Amorphous atomic mobility

Crystals, however, are not always so perfectly ordered. Atomic mobility exists within the crystal lattice however, it is greatly reduced relative to the amorphous state. Partial loss of solvent from the lattice can result in static disorder within the lattice where the atomic positions of a given atom can deviate slightly within one asymmetric unit of the unit cell relative to another. Lattice strain and defects occur for many reasons. Solvents can be present within channels of the lattice in sites not described by the symmetry of the crystal structure itself, resulting in disordered solvent molecules or incommensurate structures and potentially nonstoichiometric solvates or hydrates. 

The discussion up to this point has focused on the role of free surfaces and internal interfaces, such as grain boundaries, in mass diffusion. Surfaces produced internally in the material as a consequence of permanent deformation and damage induced by stress can also serve, in some cases, as paths along which enhanced atomic diffusion may occur. In amorphous solids undergoing active plastic flow, such increased atomic mobility along shear bands can result in the formation of nanocrystalline particles locally at the bands. An example of such crystallization process is illustrated in this section for the case of a bulk amorphous metallic alloy subjected to quasi-static nanoindentation at room temperature. 

Point defects have a significant role in intermetallic compounds, as they control many properties of technological importance, such as atomic diffusion, high-temperature creep and other mechanical properties, sintering, behavior under irradiation, and in particular irradiation-induced crystalline-to-amorphous transitions. Introduction of point defects by irradiation has even allowed one to obtain an ordered phase (FeNi) in a system where it was hindered by the low atomic mobility (Neel et al., 1964 Koczak et ai, 1971). 

In disordered materials such as amorphous silicon, the mobility is so low that it would correspond to a mean free path lower than the distance between atomic sites, which is not physically pertinent. In a classical paper, Anderson [20 has shown that disorder in a solid may result in a localization of the states, in which case the one-electron wave function takes an exponential form... 

Ni3C decomposition is included in this class on the basis of Doremieux s conclusion [669] that the slow step is the combination of carbon atoms on reactant surfaces. The reaction (543—613 K) obeyed first-order [eqn. (15)] kinetics. The rate was not significantly different in nitrogen and, unlike the hydrides and nitrides, the mobile lattice constituent was not volatilized but deposited as amorphous carbon. The mechanism suggested is that carbon diffuses from within the structure to a surface where combination occurs. When carbon concentration within the crystal has been decreased sufficiently, nuclei of nickel metal are formed and thereafter reaction proceeds through boundary displacement. 

The flexibility of amorphous polymers above the glassy state, where segmental mobility is possible, is governed by the same forces as melt viscosity and is dependent on a wriggling type of segment motion in the polymer chains. This flexibility is increased when many methylene groups (—CH2—) or oxygen atoms (—O—) are present. Thus, the flexibility of aliphatic polyesters usually increases as m is increased ... 

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