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Materials in general

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Title: 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) P polymers
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
amorphous
crystalline
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 (a, ß, ?).
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 lt 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.
32
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33
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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
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