Dislocations pipe diffusion

The dislocation core is always a more open region of the crystal. The number of bonds will be lower or there will be preexisting broken bonds. Thus, vacancies, interstitials, and other point defects can move more easily in the dislocation core. This leads to the concept of dislocation pipe diffusion. 

Dislocation pipe diffusion may be quite important in ceramics because there are generally many more point... 

The elevated temperature coarsening of fine banded dispersions of VC is associated with the presence of dislocations and it was found that the overall kinetics for flic coarsening of VC in ferrite at 725°C obeyed a law of the form x t which is in qualitative agreement with flie predictions ofthe theory for the coarsening of particles by dislocation pipe diffusion [1975Dun]. 

In one dimensional diffusion experiments (e.g., starting with a thin film source of A on a B crystal surface) one often finds an exponential decrease in the A concentration at the far tail of the concentration profile. This behavior has been attributed to pipe diffusion along dislocation lines running perpendicular to the surface. Models have been introduced which assume a (constant) pipe radius, rp, inside which Dl = p-D, b and p denoting here bulk and dislocation respectively. P values of 103 have been obtained in this way. It is difficult to assess the validity of these observations. The model considerably simplifies the real situation. During diffusion annealing, the structure of the dislocation networks is likely to change because of self-stress (see Chapter 14) and chemical interactions. 

We have pointed out before that during creep, demixing of solid solutions is to be expected. Creep in compounds, however, occurs in such a way that the rate is determined by the slowest constituent since complete lattice molecules have to be displaced and the various constituent fluxes are therefore coupled. If extra fast diffusion paths operate for one (or several) of the components in the compound crystal, the coupling is cancelled. Therefore, if creep takes place in an oxide semiconductor surrounded by oxygen gas, it is not necessarily the slow oxygen diffusion that determines the creep rate. Rather, the much faster cations may determine it if oxygen can be supplied to or taken away from the external surfaces via dislocation pipes. 

Solution. As usual, take the boundary as a slab that is 5 thick. In considering diffusion along the tilt axis, any contribution of the crystal regions in the slab can be neglected and only the contributions of the dislocation pipes are included because D° DXL. The flux through a unit cross-sectional area of the boundary slab is then... 

Solution. The transverse diffusion rate is controlled by the relatively slow crystal diffusion rate because the diffusing atoms must traverse the patches of perfect crystal between the dislocation pipes. Therefore, when the dislocations are discretely spaced, a good approximation is the simple result... 

X. Tang, and K.P.D. Lagerof, and A.H. Heuer, Determination of pipe diffusion coefficients in undoped and magnesia doped sapphire (a-A1203) a study based on annihilation of dislocation dipoles, J. Am. Ceram. Soc. 86, 560-565 (2003). 

Dislocation climb by pipe diffusion 0 5 Dislocation core... 

Dislocations are often most important in ceramic materials for what they do not do they do not usually move very easily unless the temperature is very high. Dislocations can be used as local probes, for example, when we look at loop aimealing to learn about pipe diffusion versus bulk diffusion. It is important to understand dislocations if we want to understand structured interfaces, whether grain boundaries or phase boundaries, since these often consist of arrays of dislocations (or even dislocated arrays of dislocations). When materials are used for electronic applications, dislocations become quite common because these materials are often produced by epilayer growth. The discovery of the light-emitting properties of GaN suddenly made all the work on dislocations and interfaces in AIN relevant again. 

The serial sectioning technique was used to study tracer diffusivity in samples having dislocation densities of between 10 and lO /cm. The results indicated that the pipe diffusivity could be described by ... 

Pipe diffusion (semiconductor technology) Rapid diffusion along a dislocation. 

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