L05D: Observation of defects

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L05D Observation of defects

• Many methods of observing defects in and on
  solids, with improvements and new techniques
  being developed all the time.
• What technique is best depends on the size of the
  feature you want to see and the type of material.
• Surface features seen with the naked eye can
  reveal a lot, e.g.
• The symmetry of the crystal lattice may be
  manifested in the crystal shape.
• The shape may reveal the presence of grain
  boundaries.
• Very small features can be seen by looking at
  light reflected off the surface, such as grains
  down to 1 mm and the emergence of screw
  dislocations.
• Roughening the surface mechanically or by etching
  can accentuate orientation differences in
  reflected light, as well as preferential etching
  of grain boundaries to outline them.For small
  grains might needan optical microscope with
  reflected light at different angles.Also known
  as incident light ormetallurgical microscopy.
  No color.

Last modified by W.R. Wilcox, Clarkson
University, on October 20, 2013
2
Optical microscopy by reflected light(e.g. a
metallurgical microscope with light from below)
Useful up to 1000X magnification. Depth of
field decreases as mag increases Polishing
removes surface features (e.g., scratches)
Etching or sand blasting changes reflectance
depending on grain orientation.
3
Incident light microscopyhttp//en.wikipedia.org/
wiki/Optical_microscope

• Basic limitation is the wavelength of light, 0.2
  ?m 200 nm
• But interference effects can be used to see much
  smaller step heights in incident light.
• For opaque engineering materials, the solid is
  first cut, polished mechanically, possibly
  etched, and then can be viewed by various
  methodse.g., stainless steel http//www.cartech.
  com/techarticles.aspx?id1450

Brass
http//www.pearson-studium.de/books/3827370597/cd0
1/Gallery/Images/Copper/VanderVoort-Readme.pdf
For some materials, the polarization of reflected
light depends on orientation, so a polarizing
microscope can be used to reveal grains and twins
in color without a colorizing stain.
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Transmitted light microscopy

• For transparent materials, shine the light up
  through the sample.
• Many variations in the optics are possible.
• The colors obtained in a polarizing microscope
  reflect both the grain orientation and thickness,
  e.g.
• http//www.microscopy-uk.org.uk/mag/indexmag.html?
  http//www.microscopy-uk.org.uk/mag/artdec11/db-vi
  deo1.html
• http//www.olympusmicro.com/galleries/polarizedlig
  ht/index.html

Alkalic Syenite
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Other methods of optical microscopy

• Polarized light
• metallographic microscopes often use polarized
  light to increase contrast using reflected
  (incident) lighting.
• Also used for transparent samples such as some
  ceramics and polymers using transmitted light.
  Can see strain.
• Dark-field
• Light comes in only from an angle. Only the
  light reflected in the direction of the objective
  lens is seen. All else is dark.
• Ways to beat the resolution and depth of field
  limits imposed by the wavelength of light and/or
  the need to focus
• Interference microscopy (several types)
• Scanning optical microscopy http//en.wikipedia.
  org/wiki/Near-field_scanning_optical_microscope
• Confocal microscopy http//en.wikipedia.org/wiki/
  Confocal_microscopy

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Other types of microscopy

• For higher resolution need shorter wavelength
  than light
• X-Rays? Difficult to focus, but x-ray topography
  useful for dislocations
• Transmitted electrons
• wavelengths about 3 pm (0.003 nm) depending on
  energy
• (Magnification up to 1,000,000X)
• Electron beam focused by magnetic lenses
• Requires very thin flat samples for electrons to
  go through focus
• Can make carbon replicas of the surface of thick
  materials or particles

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Transmission electron microscopyhttp//en.wikiped
ia.org/wiki/Transmission_electron_microscopy

• Very thin samples so that electrons can pass
  through them.
• High resolution TEM allows resolution of atoms.
• Can see individual dislocation lines, e.g. in Ti

Si EBSP (electron backscatter diffraction) http/
/www.sciencedirect.com/science/article/pii/S135964
5404004410
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Scanning electron microscopy (SEM)http//en.wikip
edia.org/wiki/Scanning_electron_microscope

• A serious limitation of optical and electron
  microscopy is that the depth of field is on the
  order of the resolution. As you go up in
  magnification the depth that is in focus becomes
  smaller and smaller so only view planar surfaces.
• Modern electronics enables imaging of non-planar
  surfaces.
• In SEM a finely focused electron beam is raster
  scanned across the surface in synchronization
  with a display device such as a monitor.
• A detector is used to detect scattered electrons
  reflected by the sample, or secondary electrons
  or x-rays emitted by the sample. The detector
  signal is fed to the display device, thereby
  yieldingan image of the surface. For example

http//www.rcsms.auckland.ac.nz/uoa/home/rcsms/rcs
ms-facilities/rcsms-sem
fracture surfaces http//www.met-tech.com/SEM1.ht
ml
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Scanning electron microscopy of surfaces

• 1 nm to 20 nm resolution.
• Several detection methods
• Secondary electrons
• Back-scattered electrons
• Because focusing not required, sample need not be
  planar.
• To avoid charging of the sample, it must either
  be an electrical conductor or be coated with a
  thin conducting film, such as gold.

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Energy-dispersive x-ray spectroscopyhttp//en.wik
ipedia.org/wiki/Energy-dispersive_X-ray_spectrosco
py

• Detection of x-rays emitted in an SEM enables
  analysis of composition (acronyms EDS, EDX,
  XEDS).
• For example, Portland cement Image is 256 µm x
  200µm.

CCaO, SSiO2, AAl2O3, FFe2O3 all hydrated
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Scanning probe microscopyhttps//en.wikipedia.org
/wiki/Scanning_probe_microscopy

• Atomically sharp probe that is raster scanned
  along a surface in synchronization with a
  display.
• Many types of tips, which may tap the surface or
  be slightly above it.
• Can sense many types of atomic fields
  electronic, magnetic, etc.
• Can see individual atoms and even move them
  around.
• Tend to be slow, cover a small area, and require
  a very planar surface.

Tilt grain boundary in SrTiO3 001(210) by STM
http//www.sciencedirect.com/science/article/pii/S
0304399111002075
Vacancy on Ge (111) by STM http//www.sciencedirec
t.com/science/article/pii/S0039602801010573
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Scanning Tunneling Microscopy(STM or STEM)
Atoms and molecules can be imaged and also
moved on a surface!
? http//www.youtube.com/watch?featureplayer_emb
eddedvoSCX78-8-q0
Many other sensing methods have been developed
for surface scanning.
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Dislocation etch pits

• With some etchants, pits are formed where
  dislocations emerge at a surface.
• For example, InP. Line is 0.1 mm.

• If the numbers are not large and not aligned in
  low-angle grain boundaries, the number of pits
  per unit area can be counted to give an etch pit
  density, which is often referred to as the
  dislocation density.
• Its equivalent to the total lengths of all
  dislocations per unit volume.
• May be as high as 1012/cm2

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Characterization of grain size

• ASTM International standard methods
  http//materials.mcmaster.ca/faculty/malakhov/4L04
  /ReferenceMaterials/ASTM20standards/ASTM20E112-1
  020Average20Grain20Size.pdf
• ASTM grain size number, n (log N/log 2) 1
  where N is the number of grains per square
  inchat 100X.
• Methods to measure N
• Planimetric Count grains in given area
• Intercept Count the number of grain boundaries
  intersecting a line.
• Above by hand or with image analysis software.
• For equiaxed grains compare image at 100X with