title: Morphology and structural characteristics of hydroxyapatite whiskers: effect of the initial Ca concentration, Ca/P ratio and pH.
abstract: Hydrothermal homogeneous precipitation combines the best characteristics of the hydrothermal and homogeneous precipitation methods, and allows long and uniform hydroxyapatite (HA) whiskers, with a high aspect ratio and high crystallinity, to be obtained. Their morphology and structural characteristics depend on the initial Ca/P ratio (iCa/P) and pH (ipH), as well as the initial calcium concentration (i[Ca]). Variation in these values had no effect on constitution, which was crystallographically indistinguishable from HA. Ca/P ratio steadily improved with increases in both ipH and iCa/P, but was independent of i[Ca]. Uniform whiskers were obtained at high iCa/P and low ipH, or at high ipH and low iCa/P. Whiskers with a mean length of 96-140 μm and an aspect ratio of 96-136 were obtained at ipH=2-3 and iCa/P=1.67-2. At a low ipH and low iCa/P, irregular plate-like particles and branch-like whiskers were formed, while a high ipH favoured the formation of lath-like HA at high iCa/P. Preferred growth along the c-axis was more intense at higher iCa/P and ipH as well as at low i[Ca]. However, under these conditions, the crystal growth habit was also changed, showing preferred growth along both the c- and a-axes. The increase in whisker width over the general value obtained was abrupt at low i[Ca] and high iCa/P.
authors.affiliation: Department of Health Technology and Informatics, The Hong Kong Polytechnic University, Hong Kong SAR, China.
authors.forename: Hongquan
authors.initials: H
authors.lastname: Zhang
authors.forename: Brian W
authors.initials: BW
authors.lastname: Darvell
mesh_terms: D002118:Calcium / Q000737:chemistry*; D003460:Crystallization; D003461:Crystallography; D017886:Durapatite / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008855:Microscopy, Electron, Scanning; D010710:Phosphates / Q000737:chemistry*; D013816:Thermodynamics; D014961:X-Ray Diffraction / Q000379:methods
chemical_list: D010710:Phosphates; D017886:Durapatite; D002118:Calcium