Introduction to Materials - Sample Preparation Topic for Transmission Electron Microscopy - page 14

depositing films in thin layers or multiple layers, or the flux method for single
crystals), the thermal treatment conditions will be a determinant with regard to the
growth mechanisms of the microstructure formed. Thus, parameters such as sin-
tering temperature, controlled oxidation, reduction, neutral atmospheric conditions,
heating and cooling rates, mechanical pressure exerted during formation, temper-
ature gradients, and annealing times at high or low temperature all result in the
formation of a material that is more or less homogeneous from a textural, structural,
and chemical viewpoint, with very specific physical properties.
The formation of most materials involves thermal treatments at relatively high
temperatures (573-2,273 K). Material transport via diffusional processes during
thermal treatment will lead to the formation of two sorts of microstructural defects
during crystal growth at high temperature: point defects (vacancies, interstitials) and
extended defects such as twins, dislocations, stacking faults, chemical reactions at
the interfaces, secondary solid or liquid phases. Other defects such as microcracks,
second-phase precipitations, and phase transformation twins can also be formed
during cooling.
Regardless of whether the material studied is a metal, alloy, semiconductor,
ceramic, polymer, or mixed-composite material, its formation method will be
selected based on the specific physical application desired. In order to respond to
this “material problem,” it is necessary to characterize the material through observa-
tions and different types of analyses. To obtain a given property, a formation method
is chosen for study in the TEM in order to determine its properties and structure. To
do this, it will be necessary to select the preparation method for its characterization.

3.2 Materials Microstructures

The different types of material microstructures are represented in Figs. 2.2, 2.3, and
2.4. They are classified according to the material's macroscopic organization: bulk
(Fig. 2.2) , thin layer and multilayer (Fig. 2.3) , and fine particles (Fig. 2.4) .
Figure 2.2 shows a classification that is arranged by increasing degree of crys-
tallinity, from amorphous materials up to single crystals and bicrystals. What
differentiates them from one another is the extent of crystal order. This order is
minimal (on the order of interatomic distances) in an amorphous material and
increases from poorly organized materials up to textured materials and then to
microcrystallized materials (0.1-1

µ

m), polycrystalline materials (several microm-
eters), which may be textured, and lastly to single crystals or bicrystals (several
millimeters). A microstructural defect in an amorphous material will be crystalline
in nature, whereas defects in crystalline materials will be precipitates or amorphous
phases, dislocations, vacancies, twins, crystalline or chemical second phases, etc.
A single microstructural defect can act effectively for one type of property and be
catastrophic for another type.
In Fig. 2.3, we can see that the thin layers (amorphous or crystalline) can be
self-supported or on a monocrystalline, polycrystalline, or amorphous substrate.
Lastly, Fig. 2.4 shows the different forms of fine particles or single particles.