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Now showing 1 - 10 of 23
  • (2001) Pradhan, A; Feng, Y; Zhao, Yong; Koshizuka, N; Zhou, L; Zhang, P; Liu, X; Ji, P; Du, J; Liu, Chengfei
    Journal Article
    We report on the transport and magnetization properties of MgB2 wires fabricated by a powder-in-tube ~PIT! technique. Temperature and magnetic-field-dependent resistivity displays a high conductivity and upper critical field Hc2 generally observed in dense samples. The electronic mass anisotropy g'1.360.15 predicts some texturing in the wire. Our data on transition temperature TC , Hc2 , and both magnetic and transport critical current density Jc indicate that MgB2 can be manufactured in a wire form using a PIT technique and required engineering Jc can be achieved on further optimization.

  • (2003) Cheng, CH; Zhao, Yong
    Journal Article
    Grain boundaries (GBs) of YBa2Cu3Oy (YBCO) are preferentially doped with Ag by the solid-state diffusion method. The distribution of Ag is highly localized in the GB. As a result of the Ag doping, the Jc-H behavior of YBCO textured polycrystalline samples is significantly improved, and at 60 K and 7 T, Jc is tripled. It is proposed that Ag could partially replace the Cu in CuO2 planes near GBs, therefore reducing the geometric distortion of Cu-O bonds and making the effective GBs thinner.

  • (2003) Zhao, Yong; Cheng, Cui; Rui, X; Zhang, H; Munroe, Paul; Zeng, H; Koshizuka, N; Murakami, M
    Journal Article
    MgB2-diamond nanocomposite superconductors have been synthesized by addition of nanodiamond powder. Microstructural analysis shows that the nanocomposite superconductor consists of tightly packed MgB2 nanograins (~50-100 nm) with highly dispersed and uniformly distributed diamond nanoparticles (~10-20 nm) inside the grains. The Jc-H and Hiir-T characteristics have been significantly improved in this MgB2-diamond nanocomposite, compared to MgB2 bulk materials prepared by other techniques. Also, the Jc value of 1x104 A/cm2 at 20 K and 4 T and the Hirr value of 6.4 T at 20 K have been achieved.

  • (2002) Zhao, Yong; Feng, Y; Machi, T; Cheng, C; Huang, D; Fudamoto, Y; Koshizuka, N; Murakami, M
    Journal Article
    We studied the Ti doping effect on the superconducting properties and microstructure of sintered MgB2 bulk polycrystals. Using Ti as a sintering assistant, we have fabricated MgB2 bulk materials consisting of fine 10 nm scale particles and achieved a high Jc of 1MA/cm2 in self-field and 3 x 105 A/cm2 in 1T at 20 K. The large enhancement of Jc is explained by the excellent connection between grains and the high density of pinning centers served by both grain boundaries and MgO nanoparticles.

  • (2002) Zhao, Yong; Cheng, C; Machi, T; Koshizuka, N; Murakami, M
    Journal Article
    We investigated the degradation of critical current density (Jc) and irreversibility field (Hirr) of pure and Ti-doped MgB2 bulks by exposing the samples to water for a total of 10, 48, 100, and 124 h. We found that both Jc and Hirr of the samples were degraded by exposure to water, in various degrees depending on the doping level. In the same exposure duration, the Ti-doped samples exhibited a less sensitivity to water, which was confirmed by the less degradation of Jc and Hirr. Our results demonstrate that Ti doping is not only effective in improving the superconducting properties of MgB2 but also in enhancing its chemical stability against water.

  • (2002) Fu, B; Feng, Y; Yan, G; Zhao, Yong; Pradhan, A; Cheng, C; Ji, P; Liu, X; Liu, C; Zhou, L; Yau, K
    Journal Article
    Ti-doped MgB2/Cu tapes with Ta as a buffer layer were prepared through the in situ powder in tube method by using Mg, Ti, and B powders. The phase compositions, microstructure features, and superconducting properties were investigated by x-ray diffraction, scanning electron microscope, and superconducting quantum interference device magnetometer. It is found that TiB2 phase was formed in Ti-doped MgB2 tape. Magnetization measurement results show that the critical transition temperature of MgB2/Ta/Cu tape with Ti doping is around 38 K. The irreversibility field Hirr and critical current density Jc can be greatly enhanced by Ti doping. Hirr of the Mg0.9Ti0.1B2 tape is as high as 7.4 T at 10 K. The high critical current density Jc of 1.5x106 A/cm2 (10 K, self field) and 9.3x105 A/cm2 (20 K, self field) are obtained in the Mg0.9Ti0.1B2 tape. In addition, a suitable amount of Ti doping can lead to a high density and fine grain size of MgB2, which may be the reason for high Jc in Ti-doped MgB2 tapes.

