Effect of ion exchange on the sinter–crystallisation of low expansion Li₂O.Al₂O₃. SiO₂ glass-ceramics

In this work we propose an ion exchange (IE) treatment to minimize surface crystallization and thus improve the sintering of compacts made of glass particles. This concept was tested by subjecting glass powders of the LiO₂–Al₂O₃–SiO₂ (LAS) system to molten KNO₃ IE treatment aimed at developing a low thermal expansion glass-ceramic via sinter–crystallisation. The relative densities of treated and untreated glass powder compacts sintered at different temperatures were compared and suitable IE and sintering temperatures were selected. Optical microscopy, x-ray diffraction, dilatometry and scanning electron microscopy were used to characterise the sintered samples. The proposed IE decreased the optimum sintering temperature from 1100 to 1000°C and yet allowed the formation of virgilite, a desirable β-quartz solid solu- tion having a negative thermal expansion coefficient. The compositional changes in surface of the glass particles caused by IE diminished the crystallisation tendency, which resulted in a smaller volume fraction of virgilite, thus increasing the overall thermal expansion coefficient of the resulting glass-ceramic. However, by mixing IE-treated with untreated powder this effect has been controlled leading to a glass-ceramic with low residual porosity (<1·5%) and a relatively low thermal expansion coefficient (1·6×10^–6°C^–1 between 40 and 500°C).