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The viscosity of silica glasses with different water contents was measured by penetration of sapphire microspheres in the range from 1012-1014·7 Pas at ambient pressure. Commercial silica glasses were used together with hydrous silica glasses, which were prepared by melting glass powder plus water in an internally heated pressure vessel at 2-3 kbar and 1350-1450°C. The temperature dependence of the viscosity of silica glasses with total water contents Cw of 5·4×10-4, 7·0×10-4, 1·00×10-2, and 1·55×10-2(Cw in mass fractions) can be described by Arrhenius equations. From the results, the linear dependence of the T12 isokom (K) with the logarithm of the water content was determined as T12=192-409 logCw. The analysis of the compositional dependence of the T12 isokom in the H2O-Na2O-SiO2 system reveals a temperature depression of up to 200 K for mixed water- and sodium-bearing glasses compared to H2O-SiO2 and Na2O-SiO2 glasses with same molar silica content. From these findings we conclude that protons may contribute significantly to the mixed alkali effect in glasses.
In the present study we have investigated whether the effect of water on properties of borate glasses resembles that of alkali oxide. Soda-lime-borate glasses with nominal compositions of x Na2O, 10 CaO, (90-x) B2O3 (x = 5, 15 and 25 mol%) were doped with up to 8 wt.% H2O by processing glass powder + distilled water in platinum capsules in an internally heated gas pressure vessel at 1523 K and 500 MPa. The water content of hydrous glasses was determined by Karl-Fischer titration and near-infrared spectroscopy. The glass transition temperature T-g. was derived from DTA and micropenetration experiments for which the effect of water loss at the surface of the hydrous glasses was studied. Heating glass samples at 10 K min(-1) in the DTA resulted in T-g values which are close to T-12 isokom temperatures confirming the equivalence of enthalpy relaxation and viscous relaxation for borate glasses. For all three glass series it is shown that T-g strongly decreases whereas the liquid fragility strongly increases upon the addition of water. These findings reveal that H2O primarily causes breaking of B-O-B bonds rather than supporting 4-fold coordinated boron as it is well-known for alkali oxides in this concentration range. (C) 2015 Elsevier B.V. All rights reserved.