Synchrotron X-ray Radiography Research Articles

Motion of Solid Grains During Magnetic Field-Assisted Directional Solidification

In this paper, we report the visible evidence for thermoelectric magnetic forces (TEMFs) during magnetic field-assisted directional solidification, and their potential to control the motion of solid grains (dendrite fragments or equiaxed grains). These motions are observed by means of synchrotron X-ray radiography and compared with analytic calculations for a spherical particle’s motion driven only by TEMFs, which confirms that the observed solid grain motions are the combined result of the TEMFs and gravity. We also carried out corresponding 3D numerical simulations to validate the calculations and further prove our conclusion that TEMF acts on the solid grain and affects its motion trajectory.

An in-situ method to estimate the tip temperature and phase selection of secondary Fe-rich intermetallics using synchrotron X-ray radiography

A method was developed based on synchrotron X-ray radiography to estimate tip temperatures of 203 Fe-rich intermetallic compound (IMC) particles during directional solidification of Al-Cu-Fe alloys. Tip temperatures showed two distinct IMC populations corresponding to Al6(Fe,Cu) and Al7Cu2Fe, validated by post-solidification microscopy. The IMC tip undercooling of each growing IMC was estimated by comparison with its equilibrium formation temperature. The effect of Fe concentration, cooling rate and grain refiner additions on the IMC tip undercooling was studied. With a large number of IMC particles measured, we show the relationship between solidification parameters, secondary IMC selection and tip undercooling.

Microstructural evolution and growth behavior of intermetallic compounds at the liquid Al/solid Fe interface by synchrotron X-ray radiography

The growth of Inter-Metallic Compounds (IMCs) with dissolution was investigated on the liquid Al/solid Fe interconnection by using synchrotron radiation real-time imaging technology. At the initial stage of holding at 850°C, a layer of the η-Fe2Al5 phase formed at the interface, and tongue-like η grew into α-Fe. The tongue-like η phase had a stronger (001) texture than the layered phase. The coalescence of tongue-like η started at the bottom, followed by the dissolution of IMCs, which leads to a further increase in its growth rate. During solidification stage, the dotted, needle-like and flake θ-FeAl3 phases were sequentially formed.

In-situ synchrotron X-ray radiography observation of primary Al2Cu intermetallic growth on fragments of aluminium oxide film

While using synchrotron X-ray radiography for studying ultrasonic melt processing, the growth of the primary Al2Cu intermetallic phase on fragments of aluminium oxide film during solidification of a hypereutectic Al-35% Cu alloy was observed in-situ and in real time with adequate spatial and temporal resolution. This provides the first direct evidence for the nucleation of a primary phase on oxide particles during solidification of real metallic alloys. Comparison of the primary Al2Cu intermetallics grown in the melt with and without fragments of aluminium oxide film reveals considerable refinement, which demonstrates that the enhanced nucleation of a primary phase on oxide particles promotes structure refinement.

Simultaneous characterization of oxygen transport resistance and spatially resolved liquid water saturation at high-current density of polymer electrolyte membrane fuel cells with varied cathode relative humidity

In this study, the impacts of relative humidity on liquid water accumulation and mass transport resistance at high current densities were investigated for polymer electrolyte membrane (PEM) fuel cell cathode gas diffusion layers (GDLs). Through-plane liquid water saturation distributions were measured in situ using synchrotron X-ray radiography while simultaneously performing limiting current-based characterizations of oxygen transport resistance. In experiments with inlet cathode reactant relative humidity ranging from 0% to 100%, high local saturations (>0.6) were consistently observed in the GDL region adjacent to the flow field lands when the fuel cell reached limiting current. High land-region saturation is a possible indication of lower local temperatures. In the carbon fiber substrate portion of the GDL (i.e. excluding the MPL), the liquid water volume in the regions adjacent to the flow field channels was consistently small relative to the water volume adjacent to the land regions. We observed a trend of increasing liquid water saturation of the GDL adjacent to the channels with increasing levels of cathode inlet relative humidity, along with corresponding small increases in total oxygen transport resistance.

