Phase Equilibria and Phase Diagrams in Oxide Systems

Glass production usually involves melting of a mixture of solid starting materials. Although typical glasses are homogeneous, liquid immiscibility is common in a broad range of glass-forming melt compositions. This chapter thus reviews crystal–liquid and liquid–liquid phase transitions and equilibria relevant for glassmaking and research. The principles of chemical thermodynamics and especially the concept of Gibbs free energy and the phase rule are used to predict the direction of chemical reactions and the stable state of a system of a given chemical composition at given pressure and temperature. Phase diagrams then display in a graphical form melting/crystallization and mixing/unmixing relationships in temperature–composition (T–x) coordinates. Binary (two-component) phase diagrams showing liquid stability field and its relationship to crystal phases are the simplest diagrams relevant to glassmaking. The topological types of the binary diagrams are primarily defined by mutual solubility of the components in liquid and solid states. With an increasing number of components, the basic types of phase equilibria remain the same, so ternary diagrams and diagrams for systems with a greater number of components can be viewed as various combinations of binary topologic relationships. Phase equilibria in binary systems of silica with the oxides of alkalis and alkaline earth elements and the ternary systems Na2O–CaO–SiO2 and Na2O–B2O3–SiO2 are discussed in more detail in view of their particular importance for glass production.