Position-Tagged Spectrometry:

What you need when you need to go back

One of the most powerful aspects of Position-Tagged Spectrometry (PTS) is its ability to retrieve new information from data taken previously. An illustration of this concept was found in a specimen of titania (TiO2) used as an electronic ceramic. Although the PTS data were taken in the past, over a year later new information was discovered to explain the behavior of the material in service.

What is PTS?

Position-tagged Spectrometry is an X-ray spectroscopic method, whereby X-ray photons generated by the scanning electron beam in an SEM are tagged with the position of their origin. The computer that controls the beam also receives the output of the X-ray detector and combines the position of the beam with the measured energy into one file. From that file, data can be extracted to form images, maps, or spectra.

The specimen shown in Figure 1 does not have a uniform grain size. The ceramic was formed from a 0.5 micron powder and was sintered at 1150° for one hour. One can see areas in the microstructure that have only small particles, 0.5-2.0 micron in diameter, representing the original powder. In other areas the grains have grown much larger. There is also some porosity, and the dielectric constant of the material depends on both grain size and porosity. Originally when the specimen was first studied, it was thought that the lack of growth of some particles reflected an inhomogeneous distribution of the sintering agent (Bi2Ti2,O5). However no bismuth was detected in X-ray maps nor in any specific region of the material.

Elemental Map

Over a year later, the investigators constructed spectra from various regions of the microstructure from the data in a PTS file. In some of these spectra, Mg, Si, Ca and traces of other elements were found. On that basis, maps were created for these elements long after the original session on the microscope.

Figure 2 shows the maps along with a secondary electron image of the uncoated specimen for comparison. It should be noted that all maps and images reconstructed from PTS are in perfect registry, because they come from the same file, where all data are stored by pixel.
The maps show the distribution of Si, Mg, and Ca. The original PTS file was collected in 15 minutes at 12,000 counts/second at 8 kV on a field-emission SEM. Although all three elements seem to be associated with regions that show a lack of grain growth, they are not all present in the same places. With PTS it is possible to construct an X-ray spectrum from any region-no matter how the region is defined. For example, a region could be defined as simply as a box drawn on the image or as complexly as image processing allows. In this
case, a spectrum consisting of the sum of all pixels high in silica is appropriate to show what type of contamination is on the boundaries of the small grains.

The Si map from Figure 2 was used to create the spectra shown in Figure 3. The solid spectrum is from the entire region over which the maps were collected. The line spectrum consists of the sum of the spectra from all pixels containing the contamination. Its height was normalized to the continuum of the first spectrum for comparison. Note there are over 180,000 counts/full scale. It is clear that the small particles are surrounded by a material rich in silica.

Problem Solved

It turns out that an organic binder containing Mg, Ca, and Si was added in the amount of 1% to control the forming behavior before firing, and it failed to burn out completely or uniformly. As a consequence, the particles in some areas were prevented from sintering with others, and this non-uniform grain structure was the result, causing the anomalous dielectric properties.

Another problem solved by PTS! Because the data were preserved in a PTS file, information could be extracted later. With PTS, it is possible to map any element or any region at any time during, or after, collection. Moreover, it is routine to create spectra from regions such as grain/particle boundaries, surfaces, phases, etc., all with suitable statistics for further analysis.