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Eingeladener Vortrag
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The manufacture of sintered glasses and glass-ceramics, glass matrix composites, and glass-bounded ceramics or pastes is often affected by gas bubble formation. Against this background, we studied sintering and foaming of barium silicate glass powders used as SOFC sealants using different powder milling procedures. Sintering was measured by means of heating microscopy backed up by XPD, differential thermal analysis, vacuum hot extraction (VHE), and optical and electron microscopy. Foaming increased significantly as milling progressed. For moderately milled glass powders, subsequent storage in air could also promote foaming. Although the powder compacts were uniaxially pressed and sintered in air, the milling atmosphere significantly affected foaming. The strength of this effect increased in the order Ar ≈ N2 < air < CO2. Conformingly, VHE studies revealed that the pores of foamed samples predominantly encapsulated CO2, even for powders milled in Ar and N2. Results of this study thus indicate that foaming is caused by carbonaceous species trapped on the glass powder surface. Foaming could be substantially reduced by milling in water and 10 wt% HCl.
Boron oxide glasses usually show low glass transition temperature Tg due to the three-fold oxygen coordination of boron. Adding of alkali and alkaline earth oxides to the glass composition will not decrease but increase Tg due to a change in boron coordination from trigonal to tetrahedral, known as the boron anomaly. Only for higher fractions of alkali oxides, non-bridging oxygens (NBO) are progressively formed in competition with tetrahedrally coordinated boron, which leads to a decrease in viscosity.
ln contrast to this well-known behavior of alkali oxides (R2O), there is little known about adding H2O to borate glasses. The present work therefore aims in shedding light on the rheological properties of hydrous soda lime borate glasses with particular focus on the role of water. For doing so, we determined Tg as a function of Na2O and H2O content using differential thermal analysis (DTA) backed up by micropenetration viscosity measurements. Results show that water decreases Tg for all glasses and water concentrations under study (< 8 wt.% total water). Obviously, water mostly causes the formation of NBO having no significant influence on boron coordination as seen for alkaline.
Boratgläser zeigen im allgeneinen eine niedrige Glasübergangstemperatur, Tg, auf Grund der 3fach Koordination des Bors mit Sauerstoff. Der Zusatz von Alkali- und Erdalkalioxiden zur Glaszusammensetzung führt zunächst zu einem Anstieg von Tg auf Grund des Koordinationswechsels des Bors von der 3fach zur 4fach Koordination, was als Borsäureanomalie bekannt ist. Bei hohen Alkalioxidgehalten werden dann zunehmend Nichtbrückensauerstoffe (NBO) auf Kosten des tetraedrisch koordinierten Bor gebildet, was zur Senkung der Viskosität führt. Im Gegensatz zu diesem bekannten Verhalten der Alkalioxide (R2O) ist wenig über die Wirkung von Wasser in Boratgläsern bekannt. Daher sollen in dieser Arbeit die rheologischen Eigenschaf-ten von wasserreichen Kalk-Natron-Boratgläsern unter spezieller Berücksichtigung der Rolle des Wassers näher betrachtet werden. Zu diesem Zweck wurde Tg in Abhängigkeit des Na2O- und H2O-Gehalts mittels Differentialthermoanalyse (DTA) und Mikropenetrationsviskosimetrie untersucht. Die Ergebnisse zeigen, dass im Gegensatz zu Na2O Wasser in allen untersuchten Glä-sern und H2O-Konzentrationsbereichen (< 8 Gew.%) Tg deutlich verringert. Diese Beobachtung weist darauf hin, dass Wasser primär zur Bildung von NBO führt und kaum zu einem Koordina-tionswechsel des Bors beiträgt.
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.