Preparation and properties of R₂O - CaO - SiO₂ - Si₃N₄ oxynitride glasses and melts

Abstract

Oxynitride glasses, or rather melts, of the molar composition (74-3x) SiO₂, xSi₃N₄, 16 R₂O, 10 CaO with R₂O = Na₂O, K₂O show a significant increase in the glass transition temperature Tg, viscosity η and density ρ with increasing nitrogen content, whereas the thermal expansion coefficient A decreases. These data confirm the assumptions of the structural model in which trivalent nitrogen Substitutes for bivalent oxygen to produce a more tightly-linked glass network and most, if not all, of the nitrogen in the glass is linked to Silicon. ²⁹Si-MAS-NMR measurements on glasses of the System Na₂O-CaO-SiO₂-Si₃N₄ prove that the number of links between the [SiO₄]⁴¯ tetrahedra increase with increasing nitrogen content. Mixed alkali glasses, or rather melts, of the molar composition (74-3x) SiO₂, xSi₃N₄, 16(R₂O + K₂O), 10 CaO with R2O = Na₂O, Li₂O and different ratios of Na₂O:K₂O or Li₂O:K₂O show a deviation from the linearity in the glass transition temperature Tg, thermal expansion α, viscosity η and density ρ, which is shown in a minimum or maximum in the curves at a molar composition of about 10 R₂O:6 K₂O.