ABSTRACT
In‐situ synchrotron X‐ray computed microtomography with sub‐micrometer voxel size was used to study the decomposition of gas hydrates in a sedimentary matrix. Xenon‐hydrate was used instead ...of methane hydrate to enhance the absorption contrast. The microstructural features of the decomposition process were elucidated indicating that the decomposition starts at the hydrate‐gas interface; it does not proceed at the contacts with quartz grains. Melt water accumulates at retreating hydrate surface. The decomposition is not homogeneous and the decomposition rates depend on the distance of the hydrate surface to the gas phase indicating a diffusion‐limitation of the gas transport through the water phase. Gas is found to be metastably enriched in the water phase with a concentration decreasing away from the hydrate‐water interface. The initial decomposition process facilitates redistribution of fluid phases in the pore space and local reformation of gas hydrates. The observations allow also rationalizing earlier conjectures from experiments with low spatial resolutions and suggest that the hydrate‐sediment assemblies remain intact until the hydrate spacers between sediment grains finally collapse; possible effects on mechanical stability and permeability are discussed. The resulting time resolved characteristics of gas hydrate decomposition and the influence of melt water on the reaction rate are of importance for a suggested gas recovery from marine sediments by depressurization.
Key Points
Time resolved, microstructural aspects of pressure‐induced gas hydrate dissolution in sediments followed with in situ synchrotron tomography
A diffusion‐limit on the liberated gas transport through the gas‐enriched water and a resulting gas concentration gradient is indicated
Possible effects of the microstructural behavior of hydrate decomposition on mechanical stability and permeability properties are discussed
3D graphene aerogel (GA) integrated with active metal or its derivatives has emerged as a novel class of multifunctional constructs with range of potential applications. However, GA fabricated by ...self‐assembly in the liquid phase still suffers from low conductivity and poor knowledge related to spatial active phase distribution and 3D structure. To address these issues, a facile approach involving in situ integration of 1D silver nanowire (AgNW) during gelation of graphene oxide flakes is presented. AgNWs prevent the restacking of graphene sheets and act as an efficient electron highway and Ag source for deposition of ultrasmall Ag nanocrystals (AgNCs). When applied as the cathodic electrocatalyst in a zinc–air battery, the 3D GA integrated with 0D AgNCs and 1D AgNWs permit ultrahigh discharge rates of up to 300 mA cm−2. Moreover, for the first time, with the help of phase‐contrast X‐ray computed microtomography, the interconnected porous network of millimeter‐sized GA and a full‐field view of the macrodistribution of Ag is delivered, offering the vitally complementary macroscopic structure information, which has been missing in previous reports.
X‐ray computed microtomography (X‐ray μCT) reveals the macrodistribution of active phase (i.e., Ag) and supermacroporous structure in the self‐assembled graphene aerogel for the first time. The interconnected 3D Ag network is the key to achieve high oxygen reduction reaction activity and utrahigh rate performance in Zn–air batteries.
The use of X-ray computed tomography (CT), exploiting both synchrotron and laboratory sources, has grown significantly over the last decade, driven primarily by improvements in spatial resolution, ...reduction in acquisition time and the increasing availability of laboratory X-ray CT systems. It is now able to provide highly accurate three-dimensional (3D) inspections of fibre architectures, manufacturing defects and in-service damage accumulation non-destructively, allowing the user to examine cross-sections that would previously have required laborious and skilled mechanical sectioning with the potential for inducing damage or loss of material. Further, by repeated acquisition of 3D images, it has opened new opportunities for time-lapse studies. This feature article reviews the technical aspects relating to the X-ray CT imaging of composites such as obtaining sufficient contrast, examination of thin panels, sample size/resolution issues, quantification of damage and defects, and image-based modelling. The capability of X-ray CT to provide important information is considered across applications ranging from manufacturing processes, through tensile and compression loading to fatigue and impact damage. The complementary advantages of laboratory and synchrotron X-ray CT are examined with a view to identify new opportunities and challenges.
Water acquisition is thought to be limited to the unsuberized surface located close to root tips. However, there are recurring periods when the unsuberized surfaces are limited in woody root systems, ...and radial water uptake across the bark of woody roots might play an important physiological role in hydraulic functioning.
