Lecture_7.pdf - IMPERFECTIONS IN CERAMICS Imperfections in...

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IMPERFECTIONS IN CERAMICS Imperfections in crystal are classified according to their geometry and shape into point, line, and planar defects . P oint defect - defined as any lattice point which is not occupied by the proper ion or atom needed to preserve the long-range periodicity of the structure. Linear defects - defects that cause lattice distortions centered on a line such as dislocations Planar defects- are surface imperfections in polycrystalline solids that separate grains or domains of different orientations and include grain and twin boundaries. In addition, there are three-dimensional bulk defects such as pores, cracks, and inclusions -these defects are critical in determining the strength of ceramics.
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Point Defects In metals, both vacancies and interstitials are possible. since ceramic materials contain ions of at least two kinds, defects for each ion type may occur. For example, in NaCl, Na interstitials and vacancies and Cl interstitials and vacancies may exist. The anion is relatively large, and to fit into a small interstitial position, substantial strains on the surrounding ions must be introduced.
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Stoichiometric defects ones in which the crystal chemistry, i.e., the ratio of the cations to anions, does not change, and they include, Schottky and Frenkel defects Stoichiometry may be defined as a state for ionic compounds wherein there is the exact ratio of cations to anions as predicted by the chemical formula. Eg; NaCl is stoichiometric if the ratio of ions to ions is exactly 1:1. A1 2 O 3 is 2:3, etc. Nonstoichiometric defects These defects form by the selective addition or loss of one (or more) of the constituents of the crystal and consequently lead to a change in crystal chemistry and the notion of nonstoichiometry In reality, using thermodynamic arguments the composition of every compound must vary within its existence regime. For example, if an oxide were annealed in a high oxygen partial pressure, it is assume that the number of oxygen atoms should be relatively greater than the number of cations. Conversely, if the oxygen partial pressure were very low, one would expect the cation concentration to be higher Extrinsic defects These are defects created as a result of the presence of impurities in the host crystal.
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In a pure binary compound, the following lattice defects, shown schematically in Fig. 6.1, exist: 1. Vacancies: sites where an atom is missing. These can occur on either sublattice. 2. Interstitial atoms: atoms found in sites that are normally unoccupied. 3. Misplaced atoms: types of atoms found at a site normally occupied by other types. This defect is only possible in covalent ceramics, where the atoms are not charged.
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  • Fall '16
  • Unkwnow
  • Ion, Crystallographic defect, grain boundary

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