Volume nucleation

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. 

In Section 1.5.1, the crystallization of non-steady-state processes will be examined using mica glass-ceramics. The relationship between the observed non-steady-state time lag and phase separation will be demonstrated. The basic relationship between nucleation and microimmiscibility will be discussed in the following section. 

An epitaxial effect based on similar lattice parameters (McMillan 1979) presented a theoretical explanation for the favorable nucleating action of metals. Furthermore, mechanical strain was present at the substrate-glass interface, producing a high interfacial energy, as the coefficients of thermal expansion of the metal and the new nucleus were substantially different. As a result, catalyzation of nucleation could also be expected. 

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. 

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Transfomation from a meta-stable phase, such as supersaturated solution, to a thermodynamically more favorable phase requires first the crystal nucleation of a germ of the new phase. According to the classical nucleation theory, the volume nucleation rate J (cm" sec ), describing the number of nuclei(i.e., a critical germ) formed per volume per time, is given by ... 

If we are interested in the nucleation of apeu-ticle prior to completing the solid state reaction, we need to distinguish between surface and volume nucleation of the particle, since these are the major methods of which we can perceive. Several cases are shown in the following diagram. 

The growth kinetics describes the nucleation processes on the atomic scale. Thermally activated processes as adsorption, desorption, and diffusion at the surface and in the volume, nucleation, and crystallization/ recrystallization determine the film structure and can be controlled by the substrate temperature and the growth rate. Using a diagram ln(J ) over 1/ T, R being the deposition rate and T the growth temperature, three different growth modes (epitaxial, polycrystalline, and amorphous) can be... 

Surface nucleation. Thus far, we have only considered volume nucleation, implying that the number of catalytic impurities in a droplet is proportional to its volume. Another possibility is nucleation at the inside of the droplet boundary, if that surface is catalytic for nucleus formation. This is called heterogeneous surface nucleation. An example of the resulting relation between ymax and Tc is given in Figure 14.9b, curve S. Over a very... 

A curve as steep as shown in the figure is, however, not very common. It needs a pure surfactant giving a fully packed interfacial layer, and it works best at temperatures below the chain crystallization temperature (see Section 10.3.1). Most surfactants are mixtures (or are at least impure), or the concentration is too small to obtain a plateau value of the surface load. In such cases, the interfacial layer may contain patches that are catalytic for nucleation. This then means that the droplets contain catalytic impurities, but their number would be proportional to droplet surface area rather than volume. In the simplest case, Eq. (14.14) is to be modified by replacing vAcat by A at7td 3/4, where the superscript S indicates that the number is per unit area. The curves obtained then are rather similar to those for volume nucleation, and it needs painstaking experiments to determine which of the two mechanisms prevails. 

Most apatite glass-ceramics have been developed as biomaterials. This section, however, addresses biocompatible glass-ceramics without bioactivity. Volume nucleation and crystallization are used to produce these glass-ceramics. In addition, phase separation processes in the base glass are important for nucleation. 

Glass-ceramics with a leucite main crystal phase (Section 2.2.9) are also produced according to the mechanism of controlled surface crystallization of the opal base glass since volume nucleation cannot be controlled. In this case, it is important for the crystallites to achieve a high nuclei density and to uniformly precipitate into the glassy matrix. The coast-and-island microstructure has been specifically developed as the transitional stage (Holand et al., 1996b). 

The idea to produce ceramic-like materials with a fine microstructure by controlled devitrificaton of base glasses was soon extended to procedures other than the controlled volume nucleation and crystallization of base glasses. Relatively fine-grained glass ceramics can also be obtained by sintering and crystallization of glass powders to dense bodies. 

R. S. Gordon, University of Utah) I am curious about these models by Avrami. Do you have to make the assumption that this is a volume nucleation or can it be a random nuclea-tion on a surface If it is a volume nucleation, you undoubtedly have problems getting the gases away, and then how applicable are these models for a thermal decomposition reaction ... 

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