Viscosity‐structure relationship of alkaline earth silicate melts containing manganese oxide and calcium fluoride

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The effect of CaF 2 on the viscosities of the MO‐SiO 2 ‐ x MnO‐ y CaF 2 melts (M[=Ca or Ba]O/SiO 2 = 0.4‐0.5, x = 10[±1] or 40[±2] mol%, y = 0‐15 mol%) have been studied at high temperatures. At MnO = 40[±2] mol%, the viscosity of Ba‐Mn‐silicate melts is lower than that of Ca‐Mn‐silicate melts because Mn 2+ is a strong network modifier in the former system. However, the viscosities of both alkaline earth silicate melts with MnO = 10[±1] mol% are not affected by cation type because MnO activity is too low to affect the viscosity of the melts. The CaF 2 addition decreases the viscosities of the Ba‐Mn‐silicate melts with low‐MnO contents, while it has less impact on the viscosities of the melts with high‐MnO contents. The effect of CaF 2 on the relationship between viscosity and the structure of the silicate melts is quantitatively analyzed using the micro‐Raman spectra of quenched glass samples. The NBO/Si ratio of alkaline earth silicate melts, which indicates the degree of polymerization of the silicate networks, has a linear relationship with the activation energy for the viscous flow of the melts.