Order in materials. Making Solid Stuff. Primary Bonds Summary. How do they arrange themselves? Results from atomic bonding. What are they?

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1 Making Solid Stuff Primary Bonds Summary What are they? Results from atomic bonding So the atoms bond together! Order in materials No long range order to atoms Gases little or no interaction between components How do they arrange themselves? Liquids Short range order Inorganic and organic glasses Solids Long range order Metals, ceramics & polymers

2 Packing atoms together Long Range Atomic Order Crystalline materials... atoms pack in periodic, 3D arrays typical of: -metals -many ceramics -some polymers Short Range Atomic Order Noncrystalline materials... atoms have no periodic packing occurs for: -complex structures -rapid cooling "Amorphous" = Noncrystalline Si crystalline SiO2 Adapted from Fig. 3.18(a), Callister 6e. Oxygen noncrystalline SiO2 Adapted from Fig. 3.18(b), Callister 6e. From Callister 6e resource CD. Stacking atoms together Crystal Structure Stacking Oranges Hard Sphere Model Atoms in a crystal represented by hard sphere each atom is surrounded by as many other atoms as possible i.e minimum energy state Gives rise to coordination number number of contacting neighbours any one atom has This is a function of directionality of What controls the nearest number of atoms?

3 Hard Sphere Model Simple case - Nondirectionally bonded atoms of equal size Relative atom size # of atoms around each atom Directionality of bond Metals & noble elements expect to solidify in closest packed arrangement as possible WHY? # of bonds per unit vol maximised hence bonding energy per unit volume minimised How can these oranges pack? What is the maximum number of spheres that can pack around one sphere? Such a structure is said to be CLOSE PACKED

4 Two such close packed arrangements Close packing of atoms HCP Tetrahedral site Octahedral site face centred cubic FCC Hexagonal close packed HCP These names come from the geometry that results Accounts for about 2/3 of all metals All the noble metals at low T Hexagonal Closed-Packed Crystal Structure - HCP Figure 1.4 The hexagonal close-packed (hcp) crystal structure: (a) unit cell; and (b) single crystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials,Vol. 1, John Wiley & Sons, 1976.

5 FCC Close packing of atoms Face-Centered Cubic (FCC) fc c

6 Body-Centered Cubic (BCC) From Callister 6e resource CD. HCP Coordination Number =? Simple Cubic FCC BCC Number of atoms per unit cell?

7 Napoleon Caught With His Pants Down Scott of the Antarctic A (gradual) phase change occurs from white tin (tetragonal) to gray tin (cubic) Disintegration of tin dishes and cutlery in cold weather expeditions, kerosene containers (Captain Robert Scott's Antarctic expedition) Tin Plague Tetragonal Cubic

8 Imperfections 30 What can go wrong? 31 What can go wrong? 32

9 Vacancies: First Direct Experimental Observation How many vacancies in, say, 1m 3 of Cu at 1000 C? ND = x sites = 2.2x vacancies Impurities Foreign atom pure elements not possible max purity % 1 atom in 10 6 is impurity imperfection in crystal structure where can these foreign atoms go? Impurities Interstitial Substitute 35 36

10 Impurities Impurities Foreign atom Foreign atom Interstitial Substitute Interstitial Substitute Remember or Cu Sn bronze rods? Remember, these imperfections are not always detrimental Give rise to: substitutional solid solutions interstitial solid solutions Provide unique properties unobtainable with the parent metals Impurities 39

11 Summary Line Defects impurities 41 TEM of titanium dark lines are dislocations X dislocations aid plastic deformation three types edge screw mixed Dislocations are formed -solidification - plastic deformation - thermal stresses from cooling 42 Edge Dislocation Half plane of atoms inserted into lattice distortion of lattice 43 More than 1 type of atom?

12 Ionic Ceramics Ionic Ceramics Ions pack together as densely as possible to lower overall energy electrostatic attraction in all directions cations want to maximize # of neighboring anions and vice versa. Limitations to dense packing: relative sizes of ions and necessity to maintain charge neutrality Charge neutrality e.g. Ca Ca 2+ F F - CaF 2 Linear triangular tetrahedral octahedral CsCl Example NaCl r Cs = nm r Na = nm r Cl = nm r Cl = nm radius ratio = 0.92 radius ratio = structure: SC structure: FCC cubic And, of course, a co-ordination # of 12 gives HCP or FCC

13 Coordination # And Ionic Radii MgO MnS LiF FeO Coordination # increases with Issue: How many anions can you arrange around a cation? r cation r anion Coord # < ZnS (zincblende) Examples: Ionic Ceramics NaCl (sodium chloride) CsCl (cesium chloride) In ionic crystal, during the formation of the defect the overall electrical neutrality has to be maintained (or to be more precise the cost of not maintaining electrical neutrality is high) Frenkel defect Schottky defect Pair of anion and cation vacancies E.g. Alkali halides Cation being smaller get displaced to interstitial voids E.g. AgI, CaF 2 This kind of self interstitial costs high energy in simple metals and is not usually found

14 Other defects due to charge balance If Cd 2+ replaces Na + one cation vacancy is created Directionally bonded atoms of equal size Materials with directional bonds have geometry controlled by bond angles, e.g. diamond Schematic Covalent Ceramics SiC Example: Covalent Ceramics Position and number of neighbours rigidly fixed by directional nature of bonds Energy is minimised, not by dense packing, but by forming chains, sheets or 3D networks often these are non-crystalline The results are quite different structures to ionic ceramics and also different properties

15 Examples: Covalent Ceramics BCC metals have some covalency to their bond Ceramic Structures Comparison metals v ceramics Metals Ceramics Generally more complex than metals Will be predominately ionic or covalent CaF 2 89% ionic MgO 73 NaCl 67 Al 2 O 3 63 SiO2 51 Si 3 N 4 30 ZnS 18 SiC 12