Transmission Electron Microscopic Study on the Migration of Ca Ions in Ca-Deficient Hydroxyapatite during Thermal Decomposition

The migration of Ca ions during thermal decomposition of Ca-deficient hydroxyapatite (Ca-def HAp) involving the formation of β-TCP was discussed in the present paper. The migration of Ca ions at high temperature was examined by studying the thermal behavior of Sr ions in Sr-substituted Ca-def HAp using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (TEM-EDX). Substitutions of Ca ions in Ca-def HAp by Sr ion were carried out by hydrothermal treatment in Sr(NO3)2 solution. The composition of samples obtained by hydrothermal treatment was identified as Ca9.27Sr0.27(HPO4)0.46(PO4)5.54(OH)1.54. In addition, it was considered that Sr ions would be in Ca(II) site in Sr-substituted Ca-def HAp lattice. Since the thermal behavior of Sr-substituted Ca-def HAp was almost the same as that of Ca-def HAp, the migration of Sr ions in Sr-substituted Ca-def HAp during thermal decomposition could be regarded as the same as that of Ca(II) ions in Ca-def HAp. By TEM and TEM-EDS analysis, it was revealed that Sr ions were detected only in β-TCP phase after thermal decomposition. Therefore, these results suggested that β-TCP phase would be formed by the priority migration of Ca(II) ions in Ca-def HAp.