2:30 PM - 2:45 PM
[SCG45-04] Quantitative chemical analysis of heulandite by SEM-EDS - How to measure the local chemical composition of zeolites sensitive to electron beam damage?
Keywords:Zeolite, HEU-type zeolite, SEM-EDX, Raster scan
Energy-dispersive X-ray spectroscopy combined with scanning electron microscopy (SEM-EDS) is a useful tool to obtain elemental compositions of solid materials without destroying or modifying the original state of the samples. However, since the focused electron beam is continuously irradiated to a small area, there is a possibility of unexpected sample damage. Zeolite is one of the materials that are particularly susceptible to surface damage by electron beam, and to obtain accurate chemical quantification data the sample damage must be suppressed. In this study, we investigated the relationship between beam irradiation mode (point analysis / area analysis), beam current and acquisition time and sample damage in SEM-EDS analysis of natural heulandite as an example.
In the point analysis, charged up occurred at the analyzed point after irradiation, and elongated damage traces parallel to (010) of heulandite were observed around the measurement point. This indicates that the sample was rapidly heated under the electron probe and dehydration occurred near the sample surface. During this process the crystal structure of the heulandite was also locally destroyed, and some of the constituent elements evaporated into the high vacuum sample chamber, which significantly affected the quantitative values and lowered the accuracy of the analysis. The damage to the sample surface increases in proportion to the electron beam irradiation time, but the shorter the irradiation time, the lower the count of characteristic X-rays, resulting in a lower signal-to-noise (S/N) ratio, which leads to poor quantitative accuracy (repeatability). Therefore, it is necessary to collect sufficient X-ray counts for quantitative calculation while minimizing the damage to the sample surface.
Then, we attempted to use the area analysis mode, in which measurement is performed while scanning electron beams on the sample surface. In area analysis (raster scan), the irradiation time per point is much shorter, so damage can be reduced (Yoshihara, 2000; Ohfuji and Yamamoto, 2015). In the surface analysis, no damage to the sample surface was observed at low magnifications of × 5,000 (16 μm×24 μm) or less, but at higher magnifications, surface damages similar to those seen in the point analysis were formed around the analyzed area. However, at magnifications between × 3000 and ×40,000, an index of quantitative accuracy for zeolite, E * was within the range of ±10%, and the total mass concentration was around 90%, which is consistent with the ideal composition of heulandite
In the point analysis, charged up occurred at the analyzed point after irradiation, and elongated damage traces parallel to (010) of heulandite were observed around the measurement point. This indicates that the sample was rapidly heated under the electron probe and dehydration occurred near the sample surface. During this process the crystal structure of the heulandite was also locally destroyed, and some of the constituent elements evaporated into the high vacuum sample chamber, which significantly affected the quantitative values and lowered the accuracy of the analysis. The damage to the sample surface increases in proportion to the electron beam irradiation time, but the shorter the irradiation time, the lower the count of characteristic X-rays, resulting in a lower signal-to-noise (S/N) ratio, which leads to poor quantitative accuracy (repeatability). Therefore, it is necessary to collect sufficient X-ray counts for quantitative calculation while minimizing the damage to the sample surface.
Then, we attempted to use the area analysis mode, in which measurement is performed while scanning electron beams on the sample surface. In area analysis (raster scan), the irradiation time per point is much shorter, so damage can be reduced (Yoshihara, 2000; Ohfuji and Yamamoto, 2015). In the surface analysis, no damage to the sample surface was observed at low magnifications of × 5,000 (16 μm×24 μm) or less, but at higher magnifications, surface damages similar to those seen in the point analysis were formed around the analyzed area. However, at magnifications between × 3000 and ×40,000, an index of quantitative accuracy for zeolite, E * was within the range of ±10%, and the total mass concentration was around 90%, which is consistent with the ideal composition of heulandite