Do it with electrons ! II.

Name: Do it with electrons ! II.
Uploaded: 2017-11-26T16:31:05+00:00
Duration: PTM12S36
Channel: Alma Wragge
Description: Do it with electrons ! II.

Do it with electrons ! II

TEM - transmission electron microscopy
Typical accel. volt. = kV (some instruments 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)

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

Examples Precipitation in an Al-Cu alloy

Examples dislocations in superalloy SiO2 precipitate particle in Si

Examples lamellar Cr2N precipitates in stainless steel electron diffraction pattern

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 (< micron - depends on material, voltage)

Specimen preparation Foils 3 mm diam. disk very thin (< micron - depends on material, voltage) mechanical thinning (grind) chemical thinning (etch) ion milling (sputter) examine region around perforation

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

Diffraction 2q ≈ sin 2q = R/L l = 2d sin q ≈ d (2q) R/L = l/d Rd = lL specimen image plane L is "camera length" lL is "camera constant"

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

Diffraction Symmetry of diffraction pattern reflects symmetry of crystal around beam direction [111] in cubic [001] in hexagonal Example: 6-fold in hexagonal, 3-fold in cubic Why does 3-fold diffraction pattern look hexagonal?

Diffraction P cubic reciprocal lattice layers along [111] direction 0-level l = +1 level l = -1 level 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

Diffraction Cr23C6 - F cubic a = Å Ni2AlTi - P cubic a = 2.92 Å

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

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 Ewald sphere Only zero level in position to reflect

Indexing electron diffraction patterns Measure R-values for at least 3 reflections

Indexing electron diffraction patterns

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]

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

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 polycrystalline BaTiO3 spotty Debye rings

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

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

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

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

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

Dark field imaging Instead of main beam, use a diffracted beam Move aperture to diffracted beam or tilt incident beam

Dark field imaging Instead of main beam, use a diffracted beam Move aperture to diffracted beam or tilt incident beam strain field contrast

Dark field imaging Instead of main beam, use a diffracted beam Move aperture to diffracted beam or tilt incident beam

Lattice imaging 铝钌铜合金 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

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).

Do it with electrons ! II.

Similar presentations

Presentation on theme: "Do it with electrons ! II."— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Do it with electrons ! II.

Similar presentations

Presentation on theme: "Do it with electrons ! II."— Presentation transcript:

Similar presentations

About project

Feedback