TEM transmission electron microscopy in depth

TEM - transmission electron microscopy
Typical accel. volt. = 100-400 kV
(some instruments - 1-3 MV)
Spread broad probe across
specimen - form image from
transmitted electrons
Diffraction data can be obtained
from image area
Many image types possible (BF, DF,
HR, ...) - use aperture to select
signal sources
Main limitation on resolution -
aberrations in main imaging lens
Basis for magnification - strength
of post- specimen lenses

Instrument components
Electron gun (described previously)
Condenser system (lenses &
apertures for controlling
illumination on specimen)
Specimen chamber assembly
Objective lens system (image-
forming lens - limits resolution;
aperture - controls imaging
conditions)
Projector lens system (magnifies
image or diffraction pattern onto
final screen)

Examples
Matrix - '-Ni2AlTi
Precipitates - twinned L12 type '-Ni3Al

Examples
Precipitation in an
Al-Cu alloy

Examples
dislocations
in superalloy
SiO2 precipitate
particle in Si

Examples
lamellar Cr2N
precipitates in
stainless steel
electron
diffraction
pattern

Specimen preparation
Foils
3 mm diam. disk
very thin (<0.1 - 1 micron - depends on material, voltage)
Types
replicas
films
slices
powders, fragments
foils
as is, if thin enough
ultramicrotomy
crush and/or disperse on carbon film

Specimen preparation
Foils
3 mm diam. disk
very thin (<0.1 - 1 micron - depends on material, voltage)
mechanical thinning (grind)
chemical thinning (etch)
ion milling (sputter)
examine region
around perforation

Diffraction
Use Bragg's law -  = 2d sin 
But much smaller
(0.0251Å at 200kV)
if d = 2.5Å,  = 0.288°

Diffraction
2 ≈ sin 2 = R/L
 = 2d sin  ≈ d (2)
R/L = /d
Rd = L
L is "camera length"
L is "camera constant"
image plane
specimen

Diffraction
Get pattern of spots around transmitted beam from one grain (crystal)

Diffraction
Symmetry of diffraction pattern reflects
symmetry of crystal around beam direction
Why does 3-fold diffraction pattern look hexagonal?
[111] in cubic [001] in hexagonal
Example:
6-fold in hexagonal, 3-fold in cubic

Diffraction
Note: all diffraction
patterns are
centrosymmetric,
even if crystal structure
is not centrosymmetric
(Friedel's law)
Some 0-level patterns
thus exhibit higher
rotational symmetry than
structure has
P cubic reciprocal lattice
layers along [111] direction
0-level
l = +1 level
l = -1 level

Diffraction
Cr23C6 - F cubic
a = 10.659 Å
Ni2AlTi - P cubic
a = 2.92 Å

Diffraction - Ewald construction
Remember crystallite size?
when size is small, x-ray reflection is broad
To show this using Ewald construction, reciprocal lattice points
must have a size

Diffraction - Ewald construction
Also,  very small, 1/ very large
Many TEM specimens are thin in one direction - thus, reciprocal
lattice points elongated in one direction to rods - "relrods"
Ewald
sphere
Only zero level in
position to reflect

Indexing electron diffraction patterns
Measure R-values for at least 3 reflections

Index other reflections by vector sums, differences
Next find zone axis from cross product of any two (hkl)s
(202) x (220) ——> [444] ——> [111]

Find crystal system, lattice parameters, index pattern, find zone axis
ACTF!!! Note symmetry - if cubic, what
direction has this symmetry (mm2)?
Reciprocal lattice unit cell
for cubic lattice is a cube

Why index?
Detect epitaxy
Orientation relationships at grain boundaries
Orientation relationships between matrix & precipitates
Determine directions of rapid growth
Other reasons

Polycrystalline regions
polycrystalline BaTiO3
spotty Debye rings

Indexing electron diffraction patterns - polycrystalline regions
Same as X-rays – smallest ring - lowest  - largest d
Hafnium (铪)

Indexing electron diffraction patterns - comments
Helps to have some idea what phases present
d-values not as precise as those from X-ray data
Systematic absences for lattice centering and
other translational symmetry same as for X-rays
Intensity information difficult to interpret

Sources of contrast
Diffraction contrast - some grains diffract more strongly than
others; defects may affect diffraction
Mass-thickness contrast - absorption/
scattering. Thicker areas or mat'ls w/
higher Z are dark

Bright field imaging
Only main beam is used. Aperture in back focal plane blocks
diffracted beams
Image contrast mainly due to subtraction of intensity from the
main beam by diffraction

What else is in the image?
Many artifacts
surface films
local contamination
differential thinning
others
Also get changes in image because of
annealing due to heating by beam

Dark field imaging
Instead of main
beam, use a
diffracted beam
Move aperture to
diffracted beam
or tilt incident
beam

Dark field imaging
Instead of main beam, use a diffracted beam
Move aperture to diffracted beam or tilt incident beam
strain field contrast

Dark field imaging
Instead of main beam, use a diffracted beam
Move aperture to diffracted beam or tilt incident beam

Lattice imaging
Use many diffracted beams
Slightly off-focus
Need very thin specimen region
Need precise specimen alignment
See channels through foil
Channels may be light or dark in image
Usually do image simulation to
determine features of structure
铝钌铜合金

Examples
M23X6 (figure at top
left).
L21 type '-Ni2AlTi
(figure at top center).
L12 type twinned '-
Ni3Al (figure at bottom
center).
L10 type twinned NiAl
martensite (figure at
bottom right).

TEM transmission electron microscopy in depth

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