Formation Mechanisms in β-Ca3(PO4)2-ZnO Composites: Structural Repercussions of Composition and Heat Treatments

Composites with varied proportions of beta-Ca-3(PO4)(2) and ZnO were obtained through an in situ aqueous precipitation method under slightly basic (pH approximate to 8) conditions. The formation of beta-Ca-3(PO4)(2) phase starts at an early heat-treatment stage (similar to 800 degrees C) and incorporates Zn2+ ions at both Ca2+(4) and Ca2+(5) sites of the lattice up to its occupancy saturation limit. The incorporation of Zn2+ in the beta-Ca-3(PO4)(2) lattice enhances its thermal stability delaying the allotropic beta-Ca-3(PO4)(2)->alpha-Ca-3(PO4)(2) phase transformation. The excess zinc beyond the occupancy saturation limit precipitates as Zn(OH)2 and undergoes dehydroxylation to form ZnO at elevated temperatures. The presence of ZnO in the beta-Ca-3(PO4)(2) matrix yields denser microstructures and thus improves the mechanical features of sintered composites up to an optimal ZnO concentration beyond which it tends to exert an opposite effect.