Welcoming a new Main Editor of Journal of Synchrotron Radiation.

Introducing a new Main Editor of JSR.

Materials Research & Measurement Needs for Ceramics Additive Manufacturing.

We report on a recent workshop dedicated to additive manufacturing (AM) of ceramics that was held at the National Institute of Standards and Technology (NIST) in November 2019. This two-day all-invited meeting brought together experts from industry, government agencies and academia to review the state of the field and identify the most pressing applied materials research and metrology issues which, if addressed, could accelerate the incorporation of AM methods into commercial ceramic manufacturing. Besides the AM technologies, the discussions included consideration of the necessary post-processing steps. We highlight some of the successes and challenges for the adoption of ceramics AM on an industrial scale, as viewed by the workshop participants. We also propose actions for the ceramic community to facilitate the wider commercialization of these fabrication methods.

Nucleation and early stage crystallization in barium disilicate glass.

Barium disilicate is one of the glass-ceramic systems where internal nucleation and crystallization can occur from quenched glass upon heat treatment without requiring nucleating agents. The structural origin of the nano-clusters formed during low temperature heat treatment is of great interest in gaining a fundamental understanding of nucleation kinetics in silicate glasses. Here, we present experimental investigations on the low temperature heat treatment of barium disilicate (BaO·2SiO2) glass. Several experimental techniques were used to characterize the structural nature of barium disilicate glasses that were heat treated between the glass transition temperature, Tg, and the peak temperature of crystal growth, Tcr. The data show that small amounts of crystallites including BaSi2O5 as well as other higher Ba/Si ratio phases are formed. Moreover, unlike that reported for lower BaO content (BaO<33mol%) barium silicate glass or the analogous Li2O-SiO2 glasses, no clear evidence is observed for liquid/liquid phase separation in barium disilicate glass.

Shale pore alteration: Potential implications for hydrocarbon extraction and CO2 storage.

Shale unconventional reservoirs are currently and expected to remain substantial fossil fuel resources in the future. As CO2 is being considered to enhance oil recovery and for storage purposes in unconventional reservoirs, it is unclear how the shale matrix and fractures will react with CO2 and water during these efforts. Here, we examined the Utica Shale and its reactivity with CO2 and water using scanning electron microscopy, N2 and CO2 sorption isotherms, mercury intrusion porosimetry, and X-ray scattering methods. During CO2 exposure, the presence of water can inhibit CO2 migration into the shale matrix, promote carbonate dissolution, and dramatically change the pore scale variability by opening and closing pore networks over the macro- to nano-scale range. These alterations in the shale matrix could impact flow pathways and ultimately, oil recovery factors and carbon storage potential.

High-efficiency coherence-preserving harmonic rejection with crystal optics.

This work reports a harmonic-rejection scheme based on the combination of Si(111) monochromator and Si(220) harmonic-rejection crystal optics. This approach is of importance to a wide range of X-ray applications in all three major branches of modern X-ray science (scattering, spectroscopy, imaging) based at major facilities, and especially relevant to the capabilities offered by the new diffraction-limited storage rings. It was demonstrated both theoretically and experimentally that, when used with a synchrotron undulator source over a broad range of X-ray energies of interest, the harmonic-rejection crystals transmit the incident harmonic X-rays on the order of 10-6. Considering the flux ratio of fundamental and harmonic X-rays in the incident beam, this scheme achieves a total flux ratio of harmonic radiation to fundamental radiation on the order of 10-10. The spatial coherence of the undulator beam is preserved in the transmitted fundamental radiation while the harmonic radiation is suppressed, making this scheme suitable not only for current third-generation synchrotron sources but also for the new diffraction-limited storage rings where coherence preservation is an even higher priority. Compared with conventional harmonic-rejection mirrors, where coherence is poorly preserved and harmonic rejection is less effective, this scheme has the added advantage of lower cost and footprint. This approach has been successfully utilized at the ultra-small-angle X-ray scattering instrument at the Advanced Photon Source for scattering, imaging and coherent X-ray photon correlation spectroscopy experiments. With minor modification, the harmonic rejection can be improved by a further five orders of magnitude, enabling even more performance capabilities.

Effect of heat treatment on the microstructural evolution of a nickel-based superalloy additive-manufactured by laser powder bed fusion.

