Variation in crystallinity of hydroxyapatite and the related calcium phosphates by mechanical grinding and subsequent heat treatment

Abstract

The crystallinity of hydroxyapatite (HAp) and the related calcium phosphates for regenerating hard tissues was controlled by the mechanical grinding (MG) method and subsequent heat treatment. The HAp, carbonate-apatite (CO3Ap), fluorapatite (FAp), and α- and β-tricalcium phosphates (α-TCP and β-TCP, respectively) and tetracalcium diphosphate monoxide (TTCP) were used as initial materials. Variations in crystallinity and crystal structure were examined by the X-ray diffraction (XRD) method during MG and the following heat treatment. The crystallinity, based on crystallite size and crystal elastic strain, decreased with grinding time, and the decreasing rate depended on the type of calcium phosphate; crystallographic diffraction peaks disappeared more rapidly in CO3Ap than in FAp. The change in the morphology of powder during MG was influenced by the primary particle size of the first-stage product; α-TCP, β-TCP, and TTCP powders composed of large particles were predominantly shattered into small pieces and then gathered during MG, while the crystal strain in the HAp, CO3Ap, and FAp powders was mainly accumulated without significant refinement of crystallite size. The thermal-recovery process of crystallinity and crystal structure in the milled powders was investigated. The crystallinity of HAp, CO3Ap, and FAp powders recovered depended on annealing temperature. The novel phase of β’-TCP with higher ordering than β-TCP appeared during heat treatment from the amorphous state of α-TCP or β-TCP obtained during MG. The MG and subsequent heat treatment were, finally, concluded to be an effective process for controlling the crystallinity and changing crystal structure in calcium phosphate powders.