A Comparison of Felt-Type and Paper-Type Gas Diffusion Layers for Polymer Electrolyte Membrane Fuel Cell Applications Using X-Ray Techniques

This work presents a comparison between carbon felt-type and paper-type gas diffusion layers (GDLs) for polymer electrolyte membrane (PEM) fuel cells in terms of the similarities and the differences between their microstructures and the corresponding manner in which liquid water accumulated within the microstructures during operation. X-ray computed tomography (CT) was used to investigate the microstructure of single-layered GDLs (without a microporous layer (MPL)) and bilayered GDLs (with an MPL). In-operando synchrotron X-ray radiography was used to visualize the GDL liquid water accumulation during fuel cell operation as a function of current density. The felt-type GDLs studied here exhibited a more uniform porosity in the core regions, and the carbon fibers in the substrate were more prone to MPL intrusion. More liquid water accumulated in the felt-type GDLs during fuel cell operation; however, when differentiating between the microstructural impact of felt and paper GDLs, the presence of an MPL in bilayered GDLs was the most influential factor in liquid water management.

The Impacts of Microporous Layer Degradation on Liquid Water Distributions in Polymer Electrolyte Membrane Fuel Cells Using Synchrotron Imaging

The polymer electrolyte membrane (PEM) fuel cell, featuring high power density and zero-local emissions, is a favorable candidate for replacing the traditional internal combustion engine in automotive vehicles. The gas diffusion layer (GDL) in the PEM fuel cell facilitates water management and transport of reactant gases. The GDL is typically coated with a microporous layer (MPL) to improve water management and minimize contact resistance at the catalyst layer/GDL interface, further enhancing overall cell performance (1, 2). Achieving long-term durability is one of the major obstacles to the realization of commercial success for PEM fuel cells. The majority of PEM fuel cell durability research has been focused on the impacts of catalyst layer and membrane degradation on the cell performance, rather than the GDL. However, high quantities of liquid water accumulation in the GDL were reported in a fuel cell with aged (degraded) GDLs compared to a fuel cell with pristine GDLs, leading to significant performance losses through increased mass transport resistance (3). Furthermore, the MPL experienced up to twice the mass loss from the carbon substrate attributed to a carbon-corrosion based accelerated degradation protocol (4). Therefore, a clear understanding of the effects of MPL degradation on the liquid water behavior via in operando visualization is essential. In this study, as-received pristine GDLs were immersed into a heated hydrogen peroxide solution to perform an accelerated degradation process. The porosity distributions of pristine and degraded GDLs were characterized using desktop micro-computed tomography. Pristine and degraded GDLs were then assembled identically in a customized fuel cell for performance comparison and in operando liquid water visualizations conducted using synchrotron X-ray radiography. Synchrotron X-ray radiography has been demonstrated as a highly precise and accurate tool for in operando visualizing liquid water distribution in PEM fuel cells (5). It was found that the presence of the MPL was beneficial for water management in the GDL, further enhancing the cell performance. However, the MPL itself was more prone to degradation (compared to the substrate), leading to significant performance losses. Figure 1 presents the experimental through-plane liquid water distributions in pristine and degraded Sigracet 29 BC GDLs at an operating current density of 2.0 A/cm2. The leftmost location (x = 0 μm) represents the through-plane position of the catalyst layer/MPL interface and the rightmost location (x= 162.5 μm) represents the through-plane position of GDL/flow field interface. Degradation of the MPL resulted in higher quantities of liquid water in the MPL region, as well as in the carbon substrate region. The impacts of MPL degradation on liquid water behavior will be presented in this work, further highlighting impacts of degradation on liquid water management in long-term fuel cell operation. Reference: 1. J. Lee, J. Hinebaugh and A. Bazylak, J.Power Sources., 227(2013). 2. I. Zenyuk, E. Kumbur and S. Litster, J.Power Sources., 241(2013). 3. H. Liu, M. G. George, N. Ge, R. Banerjee, S. Chevalier, J. Lee, P. Shrestha, D. Muirhead, J. Hinebaugh, R. Zeis, M. Messerschmidt, J. Scholta and A. Bazylak, ECS Trans., 75,14(2016). 4. Liu H., George M.G., Messerschmidt M., Zeis R., Kramer D., Scholta J. and A. Bazylak, Journal of the Electrochemical Society.,(Submitted). 5. S. Chevalier, N. Ge, J. Lee, R. Banerjee, H. Liu, G. George, P. Shrestha, D. Muirhead, J. Hinebaugh and Y. Tabuchi, J.Power Sources., (2016). Figure 1

