Chapter Lecture 23 Structures and Properties of Ceramics ME 330 Engineering Materials Crystal structures Defects in Ceramics Mechanical properties Read Chapter 12 Chapter ISSUES TO ADDRESS... Structures of ceramic materials: How do they differ from those of metals? Point defects: How are they different from those in metals? Impurities: How are they accommodated in the lattice and how do they affect properties? Mechanical Properties: What special provisions/tests are made for ceramic materials? Chapter 12: Structures & Properties of Ceramics Chapter Ceramics Compounds between metallic and nonmetallic materials; composed of at least two elements => their crystal structures more complex than those of metals Term ceramics comes from Greek work keramicos burnt stuff Desirable properties are achieved through high temperature heat treatment process firing Traditional ceramics (primary raw material clay): china, porcelain, bricks, tiles, glasses New ceramic materials: electronic, computer, communication, aerospace and other applications Chapter Crystal structures Atomic bonding is generally ionic When atomic bonding is predominantly ionic, crystal structures are composed of ions Cations metallic ions (positively charged, give up electrons) Anions nonmetallic ions (negatively charged) Two characteristics of component ions which influence crystal structure: Magnitude of electrical charge on each ion component (crystal must be electrically neutral, chemical formula must reflect that balance) Relative sizes of cations and anions Chapter Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity. 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. Large vs small ionic bond character: Ceramic Bonding SiC: small CaF 2 : large Chapter Which sites will cations occupy? Site Selection 1.Size of sites does the cation fit in the site? 2.Stoichiometry if all of one type of site is full, the remaining have to go into other types of sites. 3.Bond Hybridization Chapter Ionic Bonding & Structure 1. Size - Stable structures: --maximize the # of nearest oppositely charged neighbors. Adapted from Fig. 12.1, Callister 7e unstable Charge Neutrality: --Net charge in the structure should be zero. --General form: stable CaF 2 : Ca 2+ cation F - F - anions + A m X p m, p determined by charge neutrality Chapter Coordination # (# of nearest neighbors) increases with Coordination # and Ionic Radii Adapted from Table 12.2, Callister 7e. 2 r cation r anion Coord # < linear triangular TDTD OHOH cubic Adapted from Fig. 12.2, Callister 7e. Adapted from Fig. 12.3, Callister 7e. Adapted from Fig. 12.4, Callister 7e. ZnS (zincblende) NaCl (sodium chloride) CsCl (cesium chloride) r cation r anion Issue: How many anions can you arrange around a cation? T D tetrahedron, O H - octahedron Chapter Cation Site Size Determine minimum r cation /r anion for O H site (C.N. = 6) a 2r anion Chapter Site Selection II 2.Stoichiometry If all of one type of site is full, the remainder has to go into other types of sites. Ex: FCC unit cell has 4 O H and 8 T D sites. If for a specific ceramic each unit cell has 6 cations and the cations prefer O H sites 4 in O H 2 in T D Chapter Site Selection III 3.Bond Hybridization significant covalent bonding the hybrid orbitals can have impact if significant covalent bond character present For example in SiC X Si = 1.8 and X C = 2.5 ca. 89% covalent bonding both Si and C prefer sp 3 hybridization Therefore in SiC get T D sites Chapter On the basis of ionic radii, what crystal structure would you predict for FeO? Answer: based on this ratio, --coord # = 6 --structure = NaCl Data from Table 12.3, Callister 7e. Example: Predicting Structure of FeO Ionic radius (nm) Cation Anion Al 3+ Fe Ca 2+ O 2- Cl - F - Chapter Rock Salt Structure Same concepts can be applied to ionic solids in general. Example: NaCl (rock salt) structure r Na = nm r Na /r Cl = cations prefer O H sites Adapted from Fig. 12.2, Callister 7e. r Cl = nm Chapter MgO MgO has also the NaCl structure O 2- r O = nm Mg 2+ r Mg = nm r Mg /r O = cations prefer O H sites So each oxygen has 6 neighboring Mg 2+ Adapted from Fig. 12.2, Callister 7e. Chapter AX Crystal Structures AXType Crystal Structures include NaCl, CsCl, and zinc blende Adapted from Fig. 12.3, Callister 7e. Cesium Chloride structure: cubic sites preferred So each Cs + has 8 neighboring Cl - Chapter AX Crystal Structures So each Zn 2+ has 4 neighboring O 2- Zinc Blende structure Adapted from Fig. 12.4, Callister 7e. Size arguments predict Zn 2+ in O H sites, In observed structure Zn 2+ in T D sites Why is Zn 2+ in T D sites? bonding hybridization of zinc favors T D sites Ex: ZnO, ZnS, SiC Chapter AX 2 Crystal Structures Fluorite structure Calcium Fluorite (CaF 2 ) cations in cubic sites UO 2, ThO 2, ZrO 2, CeO 2 antifluorite structure cations and anions reversed Adapted from Fig. 12.5, Callister 7e. Chapter ABX 3 Crystal Structures