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Ch 12 pt 1

Ch 12 pt 1 - Chapter 12 Structure& Properties of...

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Unformatted text preview: Chapter 12: Structure & Properties of Ceramics ISSUES TO ADDRESS... Structures of Ceramic materials: How do they differ from those of metals? Chapter 12 - 1 ANNOUNCEMENTS Reading: Chapter 12; Sections 12.1-12.4 only First Midterm Exam: 8:00 am, Tuesday May 4, Main Auditorium Don’t be late… Recitation Problems: #s 12.2, 12.3, 12.5, 12.15, 12.19 There will be no quiz in Week 6 since we have Midterm I, so this topic will be included in Midterm II Chapter 12 - 2 CERAMICS - General • Composition: Compounds of metallic + non-metallic elements: o e.g. Al2O3, ZrO2, MgO, TiO2, SiC, WC, B4C, ZnS, TiB2… and carbon… Traditional ceramics - china, porcelain, bricks, tiles, glasses o many based on clays Technical ceramics - oxides, carbides, nitrides o used in electronics, computers, aerospace… • Structure/Composition: At least 2 elements - sometimes more (BaTiO3, Ti3SiC2) more complex structures than metals Determined by bonding: o ranges from purely ionic to purely covalent o many exhibit combination of ionic & covalent Chapter 12 - 3 Ceramic Bonding • Bonding: Mostly ionic, varying degrees of covalency % ionic character increases with difference in electronegativity (Equation 2.10, Fig. 2.7). • Large vs. small ionic bond character: CaF2: Large SiC: Small Adapted from Fig. 2.7, Callister 7e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University. Chapter 12 - 4 Ceramic Crystal Structures If Predominantly Ionic: Consider as electrically charged ions, not atoms +ve charged metallic ions (cations), e.g. Ca2+ o Given up valence e- to non-metallic anions o Generally smaller than anions - ve charged non-metallic ions (anions), e.g. Fo Accepted valence e- from metallic cations o Generally larger than cations Chapter 12 - 5 Ceramic Crystal Structures Key Factors Influencing Structure: – Magnitude of electrical charges: Crystal must be electrically neutral overall Required charge balance dictates ratio of # cations to anions, e.g. for CaF2 need what? Ca2+ + 2 FAbove implies possible AmXp formula (where m and/or p 1) – Relative sizes (ionic radii) of cations and anions: Cations smaller than anions, thus ratio rC/rA generally <1 When e- removed, remaining valence e- more tightly bound to nucleus, and vice-versa Each cation - prefers max. # nearest-neighbor anions Each anion - prefers max # nearest neighbor cations Determines how ions pack together in crystal Chapter 12 - 6 Ceramic Crystal Structures Close-Packed Structures: – Recall metals…close-packed planes of atoms: AB-AB-AB- and ABC-ABC- stacking of planes resulted in HCP and FCC structures, respectively – In oxide ceramics, can also have close-packed planes…of ions, & unit cells: • Usually anions. Why? • Non-metallic anions (-) much larger than metallic cations (+) – Pack like atoms in metals – e.g. close-packed oxygen anions in a lattice (FCC-like) • Smaller metallic cations may thus fit into the spaces (i.e. the interstitial sites) of the oxygen lattice • Two possible types of interstitial site… Chapter 12 - 7 ...
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