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At Westmoreland Mechanical Testing
& Research ASTM E 1508 is the most commonly employed
specification for Scanning Electron Microscope / Energy Dispersive Spectroscopy.
Energy Dispersive Spectroscopy or EDS is a method of quantifying the elemental
constituents of a sample by measuring with an Electron Scanning Microscope the
number of x-rays produced by a solid sample when irradiated by electrons versus
the energy of those x-rays. Feature or phases as small as 1um can be analyzed.
When the sample is bombarded by the electron beam of the
Scanning Electron Microscope, electrons are ejected from the atoms comprising
the sample's surface. A resulting electron vacancy is filled by an electron from
a higher shell, emitting an x-ray to balance the energy difference between the
two electrons.
The Energy Dispersive Spectroscopy's x-ray detector
measures the number of emitted x-rays compared to their energy. The energy of
the x-ray is characteristic of the element from which the x-ray was emitted. A
spectrum of the energy versus relative counts of the detected x-rays is obtained
and evaluated to determine quality and quantity of elements present in the
sample.
Elements with atomic numbers from beryllium to uranium can
be detected. Depending on the element and matrix, the minimum detection limits
vary.
Quantitative results are readily obtained without standards.
However, the accuracy of standard-less quantitative analysis is highly dependent
on the sample. Greater accuracy is obtained using known standards with similar
structure and composition as the unknown sample.
Line profile analysis:
The Scanning Electron Microscope's electron beam
is scanned along a pre-determined line across the sample while x-rays are
detected for specific positions along that line. Analysis of the x-ray energy
spectrum at each position provides plots of the relative elemental concentration
for each element compared to position along the line.
Elemental mapping:
Characteristic x-ray intensity is measured
relative to lateral position on the sample surface. Variations in x-ray
intensity then indicate the relative elemental concentrations across the
surface. Maps are recorded using image brightness intensity as a direct function
of the local concentration of the element or elements present. Lateral
resolution of about 1µm is possible.
SEM/EDS is typically used
for:
Surface contamination analysis
Corrosion evaluations
Coating composition analysis
Rapid material alloy identification
Small component material analysis
Phase identification and distribution
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