The Impacts of Microporous Layer Degradation on Liquid Water Distributions in Polymer Electrolyte Membrane Fuel Cells Using Synchrotron Imaging

In this work, commercial gas diffusion layers (GDLs) both with and without microporous layers (MPLs) were degraded through an accelerated degradation process. Pristine and degraded GDLs were each assembled in a customized fuel cell to investigate MPL degradation impacts on the distribution of liquid water via synchrotron X-ray radiography. The presence of an MPL reduced the liquid water accumulation within the GDL, which provided benefits for the performance of the fuel cell. However, the MPL was more strongly affected by the degradation process compared to the carbon fiber substrate, leading to an increased mass transport resistance and a notable performance degradation. In the degraded GDL with an MPL, higher quantities of liquid water accumulated within the GDL, as well as at the catalyst layer/MPL, MPL/carbon fiber substrate, and GDL/flow field interfaces.

<i>In Situ</i> Observation of Diffusion Behavior and Microstructural Evolution on Interfaces in Al/Cu Bimetal

Synchrotron X-ray radiography was used to in situ study the diffusion behavior and microstructural evolution of Al/Cu bimetal. The interface diffusion, dendritic/eutectic growth and the formation of intermetallic compounds around the Al/Cu bimetal interface were analyzed. During the isothermal diffusion process, a liquefied transition zone at the interface with a concentration gradient was formed when the Cu concentration exceeded eutectic composition of Al-Cu alloy. During the solidification of transition zone, the growth sequence of α-Al dendrites and eutectic structure were mainly dominated by the variation of Cu concentration and thermal field according to the temperature of the liquidus line of the equilibrium phase diagram. Finally, the transition zone around the interface were identified to be I (α-Al), II (Al+Al2Cu), III (Al2Cu) and IV (Al2Cu, AlCu and Al4Cu9), respectively.

Grain nucleation and growth behavior of a Sn-Pb alloy affected by direct current: An in situ investigation

In situ synchrotron X-ray radiography was used to study the effect of direct current (DC) on the grain nucleation and growth of Sn-50 wt.%Pb alloy. The results showed that applying DC adequately during solidification could effectively enhance the grain nucleation and inhibit its growth. Imaging of comparative experiments with varying DC intensity indicated that the final grain size, determined by the competition between grain nucleation and growth, was sensitively dependent on the DC intensity. It was found that the average grain size was decreased from 1632 to 567μm with DC density of 1.5A/mm2 compared to the case without DC. Beyond this value, raising the current density may cause a significant decrease in the nucleation rate, and thus lead to a coarsening of the grain structure.

(Invited) Imaging Techniques for Investigating Multiphase Transport Behaviour in Polymer Electrolyte Membrane Fuel Cells

The hydrogen polymer electrolyte membrane (PEM) fuel cell provides enormous potential for a future with clean energy. However, due to cost and inefficiency barriers, PEM fuel cells have not yet reached widespread commercial adoption. Mass transport limitations arising from liquid water flooding in low temperature PEM fuel cells and acid leaching in high temperature PEM fuel cells leads to inefficiencies. If these issues become resolved, smaller and more reliable devices could be produced at a lower cost. Mass transport limitations can be minimized through the development of optimized materials, which have tailored pore structures, connectivities, conductivities, and surface wettabilities. The porous materials in PEM fuel cells could be customized for mass transport with detailed information about their structure and the dominating mass transport mechanisms that result from these structures. In this talk, X-ray based visualization techniques, such as microcomputed tomography, synchrotron X-ray radiography, and neutron radiography, for investigating multiphase transport in PEM fuel cells will be discussed.

