Next: Image Processing Up:
Experimental Method Previous:
Sample Preparation
SEM Imaging
When viewed in the SEM, signals emitted from the sample as a result of the
specimen-primary electron beam interaction include backscattered electrons
(BE) and X-rays. For this study, an accelerating voltage of 12 kV and probe
current of about 2 nA was used for collecting the BE images, while probe
currents of about 10 nA were used for X-ray imaging.
In the BE images, brightness is proportional to the average atomic number
of a phase. For the major phases
present in portland cement, the phases from brightest to darkest are
tetracalcium aluminoferrite (C4AF),
tricalcium silicate (C3S), tricalcium
aluminate (C3A) and dicalcium silicate
(C2S), gypsum, and the resin-filled voids.
Because the BE signal is weak, BE images are inherently noisy so that often
the noise is reduced by averaging images of the same field of view. Phase
identification can then be attempted by segmenting the BE image based on
analysis of the greylevel histogram (a histogram of the number of pixels in
the image assigned to each greylevel or intensity). While some phase
segmentation is possible, a completely accurate separation cannot be based
solely on the BE image since several of the phases (such as C
3A and C2S) have similar
intensities ( values) even though they
differ widely in chemical composition.
To supplement the information content of the BE image, X-ray images are
obtained for the elements calcium, iron, aluminum, and sulfur. A typical
X-ray spectrum from an energy dispersive X-ray detector appears in
Figure 1 as a plot of signal intensity vs energy
level. X-ray images are created by slowly scanning the specimen while
measuring X-ray counts within energy windows encompassing a peak for the
element of interest. Typically, about three hours of scan time is required to
obtain a set of 512* 400 X-ray images. These images of X-ray signal intensity
can be processed and analyzed in the same manner as the BE image, and can even
be combined with the BE image.
Figure 1. X-ray spectrum of tetracalcium aluminoferrite indicating a composition of calcium (Ca), iron (Fe), aluminum (A1), and oxygen (O).
The vertical bars indicate the peak window region used for generating an X-ray
image
Next: Image Processing Up:
Experimental Method Previous:
Sample Preparation