Crystallites plastic deformation

Increases in rms microstrain (e) with specimen elongation suggests that the crystallites are deforming plastically. Dislocation density in the PTFE is proportional to (s ). At the highest elongation studied, this density is of the order of IO " cm-2.19... 

Another technique used for obtaining macroscopically polar films involves mechanical extension of the material. Uniaxial plastic deformation induces a destruction of the original spherulitic structure into an array of crystallites in which the molecules are oriented in the deformation direction. In case of PVF2 when such deformation takes place below 90 °C the original tg+ tg chains are forced into their most extended possible conformation which is all-trans [32]. 

During milling, the fragmentation of polycrystalhne material and primary crystallites proceeds until the fragmentation limit is reached. The particles undergo mainly plastic deformations and accumulate defects, which leads to an amorphization of the solid. 

The top-down approach starts with a bulk material and attempts to break it down into nanoscaled materials through physical methods. Hence, most of these techniques are really forms of fabrication rather than synthesis. For nanostructured bulk phases, including powders, the common methods are milling, devitrification of metallic glass, and severe plastic deformation. For nanocrystalline thin films (films with nanosized crystallites), methods include thermal vaporization (under high vacuum), laser ablation, and sputtering (thermal plasma), all of which were... 

As shown schematically in Figure 13.4c, the strain can change among different crystallites, e.g. as a consequence of plastic deformation in an elastically anisotropic medium, but can also vary across each crystallite, e.g. because of the presence of dislocations (the two terms are sometimes referred to as strain of the second and third kind, respectively). Notably, a strain broadening effect can be observed even if the macrostrain (mean value of the strain distribution) is zero (Figure 13.4c), as in a powder sample. Otherwise, the simultaneous effect of macrostrain and microstrain results in a shift and broadening of the diffraction profile (Figure 13.4d). 

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