X-ray Computed Tomography Imaging Techniques Research Articles

Effect of Accelerated Stress Testing Conditions on Combined Chemical and Mechanical Membrane Durability in Fuel Cells

Understanding membrane degradation induced by combined chemical and mechanical stresses is critical to designing durable polymer electrolyte membrane fuel cells. Accelerated stress tests (ASTs) are usually designed and carried out to study membrane degradation and identify stresses leading to it. In this work, a customized small-scale fuel cell fixture designed for in situ X-ray computed tomography (XCT) imaging is utilized to study the impact of different AST conditions on combined chemical and mechanical membrane durability. The XCT imaging technique allows the acquisition of a tomographic dataset yielding an integrated 3D image stack, which in turn, is used to analyze and compare global membrane degradation mechanisms. It was identified that cell temperature and relative humidity (RH) strongly influence the chemical membrane degradation rate, whereas the mechanical degradation rate was promoted by RH cycles with high amplitude and short period, which were dynamically diagnosed through a single frequency electrochemical impedance spectroscopy technique developed to track membrane hydration. When applied consecutively, the high chemical and mechanical stress intensities produced a joint chemo-mechanical failure mode with distinct evidence of chemical (thinning) and mechanical (fatigue-fracture) contributions in a relatively short time. The proposed AST is thus recommended for chemo-mechanical membrane durability evaluation in fuel cells.

Titanium porous-transport layers for PEM water electrolysis prepared by tape casting

While the porous-transport layer (PTL) is a key component in PEM electrolyzers, it is one of the most underexplored due to limited available structures. In this work, we present a novel PTL design for PEM water electrolyzers enabled by a cost-effective, scalable tape-casting technique. A precise control of the PTL pore structure is achieved by incorporating poreformers of various sizes, and by varying the titanium and poreformer ratio. The structures are characterized with SEM and synchrotron X-ray computed tomography imaging techniques. Comprehensive electrochemical performance analysis demonstrates that higher titanium loading provides improved contact at the catalyst-layer/PTL interface but suffers from severe mass-transport losses due to gas bubbles. We solve this mass-transport problem by mixing in large poreformer beads that produce a highly porous structure with excellent gas removal properties yet still maintaining mechanical integrity. The PTL fabricated with 60:40 Ti:PMMA ratio and 60 μm PMMA bead size outperformed the standard commercial Ti powder-based PTL by 62 mV at 4 A/cm2.

Fabrication of gold-immobilized quantum dots/silica core–shell nanoparticles and their multimodal imaging properties

This work describes the fabrication of quantum dot (QD)/silica (SiO2) core–shell particles immobilized with Au nanoparticles and the evaluation of their abilities in fluorescence and x-ray computed tomography (CT) imaging. The QD nanoparticles were coated with silica shells by a sol-gel method using tetraethyl orthosilicate (QD/SiO2). Reduction of Au3+ in a citrate aqueous solution gave an Au nanoparticle colloidal solution. (3-Mercaptopropyl)trimethoxysilane thiolated the QD/SiO2 particle surface (QD/SiO2/SH). Immobilization of the Au nanoparticles on the QD/SiO2/SH particle surface was performed with the addition of the Au nanoparticle colloidal solution to the QD/SiO2/SH nanoparticle colloidal solution (QD/SiO2/SH/Au). Surface-PEGylated QD/SiO2/SH/Au particles were fabricated using thiol-terminated polyethylene glycol (PEG) (QD/SiO2/SH/Au/PEG). The QD/SiO2/SH/Au/PEG nanoparticle colloidal solution was imaged by fluorescence and x-ray CT imaging techniques. The CT value per unit concentration of Au (M) for the QD/SiO2/SH/Au/PEG nanoparticle colloidal solution was 1.5 × 104 HU/M. The QD/SiO2/SH/Au/PEG nanoparticle colloidal solution was injected into the tail vein of a mouse and circulated in the blood, although some QD/SiO2/SH/Au/PEG nanoparticles were trapped and remained in the liver and spleen.

