Chapter 12 Ceramic Structures PDF

Chapter 12: Structures & Properties of Ceramics 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 aff...

Chapter 12: Structures & Properties of Ceramics 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 - 1  Ceramics Compounds of metallic and non-metallic elements mainly oxides, nitrides, and carbides. – Examples: Al2O3 (alumina), SiC (silicon carbide), Si3N4 (silicon nitride), and ZrO2 (zirconia). Insulative to electricity and heat. Better resistance to high temperatures and harsh environments than metals and polymers. Hard and brittle. Desirable properties of ceramics are achieved through a high temperature heat treatment called firing. – Traditional Ceramics: china clay, porcelain, bricks, tiles, glasses, cement. – Advanced Ceramics: used in electronic, computer, communication, aerospace, and other industries. Chapter 12 - 2 Space shuttle is protected by ceramic tiles. Refractory Brickwork in kilns www.fastfire.co.uk/ f_brick.htm http://rayjasm.tripod.com/sts Piston tops are coated High purity alumina and with ceramic coatings zirconia used in spark plugs www.engineceramics.com/ fullservice.htm Chapter 12 - 3 www.ceramicstoday.com/ articles/zamek.htm  Ceramic Bonding Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity. 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 Oxide structures – oxygen anions much larger than metal cations – close packed oxygen in a lattice (usually FCC) – cations in the holes of the oxygen lattice Which sites will cations occupy? 1. Size of sites – does the cation fit in the site 2. Stoichiometry – if all of one type of site is full the remainder have to go into other types of sites. 3. Bond Hybridization Chapter 12 - 5  Ionic Bonding & Structure 1. Size - Stable structures: --maximize the # of nearest oppositely charged neighbors. - - - - - - + + + Adapted from Fig. 12.1, Callister 7e. - - - - - - unstable stable stable Charge Neutrality: --Net charge in the F- structure should CaF 2 : Ca 2+ + cation anions be zero. F- --General form: A m Xp m, p determined by charge neutrality Chapter 12 - 6  Coordination # and Ionic Radii r cation Coordination # increases with r anion Issue: How many anions can you arrange around a cation? r cation Coord ZnS r anion # (zincblende) Adapted from Fig. < 0.155 2 linear 12.4, Callister 7e. 0.155 - 0.225 3 triangular NaCl (sodium 0.225 - 0.414 4 TD chloride) Adapted from Fig. 12.2, Callister 7e. 0.414 - 0.732 6 OH CsCl (cesium 0.732 - 1.0 8 cubic chloride) Adapted from Fig. Adapted from Table 12.2, 12.3, Callister 7e. Callister 7e. Chapter 12 - 7  AmXp STRUCTURES Consider CaF2 : Based on this ratio, coord # = 8 and structure = CsCl. Result: CsCl structure w/only half the cation sites occupied. Only half the cation sites are occupied since #Ca2+ ions = 1/2 # F- ions. Adapted from Fig. 12.5, Callister 6e. Chapter 12 -8  Cation Site Size Determine minimum rcation/ranion for OH site (C.N. = 6) a  2ranion Chapter 12 - 9  Site Selection II 2. Stoichiometry – If all of one type of site is full the remainder have to go into other types of sites. Ex: FCC unit cell has 4 OH and 8 TD sites. If for a specific ceramic each unit cell has 6 cations and the cations prefer OH sites 4 in OH 2 in TD Chapter 12 - 10  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 XSi = 1.8 and XC = 2.5 ca. 89% covalent bonding both Si and C prefer sp3 hybridization Therefore in SiC get TD sites Chapter 12 - 11  Example: Predicting Structure of FeO On the basis of ionic radii, what crystal structure would you predict for FeO? Cation Ionic radius (nm) Answer: Al 3+ 0.053 Fe 2+ 0.077 Fe 3+ 0.069 Ca 2+ 0.100 based on this ratio, --coord # = 6 Anion --structure = NaCl O2- 0.140 Cl - 0.181 F- Data from Table 12.3, 0.133 Callister 7e. Chapter 12 - 12  MgO and FeO MgO and FeO also have the NaCl structure O2- rO = 0.140 nm Mg2+ rMg = 0.072 nm rMg/rO = 0.514  cations prefer OH sites Adapted from Fig. 12.2, Callister 7e. So each oxygen has 6 neighboring Mg2+ Chapter 12 - 13  AX Crystal Structures AX–Type Crystal Structures include NaCl, CsCl, and zinc blende Cesium Chloride structure: rCs  0.170  0.939 rCl 0.181  cubic sites preferred So each Cs+ has 8 neighboring Cl- Adapted from Fig. 12.3, Callister 7e. Chapter 12 - 14  AX Crystal Structures Zinc Blende structure Size arguments predict Zn2+ in OH sites, In observed structure Zn2+ in TD sites Why is Zn2+ in TD sites? – bonding hybridization of zinc favors TD sites So each Zn2+ has 4 neighboring O2- Adapted from Fig. 12.4, Callister 7e. Ex: ZnO, ZnS, SiC Chapter 12 - 15  AX2 Crystal Structures Fluorite structure Calcium Fluorite (CaF2) cations in cubic sites UO2, ThO2, ZrO2, CeO2 antifluorite structure – cations and anions reversed Adapted from Fig. 12.5, Callister 7e. Chapter 12 - 16  ABX3 Crystal Structures Perovskite structure Ex: complex oxide BaTiO3 Fig. 12.6, Callister & Rethwisch 9e. Chapter 12 - 17 17 Ceramic Density Computation Number of formula units/unit cell Volume of unit cell Chapter 12 - 18  Silicate Ceramics Most common elements on earth are Si & O Si4+ O2- Adapted from Figs. 12.9-10, Callister 7e. crystobalite Crystalline SiO2 (silica) structures