Equilibrium Phase Diagrams

Abstract

Most ceramics are multicomponent materials and the most stable phase at any temperature will be the one with the lowest free energy, G. One use of phase diagrams is to represent the phase or phases we might expect to be present as a function of temperature. There are a large number of books just concerned with this one topic. Much work was carried out in the 1950s and 1960s, but many systems have remained almost completely unexplored and it is not a well-funded area in the United States now. The lack of effort is in spite of the demonstration that new complex ceramics, such as the high-temperature superconductors YBCO and BiSCCO and the magnetic manganates, possess extraordinary, and potentially very useful, properties. Much of the classical work on phase equilibria has actually been concerned with processing metals. Thus the Fe-O phase diagram is perhaps the most thoroughly characterized because of its importance to the iron and steel industry. A word to keep in mind throughout our discussion is equilibrium: we are talking about equilibrium phase diagrams. Often we use a phase diagram as a guide to processing. If the process is in progress then it is not in equilibrium. And, by definition, a chemical reaction is not an equilibrium process. If a reaction is exothermic then a rise in temperature favors the reactants. Although most of the phase diagrams we use in ceramics are for a pressure of 1 atmosphere, in one-component systems such as carbon, pressure is a very important variable. It tells us what pressure we need for direct synthesis of diamond. In metal-oxygen diagrams the partial pressure of oxygen determines what is the stable form of the oxide.