Batch hydrothermal synthesis of nanocrystalline, thermostable hydroxyapatite at various pH and temperature levels

This paper addresses the powder synthesis of nanocrystalline hydroxyapatite (HA) in a hydrothermal autoclave reactor using deionized water and the chemical reactants Ca(NO3)2 and (NH4)2HPO4 at varying pH values (7-11) and temperatures ranging from 50 to 130 OC. Analysis of the synthesized powder products included XRD (X-ray diffraction), FTIR (Fourier transform infrared spectroscopy), SEM/EDX (scanning electron microscopy), and TG/DTG (thermogravimetry). Hexagonal and monoclinic hydroxyapatite crystallized mostly after 24-h hydrothermal processing at different initial pH values and temperatures (50 to 130°C). When the temperature rose from 50 to 110 OC at a fixed pH of 10, the hexagonal phase became more prevalent while the monoclinic phase decreased proportionally. The monoclinic phase crystallite size reduced from 231.08 to 31.51 nm. Despite a constant temperature of 80 OC, pH levels ranging from 7 to 11 revealed substantial hexagonal and monoclinic hydroxyapatite with minimal monetite and brushite. FTIR spectroscopy confirmed the functional groups associated with hydroxyapatite spectra. The SEM method also revealed HA powder with spherules and anhedral agglomerates having Ca/P ratios (1.1-2.15). TG/DTG results confirmed that hydrothermal-derived HA nanosized powders are thermally stable microstructures for biomedical applications.