Structural evolution of sol-gel-derived hydroxyapatite

Structural evolution upon transformation of sol to gel, and gel to final ceramic during the synthesis of hydroxyapatite is investigated using Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), thermal behavior (DTA and TGA), and electron microscopy examination (SEM/TEM). The sol was first thermally aged at 45°C for various time periods up to 120 min. The colloidal sol, which may have an oligomeric structure, was relatively stable against coagulation. Upon drying, the sol particles consolidated into dry gel through van der Waals attraction, and showed X-ray amorphous phosphate structure. The solid gels showed a particulate microstructure, composed of primary particles of about 8-10 nm in diameter. The amorphous gel transformed into crystalline apatite at temperatures > 300°C. The calcined gels showed a nano-scale microstructure, with grains of 20-50 nm in diameter. Through an appropriate heat treatment between 300 and 400°C, the apatite prepared using current process exhibits a nano-scale, low-crystallinity, carbonated apatitic structure, which closely resembles that of human bone apatite.