Abstract
Various characterisation techniques were used to study the composition of the glass series 55-P2O5-2Cr2O3-(43-x) Na2O-xPbO (with 8 ≤ x ≤ 38; mole %) in terms of chemical durability, IR spectroscopy and scanning electron microscopy (SEM). The change in the dissolution rate as a function of time when the studied glasses were kept submerged in distilled water at 90°C for 20 days showed an improvement in the chemical durability when Na2O content was substituted to PbO content. IR spectroscopy revealed a structural change from ultraphosphate groups to pyrophosphate, orthophosphate and probably ring metaphosphate groups. SEM revealed the existence of two phases: a vitreous phase and a crystalline phase. The presence of Cr2O3, even in small amounts, seems to play an important role in the formation of crystallites in the glass network. The improved chemical durability is attributed to the replacement of the easily hydrated Na-O-P and P-O-P bonds by covalent and resistant Pb-O-P bands. Both the increase in PbO content and in the Pb + Cr/P ratio causes an increase in the number of covalent Pb-O-P and Cr-O-P bonds, making the glass structure more rigid. The increase of the covalent Pb-O-P bands leads to a clear evolution of the structure and chemical resistance, caused by grain-boundary resistance as a result of glass crystallisation. The IR spectra indicate that the increase in PbO content favours the formation of isolated PO3-4 orthophosphate groups at the expense of pyrophosphate groups. The radical change in the structure from ultraphosphate groups to pyrophosphate and orthophosphate groups seems to be the cause of the formation of crystallites. The existence of crystallites in these glasses results in a marked improvement in their chemical durability. However, when the crystallites exceed a certain limit, the equilibrium between the glass bath and these crystallites is no longer maintained; we notice, once, a decrease in the chemical durability.
Highlights
Phosphate glasses have been investigated principally because of their relatively low processing temperatures (1000 ̊C - 1200 ̊C) compared with borosilicate glasses (1200 ̊C - 1500 ̊C) and their relatively low softening temperature and liquid viscosity
The study of the chemical durability and structural change along the series of phosphate glasses 55P2O5-2Cr2O3-(43-x) Na2O-xPbO shows that an increase in PbO content in the glass network leads to an improvement in chemical durability which is explained by a radical change in the structure
This appears to be explained by the presence of phosphoric polyacid or phosphoric acid in the solution caused by a small degradation of S4 glass, which results in a slight decrease in the chemical durability
Summary
Phosphate glasses have been investigated principally because of their relatively low processing temperatures (1000 ̊C - 1200 ̊C) compared with borosilicate glasses (1200 ̊C - 1500 ̊C) and their relatively low softening temperature and liquid viscosity. The study of the chemical durability and structural change along the series of phosphate glasses 55P2O5-2Cr2O3-(43-x) Na2O-xPbO (with 8 ≤ x ≤ 38 mol %) shows that an increase in PbO content in the glass network leads to an improvement in chemical durability which is explained by a radical change in the structure. This change is due to the increase of Pb-O-P covalent bonds in the glass. The IR spectra show a radical change in the structure from ultraphosphate groups to orthophosphate groups
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