《材料科学基础英文版课件2.ppt

Point Defects – Point Defects in Ceramics Point Defects – Point Defects in Ceramics (1) Vacancies Cation vacancies and anion vacancies Interstitials Cation interstitials and anion interstitials Point Defects – Point Defects in Ceramics (2) Basic rule – Electroneutrality Equal numbers of positive and negative charges must be maintained on the formation of vacancies and interstitials Point Defects – Point Defects in Ceramics (3) Point Defects – Point Defects in Ceramics (4) Concepts: Stoichiometry: a state for ionic compounds where there is an exact ratio of cations to anions as shown by the chemical formula If no other defects exist, the ratio of cations to anions is not changed by the formation of a Frenkel (cation vacancy-cation interstitial pair) or Schottky (cation vacancy-anion vacancy pair) defect. So the material is in Stoichiometry Point Defects – Point Defects in Ceramics (5) Nonstoichiometry: a state for ionic compounds where there is no exact ratio of cations to anions as shown by the chemical formula Point Defects – Point Defects in Ceramics (6) Impurities in ceramics Solid solutions – substitutional and interstitial Point Defects – Point Defects in Ceramics (7) Substitutional solid solutions: Impurity-cations replace host-cations; impurity-anions replace host anions Interstitial solid solutions: the impurity must be quite small In any case, the electroneutrality must be maintained Point Defects – Point Defects in Ceramics (8) e.g. To form a NaCl-Ca solid solution Na+, Cl-, and Ca2+ ions When a Ca2+ ion is dissolved into NaCl, the Ca2+ ion replaces an Na+ ion, producing an extra positive charge - not electroneutral To maintain electroneutrality, removal of one Na+ ion - creating one Na+ ion vacancy or formation of a Cl- interstitial, but difficult Specification of Composition (1) Usually use two ways to specify the composition (or concentration) of an alloy in terms of its components Weight percent (wt%) Atomic percent (at%)