Formation Mechanisms in β-Ca 3 (PO 4 ) 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.