Short-circuit diffusion

For solid phase sintering, there are fonr ways of diffusion i) surface diffusion, ii) volnme diffusion (often called lattiee diffusion), iii) vapor phase transport (evaporation-eondensation), and iv) grain boundary diffusion the boundaries are very disturbed areas, which allow diffusion short-circuits . For liquid phase sintering, we must add dissolution-reprecipitation effects or a vitreous flow. Finally, for pressure sintering the pressure exerted allows the plastic deformation of the crystallized phases and the viscous flow of the amorphous phases. 

When sunlight falls on a p—n junction solar cell while it is short-circuited, the magnitude of remains essentially the same as it was in darkness. Because the diffusion of majority current only varies with lA, the majority current does not change. However, additional minority carriers are formed by... 

The technical problem in die high teiiiperamre application of Si3N4 is that unlike the pure material, which can be prepared in small quantities by CVD for example, die commercial material is made by sintering the nitride with additives, such as MgO. The presence of the additive increases the rate of oxidation, when compared with the pure material, by an order of magnitude, probably due to the formation of liquid magnesia-silica solutions, which provide short-circuits for oxygen diffusion. These solutions are also known to reduce the mechanical strength at these temperatures. 

Diffusion in the bulk crystals may sometimes be short circuited by diffusion down grain boundaries or dislocation cores. The boundary acts as a planar channel, about two atoms wide, with a local diffusion rate which can be as much as 10 times greater than in the bulk (Figs. 18.8 and 10.4). The dislocation core, too, can act as a high conductivity wire of cross-section about (2b), where b is the atom size (Fig. 18.9). Of course, their contribution to the total diffusive flux depends also on how many grain boundaries or dislocations there are when grains are small or dislocations numerous, their contribution becomes important. 

The addition of up to 15% Mn to pure Fe, under sulphidising conditions at 1073 K, leads to a small increase in the scaling rate At 2% Mn, MnS forms as stringers in the subscale, but these do not form a coherent layer even at a concentration of 15% Mn. The increase in scaling rate is possibly due to increased short-circuit diffusion, since metal diffusion in MnS is much slower than in FeS. ... 

It would appear that the effects of impurities at the grain boundary must be either (a) to increase the diffusion rates or (b) to influence the microstructure and increase the number of short-circuit paths. However, theoretical modelling of the grain boundary structure by Duffy and Tasker and... 

Figure 5.2 Diffusion couple formed by two crystals separated by radioactive material (a) initially and (b) after heating. Short-circuit diffusion occurs down extended defects, and diffusion into the bulk occurs both from the surface and laterally from extended defects.

Figure 5.3 Schematic representation of the penetration profile for bulk, grain boundary, and dislocation diffusion in a polycrystalline solid. The initial part of the curve is bell shaped, and the part due to short-circuit diffusion is made up of linear segments. The insets show the distribution of the tracer in the sample.

To evaluate the bulk diffusion coefficient, using Eq. (5.1) or (5.2), it is necessary to subtract the short-circuit diffusion contribution from the total concentration profile. Ideally, the concentration profile due to bulk diffusion will take on the shape of a bell, gradually flattening as the time of diffusion increases (Fig. 5.5). The solution to the... 

Volume diffusion refers to the transport of atoms through the body of a solid. It is also called lattice or bulk diffusion. In amorphous or glassy solids and in cubic crystals, the speed of diffusion in all directions is the same and is said to be isotropic. In all other crystals, the rate of volume diffusion depends upon the direction taken and is anisotropic. Volume diffusion is usually much slower than short-circuit diffusion, which refers to diffusion along two- and three-dimensional imperfections in the material. 

Short-circuit diffusion refers to diffusion along ... 

For the diffusion couple experimental arrangement (Fig. 5.2), (ignoring all short-circuit diffusion), the solution is... 

Defects in a SCR, which is present under reverse bias, can be tested in a similar way. Figure 10.6 c shows the same wafer as in Fig. 10.6 e after removal of the oxide and under cathodic polarization in the dark. Hydrogen bubbles caused by the dark current now decorate nickel silicide precipitates that short-circuit the SCR. Nickel precipitates are known to increase the dark current of a p-type Si electrode under reverse bias by orders of magnitude [Wa4]. If the bias is increased the copper silicide precipitates also become visible, as shown in Fig. 10.6 d. This method, like defect etching (Fig. 10.4f), is only sensitive to precipitated metals. Metals that stay in solution, like iron, do not show up in defect mapping and have to be determined by other methods, for example diffusion length mapping. 

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