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MNL17-EB/Jun
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Sheehan 1

THERE ARETWOMAINTYPESof electron microscopes, the scanning electron microscope (SEM) and the transmission electron microscope (TEM)
...
The resolving power of electron
microscopes is three orders of magnitude greater then lightoptical microscopes
...
2 nm [1]
...
Contrast develops in TEM by electrons transmitted through thin specimens and therefore variations in structure and composition
are examined
...
1
...
The TEM collects X-rays, elastically scattered electrons, and inelastically
scattered electrons from thin and ultra-thin specimens
...
These are known as scanning
transmission electron microscopes (STEM) or analytical
electron microscopes (AEM)
...
TEM quickly became widely used to image replicas of
surfaces at resolution far better than was possible with the
first SEM and therefore further attempts to develop SEM
were regarded as a waste of time
...
Immediately
SEM became widely used to examine coatings, as seen in
American Chemical Society Symposia on SEM of coatings in
1968 [3] and again in 1972 [4]
...
Therefore SEM will receive
most of the attention in this chapter
...
The SEM section begins with a discussion
of image formation and signal collection; these must be
understood to correctly interpret SEM images
...


are discussed
...
Also included is a discussion of X-ray microanalysis, used to identify
qualitatively or quantitatively the elements present in a sample
...
As resolution of SEM has
improved, it has become critical to coat samples with ultrathin films (1 to 5 nm) that do not blanket surface details to be
imaged
...
The last section discusses some of the wide
range of applications of electron microscopy to coatings,
from practical problems of identifying contaminants, to fundamental research of latex coalescence and adhesion
...
Depth of
focus is large, so rough surfaces can be imaged
...

An image of a sample in the SEM is formed on a cathode ray
tube (CRT) by a dot-mapping process
...
The same scan
generator deflects the electron beam across both the sample
and the CRT and therefore the signal displayed on the CRT is
a magnified image of the scanned area
...
2
...
The condenser lenses form the beam
into a finely focused probe
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The objective lens focuses
the beam on the sample
...
Details of the function
of each component in the SEM are discussed below
...
The SE signal is produced when bound electrons are removed as a result
of inelastic scattering of the electron beam
...
astm
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1-Signal types generated by electron beams impinging on a sample of
varying thickness
...

the SE signal depends partly on the orientation of the sample
with respect to the electron beam, which is why SE images
are topographical
...

Electrons are elastically scattered when they penetrate the
electron clouds of atoms and change direction, a result of
coulombic interaction with the positively charged nuclei,
known as Rutherford scattering
...
The probability of a hackscatter event depends on the size of the nuclei and therefore
backscatter contrast depends on the distribution of average
atomic number in the sample
...
The yield of BE varies by only about
10% over an accelerating-potential range of 10 to 50 kV [5]
...
3
...
The main components of the ET
detector are a collector grid, a scintillator, and a photomultiplier
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A 10-kV bias on the scintillator accelerates SEs
enough to cause photon emission in the light pipe
...

Secondary electrons are classified into four types, shown
schematically in Fig
...
SE-I are generated by direct interaction with the primary beam and therefore carry the highest
resolution information
...
In 1940, before the
first SEM images were collected, yon Ardenne correctly predicted that SE-2 would limit resolution: SE-2 cannot be de-

tected separately from SE-1
...

SE-3 are generated by backscatter electrons that strike the
objective lens pole piece and the specimen chamber walls
...
SE-3 generated at the objective lens
pole piece can be reduced with a shield made of low SEemitting material, such as carbon-coated aluminum, placed
just below the objective lens
...
To
increase atomic number contrast, BEs that strike the shield
can he converted into SEs when the shield is positively biased
or covered with a material that emits a large number of
secondary electrons
...
Peters [7] discussed high-resolution SE image formation
...
One is the ET
detector described above, an inefficient way to collect BE: the
geometric collection efficiency of BE in an ET detector is only
about 1 to 10% [5]
...

Collection efficiency is improved if a disk-shaped detector is
placed at the bottom of the objective lens, with a hole in the
middle for the electron beam
...

To demonstrate the importance of understanding how
SE 1, SE2, SE3, and BE generate contrast in the SEM, three
micrographs of the same area of a 1:6 styrene-butadiene
latex:calcium carbonate coating formulation, air-dried from
80 wt%, are shown in Fig
...
Micrographs in Figs
...
In Fig
...

