Do it with electrons II TEM transmission electron

Do it with electrons ! II

TEM - transmission electron microscopy Typical accel. volt. = 100 -400 k. V (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 (imageforming 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 (imageforming lens - limits resolution; aperture - controls imaging conditions) Projector lens system (magnifies image or diffraction pattern onto final screen)

TEM - transmission electron microscopy Examples Matrix - '-Ni 2 Al. Ti Precipitates - twinned L 12 type '-Ni 3 Al

TEM - transmission electron microscopy Examples Precipitation in an Al-Cu alloy

TEM - transmission electron microscopy Examples dislocations in superalloy Si. O 2 precipitate particle in Si

TEM - transmission electron microscopy Examples lamellar Cr 2 N 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)

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 - = 2 d sin But much smaller (0. 0251Å at 200 k. V) if d = 2. 5Å, = 0. 288°

TEM - transmission electron microscopy Diffraction 2 ≈ sin 2 = R/L = 2 d sin ≈ d (2 ) specimen R/L = /d Rd = L L is "camera length" L is "camera constant" image plane

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 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 l = +1 level 0 -level l = -1 level

TEM - transmission electron microscopy Diffraction Cr 23 C 6 - F cubic a = 10. 659 Å Ni 2 Al. Ti - P cubic 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 (mm 2)? 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 Ba. Ti. O 3 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 M 23 X 6 (figure at top left). L 21 type '-Ni 2 Al. Ti (figure at top center). L 12 type twinned 'Ni 3 Al (figure at bottom center). L 10 type twinned Ni. Al martensite (figure at bottom right).