Materials Science Forum Vols. 546-549

Abstract: For the high temperature superconducting (HTS) magnetic levitation (Maglev) test vehicle in china, the present NdFeB PMG has the symmetrical magnetic field distribution on the upper and lower surface. However, the vehicle only utilizes its upper magnetic field. 50% PMG magnetic field energy goes to waste, so the onboard HTSC arrays haven’t reached the best levitation performance. In order to make the HTS/PMG maglev system more efficient and reasonable, a new HTS/PMG Maglev design has been proposed based on the present PMG and the Halbach array PMG, whose PMG is called as hybrid PMG. Firstly, three magnetic field distributions of three kinds of PMG are compared using FEM. It is found that the magnetic field distribution of the hybrid PMG is more efficient for the HTSC’s maglev. The concentrating upper surface magnetic field is stronger to improve the load capability of the system. Numerical analysis and experiment are close for the present HTS/PMG system. More calculation shows that the bulk YBaCuO HTSC with the hybrid PMG has significantly better levitation performance than that with the other two PMGs. The usage of the onboard HTSC arrays is improved much and the load capability of the HTS/PMG Maglev vehicle is upgraded with the hybrid PMG.

Abstract: We apply the Lee-Low-Pines-Huybrechts variational method to study the properties of the two and three dimensional bipolaron in a quantum dot. The ground-state (GS) and two types of excited-state energies of the Fröhlich bipolaron for the whole range of electron-phonon coupling constants can be obtained. Compared with the Franck-Condon excited state, the first relaxed excited state has a lower energy. Effects of quantum dot confinement on the excitation energies of the bipolaron are given.

Abstract: Through the review of the correlation on the isotope effects research and superconductivity mechanism research as well as the progress on the study of the isotope effects in the high Tc copper oxide superconductors, we have found the fundamental factors that cause the different changes of the isotope effect value in different dope region in high Tc cupper oxide superconductors. We point the high-temperature superconductivity mechanism possible origin phonons mediation electrons.

Abstract: The Phase formation in the precursory powder, the particle size and the homogenization phase distribution have been investigated in this paper. Size of the powder particle has been reduced by a ball-milling process. The effects of the ball-milling on the characteristic of the powder have been studied by XRD. The modality and distribution of 2212, Ca2CuO3 and CuO in the powder were observed by SEM. Results show that the Ca2CuO3 particle ground was about 200nm in size. The precursory powder with the non-superconducting phases distributed uniformly in the mixture powder was obtained for Bi-2223 long tape.

Abstract: Three kinds of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3O10 ((Bi,Pb)-2223) monofilamentary tapes were fabricated by powder-in-tube (PIT) process under different conditions. The phase evolutions during melting and reformation of (Bi,Pb)-2223 core in them have been investigated by ex-situ X-ray diffraction (XRD) analysis. The short tapes were melted at 855°C, 858°C in 7.5% O2 balanced with argon, and then slowly cooled to 801°C and 804°C at 1.5°C/h, respectively. The results show that the (Bi,Pb)-2223 phase can reform directly from the melt in all three tapes if the appropriate melting temperatures are used. However, the ceramic cores underneath the silver sheath exhibit different melting and recrystalization behaviors, as the differences of phase compositions in the original tapes before melted.

Abstract: The critical current versus tensile strain characteristics of mono- and multi-filament Bi-2223 tapes with Ag and AgMn and AgMg sheath has been studied. The strain tolerance ε0.9 of naked mono- and 37-filament Bi-2223 tapes were 0.13% and 0.20%, respectively. Ag sheath increased the strain tolerance ε0.9 of critical current by 0.08% and 0.16% for mono-filament and 37-filament Bi-2223/Ag tapes. The ε0.9 of Bi-2223 with Ag and AgMn and AgMg sheath increased with filament number. AgMn and AgMg sheath, increasing ε0.9 of Bi-2223 tapes by 0.07-0.09%, are effective to improve strain tolerance of critical current for multifilamentary Bi-2223 tapes.

Abstract: High Tc superconductor (HTS) technology has been used to develop an advanced high Q resonant circuit and its devices. With a HTS, a very high Q circuit can be achieved; consequently special aspects such as high voltage generation and high current control can be theoretically and practically realized. Theoretical study has been carried out, as well as a practical approach has been made for the concept verification. This paper describes the theory of this high Q resonant circuit and the operational principle of its high voltage generation and current control.

Abstract: Minimum propagation current of Bi-2223/Ag superconducting multifilament tape was studied in detail, including experimental investigation and numerical calculation using finite element method (FEM). A set of experimental system has been established, quench propagation velocities with various transport current have been measured and minimum propagation current has been obtained in the condition of N2 gas cooling. The effect of parallel background magnetic field on minimum propagation current has also been studied experimentally. In addition, minimum propagation current at different operating temperature has been gained by the simulated calculation using FEM.It has been found that the high temperature superconductor is stable if only transport current is smaller than minimum propagation current of the time no matter what condition it is in. Minimum propagation current can be considered as an important stability criterion of high temperature superconductor.