This is possible because the exit depth of BE from a latex
film is about 1/xm at 20 kV
...
4B, the calcium carbonate

CHAPTER 7 2 - - S H E E H A N ON ELECTRON MICROSCOPY 8 1 7

FIG
...
Adapted
from Thomas [1]
...
Comparison of Figs
...
Also, SE images at lower
accelerating voltage better represent topography because the
exit depth of BE is smaller
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4C shows the same area as Figs
...

Because no SEs were detected, the latex binder at the sample
surface generated no contrast
...
This example illustrates the importance of understanding image formation in the SEM in
order to correctly interpret images
...


4A would lead one to the false conclusion that the latex film
does not cover all of the calcium carbonate particles
...
An X-ray photon is produced when an
electron from an adjacent shell drops to an inner shell to
replace the ionized electron
...

There are two types of X-ray detectors, the wavelength
dispersive spectrometer (WDS) and the energy dispersive
spectrometer (EDS)
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X-rays that satisfy Bragg's law for the
crystal are diffracted to a proportional counter
...
3-Types of secondary electrons collected by the
Everhart-Thornley detector
...
Adapted from
Reimer [6]
...
The advantage of
WDS is high-energy resolution, 6 to 30 eV
...

Today, EDS is installed on most SEMs
...
The energy of incoming X-rays is
measured from the number of electron-hole pairs generated
in the silicon detector
...
The crystal is
cooled with liquid nitrogen to reduce the leakage current
...
However, the whole energy spectrum in EDS can be recorded simultaneously and therefore
elements in a sample can be quickly identified
...
Windowless and ultra-thin window detectors
extend the detection range down to Z = 4 (Be)
...
The
concentration of each element in the sample is the ratio of the
X-ray intensity of each element in the sample to that in the
standard
...
These include backscattering and the
stopping power (Z), X-ray absorption (A), and fluorescence
(F)
...
All EDS spectrometers available from commercial vendors have computer programs that calculate ZAF corrections
...


FIG
...
(A) SE image at 20 kV; (B) SE image
at 5 kV; (C) BE image at 20 kV
...


Electron Guns
The ultimate resolution of an SEM is determined by the
type of cathode in its electron gun
...
High current density and a small first
crossover are needed for high-resolution imaging
...
Thermionic excitation occurs when a filament is
heated to near its melting point, allowing electrons to overcome their work function and eject themselves into the vacuum
...
Tungsten cathodes consist of a wire bent like a hairpin
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However, they require frequent replacement: every
2 to 40 h
...
One advantage of
LaB 6 over tungsten is its higher current density, 20 to 50 M
cm 2, compared to tungsten at about 3 A/cm 2
...
Together, these make the
resolution of LaB6-equipped SEMs higher
...

However, LaB6 guns require more frequent and more precise
alignment than tungsten guns
...

Field emission occurs when a huge potential gradient
(>107 Wm) is applied to a tungsten tip of about 0
...
No thermal energy is needed to lift the electrons over
the barrier
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The current density is around 10~
A/cm 2, three orders of magnitude greater than LaB 6 cathodes
...

Gas molecules that strike the tip cause the work function to
rise and the emission to fall
...
Even in a clean vacuum, gas
molecules eventually cover the tip
...
Field emitters that operate at
around 1200 K are less susceptible to gas molecules, and
field-emission is stable at pressures up to 10 -7 Pa [6]
...
Until recently, low-voltage SEM was
limited in resolution mainly by large chromatic aberrations
in objective lenses, which arise from the large energy spread
of electron beams from thermionic cathodes
...

The merits of low-voltage SEM with FE cathodes were discussed by Pawley [8]
...
SEMs with
thermionic electron guns have two condenser lenses, as
shown in Fig
...
Each condenser lens and spray aperture
demagnifies the probe
...

Two condenser lenses instead of one are advantageous because more electrons are collected and focused into the
probe, and the probe diameter is smaller
...
The first spray aperture is
usually fixed, and the second is selectable to varying diameters
...
Higher probe currents are needed for EDS and

819

some backscatter detectors
...
Many mode m SEMs do not have an aperture in the objective lens, and
therefore the second condenser aperture is often called an
"objective" aperture because it is the aperture closest to the
sample
...

