lower critical current density (J_C) in a high magnetic field than Nb-based superconductive materials. We have developed the original method of three-dimensional penetration casting (3DPC) to fabricate the MgB₂/Al composite materials. In the composite material we made, MgB₂ particles dispersed to the matrix uniformly. Thus, these composite materials can be processed by machining, extrusion and rolling. The T_C was determined by electrical resistivity and magnetization to be about 37～39K. In this work, we made composite material with ground MgB₂ particle with the purpose of extruding thinner wires of composite material, successfully produced φ1mm wire and changed the matrix from pure Al to Al-In alloy. J_C of composite materials with the matrix of Al-In alloy was calculated from the width of the magnetic hysteresis based on the extended Bean model. The result was better than that of MgB₂/Al composite material without Indium. Microstructures of these samples had been confirmed by SEM observation.

Abstract: MgB₂ has the higher critical temperature of superconducting transition (T_C : 39K) among the intermetallic compound superconductive materials, however, MgB₂ is hard for practical use because of its unworkable and lower critical current density (J_C) in a high magnetic field than Nb-based superconductive materials. We have developed the original method of three-dimensional penetration casting (3DPC) to fabricate the MgB₂/Al composite materials. In the composite material we made, MgB₂ particles dispersed to the matrix uniformly. Thus, these composite materials can be processed by machining, extrusion and rolling. The T_C was determined by electrical resistivity and magnetization to be about 37～39K. In this work, we made composite material with ground MgB₂ particle with the purpose of extruding thinner wires of composite material, successfully produced φ1mm wire and changed the matrix from pure Al to Al-In alloy. J_C of composite materials with the matrix of Al-In alloy was calculated from the width of the magnetic hysteresis based on the extended Bean model. The result was better than that of MgB₂/Al composite material without Indium. Microstructures of these samples had been confirmed by SEM observation.