Elemental segregation is a ubiquitous phenomenon in additive-manufactured (AM) parts due to solute rejection and redistribution during the solidification process. Using electron microscopy, in situ synchrotron X-ray scattering and diffraction, and thermodynamic modeling, we reveal that in an AM nickel-based superalloy, Inconel 625, stress-relief heat treatment leads to the growth of unwanted δ-phase precipitates on a time scale much faster than that in wrought alloys (minutes versus tens to hundreds of hours). The root cause for this behavior is the elemental segregation that results in local compositions of AM alloys outside the bounds of the allowable range set for wrought alloys. In situ small angle scattering experiments reveal that platelet-shaped δ phase precipitates grow continuously and preferentially along their lateral dimensions during stress-relief heat treatment, while the thickness dimension reaches a plateau very quickly. In situ XRD experiments reveal that nucleation and growth of δ-phase precipitates occur within 5 min during stress-relief heat treatment, indicating a low nucleation barrier and a short incubation time. An activation energy for the growth of δ phase was found to be (131.04 ± 0.69) kJ mol-1. We further demonstrate that a subsequent homogenization heat treatment can effectively homogenize the AM alloy and remove the deleterious δ phase. The combined experimental and modeling methodology in this work can be extended to elucidate the phase evolution during heat treatments in a broad range of AM materials.

Structure and Dynamics of Bimodal Colloidal Dispersions in a Low-Molecular-Weight Polymer Solution.

We present an experimental study of the structural and dynamical properties of bimodal, micrometer-sized colloidal dispersions (size ratio ≈ 2) in an aqueous solution of low-molecular-weight polymer (polyethylene glycol 2000) using synchrotron ultra-small angle X-ray scattering (USAXS) and USAXS-based X-ray photon correlation spectroscopy. We fixed the volume fraction of the large particles at 5% and systematically increased the volume fraction of the small particles from 0 to 5% to evaluate their effects on the structure and dynamics. The bimodal dispersions were homogenous through the investigated parameter space. We found that the partial structure factors can be satisfactorily retrieved for the bimodal colloidal dispersions using a Percus-Yevick hard-sphere potential when the size distributions of the particles were taken into account. We also found that the partial structure factor between the large particles did not exhibit a significant variation with increasing volume fraction of the small particles, whereas the isothermal compressibility of the binary mixture was found to decrease with increasing volume fraction of the small particles. The dynamics of single-component large-particle dispersion obey the principles of de Gennes narrowing, where the wave vector dependence of the interparticle diffusion coefficient is inversely proportional to the interparticle structure factor. The dynamics of the bimodal dispersions demonstrate a strong dependence on the fraction of small particles. We also made a comparison between the experimental effective dynamic viscosity of the bimodal dispersion with the theoretical predictions, which suggest that the complex mutual interactions between the large and small particles have a strong effect on the dynamic behaviors of bimodal dispersions.

Application of Finite Element, Phase-field, and CALPHAD-based Methods to Additive Manufacturing of Ni-based Superalloys.

Numerical simulations are used in this work to investigate aspects of microstructure and microseg-regation during rapid solidification of a Ni-based superalloy in a laser powder bed fusion additive manufacturing process. Thermal modeling by finite element analysis simulates the laser melt pool, with surface temperatures in agreement with in situ thermographic measurements on Inconel 625. Geometric and thermal features of the simulated melt pools are extracted and used in subsequent mesoscale simulations. Solidification in the melt pool is simulated on two length scales. For the multicomponent alloy Inconel 625, microsegregation between dendrite arms is calculated using the Scheil-Gulliver solidification model and DICTRA software. Phase-field simulations, using Ni-Nb as a binary analogue to Inconel 625, produced microstructures with primary cellular/dendritic arm spacings in agreement with measured spacings in experimentally observed microstructures and a lesser extent of microsegregation than predicted by DICTRA simulations. The composition profiles are used to compare thermodynamic driving forces for nucleation against experimentally observed precipitates identified by electron and X-ray diffraction analyses. Our analysis lists the precipitates that may form from FCC phase of enriched interdendritic compositions and compares these against experimentally observed phases from 1 h heat treatments at two temperatures: stress relief at 1143 K (870 °C) or homogenization at 1423 K (1150 °C).

Homogenization Kinetics of a Nickel-based Superalloy Produced by Powder Bed Fusion Laser Sintering.