The scan coils deflect the electron beam across the sample
in a raster
...
The probe is focused at the sample surface by
the objective lens
...
" The
depth of focus is inversely proportional to the working distance
...
So high-magnification imaging should be done at short working distances, and lowmagnification imaging should be done at long working distances, where depth of focus is high
...
Coatings should be thinner
than the size of the smallest surface details to be imaged
...
"
However, great care is required to coat samples with ultrathin (<5 nm) continuous films needed in order to take advantage of the high resolution now available from LaB6 and FEequipped SEMs
...
During nucleation, metal atoms strike the
sample surface and diffuse until they find a nucleation site,
often a surface inhomogeneity, where it is energetically favorable for them to stick
...
As growth continues, adjacent grains touch and
eventually begin to coalesce
...
The area
density of nucleation sites determines the size of grains required to form a continuous film
...
When there are many nucleation sites; tiny grains
grow together and form a continuous film
...

Coating techniques for SEM include thermal evaporation,
d-c plasma sputtering, and ion beam sputtering
...
Films formed by
thermal evaporation have large grains compared to other
techniques, but grain size decreases with decreasing substrate temperature [10]
...
But as the resolution
of SEMs improved, the grain size of metal coatings became a
limit to resolution
...
D-c magnetron sputtering is one of them
...
Ionization efficiencies and,
hence, sputtering rates are increased with an annular magnetic field that surrounds the cathode to concentrate electrons in the cathode area [11]
...
However, grain size is sensitive to contamination [12], and artifacts are often present [13]
...

Many models are available and prices vary widely
...

Care should be taken not to allow rotary-pumped coaters to
reach their ultimate pressure, otherwise pump oil can disrupt
nucleation when it contaminates the target and the sample
...

A more recent development in metal coating for high-resolution SEM is ion beam sputtering
...
Mean grain sizes of 1 nm
have been obtained for both gold and platinum [15], too small
to be resolved with SEM
...
It is speculated that high kinetic
energy metal atoms implant themselves into sample surfaces,
causing increased nucleation density and small grains
...

Metal film thickness should be measured to insure that
only enough metal is deposited on the sample to form a
continuous film
...
Film thickness sensitivity is equivalent
to about 0
...
Methods of film-thickness determination were discussed by Flood [16]
...
This limits conventional SEM to dry samples
...
One solution is to lower vapor pressures by freezing and examine samples in an SEM equipped
with a stage cooled by liquid nitrogen
...
The
reader is also referred to Echlin [20] for more on techniques
of cryo-electron microscopy
...
There each sample is fractured, sometimes etched, and then coated with a thin metal film
...
The
challenges of high-resolution cryo-SEM are to freeze fast
enough to prevent noticeable growth of ice crystals, transfer
without condensing water vapor on the fracture surfaces,
coat frozen-hydrated samples with an ultra-thin, continuous
metal film, and image in the SEM without too much buildup
of contaminants
...
Complementary fracture faces of a frozen-fractured pigment/latex
coating formulation are shown in Fig
...
Kaolin and calcium
carbonate particles can be identified by their morphologies
...
Close examination of the complementary faces shows that fracture propagated around the pigment particles
...
A cross section of a fractured drop of a 20
wt% aqueous suspension of styrene-butadiene latex is shown
in Fig
...
The drop was placed on a glass slide, air-dried for 3
rain, and plunge-frozen into liquid ethane
...
5-Cryo-SEM image of complementary fracture faces of a coating formulation
...
m [18]
...
6-Cross section of a partly dried drop of styrene-butadiene latex
...
Bar = 1 p
...


Fig
...

Latex that was deformed during fracture is seen on the cross
section
...


Environmental Scanning Electron Microscopy
Cryo-techniques are one way to image volatile samples in
the SEM
...
In ESEM, samples are imaged
in a gaseous environment at pressures up to 2
...
This is
made possible by a combination of a differentially pumped
vacuum system, small travel distances for the electron beam
in high-pressure gas, and a new biased specimen current
detector known as a gaseous detector device (GSD)
...
Therefore, metal coating is not required for electrically insulating
samples
...
But one cannot expect this resolution from
electrically insulating samples without metal coating because
SE contrast is low from electrical insulators in ESEM
...
This makes accessible dynamic imaging of wetting and drying by controlling sample temperature and water-vapor pressure
...
7A
...
3 kPa and lowering sample temperature
to 12~ An image is shown in Fig
...
Most of the fibers are
covered with water
...
Water molecules are radiolyzed by the electron beam into mainly
hydrogen and hydroxyl radicals, which diffuse into specimens and sometimes damage them [22-24]
...
7-Two ESEM micrographs of the same area of paper (A) before wetting; (B) wet
...


822

PAINT AND COATING TESTING MANUAL

TRANSMISSION ELECTRON MICROSCOPY
The TEM produces images from electrons that transmit
themselves through thin and ultra-thin samples, as shown in
Fig
...
High-resolution TEMs resolve better than 0
...

