Metal Structure Atoms held together by metallic bonding Crystalline structures in the solid state, almost without exception BCC, FCC, or HCP unit cells Bodycentered cubic (BCC) Chromium, Iron, Molybdenum, Tungsten Facecentered cubic (FCC) Aluminum, Copper, Gold, Lead, Silver, Nickel Hexagonal closepacked (HCP) Magnesium, Titanium, Zinc 48
Ceramic Structure Most ceramics have crystal structure, while glass (SiO 2 ) is amorphous Molecules characterized by ionic or covalent bonding, or both % ionic character increases with difference in electronegativity 49
Ceramic Bonding Large vs small ionic bond character: H 2.1 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 Be 1.5 Mg 1.2 Ca 1.0 Sr 1.0 Ba 0.9 Ra 0.9 Ti 1.5 Cr 1.6 CaF2: large SiC: small Fe 1.8 Ni 1.8 Zn 1.8 C 2.5 Si 1.8 Table of Electronegativities As 2.0 F 4.0 Cl 3.0 Br 2.8 I 2.5 At 2.2 He Ne Ar Kr Xe Rn 50
Ionic Bonding & Structure Charge Neutrality: Net charge in the structure should be zero. General form: AmXp CaF2: Ca 2+ cation + F anions F m, p determined by charge neutrality Stable structures: maximize the # of nearest oppositely charged neighbors. + unstable + stable + stable 51
Coordination # & Ionic Radii r cation r anion Coordination # increases with Issue: How many anions can you arrange around a cation? r cation r anion Coord # <.155 2 ZnS (zincblende).155.225.225.414.414.732.7321.0 3 4 6 8 NaCl (sodium chloride) CsCl (cesium chloride) 52
Ex: Predicting Structure of FeO On the basis of ionic radii, what crystal structure would you predict for FeO? Cation Al 3+ Fe 2+ Fe 3+ Ca 2+ Anion O 2 Cl F Ionic radius (nm) 0.053 0.077 0.069 0.100 0.140 0.181 0.133 Answer: r cation r anion = 0.077 0.140 = 0.550 based on this ratio, coord # = 6 structure = NaCl 53
Consider CaF2 : A m X p Structures r cation r anion = 0.100 0.133 0.8 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 #Ca 2+ ions = 1/2 # F ions. 54
Polymer Microstructure Polymer = many mers H H mer H H H H H H mer H H H H H H C C C C C C C C C C C C H H H H H H H Cl H Cl H Cl Polyethylene (PE) Polyvinyl chloride (PVC) mer H H H H C C C C C C H CH3 H CH3 H CH3 Polypropylene (PP) Adapted from Fig. 14.2, Callister 6e. Covalent chain configurations and strength: secondary bonding Linear Branched CrossLinked Network Direction of increasing strength 55
Molecular Weight & Crystallinity Molecular weight, Mw: Mass of a mole of chains. smaller Mw larger Mw Tensile strength (TS): often increases with Mw. Why? Longer chains are entangled (anchored) better. % Crystallinity: % of material that is crystalline. TS and E often increase with % crystallinity. Annealing causes crystalline regions to grow. % crystallinity increases. crystalline region amorphous region 56
Summary Metals are usually crystalline in BCC, FCC or HCP. Ceramic materials have mostly covalent & some 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. Most polymers are based on carbon and are therefore considered organic chemicals. Both amorphous and crystalline structures are possible, although the tendency to crystallize is much less than for metals or nonglass ceramics. 57