Novel Glass Formation in the Ca–Si–Al–O–N–F System

Abstract

Ca–Si–Al–O–N–F glass formation regions have been mapped on 6Ca2+–3Si4+–4Al3+ ternary diagrams at fixed O:N:F of 79:20:1 and 75:20:5 (in eq%). For both fluorine contents, glass formation regions extend toward more Ca‐rich compositions at 1650°C compared with the Ca–Si–Al–O–N system at 1700°C. Ca–Si–Al–O–N–F melting temperatures are 150°–800°C lower than equivalent Ca–Si–Al–O compositions due to fluorine incorporation into the melt. Si‐rich compositions, with both 1 and 5 eq% F, form porous glasses due to loss of SiF4. Reaction mechanisms were studied by firing two compositions (0 or 5 eq% F) with identical cation ratios in the range of 900°–1650°C at 100°C intervals and following phase development using X‐ray diffraction. For Ca–Si–Al–O–N compositions, initial liquid formation occurs at >1200°C with complete dissolution of Si3N4 at 1300°C. For Ca–Si–Al–O–N–F compositions, reactions occur at lower temperatures with Si3N4 dissolution at 1200°C. At 20 eq% N, glasses with maximum fluorine content of ∼7 eq% were obtained. At 5 eq% F, the solubility limit for N is ∼25 eq%. At 1 eq% F, the maximum nitrogen content can be substantially increased to 40 eq% N. Incorporation of both nitrogen and fluorine in Ca–Si–Al–O glasses extends glass formation regions through combinations of lower melting temperatures (fluorine) and higher viscosities (nitrogen).

Properties ofCa–(Y)–Si–Al–O–N–FGlasses: Independent and Additive Effects of Fluorine and Nitrogen

“…This was expected beforehand as the cation composition lies inside the glass‐forming region for 20 eq.% nitrogen (0 eq.% fluorine) reported by Drew et al . for calcium–sialon glasses and also within the glass‐forming region for 20 eq.% nitrogen and 5 eq.% fluorine reported by Hanifi et al . for Ca–Si–Al–O–N–F glasses.…”

“…The compositions of thirty glasses in the Ca–(Y)–Si–Al–O–N–F system that were used in this study are given in Table . The Ca:Al and Al:Si ratios chosen are expected to maintain Al in fourfold coordination as they are >1:2 and <1:1, respectively . Therefore, Al atoms are expected to enter the glass network as [AlO 4 ] − tetrahedra leading to preferential formation of Al–F bonds over the undesired Si–F bonds, which can lead to fluorine loss as volatile SiF 4 .…”

“…The Ca:Al and Al:Si ratios chosen are expected to maintain Al in fourfold coordination as they are >1:2 and <1:1, respectively. 33,35 Therefore, Al atoms are expected to enter the glass network as [AlO 4 ] À tetrahedra leading to preferential formation of Al-F bonds over the undesired Si-F bonds, which can lead to fluorine loss as volatile SiF 4 . Furthermore, maxi-mum nitrogen, and fluorine substitution levels for oxygen were chosen so that they were both similar (~8.4 at.%).…”

“…Addition of fluorine to oxynitride glasses has been reported by Hanifi et al ., introducing a new family of Ca–Si–Al–O–N–F glasses. The present work involves a systematic investigation of the effects of oxygen replacement by fluorine and nitrogen simultaneously on the properties of calcium– and calcium–yttrium‐modified alumino–silicate glasses with a constant cation composition.…”

Compositional effects on the crosslink density of Ca–(Mg)–(Y)–Si–Al–oxyfluoronitride glasses

“…Twenty years later, the first systematic studies were carried out in order to determine the combined effect of adding nitrogen and fluorine to aluminosilicate glasses on glass formation and properties . The authors explored the glass‐forming region in the Ca–Si–Al–O–N–F system and compared it with the one obtained 26 years earlier in the Ca–Si–Al–O–N system . The addition of fluorine, even in small quantities (1 eq.%), was shown to expand the glass‐forming region, facilitating the dissolution of the modifier cations and nitrogen into the melt.…”