RE₂O₃-alkaline earth-aluminosilicate fiber glasses: Melt properties, crystallization, and the network structures

Glasses and/or glass fibers of a new glass system, RE₂O₃ (RE = Sc,Y, La)-MgO-CaO-Al₂O₃-SiO₂, were studied in detail, covering rare earth (RE) effects on high temperature viscosity, liquidus temperature and crystalline phases, softening and glass transition temperature, and speciation reactions of aluminum (AlO_x, x = 4, 5) and silicate network (Qⁿ, n = 4, 3, 2) using ²⁷Al and ²⁹Si MAS NMR and Raman spectroscopic methods. Rare earth oxides (RE₂O₃) were shown to have a dual functionality, i.e., decreasing melt viscosity like a network modifier depolymerizing glass network, but increasing viscosity at low temperature, i.e., glass softening temperature (T_soft) and transition temperature (T_g), strengthening the glass network functioning like Al₂O₃. Liquidus temperature (T_Liq) was found to be sensitive to the type of RE₂O₃ modifications in terms of their ionic field strength (IFS); Sc³⁺ of the highest IFS increased, but La³⁺ of the lowest IFS decreased T_Liq. Effect of Y³⁺ (intermediate IFS) on T_Liq resembles Sc³⁺ only at higher concentration. The overall results suggest that RE ions of different IFS have different “true” solubility limits in the host glass matrix, above which RE₂Si₂O₇ crystallizes. All of the glasses with and without RE₂O₃ contained predominately four-fold coordinated aluminum, AlO₄; yet replacement of CaO by RE₂O₃ increased AlO₅ according to ²⁷Al MQMAS NMR spectroscopic measurements. Raman spectroscopic results suggested that depolymerization of the silicate network.