Development of Glass Ceramics

Low-expansion glass ceramics are widely used as precision parts (see Chap. 4), cooktop panels, stove windows, and cookware. In this section we will focus on the development of transparent glass ceramics used as stove windows (colourless) and cooktop panels (tinted) glass ceramics for cookware are also briefly discussed within this section. 

Glass ceramics are polycrystalline solids formed from suitable parent glasses by a controlled heat treatment, the so-called ceramization. The glass ceramics consist of varying crystalline phases and a residual glass phase, depending on the base glass composition. 

Property Field of application Cooktop panel Stove window Cookware 

Depending on the application of low-expansion glass ceramics - we are now focussing on tinted cooktop panels and colourless stove windows - further physical and chemical properties are required from both the production and the user side. To lower production costs, fast and effective processing is required, for example, tank melting, on-line hot forming of panels, as well as fast nucleation rates and crystal growing are absolutely necessary. Moreover, the application of decoration, which, for example, indicates the hot zones of a final cooktop panel, should be performed at the same time as ceramization takes place, without any influence on the production quality. 

Furthermore, the glass ceramic used as a cooktop panel has to meet transmission specifications it has to be nearly opaque in the visible spectrum to avoid a direct view to the electric heating elements of the cooktop assembly. On the other hand, the glass ceramic should have a high transmission for infrared radiation to support the overall heating performance. 

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Although modulus of rupture is a useful comparative measure if surface conditions are specified, fracture toughness is a more useful property, especially for the development of glass ceramic compositions and crystallization heat treatments. 

Other work reviewed by Prewo and co-workers [105-108] includes the development of glass ceramic matrices with significantly improved properties. 

Kawai, S. (1999). Development of glass-ceramic material for multilayer substrate in submillmeter-wave application, Proceedings of 12th Fall Meeting of the Ceramic Society of Japan , p. 397. 

The processes in question are thus of a highly complex nature. A comprehensive mathematical theory describing all the diflFerent processes does not yet exist. As a result, with todays standard of knowledge concerning the development of glass-ceramics, nucleation is assumed to be influenced by two neral factors ... 

The successful development of glass-ceramics is thus dependent on highly controlled synthesis procedures and thorough analysis of phase development processes with high-resolution analytical methods (e.g., electron microscopy, X-ray diffraction, thermal and chemical methods). 

Reference will be made to additional theoretical work. By describing the kinetic processes, criteria are compiled that will be of practical use in the development of glass-ceramics. Several examples of specialized glass-ceramics will be used to illustrate these findings. 

During synthesis and experimentation in the development of glass-ceramics, the determination of N—t functions and comparison with the model functions in Fig. 1-29, for example, are of importance in trying to establish the mechanism involved. Determination of the non-steady-state time lags may be of particular significance. 

Predominantly simplified model glasses with stoichiometric compositions were used to test the different nucleation theories. Multicomponent glasses were examined in few cases. Thus simple glass systems that are significant for the development of glass-ceramics will be discussed at this stage. 

In the development of glass-ceramics, two mechanisms are generally used volume and surface nucleation. The mechanism of volume nucleation will be examined in detail. The role of non-steady-state processes, phase separation reactions, and heterogeneous nucleating agents are critical. Surface nucleation is evaluated for controlling nucleation processes. 

The mechanism of volume crystal nucleation is predominantly applied in the development of glass-ceramics. The development of the first glass-ceramic by Stookey (1959) demonstrated how crystals of uniform size can be precipitated in the glass matrix with controlled volume nucleation. 

These different types of crystallization together have permitted the development of glass-ceramics with special microstructures and previously unknown properties. 

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. 

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. 

The development of glass-ceramics with a barium disilicate primary crystal phase was conducted in two diflFerent ways. The first type of glass-ceramic was produced from base glasses with a composition almost corresponding to the stoichiometric composition of barium disilicate. The second type of glass-ceramic with sanbornite crystals was developed from multicomponent glasses using controlled crystallization. 

The most important and economically most significant developments of glass-ceramics from the Si02-Al203-Zn0-Mg0 system were achieved by Pinckney and Beall (1997) and Beall and Pinckney (1999). The objective of their research involving the Si02—Al203-Zn0-Mg0 system was to develop a... 

As stated at the beginning of this book and supported by a number of examples, controlled crystallization of several main crystal phases has been shown to permit the combination of different properties in the development of glass-ceramics, according to special requirements. 

Metaphosphate glasses are of particular interest in the development of glass-ceramics and glass-ceramic fibers (Griffith 1995) by the controlled crystallization of phosphate base glasses. These glasses are composed of (PO ) tetrahedra chains. In chemical terms, these structural units must be called condensed phosphates. 

According to Beall and Pinckney (1999), three important criteria must be observed in the development of glass-ceramics with zero expansion. The first two requirements involve low scattering, while the third requirement involves low ionic or atomic absorption. First, the index of refraction of the glass and the crystal phase must be almost identical. Second, the crystallite... 

In the development of glass-ceramics of the P-quartz solid solution type, all three requirements were met similarity of the index of refraction between the glass matrix and the crystals, very small particle size, and low absorption. The solid-state reactions that are involved in the formation of P-quartz solid solution glass-ceramic are presented in Section 2.2.2. The chemical composition of different products are also discussed in this section. The special nanostructure of this glass-ceramic is described in Section 3.2.1. 

These requirements have been met in the development of glass-ceramics. An effective production process in technical dimensions has been developed. Furthermore, most of the required properties have also been achieved. 

These desired applications determine the main requirements to be fiilfilled in the development of glass-ceramics for dental applications. The main objective is to produce a new biomaterial, the properties of which correspond to those of natural teeth. The most important properties are mechanical properties, biochemical compatibility with the oral environment, and a degree of translucency, shade, opalescence, and fluorescence similar to that of natural teeth. An abrasion resistance similar to that of natural teeth must also be achieved. The new biomaterial must demonstrate higher chemical durability than natural teeth, to prevent it from being susceptible to decay. 

Because of the unique property achievable with low-expansion glass ceramics this material class attracted most attention at the beginning of the development of glass ceramics but of course many researchers tried to apply the basic ideas of the formation of glass ceramics to other composition fields, and often they were very successful. Nowadays there is no principal reason why the glass ceramic approach could not work in other composition fields, although the specific details have to be worked out for each field separately. 

Very soon, i.e., just at the very beginning of the development of glass ceramic cooktops, it became evident to Schott that delivery and optimization of only the glass ceramic panel itself - without any assistance to our customers - would not be sufficient to achieve success in the market for this success all compoments/parts of the cooktop - as presented in Fig. 3.2 (and in even more detailed with its surroundings in Fig. 3.3 ) - had to be developed and optimized in perfect harmony with each other. This labourious task was performed in cooperation with our customers, the heating element manufacturers, as well as with cookware and cleaning material manufacturers. 

To give the reader a view of the extraordinary material glass ceramic , the volume opens with a general survey of the development of glass ceramics and their important fields of application and the aims, limits, and the current state of new developments. 

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