Lithium disilicate glass-ceramics

Since 1959, however, the principle of heterogeneous nucleation with metals has been successfiilly applied in the development of only a few glass-ceramics. To produce lithium disilicate glass-ceramics, McCracken et al. 

As mentioned in the History section, lithium disilicate glass-ceramic was the first glass-ceramic that Stookey (1953, 1959) developed. The fimdamen-tal research conducted by Stookey provided a basis for the large-scale development of glass-ceramics in a variety of chemical systems. Furthermore, other materials systems based on lithium disilicate have been also developed according to his findings. 

Barret and Hench (1980) and Wu (1985) improved the chemical durability of lithium disilicate glass-ceramics to a significant extent by incorporating additions such as AiPj and K2O to the stoichiometric base glass. The objective of improving the chemical durability of this glass-ceramic was to render the material suitable for use as a biomaterial in human medicine and, in particular, as a restorative material in dentistry. 

It must be noted that a significant improvement of the chemical durability of lithium disilicate glass-ceramics was achieved later in the development of glass-ceramics with nonstoichiometric compositions. 

Lithium disilicate glass-ceramics demonstrate a relatively high linear coefficient of thermal expansion of approximately 105 x 10 K h This property is favorable for the fabrication of special composite materials, e.g., for sealing to metal substrates in the electrical industry (Beall 1993). 

Beall (1993) and Echeverrfa (1992) achieved notable results in the development of a new lithium disilicate glass-ceramic. The new material is distinguished by the following three characteristics ... 

Using a special hot-press procedure, Schweiger et al. (1998) and Frank et al. (1998) also developed a powder-processed lithium disilicate glass-ceramic. To optimize the viscous properties for the hot-press procedure at approximately 920°C, components such as La2 3 hlgO, and pigments were added to the main components Si02, ZnO. The fabrication... 

The preferential crystallization mechanism is that of volume crystallization. However, surface reactions cannot be neglected when considering crystallization and nucleation in powder compacting and subsequent sintering and crystallization. In these processes, water has a special effect on the production of lithium disilicate glass-ceramics, as demonstrated by Helis and Shelby (1983) and Davis (1997). 

In a study on the effects of oxidation on crystallization, Keding and Russel (1997) found that reduction produced the ion, which acted as a nucleating agent. Russel (1997) managed to control the orientation of the main crystal phases in a specific axial direction in various glass-ceramic systems. The fresnoite system as well as apatite glass-ceramics and lithium disilicate glass-ceramics were particularly suitable. 

Figure 3-25 EGA of a lithium disilicate glass-ceramic powder compact in comparison to DTA.

To meet this demand, lithium metasilicate/lithium disilicate glass-ceramics of the Fotoform /Fotoceram type hx>m Corning Glass Works were used. The traditional manufacturing process for Fotoform /Fotoceram products was further developed by Borrelli and Morse (Beall 1993) and specially adapted. 

The development of the lithium disilicate glass-ceramic, its mechanisms of controlled crystallization, as well as the microstructure formation, and its properties are described in Section 2.1.1. The application of the glass-ceramics in high-precision equipment components and in the electrical industry is addressed in Section 4.1.2. 

McCracken W.J., Clark D.E., and Hench L.L., "Aqueous Durability of Lithium Disilicate Glass-Ceramics," Ceram. Soc. Bull., 61, 1218-29 (1982). 

Figure 5. Edge chip results for a lithium disilicate glass ceramic (e.max CAD).

If defbrmadon processes dominate the edge chipping process, the az term in eq. 8 should dominate. This is confirmed by outcomes shown in Table 2, and Figures 5, 6, and 8. The corresponding power law fit gives exponents between 1.6 to 1.8 for the lithium disilicate glass ceramic (Fig. 5), the two filled-composite materials (Fig. 6) and the two laminated ceramic composites (Fig 8). 

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