Most common elements on earth are Si & O SiO 2 (silica) crystalline structures include quartz, crystobalite, & tridymite The strong Si-O bond leads to a strong, high melting material (1710ºC) Silicate Ceramics Si 4+ O 2- Crystobalite – Polymorph of SiO 2 SiO 4 4- tetrahedron is the fundamental building block of all silicates covalent

Amorphous 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 “Fused Silica” or “vitreous silica” Network formers – Silica tetrahedron Other oxides like B 2 O 3, GeO 2 Network modifiers – other cations that are incorporated into and modify SiO 4 4- network Intermediates – other oxides (TiO 2 or Al 2 O 3 ), not network formers, substitute for silicon and become part of and stabilize the network Change melting point with modifiers and intermediates

–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 – Minerals Mg 2 SiO 4 Ca 2 MgSi 2 O 7 Forsterite Akermanite

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 =

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 Waal’s forces Adapted from Fig , Callister 7e.

Frenkel Defect -- a cation is out of place. Schottky Defect -- a paired set of cation and anion vacancies. Equilibrium concentration of defects Defects in Ceramic Structures Schottky Defect: Frenkel Defect

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

Ceramic Phase Diagrams MgO-Al 2 O 3 diagram: 

Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials. 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

3-point bend test to measure room T strength. 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 R3R3 Si nitride Si carbide Al oxide glass (soda) Material  fs (MPa) E(GPa) x F FfFf  fs 

Chapter 14 – Polymers What is a polymer? Poly mer many repeat unit CCCCCC HHHHHH HHHHHH Polyethylene (PE) Cl CCCCCC HHH HHHHHH Polyvinyl chloride (PVC) HH HHHH Polypropylene (PP) CCCCCC CH 3 HH H repeat unit repeat unit repeat unit

Polymer Composition Most polymers are hydrocarbons – i.e. made up of H and C Saturated hydrocarbons –Each carbon bonded to four other atoms C n H 2n+2

Unsaturated Hydrocarbons Double & triple bonds relatively reactive – can form new bonds –Double bond – ethylene or ethene - C n H 2n 4-bonds, but only 3 atoms bound to C’s –Triple bond – acetylene or ethyne - C n H 2n-2

Isomerism –two compounds with same chemical formula can have quite different structures Ex: C 8 H 18 n-octane 2-methyl-4-ethyl pentane (isooctane) 

Chemistry of Polymers Free radical polymerization Initiator: example - benzoyl peroxide

Polyethylene Adapted from Fig. 14.1, Callister 7e. Note: polyethylene is just a long HC - paraffin is short polyethylene

Bulk or Commodity Polymers

MOLECULAR WEIGHT M w is more sensitive to higher molecular weights Molecular weight, M i : Mass of a mole of chains. Lower M higher M Fraction Mean of size range Weight fraction

Molecular Weight Calculation Example: average mass of a class

Degree of Polymerization, n n = number of repeat units per chain n i = 6 mol. wt of repeat unit i Chain fraction

End to End Distance, r

Covalent chain configurations and strength: Direction of increasing strength Molecular Structures BranchedCross-LinkedNetworkLinear secondary bonding

Polymers – Molecular Shape Conformation – Molecular orientation can be changed by rotation around the bonds –note: no bond breaking needed Adapted from Fig. 14.5, Callister 7e.

Polymers – Molecular Shape Configurations – to change must break bonds Stereoisomerism mirror plane

Tacticity Tacticity – stereoregularity of chain isotactic – all R groups on same side of chain syndiotactic – R groups alternate sides atactic – R groups random

cis/trans Isomerism cis cis-isoprene (natural rubber) bulky groups on same side of chain trans trans-isoprene (gutta percha) bulky groups on opposite sides of chain

Copolymers two or more monomers polymerized together random – A and B randomly vary in chain alternating – A and B alternate in polymer chain block – large blocks of A alternate with large blocks of B graft – chains of B grafted on to A backbone A – B – random block graft alternating