Short-circuit paths

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... 

Solely from a conventionally measured impedance spectrum it is neither possible to draw conclusions on the existence of short-circuiting paths, nor to determine grain... 

Chromia is thought to grow via outward diffusion of cahons from the metal to the oxide/ gas interface. When the scale formation proceeds by diffusion along short-circuit paths such as grain boundaries in the oxide layer, Eq. 1 describes the usual diffusion controlled oxidation kinetics ... 

Simple grain boundaries in metals have been exhaustively considered from the standpoint of empirical descriptions of the total energy. The logic of much of this work is founded on the contention that prior to understanding the susceptibility of boundaries to segregation and fracture or their role as short-circuit paths for diffusion, it is necessary to first understand the structures themselves. The majority of this work has emphasized bicrystals. 

Stage 2 also follows logarithmic kinetics, reflecting competition between parabolic oxide growth and short circuit diffusion down preferred channels. Initially, the short circuit paths account for the early observed rapid scale growth. A transition is later observed to parabolic kinetics, which marks the onset of the third stage of scale growth in hot salt accelerated oxidation of -y-TiAl. 

A10mm PVC sheathed mineral insulated (Ml) copper cable is short-circuited when connected to a 400 V supply. The impedance of the short-circuit path is 0.1 Cl. Calculate the maximum permissible disconnection time and show that a 50 A Type B MCB to BS EN 60898 will meet this requirement. 

Fig. 1.87 Voids at an oxide/metal interface. They may grow by (a) condensation of vacancies from the metal as well as the local oxide they impede transport as shown in Fig. 1.85. However, voids which become filled with a carrier gas (b) may act as short-circuit paths by the reactions ai A, B.C and >, given in the text (after Birks and Rickert ). Alternatively (c) gaseous by-products of oxidation may maintain fissures in the oxide (after Boggs and Kachik ...

In practice it is usually necessary to make provisions to avoid the transmission of vibrations via paths that bypass the isolated base. For this purpose it usually is desirable to provide flexible sections of piping, electrical conduit, cable, and ducts between the isolated machines and the unisolated surroundings. (Appropriate devices especially designed for vibration isolation are commercially available devices such as loops and bellows that are primarily designed to accommodate thermal expansion rarely are adequate for vibration isolation.) Similarly, hold-down bolts through isolators, and similar arrangements that in effect serve as vibration short-circuit paths around the isolators, must be removed or themselves isolated, e.g., with soft rubber sleeves and washers. 

The commonly observed decrease in brought about by a reduction in the grain size or an increase in the dislocation density at the alloy surface (i.e., because of mechanical deformation via abrasion, shot peening, etc.) has also been interpreted by use of Wagner s criterion given in Eq. (5-19) (Rapp, 1965). According to the interpretation, an increase in the amount of short-circuit diffusion paths causes a decrease in the DxlD ratio and, hence, a decrease in Ng . It is not entirely clear, however, why Dq should increase to a much greater extent than Dx when more short-circuit paths are introduced. It is very likely that a more complete explanation also requires consideration of the kinetics and behavior of BXj, nu-cleation. 

As seen in Fig. 4.11(c), the amount of oxide formation at the Si02/Si interface increases with oxidation time. The SIMS data suggest that the inward diffusion of oxygen probably occurs along short-circuit paths, such as micropores extending to the Si02/Si interface. 

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