High-Resolution Electron Microscopy and Convergent-Beam Electron Diffraction of Sintered Undoped Hydroxyapatite

Transmission electron microscopy (TEM) was used to characterize the microstructure of sintered undoped hydroxyapatite (OHAp). Conventional TEM observations were accom-panied by high-resolution electron microscopy (HREM) and convergent-beam electron diffraction (CBED) studies. CBED analysis enabled the determination of the space group of the OHAp, P6₃/m. Densification of the material was performed by pressureless sintering at 1250°C for 30 min. The undoped sample was comprised of apatite as the only crystalline phase, in addition to a small volume fraction (4%) of closed porosity. In general, the observed microstructure was homogeneous and consisted of equiaxed apatite grains with an average grain diameter of }1–2μm. No indication of a residual amorphous phase, which may have formed during sintering, was observed at multigrain junctions. HREM studies on grain boundaries also revealed that no intergranular glass film was present along two-grain junctions, indicating that densification proceeded without a liquid phase. A slightly disordered region at the interfaces was observed, suggesting an extended grain-boundary structure.