Page 48 - Handbook of Properties of Textile and Technical Fibres

Testing and characterization of fibers 29

sort of measurement. However, a rigorous calibration of the scale bar is required.
SEMs can give the distance between any two selected points on the image, greatly
simplifying the measurement.
The collection of electrons on the surface of a specimen, which is known as
charging, will cause perturbations in the electromagnetic field within the SEM and pre-
vent useable photographs being made. This is a particular problem with nonconducting
specimens, which is the case for most fibers. It is usually necessary, therefore, to
sputter on a conductive coating (e.g., AuePd) to prevent charging from the electron
beam. As the thickness of the coating is normally one or two atomic layers and the fiber
diameter is in the micrometric range, negligible error is introduced. In some instances,
nonconducting fibers can be imaged in a noncoated state if very low accelerating volt-
ages are used. Eqs. (2.7) and (2.9) show that the resolving power is proportional to
V 0:5 .
SEMs provide several different opportunities to study the surface of fibers. Imaging
of the fiber surface may be accomplished in the SEM using any of the three different
by-products of the incident beamdprimary and secondary electrons and characteristic
X-rays.

2.3.1.1 Elemental contrast

The yields of both backscattered and secondary electrons depend on the atomic num-
ber, Z, of the atoms on the fiber surface (Campbell and White, 1989). For backscattered
2
electrons the yield varies roughly as Z . The relative yield may be used to provide a
map of the distribution of different elements on the surface of a fiber. The signals
from secondary electrons, which are of lower energy, are removed by using multiple
detectors and adding or subtracting the signals recorded by each.

2.3.1.2 X-ray maps

X-rays are emitted when an outer shell electron falls into the gap created by the pro-
duction of a secondary electron; the energy of the X-ray is determined by the differ-
ence in binding energy between the two shells. The binding energy is a function of
the nuclear charge, and hence the atomic number Z. By measuring the energy of the
emitted X-rays, the identity of elements on the fiber surface may be determined.
The technique is capable of detecting boron and heavier elements.

2.3.1.3 Surface topography

The secondary electron yield is very sensitive to the surface contours, and defiladed
secondary electrons will be reabsorbed by the material and not be available for collec-
tion by the detection device. Secondary electrons formed on a high point will be
collectable. In the former case the secondary electron yield will be low and in the latter,
high, providing surface contrast. Identical to the case of elemental contrast, the signals
from backscattered electrons are filtered out through the use of multiple detectors.
The creation of artifacts by the electron beam has the potential to cause problems
when studying surface topography. Chain scission and polymer decomposition can