Types of contrast include scattering, diffraction, and phase
contrast
...
The maximum useful
thickness of metal foils and crystalline materials at 100 kV is
50 to 200 nm [25]
...
1
...
Scattering contrast is also known as mass-thickness contrast because the amount of scattering depends on
specimen thickness and atomic number
...
In the bright-field mode, electrons that are
scattered at larger angles are intercepted by the objective
aperture
...
In the dark-field mode, only the electrons
that are scattered at large angles are used to form the image
...

Another type of contrast caused by elastic scattering is
known as phase contrast, the result of interference between
the scattered and the unscattered electrons
...

Diffraction contrast results when Bragg reflections from
crystalline planes interfere with the primary beam
...

Inelastic interactions in TEM are plasmon excitations and
inner-shell ionizations
...
Electronic structures are indicated by plasmon losses, whose energies are less than 50 eV
...

Optics
A schematic of an electron-optical column of a TEM is
shown in Fig
...
The electron-optical column of the TEM
differs from the SEM in several ways
...
Instead, the condenser lens
system illuminates the entire area to be imaged and a set of
projector lenses beneath the sample focus the transmitted
electrons on a phosphor screen, where the image can be
viewed
...
Instead, the condenser lens system in TEM illuminates the entire area of
interest with the highest electron density that will not damage
the sample
...
Fourth, higher

accelerating potentials (100 to 1000 kV) in TEM are needed to
transmit electrons through thin samples
...

In most imaging modes, Condenser Lens 2 is excited and
Condenser Lens 1 is shut off
...

The purpose of the projector lens system is to project the
transmitted electrons into an image on the phosphor viewing
screen or photographic emulsion
...
The objective aperture blocks scattered electrons in the bright-field mode, discussed above
...


APPLICATIONS TO COATINGS
Film Thickness Measurements
Film thickness measurement requires cross sections
...
The simplest
is to cut by hand with a razor blade
...
Cross sections of coated paper are made with a microtome after embedding the paper in
epoxy [26]
...
9
...

ASTM Test Method for Microscopic Measurement of Dry
Film Thickness of Coatings on Wood Products (D 2691) states
that the specimen may be sliced with a microtome or a razor
blade
...
Also, the diamond
knife in a microtome is easily damaged by hard paint films
...


Failures and Defects
Pits, cracks, and loss of adhesion can be readily examined
with SEM
...
Here
are some examples: SEM examination of a paint film that
failed rapidly in a salt spray test revealed a textured surface
that rusted rapidly in the pitted areas
...

Poor scrubbability in a latex paint was due to lack of coalescence, readily apparent after SEM examination revealed
many cracks in the film
...
A glossy varnish which turned out flat was found to be
laden with diatomaceous earth from a broken filter [28]
...
Ion distributions were mapped quantitatively by electron microprobe analysis equipped with wavelength dispersive detectors
...
Welding splatter was
identified as a cause of chipping of paint on a steel building

CHAPTER 7 2 - - S H E E H A N ON ELECTRON MICROSCOPY 823

FIG
...
Adapted from Thomas [1]
...
9-Microtome-sectioned paper showing thickness of the coating and its penetration into the sheet
...
m
...
Matsubayashi [26]
...
It was also shown that the EDS identified plaster dust,
fly ash, and S02 as contaminants that caused adhesive failure
in other samples
...
Some pigment particles are smaller
than 0
...
Pigment particles can also be identified in
SEM by their EDS spectra if the particles are larger than a
few micrometers
...

Figure 10 shows images of kaolin clay taken with SEM
(10A) and TEM (10B)
...
The SE contrast is topographical, and the TEM contrast depends on mass-thickness
...


Pigment Particle Sizing
The microscope is the best and also the most tedious
method of determining the particle size of pigments
...
10-Kaolin clay imaged in (A) SEM; (B) TEM
...
5/~m
...
P
...


size without regard to size distribution
...
However, microscopic
methods are slow and laborious, a disadvantage [28]
...
The reticle is an
aid to manually counting the number of particles in each size
class
...


resolution SEM extents of latex deformation resulting from
particle-substrate adhesion and compared their results to
theoretical predictions
...
Matsubayashi (Nippon Zeon Co
...
,
Kawasaki, Japan) for supplying the SEM micrograph of
cross-sectioned, coated paper and R
...
Gursky (Unilever
Research, Edgewater, N J) for supplying the TEM micrograph
of kaolin clay
...
Cross sections were
made by fracturing in liquid nitrogen
...
Examination of complimentary
fracture faces was required to see if leached pigment or fracture was responsible for voids
...


