Initial Stages of Bulk Phase Formation

As the name implies, continuum mechanics is predicated on the hypothesis that one may describe the properties and behaviour of physical systems entirely in terms of continuous functions of position and time, at least for a single pure component within a single bulk phase (gas, liquid, or solid). Continuum mechanics makes no reference to the fact that real materials are composed of atoms or molecules. Strictly speaking, rationalization of transport coefficients (like viscosity, thermal conductivity, and diffusivity) in terms of molecular behaviour lies within the realm of statistical mechanics and molecular simulations. Continuum mechanics begins to lose its validity when the characteristic length and time scales in a physical system become comparable to molecular scales. Continuum mechanics thus cannot describe the initial stages of SEI film formation composed of many simultaneous, discrete, discontinuous molecular events. 

The kinetics of the initial stage of 3D Me-S bulk alloy formation process can be affected by nucleation and growth phenomena. A typical example is the formation of the fi phase of 3D Li-Al bulk alloy in the systems Al(polycrystalline)/molten Li, Cf and A1 (polycrystalline)/LP, 0104, propylene carbonate [3.345, 3.346]. In both systems, non-monotonous current transients were observed in the initial stage of alloy formation as shown in Fig. 3.66 [3.345]. 

Me UPD on foreign substrates S involving the formation of well-ordered 2D Meads overlayers, 2D Me-S surface alloys, and/or 3D Me-S bulk alloy phases in the underpotential (undersaturation) range represent the initial stage of Me deposition in systems with high Me-S interaction energy. 

Structure and orientation of a Me deposit on S in the initial stage of 3D Me bulk phase formation can be either independent of or influenced by the surface structure of S, which can be modified by 2D Meads overlayer formation and/or 2D Me-S surface alloy phase formation in the UPD range. Epitaxial behavior of 2D and 3D Me phases exists if some or all of their lattice parameters coincide with those of the top layer of S. The epitaxy is determined by a minimum of the Gibbs function at constant temperature and pressure. 

In a recent review Gasgnier (1980) has discussed the problem of the existence of Ln-monoxides in the bulk phase. He showed conclusively that the experimental results invoked to indicate the formation of solid state Ln-monoxides can either be attributed to bulk dihydrides or to partial substitution of nitrogen instead of oxygen in the sesquioxides (Felmlee and Eyring, 1968 Work and Eick, 1972). Thus, with the exception of EuO, the existence of solid Ln-monoxides has been denied. The question, however, arises what happens during the initial stages of rare earth metal oxidation, when kinetic effects dominate the behavior of the rare earth-oxygen system ... 

The results of this investigation indicate the formation of a product other than the Ca2Si03 phase in the initial stages of the reaction, possibly due to the presence of water adsorbed on the surface of the reactants. It is thought that the presence of water vapor promoted the formation of a metastable solid solution of calcium in quartz, as indicated by the electron microprobe results, the X-ray lattice parameter studies, and the optical micrographic evidence. Upon elimination of the bulk of the water the reaction proceeded to the formation of what is thought to be the Ca2Si04 phase. 

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