Investigating the effects of gas diffusion layer substrate thickness on polymer electrolyte membrane fuel cell performance via synchrotron X-ray radiography

Synchrotron X-ray radiography was used to visualize the liquid water accumulation in polymer electrolyte membrane (PEM) fuel cells to compare the impact of carbon substrate thickness on water management. A differential fuel cell with an active area of 0.68cm2 and rib/channel width of 0.2mm was custom-made to provide 1-dimensional (1D) conditions over the active area. The fuel cell with the thin substrate (TGP-H-030) outperformed the fuel cell with the thick substrate (TGP-H-060). The fuel cell with the thinner substrate exhibited a higher limiting current density, less liquid water in the microporous layer (MPL)-substrate transition region, and reduced oxygen transport resistance measured through electrochemical impedance spectroscopy (EIS). The compression behaviour of each GDL was also investigated through two consecutive fuel cell assemblies. The pressure in the second assembly was lower than that for the initial assemblies for both GDLs, and this significant change in assembly pressure was more pronounced for the thinner GDL (TGP-H-030). The resulting interfacial contact between the catalyst layer and the GDL was degraded, which manifested in the microscale displacement of fuel cell materials during operation (detected as a negative liquid water thickness). While the thinner GDL provided superior performance, the long term effects of material deformation may exacerbate a heterogeneous distribution of liquid water that could also impact the performance.

Novel electrospun gas diffusion layers for polymer electrolyte membrane fuel cells: Part II. In operando synchrotron imaging for microscale liquid water transport characterization

Abstract This is the second paper in a two-part series in which we investigate the impact of the gas diffusion layer structure on the liquid water distribution in an operating polymer electrolyte membrane (PEM) fuel cell through the procedures of design, fabrication, and testing of novel hydrophobic electrospun gas diffusion layers (eGDLs). In this work, fibre diameters and alignment in eGDLs are precisely controlled, and concurrent synchrotron X-ray radiography and electrochemical impedance spectroscopy (EIS) are used to evaluate the influence of the controlled eGDL parameters on the liquid water distribution and on membrane liquid water content. For eGDLs with small fibre diameters (150–200 nm) and correspondingly smaller pore sizes, reduced liquid water accumulation under the flow field ribs is observed. However, more liquid water is pinned onto the eGDL – at the interface with flow field channels. Orienting fibre alignment perpendicular to the flow field channel direction leads to improved eGDL-catalyst layer contact and prevents rib-channel membrane deformation. On the other hand, eGDLs facilitate significant membrane dry-out, even under highly humidified operating conditions at high current densities.

High-speed, two-dimensional synchrotron white-beam x-ray radiography of spray breakup and atomization.

High-speed, two-dimensional synchrotron x-ray radiography and phase-contrast imaging are demonstrated in propulsion sprays. Measurements are performed at the 7-BM beamline at the Advanced Photon Source user facility at Argonne National Laboratory using a recently developed broadband x-ray white beam. This novel enhancement allows for high speed, high fidelity x-ray imaging for the community at large. Quantitative path-integrated liquid distributions and spatio-temporal dynamics of the sprays were imaged with a LuAG:Ce scintillator optically coupled to a high-speed CMOS camera. Images are collected with a microscope objective at frame rates of 20 kHz and with a macro lens at 120 kHz, achieving spatial resolutions of 12 μm and 65 μm, respectively. Imaging with and without potassium iodide (KI) as a contrast-enhancing agent is compared, and the effects of broadband attenuation and spatial beam characteristics are determined through modeling and experimental calibration. In addition, phase contrast is used to differentiate liquid streams with varying concentrations of KI. The experimental approach is applied to different spray conditions, including quantitative measurements of mass distribution during primary atomization and qualitative visualization of turbulent binary fluid mixing.

Initiation and growth kinetics of solidification cracking during welding of steel

Solidification cracking is a key phenomenon associated with defect formation during welding. To elucidate the failure mechanisms, solidification cracking during arc welding of steel are investigated in situ with high-speed, high-energy synchrotron X-ray radiography. Damage initiates at relatively low true strain of about 3.1% in the form of micro-cavities at the weld subsurface where peak volumetric strain and triaxiality are localised. The initial micro-cavities, with sizes from 10 × 10−6 m to 27 × 10−6m, are mostly formed in isolation as revealed by synchrotron X-ray micro-tomography. The growth of micro-cavities is driven by increasing strain induced to the solidifying steel. Cavities grow through coalescence of micro-cavities to form micro-cracks first and then through the propagation of micro-cracks. Cracks propagate from the core of the weld towards the free surface along the solidifying grain boundaries at a speed of 2–3 × 10−3 m s−1.

