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

Composites with varied proportions of β-Ca3(PO4)2 and ZnO were obtained through an in situ aqueous precipitation method under slightly basic (pH ≈ 8) conditions. The formation of β-Ca3(PO4)2 phase starts at an early heat-treatment stage (∼800 °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 β-Ca3(PO4)2 lattice enhances its thermal stability delaying the allotropic β-Ca3(PO4)2→α-Ca3(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 β-Ca3(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.