Xinsheng Yang

Low-field magnetoresistance in La0.7Sr0.3MnO3/Ta2O5 composites

Two-phase composites La0.7Sr0.3MnO3/Ta2O5 are synthesized and their magnetization and low-field magnetoresistance (LFMR) characteristics are investigated. It is found that a small amount of Ta ions enter into LSMO grains near the grain surface region, resulting in the reduction of the cell volume, and consequently, an extra reduction of the saturate magnetization. The paramagnetism-to-ferromagnetism transition temperature, TC, does not show significant change. The LFMR enhancement is observed over a wide range of temperature from 50 to 350?K when the second phase material (Ta2O5) is introduced. The magnetic disorder caused by the secondary phase at grain boundaries and the Ta doping effect on the surface of LSMO grains are believed to contribute to the enhanced MR.

Design and Development of a Cryogen-Free Superconducting Prototype Generator With YBCO Field Windings

A prototype machine has been developed to study large-capacity wind turbine generators using high-temperature superconducting technology. The superconducting field windings, using 2G YBCO wires, are used in the rotor, whereas the conventional technology is employed in the stator. The racetrack superconducting magnets are cooled directly by two cryo-refrigerators and the operating temperature is 30 K. The cold heads of the refrigerators are installed on the rotor, which is driven by a drive motor, rotating synchronously with a vacuum chamber. A rotary gas seal is employed as the gas slip ring connecting the rotary cold head to a stationary compressor. This paper also presents the electromagnetic characteristics of the prototype machine and the feasibility of the cooling system. Experimental results of back electromotive force are shown at field current from 10 to 100 A and rotating speed from 10 to 30 r/min. The electromagnetic design, based on the finite-element method (FEM), is available in this paper, and the FEM data are coincident with the experimental results very well.

Composite varistors based on epoxy resin/La0.8Sr0.2MnO3

Polymer/perovskite manganese oxide (epoxy resin/La0.8Sr0.2MnO3) composites are prepared using bonded method. There is no reaction between La0.8Sr0.2MnO3 and the polymer. The nonlinear current–voltage property is significantly affected by the content of polymer. The resistivity and nonlinear coefficient increase with the increase of polymer content. The resistivity of the composites is 5–9 orders of magnitude higher than that of the sintering ceramics. The nonlinear electrical behavior is an intrinsic property. Compared with conventional sintered ceramic varistors, the polymer/manganese oxide composite varistors have greater nonlinear coefficient above 45.

Electrical and magnetic transport properties of Co-doped Bi2Se3 topological insulator crystals

Transition metal doped topological insulators (TI) CoxBi2-xSe3 were prepared and the phase structure, electrical and magnetic transport properties, Hall mobility and Hall resistivity were studied. The lattice constant c decreased with the increasing Co concentration. All samples exhibited weak metallic resistivity. With increasing Co concentration, the value of ρxx increased, reaching a maximum at x = 0.02, and then decreased. The resistivity data could be fitted by different formulas below and above 30 K in all samples result from different mechanism corresponding to different temperature regions. The magnetoresistance (MR) of Bi2Se3 TI showed a change dependence of concentration. The MR was enhanced by small quantity of Co doping, attaining a maximum value in Bi1.98Co0.02Se3 crystal; but MR was suppressed in high-doped sample (x > 0.02). Those results were ascribed to the rivalry between the phonon scattering effect and the magnetic ordering in single crystals caused by Co ions. For low-doped samples doped, the contribution of phonon scattering was much higher than that of magnetic ordering, leading to larger MR value. However, in samples doped with higher Co concentrations, the magnetic ordering effect became dominant and caused the MR decreases. The MR increased with decreasing temperature and increasing magnetic field. The Hall mobility data indicated that incorporating Co in Bi2Se3 single crystal can reduce native defect of Bi2Se3.