Additively manufactured (AM) metal components often exhibit fine dendritic microstructures and elemental segregation due to the initial rapid solidification and subsequent melting and cooling during the build process, which without homogenization would adversely affect materials performance. In this letter, we report in situ observation of the homogenization kinetics of an AM nickel-based superalloy using synchrotron small angle X-ray scattering. The identified kinetic time scale is in good agreement with thermodynamic diffusion simulation predictions using microstructural dimensions acquired by ex situ scanning electron microscopy. These findings could serve as a recipe for predicting, observing, and validating homogenization treatments in AM materials.

Towards understanding the microstructural and structural changes in natural hierarchical materials for energy recovery: In-operando multi-scale X-ray scattering characterization of Na- and Ca-montmorillonite on heating to 1150 °C.

Understanding the changes in the microstructures and structures of clays with varying intercalated metal ions at elevated temperatures is of importance for many applications ranging from the recovery of shale gas from unconventional formations to developing effective nuclear waste containment technologies, and engineering materials such as ceramics for fuel cell applications. In this study, synchrotron-based in-operando multi-scale X-ray scattering analyses are used to determine dynamic microstructural and crystal structural changes in Na- and Ca-montmorillonite on heating from 30 °C to 1150 °C. Larger cations such as Ca2+ confer more defined morphological regimes compared to Na+ ions in compacted clays, as evident from the ultra-small-angle X-ray scattering results. The hierarchical morphology of clays is characterized to distinguish between nano-scale interlayer swelling porosity, meso-scale porosity, and intergranular pore spaces between powdered clay grains. On heating from ambient temperature to 200 °C, the removal of interlayer water reduced the basal distances to 9.6 Å. On further heating to 800 °C, gradual dehydroxylation of the clay sheets is evident from the structural changes. The effects of sintering at temperatures greater than 800 °C are evident from significant reductions in the intrinsic porosities of the clay sheets, and the formation of newer phases such as mullite. By connecting the in-operando microstructural and structural changes across spatial scales ranging from micrometers to Angstroms, the possibility of engineering high temperature processes for achieving morphologies and chemical compositions of interest is presented.

In Situ Structural Characterization of Ageing Kinetics in Aluminum Alloy 2024 across Angstrom-to-Micrometer Length Scales.

The precipitate structure and precipitation kinetics in an Al-Cu-Mg alloy (AA2024) aged at 190 °C, 208 °C, and 226 °C have been studied using ex situ Transmission Electron Microscopy (TEM) and in situ synchrotron-based, combined ultra-small angle X-ray scattering, small angle X-ray scattering (SAXS), and wide angle X-ray scattering (WAXS) across a length scale from sub-Angstrom to several micrometers. TEM brings information concerning the nature, morphology, and size of the precipitates while SAXS and WAXS provide qualitative and quantitative information concerning the time-dependent size and volume fraction evolution of the precipitates at different stages of the precipitation sequence. Within the experimental time resolution, precipitation at these ageing temperatures involves dissolution of nanometer-sized small clusters and formation of the planar S phase precipitates. Using a three-parameter scattering model constructed on the basis of TEM results, we established the temperature-dependent kinetics for the cluster-dissolution and S-phase formation processes simultaneously. These two processes are shown to have different kinetic rates, with the cluster-dissolution rate approximately double the S-phase formation rate. We identified a dissolution activation energy at (149.5 ± 14.6) kJ mol-1, which translates to (1.55 ± 0.15) eV/atom, as well as an activation energy for the formation of S precipitates at (129.2 ± 5.4) kJ mol-1, i.e. (1.33 ± 0.06) eV/atom. Importantly, the SAXS/WAXS results show the absence of an intermediate Guinier-Preston Bagaryatsky 2 (GPB2)/S″ phase in the samples under the experimental ageing conditions. These results are further validated by precipitation simulations that are based on Langer-Schwartz theory and a Kampmann-Wagner numerical method.

Simultaneous multiplexed materials characterization using a high-precision hard X-ray micro-slit array.

