X-ray micro computed tomography for 3D visualization

https://doi.org/10.1039/D2TA02897B

“To visualize and quantify the volume of NaNO₃ in the grooves of the model samples after CO₂ capture we turned to X-ray micro computed tomography (μCT), Fig. 2. A 3D render of the NaNO₃–MgO_B system after 5 hours of exposure to CO₂ at 330 °C, NaNO₃–MgO_B_5hours-CO2, is shown in Fig. 2a. From the 3D render, it is clear that NaNO₃ no longer fills the entire volume of the groove, thus confirming a volumetric loss of NaNO₃ during carbonation. Moreover, the surface of the remaining volume of NaNO₃ forms a concave meniscus, which is an indication that the molecules of the liquid (molten NaNO₃) show stronger adhesion to the wall material (MgO) than to each other. A quantification of the volumetric NaNO₃ loss was performed by segmentation of MgO and NaNO₃ using their X-ray contrast. The volume fraction of NaNO₃ inside the groove is calculated by summing up the number of voxels occupied by NaNO₃ and dividing it by the total number of voxels inside the groove. MgCO₃ and NaNO₃ have a similar X-ray contrast, which hindered differentiation of these two materials. Therefore, the volume of MgCO₃ is included in the volume of NaNO₃, however it has a negligible contribution as the volume of MgCO₃ is significantly smaller than that of NaNO₃. Fig. 2b and c plot the volume fraction of NaNO₃ in the groove as a function of, respectively, time and depth for the sample NaNO₃–MgO_B. Our results plotted in Fig. 2b show that the loss of NaNO₃ during carbonation accelerates (volume fraction of NaNO₃ is 100% for the as-prepared sample and decreases to 99 vol% after 1 hour, while after 7 hours of carbonation there is 0 vol% NaNO₃ in the groove of a depth of 122 μm). The accelerated loss is further evident through a second order polynomial fit to the data, as shown by the dashed line in Fig. 2b. Considering a fixed carbonation time of 5 hours, we observe that in shallower grooves the volume fraction of NaNO₃ is lower compared to deeper grooves, pointing to a faster volumetric loss of NaNO₃ in shallower grooves (Fig. 2c). The experiments are carried out at 330 °C (below the boiling point of NaNO₃ of 380 °C) and the loss of NaNO₃ is ascribed to its evaporation (the vapor pressure of NaNO₃ at 330 °C is 3 mPa).²² Raman micro-spectroscopy of the NaNO₃-filled grooves revealed that NaNO₃ did not change its phase during carbonation (where evaporation has occurred).”

“Fig. 2 Quantification of the volume fraction of NaNO₃ in the grooves of a MgO(100) single crystal after carbonation using 3D X-ray micro computed tomography. (a) 3D render of three grooves in the NaNO₃ filled MgO sample (NaNO₃–MgO_B, groove depths of d_grooves = 84, 44, 22 μm) showing a volumetric loss of NaNO₃ after 5 hours of carbonation at 330 °C in a CO₂ rich atmosphere (CO₂ flow of 80 ml min⁻¹ with a purge flow of N₂ 25 ml min⁻¹) i.e. NaNO₃–MgO_B_5hours-CO2. Volume fraction of NaNO₃ in the grooves as a function of (b) carbonation time (groove with a depth of 122 μm) and (c) volume fraction as function of groove depth after 5 hours of carbonation at 330 °C in CO₂. Second order polynomial fits are plotted as dashed lines.”