Using X-ray microcomputed tomography (microCT) and hydraulic conductivity measurements (Lp
r), we examined water uptake capacity of suberized woody roots in vivo and in excised samples.
Bark hydration in grapevine woody roots occurred quickly upon exposure to water (c. 4 h). Lp
r measurements through the bark of woody roots showed that it is permeable to water and becomes more so upon wetting. After bark hydration, microCT analysis showed that absorbed water was utilized to remove embolism locally, where c. 20% of root xylem vessels refilled completely within 15 h. Embolism removal did not occur in control roots without water.
Water uptake through the bark of woody roots probably plays an important role when unsuberized tissue is scarce/absent, and would be particularly relevant following large irrigation events or in late winter when soils are saturated, re-establishing hydraulic functionality before bud break.
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Traditionally, the development of a new solid dosage form is formulation-driven and less focus is put on the design of a specific microstructure for the drug delivery system. However, ...the compaction process particularly impacts the microstructure, or more precisely, the pore architecture in a pharmaceutical tablet. Besides the formulation, the pore structure is a major contributor to the overall performance of oral solid dosage forms as it directly affects the liquid uptake rate, which is the very first step of the dissolution process. In future, additive manufacturing is a potential game changer to design the inner structures and realise a tailor-made pore structure. In pharmaceutical development the pore structure is most commonly only described by the total porosity of the tablet matrix. Yet it is of great importance to consider other parameters to fully resolve the interplay between microstructure and dosage form performance. Specifically, tortuosity, connectivity, as well as pore shape, size and orientation all impact the flow paths and play an important role in describing the fluid flow in a pharmaceutical tablet. This review presents the key properties of the pore structures in solid dosage forms and it discusses how to measure these properties. In particular, the principles, advantages and limitations of helium pycnometry, mercury porosimetry, terahertz time-domain spectroscopy, nuclear magnetic resonance and X-ray computed microtomography are discussed.
Porous media combustion (PMC) relies on internal heat recirculation in an open-cell ceramic foam matrix to enhance the flame speed of fuels with poor combustion properties. Volume-averaged ...simulations are often used to study the combustion performance and pollutant emissions of such systems. However, due to the varying complexity of matrix geometries found in practical burners, as well as the wide range of closure models for the constitutive relations of the solid phase, contradicting statements about the predictive accuracy of these volume-averaged models can be found in the literature. In this work, we propose an open-source modeling framework for accurate volume-averaged PMC simulations by using first-principles methods to determine effective properties used in closure models. This framework relies on adequately characterizing the topology of the solid matrix, using commonly available X-ray computed microtomography. With this approach, significant improvements in accuracy are reported compared to empirical models from the literature. The framework based on first-principle evaluations of constitutive relations is compared against experimental measurements conducted on an interface-stabilized burner operated with premixed NH3/H2-air. The model shows good agreement for exhaust gas composition and stability limits. The proposed simulation framework performs significantly better than state-of-the-art techniques that employ commonly used empirical correlations for effective matrix properties.
We present a new open-source simulation framework for improved characterization of porous media combustion. By utilizing µCT techniques, accurate effective matrix properties can be determined from first-principle simulations. These effective properties are used in closure models for 1D volume-averaged reacting flow simulations using appropriate sub-models for heat recirculation. This modeling framework is able to reliably predict stability limits while conventional closure models yield erroneous trends. Assessment of the resulting modeling framework is performed using experiments with exhaust gas characterization performed on a NH3/H2-air porous media burner.
X‐ray microtomography (micro‐CT) provides a nondestructive way for estimating rock properties such as relative permeability. Relative permeability is computed on the fluid distributions generated on ...three dimensional images of the pore structure of a rock. However, it is difficult to numerically reproduce actual fluid distributions at the pore scale, particularly for a mixed‐wet rock. Recent advances in imaging technologies have made it possible to directly resolve a large field of view for arbitrary wetting conditions. Herein, the objective of this study is to evaluate relative permeability computations on imaged fluid distributions under water‐wet and mixed‐wet conditions. By simultaneously injecting oil and brine on a Bentheimer sandstone before and after wettability alteration, imaged fluid distributions are obtained under steady state conditions. Then relative permeability computations performed on imaged fluid distribution are compared with experimental data obtained on the same rock. We find that relative permeabilities computed directly from imaged fluid distributions show agreement with experimental data in water‐wet rock while for mixed‐wet rock, the imaged connected pathways provided a poor estimate of relative permeability. Analysis of imaged fluid distributions and connectivity demonstrates that under mixed‐wet conditions, increased dynamic connectivity and ganglion dynamics result in non‐equilibrium effects at the fluid‐fluid interface. These effects result in more energy dissipation during fractional flow in mixed‐wet systems and thus lower effective permeability than water‐wet rock at the same saturation.
