do it with electrons ! ii do it with electrons ! ii

Do it with electrons !

II

Do it with electrons !

II

TEM - transmission electron microscopyTEM - 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

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)

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)


ExamplesExamples

Matrix - '-Ni2AlTi

Precipitates - twinned L12 type '-Ni3Al

Matrix - '-Ni2AlTi

Precipitates - twinned L12 type '-Ni3Al


ExamplesExamples

Precipitation in anAl-Cu alloyPrecipitation in anAl-Cu alloy


ExamplesExamples

dislocationsin superalloydislocationsin superalloy

SiO2 precipitate particle in SiSiO2 precipitate particle in Si


ExamplesExamples

lamellar Cr2N precipitates in stainless steel

lamellar Cr2N precipitates in stainless steel

electron diffraction pattern

electron diffraction pattern


Specimen preparationSpecimen preparation

Foils3 mm diam. disk very thin (<0.1 - 1 micron - depends on

material, voltage)


material, voltage)

Typesreplicasfilmsslices

powders, fragmentsfoils

Typesreplicasfilmsslices

powders, fragmentsfoils

as is, if thin enough ultramicrotomy

crush and/or disperse on carbon film

as is, if thin enough ultramicrotomy

crush and/or disperse on carbon film


Specimen preparationSpecimen preparation


material, voltage)

mechanical thinning (grind)chemical thinning (etch)ion milling (sputter)


material, voltage)

mechanical thinning (grind)chemical thinning (etch)ion milling (sputter)

examine region

around perforationexamine region

around perforation


DiffractionDiffraction

Use Bragg's law - = 2d sin

But much smaller

(0.0251Å at 200kV)

if d = 2.5Å, = 0.288°

Use Bragg's law - = 2d sin

But much smaller

(0.0251Å at 200kV)

if d = 2.5Å, = 0.288°



2 ≈ sin 2 = R/L = 2d sin ≈ d (2)

R/L = /d

Rd = L

2 ≈ sin 2 = R/L = 2d sin ≈ d (2)

R/L = /d

Rd = L

L is "camera length"

L is "camera constant"

L is "camera length"

L is "camera constant"

image planeimage plane

specimenspecimen



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



Symmetry of diffraction pattern reflectssymmetry of crystal around beam direction

Symmetry of diffraction pattern reflectssymmetry of crystal around beam direction

Why does 3-fold diffraction pattern look hexagonal?Why does 3-fold diffraction pattern look hexagonal?

[111] in cubic [001] in hexagonal

Example: 6-fold in hexagonal, 3-fold in cubic

Example: 6-fold in hexagonal, 3-fold in cubic

TEM - transmission electron microscopyTEM - transmission electron microscopyDiffractionDiffraction

Note: all diffraction patterns are centrosymmetric, even if crystal structure is not centrosymmetric (Friedel's law)

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

Some 0-level patterns thus exhibit higher rotational symmetry than structure has

P cubic reciprocal latticelayers along [111] direction

0-level

l = +1 level

l = -1 level



Cr23C6 - F cubica = 10.659 Å

Cr23C6 - F cubica = 10.659 Å

Ni2AlTi - P cubica = 2.92 Å

Ni2AlTi - P cubica = 2.92 Å


Diffraction - Ewald constructionDiffraction - 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

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 constructionDiffraction - Ewald construction

Also, very small, 1/ very largeAlso, very small, 1/ very large

Many TEM specimens are thin in one direction - thus,

reciprocal

lattice points elongated in one direction to rods

- "relrods"

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

Only zero level in position to reflect


Indexing electron diffraction patternsIndexing electron diffraction patterns

Measure R-values for at least 3 reflectionsMeasure R-values for at least 3 reflections



Index other reflections by vector sums, differencesIndex other reflections by vector sums, differences

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

(202) x (220) ——> [444] ——> [111]

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 axisFind crystal system, lattice parameters, index pattern, find zone axis

ACTF!!!ACTF!!! Note symmetry - if cubic, what direction has this symmetry (mm2)?

Note symmetry - if cubic, what direction has this symmetry (mm2)?

Reciprocal lattice unit cell for cubic lattice is a cube

Reciprocal lattice unit cell for cubic lattice is a cube


Why index?Why index?

Detect epitaxyOrientation relationships at grain boundariesOrientation relationships between matrix & precipitatesDetermine directions of rapid growthOther reasons

Detect epitaxyOrientation relationships at grain boundariesOrientation relationships between matrix & precipitatesDetermine directions of rapid growthOther reasons


Polycrystalline regionsPolycrystalline regions

polycrystalline BaTiO3 spotty Debye rings

polycrystalline BaTiO3 spotty Debye rings


Indexing electron diffraction patterns - polycrystalline regionsIndexing electron diffraction patterns - polycrystalline regions

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

Hafnium ( 铪 )


Indexing electron diffraction patterns - commentsIndexing electron diffraction patterns - comments

Helps to have some idea what phases present

d-values not as precise as those from X-ray data

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

Systematic absences for lattice centering and other translational symmetry same as for X-rays

Intensity information difficult to interpret


Sources of contrastSources of contrast

Diffraction contrast - some grains diffract more strongly than others; defects may affect diffraction

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

Mass-thickness contrast - absorption/ scattering. Thicker areas or mat'ls w/ higher Z are dark


Bright field imagingBright 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

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?What else is in the image?

Many artifacts surface films local contamination differential thinning others

Many artifacts surface films local contamination differential thinning others

Also get changes in image because ofannealing due to heating by beam

Also get changes in image because ofannealing due to heating by beam


Dark field imagingDark field imaging

Instead of main beam, use a diffracted beam

Move aperture to diffracted beam or tilt incident beam









strain field contrast


Lattice imagingLattice imaging

Use many diffracted beams

Slightly off-focus

Need very thin specimen region

Need precise specimen alignment

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 todetermine features of

structure

See channels through foil

Channels may be light or dark in image

Usually do image simulation todetermine features of

structure

铝钌铜合金


ExamplesExamples

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).

do it with electrons ! ii do it with electrons ! ii

Documents

specimen form image

image area

main imaging lens basis

al slide

specimen lenses typical

final screen slide

si slide

image types possible