Diffusion coefficient of defect

Therefore, the detailed analysis of concentration of defects in surface-adjacent layer and in the volume of adsorbent as well as assessment of the values of diffusion coefficients of defects and particles of various gases in material of adsorbent are very important for understanding the processes of both reversible and irreversible change in electrophysical characteristics of semiconductor during low temperature (if compared to the temperature of creation of defects) interaction with gaseous phase. 

Diffusion of the molecular gases can be compHcated by reactions with the glass network, especially at the sites of stmctural defects. The diffusion coefficient of water, for example, shows a distinct break around 550°C (110). Above 550°C, the activation energy is approximately 80 kj /mol (19 kcal/mol), but below 550°C, it is only 40 kJ/mol (9.5 kcal/mol). Proposed explanations for the difference cite the fact that the reaction between water and the sihca network to form hydroxyls is not in equiUbrium at the lower temperatures. 

This formula for ki can be cast into another form by using equations 1.169 and 1.170. We note first that in these latter equations K is the concentration of defects in CujO at 1 atm pressure of oxygen, so that (A ),Q) is the self-diffusion coefficient of Cu in CujO at this oxygen pressure. Call this self-diffusion coefficient Z7 , then... 

From the formation reaction of protonic defects in oxides (eq 23), it is evident that protonic defects coexist with oxide ion vacancies, where the ratio of their concentrations is dependent on temperature and water partial pressure. The formation of protonic defects actually requires the uptake of water from the environment and the transport of water within the oxide lattice. Of course, water does not diffuse as such, but rather, as a result of the ambipolar diffusion of protonic defects (OH and oxide ion vacancies (V ). Assuming ideal behavior of the involved defects (an activity coefficient of unity) the chemical (Tick s) diffusion coefficient of water is... 

Numerous studies by other workers (I, 10) have shown that the releases of iodine and the noble-gas fission products from pyrolytic carbon-coated fuel particles are controlled by diffusion of these nuclides through grain boundaries, cracks, and defects in the isotropic pyrolytic carbon coating. When coatings are intact, however, the release of these fission product nuclides is low. However, the pyrolytic carbon coating constitutes only a delaying barrier to the metallic nuclides barium and strontium through which they diffuse with diffusion coefficients of the order of 10 9 cm.2/sec. (at — 1400°C.). The steady-state release of these metallic nuclides is controlled instead by diffusion out of the fuel kernel,... 

In order to do so we must first evaluate the chemical diffusion coefficients of the pair of majority defects (e.g., V and h ) in the semiconducting oxide A O. The coupling of the defect fluxes (jh--2jV = 0) to maintain electroneutrality results in a chemical diffusion coefficient Dv. This controls the change in nonstoichiometry, <5( ,/)> through defect transport and reads... 

The addition of microspheres lowers the glass transition temperature of the epoxy binder (Fig. 13). This seems to be because the filler causes defects in the matrix network. Equal diffusion coefficients of filled and unfilled epoxy binder indicates, therefore, that the diffusion processes are insensitive to binder changes. The sorption of water by epoxy resins is in fact known to depend mainly on their polarity and only slightly on the three-dimensional compactness of the network. 

As mentioned in Section 9.4, Griggs and Blade (1964) observed a dramatic weakening of these crystals when deformed at 1.5 GPa confining pressure in the presence of water at Ts 800 C. However, Mackwell and Paterson (1985) were unable to reproduce this result at a confining pressure of 300 MPa. They interpreted the absence of weakening as due to a decrease in the diffusion coefficient of the water-related defects involved in the Griggs (1974) mechanism, which was reviewed briefly in Section 9.4. 

Experimental and theoretical studies on H diffusion in silicon at lower temperatures, where trapping of hydrogen at defects and impurities and H2-molecule formation are significant, discovered much lower effective diffiisivities. Values for the H-diffiision coefficient in silicon expected from an extrapolation of the diffusion coefficient of (11.1) to lower temperatures are several orders of magnitude higher than experimentally obtained diffiisivities. This is illustrated in Fig. 11.2, which shows (11.1) and (11.2) extrapolated to low temperatures with experimentally determined values from Johnson et al. (1986). 

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