The needs both for increased experimental throughput and for in operando characterization of functional materials under increasingly realistic experimental conditions have emerged as major challenges across the whole of crystallography. A novel measurement scheme that allows multiplexed simultaneous measurements from multiple nearby sample volumes is presented. This new approach enables better measurement statistics or direct probing of heterogeneous structure, dynamics or elemental composition. To illustrate, the submicrometer precision that optical lithography provides has been exploited to create a multiplexed form of ultra-small-angle scattering based X-ray photon correlation spectroscopy (USAXS-XPCS) using micro-slit arrays fabricated by photolithography. Multiplexed USAXS-XPCS is applied to follow the equilibrium dynamics of a simple colloidal suspension. While the dependence of the relaxation time on momentum transfer, and its relationship with the diffusion constant and the static structure factor, follow previous findings, this measurements-in-parallel approach reduces the statistical uncertainties of this photon-starved technique to below those associated with the instrument resolution. More importantly, we note the potential of the multiplexed scheme to elucidate the response of different components of a heterogeneous sample under identical experimental conditions in simultaneous measurements. In the context of the X-ray synchrotron community, this scheme is, in principle, applicable to all in-line synchrotron techniques. Indeed, it has the potential to open a new paradigm for in operando characterization of heterogeneous functional materials, a situation that will be even further enhanced by the ongoing development of multi-bend achromat storage ring designs as the next evolution of large-scale X-ray synchrotron facilities around the world.

Dissolution, agglomerate morphology, and stability limits of protein-coated silver nanoparticles.

Little is understood regarding the impact that molecular coatings have on nanoparticle dissolution kinetics and agglomerate formation in a dilute nanoparticle dispersion. Dissolution and agglomeration processes compete in removing isolated nanoparticles from the dispersion, making quantitative time-dependent measurements of the mechanisms of nanoparticle loss particularly challenging. In this article, we present in situ ultra-small-angle X-ray scattering (USAXS) results, simultaneously quantifying dissolution, agglomeration, and stability limits of silver nanoparticles (AgNPs) coated with bovine serum albumin (BSA) protein. When the BSA corona is disrupted, we find that the loss of silver from the nanoparticle core is well matched by a second-order kinetic rate reaction, arising from the oxidative dissolution of silver. Dissolution and agglomeration are quantified, and morphological transitions throughout the process are qualified. By probing the BSA-AgNP suspension around its stability limits, we provide insight into the destabilization mechanism by which individual particles rapidly dissolve as a whole rather than undergo slow dissolution from the aqueous interface inward, once the BSA layer is breached. Because USAXS rapidly measures over the entire nanometer to micrometer size range during the dissolution process, many insights are also gained into the stabilization of NPs by protein and its ability to protect the labile metal core from the solution environment by prohibiting the diffusion of reactive species. This approach can be extended to a wide variety of coating molecules and reactive metal nanoparticle systems to carefully survey their stability limits, revealing the likely mechanisms of coating breakdown and ensuing reactions.

Structure and dynamics studies of concentrated micrometer-sized colloidal suspensions.

We present an experimental study of the structural and dynamical properties of concentrated suspensions of different sized polystyrene microspheres dispersed in glycerol for volume fraction concentrations between 10% and 20%. The static structure, probed with ultrasmall-angle X-ray scattering, shows a behavior very similar to that of hard spheres. The equilibrium dynamics is probed with ultrasmall-angle X-ray scattering-X-ray photon correlation spectroscopy, a new technique that overcomes the limits of visible light-scattering techniques imposed by multiple scattering and is suitable for studies of optically opaque materials containing micrometer-sized structures. We found that the intensity autocorrelation functions are better described by a stretched exponential function and microspheres in a concentrated suspension move collectively. We also found that the inverse of the effective diffusion coefficients displays a peak with respect to the scattering vector that resembles the peaks in the static structure factors, which indicates that a long-lived, low free-energy state exists. The relaxation time is approximately inversely related to scattering vector, a behavior consistent with models that describe the dynamics in terms of random, local structural arrangements in disordered media.

Measurement, standards, and data needs for CO2 capture materials: a critical review.

The commercial deployment of cost-effective carbon capture technology is hindered partially by the lack of a proper suite of materials-related measurements, standards, and data, which would provide critical information for the systematic design, evaluation, and performance of CO2 separation materials. Based on a literature search and conversations with the carbon capture community, we review the current status of measurements, standards, and data for the three major carbon capture materials in use today: solvents, solid sorbents, and membranes. We highlight current measurement, standards and data activities aimed to advance the development and use of carbon capture materials and major research needs that are critical to meet if innovation in carbon capture materials is to be achieved. The review reveals that although adsorbents are considered to have great potential to reduce carbon capture cost, there is no consensus on the experimental parameters to be used for evaluating sorbent properties. Another important finding is the lack of in situ experimental tools for the structural characterization of solid porous materials during CO2 adsorption, and computational methods that would enable a materials-by-design approach for their development.

