Glide shear mechanism

Fig. 20. (a) Active sites observed by in situ atomic-resolution ETEM structural modification of VPO in n-butane along (201) indicates the presence of in-plane anion vacancies (active sites in the butane oxidation) between vanadyl octahedra and phosphate tetrahedra. (b) Projection of (010) VPO (top) and generation of anion vacancies along (201) in n-butane. V and P are denoted. Bottom model of novel glide shear mechanism for butane oxidation catalysis the atom arrowed (e.g., front layer) moves to the vacant site leading to the structure shown at the bottom. 

Novel glide shear mechanism in anion-deficient oxides... 

The results that CS planes (which eliminate anion vacancies in supersaturation by shearing and collapsing the lattice) are detrimental to catalysis are also consistent with me fact mat if the catalyst structure continues to collapse to form CS planes, after a period of time me catalyst is no longer an efficient oxidation catalyst. An efficient catalyst is essential for prolonged catalytic activity. This has led to me discovery of a novel glide shear mechanism (Gai et al 1995, Gai 1997). The role of mis mechanism in mixed-metal practical (commercial) catalysts will be examined when we discuss butane oxidation technology. 

The EM studies show that the novel glide shear mechanism in the solid state heterogeneous catalytic process preserves active acid sites, accommodates non-stoichiometry without collapsing the catalyst bulk structure and allows oxide catalysts to continue to operate in selective oxidation reactions (Gai 1997, Gai et al 1995). This understanding of which defects make catalysts function may lead to the development of novel catalysts. Thus electron microscopy of VPO catalysts has provided new insights into the reaction mechanism of the butane oxidation catalysis, catalyst aging and regeneration. 

Flinn et al. [30] describes an experimental impact technique in which <100)-oriented LiF single crystals ( 8 ppm Mg) are loaded in a controlled manner and the multiplication of screw dislocations is measured. The peak shear stress in this relatively soft material is 0.01 GPa. For shear impulses exceeding approximately 40 dyne s/cm, dislocation multiplication is adequately described by the multiple-cross-glide mechanism [(7.24)] with m = l/bL = (2-4) X 10 m, in reasonable agreement with quasi-static measurement [2]. 

The dislocation cannot glide upwards by the shearing of atom planes - the atomic geometry is wrong - but the dislocation can move upwards if atoms at the bottom of the half-plane are able to diffuse away (Fig. 19.2). We have come across Fick s Law in which diffusion is driven by differences in concentration. A mechanical force can do exactly the same thing, and this is what leads to the diffusion of atoms away from the... 

The pyroxenes are the most abundant minerals, after olivine, in perido-tites, which are the dominant constituents of the upper mantle. It is not surprising, therefore, that there has been considerable interest in the mechanical properties of the pyroxenes, and a review has recently been given by Doukhan et al. (1986). The orthorhombic pyroxenes deform by slip and by a shear transformation that produces monoclinic lamellae (one or a few unit cells thick) parallel to (100). Coe and Kirby (1975) and McLaren and Etheridge (1976) have shown that the shear transformation is achieved by the glide of partial dislocations of b = 0.83[001] in (100), which leave partial dislocations of b = 0.17[001] terminating the shear lamellae. The dominant slip system is (100) [001]. Recent TEM observations by van Duysen, Doukhan, and Doukhan (1985) suggest that the dislocations associated with this slip system may be dissociated into four partials and that the slip system (100) [010] may also be activated. These observations are discussed in Section 9.9.1. 

The deformation and damage mechanisms in creep of ceramics and hard materials are similar to those in metals [150,151]. Under normal loading conditions (in the absence of severe elastic constraint) ceramics fracture at room temperature before any significant plastic flow. Dislocation glide in ionically bonded ceramics is complicated by the presence of both anions and cations, which create electrostatic (Coulombic) barriers to shear. As in metals, three creep regimes have been identified. The initial high strain-rate, observed on applying the load, decreases rapidly... 

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