Perovskite Ex: complex oxide BaTiO 3 Adapted from Fig. 12.6, Callister 7e. Chapter Mechanical Properties We know that ceramics are more brittle than metals. Why? Consider method of deformation slippage along slip planes in ionic solids this slippage is very difficult too much energy needed to move one anion past another anion Chapter Ceramic Density Computation Number of formula units/unit cell Volume of unit cell density A c sum of atomic weights of all cations in the formula unit A A sum of atomic weights of all anions in the formula unit V c unit cell volume N A Avogadros number, x formula units/mol Chapter Silicate Ceramics Most common elements on earth are Si & O SiO 2 (silica) structures are quartz, crystobalite, & tridymite The strong Si-O bond leads to a strong, high melting material (1710C) Si 4+ O 2- Adapted from Figs , Callister 7e. crystobalite Chapter Amorphous Silica Silica gels - amorphous SiO 2 Si 4+ and O 2- not in well-ordered lattice Charge balanced by H + (to form OH - ) at dangling bonds very high surface area > 200 m 2 /g SiO 2 is quite stable, therefore unreactive makes good catalyst support Adapted from Fig , Callister 7e. Chapter Silica Glass Dense form of amorphous silica Charge imbalance corrected with counter cations such as Na + Borosilicate glass is the pyrex glass used in labs better temperature stability & less brittle than sodium glass Chapter Combine SiO 4 4- tetrahedra by having them share corners, edges, or faces Cations such as Ca 2+, Mg 2+, & Al 3+ act to neutralize & provide ionic bonding Silicates Mg 2 SiO 4 Ca 2 MgSi 2 O 7 Adapted from Fig , Callister 7e. Chapter Layered Silicates Layered silicates (clay silicates) SiO 4 tetrahedra connected together to form 2-D plane (Si 2 O 5 ) 2- So need cations to balance charge = Adapted from Fig , Callister 7e. Chapter Kaolinite clay alternates (Si 2 O 5 ) 2- layer with Al 2 (OH) 4 2+ layer Layered Silicates Note: these sheets loosely bound by van der Waals forces Adapted from Fig , Callister 7e. Chapter Carbon Forms Carbon black amorphous surface area ca m 2 /g Diamond tetrahedral carbon hard no good slip planes brittle can cut it large diamonds jewelry small diamonds often man made - used for cutting tools and polishing diamond films hard surface coat tools, medical devices, etc. Adapted from Fig , Callister 7e. Chapter Carbon Forms - Graphite Graphite - layer structure aromatic layers weak van der Waals forces between layers planes slide easily, good lubricant Adapted from Fig , Callister 7e. Chapter Carbon Forms Fullerenes and Nanotubes Fullerenes (bulkyballs) or carbon nanotubes wrap the graphite sheet by curving into ball or tube Buckminister fullerenes Like a soccer ball C 60 - also C 70 + others Adapted from Figs & 12.19, Callister 7e. Chapter Frenkel Defect -- a cation is out of place. Shottky Defect -- a paired set of cation and anion vacancies. Equilibrium concentration of defects Adapted from Fig , Callister 7e. (Fig 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.) Defects in Ceramic Structures Shottky Defect: Frenkel Defect Chapter Impurities must also satisfy charge balance = Electroneutrality Ex: NaCl Substitutional cation impurity Impurities Na + Cl - initial geometryCa 2+ impurityresulting geometry Ca 2+ Na + + Ca 2+ cation vacancy Substitutional anion impurity initial geometry O 2- impurity O 2- Cl - anion vacancy Cl - resulting geometry Chapter Ceramic Phase Diagrams MgO-Al 2 O 3 diagram: Adapted from Fig , Callister 7e. Similar to lead-magnesium diagram Find two eutectic points Chapter Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials. Adapted from Fig , Callister 7e. Measuring Elastic Modulus F L/2 d = midpoint deflection cross section R b d rect.circ. Determine elastic modulus according to: F x linear-elastic behavior F slope = E = F L 3 4bd 3 = F L 3 12 R 4 rect. cross section circ. cross section Chapter point bend test to measure room T strength. Adapted from Fig , Callister 7e. Measuring Strength F L/2 d = midpoint deflection cross section R b d rect.circ. location of max tension Flexural strength: Typ. values: Data from Table 12.5, Callister 7e. rect. fs 1.5F f L bd 2 F f L R3R3 Si nitride Si carbide Al oxide glass (soda) Material fs (MPa) E(GPa) x F FfFf fs Chapter Measuring Elevated T Response time Elevated Temperature Tensile Test (T > 0.4 T m ). creep test slope = ss = steady-state creep rate. x Chapter Ceramic materials have covalent & ionic bonding. Structures are based on: -- charge neutrality -- maximizing # of nearest oppositely charged neighbors. Structures may be predicted based on: -- ratio of the cation and anion radii. Defects -- must preserve charge neutrality -- have a concentration that varies exponentially w/T. Room T mechanical response is elastic, but fracture is brittle, with negligible deformation. Elevated T creep properties are generally superior to those of metals (and polymers). Summary