Selected advances in small-angle scattering and applications they serve in manufacturing, energy and climate change.

Innovations in small-angle X-ray and neutron scattering (SAXS and SANS) at major X-ray and neutron facilities offer new characterization tools for researching materials phenomena relevant to advanced applications. For SAXS, the new generation of diffraction-limited storage rings, incorporating multi-bend achromat concepts, dramatically decrease electron beam emittance and significantly increase X-ray brilliance over previous third-generation sources. This results in intense X-ray incident beams that are more compact in the horizontal plane, allowing significantly improved spatial resolution, better time resolution, and a new era for coherent-beam SAXS methods such as X-ray photon correlation spectroscopy. Elsewhere, X-ray free-electron laser sources provide extremely bright, fully coherent, X-ray pulses of <100 fs and can support SAXS studies of material processes where entire SAXS data sets are collected in a single pulse train. Meanwhile, SANS at both steady-state reactor and pulsed spallation neutron sources has significantly evolved. Developments in neutron optics and multiple detector carriages now enable data collection in a few minutes for materials characterization over nanometre-to-micrometre scale ranges, opening up real-time studies of multi-scale materials phenomena. SANS at pulsed neutron sources is becoming more integrated with neutron diffraction methods for simultaneous structure characterization of complex materials. In this paper, selected developments are highlighted and some recent state-of-the-art studies discussed, relevant to hard matter applications in advanced manufacturing, energy and climate change.

Time-dependent CO2 sorption hysteresis in a one-dimensional microporous octahedral molecular sieve.

The development of sorbents for next-generation CO(2) mitigation technologies will require better understanding of CO(2)/sorbent interactions. Among the sorbents under consideration are shape-selective microporous molecular sieves with hierarchical pore morphologies of reduced dimensionality. We have characterized the non-equilibrium CO(2) sorption of OMS-2, a well-known one-dimensional microporous octahedral molecular sieve with manganese oxide framework. Remarkably, we find that the degree of CO(2) sorption hysteresis increases when the gas/sorbent system is allowed to equilibrate for longer times at each pressure step. Density functional theory calculations indicate a "gate-keeping" role of the cation in the tunnel, only allowing CO(2) molecules to enter fully into the tunnel via a highly unstable transient state when CO(2) loadings exceed 0.75 mmol/g. The energy barrier associated with the gate-keeping effect suggests an adsorption mechanism in which kinetic trapping of CO(2) is responsible for the observed hysteretic behavior.

Challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions.

The reported size distribution of silver nanoparticles (AgNPs) is strongly affected by the underlying measurement method, agglomeration state, and dispersion conditions. A selection of AgNP materials with vendor-reported diameters ranging from 1 nm to 100 nm, various size distributions, and biocompatible capping agents including citrate, starch and polyvinylpyrrolidone were studied. AgNPs were diluted with either deionized water, moderately hard reconstituted water, or moderately hard reconstituted water containing natural organic matter. Rigorous physico-chemical characterization by consensus methods and protocols where available enables an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provides a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. An approach to developing routine screening is also presented.

Density Functional Theory Study of the Structure of the Pillared Hofmann compound Ni(3-Methy-4,4'-bipyridine)[Ni(CN)4] (Ni-BpyMe or PICNIC-21).

We use dispersion-corrected density functional theory (DFT) to investigate the structure of the pillared Hofmann compound Ni(3-Methy-4,4'-bipyridine)[Ni(CN)4] (Ni-BpyMe for short, or PICNIC-21). We model the disorder found in experimental X-ray structure refinement via an ensemble of supercells with ordered ligand orientation configurations. The ensemble-averaged structure agrees very well with experiment, except for the positions of the methyl group hydrogen atoms. While the dihedral angles between the bipyridal rings of each BpyMe ligand of the averaged structure is 90°, the local dihedral angles are about 80°. DFT screening of configurations where the crystallographic a/b ratio is constrained to equal 1 fail to find the configurations that are most stable when a/b is set to its distorted experimental value of a/b = 0.86, demonstrating the difficulty of solving pillared Hofmann structures purely theoretically without experimental input. The waviness of the Ni(CN)2 sheets is explained as a tendency to maximize dispersion interactions between these sheets and the methyl pyridine rings. This waviness leads to greater residual pore space and greater adsorbate uptake at low pressure compared with the analogous pillared compound Ni-Bpene (PICNIC-60).