In a previous article, we addressed how the nature of heterogeneous structures – like NdFeB magnets used in the automotive industry – makes them difficult to analyze. These magnets are comprised of Neodymium (Nd), Iron (Fe) and Boron (B) but other added elements can change the magnetic properties of the material as well as its corrosion properties, and in certain cases improve utility at higher temperatures. Thus, it is important to know the exact mixture of elements.
We identified a Scanning Electron Microscope (SEM) equipped with an Energy Dispersive X-ray Spectrometer (EDS) as a tool well suited for this kind of microanalysis. We touched upon one technique, mapping, using Spectral Imaging (SI) as a more sophisticated analysis technique that can be used. Let’s delve deeper into the mapping technique.
In this case the field of view is selected by the analyst, then the computer rasters the electron beam in a grid pattern across the sample. At each node in the grid a spectrum is measured and stored. The result is a two dimensional array of spectra that illustrates the elemental composition of this area.
The main purpose of mapping is to reveal the distribution of an element in the field of view. Because the technique analyzes a large area it does not depend on the analyst locating a feature by eye for inspection. Every part of the sample is analyzed.
Figure 1 shows an image of the area mapped and Figure 2 shows the spectrum acquired from the entire field of view. This shows the elements that may be found in this region
This spectrum reveals the presence of Nd, Pr, Fe, Nb, Co, O, C, and a trace of Dy. Figure 3 shows maps of the discovered elements except for Dy which is present at too low a level to map easily in this particular sample and C which is likely merely a contaminant. These are processed maps in which background information and peak interference overlaps have been removed.
Maps reveal the size and shape of the inclusions containing each element. These maps clearly show that the Nb and Co are located in just a few areas, while Fe and Nd are abundant throughout the map. Because a full spectrum is stored at each point in the map it is easy to extract a spectrum from a feature to confirm the composition. Figure 4 shows the spectra extracted from Nb and Co rich regions confirming the correctness of those identifications. Displays of Net Counts values for specific elements requires knowing that one should look for those elements. Another technique parses the Spectral Imaging data set automatically and can find elements that weren’t expected to be present.
There is X-ray microanalysis system software available that finds element groupings in the field of view then produces a map of the abundance of that set of elements across the field of view. For a simple example consider a sample that has three kinds of magnesium silicates – olivine, pyroxene and serpentine. All contain Mg and Si but in different ratios. Simple mapping would show maps for Mg and Si. The software would identify each ratio as a different component and produce a map showing the distribution for each component.
The software phase mapping groups together nodes in the map with similar values and flags it as a unique phase. The software then calculates the percent area of that phase in the image and collects all of the individual spectra from each point in the phase and sums them into one spectrum. The components look like spectra but actually mathematically derived shapes.
For more specifics, download SEM/EDS Techniques for Analyzing NdFeB Magnet Material in the Automotive Industry for a detailed explanation of mapping and each measurement technique as well as images, spectra, maps showing elemental distributions in a region of the sample, and data tables.