Variation in crystallinity of hydroxyapatite and the related calcium phosphates by mechanical grinding and subsequent he
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CIUM hydroxyapatite (HAp) and the related calcium phosphates have been of considerable interest as biocompatible materials for bone replacement, because they are the dominant mineral constituents of bone, teeth, and their related components.[1,2] Synthetic HAp ceramics are the most promising candidates for bone graft, but the mechanical and biochemical properties are different from those of biological apatite. Moreover, rapid substitution of human tissue by the HAp ceramics cannot be expected through remodeling for hard tissues, because of its low solubility. One of the most significant differences between synthetic and biological apatites is their crystallinity, such as crystallite size, strain, and/or perfection.[1–4] The crystallinity also differs in human hard tissues; for example, human enamel and synthetic apatite ceramics show higher crystallinity than human dentin and bones. Since the solubility of apatites is closely related to the crystallinity, control of the crystallinity is important for improvement of their dissolution in human body. Crystallinity control is often achieved in a wet fabrication process, depending on temperature, additional element, and pH.[5] However, it is difficult to control the crystallinity in the calcium phosphates facilitated in the drying process. In TAKAYOSHI NAKANO, Associate Professor, and YUKICHI UMAKOSHI, Professor, are with the Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-Oka, Suita, Osaka 565-0871, Japan. Contact e-mail: nakano@ mat.eng.osaka-u.ac.jp ATSUYUKI TOKUMURA, Engineer, is with the Daihatsu Motor Co., Ltd., Osaka 563-0044, Japan. Manuscript submitted April 30, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
contrast, mechanical grinding (MG) is known to decrease the crystallinity of metals in the powder condition.[6] This method is expected to introduce a large amount of plasticstrain energy and lattice defects and/or to create a nonequilibrium phase in powders. Indeed, the crystallinity of HAp was reported to decrease effectively during MG.[7] In this study, MG and a subsequent heat treatment were performed on HAp and the related calcium phosphates such as carbonate-apatite (CO3AP), fluorapatite (FAp), ␣ - and  -tricalcium phosphates (␣ -TCP and  -TCP, respectively), and tetracalcium diphosphate monoxide (TTCP), focusing on the morphology and mechanical/thermal stability of their powders. II. EXPERIMENTAL PROCEDURE The CO3Ap (HAP-200), FAp, ␣ -TCP,  -TCP, and TTCP powders were supplied by Taihei Chemical Industrial Co., Ltd. (Naro, Japan), and the HAp powder was provided by Sumitomo Pharmaceuticals Co., Ltd. (Osaka, Japan). The HAp, CO3Ap, and FAp powders were synthesized using mixed slurries in the wet process, while ␣ -TCP,  -TCP, and TTCP were obtained as sintering products by heating at 1300 ⬚C, 900 ⬚C, and 1450 ⬚C, respectively. The details can be seen in References 8 and 9. It should be noted that the CO3Ap used in this study was HAp partially replaced by CO32⫺ ions. Each powder was mec
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