Transmission electron microscopic study of tetracalcium phosphate surface-treated in diammonium hydrogen phosphate solution

The present study investigates the changes in microstructure and microchemistry (particularly Ca/P ratio) during whisker formation on the surface of a monolithic TTCP powder in a basic phosphoric acid solution. The XRD results indicate that when TTCP powder was treated for 10 min or less in (NH ₄) ₂HPO ₄ solution, the monolithic TTCP phase remains unchanged. When treated for 30 min, apatite whiskers appear, which continue to grow with treating time. When treated for 24 h, the apatite phase becomes dominant. TEM results show that on the surface of 1 min-treated TTCP are observed a large amount of globular-shaped fine particles which are substantially amorphous with an average Ca/P ratio of about 1.2. When treated for 10 min, whiskers primarily having a crystal structure of TTCP with an average Ca/P ratio of about 2.1 are observed to grow radially from TTCP surface. With time the whiskers continue to grow in length and width. The whiskers on 24h-treated surface become Ca-rich apatite with an average Ca/P ratio of about 1.8. Both TTCP and apatite whiskers are essentially non-stoichiometric in chemistry.