Building a foundation for the future, staying cool, and rocket science all with the aid of high energy x-rays
Angus P. Wilkinson
School of Chemistry and Biochemistry, Georgia Institute of Technology,
Atlanta, GA 30332-0400
High energy x-ray diffraction is an extremely valuable tool for probing a wide variety of materials systems that are of economic, environmental, security and fundamental interest. The great penetrating power of x-rays with > 60 KeV energy enables studies where sample or sample environment penetration is a key issue. Highly penetrating radiation also facilitates the minimization of systematic errors in structural studies leading to very high quality results. Access to high Q data provides a level of structural detail that can not be obtained in other ways. High energy tunable x-ray sources are potentially of use for resonant scattering studies at the Kedges of heavy elements. Lower energy resonant scattering studies for these elements may be a less appealing proposition due to problems with severe absorption by the sample or the need to use substantial sample containers if the specimen is, say, radioactive.
To fully exploit the possibilities offered by high energy x-rays, it is very important that sufficient beam time on an optimized beam line(s) is made available. This optimization should include options for efficient very fast readout detectors, very high flux on the samples, very good energy stability and band passes that are well matched to the experiment being undertaken (small
band pass for resonant scattering, larger band pass for real time studies etc.)
The value of good sample penetration can be illustrated using our studies of phase distribution inside cement paste cylinders that had been exposed to environmental sulfate. Here EDXRD, with x-rays of up to ~ 90 keV, were used to penetrate through up to ~ 1 cm of cement paste and rapidly provide high spatial resolution (~200 µm) depth profiles of the phases that
were present (see Fig. 1)
Figure 1. The distribution of ettringite, gypsum and calcium hydroxide is shown as a function depth below thesurface of a cement paste cylinder that had been exposed to sodium sulfate. This information could be obtained in ~30 minutes of beam time, by EDXRD, and correlated with sample microstructure in a completely nondestructivefashion by comparing with µCT scans of the same sample.
Sulfate attack on concrete structures is a significant issue as far as the long term durability of concrete structures is concerned. Premature failure due to sulfate damage can lead to severe economic and environmental problems.
Good penetration is also of considerable value for in-situ studies of transformations. We are involved in a series of diffraction studies examining the real time hydration of oil well cements at the high pressures and temperatures that are encountered in real wells. These studies make use of high energy x-rays to penetrate substantial sample environment and provide a detailed picture of the cement chemistry that is occurring (Fig. 2). This data provides an opportunity to rationally design additives and improve cement performance. The performance of oil well cements is a significant economic and environmental issue. Cement jobs on oil wells are very costly to perform, and improved cement characteristics could reduce this cost. Additionally, failure of the cement can lead to the release of pollutants into the environment.
Figure 2. Small titanium autoclaves used for our early cement hydration studies are shown on the left and example diffraction data obtained using these autoclaves and 65 keV x-rays are shown on the right. The ability to penetrate the autoclave and sample is crucial.
Our plans for future in-situ studies include the examination of solid fuel rocket nozzle erosion in real time (< 100 ms time resolution). This work will require highly penetrating radiation to go through windows that are exposed to high pressures and temperatures, and very high fluxes on sample to achieve the desired time resolution. The goal is to provide some level of
understanding regarding what happens to the throat of the nozzle while it is being eroded. This future work is part of a much larger effort to improve the performance of missile motors and it is
of direct relevance to national security.
We have recently exploited the specialized high energy optics at 1-ID to perform a successful resonant diffraction study of the Pb/Bi distribution in Pb5Bi6Se14 (Figure 3) using the Pb and Bi K edges at ~ 90 keV. Pb5Bi6Se14 was a potential candidate for thermoelectric cooling applications and the use of the K-edges, rather than the LIII edges, was dictated by the
combination of very high sample absorption at LIII edge energies and extreme preferred orientation problems. Such high energy resonant scattering experiments are potentially of great value in studying a wide variety of heavy element materials and they are fully compatible with substantial sample environment and sample containers such as those that might be used for
actinide samples. Resonant scattering at very high energies requires optimized optics providing a band pass that is matched to the core hole lifetime broadening at the edge and very good energy stability.
Figure 3. Resonant diffraction data collected at the K-edges of heavy elements provides a powerful tool for determining the distribution of heavy metal in complex structures such as that of Pb5Bi6Se14.