Key Points
Nonwetting phase is less connected under mixed‐wet than water‐wet conditions
Dynamic connectivity is observed more frequently under mixed‐wet conditions
Energy balance demonstrates higher propensity for interface creation under mixed‐wet conditions
Platinum (Pt)‐tipped electrodes are frequently employed to measure the soil redox potential (EH). Thereby, the timely transition from reducing towards oxidising soil conditions is one of the most ...important biogeochemical changes that can occur in soil. This condition is mainly linked to the air‐filled pore volume (ε) and pore geometries. However, even when the Pt electrodes are located in close vicinity to each other, EH readings behave non‐uniformly, presumably due to the millimetre scaled heterogeneity of pore spaces controlling oxygen (O2) availability and transport. In this study, we examined the ε distribution and pore connectivity in the close vicinity of a Pt electrode during an artificial evaporation experiment using an undisturbed soil sample (Ah‐horizon, Calcaric Gleysol). We combined physio‐chemical methods with non‐destructive X‐ray computed microtomography (μCT) and 3D‐image analysis. μCT scans were conducted at three‐time points, that is, reducing conditions with EH < −100 mV (CT‐1), the transition from reducing towards oxidising conditions with an EH increase > 5 mV h−1 (CT‐2), and oxidising conditions with EH > 300 mV (CT‐3). We observed that the shift from reducing towards oxidising conditions took place at an air‐filled porosity (εCT) of ~0.03 cm3 cm−3, which matches very with gravimetrically calculated data obtained by tensiometry of ε ~0.05 cm3 cm−3. Besides the relation of EH and ε, image analysis revealed that a connected εCT (εCT_conn) of ~0.02 cm3 cm−3 is needed to enable enhanced O2 diffusion from the soil surface towards the Pt surface and facilitate a straightforward EH response. We conclude that εCT_conn is a critical parameter to assess aeration processes in temporarily water‐saturated soils to characterise a switch in redox conditions.
Highlights
Usually, soil redox dynamics are related to the air‐filled porosity (εCT) but here its connected portion (εCT_conn) was found more relevant.
3D X‐ray computed microtomography imaging close to a redox electrode enabled us to understand the soil aeration process.
Connected εCT (εCT_conn) of ~0.02 cm3 cm−3 facilitated oxidising soil conditions.
εCT_conn is a critical parameter to assess the aeration process in temporarily water‐saturated soils.
High‐resolution microstructural analysis of porphyroblast inclusion trails integrated with Sm‐Nd garnet geochronology has provided new insight into the tectonic history of the Betic‐Rif orogen. Three ...principal age groups of porphyroblasts are demonstrated with distinctly oriented inclusion‐trails. Inclusion‐trail curvature axes or “FIA” (Foliation Inflexion/Intersection Axes) are shown to represent “fossilized” crenulation axes from which a succession of different crustal shortening directions can be deduced. The regional consistency of microstructural orientations and their geometric relationship with multiple sets of macroscopic folds reveal the composite character of the Gibraltar Arc formed by a superposition of different folding directions and associated lineations. Bulk‐garnet ages of 35–22 Ma obtained from five micaschist samples of the Alpujarride‐Sebtide complex (ASC) and of 35–13 Ma from four micaschists of the Nevado‐Filabride complex (NFC) allow to deduce NNE‐SSW directed shortening in the Late Eocene changing to NW‐SE shortening in the early Oligocene, alternating with suborthogonal NE‐SW shortening during the Miocene. These directions can be related to a major swing in the direction of relative Africa‐Iberia plate‐motion known from kinematic modeling of magnetic seafloor anomalies, and subsequent dynamic interference between plate convergence and suborthogonal “tectonic escape” of the Alboran Domain. Coupled to previously established P‐T‐t paths, the new garnet ages support a common tectono‐metamorphic evolution of the ASC and NFC as laterally equivalent orogenic domains until, in the Miocene, the second became re‐buried under the first.
