Glass-ceramics theory

Note that in general the nitrides and carbides of Si, with their lower thermal expansion coefficients, are more resistant to thermal shock than oxides. In theory, a material with zero thermal expansion would not be susceptible to thermal shock. In practice, a number of such materials do actually exist commercially, including some glass-ceramics that have been developed which, as a result of thermal expansion anisotropy, have extremely low a s (see Ch. 4). Another good example is fused silica which also has an extremely low a and thus is not prone to thermal shock. 

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 ... 

Findings from the theories of nucleation have also contributed to the optimization of these comprehensive experimental investigations. As a result, glass-ceramics with improved properties have been produced, specifically glass-ceramics with specific solid-solution limits like j3-spodumene and stuffed P-quartz or with stoichiometric compositions like cordierite and lithium disilicate. Initial results for the successfU application of the theory of nucleation to multicomponent glass-ceramics with a nonstoichiometric composition were achieved with mica and anosovite glass-ceramics. 

Weinberg (1992a), Weinberg et al. (1997), and Zanotto (1997), reported in detail on transformation kinetics via nucleation and crystal growth. The standard theory of this type of phase transformation kinetics was developed by Johnson and Mehl and Avrami and Kolmogorov (see Weinberg et al., 1997). Therefore, this theory is called the JMAK theory. The JMAK equation (Eq. 1-6) is universal and applicable to glass-ceramics. 

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. 

Controlled crystal growth also involves a functional relationship between heat treatment and the properties of the glass-ceramic such as the linear thermal expansion coefficient and optical properties. By using fundamental theories of optical spectroscopy on P-spodumene... 

In the following, a survey is given about the phenomenon of nucleation including a short review of the historical development of investigations of nucleation. (Extensive presentations of the theory of nucleation have repeatedly been described in the literature [2.40,41].) Following this, the determination of nucleation kinetics in a parent glass of a commercial lithia alumino-silicate glass ceramic will be presented. (For references to this topic see [2.42-44].)... 

The control of the size of the particles in glass-ceramics is fundamental as it plays a key role in the transparency of the glass-ceramics. Indeed, according to Rayleigh s theory, the intensity of light scattered by a particle follows the relation ... 

Materials science has in its time suffered a great deal of the second type of criticism. Thus Calvert (1997) asserts that metallurgy remains a proper discipline, with fundamental theories, methods and boundaries. Things fell apart when the subject extended to become materials science, with the growing use of polymers, ceramics, glasses and composites in engineering. The problem is that all materials are different and we no longer have a discipline. ... 

Ernsberger, F.M. (1963) Current status of the Griffith crack theory of glass strength, in Progress in Ceramic Science, vol. 3, ed. Burke, J.E. (Pergamon, Oxford) p. 58. 

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