  • (2002) Zhao, Yong; Huang, D; Feng, Y; Cheng, C; Machi, T; Koshizuka, N; Murakami, M
    Journal Article
    The microstructure of the Ti-doped MgB2 which shows a significantly improved critical current density, Jc [Appl. Phys. Lett. 79, 1154 (2001)], is investigated. It is found that Ti does not occupy the atomic site in the MgB2 crystal structure, but forms a thin TiB2 layer (with a thickness about one unit cell of TiB2) in the grain boundaries of MgB2. Besides, MgB2 grains are greatly refined by Ti doping, forming a strongly coupled nanoparticle structure. It is argued that the unique microstructure of the MgB2 nanoparticles with TiB2 nanograin boundaries may take responsibility for the enhancement of Jc in the Ti-doped MgB2 bulk superconductor.

  • (2001) Zhao, Yong; Feng, Y.; Cheng, C.H.; Zhou, L.; Wu, Y.; Machi, T.; Fudamoto, Y.; Koshizuka, N.; Murakami, M.
    Journal Article
    Ti-doped MgB2 superconductors with different doping levels were prepared by solid-state reaction at ambient pressure. The density, diamagnetic signal, and Jc of the samples change significantly with the doping level, with the best result achieved at cursive chi = 0.1. At 5 K, the Jc reaches 2 x 106 A/cm2 in the self-field and 5 x 104 A/cm2 in 5 T. At 20 K, the Jc is still as high as 1.3 x 106 A/cm2 in the self-field and 9.4 x 104 A/cm2 in 2 T. It is observed that partial melting occurs in the Ti-doped samples, resulting in an excellent grain connection and extremely high density. In addition, some fine particles (with sizes from 10 to 100 nm) of the second phases induced by Ti doping are distributed in the MgB2 matrix, and this may play an important role in flux pinning enhancement.

  • (2001) Feng, Y; Zhao, Yong; Sun, Y; Liu, F.C.; Fu, B.Q.; Zhou, L.; Cheng, C.H.; Koshizuka, N; Murakami, M
    Journal Article
    We present the superconducting properties and phase compositions of Mg1-xZrxB2 bulk samples fabricated by a solid-state reaction at ambient pressure. It is found that a small amount of Zr atoms may be introduced into the lattice of MgB2, while the majority of them forms ZrB2 phase. The Mg0.9Zr0.1B2 sample shows the highest JC of 2.1x106 A/cm2 in 0.56 T at 5 K and 1.83x106 A/cm2 in self-field at 20 K, higher irreversibility field and larger upper critical field in MgB2 bulk samples. The combination of good grain connection, the reduction of grain size and small ZrB2 particles in the sample may be responsible for the significant enhancement of JC in Zr-doped samples. This technique has a great potential to prepare high performance MgB2 bulk samples and wires on an industrial scale.

  • (2000) Zhang, Guangqing; Cheng, L; Qin, X; Zhao, Yong
    Journal Article
    The influence of the interaction between two structural blocks on superconductivity was studied by calculating the cohesive energy in Bi-system superconductors, Bi2Sr2CuOy (2201), Bi2Sr2CaCu2Oy (2212), and the Bi2Sr2Ca2Cu3Oy (2223) phases, and Hg-system superconductors, HgBa2CuOy (1201), HgBa2CaCu2Oy (1212), HgBa2Ca2Cu3Oy (1223), HgBa2Ca3Cu4Oy (1234), and HgBa2Ca4Cu5Oy (1245), respectively. We developed a program to calculate the combinative energy between Cu-O planes and remaining parts, and between blocks in the superconductors. The result indicates that if we consider the cell as two blocks combined together, a close relationship among the combinative energy between the two blocks, the value of Tc, and the number of the Cu-O planes in the Bi- and Hg-system superconductors is established. The result gives an interesting way to understand the change of the value of Tc as the number of the Cu-O planes. In contrast, the combinative energy between the Cu-O planes and the remains obtained from the method separating all the Cu-O planes from the cell and leaving some discrete remaining parts does not show any relationship with the value of T0. This means that considering the cell as the two blocks is more reasonable, and the interaction between the two blocks plays an important role in superconductivity.