https://doi.org/10.1039/C6FD00047A
“X-ray tomography (XRT) experiments were performed on the micro end-station of the TOMCAT beamline at the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland.21 The radiographs were acquired with a magnification of ×20, pixel size of 0.33 μm, exposure time of 125 ms, rotation range of 0–180°, and at an energy of 20 keV. For each measurement, 1001 projections were collected. While this resolution means that only macro porosity can be measured, this is still a valuable source of information, especially given the reliance in the literature on nitrogen absorption surface area measurements which only measure micro and meso porosity.
In situ variable temperature (VT) measurements were achieved through the use of a specially designed infrared furnace, comprising a 250 W infrared heater with elliptical reflector (Research Inc., MN, USA), with the sample enclosed in a water-cooled chamber.22 The heater provides a highly focussed heat spot of ∼5 mm, and a variable AC power source modulates the heater power and therefore the temperature of the sample. Additionally, the chamber has an inlet for gas flow, allowing a variable gas atmosphere to be exposed to the sample. In our experiments, the inlet was switched between continuous flows of pure N2 and CO2, moderated by flowmeters.
The samples were mounted on stainless steel pins, and placed within the furnace under an initial flow of N2. Tomographic measurements were taken at temperatures of 293 K, 473 K, 673 K, 823 K, 1023 K, and then 10 measurements at 1223 K with each measurement taking approximately 5 minutes. After the measurements at 1223 K, the sample was cooled to 923 K and the gas flow switched to CO2, with a further 10 measurements taken at this temperature. A further 10 measurements were then recorded at 1223 K under N2.
The experimental tomographs were reconstructed using the gridrec algorithm.23 Three dimensional datasets were constructed using the Avizo Fire software (Version 5, FEI Visualization Science Group, France), and subsequent visualisation and phase segmentation were carried out using the same software. The phases were segmented based on grayscale ranges, as the most dense phase is expected to attenuate the X-rays to the greatest extent and therefore will have the highest grayscale values. By visual inspection, the phases were binned according the following ranges: 0–10 000 (pores), 10 000–17 000 (CaCO3) and 17 000–65 000 (CaO), with these ranges used for the entire analysis.”