Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 93

can be plotted. Recordings can be used to observe an increase in current using
low potentials (cathodes with hydrogen release), which reaches a plateau (called
the “polishing plateau”). Then, the current increases again and there is an oxygen
release. Before the plateau, the surface presents localized etching on the high density
crystal planes; after the plateau, small holes can be observed. A good polishing con-
dition is situated at the last third of the plateau, where the apparent resistance of the
electrolytic cell is greatest.
What is happening? The anions of the solution lose their hydration water around
the anode (metal) and are adsorbed at the electrolyte-metal interface, forming a
viscous layer. Often, the anions combine with the metal to form crystal salts that
can be observed in the microscope using polarized light. They disappear once the
current is interrupted.
Electropolishing is used to polish surfaces, thereby eliminating any damage
caused by the material production technique or the preliminary preparation tech-
nique (sawing, mechanical polishing, etc.).
Electrochemical thinning is used to obtain a thin slice.
4 Ionic Action
4.1 Ionic Abrasion Principles
Ionic abrasion results from the interaction between the material and ionic particles.
Ions created by an electrical discharge are accelerated under a few kiloelectron volts
(0.5-6) in a focused beam with a Gaussian current density. This ion beam is directed
at the surface of the sample, in the area to be thinned. When an ion meets a material
surface, it penetrates the material until it successively hits different atoms (or ions
in the case of ionic compounds) with sufficient energy to displace them. In turn, the
atoms of the lattice will be projected into the solid, resulting in new collisions that
cause the atoms on the surface to tear; this is the phenomenon of pulverization. The
output of pulverization is characterized by the ratio of the number of atoms torn to
the number of incident ions.
Under normal incidence, the pulverization output depends on the energy of the
incident ion. Below a certain energy level (10-40 eV) no atoms are expelled. The
output increases to reach a maximum of between 2 and 30 keV, a range in which an
ion tears between 1 and 50 atoms (this depends on the nature of the target and the
energy of the ions). If the energy is too strong, the ions enter the material without
tearing atoms. There is an implantation of ions and this occurs more if the material
is composed of light elements. In an initial approximation, the pulverization output,
below the maximum, is proportionate to the logarithm of the acceleration voltage
(Fig. 5.8) .
The higher the mass of the incident ion (rare gas or metal), the stronger the pul-
verization output. The higher the index of the crystal plane, the faster the tearing
speed, but the maximum on the curve is displaced toward high energies because