吉林大学《材料科学与工程基础》chapter 5.pptVIP

Chapter 5 Imperfections in Solids Defect structure: Relating to the kinds and concentrations of vacancies and interstitials in a ceramic compound 5.3 POINT DEFECTS IN CERAMICS Electroneutrality: The state of having exactly the same numbers of positive and negative electrical charges 5.3 POINT DEFECTS IN CERAMICS Stoichiometry:For ionic compounds,the state of having exactly the ratio of cations to anions specified by the chemical formula. Nonstoichiometry may occur for some ceramic materials in which two valence (or ionic) states exist for one of the ion types. Iron oxide (FeO) is one such material 5.4 IMPURITIES IN SOLIDS IMPURITIES IN METALS : Inescapability of impurities Alloys:A metallic substance that is composed of two or more elements. impurity atoms have been added intentionally to impart specific characteristics to the material. Formating solid solutions and second phase. 5.4 IMPURITIES IN SOLIDS 5.4 SOLID SOLUTIONS Factors determine the solubility of Substitutional solid solution. Atomic size factor. For appreciable quantities of a solute, the difference in atomic radii between the two atom types is less than about 15%. Crystal structure. For appreciable solid solubility the crystal structures for metals of both atom types must be the same. Electronegativity. Electropositive difference form an intermetallic compound 4. Valences. Other factors being equal, a metal will have more of a tendency to dissolve another metal of higher valency than one of a lower valency. For example: copper and nickel completely soluble in one another 5.4 SOLID SOLUTIONS For interstitial solid solutions impurity atoms fill the voids or interstices among the host atoms Consequently, the atomic diameter of an interstitial impurity must be substantially smaller Normally, the maximum allowable concentration of interstitial impurity atoms is low (less than 10%). For example: carbon forms an interstitial solid