Transmission electron microscopy (TEM) was used toreported to decompose to tricalcium phosphate, Ca 3 (PO 4 ) 2 , and tetracalcium phosphate, Ca 4 O(PO 4 ) 2 , according to 16-21 characterize the microstructure of sintered undoped hydroxyapatite (OHAp). Conventional TEM observations were accomCa 10 (PO 4 ) 6 (OH) 2 → 2Ca 3 (PO 4 ) 2 ϩ Ca 4 O(PO 4 ) 2 ϩ H 2 O panied by high-resolution electron microscopy (HREM) and convergent-beam electron diffraction (CBED) studies. CBEDWhen sintering under moisture, OHAp also can be stable at analysis enabled the determination of the space group of the temperatures Ͼ1300ЊC, indicating that the sintering atmoOHAp, P6 3 /m. Densification of the material was performed sphere has a strong influence on the decomposition temperaby pressureless sintering at 1250؇C for 30 min. The undoped ture. 21 According to van Landuyt et al., 16 temperatures even sample was comprised of apatite as the only crystalline higher than 1300ЊC can be used during densification without phase, in addition to a small volume fraction (4%) of closed the decomposition of OHAp when the holding time is mainporosity. In general, the observed microstructure was tained below the incubation time for decomposition. The maxihomogeneous and consisted of equiaxed apatite grains with mum temperature-time cycle applicable without an appreciable an average grain diameter of }1-2 m. No indication of a amount of phase transformation is 1450ЊC for 3 h; at 1500ЊC, residual amorphous phase, which may have formed during the decomposition of OHAp has been shown to start after ϳ1 h.
sintering, was observed at multigrain junctions. HREMGrain boundaries in different ceramic materials, 22-27 as studies on grain boundaries also revealed that no intergranwell as ceramic-metal interfaces, 28 have been studied by highular glass film was present along two-grain junctions, indiresolution electron microscopy (HREM). Using the same techcating that densification proceeded without a liquid phase.nique, several structural defects have been recently detected A slightly disordered region at the interfaces was observed, inside human-tooth-enamel OHAp crystals. 29,30 In this paper, suggesting an extended grain-boundary structure.the nanostructure of an undoped (without the addition of sintering aids) sintered OHAp material also is characterized by HREM and convergent-beam electron diffraction (CBED). The