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Imperfections in Solids

Lecture # 9. Imperfections in Solids. Point Defects. VACANCIES AND SELF-INTERSTITIALS (The simplest of the point defects is vacancy .). A self- interstitialis an atom from the crystal that is crowded into an interstitial site. N= the total number of atomic sites.

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Imperfections in Solids

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  1. Lecture # 9 Imperfectionsin Solids Point Defects VACANCIES AND SELF-INTERSTITIALS (The simplest of the point defects is vacancy .) A self-interstitialisan atom from the crystal that is crowded into an interstitial site.

  2. N= the total number of atomic sites. QV = energy required for the formation of a vacancy. T = the absolute temperature in Kelvin. K = Boltzmann’s constant. is J/atom-K

  3. Intended learning Outcomes: 1- Structure of ceramic materials. 2-Given the chemical formula for ceramic compound and the ionic radii of its component ions, predict the crystal structure. 3- Impurities in ceramics. 4-Mechanical properties of ceramics. 5- Application and processing of ceramics. Structure and properties of ceramicsApplication and processing of ceramics

  4. CERAMIC CRYSTAL STRUCTURES • ceramics are composed of at least two elements, and often more, their • crystal structures are generally more complex than those for metals. • Table 3.2 presents the percent ionic character for several common ceramic materials;

  5. For example, in calcium fluoride, each calcium ion has a 2 charge (Ca2), and associated with each fluorine ion is a single negative charge (F). • The second criterion involves the sizes or ionic radii of the cations and anions, • rC and rA, respectively. • cations are ordinarily smaller than anions, and, consequently, the ratio • rC/rA is less than unity

  6. 1-AX-TYPE CRYSTAL STRUCTURES • Rock Salt Structure, Cesium Chloride Structure, Zinc Blind Structure

  7. 2-AmXp-Type Crystal Structures: Ca F2

  8. 3-AmBnXp-TYPE CRYSTAL STRUCTURES

  9. Ceramic Density Computations

  10. SILICATE CERAMICS: • Each atom of silicon is bonded to four oxygen atoms, which are situated at the corners of the tetrahedron; the silicon atom is positioned at the center. Since this is the basic unit of the silicates. i

  11. COORDINATION # AND IONIC RADII • Coordination # increases with Issue: How many anions can you arrange around a cation? Adapted from Fig. 12.4, Callister 6e. Adapted from Fig. 12.2, Callister 6e. Adapted from Fig. 12.3, Callister 6e. Adapted from Table 12.2, Callister 6e.

  12. DEFECTS IN CERAMIC STRUCTURES • Frenkel Defect --a cation is out of place. • Shottky Defect --a paired set of cation and anion vacancies. Adapted from Fig. 13.20, Callister 5e. (Fig. 13.20 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.) See Fig. 12.21, Callister 6e.

  13. IMPURITIES • Impurities must also satisfy charge balance • Ex: NaCl • Substitutional cation impurity • Substitutional anion impurity

  14. TYPES OF CERAMICS

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