TEM by huanghengdong

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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

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
TEM - transmission electron microscopy

Examples




       dislocations               SiO2 precipitate
       in superalloy              particle in Si
TEM - transmission electron microscopy

Examples


           lamellar Cr2N
           precipitates in
           stainless steel




              electron
              diffraction
              pattern
TEM - transmission electron microscopy

Specimen preparation

 Types
    replicas
    films                      as is, if thin enough
    slices                     ultramicrotomy
    powders, fragments         crush and/or disperse on carbon film
    foils

  Foils
      3 mm diam. disk
      very thin (<0.1 - 1 micron - depends on material, voltage)
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)



                   examine region
                 around perforation
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


        2 ≈ sin 2 = R/L
                                         specimen
         = 2d sin  ≈ d (2)

        R/L = /d

        Rd = L


                                         image plane
        L is "camera length"

        L is "camera constant"
TEM - transmission electron microscopy

Diffraction




Get pattern of spots around transmitted beam from one grain (crystal)
TEM - transmission electron microscopy

Diffraction

  Symmetry of diffraction pattern reflects
     symmetry of crystal around beam direction

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




                          [111] in cubic    [001] in hexagonal

       Why does 3-fold diffraction pattern look hexagonal?
TEM - transmission electron microscopy
Diffraction
                                         P cubic reciprocal lattice
                                        layers along [111] direction
 Note: all diffraction
 patterns are
 centrosymmetric,
 even if crystal structure   l = +1 level
 is not centrosymmetric
 (Friedel's law)


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

                             l = -1 level
TEM - transmission electron microscopy

Diffraction




              Cr23C6 - F cubic   Ni2AlTi - P cubic
               a = 10.659 Å         a = 2.92 Å
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
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
                                                                    Ewald
                                                                    sphere
TEM - transmission electron microscopy

Indexing electron diffraction patterns

    Measure R-values for at least 3 reflections
TEM - transmission electron microscopy

Indexing electron diffraction patterns
TEM - transmission electron microscopy

Indexing electron diffraction patterns




        Index other reflections by vector sums, differences

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




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
TEM - transmission electron microscopy

Polycrystalline regions




                                   polycrystalline BaTiO3
                                    spotty Debye rings
TEM - transmission electron microscopy

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




                                                             Hafnium (铪)
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
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

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
TEM - transmission electron microscopy

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).

								
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