Materials in general

1. Materials in general

2. The role of materials in every civilisation has always been substantial: The outstanding material has characterised each age (stone age, iron age, bronze age, etc.) The present one is the age of “many materials”

3. Classical partition of materials: structural and functional With biomaterials: both Structural materials prevail An example of functional material: drug release

4. Main features of structural solids are the mechanical properties. • Qualitatively: • Ductility: ability to elongate under stress • Fragility (brittleness): a solid which does not change shape upon stress but fractures instead is fragile • Hardness: a measure of the resistance of a material to cutting, incision or o penetration • Also: • Electrical Conductivity • Thermal Conductivity

5. TThree main groups of structural materials, according to the structure and type of bonds between atoms: M metals and their alloys G glasses and ceramics (same composition, but D different atomic arrangements) Ppolymers

6. Metals usually are: • opaque and light-reflecting • ductile • good heat and current conductors • crystalline • easily workable

7. Ceramics: • are fragile • have high hardness • may be trasparent to light • are electrical and thermal insulators • may be used at high temperatures and in harsh ambients (refractories)

8. Most polymers: • Do not stand high temperatures • Are insulators • Many are deformable • Some are elastomers (rubber)

9. Chemical bonds in solids

10. Solids without translational periodicity Amorphous Crystalline Solids Solids with translational periodicity crystalline amorphous

11. Amorphous solids Most common: glasses Also polymers (crystalline patches)

12. An example of translational symmetry (from C. Escher)

13. The structure of crystalline solids is represented by LATTICES: The REPEAT UNIT or UNIT CELL: the smallest structural unit keeping the lattice symmetry, repeated indefinitely in space In 3D the unit cell is defined by three periods (a, b, c) and three angles (α, β, γ).

14. The seven crystal classes and the forteen Bravais lattices

15. Determination of crystal structure: X-rays diffraction Atoms in crystalline solids are at distances of the order of the X-ray wavelength  diffraction phenomena Alternative description: reflection by parallel planes Powerful method of investigation

16. Ionic solids A set of ions with different charge bound together y electrostatics Common salt, NaCl , is an ionic solid. Typical rocksalt structure (cubic unit cell with Na+ ions placed at corners and at the middle of the cube faces). The same for Cl- ions. Structure FCC

17. With chemically similar compounds the lattice structure may change as a function of the relative dimensions of the ions. In CsCl each Cs+ ion is surrounded by eight Cl-. Unit cell: BCC (body centered cubic).

18. + + + - - - - + - + - + + + + - - - Structure accounts for observed properties • hardness • High melting points • Brittleness fracture line

19. Covalent (macromolecular) solids • Atoms bound by covalent bonds (sharing of an electron pair) to form potentially infinite structure: • 1D (chain, polymers) • 2D (graphite) • 3D (diamond, quartz)

20. Diamond A single infinite molecule with each C atom in sp3 hybridization bound to four neighboring atoms forming a tetrahedron

21. Graphite Made of parallel layers where C atoms in sp2 hybridization form hexagons Each atom still has an unpaired electron in a p orbital perpendicular to the plane the overlap of which forms a  bond extending over the whole surface. Graphite  good electrical conductor  Solid lubricant

22. Quartz Made of tetrahedra where Si is at the center and O at the four corners. O atoms join two tetrahedra together

23. silica polymorphs

24. Metallic solids Atoms all have the same electronic structure. Predictable structures are the dense ones

25. The two dense structures of metals

26. All properties amenable to the structure! • Malleability and ductility: metallic bond not directional • Electrical Conductivity: presence of mobile electrons close to the Fermi surface • Resistance increasing with temperature: scattering of electrons by the thermal motion of positive ions • Thermal Conductivity: proportional to electrical conductivity (electrons as carriers) • Alloying: easy mixture of different atoms

27. GLASS Amorphous material obtained through the progressive stiffening (increase in viscosity) of a liquid which did not crystallize upon cooling

28. Solidification of a crystalline material Solidification of a vitreous material

29. To make a glass from a liquid: • The cooling rate (at T < Tmelt) must be higher than the crystallization rate • In principle, all substances may give rise to a vitreous state. In practice: • silicates • poly-alcohols (sugars) • polymers

30. (a) Crystalline solid (b) solid in an amorphous state Black dots: tetrahedral atoms (Si or other lattice former atom: Al, Fe, B, Ti, etc)

31. Lattice modifier atoms Alkali, alkali-earth cations: Na, Ca, etc.

34. OPTICAL FIBRES

35. Polymers Generally classified according to: structure, properties and usage into: Thermoplastic polymers: made by macromolecules, linear or branched. Reversible softening with heat. Network structured polymers: with a three dimensional structure: made by a giant single macromolecule  thermosetting No melting, decomposition instead

36. Thermoplastic Polymers Either amorphous or semi-crystalline form

37. Network structured polymers • elastomers (rubbers): linear polymers with a limited amount of cross-linking, introduced by post-polymerization curing reaction, causing: • 1) a 3D structure • 2) elasticity • thermosetting resins with high cross-linking degree. Higher mechanical properties (stiff, fragile and temperature-resistant)

38. According to the location of substituents in the alkylic chain, linear polymers may be: i) isotactic, syndiotactic or atactic

39. The end