Pipe diffusion

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. 

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). 

A shortcoming of both NRA and SIMS is that the spatial resolution is not high in all three directions. While depth resolutions on the order of a few hundred nanometers or less are routine, the profiles are measured over surface areas of a few hundred square micrometers or even millimeters. Thus, the concentration profile can be an average from various localized reaction or fast diffusion (e.g. pipe diffusion). The consequences of dual-diffusion mechanisms on the geometry of isotopic profiles is discussed in more detail below. There is no available method to by pass these problems for isotopic diffusion, although for elemental diffusion, alytical Transmission Electron Microscopy (ATEM) offers an alternative (Meissner et al. 1997). 

A gas will not stay in the basement. For example, natural gas, or methane, is a fuel burned in gas furnaces, hot-water heaters, and stoves. Gas that leaks from a gas pipe diffuses throughout the house. Because natural gas is odorless, companies that supply the fuel include a compound with a distinct odor. Adding odor to natural gas warns the home-owner of the leak. The customer has time to shut off the gas supply, open windows to allow the gas to diffuse, and call the gas company to report the leak. 

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

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... 

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. 

Diffused aeration system, installed including all piping, diffusers, blowers, air filters and blower house. Fully installed cost 820 000 at air flow rate = 0.7 m s with n = 0.73 for the range 0.25-15. 

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]. 

It was also deduced that pipe diffusion occurred which could be described by ... 

This activation energy was consistent with that for the grain boundary diffusion of P and B, as well as the pipe diffusion of As in Si. 

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 ... 

The pipe diffusion results could be described by the expression ... 

---