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Do it with electrons ! II. 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

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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


TEM - transmission electron microscopy

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)


TEM - transmission electron microscopy

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)



TEM - transmission electron microscopy

Examples

Matrix - '-Ni2AlTi

Precipitates - twinned L12 type '-Ni3Al


TEM - transmission electron microscopy

Examples

Precipitation in an

Al-Cu alloy


dislocations

in superalloy

SiO2 precipitate

particle in Si

TEM - transmission electron microscopy

Examples


TEM - transmission electron microscopy

Examples

lamellar Cr2N precipitates in

stainless steel

electron diffraction pattern


TEM - transmission electron microscopy

Specimen preparation

Types

replicas

films

slices

powders, fragments

foils

as is, if thin enough

ultramicrotomy

crush and/or disperse on carbon film

Foils

3 mm diam. disk

very thin (<0.1 - 1 micron - depends on material, voltage)


examine region

around perforation

TEM - transmission electron microscopy

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)


TEM - transmission electron microscopy

Diffraction

Use Bragg's law -  = 2d sin 

But much smaller

(0.0251Å at 200kV)

if d = 2.5Å,  = 0.288°


TEM - transmission electron microscopy

Diffraction

2q ≈ sin 2q = R/L

l = 2d sinq ≈ d (2q)

R/L = l/d

Rd = lL

specimen

image plane

L is "camera length"

lL is "camera constant"


TEM - transmission electron microscopy

Diffraction

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


Example:

6-fold in hexagonal, 3-fold in cubic

[111] in cubic

[001] in hexagonal

TEM - transmission electron microscopy

Diffraction

Symmetry of diffraction pattern reflects

symmetry of crystal around beam direction

Why does 3-fold diffraction pattern look hexagonal?


P cubic reciprocal lattice

layers along [111] direction

l = +1 level

0-level

l = -1 level

TEM - transmission electron microscopy

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


Cr23C6 - F cubic

a = 10.659 Å

Ni2AlTi- P cubic

a = 2.92 Å

TEM - transmission electron microscopy

Diffraction


TEM - transmission electron microscopy

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


Ewald

sphere

TEM - transmission electron microscopy

Diffraction - Ewald construction

Many TEM specimens are thin in one direction - thus, reciprocal

lattice points elongated in one direction to rods - "relrods"

Also,  very small, 1/ very large

Only zero level in position to reflect


Measure R-values for at least 3 reflections

TEM - transmission electron microscopy

Indexing electron diffraction patterns


TEM - transmission electron microscopy

Indexing electron diffraction patterns


Next find zone axis from cross product of any two (hkl)s

(202) x (220) ——> [444] ——> [111]

TEM - transmission electron microscopy

Indexing electron diffraction patterns

Index other reflections by vector sums, differences


TEM - transmission electron microscopy

Indexing electron diffraction patterns

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


TEM - transmission electron microscopy

Why index?

Detect epitaxy

Orientation relationships at grain boundaries

Orientation relationships between matrix & precipitates

Determine directions of rapid growth

Other reasons


polycrystalline BaTiO3 spotty Debye rings

TEM - transmission electron microscopy

Polycrystalline regions


Hafnium (铪)

TEM - transmission electron microscopy

Indexing electron diffraction patterns - polycrystalline regions

Same as X-rays – smallest ring - lowest  - largest d


TEM - transmission electron microscopy

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


TEM - transmission electron microscopy

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


TEM - transmission electron microscopy

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


TEM - transmission electron microscopy

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


TEM - transmission electron microscopy

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


TEM - transmission electron microscopy

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


TEM - transmission electron microscopy

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


TEM - transmission electron microscopy

Dark field imaging

Instead of main beam, use a diffracted beam

Move aperture to diffracted beam or tilt incident beam


strain field contrast

TEM - transmission electron microscopy

Dark field imaging

Instead of main beam, use a diffracted beam

Move aperture to diffracted beam or tilt incident beam


TEM - transmission electron microscopy

Dark field imaging

Instead of main beam, use a diffracted beam

Move aperture to diffracted beam or tilt incident beam


钌铜合金

TEM - transmission electron microscopy

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


TEM - transmission electron microscopy

Examples

M23X6 (figure at top left).

L21 type b'-Ni2AlTi (figure at top center).

L12 type twinned g'-Ni3Al (figure at bottom center).

L10 type twinned NiAl martensite (figure at bottom right).


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