MgCO3 growth in NaNO3–MgO using FIB-SEM

https://doi.org/10.1039/D2TA02897B

“Subsequently we used plasma FIB-SEM to obtain a smooth cross-section of a groove allowing for high quality stacks of SEM images of cross-sections through a groove (ca. 300 μm × 200 μm). This is shown schematically in Fig. 4a and b. A series of SEM micrographs taken after different times of plasma FIB milling along the length of the grooves are shown in Fig. 4c–f. Note that slice #1, shown in Fig. 4c, is at the same location as Fig. 3d (lower magnification) and the blue colour is used to visually ease the identification of the location of MgCO₃. In the ESI S5,† we demonstrate how we use EDX analysis (Na, Mg, O) to identify the location of MgCO₃ and use this information to colour the SEM images in Fig. 4 with blue to highlight the structures that are composed of MgCO₃. Three additional slices were milled by plasma FIB to provide 3D insight into the structure and location of MgCO₃. Importantly, these additional FIB cuts demonstrate that (i) MgCO₃ grows at the NaNO₃/MgO interface, i.e. at the wall of the groove, and (ii) MgCO₃ grows both close, but also far away from the triple phase boundary (TPB). These observations are in agreement with earlier reports indicating that MgCO₃ grows at the buried NaNO₃/MgO interface.^15,16 Indeed, our results establish that the TPB is not the primary location for MgCO₃ nucleation and growth, as we do not see a higher fraction of MgCO₃ particles at the TPB compared to other locations of the NaNO₃/MgO interface. Additional analysis by SEM provides more evidence that MgCO₃ nucleates and grows both close and far away from the TPB (vide infra).”

“Fig. 4 Scanning electron micrographs of cross-sections obtained by plasma focused ion beam (FIB) milling taken at four different distances within the groove length (mechanically cleaved followed by broad ion beam (BIB) cross-section polishing). (a) The turquoise box marks the region of the model structure that was FIB milled. (b) Close up showing the location of slice #4 given in (c). (c–f) Series of cross-sectional SEM images whereby each slice is taken at a specific distance from the mechanical cleaved cross section of the sample NaNO₃–MgO_A_5hours-CO2. The MgCO₃ phase is highlighted in blue for better visibility based on EDX analysis (ESI Fig. S2†). Slices #1–4 were taken at distances of ca. 0 μm, 5 μm, 10 μm and 40 μm from the mechanically cleaved surface, respectively.”