Ch 12 pt 3

Ch 12 pt 3 - AmBnXp Crystal Structures • >1 type...

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Unformatted text preview: AmBnXp Crystal Structures • >1 type of cation, e.g. A and B • Perovskite structure… • e.g.: complex oxide BaTiO3 Cubic crystal structure Ba2+ cations at corners Ti4+ cation in center O2- anions at each face center Adapted from Fig. 12.6, Callister 7e. Chapter 12 - 24 Ceramic Density Computation Number of formula units/unit cell n ( AC + AA ) = VC N A Mass of ions/unit cell = Volume of unit cell (# of cations/UC)(atomic wt. of cation) + (# of anions/UC)(atomic wt. of anion) VCNA Volume of unit cell Check out Example Problem 12.3 Chapter 12 - 25 Silicate Ceramics The most common elements on Earth are Si & O: Si4+ O2- Adapted from Figs. 12.9-10, Callister 7e. • • • • Crystobalite Basic unit is SiO44- tetrahedron rather than unit cells… – Each Si atom bonded to 4 O atoms at corners of tetrahedron – Considered as -ve charged entity (- 4 charge) Strong, directional Si-O bond (significant covalent character) leads to strong, high melting point material (Tm = 1710 ºC) 1, 2 & 3-D network silicate structures…quartz, crystobalite & tridymite (silica, SiO2) – Crystalline if tetrahedra arrayed in regular/ordered manner – Corner O atoms shared by adjacent tetrahedra, neutral Chapter 12 - 26 Open structure leads to low density (~2.65 g/cm3) Amorphous Silica - Silica Glasses • Silica Glasses - amorphous SiO2: Si4+ and O2- not in well-ordered lattice…significant randomness Charge balanced by H+ (to form OH-) at “dangling” bonds • very high surface area > 200 m2/g SiO2 is quite stable, therefore unreactive: • makes good catalyst support Add network modifiers (CaO, Na2O) to form sodium-silicate glass: • cations incorporated into/modify SiO44- network • lower Tm and viscosity Adapted from Fig. 12.11, Callister 7e. Chapter 12 - 27 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 or as cookware: • Better temperature stability & less brittle than sodium glass Chapter 12 - 28 Silicates Combine SiO44- tetrahedra by having them share 1, 2 or 3 corner O atoms to form complex structures Mg2SiO4 Ca2MgSi2O7 Adapted from Fig. 12.12, Callister 7e. +ve charged cations such as Ca2+, Mg2+ & Al3+ act to neutralize -ve charge & ionically bond tetrahedra together chain structures also possible… Chapter 12 - 29 Layered Silicates • Layered silicates (clay silicates) – SiO44- tetrahedra connected together to form 2-D plane or sheet • (Si2O5)2- repeat unit: – 3 O ions shared in each tetrahedron – net -ve charge due to unbonded O2- atom out of plane – electroneutrality via 2nd planar sheet w/ excess cations bonding to unbonded O atoms from Si2O5 sheet = Adapted from Fig. 12.13, Callister 7e. Chapter 12 - 30 Layered Silicates • Kaolinite clay alternates (Si2O5)2- layers with Al2(OH)42+ layers which make overall electrically neutral… O2- ions from (Si2O5)2- and OHions from Al2(OH)42+ layer Adapted from Fig. 12.14, Callister 7e. Strong ionic-covalent bonding within 2-layer sheet Only weak Van der Waal’s forces between sheets Chapter 12 - 31 Layered Silicates • Can vary the counter ions: – changes layer spacing – 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 wells so polluted ground water cannot seep in to contaminate water supply Chapter 12 - 32 ANNOUNCEMENTS Week 6 Recitations: Problems from Chapter 14 (Polymers) No Quiz Chapter 12 - 33 Forms of Carbon Carbon Black - amorphous surface area ~1000 m2/g Diamond: tetrahedral carbon (like zinc blende) strong covalent bonds • hard…no good slip planes • brittle…can cut it large diamonds…jewelry small diamonds: • often synthetic • used for cutting tools, polishing Adapted from Fig. 12.15, Callister 7e. diamond films: • hard surface coatings…tools, medical devices, etc. Chapter 12 - 34 Forms of Carbon - Graphite • Layered structure…aromatic (ring) layers of hexagonal C atoms: – Each C atom covalently bonded to 3 coplanar C atoms Adapted from Fig. 12.17, Callister 7e. – 4th bonding e- participates in weak Van der Waal’s forces between layers – Planes thus slide/cleave easily, good lubricant Chapter 12 - 35 – Good electrical conductor // to hexagonal sheets Newer Forms of Carbon – Fullerenes and Nanotubes • Fullerenes (1985) or Carbon Nanotubes (CNTs) (1991): – Wrap graphite (graphene) sheet by curving into ball or tube – Fullerenes: • 5 and 6 member rings - closed structures • like a soccer ball C60 - also C70 + others Adapted from Figs. 12.18 & 12.19, Callister 7e. Chapter 12 - 36 Summary • Ceramic materials have covalent & ionic bonding • Structures are based on: - need to maintain charge neutrality - maximizing # of nearest oppositely-charged neighbors • Structures may be predicted based on: - ratio of cation and anion radii…geometrical Chapter 12 - 37 From here on not included Chapter 12 - 38 Mechanical Properties We know that ceramics are more brittle than metals. Why? • Consider method of deformation: slippage along slip planes (metals) • in ionic solids (like ceramics) this slippage is very difficult • too much energy needed to move one (large) anion past another anion Chapter 12 - 39 Defects in Ceramic Structures • Frenkel Defect: a cation out of place • Shottky Defect: a paired set of cation and anion vacancies Shottky Defect: Frenkel Defect • Equilibrium concentration of defects ~e 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.) QD / kT Chapter 12 - 40 Impurities • Impurities must also satisfy charge balance = Electroneutrality • e.g.: NaCl Na + Cl - • Substitutional cation impurity cation vacancy Ca2+ Na+ Na+ initial geometry Ca2+ impurity Ca2+ resulting geometry • Substitutional anion impurity anion vacancy O2- Clinitial geometry Cl- O2- impurity resulting geometry Chapter 12 - 41 Ceramic Phase Diagrams MgO-Al2O3 diagram: ° Adapted from Fig. 12.25, Callister 7e. Chapter 12 - 42 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 d L/2 F Adapted from Fig. 12.32, Callister 7e. L/2 R b = midpoint deflection Rect. Circ. • Determine elastic modulus according to: F x slope = E= F linear-elastic behavior F L3 4bd 3 rect. cross section = F L3 12 R4 circ. cross section Chapter 12 - 43 Measuring Strength • 3-Point Bend test to measure room T strength Cross-section d b Rect. L/2 F L/2 Adapted from Fig. 12.32, Callister 7e. R = midpoint deflection Circ. location of max tension • Typical Values • Flexural Strength: fs Ff F = 1.5Ff L bd 2 x rect. = Ff L R3 Material fs(MPa) E (GPa) Si nitride 250-1000 304 Si carbide 100-820 345 Al oxide 275-700 393 Glass (soda) 69 69 Data from Table 12.5, Callister 7e. fs Chapter 12 - 44 Measuring Elevated T Response • Elevated temperature tensile test (T > 0.4Tm) Creep Test x . slope = ss = steady-state creep rate time Chapter 12 - 45 Summary • Defects in Ionic Solids (Ceramics): 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 (i.e. lower) to those of metals (and polymers) Chapter 12 - 46 ...
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This note was uploaded on 07/27/2011 for the course ENGR 134 taught by Professor Marks during the Spring '11 term at Drexel.

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