The influence of lithia content on the properties of fluorphlogopite glass-ceramics. I. Nucleation and crystallisation behaviour

Glasses based on 8SiO23Al2O3(3−S)MgO3MgF2(1−Z)BaOZK2OSLi2O where lithia was substituted for magnesia and baria for potassia were synthesised and characterised by differential scanning calorimetry, combined differental thermal analysis/thermal gravimetric analysis dilatometry and X-ray powder diffraction. The glass transition temperature (Tg) and first peak crystallisation temperature (Tp1) decreased, whilst the thermal expansion coefficient increased with increasing lithia and potassia content. The glasses exhibited optimum nucleation temperatures close to the experimentally determined Tg values indicative of crystal nucleation via prior amorphous phase separation. The first phase to form in the baria containing glasses was barium fluorphlogopite (Ba0.5Mg3(Si3AlO10)F2), whilst in the glass compositions containing both lithia and potassia, sellaite (MgF2) formed prior to formation of potassium fluorphlogopite (KMg3(Si3AlO10)F2).