Do it with electrons ! II

1. Do it with electrons ! II

2. 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

3. 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)

4. 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)

5. TEM - transmission electron microscopy Examples Matrix - '-Ni2AlTi Precipitates - twinned L12 type '-Ni3Al

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

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

8. TEM - transmission electron microscopy Examples lamellar Cr2N precipitates in stainless steel electron diffraction pattern

9. 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)

10. 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)

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

12. 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"

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

14. 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?

15. 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

16. Cr23C6 - F cubic a = 10.659 Å Ni2AlTi- P cubic a = 2.92 Å TEM - transmission electron microscopy Diffraction

17. 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

18. 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

19. Measure R-values for at least 3 reflections TEM - transmission electron microscopy Indexing electron diffraction patterns

20. TEM - transmission electron microscopy Indexing electron diffraction patterns

21. 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

22. 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

23. 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

24. polycrystalline BaTiO3 spotty Debye rings TEM - transmission electron microscopy Polycrystalline regions

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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. TEM - transmission electron microscopy Dark field imaging Instead of main beam, use a diffracted beam Move aperture to diffracted beam or tilt incident beam

34. 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

35. TEM - transmission electron microscopy Dark field imaging Instead of main beam, use a diffracted beam Move aperture to diffracted beam or tilt incident beam

36. 铝钌铜合金 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

37. 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).