Scanning Electron Microscopy (SEM)

The SEM produces and scans a finely-focused beam of electrons across the specimen and measures signals resulting from the electron beam/specimen interaction. Common signals employed in SEM analysis include secondary electrons (SE) for imaging surface topography; backscattered electrons (BE) for highlighting compositional differences; and X-rays for determining elemental composition and imaging element spatial distribution.

Backscattered electron (BE) imaging is an ideal technique for distinguishing phases in a microstructure when using flat, polished surfaces and is commonly referred to as compositional imaging. Image contrast is generated by different phases’ compositions relative to their weighted average atomic number (Z). Higher Z phases appear brighter. The polished surface necessary for compositional BE imaging is also ideally suited for quantitative analysis of microstructural features. As shown in the images below, the aluminum wire is very easy to see in the BE image whereas the SE image does not show the wire as well. BE images of polished sections are attractive when quantitative analysis of phases is desired. The SE image is better at imaging the textures present at the surface, and is therefore not used for polished sections. Almost all specimens other than polished sections are imaged in SE imaging mode.

Images from an SEM are monochrome since they reflect the electron or X-ray flux resulting from the beam/specimen interaction. Backscattered electron and X-ray imaging are the most useful imaging modes for quantitative scanning electron microscopy.

X-radiation is produced when a specimen is bombarded by high energy electrons. The X-ray energy level is displayed as the number of counts at each energy level appearing as a set of peaks on a continuous background. The positions of the peaks are characteristic of a particular element, so identification is made by peak positions and relative intensities. The X-ray signal can be used in a number of ways: A) spectrum analysis to determine which elements are present and in what concentration [See example]; B) line scan analysis to display the relative concentration changes along a line; and C) X-ray imaging (XR) of element spatial distribution and relative concentrations, and aid in phase identification [See example]. Mass concentration to a few tenths of a percent can be detected using an energy dispersive X-ray detector. Relative accuracy of quantitative analysis (using certified standards) is about ±20% for concentrations of about 1%, and ±2% for concentrations greater than 50%.

BE image of MDF cement paste with embedded aluminum reinforcing wire. [Images by Xuemei Xi and David A. Lange, UIUC]

SE image of MDF cement paste with embedded aluminum reinforcing wire. [Images by Xuemei Xi and David A. Lange, UIUC]

BE image of MDF cement paste with embedded glass reinforcing fiber. [Images by Xuemei Xi and David A. Lange, UIUC]

SE image of MDF cement paste with embedded glass reinforcing fiber. [Images by Xuemei Xi and David A. Lange, UIUC]