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  • 1
    Electronic Resource
    Electronic Resource
    DTA study for clarifying the role of silver in the formation of Bi-2223 phase in Ag-Sheathed tapes (1994)
    Springer
    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1665-1670 
    Details
    ISSN: 0392-6737
    Keywords: Superconducting magnets and magnetic levitation devices ; superconducting cables and conductors ; Superconducting perovskites and related structures ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The role of silver in catalysing and promoting the textured growth of 2223 phase in Ag-sheathed (Powder In Tube) tapes, starting from a precursor «bi-component» powder, constituted by Bi-2212 and other mixed oxides, was investigated in detail by means of DTA and X-ray analyses at the different stages of the annealing of the tapes. By monitoring the reaction it was possible to show how the presence of silver modifies the thermal behaviour of the reactants. The reduction of the temperature at which a liquid phase is formed is believed to be the key mechanism through which the presence of silver enhances the overall conversion to 2223 phase. This temperature decrease was shown (for the first time, to our knowledge) to be active only as long as some precursor (unreacted) powder was present, and not on fully reacted 2223 phase. Further investigation on 2212 phase and the other mixed oxides, separately analysed by DTA, clarified that only the thermal behaviour of the former was modified by the presence of silver. An explanation based on oxygen unbalancing in the material possibly induced, at the annealing temperature, by the presence of silver, is suggested: as a consequence of the very high solubility of oxygen in silver at high temperature, O2 would be «pumped off» the superconducting 2212 phase, with the result of lowering its melting point.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02462157
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Interatomic potentials for CaCO3 polymorphs (calcite and aragonite), fitted to elastic and vibrational data (1992)
    Springer
    Physics and chemistry of minerals 19 (1992), S. 80-87 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Calcite and aragonite have been modeled using rigid-ion, two-body Born-type potentials, supplemented by O-C-O angular terms inside the CO3 groups. A shell model has also been developed for calcite. Atomic charges, repulsive parameters and force constants have been optimized to reproduce the equilibrium crystal structures, the elastic constants and the Raman and infrared vibrational frequencies. The rigid-ion potential RIM (atomic charges:z O= -0.995e,z C = 0.985e,z Ca = 2.0e) fitted to calcite properties is able to account for those of aragonite as well. Experimental unit-cell edges, elastic constants, internal and lattice frequencies are reproduced with average relative errors of 2.1, 5.5, 2.4, 15.1% for calcite and of 0.2, 19.4, 2.5, 11.8% for aragonite, respectively. The RIM potential is suitable for thermodynamic and phase diagram simulations in the CaCO3 system, and is discussed and compared to other potentials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00198605
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Quantum-mechanical Hartree-Fock study of calcite (CaCO3) at variable pressure, and comparison with magnesite (MgCO3) (1993)
    Springer
    Physics and chemistry of minerals 20 (1993), S. 104-110 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract The static crystal energy of calcite and its structure configuration as functions of pressure were determined by ab initio all-electron periodic Hartree-Fock calculations (CRYSTAL code). Ca, O and C atoms were represented by 22, 18 and 14 atomic orbitals, respectively, in form of contracted Gaussian-type functions. Comparison between theoretical and experimental data was performed for binding energy, equilibrium unit-cell and bond lengths, bulk modulus and C 33 and C 11 + C 12 elastic constants, and vibrational frequency of the symmetrical C-O stretching mode. The agreement is generally satisfactory. A larger compressibility is observed for structural parameters of calcite than for those of magnesite coming from a similar calculation. The Ca-O and C-O chemical bonding was characterized by electron density maps and by Mulliken atomic charges; these are discussed and compared to values determined by empirical fitting of Born-type interatomic potentials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00207203
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  • 4
    Electronic Resource
    Electronic Resource
    Thermodynamic properties of CaCO3 calcite and aragonite: A quasi-harmonic calculation (1993)
    Springer
    Physics and chemistry of minerals 19 (1993), S. 472-479 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract A quasi-harmonic model has been used to simulate the thermodynamic behaviour of the CaCO3 polymorphs, by equilibrating their crystal structures as a function of temperature so as to balance the sum of inner static and thermal pressures against the applied external pressure. The vibrational frequencies and elastic properties needed have been computed using interatomic potentials based on two-body Born-type functions, with O-C-O angular terms to account for covalency inside the CO3 molecular ion. A good agreement with experimental data is generally shown by simulated heat capacity and entropy, while the thermal expansion coefficient seems to be more difficult to reproduce. The results obtained for aragonite are less satisfactory than those of calcite, but they are improved by using a potential specifically optimized on properties of that phase itself.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00203187
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