Influence of Fluorine Content on the Crystallisation of Potassium and Barium Fluorphlogopite Glass-Ceramics

Abstract

The influence of fluorine content on the nucleation and crystallization behaviour of fluormica glass-ceramics based on 8SiO2-3Al2O3-(6-X)MgO-XMgF2-(K2O/BaO) were investigated. Fluoride reduced the glass transition temperature (Tg), as well as the first peak crystallisation temperature (Tp1) in both potassium and barium series. High fluoride content (X ≥ 2) promoted bulk crystal nucleation in the potassium series. The first crystallisation peak corresponded to the crystallization of potassium fluorphlogopite in glasses with X ≥ 2.0. However, in barium glasses surface crystallization was more dominant regardless of the fluorine content with kinoshitalite found to be the main mica phase. The formulations were designed to have high aluminium content to reduce volatilisation of SiF4, therefore only small weight loss was evident from the DTA/TGA analysis, <5% in fine samples; the frit samples showed much lower weight loss. In the potassium series, the weight loss increased with increase in fluoride content. In barium series the weight loss was independent of fluoride content. No evidence was found of precursor crystal phases, such as chondrodite (2Mg2SiO4.MgF2,), or norbergite (MgSiO4.MgF2). Owing to the high alumina and silica content in both series, other aluminium and silica rich phases, e.g. cordierite and mullite, crystallised in addition to the main phase of Phlogopite. Cordierite (5SiO2.2Al2O3.2MgO) was formed readily at low fluoride content of all glasses at high temperatures. Microstructural imaging of the developed glass-ceramic showed that the growth and coarsening of the microstructure was faster in glass compositions with higher fluorine contents and with potassium rather than barium.