The Detection of Structure in Wood by X-Ray CT Imaging Technique

Medical computed tomography (CT) has been used in forestry science and the wood industry to explore the internal structures of trees in a non-destructive way. The wood material has great diversity in structure, density, and size. The software system for medical CT is not applicable to analyze and process sectional images of wood. In order to solve this problem, a CT imaging system, based on the principle of X-ray fan-beam scanning, was constructed for this study. The computer tomography technique was applied in the non-destructive testing of wood. Four kinds of representative specimens—laminated wood, multi knot logs, large diameter logs, and small diameter logs—were selected as scanned objects. The sinusoidal images and sectional images were reconstructed with scanning data. The results showed that the accuracy of CT images is determined by the information in the sinusoidal images, from which the shapes and locations of cracks and knots can be identified. The internal properties of wood in some tomography, such as the size, number, and location of cracks and knots, the number of tree rings, and the growth law of early wood and late wood can be visually observed. Finally, the feasibility and validity of the CT imaging system was tested as a non-destructive method for verifying the internal structures of wood.

3D observations of the hydraulic fracturing process for a model non-cemented horizontal well under true triaxial conditions using an X-ray CT imaging technique

To investigate the spatial evolution of fractures in non-cemented horizontal wells, cubic cement specimens were fractured in triaxial experiments to reproduce hydraulic fracturing in field applications. Systematic experiments were conducted on specimens with different azimuth angles (0°, 30°, 45°, 60° and 90°), horizontal principal stress anisotropies (10 MPa and 4 MPa) and fracturing fluid viscosities (60 mPa·s and 5 mPa·s). All the tests were performed by injecting fluid at a constant rate of 9 cc/min. High-resolution non-destructive 3D X-ray microtomography was used to record the internal evolution of the induced fractures. The X-ray computed tomography (CT) imaging technique can produce complex 3D images of fracture systems with little artificial noise. The complex fracture geometries observed are a result of the rapidly changing stress conditions at the wellbore wall during injection and of the mechanical interaction among the fractures. The magnitudes of the well azimuth angle, principal stress anisotropy and injection fluid viscosity play significant roles in the evolution of the induced fractures. For a high horizontal differential stress of 10 MPa, the fracture morphologies are primarily determined by the in situ stresses, and the fractures extend in the direction of the maximum principal stress. However, when the horizontal differential stress is 4 MPa, the fracture geometry is influenced by the combined effects of the induced stress from the open-hole wellbore, fracture internal pressure distribution, far-field stresses and azimuth angle. In general, an induced fracture is planar at both a low (0°) and high (90°) azimuth angles, while twisted fractures form at intermediate azimuth angles (30°, 45° and 60°). In addition, the injection fluid with a low viscosity of 5 mPa·s easily infiltrates the formation, inducing a fracture network, in contrast to the smooth fracture plane induced by the injection fluid with a high viscosity of 60 mPa·s. Finally, we propose a new parameter, called the dimensionless net pressure, which can reflect the combined effects of the principal horizontal stress difference, well azimuth angle and injection fluid parameters on the evolution of the hydraulic fractures. The complexity of the fracture geometries is strongly and positively correlated with the dimensionless net pressure.

Bridging the gaps; imaging of catalytic materials under reaction conditions

Industrial catalysis utilizes mainly μm to mm-sized catalyst particles in catalytic reactors instead of powders since this minimizes problems associated with for example back pressure and clogging. In recent times, efforts have been made to study and characterize these `real life' single particles so as to determine the nature of chemical species present in 2D and 3D during various stages of the catalyst lifetime such as preparation, reaction and deactivation. Traditionally this sort of analysis is performed on ex situ samples using invasive approaches which often interfere with the chemical process under study and the subsequent conclusions that can be drawn. As a result there has been a recent move towards studying these processes non-invasively and where possible, dynamically in order to understand in detail how the chemistry evolves within catalyst particles and how this and the spatial distribution of the various chemical components influence catalytic behaviour. For this purpose we have developed synchrotron-based X-ray Computed Tomography imaging techniques for studying catalytic solids in real time in order to examine how the active phases form, how they behave under reaction conditions and why they eventually deactivate.

