Structure transformation grain size effect

Frey MH, Payne DA (1996) Grain-size effect on structure and phase transformations for barium titanate. Phys Rev 6 54 3158-3168... 

As noted in previous chapters of this book, crystal structure parameters (coordination numbers, interatomic distances, unit cell volumes), characteristics of phase transformations, and physical properties of samples can depend on the size of crystal grains the detailed reviews of the size effect in different mechanical properties of nanomaterials can be found in [9,105]. In the present chapter it is shown that the size effect is especially significant for electrophysical properties of crystals, including the effect of dispersed powders on the properties of polar liquids contacting with them. 

Even though the simulation timescale accessible with atomistic MD is short, some authors succeed in modeling micelle formation starting from an initially random distribution of monomers [64]. Provided the ability to frequently repeat this computer experiment, that is, formation of an isolated micelle (or cluster), with varying number of monomers in the simulation cell, it is possible to observe the dependence of micelle shape on monomer concentration. Usually such simulations are carried out for coarse-grained molecules [65]. But even then equilibration is difficult. The collapse of randomly distributed monomers into a condensed structure may be fast however, the transformation of this structure into an equilibrium structure may be very slow. In addition, finite size effects may strongly influence the results, because often the characteristic dimensions of the observed structures usually are comparable to the size of the simulation volume. 

It may well be, however, that the colour of ferric oxide is determined by the size of the grain rather than by any variation in molecular structure.7 Thus, brown and violet samples of ferric oxide are converted into the yellowish red variety by alternate grinding and washing —processes which, in view of the chemical stability of ferric oxide, are hardly likely to effect a molecular transformation. 

Thus, as indicated above, in the submicron-sized 3Y-TZP ceramic, the stress-induced cyclic hardening, due to transformation taking place, was higher than under static deformation. NanocrystaUine 3Y-TZP softened cyclically, due to the formation of a large number of microcracks. In the submicron structures, this observation basically reflects the effects of dislocations and dislocation-dislocation interactions. In the nanocrystalline 3Y-TZP ceramic, this greater ability to accommodate plastic strain is probably due to grain-boundary sliding, since in nanocrystalline structures dislocations cannot move, because shp distances are on an atomic scale (hke the dimensions of dislocations themselves). 

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