Grain boundary, mixed

Although there are many features common to synthetic oxides and minerals, fundamental studies of the charge-transfer processes in mixed-valence compounds can only be systematically carried out on synthetic oxides of controlled stoichiometry and impurity concentration. However, with the exception of Seebeck coefficients, transport measurements require single-crystal data if quantitative interpretations are to be made. Nevertheless, conductivity data for polycrystalline samples of cubic phases are useful if the samples are dense and care has been taken to eliminate any segregation of impurities into the grain boundaries. 

The long-term consequences of a-decay will be He gas build-up, which may lead to grain boundary disintegration. Poinssot et al. (2002) have shown that the internal pressures generated over time in LWR fuels from He gas are insignificant however, the pressures that could be produced by mixed oxide (MOX) fuels may need to be considered. 

The lattice defects are classified as (i) point defects, such as vacancies, interstitial atoms, substitutional impurity atoms, and interstitial impurity atoms, (ii) line defects, such as edge, screw, and mixed dislocations, and (iii) planar defects, such as stacking faults, twin planes, and grain boundaries. 

Tilt boundaries occur if the axis of rotation between the two grains is located in the boundary (interface). In contrast, if the axis of rotation is perpendicular to the boundary, the boundary is called a twist boundary and consists of a collection of screw dislocations (Fig. 3-6b). An equation similar to Eqn. (3.14) holds for twist (and mixed) boundaries. Since dislocation theory is well understood, it is possible to quantitatively treat small-angle grain boundaries [J.P. Hirth, J. Lothe (1982)]. 

See Appendix B for descriptions of tilt, twist, and mixed grain boundaries. 

Grain boundaries can also be classified as tilt boundaries, twist boundaries, and mixed boundaries. A tilt boundary s plane is parallel to the rotation axis used to define its crystal misorientation, as in Fig. B.4c. The crystals adjoining the boundary are related by a simple tilt around this axis. A twist boundary, as in Fig. B.56, is a boundary whose plane is perpendicular to the rotation axis. The two crystals adjoining the boundary are then related by a simple twist around this axis. All other types of boundaries are considered to be mixed. 

Figure 9. Four modes of spontaneous oxygen permeation (a) in a short-circuited electrochemical cell (b) through a mixed conducting single phase, (c) through a composite phase mixture comprising an ionic and an electronic conductor, and (d) through an ionic (electronic) conductor the grain boundaries of which are predominandy electronically (ionically) conducting.

In technological applications, mixed, doped, or multi-metal oxides play an important role, for example, Mo-V-Te-Nb oxide [15] is used for selective oxidation of propane to acrylic acid. For some complex oxides, the bulk oxide structures and distribution of phases are often unknown and there is little knowledge of the atomic surface structure and composition, extent of hydroxylation, type and density of defects, and the location of dopants (homogeneously distributed, concentrated at the surface, grain boundaries, or interfaces). 

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