One of the greatest challenges of our generation is the sustainable storage of environmentally harmful by-products of energy production processes. High-level nuclear wastes and CO2 produced from the energy sectors are examples of these by-products. To ensure the environmentally benign storage of these by-products in a solid form, it is essential to understand the chemical and morphological features of the materials in which these by-products are immobilized. With recent advancements in X-ray scattering, it is now possible to map the structure and the microstructure of architected and natural materials across four decades in spatial scale. Multiscale X-ray scattering that encompasses ultrasmall-, small-, and wide-angle X-ray scattering (USAXS/SAXS/WAXS) allows us to probe material features in the spatial ranges of ~5 μm–10 nm, ~100–1 nm, and ~1 nm–0.2 Å, respectively. This connection is illustrated using two specific examples. The first example involves determination of the changes in the porosity and the structure of beidellite, a swelling clay used in the repository design for nuclear waste disposal, on heating to temperatures above 1000°C. The second example illustrates the changes in the nanoscale porosity of heat-treated serpentine after reacting with CO2 to form magnesium carbonate.
- X-ray scattering
- porous materials
- structural and microstructural changes
- hierarchical materials
- materials for acid gas storage
- contaminant removal
Developing technological solutions for the long-term and sustainable storage of environmentally harmful by-products of energy production processes is one of the critical needs of our society. Examples of these by-products include high-level nuclear wastes and CO2 produced from the energy or industrial sectors. A fundamental understanding of the chemo-morphological coupling in materials containing these immobilized environmentally hazardous by-products is essential for predicting the long-term fate of these contaminants. In this context, chemo-morphological coupling refers to the influence of structural changes in materials at the sub-nanoscale on the nano- and mesoscale morphological changes in materials. Understanding chemo-morphological connectivity in materials allows for establishing a fundamental basis for the bulk-scale properties such as the extent of contaminants immobilized in the solid matrix. The recent development of multiscale X-ray scattering techniques presents a non-invasive, reliable, and robust approach for probing the changes in the structure and morphology of materials from the atomic-to-micrometer scales.
Multiscale X-ray scattering encompasses ultrasmall-, small-, and wide-angle X-ray scattering (USAXS/SAXS/WAXS) measurements. USAXS, SAXS, and WAXS measurements provide a spatial resolution in the range of ~5 μm–10 nm, 100–1 nm, and 1 nm–0.2 Å, respectively. The four decades of spatial scale can be probed in a single measurement within a span of 3–4 min. The capability of connecting microstructural and structural changes using
One of the advantages of multiscale X-ray scattering is the ability to probe,
2. Analyses of ultrasmall and small angle scattering data for porous materials
Given the multiscale nature of the applications described in the previous section, powerful techniques such as ultra-small, small, and wide-angle X-ray scattering (USAXS/SAXS/WAXS) are needed to probe the structures of materials across scales. At the angstrom scale, wide angle X-ray scattering (WAXS) provides detailed insights into the internal structure of crystalline materials by modeling the Bragg diffraction peaks. At the nanometer scale, small angle X-ray scattering (SAXS) provides quantitative information regarding the shape and size of the scattering objects including the relative roughness of pore-solid interfaces in porous materials. By combining small angle X-ray scattering (SAXS) with ultra-small angle X-ray scattering (USAXS), it is now possible to quantify the particle and pore sizes across the material hierarchy in architected materials or the fractal morphology in heterogeneous materials from the nanometer to millimeter scale. The key relationships for quantifying the microstructures in materials using small angle scattering are discussed in the following paragraphs.
The measured scattering,
Simplification of Eq. (1) by assuming that the isotropic microstructure consists of scattering particles or features of number density,
2.1. Determination of the volume fractions using the scattering invariant
The volume fraction of a given set of scattering features is determined using the scattering invariant. The scattering invariant is obtained by integrating the small angle scattering intensity over all
2.2. Determination of the mean volumes and radius of gyration using the guinier approximation
The mean radius of gyration of narrowly dispersed and uncorrelated scattering features is determined using the Guinier approximation. The following expression relates the scattering intensity,
2.3. Determination of the morphology or roughness of solid interfaces from the porod scattering regime
The Porod region is used to probe the local structure at the higher
Scattering from rigid rods provides a Porod slope of 1. Porod slopes between 2 and 3 represent branched systems or networks also known as mass fractals. Porod slopes between 3 and 4 represent rough interfaces with a fractal dimension,
2.4. Shapes and size distributions
One of the challenges with USAXS and SAXS is the need for microstructural insights prior to the morphological quantification of a material. Information needed
3. Brief description of USAXS/SAXS instrumentation
The ability to obtain a wide
4. Morphological and structural changes in beidellite on heating
4.1. Materials and methods
