Large-angle grain boundaries

In Fig. 9.1, Dd for nondissociated dislocations is practically equal to DB, which indicates that the diffusion processes in nondissociated dislocation cores and large-angle grain boundaries are probably quite similar. Evidence for this conclusion also comes from the observation that dislocations can support a net diffusional transport of atoms due to self-diffusion [15]. As with grain boundaries, this supports a defect-mediated mechanism. 

The example in Fig. 13.4 is an extension of the model for the motion of a small-angle boundary by the glide and climb of interfacial dislocations (Fig. 13.3). Figure 13.4 presents an expanded view of the internal surfaces of the two crystals that face each other across a large-angle grain boundary. Crystal dislocations have... 

Homophase crystal/crystal interfaces are often called grain boundaries. It is customary to classify such boundaries as either small-angle grain boundaries or large-angle grain boundaries. 

It is known that clean dislocations without decoration reveal almost no recombination activity [55], but increasing decoration with impurities leads to recombination centres deep in the band gap, which significantly reduce carrier lifetime [56]. It can, therefore, be concluded that one of the most detrimental defects in EFG and SR apart from recombination active large angle grain boundaries are decorated dislocations. 

The optical and scintillation applications do not set such rigorous requirements for crystal perfection as semiconductor applications. However, the presence of low- and large-angle grain boundaries and ofindividual dislocations is potentially dangerous. The segregation of activator and impurities near such defects contributes to the optical and scintillation nonhomogeneity of the material. 

FIG. 13 Cross sections indicating spalling of portions of the SAC solder from a BGA solder joint, (a) Variations in color occur at large-angle grain boundaries in the solder joint. (Courtesy of Nokia Corp.)... 

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