Characterization of the structure of sol-gel-derived hydroxyapatite thin films.

摘要

The synthesis and processing of hydroxyapatite (Ca₁₀[PO 4]₆[OH]₂, HA) thin films was investigated. Films 10 to 900 nm thick were synthesized using solution chemistry sol-gel techniques and processed under various conditions. Analytical chemistry techniques (NMR, TLC, titration) were used to study the initial reactions. It was found that several key sol-gel synthesis steps were either absent (hydrolysis) or minimally present (network formation). These results are consistent with the findings that the films have little skeletal structure, but are composed of crystalline aggregates and inter-granular pores. Ion-implantation using silicon ions was used to densify the films from a density of 36% (of fully dense HA) to 83%. High temperature annealing was also used to densify the films, and 40% densification could be achieved after firing at 800°C. While larger densification can be reached at higher temperatures, the film transforms from HA to other calcium phosphate phases. The kinetics of transformation from the amorphous gel state was studied using X-ray diffraction (XRD), and Time-Temperature-Transformation (TTT) curves were constructed. These curves demonstrated that the transformation kinetics are extremely rapid and likely related to the short chain phosphate groups (dimers and trimers) present in the amorphous gel. Thus, high temperature sintering, even using rapid heating rates, can not be used to achieve high densities. A new technique, solute impregnation, was developed to further characterize the porosity of the films. The results of the solute impregnation tests demonstrated that (a) the films have an open pore structure and (b) high temperature firing and ion implantation decreased the porosity of the films. Studies with transmission electron microscopy demonstrated that the porosity is due to inter-granular pores, and that the grains were essentially fully dense, a result confirmed by pycnometry. These results are consistent with the observation that the initial solid lacks a strong skeletal structure. Infrared spectroscopy and solute impregnation measurements demonstrated that film properties such as crystallinity and porosity are dependent on film thickness. This study indicates that, while HA thin film properties are dependent on the initial synthesis phase, subsequent film processing can be used to control HA density and crystallinity.