many of the important properties of materials are due to
the presence of imperfections
types of imperfections
vacancy atoms
interstitial atoms
substitutional atoms
dislocations
grain boundaries
pores
list the types of point defects
vacancy atoms
interstitial atoms
substitutional atoms
list the type of line defects
dislocations
list the type of area defects
grain boundaries
list the types of volume defects
pores
vacancies define
vacant atomic sites in structure
define self interstitial
extra atoms positioned between atomic sites
dislocations
are line defects
slip between crystal planes result when dislocations move
produce permanent plastic deformation
interstitial
when a smaller different atom is placed in the atomic structure
substitutional
when a larger different atom added to the atomic structure
linear defects are
1D defects around which atoms are misaligned
edge dislocations are
extra half plane of atoms inserted in a crystal structure burges vector is perpendicular to the dislocation line
screw dislocation
spiral planar ramp resulting from shear deformation
burgers vector is paralle to the dislocation line
brugers vector b
measure of lattice distortion
its the vector necessary to close a stepwise loop around the defect i.e.
the displacement of the crystal due to the defect being there
dislocation motion requires
the successive bumping of half a plane of atoms
bonds across the slipping planes are broken and remade in succession
the grains can be from nanometers to millimetres in size and the orientations of the atomic planes are rotated with respect to the neighbouring grains. These materials are called
polycrystals
the individual grains are separated by
grain boundaries, regions that are less densely and regularly packed as compared to the bulk of the grains
low angle grain boundary
arrays of dislocations separated by areas of the strained lattice.
b = lattice translation of the crystal
tilt boundaries are
edge dislocations
twist boundaries are
screw dislocations
special or coincident grain boundaries
special orientation relation between 2 grains on either side of boundary
fraction of the total lattice slides between the 2 grains that coincide
volume 3D defects
grain boundaries with thick glass films
pores
grains themselves especially when coarsened
the overall aim of characterisation is to
gain an understanding of the microstructure at different scale lengths
macrostructure
how it looks to the eye
optical microscopes
large pores
grains and grain boundaries
crystal density
SEM
topographic and atomic number contrast
TEM
meso and nano structure
optical microscopy can be broadly divided into the following categories
reflected light
transmitted light, crossed polars
reflected light
grinding and polishing to view porosity
chemical and thermal etching to view grain boundaries
transmitted light, cross polars
parallel grinding of optical section on glass slide followed by use of a cover slip
ceramics may be sensitive to
water
porous ceramics must be mounted in
low viscosity resin to hold the sample together
chemical etching
acids or alkalis
removal of grain boundary phases
thermal etching
heat about 200c below sintering temperature for 10-30 mins
surface mass diffusion away from grain boundary leads to dark contrast in reflected light
reflect light microscopy technique
bulk samples
bright field
pores are dark
relative reflectivity R from Fresnel formula
R = (n-1)^2 / (n+1)^2
where n = refractive index
brighter phases have a higher
refractive index
transmitted light microscopy technique
thin sections that can give info on optical properties and crystal system
polarised light can be used with both
reflected and transmitted light microscopes
polarisation - when crossed. no light reaches eye. when rotated
analysers can bring to fully transmission or reflexion
in isotropic crystals or glass
light moves in all directions with equal velocity and vibrates in all directions perpendicular to the direction of propagation. when placed between crossed polars they remain dark
in uniaxially anisotropic crystals
light only moves parallel to c axis with vibrations in all directions in basal plane. when placed between crossed polars and rotated some light gets through at particular orientations
in biaxially anisotropic crystals
there are 2 directions in which light travels through the crystals
its easy to distinguish glass and cubic crystals from any other e.g. when a MgO graphite refractory sample is rotated between crossed polars
the cubic periclase stays the same while the hexagonal graphite changed intensity and colour