The Investigation Of Environmental-friendly And Fluoride-free Opal Glasses

Opal glass receives a wide range of applications in many fields such as construction, bottles, wares, art, decoration, and so on. Common emulsion agents are fluorides such as fluorite, sodium fluoride and cryolite, aluminum fluoride, potassium fluoride, and so on. However, during glass making such as raw materials processing, glass melting and molding process, the volatilization of fluorides will cause human's acute and chronic poisoning as well as environmental pollution. In addition, fluorides can react with firebrick and shorten the lifetime of kiln, which increases the cost of glass making. Such the investigation of fluorine-free opal glass is the key factor to solve above problem.In this work, influence of compositions and heat treatment of Phosphorus- containing silicate opal glass on structure and properties was systematically investigated. Microstructure and opaque mechanism was studied by using XRD, SEM, EDS and DSC method. Furthermore, to judge whether it can be applied practically or not, the physical and chemical properties such as whiteness, density, bending strength, thermal expansion behavior, chemical stability and micro hardness were measured.The results show that the excellent opaque glasses can be obtained by adjusting the ratio of SiO₂ and P₂O₅, and by introducing proper content of alkali or/and alkaline oxides. Although the substitution of 3wt% B₂O₃ for Al₂O₃ has little effect on appearance, i.e. heavy opaque glass can be obtained in both composition due to unstable phase separation and the whiteness of both glasses is larger than 80%. However, the substitution produces great change in following aspects: first, small amounts of crystal are precipitated during cooling and the particle size becomes more inhomogeneous, second, the destiny and bending strength decrease and chemical stability also reduces, thermal expansion coefficient increases, which could attribute to coexistence of [BO₃] tetragonal and [BO₄] tetrahedral coordination.The type and content of network modifiers has great influence on the microstructure and properties of opal glasses. The glass remain transparent when only Na₂O and CaO are introduced and NaCaPO₄ crystals were observed, whereas the glasses become opalescent when MgO and/or Li₂O are used, which implies addition of MgO and/or Li₂O enhance the phase-separation and crystalization. In addition, the heat treatment also has effect on morphology and properties. The size of hexagonal NaCaPO₄ increases to10-30μm after heat treatment at 600 oC/700 oC for 20h, leading to that all samples appear heavy opalescent, at the same time, bending strength, micro hardness, toughness increase obviously.Viscosity measurement results show that viscosity increases with substituting P₂O₅ for SiO₂, and this is due to formation of biger anion groups (PO₄3-)which enhances the activation enthalpy of viscous flow. A good quality of fitting of Vogel-Fulcher- Tamman (VFT), Adam & Gibbs(AG) and Avramov(AV) equations to the measured viscosity has been achieved, which is reflected by the less standard deviations than 3.03×10-6, and the order is VFT>AG>AV. In addition, the glass transition temperature(Tg) and fragility index decrease, and the configurational entropy increase with the addition of P₂O₅.