[1] Thomas, E
...
, "Electron Microscopy," Encyclopedia of Polymer
Science and Engineering, Vol
...

[2] McMullan, D
...
155, No
...
373-392
...
H
...
, "Scanning Electron Microscopy of Polymers
and Coatings," Applied Polymer Symposia, Vol
...

[4] Princen, L
...
, Ed
...
23, 1974
...
L, Scanning Electron Microscopy, 2nd ed
...

[6] Reimer, L
...

[7] Peters, K
...
, "Conditions Required for High Quality High Magnification Images in Secondary Electron-I Scanning Electron
Microscopy," SEM 1982, Vol
...
, Chicago, 1982, pp
...

[8] Pawley, J
...
136, No
...
45-68
...
A
...
8, McGrawHill, New York, 1970
...
L
...
37, No
...
34053410
...
However, the conclusion was that
SEM images point to the cause of chalking, but gloss measurements are a more quantitative and economical method
...
Demejo et al
...
E
...
, Dobson, E
...
, "Design
and Operation of a High Efficiency Magnetron Sputter Coater,"
SEM 1982, Vol
...
, Chicago, 1982, pp
...

[12] Echlin, P
...
H
...
, "Improved Resolution of
Sputter-Coated Metal Films," SEM 1980, Vol
...
,
Chicago, 1980, pp
...

[13] Holland, V
...
, "Some Artifacts Associated with Sputter-Coated
Samples Observed at High Magnifications in the Scanning Electron Microscope," SEM 1966, Vol
...
, Chicago, 1966,
pp
...

[14] Geller, J
...
, Yoskioda, T
...
A
...
II, SEM, Inc
...
355-360
...
, Clay, C
...
, and Peace, G
...
I, SEM, Inc
...

155-162
...
R
...
I, 1980, SEM, Inc
...
183-200
...
G
...
1282-1283
...
G
...
418419
...
G
...
, "High Magnification Cryogenic Scanning Electron Microscopy (Cryo-SEM) of Wet Coating Microstructures," TAPPI Journal, Vol
...
5, 1990, pp
...

[20] Echlin, P
...

[21] Danilatos, G
...
, "Review and Outline of Environmental SEM at
Present," Journal of Microscopy, Vol
...
3, 1991, pp
...

[22] Danilatos, G
...
, "Beam-Radiation Effects on Wool in the
ESEM," Proceedings of the 44th Annual Meeting of the Electron
Microscopy Society of America, San Francisco Press, 1986, pp
...

[23] Sheehan, J
...
, "Radiation Damage to Cellulose Fibers in
ESEM," Proceedings of the 49th Annual Meeting of the Electron

825

Microscopy Society of America, San Francisco Press, 1991, pp
...


[24] Sheehan, J
...
, "Assessment of Environmental Scanning Electron Microscopy for Coating Research," 1990 TAPPI Coating
Conference, TAPPI, Technology Park/Atlanta, 1990, pp
...


[25] Reimer, L
...


[26] Matsubayashi, H
...
, Takagishi, Y
...
,
"Study on Blistering by Coating Structure Analysis," 1990 TAPPI
Coating Conference, TAPPI, Technology Park/Atlanta, 1990, pp
...


[27] Whitehead, A
...
, "The Micrography of Paint Films," Journal of
the Oil and Colour Chemists' Association, Vol
...
39
...
K
...
10
...
515-521
...
S
...
, "Characterization of Coatings by
Electron Microprobe Analysis," Applied Polymer Symposia, Vol
...
61-72
...
, "Applications of Energy-DispersiveX-ray Spectrometry in Interracial Coating Failures," Applied Polymer Symposia,
Vol
...
49-59
...
H
...
R
...
16, 1971, pp
...

[32] Carter, O
...
, Schindler, A
...
, and Wormser, E
...
, "Scanning
Electron Microscopy for Evaluation of Paint Film Weatherability," Applied Polymer Symposia, Vol
...
13-25
...
H
...
L
...
A
...
23, 1974, pp
...

[34] Eckersley, S
...
and Rudin, A
...
62,
No
...
89-100
...
and Padget, J
...
, "Latex Coalescence," International
Journal of Adhesion and Adhesives, July 1982, pp
...

[36] Demejo, L
...
, Ritual, D
...
, and Bowen, R
...
, "Direct Observations of Deformations Resulting from Particle-Substrate Adhesion," Journal of Adhesion Science and Technology, Vol
...
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...




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