Beidellite procured from The Source Clay Mineral Repositories (Purdue University, West Lafayette, IN) are ground to a size smaller than 75 μm and compacted into a pellet with a thickness of about 0.5 mm. The pellet is placed in a Linkam TS1500 heating stage (Linkam Scientific Instruments Ltd., Tadworth, UK) in alignment with the synchrotron beamline. The starting and final temperatures of 30°C and 1150°C are set with a temperature ramp rate of 3°C/min. The X-ray scattering measurements are performed at the USAXS instrument at sector 9-ID at the Advanced Photon Source (APS), Argonne National Laboratory, Argonne, IL. Using this instrument, USAXS measurements are combined with pinhole-camera based SAXS measurements and with WAXS measurements. The USAXS, SAXS and WAXS data are represented as a function of the scattering or diffracted intensity,
4.2. Structural changes in beidellite on heating
One of the interesting structural features in clays is the hierarchical arrangement of the nanosheets and the characteristic interlayer basal distances. The interlayer water confined between the nanosheets contributes to the swelling behavior of these materials. In case of beidellite, a swelling clay, the characteristic interlayer basal spacing decreased from 12.5 to 9.7 Å on heating from 32 to 308°C (Figure 2). This reduction in the interlayer basal distance corresponds to the removal of the one layer of interlayer water. A progressive reduction in the intensity of the peak corresponding to the interlayer basal distance of beidellite is also noted on heating to 1150°C. The decreasing peak intensity on heating suggests a reduction in the number of interlayer nanopores on heating. On the structural front, a significant reduction in the intensity of the characteristic beidellite peak that corresponds to
4.3. Morphological changes in beidellite on heating
The USAXS and SAXS data are collected in the range (10−4 to 1.5) Å−1 (Figure 4(a)). As the temperature is increased from 32 to 1150°C, the scattering intensity increased 43, 58, and 25% at
To quantify the morphologies in beidellite on heating, the scattering contrast factor, |Δ
To quantify the changes in the morphology of beidellite, the void size distributions are modeled as spheroids with an aspect ratio of 0.2, which allows us to account for the layered morphology of clays. The size distributions are determined using the entropy maximization routine,
5. Morphological and structural changes in heat-treated serpentine on carbon mineralization
One of the approaches for permanent carbon storage is to use highly reactive Ca- and Mg-bearing substrates that can react with CO2 to form thermodynamically stable and environmentally benign calcium or magnesium carbonates [33, 34, 35, 36, 37]. Examples of reactive Ca- and Mg-bearing materials include wollastonite (CaSiO3), olivine (Mg2SiO4), and serpentine (Mg3Si2O5(OH)4). Serpentine is widely mined for extracting nickel and the use of serpentine mine tailings for accelerated carbon mineralization has been proposed . However, the kinetics of CO2 interactions with serpentine are slow. To accelerate these kinetic interactions, the heat treatment of serpentine is proposed. Heat treating serpentine to temperatures of 625°C dehydroxylates the lattice enabling its conversion to an amorphous state . This amorphous material has a higher reactivity with CO2 compared to the unreacted material. Recent studies have shown that heat-treated serpentine is effective in capturing and converting CO2 from flue gas streams to magnesium carbonate . However, the change in the pore size as the heat-treated serpentine is converted to magnesite is not well understood.
A reduction in the porosity of heat-treated serpentine as it is converted to magnesite may potentially limit the reactivity of the material by preventing the migration of Mg. In this context, quantification of the extent of reduction in the porosity of heat-treated serpentine on conversion to magnesium carbonate is useful. Combined ultrasmall, small, and wide angle X-ray scattering (USAXS/SAXS/WAXS) is particularly useful in linking changes in the porosity of heat-treated serpentine to the formation of magnesite. In this study, serpentine was heat-treated to 625°C and the resulting material was reacted at 185°C, PCO2 = 139 bar in 1.0 M NaCl and 0.64 M NaHCO3for 3 h while stirring at 300 rpm in a pressurized batch process. The extent of carbonation of heat-treated serpentine to magnesium carbonate is 48%. The particle sizes of these materials before and after carbon mineralization were determined to be in the range of 3–100 μm.
WAXS measurements of the heat-treated serpentine before and after the carbonation reaction showed the formation of magnesite. The characteristic magnesite peak corresponding to
In this chapter, we have illustrated the use of combined USAXS, SAXS, and WAXS measurements to quantify reaction-driven microstructural and structural changes involving complex architected and natural materials. These materials include beidellite, a swelling clay from the smectite family of minerals, and serpentine, a layered sheet silicate that belongs to the family of phyllosilicates. The combined USAXS, SAXS, and WAXS techniques available at Sector 9-ID at the Advanced Photon Source in Argonne National Laboratory allows for measurements that span nearly four decades in spatial scale. This chapter also illustrates the need for prior knowledge about the morphological arrangement in materials to quantify the key features such as the changes in the porosity or surface area of porous materials on reaction. In this context, recent advancements in electron microscopy and X-ray tomography have aided the quantitative interpretation of the X-ray scattering measurements discussed in this chapter.
The author gratefully acknowledges the Wisconsin Alumni Research Foundation and the College of Engineering at the University of Wisconsin, Madison. A special thanks to Dr. Jan Ilavsky and Dr. Ivan Kuzmenko, X-ray Science Division, Argonne National Laboratory, for providing experimental support for the combined USAXS/SAXS/WAXS measurements at the Advanced Photon Source. The use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, is supported by the U.S. DOE under Contract DE-AC02-06CH11357.