Vacancies migration properties

Jump reorientation models may involve activation over barriers to rotation or the migration of lattice defects or holes. Reorientation is in both cases discontinuous and changes in orientation occur-ing in one step are assumed to be large. Both types of jump reorientation models have been discussed by O Reilly [68], In his quasilattice random flight model, for example, O Reilly [69 70] assumes that the liquid structure up to the first coordination shell may be approximated by a lattice. Some of the properties of the solid state such as vacancies and translational diffusion by vacancy migration are considered present. In general difficulties arise when these jump reorientation models are compared with experimental data because several parameters are needed in the analysis. Furthermore, it appears that O Reilly [71] employs results obtained by Huntress [55] which apply only in the limit of small-step reorientation to treat the case of... 

In a few cases, vacancy migration energies have been determined by isothermal variations of a physical property measured during the thermal recovery of a quenched or electron-irradiated sample ... 

It was shown earlier that the NabaiTO-Hemirg model of creep in solids involved the migration of vacancies out of the stressed solid accompanied by counter-migration of atoms to reduce dre length of the solid in the direction of the applied stress. This property could clearly contribute to densification under an external pressure, given sufficient time of application of the stress... 

The diffusion coefficient for a given ion in a crystal is determined, as we have seen, by the atomistic properties of the ion in the structural sites where the vacancy (or interstitial) participating in the migration process is created (see eq. 4.71). The units of diffusion (and/or self-diffusion ) are usually cm sec . Pick s first law relates the diffusion of a given ion A (Jf) to the concentration gradient along a given direction X ... 

These simulation studies, based on either static lattice, MD or ab initio methods, have been able to provide deeper insight as to the fundamental solid state properties at the atomic level, particularly in the following key areas (a) defect chemistry and dopant-vacancy association, (b) mechanisms of oxygen ion migration, (c) structures and stability of surfaces (mainly (110) and (111)), (d) energetics of redox reactions... 

For ceramic materials, defects within the lattice are inextricably linked with transport properties. The diffusion of a cation in a ceramic, for example, involves the formation of vacancy or interstitial states within the crystal, and the migration of these species leads to a net transport of material through the lattice. These processes may be modeled by means of ion pair potentials in conjunction with the Mott-Littleton defect approach, direct molecular dynamics techniques, 24 or Monte Carlo methods to describe overall transport on the basis of calculated individual process statistics. 

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