Melting multicomponent system

Diffusion of ions can be observed in multicomponent systems where concentration gradients can arise. In individnal melts, self-diffnsion of ions can be studied with the aid of radiotracers. Whereas the mobilities of ions are lower in melts, the diffusion coefficients are of the same order of magnitude as in aqueous solutions (i.e., about 10 cmVs). Thus, for melts the Nemst relation (4.6) is not applicable. This can be explained in terms of an appreciable contribntion of ion pairs to diffusional transport since these pairs are nncharged, they do not carry cnrrent, so that values of ionic mobility calculated from diffusion coefficients will be high. 

Uphill diffusion of some components is reported in silicate melts (e.g., Sato, 1975 Watson, 1982a Zhang et al., 1989 Lesher, 1994 Van Der Laan et al., 1994). Recall that uphill diffusion in binary systems is rare and occurs only when the two-component phase undergoes spinodal decomposition. In multicomponent systems, uphiU diffusion often occurs even when the phase is stable, and may be explained by cross-effects of diffusion by other components. 

In multicomponent systems, compounds are frequently formed between components. The following phase diagrams are for ABC ternary system forming a binary compound AB which melts congruently, as it is stable at its melting point ... 

Valuable information on the corrosion process is provided by phase diagrams if they are available for the given system. They show whether the substances in question actually react mutually producing a melt, what is the respective saturated concentration (equilibrium melt composition), and if any new products are formed. However, phase diagrams are often not available for multicomponent systems the other characteristics determining the corrosion rate are usually also unknown. The resistance of refractories to corrosion is therefore in practice determined by tests providing useful Complex information which, however, holds exactly only for the test conditions. 

New data are presented for the P-T traces of the four phase line representing the melting point depressions in two ternary systems. These data are valuable for testing and development of predictive models of melting point depressions in multicomponent systems. 

This work demonstrates that binary data alone do not necessarily indicate some important aspects of the phase behavior, such as melting point depressions of multicomponent systems involving high pressure gases and supercritical fluids. 

A review of melting and solidification of single-component systems follows, as well as a discussion of the multicomponent systems. A more extensive treatment is given by Kaviany [7]. 

Rates of Diffusion. When the solid and liquid of a multicomponent system are in thermodynamic equilibrium, the composition of the solid wul usually differ from that of the liquid. When the system is submitted to further melting or crystallization, the composition of at least one of the phases will change in the vicinity of the contact surface. Diffusion tends to equalize the concentration differences occurring both in the solid and in the liquid phases and should, therefore, be promoted. 

A simple classification scheme of solids is given in Fig. 7.1. In order to differentiate between the types of solids, we have to consider the Gibbs phase rule, which is discussed in any physical chemistry textbook. The basic question is whether the solid substance consists of only one chemical entity (component) or more than one. Usually the component is one molecular unit, with only covalent bonded atoms. However, a component can also consist of more constituents if their concentration cannot be varied independently. An example of this is a salt. The hydrochloride salt of a base must be regarded as a one-component system as long as the acid and the base are present in a stoichiometric ratio. A deficiency of hydrochloric acid results in a mixture of the salt and the free base, which behave as two completely different substances (i.e. two different systems). Polymorphic forms, the glassy state, or the melt of the base (or the salt) are considered as different phases within such a system (a phase is defined as the portion of a system that itself is homogeneous in composition but physically distinguishable from other phases). When the base (or salt) is dissolved in a solvent, a new system is obtained this is also tme when a solvent is part of the crystal lattice, as in the case of a solvate. Thus, each solvate represents a different multicomponent system of a compound, whereas, polymorphic forms are different phases. The variables in the solvate are the kind of solvate (hydrate. 

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