Structural Perceptions and Mechanical Evaluation of ß-Ca3(PO4)2/c-CeO2 Composites with Preferential Occupancy of Ce3+ and Ce4.

The role of cerium in the formation of stable ß-Ca3(PO4)2/c-CeO2 composites and their structural analysis with varied compositional ratios were investigated. The composite formation was attempted through an in situ precipitation technique, and the gradual structural changes during heat treatments to yield the pure form of ß-Ca3(PO4)2/c-CeO2 composites was presented. The cerium was found in Ce3+ and Ce4+ oxidation states in composites. Ce3+ prefers to occupy the Ca2+(1), Ca2+(2), and Ca2+(3) sites of ß-Ca3(PO4)2, whereas, beyond the saturation occupancy limit, excess cerium prefers to crystallize in the form of thermodynamically stable cubic ceria (c-CeO2). A uniform expansion of the ß-Ca3(PO4)2 unit cell and the delayed allotropic conversion of ß-Ca3(PO4)2 → α-Ca3(PO4)2 have been detected due to the Ce3+ occupancy at the ß-Ca3(PO4)2 lattice. ß-Ca3(PO4)2/c-CeO2 composites exhibited a steady upsurge in the mechanical properties with consistent enhancement of c-CeO2 content in the composites. The overall results from the investigation imply the appropriateness of the ß-Ca3(PO4)2/c-CeO2 composites for applications in hard tissue replacements.

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

Composites with varied proportions of ß-Ca3(PO4)2 and ZnO were obtained through an in situ aqueous precipitation method under slightly basic (pH ≈ 8) conditions. The formation of ß-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 ß-Ca3(PO4)2 lattice enhances its thermal stability delaying the allotropic ß-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 ß-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.