Spinodal decomposition zone

Furthermore, for A = l/2-Amin, the term in brackets in Eqn. (12.30) has a maximum. Therefore, after a sufficient time of continuous spinodal decomposition, zones with the Amax periodicity will predominate. With other words, the decomposed solid solution exhibits periodicity in the direction, and the period length is Amax- min and max increase with strain, as can be seen from Eqns. (12.29) and (12.31). If the strain energy is high enough, Amax may become sufficiently large so that spinodal decomposition does not take place any more. 

Figure 3J2 Energy relationships between solvus and spinodal decompositions. (A) Portion of Gibbs free energy of mixing curve in zone between binodal (X ) and spinodal (X ) points. (B) Gibbs free energy variation as a consequence of compositional fluctuations around intermediate points X and X(2). ...

The presence and width of a metastable zone, in which nucleation is not spontaneous, have been discussed in Chapter 2. The thermodynamic limit of the metastable zone is a locus of points known as the spinodal curve, where spinodal decomposition replaces nucleation and crystal growth as the phase separation. In typical industrial crystallization, nucleation (and release of supersaturation) occurs at much lower supersaturations than the spinodal curve. 

An important quantity that can be calculated is the diffusion coefficient, D. As illustrated in Figure 3, nucleation and growth results in a positive diffusion coefficient, because diffusion of the separating component is measured as movement from the original concentration to the depleted zone ahead of the growing new phase domain. By contrast, spinodal decomposition diffusion is measured as the spontaneous movement from the original composition to a new, more concentrated phase. In terms of growth rate of the phase domain amplitude, R( ) and the diffusion mobility M,... 

Below ec, there are two dewetting mechanisms (a) a macroscopic film is metastable and dewets by nucleation and growth of dry zones (b) a microscopic film is unstable and spontaneously breaks into a multitude of droplets. Capillary waves are amplified and this mechanism is called spinodal decomposition, by analogy with what happens in phase transitions. 

Both the chemical reactions and the phase separation proceed under nonequilibrium conditions simply because they proceed simultaneously. After some degree of chemical conversion and cross-linking is reached, microphase separation is impeded and the system freezes in a nonequilibrium structure characterized by incomplete phase separation. Thus, by the completion of IPN formation, reactions proceed in two evolved phases. The real structure of an IPN is a multiphase one, which is determined by the coexistence of at least three phases (not in a true thermodynamic sense). Two phases are formed by networks due to phase separation. Each phase may be considered as an independent IPN in which phase separation did not take place (the state of forced compatibility), and in which mixing on the molecular level is preserved. The composition of these two phases is determined by the reaction rate and the temperature. Each phase has an average composition that does not correspond to the network ratio in the entire IPN. The third phase is the nonequilibrium transition zone from one phase to another its size depends on the conditions of phase separation. This zone may be called mesophase and may be considered as a nonequilibrium IPN of some transition composition, since the molecular level of mixing should also be preserved. For spinodal decomposition there is no sharp border between coexisting phases. The transition zone may be arbitrarily chosen in such a way that its composition corresponds to the average composition of the IPN. 

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