Browsing by Author "Hirashima, Hiroshi"

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    Electrical conductivity of vanadium phosphate glasses containing ZnO or GeO2
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1988) Hirashima, Hiroshi; Kurokawa, Haruki; Mizobuchi, Katsuo; Yoshida, Tetsuro
    Glasses containing up to 80 mol% V2O5 were obtained when ZnO : P2O5 or GeO2 : P2O5 was 1 in the mole ratio. The temperature dependence of dc conductivity, σ, was exponential with σ = σ0/Texp (-W/k Τ), σ and W of glasses containing the same amount of V2O5 were affected slightly by the kind of the additive oxides ZnO or GeO2, when V2O5 ⪆ 50 mol%. In this region, σ0 varied slightly with the change of V2O5 content, and the hopping conduction was adiabatic. When V2O5 ⪅ 50 mol%, adiabatic approximation could not be valid for the hopping conduction. σ0 and the transition probability decreased remarkably with decreasing V2O5 content, σ and W of the glasses containing the same amount of V2O5 depended on the kind of the additive oxides. The relationship between the anionic molar volume or Tg and V2O5 content indicated that the glass structure changed at V2O5 ≈ 50 mol%. The conduction mechanism was discussed in relation to the glass structure.
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    Redox equilibria and constitution of polyvalent ions in oxide melts and glasses
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1988) Hirashima, Hiroshi; Yoshida, Tetsuro; Brückner, Rolf
    It is shown that redox equilibria of polyvalent ions in glass melts cannot usually be considered without complex-formation reactions and therefore without ligand formation and coordination behaviour. In principle, a distinction should be made between ligand numbers and coordination numbers, as well as between conditions at high temperatures and those at temperatures below Tg. The equations derived and some experiments in literature indicate that most transition metal ion pairs in melts show a stronger tendeney to form complexes for the higher valency states than for the lower ones. In many cases this behaviour coincides with the ligand number of polyvalent cations in the corresponding glasses, viz. the higher valency state leads to a stronger complex formation with network-forming character and thus, to a lower coordination number than is the case with the lower valency state of a polyvalent ion. However, this behaviour often does not occur because of the very common tendeney for coordination to change during cooling from the melt to the glassy state. Those differences between the stable melt at very high temperatures, the metastable melt at intermediate temperatures and the glassy state at room temperature need to be considered and are discussed.