Grain boundary diffusivity

Obsidian and flints are natural glasses. Such samples show a uniform diffusion in any direction. In contrast, the diffusion of F into meteorites must be a function of the grain size of the material. The apparent diffusion is a mixture of volume diffusion and grain-boundary diffusion. Grain-boundary diffusion is much faster... 

Boundary diffusion Grain boundary Neck Sintering... 

Grain-boundary diffusion (grain boundary => neck) K -t- 4a ... 

Figure 1. Development of microstructural texture that can occur during hot pressing the grains are flattened in the direction of the applied pressure. For matter transport by diffusion, grain boundary sliding is necessary to accommodate the change in grain shape.

Figure 19.8 Schematic visualising the possible influence of the film stmcture on the metal diffusion. Grain boundaries might either act as fast diffusion paths or act as trapping and nucleation sites for clusters.

Grain boundary diffusion Grain boundary Neck Yes... 

Lattice diffusion Grain boundary Neck Lattice diffusivity, Z)/... 

Another key problem with diffusive sintering is that it can occur by several diffusion mechanisms as shown in Fig. 9.33. Viscous and plastic flow are two simple possibilities but another five are readily distinguished, including solid-state diffusion, grain boundary diffusion, surface diffusion, gas phase transport, and liquid layer transport. These inevitably form a neck at the particle contact, to reduce the sharp curvature of the contact region. 

The different types of defects determine the path of matter transport and diffusion along the major paths gives rise to the major mechanisms of matter transport lattice diffusion (also referred to as volume or bulk diffusion), grain boundary diffusion, and surface diffusion. 

Amorphous substances have a liquid-like supramolecular stmcture. Like liquid droplets, two amorphous particles in contact with each other tend to adopt a spherical shape. Accordingly, molecules are transported to the contact point between the two particles. The capillary and vapor pressure gradients between the particle volumes and the contact point between the two particles are the driving force for these transport processes. Linked to these local differences in capillary and vapor pressure different molecular transport mechanisms are observed. The molecules can be transported via the surrounding gas phase (evaporation and sublimation), diffusion on the surface (surface diffusion/grain boundary diffusion) or diffusion... 

J. E. Burke General Electric) There has been a lot of discussion about two mechanisms which involve dislocation movement and diffusion. Grain boundary sliding must operate under some circumstances. You have looked at many specimens very carefully have you seen any evidence of this occurring I know it is difficult in the oxide case because the closed structure varies so much in something like alumina. But in a metal such as copper at what particle size would you expect to see this dislocation movement mechanism important in sintering ... 

Several theoretical mechanisms have been proposed to explain the creep behavior for various materials these mechanisms involve stress-induced vacancy diffusion, grain boundary diffusion, dislocation motion, and grain boimdary sliding. Each leads to a different value of the stress exponent n in Equations 8.24 and 8.25. It has been possible to elucidate the creep mechanism for a particular material by comparing its experimental n value with values predicted for the various mechanisms. In addition, correlations have been made between the activation energy for creep (Qc) and the activation energy for diffusion (g Equation 5.8). 

Diffusion into and through a material is by bulk diffusion, grain boundary diffusion, and/or surface diffusion in order of increasing diffusion rates. Thin films are often used as diffusion barriers. Since in thin films, grain size is typically small compared to bulk materials, the grain boundary mechanism may dominate. However, if there is a columnar microstructure, surface diffusion may predominate. Amorphous films seem to be particularly good diffusion barrier materials since they have no grain boundaries. 

Grain boundary diffusion Grain boundaries Sintering neck Densifying sintering... 

Experimental evidence of neck growth was first demonstrated by Kuczynski, who in 1949 sintered large polycrystalline particles onto flat polycrystalline substrates. Theories, based on a two sphere model (each a single crystal), were developed to determine the rate of neck growth and the rate at which particle centres approach one another. These theories conclude that the rate of neck growth is inversely proportional to particle size raised to a power that depends on the mass transport path. Many mass transport paths were subsequently considered, bulk diffusion, surface diffusion, grain boundary diffusion, viscous flow, evaporation-condensation, liquid solution-reprecipitation, and dislocation motion (see reference 8 for a review). 

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