Materials in generalMaterials in general

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”

Classical partition of materials:

structural and functional

With biomaterials: both

Structural materials prevail

An example of functional material: drug release

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

TThree main groups of structural materials, according to the structure and type of bonds between atoms:M metals and their alloysG glasses and ceramics (same composition, but D different atomic arrangements)P polymers

Metals usually are:

• opaque and light-reflecting • ductile • good heat and current conductors • crystalline • easily workable

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)

Most polymers:

• Do not stand high temperatures• Are insulators • Many are deformable• Some are elastomers (rubber)

Chemical bonds in solids

Tipo di solido

Particelle costitutive

Tipo di forza di legame

Proprietà esempi

I onico I oni positivi (+) e negativi(-)

Attrazioni elettrostatiche

Durezza Alto punto di

fusione conduzione allo stato fuso e in soluzione

Fragilità.

NaCl, CaCO3

Covalente Molecolare

Molecole Forze intermolecolari (van der Waals)

Non duri Basso punto di

fusione Non conduttori

CO2, H2O, I 2

Covalente Reticolare

Atomi (uguali o diversi tra loro)

Legame covalente Molto duri Elevati punti di

fusione Non conduttori

C (diamante), SiO2

Metallico I oni positivi + elettroni

Attrazioni tra il reticolo e l’intera nube elettronica

Conduttori anche da solidi

Lucentezza Duttilità

malleabilità

Tutti i metalli

Solids without translational periodicity

Solids

Amorphous

CrystallineSolids with translational periodicity

amorphouscrystalline

Amorphous solids

Also polymers

(crystalline patches)

Most common: glasses

An example of translational symmetry

(from C. Escher)

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).

The seven crystal classes and the forteen Bravais lattices

Determination of crystal structure: X-rays diffraction

Alternative description: reflection by parallel planes

Atoms in crystalline solids are at distances of the order of the X-ray wavelength

diffraction phenomena

Powerful method of investigation

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

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).

Structure accounts for observed properties

• hardness• High melting points• Brittleness

+-+

+-

-

+-+

+-

-

+-+

+-

-

fracture line

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)

Diamond

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

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

Quartz

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

O

SiO O

OOO

Si

O

O

Si O

O

O

Si

O

silica polymorphs

Metallic solids

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

The two dense structures of metals

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

GLASS

Amorphous material obtained through the progressive stiffening (increase in viscosity) of a liquid which did not

crystallize upon cooling

Solidification of a vitreous material

Solidification of a crystalline material

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

(a) Crystalline solid (b) solid in an amorphous state

Black dots: tetrahedral atoms (Si or other lattice former atom: Al, Fe, B, Ti, etc)

Lattice modifier atoms

Alkali, alkali-earth cations: Na, Ca, etc.

OPTICAL FIBRES

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 thermosettingNo melting, decomposition instead

Thermoplastic Polymers

Either amorphous or semi-crystalline form

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)

According to the location of substituents in the alkylic chain, linear polymers may be: i) isotactic,

syndiotactic or atactic

The end