Applications of Thermodynamic Parameters to CBPC Formation

These relations reveal the nature of chemical reactions involved in formation of CBPCs, which include dissolution of oxides and acid phosphates, and their subsequent acid-base reaction to form ceramics. Therefore, the Gibbs free energy plays an important role in determining which reactions (and hence which components) are most suitable in forming ceramics. 

Another important thermodynamic parameter implicit in the above relations is the net change in the enthalpy, LH. It is a measure of the heat generated or absorbed during a chemical reaction. Following the notation of Eq. 6.12, we can write it as 

In most CBPC fabrication processes, the pressure is a constant, but the temperature of the system changes due to evolution or absorption of heat. In such cases, following Eq. 6.9, we obtain 

Because the CBPC process is based on slow dissolution of the components, spontaneous dissolution of oxides is not desirable in the ceramic formation. This implies thatEq. 6.18 is a requirement for a dissolution reaction that is useful in forming a ceramic. Consider, for example, dissolution of MgO in a neutral medium given by Eq. 5.9c 

The AG values of individual components participating in these reactions may be obtained from Appendix B, and are — 569.57, — 238.59, —456.01, and — 157.3 kJ/mol for MgO, H2O, Mg (aq), and (OH), respectively. These values yield AG = 37.55 kJ/mol, a nonspontaneous reaction. For this reason, MgO can be directly used to form a ceramic in a near neutral medium. If, however, the same is calculated in an acidic medium using Eq. 5.9a, we obtain a spontaneous reaction, because AG = —125.03 kJ/mol. For this reason, one cannot use phosphoric acid for making CBPC products of Mg without some neutralization. The same is tme for most divalent metal oxides. 

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