Apatite-leucite glass-ceramics crystallization

After nucleation in the apatite-leucite glass-ceramic, however, the apatite grows anisotropicaily in a preferred orientation into needlelike apatite (Holand et al., 1995). Previously, these crystals had only been produced under hydrothermal conditions (Newesely 1972 Jaha et al., 1997). Their morphology corresponds to that of the hydroxyapatite in natural teeth (enamel). 

Figure 2-33 Number (N) and length (L) of needlelike apatite crystals as a function of time for apatite-leucite glass-ceramics.

As reported the mechanisms of the nucleation and crystallization of needlelike apatite in apatite-leucite glass-ceramics is very difficult. Therefore, the mechanism was investigated in detail and the main results can be explained by the following two investigations. 

Figure 2-40 Crystal formation in apatite-leucite glass-ceramic (A NaCaP04, B Na2Ca4(P04)2Si04, C KAISigOg, D unknown phase, E C g PO JF.

Figure 2-41 SEM image (etching 10 sec, 3% HF) of the microstructure of apatite-leucite glass-ceramic for dental restorations. Heat treatment of the glass powder at SSO C/I h and 1050°C/1 h. The apatite crystals measure approximately 0.1-0.5 and 1-2 pm in diameter. The leucite crystals measure approx. 2 pm.

A typical apatite-leucite glass-ceramic is presented in Section 2.4.6. This section shows that the characteristic part of phase formation in the glass-ceramic is the controlled crystallization of the leucite and apatite crystal phases. The formation of the crystals and the typical microstructure of the glass-ceramic with the needlelike apatite crystals are described in Sections 2.4.6 and 3.2.12. 

Szabo I., Barnab S., Volksch G., and Holand W, "Crystallization and Color of Apatite-Leucite Glass-Ceramic," Glastech. Ber. Glass Sci. TechnoL, 73 [Cl] 354—57 (2000). 

This crystal growth mechanism is linked to surface nucleation. The success-fill application of the mechanisms of surface nucleation and crystallization in the development of cordierite (Semar and Pannhorst, 1991), apatite-wollastonite (Kokubo 1991), and leucite glass-ceramics (Holand et al., 1995a) has already been mentioned in Section 1.4. 

Heat treatment of glass powders results in the precipitation of leucite from the surface of the glass-ceramics and the volume crystallization of needlelike apatite. 

Figure 3-8 SEM image showing dendritic surface crystallization of leucite in leucite-apatite glass-ceramics at 900°C, heat-treated for 1 h, etched for 10 sec with 3% HF (Haiand et al., 2000c).

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