Chapter 12: Structures & Properties of Ceramics ISSUES TO ADDRESS... • How do the crystal structures of ceramic materials differ from those for metals? • How do point defects in ceramics differ from those defects found in metals? • How are impurities accommodated in the ceramic lattice? • In what ways are ceramic phase diagrams different from phase diagrams for metals? • How are the mechanical properties of ceramics measured, and how do they differ from those for metals?

Atomic Bonding in Ceramics -- ______________________________________. -- % ionic character __________ with difference in electronegativity of atoms. • Degree of ionic character may be large or small: CaF2: large SiC: small

Ceramic Crystal Structures Oxide structures oxygen anions ________ than metal cations close _______ oxygen in a ______ (usually ____) cations fit into ________ sites among ______ ions

Factors that Determine Crystal Structure 1. Relative sizes of ions – ________________________: --maximize the # of ____________________________. - + ________ - + - + Adapted from Fig. 12.1, Callister & Rethwisch 9e. ______ stable 2. Maintenance of Charge Neutrality : --_________________ should be zero. --Reflected in chemical formula: CaF 2 : Ca 2+ cation F - anions + A m X p m, p values to achieve charge neutrality

Coordination Number and Ionic Radii cation anion • __________ Number increases with To form a ______ structure, how many anions can surround around a cation? Adapted from Fig. 12.2, Callister & Rethwisch 9e. Adapted from Fig. 12.3, Callister & Rethwisch 9e. Adapted from Fig. 12.4, Callister & Rethwisch 9e. ZnS (zinc blende) NaCl (sodium chloride) CsCl (cesium r cation anion Coord. Number < 0.155 2 linear 0.155 - 0.225 3 _______ 0.225 - 0.414 4 tetrahedral 0.414 - 0.732 6 octahedral 0.732 - 1.0 8 _____ Adapted from Table 12.2, Callister & Rethwisch 9e.

Computation of Minimum Cation-Anion Radius Ratio Determine ________ rcation/ranion for an octahedral site (C.N. = __) __________

Bond Hybridization Bond Hybridization is possible when there is significant ____________ bonding ____________________________ For example for SiC XSi = 1.8 and XC = 2.5 ~ 89% ___________ bonding Both Si and C prefer sp3 hybridization Therefore, for SiC, Si atoms occupy _______________ sites

Example Problem: Predicting the Crystal Structure of FeO • On the basis of ionic radii, what ___________________ would you predict for FeO? Cation Anion Al 3+ Fe 2 + Ca 2+ O 2- Cl - F Ionic radius (nm) 0.053 0.077 0.069 0.100 0.140 0.181 0.133 • Answer: based on this ratio, -- coord # = __ because 0.414 < 0.550 < 0.732 -- crystal structure is ____ Data from Table 12.3, Callister & Rethwisch 9e.

Rock Salt Structure Same concepts can be applied to ____ solids in general. Example: NaCl (rock salt) structure rNa = 0.102 nm rCl = _____ nm rNa/rCl = _________ cations (Na+) prefer __________ sites Adapted from Fig. 12.2, Callister & Rethwisch 9e.

MgO and FeO MgO and FeO also have the NaCl structure O2- rO = 0.140 nm Mg2+ rMg = 0.072 nm rMg/rO = _______ _______ prefer octahedral sites Adapted from Fig. 12.2, Callister & Rethwisch 9e. So each Mg2+ (or Fe2+) ______ neighbor oxygen atoms

AX Crystal Structures AX–Type Crystal Structures include NaCl, CsCl, and zinc blende Cesium Chloride structure:  Since 0.732 < ____ < 1.0, _____ sites preferred So each Cs+ has __ neighbor Cl- Fig. 12.3, Callister & Rethwisch 9e.

AX2 Crystal Structures Fluorite structure Calcium ______ (CaF2) Cations in _______ sites UO2, ThO2, ZrO2, CeO2 ___________ structure – positions of cations and anions reversed Fig. 12.5, Callister & Rethwisch 9e.

ABX3 Crystal Structures Ex: complex oxide _________ Fig. 12.6, Callister & Rethwisch 9e.

VMSE Screenshot – Zinc Blende Unit Cell

Density Computations for Ceramics Number of formula units/unit cell __________ number Volume of unit cell = sum of atomic weights of ____________________ = sum of atomic weights of ____________________

Silicate Ceramics Most common ___________________________ SiO2 (silica) _________________ forms are quartz, crystobalite, & tridymite The strong Si-O bonds lead to a high ____________ temperature (1710ºC) for this material Si4+ O2- Figs. 12.9 & 12.10, Callister & Rethwisch 9e crystobalite

Silicates Bonding of adjacent SiO44- accomplished by the sharing of common _____________________ Adapted from Fig. 12.12, Callister & Rethwisch 9e. Mg2SiO4 Ca2MgSi2O7 Presence of cations such as Ca2+, Mg2+, & Al3+ 1. maintain charge ____________, and 2. ___________ bond SiO44- to one another

Glass Structure • Basic Unit: Glass is ____________ (_________) • ____________ is SiO2 to which no impurities have been added • Other common _________ contain impurity ions such as Na+, Ca2+, Al3+, and B3+ Si0 4 tetrahedron 4- Si 4+ O 2 - • Quartz is _____________ SiO2: Si 4+ Na + O 2 - (soda glass) Adapted from Fig. 12.11, Callister & Rethwisch 9e.

