Grain diffusion

Considering the physical features of the model, two parameters are involved Xg, the time required for complete conversion of the grain in the [A] environment and x, the time for complete conversion of the particle by diffusion if the grain conversion process is extremely fast. In the limiting case of grain diffusion controlling, the simple homogeneous model is recovered. The... 

The discussion above considered only simple whole grain or sub-grain diffusion when lattice diffusion was the transporting mechanism. In recent years, the advent of laser analysis has demonstrated just how variable the actual within grain patterns can be (Fig. 5) (e.g., Hodges et al. 1994 Kelley and Turner 1991 Kramar et al. 2001 Lee et al. 

Kramar N, Cosca MA, Hunziker JC (2001) Heterogeneous Ar-40 distributions in naturally deformed muscovite In situ UV-laser ablation evidence for micro structurally controlled intra-grain diffusion. Earth Planet Sci Lett 192 377-388... 

Figure 7.5 In polycrystals, diffusional creep may occur by diffusion through the grains (diffusivity D ) or along the grain boundaries (diffusivity D ). The two possible atom diffusion paths are shown by the arrows. (Adapted from Cook and Pharr, 1994, reproduced eourtesy of VCH Publishers, Weinheim, Germany.)...

For polycrystalline solids, additional effects must be considCTed. The location of the pores will influence the gas diffusion path. When the pores are located within the grains, diffusion will depend on the gas solubility in the crystal lattice. For the more common case in which the pores are located at the grain boundaries, diffusion of the gas through the disordered grain boundary region provides an important additional path. 

When colloidal palladium oxide is prepared in advance (methods 3 in Table 1), the enhanced capability of such species to adsorbing provides their deposition onto the exterior surface of catalyst grains. Diffusion limitations for rapid catalytic processes diminish thereby (of. the R factors for different catalysts in Table 1). Naturally, the active component is forced to penetrate more towards the grain ooi e if incipient wetness impregnation is used Instead of adsorption (method 3.2). 

Figure C2.11.6. The classic two-particle sintering model illustrating material transport and neck growtli at tire particle contacts resulting in coarsening (left) and densification (right) during sintering. Surface diffusion (a), evaporation-condensation (b), and volume diffusion (c) contribute to coarsening, while volume diffusion (d), grain boundary diffusion (e), solution-precipitation (f), and dislocation motion (g) contribute to densification.

Transmission electron microscopy (tern) is used to analyze the stmcture of crystals, such as distinguishing between amorphous siUcon dioxide and crystalline quartz. The technique is based on the phenomenon that crystalline materials are ordered arrays that scatter waves coherently. A crystalline material diffracts a beam in such a way that discrete spots can be detected on a photographic plate, whereas an amorphous substrate produces diffuse rings. Tern is also used in an imaging mode to produce images of substrate grain stmctures. Tern requires samples that are very thin (10—50 nm) sections, and is a destmctive as well as time-consuming method of analysis. 

Under equiUbrium conditions, magnesium can contain as much as 12.7% aluminum in soHd solution at the eutectic temperature. However, the slow diffusion of aluminum to the grain boundary leads to a coring effect in primary crystals and a hard-phase magnesium—aluminum compound(17 12)... 

Diffusion is based mainly on the diffusion of vacancies grain boundaries may act as sinks for these vacancies. This vacancy movement and annihilation cause the porosity of the powder compact to decrease during sintering. 

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