material
3. Transmission Electron Microscopy
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22
3.1 Historical Introduction
The resolution ? of a microscope is defined as the distance between two details just
separable from one another. It can be calculated using the Abb theory of images formation
for optic systems. For incoherent light or electron beam:
where ? is the wavelength of the light, and ? the maximum angle between incident and
deflected beam in the limit of the lens aberrations.
For optical microscopy, the resolution is therefore limited by the wavelength of light
(410-660 nm). The X or ? rays have lower wavelength, but unfortunately, high-performance
lenses necessary to focus the beam to form an image do not exist yet (however, X-rays can
reveal structural information of materials by diffraction techniques). In 1923, De Broglie
showed that all particles have an associated wavelength linked to their momentum:
where m and v are the relativist mass and velocity respectively, and h the
Plank’s constant. In 1927, Hans Bush showed that a magnetic coil can focus an electron beam
in the same way that a glass lens for light. Five years later, a first image with a TEM was
obtained by Ernst Ruska and Max Knoll [42]. In a TEM, the electrons are accelerated at high
voltage (100-1000 kV) to a velocity approaching the speed of light (0.6-0.9 c); they must
therefore be considered as relativistic particles. The associated wavelength is five orders of
magnitude smaller than the light wavelength (0.04-0.008 ?). Nevertheless, the magnetic lens
aberrations limit the convergence angle of the electron beam to 0.5° (instead of 70° for the
glass lens used in optics), and reduce the TEM resolution to the ? order. This resolution
enables material imaging (section 3.5) and structure determination at the atomic level (section
3.6 and 3.7). In the 1950s, Raymond Castaing developed an electron probe and X-ray detector
for the chemical analyses. A modified version of his technique, the energy dispersive
spectrometry EDS (section 3.3) is nowadays usually added to the TEM. Many different
techniques based on TEM are used in materials science. Some of them will be detailed in the
following sections.
3.2 Preparation of the TEM Samples
For TEM observations, thin samples are required due to the important absorption of the
electrons in the material. High acceleration voltage reduces the absorption effects but can
cause radiation damage (estimated at 170 kV for Al). At these acceleration tensions, a
maximum thickness of 60 nm is required for TEM and HREM observations and
quantifications. For Al alloys, this thickness can be obtained by electropolishing with a
solution of 20% nitric acid and 80% methanol, but this method is not convenient for the
(Rayleigh criterion) (3.1) ? 0.61?
? sin
------------- =
? h mv ? =
3.3. Chemical Analyses by EDS
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preparation of AMCs due to the low reactivity of the reinforcements in comparison to the
unreinforced Al alloys. For AMCs, the following mechanical method was used: TEM foil
specimens were prepared by mechanical dimpling down to 20 µm, followed by argon ion
milling (Fig. 3.2) on a Gatan Duo-Mill machine, operating at an accelerating voltage of 5 kV
and 10° incidence angle, with a liquid nitrogen cooling stage to avoid sample heating and
microstructural changes associated with the annealing effect. Such effects have been
experienced on first samples prepared on an ion mill without cooling stage (PIPS), resulting in
an unexpected and substantial coarsening of the precipitation state.
Another preparation method called focus ion beam FIB has been tried. A thin slice of the
sample was cut by an ion beam on a scanning ion microscope. Unfortunately, the large
thickness of the sample (> 150 nm) impeded good HREM studies. The small observable area
(100 nm x 100 nm) permitted to study only one or two grains, which is generally not enough if
a special grain orientation is required.
3.3 Chemical Analyses by EDS
The first step in phase identification before the analysis of the diffraction patterns is a
chemical analysis that can been done in a TEM microscope by X-rays energy dispersive
spectrometry EDS, or electron energy loss spectrometry EELS. In addition to many other
advantages such as the possibility of obtaining information on the chemical bonding and its
Ar
3 mm
300 µm
100 µm
20 µm
mechanical grinding
+
polishing
dimple grinding
ion milling
grinding wheel
sample cutting
Fig. 3.2 TEM sample preparation of AMCs.
3. Transmission Electron Microscopy
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24
good spatial resolution, EELS is particularly appropriated for light elements (Z < ZAl), but the
identification of the chemical elements and the interpretation of the spectra are not as
straightforward as in EDS which remains a quick method for identifying and quantifying the
elements thanks to user-friendly software. In this work, EDS has been widely used for the
identification and, to a lesser degree, for quantification. Basic knowledge of EDS theory is
required to be aware about the limitation and the resolution of this technique [43, 44, 45, 46].
The X-ray microanalyses date from 1950’s with the thesis of R. Castaing who built a
microprobe on a wave dispersive spectrometre (WDS). This was followed in 1956 by the
work of Cosslet and Duncumb who developed it on a scanning electron microscope SEM.
EDS is now quasi-systematically associated with TEM to constitute a powerful set called
analytical electron microscopy AEM [47].
Inelastic interactions between electrons and matter give different kinds of signals:
secondary electrons, Auger electrons, X-rays, light and lattice vibrations (Fig. 3.1). The X-ray
energy corresponds to a difference between two energy levels of the electron cloud of an atom
(K, L.). Since these levels are quantified, the X-ray energy spectrum represents the signature
of the atom (Fig. 3.3a). The X-rays are detected by a semi-conductor and processed by a
detector protected by a ultrathin window (Fig. 3.3b) and cooled at liquid nitrogen temperature
to avoid the thermal noise and the diffusion of the dopant in the semi-conductor. An EDS
spectrum is constituted by a background produced by the Bremsstrahlung X-rays and by
peaks characteristic to the chemical elements of the material, as shown in Fig. 4.4c.
The identification is quite straightforward for elements beyond C when the peaks do not
overlap. For lighter elements, the energy of relaxation of excited atoms is in great part carried
off as the kinetics energy of Auger electrons (94% of the relaxation process). Moreover, the
potential emitted X-rays are in great part absorbed by the window. If there is an overlapping of
the peaks, a deconvolution is required, and gives poor results for close elements, such as Mg
Vacuum
Conduction band
Valence