materials in general
DESCRIPTION
Materials 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: - PowerPoint PPT PresentationTRANSCRIPT
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