have 3 polymorphic forms: quartz, crystobalite, & tridymite The strong Si-O bond leads to a high melting material (1710ºC) Chapter 12 - 19  Silicates Bonding of adjacent SiO44- accomplished by the sharing of common corners, edges, or faces Adapted from Fig. 12.12, Callister & Rethwisch 9e. Mg2SiO4 Ca2MgSi2O7 Presence of cations such as Ca2+, Mg2+, & Al3+ 1. maintain charge neutrality, and 2. ionically bond SiO44- to one another Chapter 12 - 20 20  Glass Structure Basic Unit: Glass is noncrystalline (amorphous) 4- Fused silica is SiO2 to which no Si0 4 tetrahedron impurities have been added Si 4+ Other common glasses contain O2- impurity ions such as Na+, Ca2+, Al3+, and B3+ Quartz is crystalline Na+ SiO2: Si 4+ O2- (soda glass) Adapted from Fig. 12.11, Callister & Rethwisch 9e. Chapter 12 - 21  Layered Silicates Layered silicates (clay silicates) – SiO4 tetrahedra connected together to form 2-D plane Adapted from Fig. (Si2O5)2- 12.13, Callister 7e. So need cations to balance charge = Chapter 12 - 22  Layered Silicates Kaolinite clay alternates (Si2O5)2- layer with Al2(OH)42+ layer Adapted from Fig. 12.14, Callister 7e. Note: these sheets loosely bound by van der Waal’s forces Chapter 12 - 23  Layered Silicates Can change the counterions – this changes layer spacing – the layers also allow absorption of water Micas KAl3Si3O10(OH)2 Bentonite – used to seal wells – packaged dry – swells 2-3 fold in H2O – pump in to seal up well so no polluted ground water seeps in to contaminate the water supply. Chapter 12 - 24  Carbon Forms Carbon black – amorphous – surface area ca. 1000 m2/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 Adapted from Fig. hard surface coat – tools, 12.15, Callister 7e. medical devices, etc. Chapter 12 - 25  Carbon Forms - Graphite layer structure – aromatic layers Adapted from Fig. 12.17, Callister 7e. – weak van der Waal’s forces between layers – planes slide easily, good lubricant Chapter 12 - 26  Carbon Forms – Fullerenes and Nanotubes Fullerenes or carbon nanotubes – wrap the graphite sheet by curving into ball or tube – Buckminister fullerenes Like a soccer ball C60 - also C70 + others Adapted from Figs. 12.18 & 12.19, Callister 7e. Chapter 12 - 27 Carbon Nanotube (CNT): Recently discovered polymorph of carbon in 1990s. Molecular structure consists of single sheet of graphite rolled into tube and at the ends capped with C60 hemispheres. It is called nanotube because the diameter of tube is at nano level (less than 100 nm). Applications Tensile strength is 50 to 200 GPa - high performance Elastic modulus is in TPa. composites Fracture strain is 5% to 20%. -in flat-panel full-color 1040 steel; TS = 800 MPa, E = 110 displays as field emiiters GPa - future applications CNT’s - diodes, TS is ~250 times higher than 1040 - transistors, steel E is ~10 times higher than 1040 steel Chapter 12 - 28  Point Defects in Ceramics (i) Vacancies -- vacancies exist in ceramics for both cations and anions Interstitials -- interstitials exist for cations -- interstitials are not normally observed for anions because anions are large relative to the interstitial sites Cation Interstitial Cation Vacancy 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.) Anion Chapter 12 - 29 Vacancy  Defects in Ceramic Structures Frenkel Defect --a cation is out of place. Shottky Defect --a paired set of cation and anion vacancies. Shottky Defect: Adapted from Fig. 12.21, Callister 7e. (Fig. 12.21 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.) Frenkel Defect Equilibrium concentration of defects Chapter 12 - 30  Imperfections in Ceramics Impurities must also satisfy charge balance = Electroneutrality Ex: NaCl Na + Cl - cation Substitutional cation impurity vacancy Ca 2+ Na + Na + Ca 2+ initial geometry Ca 2+ impurity resulting geometry Substitutional anion impurity anion vacancy O2- Cl - Cl - initial geometry O2- impurity resulting geometry Chapter 12 - 31  Ceramic Phase Diagrams MgO-Al2O3 diagram:  Adapted from Fig. 12.25, Callister 7e. Chapter 12 - 32  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 12 - 33  Measuring Elastic Modulus Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials. cross section F L/2 L/2 Adapted from Fig. 12.32, Callister 7e. d R b d = midpoint rect. circ. deflection Determine elastic modulus according to: F F L3 F L3 x E= = F  4bd 3  12  R4 slope =  rect. circ. cross cross  section section linear-elastic behavior Chapter 12 - 34  Measuring Strength 3-point bend test to measure room T strength. cross section F L/2 L/2 Adapted from Fig. 12.32, d R Callister 7e. b d = midpoint rect. circ. deflection location of max tension Flexural strength: Typ. values: Material fs (MPa) E(GPa) 1.5Ff L Ff L  fs   Si nitride 250-1000 304 bd 2 R3 Si carbide 100-820 345 F rect. Al oxide 275-700 393 Ff x glass (soda) 69 69 Data from Table 12.5, Callister 7e.  fs Chapter 12 - 35  Measuring Elevated T Response Elevated Temperature Tensile Test (T > 0.4 Tm). creep test   x. slope = ss = steady-state creep rate  time Generally,... Steady state creep rate of ceramics is lowest. Chapter 12 - 36  Summary 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). Chapter 12 - 37