Thermodynamic Basis of CBPC Formation

Thermodynamics is the basis of all chemical transformations [1], which include dissolution of chemical components in aqueous solutions, reactions between two dissolved species, and precipitation of new products formed by the reactions. The laws of thermodynamics provide conditions in which these reactions occur. One way of determining such conditions is to use thermodynamic potentials (i.e., enthalpy, entropy, and Gibbs free energy of individual components that participate in a chemical reaction) and then apply the laws of thermodynamics. In the case of CBPCs, this approach requires relating measurable parameters, such as solubility of individual components of the reaction, to the thermodynamic parameters. Thermodynamic models not only predict whether a particular reaction is likely to occur, but also provide conditions (measurable parameters such as temperature and pressure) in which ceramics are formed out of these reactions. The basic thermodynamic potentials of most constituents of the CBPC products have been measured at room temperature (and often at elevated temperatures) and recorded in standard data books. Thus, it is possible to compile these data on the starter components, relate them to their dissolution characteristics, and predict their dissolution behavior in an aqueous solution by using a thermodynamic model. The thermodynamic potentials themselves can be expressed in terms of the molecular behavior of individual components forming the ceramics, as determined by a statistical-mechanical approach. Such a detailed study is beyond the scope of this book. 

A thermodynamic model of dissolution is presented in this chapter, which relates the solubility product constant to the thermodynamic potentials and measurable parameters, such as temperature and pressure of the solution. The resulting relations allow us to develop conditions in which CBPCs are likely to form by reactions of various oxides (or minerals) with phosphate solutions. Thus, the model predicts formation of CBPCs. 

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Reaction 5.1 forms the basis for dissolution of an oxide for CBPC formation. It represents dissociation of metal oxides in which cations and anions are formed in an aqueous solution. In general, divalent metal oxides dissociate more easily than trivalent oxides, and quadrivalent oxides dissolve less easily than trivalent oxides, though some exceptions may be found to this general trend. The actual rate of dissolution will be discussed in detail as we develop a thermodynamic basis for these transformations. 

To be more specific. Chapter 2 provides an overview of Chemically Bonded Phosphate Ceramics. It is intended to streamline the earlier literature and present it in a suitable context. Since the many potential applications of CBPCs are likely to alfect the raw materials (such as phosphates) market, an overview of the raw materials, their general properties, and their manufacturing processes is given in the third chapter. Chapters 4-7 are devoted to the theoretical basis for formation of phosphate ceramics by chemical reactions, and much of the discussion in these chapters is based on thermodynamics. 

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