Slip systems geometry

More recently Mackwell, Kohlstedt, and Paterson (1985) studied the deformation of single crystals of San Carlos (Arizona) olivine deformed under hydrous conditions at 1,300 C, 300 MPa confining pressure, and 10 s strain-rate and found they were a factor of 1.5-2 weaker than those deformed in an anhydrous environment. TEM observations showed that specimens deformed under dry conditions, in an orientation such that the slip systems (001)[100] and (100)[001] would be activated, were characterized by a microstructure of generally curved dislocations and dislocation loops, but no organization into walls. The dislocation density was 10 -10 cm compared with an initial value of < 10 cm . Most of the dislocations and the loops lie approximately in the (010) plane because they are in contrast for g = 004, they probably have b = [001] dislocations with b = [010] and [100] would be out-of-contrast for this reflection. However, the slip system (010) [001] is not expected to be active. It is not clear, therefore, if these dislocations are actually involved in the deformation. The general geometry of the dislocation microstructure is not inconsistent with some climb mobility in fact, on the basis of the observations of Phakey et al. (1972), climb is certainly expected at 1,300°C. 

The photophysics of these six metallacycles has been studied in chloroform [71]. As expected for weakly interacting systems, the absorption spectra of the homonuclear species 9,10 and 9Zn, lOZn are very similar to those of the parent free-base and zinc-porphyrin chromophores in the Q-band region (Fig. 22), except for minor spectral shifts. A prominent difference between the planar and the slipped cofacial macrocycles is found in the Soret band region, in which a clear exciton splitting (of ca. 500 cm ) is present only for the latter compoimds (10 and lOZn). This result is as expected on the basis of the relative center-to-center distance in the two types of metallacycles (10.1 A in the slipped cofacial geometry as compared to 14.1 A in the planar one). The photophysics of the homo-dimers is very similar to that of the corresponding monomeric species. In particular, 9 and 10 exhibit the typical fluorescence of the free-base or zinc-porphyrin units (9 A. ,ax = 655, 716 nm, T = 5.7 ns 10 Amax = 656, 716 nm) and 9Zn and lOZn that of Zn-porphyrins (9Zn Amax = 608, 651 nm, t = 1.1 ns lOZn Amax = 600, 651 nm). The fife-times (9 and 10, 5.5 ns 9Zn and lOZn, 1.04 ns) are somewhat shortened (by 30-40%) with respect to the porphyrin components, as a consequence of the heavy-atom effect of the external ruthenium centers (see above for a detailed account of this phenomenon). 

Fig. 9.17 Sketches depicting (a) a typical lamella in a great circle of a spherulite of HOPE and (b) the geometry of a typical lamella, showing the principal slip system (100) [001] and three modes A, B, and C, namely nucleation of a monolithic screw dislocation from a narrow edge a screw-dislocation half loop, again from a narrow edge and an edge-dislocation half loop from a wide surface of a lamella, respectively, under an applied shear stress (from Argon et al. (2005) courtesy of Elsevier).

Figure 1.11. Sketch depicting the geometry of a typical lamella showing the principal chain slip system (100) [001] and the three separate modes of dislocation nucleation A monolithic screw, B screw loop and Cedge loop [159]...

The effect of temperature on the microstructures and dominant slip systems determined from textures are similar for both experimentally and naturally deformed quartzites (see [266-269]). Inferences about shp systems from textures are not unequivocal, especially because many texture studies rely only on c-axis orientations and not the full orientation distribution. There are very few TEM investigations identifying dislocation geometries in natural quartzites. The newer studies are consistent with the concept that (0001) j[2 11 0] slip dominates at lower temperatures, with a transition to 1010 112 0) slip at intermediate temperatures ( 900°C in the laboratory, 500-600°C in the Earth) and finally 1010 [0001] slip at temperatures above 600-700°C in the Earth [267]. Shp on the rhombohedral planes in a and c directions is also observed. The different transition temperatures observed for experiments and rocks recovered from natural settings are interpreted to reflect a kinetic trade-off between the temperature and strain rate. Experimental samples creep at rates 10 -10 times faster than deformation occurs in the Earth. 

Dislocation dynamics has been recently applied to minerals such as periclase [443] (Fig. 17) to explore the hardening in this material through interactions and reactions between dislocations gliding in noncoplanar slip systems and olivine [424]. Such simulations may in the future become important tools to predict plasticity based on dislocation geometry for a variety of conditions that cannot be explored experimentally. 

One very important point that must be considered in any rheological measurement is the possibility of slip during the measurements. This is particularly the case with highly concentrated dispersions, whereby the flocculated system may form a plug in the gap of the platens, leaving a thin liquid film at the walls of the concentric cylinder or cone-and-plate geometry. This behaviour is caused by some syneresis of the formulation in the gap of the concentric cylinder or cone and plate. In order to reduce sHp, roughened walls should be used for the platens an alternative method would be to use a vane rheometer. 

The overall reaction rate requires knowledge of both the diffusion and surface reaction step. The diffusion step can be estimated from a slip velocity-terminal velocity theory but values for the surface reaction rate must be obtained by experiment and this may be difficult. Slip velocities can also be calculated from the terminal velocity-slip velocity theory and these may enable overall reaction rates in more easily defined hydrodynamic systems to be used to estimate the rates obtainable in a stirred vessel. For efficient suspension geometries,... 

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