Solid materials
Physical bases of dental material
science
Classification of solid materials
crystalline
Irén Bárdos-Nagy
monocrystal
amorphous
polycrystal
porcelain veneer
moldavite
sapphire
calcite
crystal structure:
Most important characteristics of crystalline materials
- definite shape and volume
cuprite
a unique arrangement of atoms or molecules
longe – range order and symmetry
unit cell repeated periodically in 3D
The seven lattice system:
cubic
- macroscopic range order (crystals)
- periodic crystal structure, symmetry
- relatively small number of defects in the structure
- low degree of translational motion of the individual building elements
simple cubic
body-centered cubic
face-centered cubic
hexagonal
- frequent anisotropy (the physical properties depend on the direction of the measurement)
triclinic
monoclinic
orthorhombic
rhombohedral
tetragonal
Properties of crystalline materials
type of bond
building units binding energy melting point rigidity conductivity
(kJ/mol)
sodium chloride
cesium chloride
covalent
atoms
100 – 1000
high
+
-
ionic
ions
200 – 1500
high
+
-
70 – 1000
high
ductile
+
quartz
metallic
diamond
graphite
free electrons
and ions
H-bond
molecules
15 – 20
low
+
-
van der Waals
molecules
0.5 – 3
low
soft
-
tungsten carbid
graphite
fluorapatite
gold
ice
sulfur
Polymorphism - allotropy
the ability of solid material to exist in more than one form or crystal structure
polymorph forms of SiO2
allotrop forms of carbon
Polycrystalline materials
- no macroscopic range order
- large number of crystallites (grains, microscopic size crystals)
- grain boundaries (interfaces where crystals of different orientations meet)
- large number of crystal defects
α quartz
cristobalite
tridymite
coesite
- mainly isotrope property
a./ diamond
b./ graphite
c./ lonsdaleit
d.-f./ fullerenes
g. amorphous carbone
h./ carbon nanotube
Crystal defects
b/ doping
ideal crystal
Point defects
substitutional
a/ thermal
Schottky – defect (vacancy or hole)
interstitium
interstitial
Frenkel – defect (vacancy and interstitia)
Line defects
Properties of amorphous solids:
edge dislocation
no longe-range order of the position of atoms
screw dislocation
Surface defects
the crystalline and amorphous structure of silica in two dimension
Transmission electron
micrograph of dislocation
large number of structure defects
no definite shape (large viscosity liquid, supercooled liquid)
mechanical hardness
glass transition temperature Tg (the amorphous material becomes brittle
The defects strongly influence the material properties!
Al2O3 and
Al2O3 +
Cr3+
isotrope property
on cooling or soft on heating)
Ti
Fe
Cr
Ni
Co
Ag
Pd
Hg
Au
Crystallization
(metals applied in the dentistry)
high melting-point
fragile
precious metals
low melting-point
ductile
Properties of metals
-solid at room temperature (except Ga and Hg)
chromium
titanium
iron
- high luster
- relatively high density (tightly packed crystal lattice)
- large strength and toughness
nickel
cobalt
- ability to be deformed under stress without cleaving (ductile)
palladium
- good electric and thermal conductivity
silver
gold
mercury
tin
Microscopic structure of metals
Submicroscopic structure of metals
crystallites with different orientation, grains
small homogeneous particles
the properties of the material strongly depend
on the structure
hexagonal
Ti, Cd, Co, Zn
Space filling factor 74 %
face-centered cubic
Ag, Au, Pd, Pt, Al, Cu, Ni
Space filling factor 74 %
Stucture analysis: polishing (fine, rough)
chemical etching
microscopic technics
body-centered cubic
Fe, Cr
Microscopic view of metal surfaces
Space filling factor 68 %
homogeneous
Phase transitions
Classification of the phase transitions:
G = E + pV − TS
Tm and Tb strongly depend on the type
of bond between the particles
heterogeneous
Crystallization (phase transition from liquid to solid phase)
cooling curve
Supercooling (phase transition from liquid to solid phase)
Transition from the liquid to the solid state
two stages: a./ nucleus (seed crystal) formation
b./ crystal growth
nucleus formation
growth of the crystallites
solidus point
dentritic (tree) increase
liquidus point
supercooling
grain boundaries
The role of the size and the shape of the grains !!
isotrope increase
Nucleus formation
solid phase
a./ homogeneous nucleation
nucleous
liquid phase
important parameters:
- free energy +
manganese dentrites on a limestone
critical radius
of the nucleus
r*
~r2
rate of
nucleation
nucleus
(stable)
Diffusive motion
of atoms
r
embryo
(unstable)
snow crystal
the size of the nucleus
the rate of nucleus formation
Driving force
(number of nuclei)
~r3
Tm T
b./ heterogeneous nucleation (on the wall of the dish, impurities, dislocations
mainly earlier and faster crystallization)
Crystallization
The growth of the stable nuclei:
glass formation
rate of
crystallization
liquid
specific
volume
a
T = 540 °C
fast nucleation and low low rate of nucleation and
rate of crystal increase fast increase of crystal size
b
result
T = 690 °C
fine grain structure
(a)
rough grain structure
(b)
(Tm = 727 °C)
harder, stronger, not easily ductile
supercooled
liquid
glass
crystal
Tm T
monocrystal
glass
Tg
polycrystal
rate of
crystallization
metals
ceramics
solid – solid state conversions !!
Tm
T
Tm
T