Scanning Electron Microscopy (SEM):
Principle: In this technique, an electron beam is focused onto the sample surface kept in a
vacuum by electro-magnetic lenses (since electron possesses dual nature with properties of
both particle and wave an electron beam can be focused or condensed like an ordinary light)
The beam is then rastered or scanned over the surface of the sample. The scattered electron
from the sample is then fed to the detector and then to a cathode ray tube through an
amplifier, where the images are formed, which gives the information on the surface of the
sample.
Instrumentation: It comprises of a heated filament as source of electron beam, condenser
lenses, aperture, evacuated chamber for placing the sample, electron detector, amplifier, CRT
with image forming electronics, etc.
Applications: Scanning electron microscopy has been applied to the surface studies of metals,
ceramics, polymers, composites and biological materials for both topography as well as
compositional analysis. An extension (or sometimes conjunction to SEM) of this technique is
Electron Probe Micro Analysis (EPMA), where the emission of X-rays, from the sample
surface, is studied upon exposure to a beam of high energy electrons. Depending on the type
of detectors used this method is classified in to two as: Energy Dispersive Spectrometry
(EDS) and Wavelength Dispersive Spectrometry (WDS). This technique is used extensively
in the analysis of metallic and ceramic inclusions, inclusions in polymeric materials,
diffusion profiles in electronic components.
Disadvantages: The instrumentation is complicated and needs high vacuum for the optimum
performance.
No comments:
Post a Comment