Point Defects in Materials Part II: Applications to Different Materials Problems
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rformance; he illustrâtes what he means by defect engineering with examples from basic processes used in electronic materials processing. Marshall Stoneham, director of research for AEA Industrial Technology, Harwell Laboratory, is a well-known solid-state theorist with extremely broad interests in materials science and engineering. He is the author of several books on defects and defect processes in solids, and also the author of many publications. In his article on the theory of solid-state defects—a theoretical paper with no équations—Stoneham demonstrates, with many examples, that studies of materials are often studies of defects since fundamental properties of defects (point, line, and planar) control many important, practical properties of materials. Stoneham's examples cover the main classes of materials—metallic, semiconducting, ionic, and polymeric. He emphasizes the intimate relationship between theory and experiment. And he emphasizes that the rôle played by theory in establishing expérimental priorities is important and that theoretical estimâtes of significant parameters are extremely useful for showing what needs to be accomplished to improve materials properties. He also points out that theoretical or empirical models provide a framework for unraveling new situations where the critical variables hâve not yet been ascertained. Stoneham's article is peppered with many practical examples covering a wide range of problems.
Riidiger Dieckmann, professor of materials science and engineering at Cornell University and a well-known expert on point defects and mass transport in ceramic materials, covers the problem of the relationships between point defect concentrations in nonstoichiometric métal oxides and diffusion. To illustrate the principles involved he cites manganosite, M n M 0 , and spinels of the type M e ^ C ^ with Fe, Mn, and Co cations. First, Dieckmann discusses the relationship between point defects and nonstoichiometry and shows, subject to the caveat that the point defect concentrations are sufficiently small, that the concentrations of defects hâve a power law dependence on the oxygen activity. This resuit allows the investigator to vary point defect concentrations by varying the ambient pressure of oxygen; this strongly contrasts with elemental metals or metallic alloys, where the point defect concentrations or types are not affected, to first order, by the ambient pressure. The experimentally observed dependence of the point defect concentrations on the oxygen activity permits unraveling the point defect structure of métal oxides. The tracer diffusivity of an ion is directly proportional to the concentration of point defects, and the latter is in turn proportional to the oxygen activity to some power. Thus, from measurements of the tracer diffusivity of an ion as a function of the oxygen activity, one may extract the point defect mechanism for diffusion. Dieckmann demonstrates very carefully and in détail the power of thèse basic principles in obtaining detailed point defect information for
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