Page 36 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 2. Characterization of interfaces 19

Table 2.4
Techniques for studying s ,urface structures and composition"
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Technique Atomic process and type of information

Microscopy
Scanning electron An analytical SEM consists of electron optics, comprehensive signal
microscopy (SEM) detection facilities, and a high-vacuum environment. When the primary
electron beam is targctcd at the specimen, a portion of the electrons is
backscattered from the upper surface of the specimen. The electrons in
the specimen can also be excited and emitted from the upper surface
which are called secondary electrons. Both backscatterd and secondary
electrons carry the morphological information from the specimen
surface. The microscope collects these electrons and transmits the signals
to a cathode ray tube where the signals are scanned synchronously.
providing morphological information on the specimen surface.
Environmental SEMs are a special type of SEM that work under
controlled environmental conditions and require no conductive coating
on the specimen with the pressure in the sample chamber only 1 or 2
orders magnitude lower than the atmosphere.
Transmission electron TEM is composed of comprehensive electron optics, a projection system,
microscopy (TEM) and a high-vacuum environment. When a portion of high voltage primary
electrons is transmitted through an ultrathin sample, they can be
unscattered and scattered to carry the microstructural information of the
specimen. The microscopes collect the electrons with a comprehensive
detection system and project the microstructural images onto a fluorescent
screen. The ultimate voltage for a TEM can generally be from IO to 1000
keV, depending on the requirement of resolving power and specimcn
thickness.
Scanning tunneling The STM, like other scanning probe microscopes, relies on the scanning of
microscopy (STM) a sharp tip over a sample surface. When the tip and sample are very close
so that the electron clouds of tip and sample atoms overlap, a tunneling
current can be established through voltage differences applied between the
two electrodes. When a raster scan is made, the relative height coordinate z
as a function of the raster coordinate x and y reflects the surface
topography of the sample. The STM is limited to conducting materials as
it is based on the flow of electrons.
Atomic forcc microscopy In AFM, a sharp tip integrated with a soft spring (cantilever) deflects as a
(AFM) result of the local interaction forces present between the apex of the tip and
the sample. The deflection of this cantilever can be monitored at its rear
by a distance sensor. The forces existing between tip and sample, when
they are close, can be van der Waals, electrostatic or magnetic force.
Atomic-scale friction, elasticity and surface forces can also be measured.
AFM can be employed for both conductive and non-conductive
specimens, without having to apply a high vacuum, presenting a major
advantage over STM.