Week 2 : Lecture 3 and 4, Imperfections in crystals impurities. Vacancies. Grain boundaries.

The arrangement of the atoms or ions in engineered materials contains imperfections or defects. These defects often have a profound effect on the properties of materials. In this chapter, we introduce the three basic types of imperfections: point defects, line defects (or dislocations), and surface defects. These imperfections only represent defects in or deviations from the perfect or ideal atomic or ionic arrangements expected in a given crystal structure. The material is not considered defective from a technological viewpoint. In many applications, the presence of such defects is useful. There are a few applications, though, where we strive to minimize a particular type of defect. For example, defects known as dislocations are useful for increasing the strength of metals and alloys; however, in single crystal silicon, used for manufacturing computer chips, the presence of dislocations is undesirable. Often the “defects” may be created intentionally to produce a desired set of electronic, magnetic, optical, or mechanical properties. For example, pure iron is relatively soft, yet, when we add a small amount of carbon, we create defects in the crystalline arrangement of iron and transform it into a plain carbon steel that exhibits considerably higher strength.

                                                                                                                                                                                                                                                                                             Donald R. Askeland

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