Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 92

The selection of commercial polishing solutions is fairly limited; it is often nec-
essary to adjust the composition and concentration of components depending on the
metal or alloy. The electrochemical conditions (potential and current) must be cus-
tomized for each polishing cell. Indeed, they depend on the geometry of the cell and
the respective positions and the size of the cathode and anode electrodes (sample).
The cathode must always have a large surface area compared to the anode and must
be positioned across from and near the anode.
The tables available in specialized publications provide a large number of chem-
ical compositions and working conditions (type of cathode, temperature, polishing
potential, and current density) that can be adjusted.
There are three classes of electrolytes for processing most metals:
1. Perchloric acid (1-20%)/ethanol
2. Perchloric acid (5-20%)/other acids
3. Chromic acid/acetic acid solutions
3.1.1 Techniques Involving Chemical and Electrochemical Dissolution
Preliminary Chemical Polishing Preparation and Chemical Thinning Technique
The chemical polishing and thinning techniques are used to thin non-electrically
conductive materials (semiconductors, glasses, oxides, etc.) or conductive materials
that can be dissolved chemically.
This dissolution is often difficult to control, in particular for multiphase materials.
It often requires the use of high temperatures and more reactive solutions than those
used for electropolishing.
Chemical polishing is used to polish surfaces in order to eliminate any dam-
age caused by either the technique that produced the material or the preliminary
preparation technique (sawing, mechanical polishing, etc.).
Chemical thinning is used to obtain a thin slice.
Preliminary Electropolishing Preparation and Electrochemical Thinning Technique
Electrochemical polishing and thinning techniques are used to thin electrically con-
ductive materials. They often require low temperatures to prevent explosions or fire
hazards, in case high-risk chemical substances are used. Lowering the temperature
also affects the thickness of the viscous layer, controlling the polishing rate and
quality.
The material is either immersed in the electrolytic solution or placed between
two nozzles directing an electrolyte jet simultaneously at both sides of the material.
Without external action, its surface acquires an equilibrium potential with regard
to the solution, as with chemical polishing. Under electrochemical conditions, an
electrical generator or a potentiostat is used to set and regulate the surface potential.
If the potential is increased successively, the surface acquires a stable dissolution
current for each potential value. Thus, a curve called the current/voltage curve