Environmental Engineering Reference
In-Depth Information
The modes of observation under optical microscope are numerous. In addition to
observation under polarized light, other techniques are sometimes used, such as
black field reflective microscopy, phase contrast microscopy and interferential
contrast microscopy. The last two techniques allow the observation of living matter
without coloration and in a natural medium [WAS 03].
10.1.1.2
. Scanning electronic microscopy (SEM)
Electronic microscopy uses an electron beam accelerated and focused through
magnetic and electric fields instead of visible light rays. The superficial structure of
a material can be seen under magnification reaching 100,000 (
versus
1,000 for the
most powerful optical microscopes). It is a very powerful method of investigation
because it enables us to study the morphology and chemical composition of stone at
the same time when it is fitted with an X micro-analysis device.
The material to be studied is dried in a drying chamber and coated with a
conductive deposit of carbon or gold under high vacuum. It is then scanned by an
electron beam. These illuminating electrons (so-called primary electrons) cause the
emission of electromagnetic rays (X, UV, visible) and electrons from the impacted
surface. The image obtained under these radiations or electrons is observed on a
fluorescent screen and photos can be taken.
The main mode of observation of the surface is secondary electron emission. The
electrons coming from the first superficial layers or those coming from deeper layers
will be observed by the detector.
In the first case (the most common observation), we refer to secondary electrons
(properly speaking) and the resulting image is the relief of the surface scanned. The
field depth is 100 times that obtained with an optical microscope and enhances the
effect of the relief (see Figure 10.3).
In the second case, there are backscattered electrons. The resulting image
provides information on the composition (contrast of composition), even if
information on the relief remains. On a surface without any relief or polish, minerals
will be distinguished according their gray value; the heaviest minerals appearing
clear and bright. This contrast composition does not allow determination of the
chemical composition of the surface observed. For this purpose, the X microprobe
must be used (see section 10.1.1.3). When the illuminated material has the shape of
an ultrathin section (30 nm), transmitted electrons are also used. For this reason,
magnifications are much more important for observation at the nanometric scale.