Plain Language Summary
A 3D microstructural analysis of tectonic foliations preserved within garnet porphyroblasts combined with radiometric dating of these crystals has allowed reconstruction of a complex tectonic history experienced by the Betic‐Rif orogen, the strongly curved mountain belt connecting southern Spain and Morocco. Multiple porphyroblast age groups can be linked to differently oriented microfolds preserved within these metamorphic crystals. The specific ages and orientations of these microstructures measured using conventional thin sections and X‐ray tomography can be matched to known changes in the direction of convergence of the African and Iberian plates, thus demonstrating a powerful new tool for reconstructing past plate motions.
Key Points
Three sets of microfolds (Foliation Inflexion/Intersection Axis, FIA) preserved within garnet porphyroblasts of the Betic‐Rif orogen record changes in crustal shortening direction
New Sm‐Nd garnet ages reveal Eocene to Miocene tectonism in the Nevado‐Filabride complex
Integration of FIA data with garnet geochronology shown powerful tool for reconstructing past plate motions
The timing and magnitude of greenhouse gas (GHG) production depend strongly on soil oxygen (O2) availability, and the soil pore geometry characteristics largely regulate O2 and moisture conditions ...relating to GHG biochemical processes. However, the interactions between O2 dynamics and the concentration and flux of GHGs during the soil moisture transitions under various soil pore conditions have not yet been clarified. In this study, a soil-column experiment was conducted under wetting–drying phases using three pore-structure treatments, FINE, MEDIUM, and COARSE, with 0 %, 30 %, and 50 % coarse quartz sand applied to soil, respectively. The concentrations of soil gases (O2, nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4)) were monitored at a depth of 15 cm hourly, and their surface fluxes were measured daily. Soil porosity, pore size distribution, and pore connectivity were quantified using X-ray computed microtomography. The soil O2 concentrations were found to decline sharply as soil moisture increased to the water holding capacities of 0.46, 0.41, and 0.32 cm cm−3 in the FINE, MEDIUM, and COARSE, respectively. The dynamic patterns of the O2 concentrations varied across the soil pore structures, decreasing to anaerobic in FINE (<0.01 %) and MEDIUM (0.02 %), and to hypoxic (4.42 %) in COARSE. Correspondingly, the soil N2O concentration was the highest in FINE (101 μL L−1) and the lowest in COARSE (10 μL L−1), whereas the highest surface N2O flux was observed in MEDIUM (131 μg N m−2 h−1). As soil CO2 concentrations declined, CO2 fluxes increased from FINE to MEDIUM to COARSE. Most pores of FINE, MEDIUM, and COARSE were 15–80 μm, 85–100 μm, and 105–125 μm, respectively, in terms of diameter. The X-ray CT visible (>15 μm) porosity in FINE, MEDIUM and COARSE were 0.09, 0.17, and 0.28 mm3 mm−3, respectively. The corresponding Euler-Poincaré numbers were 180,280, 76,705, and −10,604, respectively, indicating higher connectivity in COARSE than in MEDIUM or FINE. In soil dominated by small air-filled porosity which limits gas diffusion and result in low soil O2 concentration, N2O concentration was increased and CO2 flux was inhibited as the moisture content increased. The turning point in the sharp decrease in O2 concentration was found to correspond with a moisture content, and a pore diameter of 95–110 μm was associated with the critical turning point between holding water and O2 depletion in soil. These findings suggest that O2-regulated biochemical processes are key to the production and flux of GHGs, which in turn are dependent on the soil pore structure and a coupling relationship between N2O and CO2. Improved understanding of the intense effect of soil physical properties provided an empirical foundation for the future development of mechanistic prediction models for how pore-space scale processes with high temporal (hourly) resolution up to GHGs fluxes at larger spatial and temporal scales.
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•O2 concentration decreased in FINE at moisture of 0.46, in COARSE at 0.32 cm cm−3.•N2O concentration was the highest in FINE at moisture of 0.42, lowest in COARSE at 0.25 cm cm−3.•The highest surface N2O flux was from MEDIUM at moisture of 0.34, and lowest was from COARSE at 0.24 cm cm−3.•95–110 μm pores play critical role in aeration or O2 depletion.
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