Experimental Investigation of Fracturing-Fluid Migration Caused by Spontaneous Imbibition in Fractured Low-Permeability Sands

SummaryDuring hydraulic-fracturing operations in low-permeability formations, spontaneous imbibition of fracturing fluid into the rock matrix is believed to have a significant impact on the retention of water-based fracturing fluids in the neighborhood of the induced fracture. This may affect the post-fracturing productivity of the well. However, there is lack of direct experimental and visual evidence of the extent of fluid retention, evolution of the resulting imbibing-fluid front, and how they relate to potential productivity hindrance. In this paper, laboratory experiments have been carefully designed to represent the vicinity of a hydraulic fracture. The evolution of fracturing fluid leakoff is monitored as a function of space and time by use of X-ray computed tomography (CT). The X-ray CT imaging technique allows us to map saturations at controlled time intervals to monitor the migration of fracturing fluid into the reservoir formation. It is generally expected for low-permeability formations (5 to 10 md) to show strong capillary forces because of their small characteristic pore radii, but this driving mechanism is in competition with the low permeability and spatial heterogeneities found in low-permeability sands. The relevance of capillarity as a driver of fluid migration and retention in a low-permeability sand sample is interpreted visually and quantified and compared with high-permeability Berea sandstone in our experiments. It is seen that although low-permeability sands are subject to strong capillary forces, the effect can be suppressed by the low permeability of the formation and the heterogeneous nature of the sample. Nevertheless, saturation values attained as a result of spontaneous imbibition are comparable with those obtained for high-permeability samples. Leakoff of fracturing fluids during the shut-in period of a well can result in delayed gas flowback and can hinder gas production. Results from this investigation are expected to provide fundamental insight regarding critical variables affecting the retention and migration of water-based fracturing fluids in the neighborhood of hydraulic fractures, and consequently affecting the post-fracturing productivity of the well.

The Impacts of Image Resolution on Permeability Simulation of Gas Diffusion Layer Using Lattice Boltzmann Method

The effect of image resolution on gas permeability through the x-ray reconstructed carbon paper gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) was examined in this paper. The 3D models of the GDL at 6 different resolutions were obtained by the x-ray computed tomography imaging technique. Each GDL image was then characterized its gas permeability through the lattice Boltzmann (LB) numerical method. The results suggest that the image resolution has a great impact on gas permeability in both principal and off-principal flow directions. The coarser resolutions can contribute to significant changes in the resulting permeability. However, it can reduce calculation time to a great extent. The results also indicate that the GDL image at the resolution of 2.72 mm provides a good compromise between computation time and accuracy.

Evaluating the aggregate structure in hot-mix asphalt using three-dimensional computer modeling and particle packing simulations

In a hot-mix asphalt (HMA) pavement, the aggregate structure serves as a backbone and is primarily responsible for resisting pavement distresses. A sound aggregate structure implies optimal packing of aggregates providing both particle–particle contact and sufficient void space to fill in asphalt. In this paper, three-dimensional particle packing concepts are applied to the study of aggregate structure in HMA. A sequential deposition packing algorithm was used for packing typical aggregate gradations. The packing fraction and the distribution of particle–particle contacts in the simulated compact were studied. The packing simulation gave satisfactory results when aggregates above a certain minimum size were considered. Regression models were established to estimate the coordination number of any size aggregate in the compact. Such studies, in conjunction with the recent advances in X-ray computed tomography imaging techniques and discrete element modeling (DEM) simulations, have tremendous potential to help develop a deeper understanding of the HMA aggregate structure, develop and optimize the various parameters that describe the aggregate structure, and relate these parameters to the performance of pavements in a scientific way.Key words: packing, aggregate structure, computer simulation, aggregate–aggregate contact, pavement performance.