Formation of Zinc Phosphate Ceramic

Solubility of various phases of zinc oxide as a function of pH. 

The literature review summarized in Chapter 2 and discussed in Ref. [10] indicates that zinc phosphate ceramics have been synthesized only in small sizes as dental cements, and no attempt has been made to cast large forms of these cements. In spite of this. 

The process of fabrication of zinc phosphate cements is very similar to that of magnesium phosphate ceramics. Direct reaction with phosphoric acid is fierce and needs to be slowed down. This is done by the following methods. 

Unlike magnesium oxide, zinc oxide is seldom used in its pure form. It is blended with MgO and often with AI2O3 to initiate some neutralization of phosphoric acid [10]. MgO will form its dihydrogen phosphate (see Chapter 9), and as shown in Chapter 11, AI2O3 will also form its hydrophosphate. This partial neutralization of the acid aids in reducing the fierce reaction between the base ZnO and the acid. Typically, 3-10 wt% MgO and 1 % AI2O3 are used. 

As in the case of MgO, the powder is precalcined at temperatures of 1000-1350°C to reduce the surface area of the particles [22]. The surface area is reduced by partial elimination of porosity of individual grains and by grain growth resulting from consolidation of amorphous content. The presence of MgO and silica in zinc oxide promotes densification by forming a solid solution of ZnO with these oxides [7,23]. In addition to this densification, compounds of lower solubility formed by the solid solutions, such as zinc silicates, are likely to reduce the overall solubility of zinc oxide. 

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