No turning round: abusing µct as an X-ray transmission microscope

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Sabina Sutton
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1 No turning round: abusing µct as an X-ray transmission microscope A. Sandor 1, C. Willax 1, N. Nestle 1 1 BASF SE Materials Physics and Analytics, GMC/R, G201, D Ludwigshafen, Germany Introduction While providing a wealth of details in real 3D, µct is not exactly a fast method, especially at high geometric resolutions. Furthermore, samples with a large in-plane aspect ratio are not ideal for tomographic studies (see [1] for a workaround in tomographic studies of such samples). By contrast, a simple transmission X-ray (TXM) image of a flat sample may already provide a wealth of information on the sample s lateral structure. Technically, the projection image taken in the µct system at an orientation of the sample parallel to the camera is just such a TXM image. Nevertheless, until recently there was no real support for the effective use of this capability in the operating software of Bruker-Skyscan s µct systems. Furthermore, some thoughts need to be given to sample handling as well in order to produce useful TXM images. Over the last few months, we have been working together with Bruker Skyscan to establish the TXM capabilities in an 1172 µct system. The main target application in this work was the fast inspection of grainy materials/powders for chemical and structural heterogeniety. On our side, a sample handling procedure and an appropriate sample holder was developed, on the Bruker side, an amendment to the instrument control software was made to produce images with an integrated electron-microscopy-style legend. Considerations for sample handling In order to obtain good TXM images of grainy media, the grains need to be spread in a thin layer (preferably a monolayer) onto a substrate which comes with minimal X-ray absorption and structure on its own. As the method is fast and thus cheap, low-cost, simple disposable sample carriers are desirable. After scanning standard lab supply stocks for possible candidates, we identified polystyrene Petri dishes as a good choice: They come with a thin bottom consisting of a completely amorphous polymer and with a stacking ring of 0.4 mm height which can be used as a spacer to define a thin packing cell which can be closed by the lid of the dish when applying it to the bottom instead of the top. Sealing the sample cell can be achieved by a ring of adhesive applied to the rim of the lid (see figure 1a). While using the compartment defined by the stacking ring as a sample cell allows filling in a powder aliquot that is not contaminated during the measurement, powders with a smaller grain size will result in images with superposition of several grains (see figure 2) which are harder to interpret and powders with grains larger than 0.4 mm will lead to problems in sealing the cell. Like this, the use of the inversely closed Petri dish as a powder sample cell is often not the optimal sample preparation. When applying an adhesive coating (e.g. a butyl acrylate dispersion) to the bottom of the Petri dish, a single layer of grains can be glued onto it e.g. by pouring the powder over the plate (see figure 1b). If a sufficiently strong adhesive is used, a single pour will be sufficient and no issues such as selective sticking of grains with special properties need to be considered. In order to hold samples produced by either way of sample preparation, a sample holder consisting of two pairs of Al forks was constructed to hold the sample in a way that the powder-covered bottom of the Petri dish sits directly over the µct s rotation axis (see figure 3).

powder formulation packed into a sample cell prepared in the stype of figure 1a; the makeshift figure

2 Figure 1: A) powder-filled inversely assembled petri dish used as a sealed sample cell B) sand grains sticking on the bottom of an adhesive-coated petri dish Figure 2: TXM image obtained on a spray dired powder formulation packed into a sample cell prepared in the stype of figure 1a; the makeshift figure legend dates back to the time when the TXM supporting features in the µct software were not yet available

shall be given and the information available from these images shall be discussed.

3 Figure 3: Sample holder with inserted petri dish in front view and side view Example Results In the following, a few examples of different grainy materials inspected by TXM shall be given and the information available from these images shall be discussed. Figure 4: Single layer of a spray-dried formulation similar to that in figure 2, overview image

layer on an adhesive, in figure 5, a detail of the same sample is shown at higher resolution.

4 Figure 5: Detail image of the sample in figure 4 Figure 6: Detail magnification out of the image in figure 5 showing the actual detail resolution In figure 4, a similar spray-dried powder formulation like that in figure 2 is shown as a single layer on an adhesive, in figure 5, a detail of the same sample is shown at higher resolution. Note that these images are 4000 x 2666 pixel images. The detail shown in figure 6 conveys an idea of the actual resolution of the image in figure 5.

From images like those shown in figures 4 to 6, one can determine the size distribution of the grains as well as information on the inner structure and porosity of the grains and the amount of broken

5 From images like those shown in figures 4 to 6, one can determine the size distribution of the grains as well as information on the inner structure and porosity of the grains and the amount of broken or otherwise damaged grains. The images shown in figure 7 illustrate another benefit of TXM images: in addition to information on the grain s shapes, also conclusions with respect to chemical heterogeneity can be drawn, especially when comparing images taken at different x-ray energy. While the salt grain mixture shown in figure 7 is of course a rather artificial example, the image of a quartz sand in figure 8 shows a some very localized concretions with strong absorption within an individual sand grain while the other grains show rather homogeneous X-ray absorption. Like this, it can be concluded that the strongly absorbing impurity is very unequally distributed over the sample. Such information can be very valid when trying to trace back the possible origin of impurities in a powder sample. Figure 7: Mixed NaI and NaCl grains (A) imaged at 49 kev with an Al filter (B) at 70 kev with a Cu/Al-filter

Figure 8: Quartz sand (0.3..0.8 mm grain size) imaged at 40 kev without filter (A) and at 49 kev with an Al filter.

6 Figure 8: Quartz sand ( mm grain size) imaged at 40 kev without filter (A) and at 49 kev with an Al filter. Comparing the two images clearly highlights the chemical impurities in the central grain. Conclusion While methodically rather a step back from tomography, the support for easytxm nevertheless brings in a new feature into the µct software that can provide many interesting insights into samples of appropriate geometry. Due to the short measuring times, TXM experiments also can be very valuable in order to identify good candidates for detailed tomographic inspection from a large set of samples that would no really be accessible to complete tomographic inspection due to the sheer number of samples. The information available from TXM images is complementary to electron microscopy and optical microscopies as it provides a shine-through projection and can also be applied to samples with heavy scattering and/or absorption in the optical range. The need for sample preparation is minimal as no vacuum treatment or immersion is required.. References: 1. Nestle N, Sandor A, Willax C, Seyffer H Just roll it μct studies of paper coatings, Bruker Skyscan Micro-CT User Meeting Oostende,

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