Influence of chemical doping on flux pinning of GdBa2Cu3O7−z films

ABSTRACT Epitaxial GdBCO films with Cobalt (Co) and BaZrO3 (BZO) doping were prepared on (00l) LaAlO3 substrate by a self-developed fluorine-free polymer-assisted metal-organic deposition (FF-PA-MOD) approach. The effect of different doping ways on structure and properties of GdBCO films was investigated. Our results show that in-field critical current density (Jc) values of doped films have been distinctly improved compared to the pure film. It is noted that Co-doped film has much larger in-field Jc values than BZO-doped film, especially at higher temperatures. Furthermore, Co-doped GdBCO film shows significantly enhanced surface smoothness and grain connectivity. It is suggested that obviously enhanced Jc–B properties of the film may be attributed to improved surface morphologies and effective flux pinning centres by dilute Co doping. This study may offer another simple and efficient way to obtain the GdBCO-based coated conductors with high current densities.

Effect of Winding Tension, Support Material, and Epoxy Impregnation on the Strain and Critical Current of YBCO Coil

The critical current (Ic) of epoxy impregnated YBCO coil is prone to degrade due to the cumulative radial tensile stress. Winding tension has the advantage to reduce the amount of epoxy resin and decrease the radial tensile stress during cool down process. In this paper, the dry and wet coils wound on copper and polytetrafluoroethylene (PTFE, Teflon) bobbin materials with various winding tensions were manufactured and tested. With increasing the winding tension, the winding strain increased but the thermal strain reduced. The low coefficient of thermal expansion for bobbin and epoxy impregnation could decrease thermal contraction of the innermost layer of coil. The winding tension effectively reduced the degradation of epoxy impregnated YBCO coil. The bobbin materials and winding tension of 10–50 N had no effect on the degradation of Ic. A proper winding tension of 20 N was successfully used to fabricate an epoxy impregnated YBCO coil of 30 turns without degradation

Effect of the Bending Strain on the Inhomogeneity of Critical Current for YBCO Coated Conductors

The inhomogeneity of critical current in long superconducting tape is one of the factors affecting the performance of the superconducting magnet. We investigate the effect of the bending strain on the inhomogeneity of critical current for the YBCO-coated conductor by a Hall sensor array system. Two kinds of the commercial YBCO-coated conductor tapes, which are prepared by rolling-assisted biaxially textured substrate (RABiTS) and ion-beam-assisted deposition (IBAD), are used for measurement. The bending strain is applied to the coated conductor tapes, and the distribution of the perpendicular component of the remanent field in the direction along the tape length after bending is measured by a Hall sensor array. The local critical current of the tape is calculated through the method of calibration and the distribution of the critical current in the long tape is described by the Weibull function. The result shows that the inhomogeneity of critical current in the long tape is significantly increased when bending strain exceeds an irreversible strain. The change of the superconducting layer structure is believed to be the cause of the critical current inhomogeneity growth.

Delamination Properties of YBCO Tapes Under Shear Stress Along the Width Direction

In YBCO coils, the increased transport current density and magnetic field will induce larger electromagnetic force, and YBCO coated conductors (CCs) may suffer interlayer delamination by shear forces, which may give rise to performance degradation of the conductor. The delamination characteristics of YBCO CCs under shear stress should be well understood for better application of YBCO tapes to high-temperature superconducting equipment. This paper describes the experimental results for different delamination strength and critical current Ic degradation behaviors of the YBCO CCs under shear stress at the unslit edge, center, and slit edge of the conductor. The results show that the average delaminated shear strengths of 4.9, 5.9, and 3.7 MPa are recorded for the unslit edge, center, and slit edge at room temperature, whereas 6.8, 7.2, and 5.1 MPa are recorded for the unslit edge, center, and slit edge at liquid nitrogen. The different degradation behaviors of Ic under shear stress are related to the location where delamination occurs in the sample. According to the finite-element method analysis for the stress distribution of the conductor under shear stress, stress concentration leads to initial delamination of the conductor. The decrease of moment arm is suggested to reduce the effect of moment on the delamination test.

ELECTRICAL TRANSPORT AND LOW-FIELD MAGNETORESISTANCE IN La0.8Sr0.2MnO3/Zn COMPOSITES

La0.8Sr0.2MnO3 (LSMO)/Zn composites were prepared by bonded method. The phase structure, electrical transport and low-field magnetoresistance (LFMR) characteristics have been investigated. The binder segregated mostly at the grain boundaries. The bonded composites exhibited enhanced LFMR, which was attributed to the enhanced intergrain spin-polarized tunneling behavior.