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  • 1
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results are presented regarding the sintering mechanisms of barium hexaferrite (BaFe12O19) and modification thereof via secondary phase additions. Additional studies are presented concerning the effect of additives and microstructural development on the resultant microwave frequency magnetic and dielectric loss properties. The increasing demands on materials for microwave application has required development of barium hexaferrites to achieve fine grain size (i.e., ≤1 μm), a high degree of c axis alignment, high density and a minimum in dielectric loss. To accomplish such a task one area of interest is that of incorporating secondary phases. Additions of SiO2, Al2O3 and CaCO3 are examined in terms of grain size, magnetic, and dielectric loss properties. Thermal analysis methods revealed a redox reaction associated with the sintering process in barium hexaferrite. The use of yttrium iron garnet (Y6Fe10O24) indicated an enhanced densification behavior in comparison to undoped barium hexaferrite which was associated to a decreased redox reaction temperature. The inclusion of the SiO2, Al2O3, and CaCO3 additions were seen to inhibit the redox reaction as measured via thermal analysis while liquid phase sintering occurred. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 353-361 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Changes in various superconducting properties as a function of silver substitution in the bulk granular high-Tc system Y-Ba-Cu-O have been investigated. In the low doping limit, Ag impurities behave as an excellent oxygen stabilizer and the critical current density is enhanced by an order of magnitude while the Tc remains practically unchanged. When the Ag content is increased to above 20%, the Tc starts to drop and the effects of Ag clusters as well as impurity phases become more important. However, the compound is still a superconductor with a Tc around 20 K by full substitution of Ag for Cu. The problems concerning the location of Ag atoms and formation of Ag clusters are examined with measurements of x-ray absorption fine structure using synchrotron radiation. Our results indicate that at least some Ag atoms occupy the Cu(1) and Cu(2) sites in the material. Magnetic field dependence shows that the critical current density of a bulk sintered high-Tc superconductor below a characteristic value H'c is mainly controlled by intergranular weak-link coupling. A shift of Hc with the addition of Ag helps explain the variation of critical current density in this compound system.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 28 (1995), S. 429-435 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The crystal structure of the YBa4Cu2(CO3)O5.5±δ (`142') oxycarbonate has been investigated by X-ray and neutron powder diffraction techniques. X-ray diffraction patterns indexed as a tetragonal P4/m unit cell with lattice parameters a = 8.1973 (4) and c = 8.0360 (5) Å. Rietveld refinements of powder neutron diffraction data indicated a distorted cubic perovskite cell in which yttrium and copper are arranged to yield a superlattice double that of the typical perovskite unit. Refinements indicate CO2−3 ions substituted on two of the four Cu sites in the proposed cell. Infrared spectroscopy results were consistent with neutron structural analysis, giving strong supporting evidence to the presence of two unique carbonate sites in the unit cell.
    Type of Medium: Electronic Resource
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