Layered Silicates Layered ______ (e.g., clays, mica, talc) SiO4 ___________ connected together to form 2-D plane A net negative charge is associated with each (Si2O5)2- unit Negative charge balanced by ________ plane rich in positively charged __________ Fig. 12.13, Callister & Rethwisch 9e.

Layered Silicates (cont) Kaolinite clay _________ (Si2O5)2- layer with Al2(OH)42+ layer Fig. 12.14, Callister & Rethwisch 9e. Note: Adjacent sheets of this type _________ bound to one another by ________________________.

Polymorphic Forms of Carbon Diamond tetrahedral bonding of carbon ______________________ ______________________ conductivity large single crystals – gem stones small ________ – used to grind/cut other materials ________ thin films hard surface coatings – used for cutting tools, medical devices, etc. Fig. 12.16, Callister & Rethwisch 9e.

Polymorphic Forms of Carbon (cont) ____________ _________ structure – parallel ________ arrays of carbon atoms weak van der Waal’s forces between layers planes slide easily over one another -- good lubricant Fig. 12.17, Callister & Rethwisch 9e.

Point Defects in Ceramics (i) • Vacancies -- vacancies exist in _______ for both ___________________ • Interstitials -- interstitials exist for __________ -- interstitials are not normally observed for ___________________ are large relative to the interstitial sites Cation Interstitial Vacancy Anion Fig. 12.18, Callister & Rethwisch 9e. (From W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, p.78. Copyright ©1964 by John Wiley & Sons, New York. Reprinted by permission of John Wiley and Sons, Inc.)

Point Defects in Ceramics (ii) • Frenkel Defect -- ____________________________________. • Shottky Defect -- ____________________________________. Shottky Defect: Frenkel Defect Fig. 12.19, Callister & Rethwisch 9e. (From W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, p.78. Copyright ©1964 by John Wiley & Sons, New York. Reprinted by permission of John Wiley and Sons, Inc.) • Equilibrium concentration of defects

Imperfections in Ceramics • ________________ (charge ______) must be maintained when impurities are present Na + Cl - • Ex: NaCl • Substitutional cation impurity without impurity Ca 2+ impurity with impurity Na + cation vacancy • Substitutional anion impurity without impurity O 2- impurity Cl - an ion vacancy with impurity

Ceramic Phase Diagrams MgO-Al2O3 diagram: Fig. 12.23, Callister & Rethwisch 9e. [Adapted from B. Hallstedt, “Thermodynamic Assessment of the System MgO–Al2O3,” J. Am. Ceram. Soc., 75[6], 1502 (1992). Reprinted by permission of the American Ceramic Society.] 

Mechanical Properties ___________ materials are more brittle than _________. Why is this so? Consider _________________________________ In crystalline, by ____________ motion In highly ionic solids, dislocation motion is difficult few _______________ resistance to motion of ions of like charge (e.g., anions) past one another

Flexural Tests – Measurement of Elastic Modulus • Room T behavior is usually _______, with _______ failure. • _____________________ often used. -- tensile tests are difficult for ________ materials. F L/2 δ = midpoint deflection cross section R b d rect. circ. Adapted from Fig. 12.30, Callister & Rethwisch 9e. • Determine ________________ according to: F x linear-elastic behavior δ slope = (rect. cross section) (circ. cross section)

Flexural Tests – Measurement of _________ Strength • ____________ test to measure room-T flexural strength. F L/2 δ = midpoint deflection cross section R b d rect. circ. location of max tension Adapted from Fig. 12.30, Callister & Rethwisch 9e. • __________ strength: • Typical values: Data from Table 12.5, Callister & Rethwisch 9e. Si nitride Si carbide Al oxide glass (soda-lime) 250-1000 100-820 275-700 69 304 345 393 Material σ fs (MPa) E(GPa) (rect. ____________) (circ. ____________)

SUMMARY • Interatomic bonding in ceramics is ionic and/or covalent. • Ceramic crystal structures are based on: -- maintaining charge neutrality -- cation-anion radii ratios. • Imperfections -- Atomic point: vacancy, interstitial (cation), Frenkel, Schottky -- Impurities: substitutional, interstitial -- Maintenance of charge neutrality • Room-temperature mechanical behavior – flexural tests -- linear-elastic; measurement of elastic modulus -- brittle fracture; measurement of flexural modulus