Growth kinetics of hexagonal sub-micrometric β-tricalcium phosphate particles in ethylene glycol

Recently, uniform, non-agglomerated, hexagonal β-tricalcium phosphate (β-TCP) platelets (diameter ≈ 400–1700 nm, h ≈ 100–200 nm) were obtained at fairly moderate temperatures (90–170 °C) by precipitation in ethylene glycol. Unfortunately, the platelet aspect ratios (diameter/thickness) obtained in the latter study were too small to optimize the strength of polymer–β-TCP composites. Therefore, the aim of the present study was to investigate β-TCP platelet crystallization kinetics, and based on this, to find ways to better control the β-TCP aspect ratio. For that purpose, precipitations were performed at different temperatures (90–170 °C) and precursor concentrations (4, 16 and 32 mM). Solution aliquots were retrieved at regular intervals (10 s–24 h), and the size of the particles was measured on scanning electron microscopy images, hence allowing the determination of the particle growth rates. The β-TCP platelets were observed to nucleate and grow very rapidly. For example, the first crystals were observed after 30 s at 150 °C, and crystallization was complete within 2 min. The crystal growth curves could be well-fitted with both diffusion- and reaction-controlled equations, but the high activation energies (∼100 kJ mol−1) pointed towards a reaction-controlled mechanism. The results revealed that the best way to increase the diameter and aspect ratio of the platelets was to increase the precursor concentration. Aspect ratios as high as 14 were obtained, but the synthesis of such particles was always associated with the presence of large fractions of monetite impurities.