Introduction to Materials - Sample Preparation Topic for Transmission Electron Microscopy - page 17

Microcrystalline materials are materials in which the order is 3D like that of
a crystal. Their dimensions vary from 100 nm to 1 m at most. In this type of

microstructure, there are a large number of interfaces. The crystal orientation varies
from one crystal to another and a structural analysis can show either the distribution
of single crystals if they are in a matrix or the crystal-amorphous structural transition
if the material is manufactured using tempering. This type of structural analysis will
provide information on the chemical distribution. These materials are either bulk or
fine particles and may be either compact or porous.
Polycrystalline materials are composed of single crystal grains whose size is
greater than 1

µ

µ

m and may extend to several hundreds of microns. In this type
of microstructure, it is important to determine whether there are orientation rela-
tionships between the grain boundaries, particular interfaces, or other interfaces,
as well as to determine if there is texturation. Characterization will also help to
determine the formation of (1) secondary phases belonging to the equilibrium phase
diagram; (2) particular microstructural defects, such as cavities formed by diffusion
mechanisms; or (3) microcracks due to crystal anisotropy and chemistry (possible
segregations). These materials are bulk and may be either compact or porous.
Single crystal or monocrystalline materials are crystals whose crystalline order
extends over three directions and may range from a few microns to a few centime-
ters. Their microstructure is linked to their crystal growth, and in particular to their
growth mechanism. For example, dislocations, which provide evidence of pyramidal
crystal growth on the surface, can be seen inside these materials. Point defects, dislo-
cation loops, and growth or annealing twins may also be detected depending on the
structure of the crystal. In certain cases, chemical inhomogeneity can be detected,
often in ionic compounds that demonstrate variations in stoichiometry, which can
lead to structural variations or to the presence of chemical impurities.
Thin layer or multilayer materials have microstructures with layer surfaces and
interfaces oriented on a substrate. Passage from a bulk 3D material to a thin 2D
layer material is characteristic of an anisotropy of one of the structure's dimen-
sions in a ratio on the order of 1:10; in this case, it is the direction perpendicular
to the substrate. The dimension in the plane of these materials can range from
a few millimeters to several centimeters, as with electronic components that are
several centimeters in size. These materials can be either amorphous or crys-
talline. Analyses of their microstructures will include the study of the surfaces,
substrate/layer interface, and layer/layer interfaces. In the case of crystalline mate-
rials, the epitaxial or heteroepitaxial growth depends on crystalline parameters that
define the connections between the atomic planes of the substrate and the layer as
well as the connections between the layers themselves. Depending on the lattice
mismatch between them, interfacial dislocations will form and lead to coherent,
semi-coherent, or incoherent interfaces (depending on the case). The formation
of roughness at the interfaces can be quantitatively measured and directly tied to
the film's growth mechanism mode (2D, 2D-3D, or 3D) and growth conditions.
The analysis of the film in the planar-longitudinal view can be used to analyze the
growth islands, while cross sections can be used to analyze the interface structure
and chemistry.