Viscosity, fragility and structure of Na2O–CaO–Al2O3–SiO2 glasses of increasing Al/Si ratio

The research and development of a new float glass with higher content of Al2O3 is essential for the commercial flat glass. The study on the workability and kinetic fragility of Na2O–CaO–Al2O3–SiO2 glass melts with different Al/Si ratios has been linked with the structure. The viscosities as a function of temperature for glass melts were derived on the basis of Vogel–Fulcher–Tamman (VFT) equation. Some characteristic temperatures and four characteristic temperature intervals of forming process in tin bath were estimated. The results showed that: adding 12 wt% Al2O3 substitute for SiO2, the melting point (Tm) increased about 35 K, entire temperature interval in tin bath narrowed down about 20 K, the shortening of workability was mainly reflected in the viscosity range of 105.75–1010 Pa s, the fragility index m increased by 15%. It reveals an inverse correlation between the workability and the fragility. The structural changes on the tetrahedron structural unit Qn (n=1, 2, 3, 4) were obtained by using Raman spectroscopy. Our analysis indicates that: the number of NBO reducing and a more polymerized structure with adding Al/Si ratios are responsible for the increase of viscosity; the tetrahedral distortion, a decrease of Q3/Q2 in the Qn species, is responsible for the increase of fragility.