Accelerated Degradation of Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers

In this work, as-received pristine GDLs were degraded (aged) through an accelerated ageing process by immersion into a 35% wt. solution of H2O2 at 90°C for 12 hours. Pristine and degraded GDLs were assembled in custom-built fuel cells for in operando synchrotron X-ray visualization. Electrochemical impedance spectroscopy (EIS) measurements were obtained, while synchrotron X-ray radiography was performed to investigate the effect of ageing on liquid water transport behavior. In both pristine and aged GDLs, the liquid water profile converged to an upper (terminal) threshold in the saturation pattern as the current density increased. The terminal steady state liquid water profile was established at a lower current density for the aged GDLs compared to that of the pristine GDLs. Higher water quantities of liquid water accumulated at the GDL/flow field channel and MPL/carbon fiber substrate interfaces of the fuel cell with an aged GDL compared to the fuel cell with a pristine GDL. The aged GDLs exhibited more liquid water accumulation at lower current densities. Increases in liquid water accumulation and reduced performance were attributed to the reduction in hydrophobicity of the GDL.

Multiwall Carbon Nanotube-Based Microporous Layers for Polymer Electrolyte Membrane Fuel Cells

The presence of multiwall carbon nanotubes (MWCNTs) corresponded to a dispersion of carbon black particles in the microporous layer (MPL) of the polymer electrolyte membrane (PEM) fuel cell. The gas diffusion layer (GDL) with a MWCNT-based MPL exhibited larger pores and a higher porosity compared to a conventional GDL, and less MPL intrusion into the GDL substrate was observed with the MWCNTs-based MPL. The GDLs were evaluated in operando in a fuel cell that was customized for concurrent liquid water visualization (synchrotron X-ray radiography) and electrochemical characterization. The MWCNT-based fuel cell exhibited higher power densities and lower mass transport resistances compared to the fuel cell with the conventional GDL; however, a higher liquid water saturation was observed for the MWCNT-based GDL. Although the liquid water saturation in the MWCNT-based GDL was higher, its higher effective porosity led to superior performance compared to the conventional fuel cell. The use of the MWCNTs-based MPL resulted in improved oxygen transport in the fuel cell, particularly at high current densities.

Transient Liquid Water Distributions in Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers Observed through In-Operando Synchrotron X-ray Radiography

In this work, in-operando synchrotron X-ray radiography was used to capture the changes in the liquid water saturation of gas diffusion layers (GDLs) during changes in operating current density. Through in-operando visualizations at high temporal and spatial resolutions, we observed that the liquid water saturation increased with increasing current density. Eventually, a threshold water content in the GDL was reached despite further increases in current density. A time lag between the change in current density and the onset of increasing GDL water content was also observed. Current density consistently reached a steady state value before the GDL water content reached steady state, and the trends in liquid water distributions in the MPL were distinct from, yet influential to the accumulation in the substrate. We present a logarithmic growth function that describes the dynamic changes in GDL water content. The formulation of the GDL liquid water content transient response to changes in operating conditions provides a new metric for designing next generation fuel cell powertrains with fast dynamic responses. Through plane water thickness profiles were also used to show that the water accumulation patterns continued to evolve for up to 15 minutes at the low current density.

Tracking Internal Temperature and Structural Dynamics during Nail Penetration of Lithium-Ion Cells

Mechanical abuse of lithium-ion batteries is widely used during testing to induce thermal runaway, characterize associated risks, and expose cell and module vulnerabilities. However, the repeatability of puncture or ‘nail penetration’ tests is a key issue as there is often a high degree of variability in the resulting thermal runaway process. In this work, the failure mechanisms of 18650 cells punctured at different locations and orientations are characterized with respect to their internal structural degradation, and both their internal and surface temperature, all of which are monitored in real time. The initiation and propagation of thermal runaway is visualized via high-speed synchrotron X-ray radiography at 2000 frames per second, and the surface and internal temperatures are recorded via infrared imaging and a thermocouple embedded in the tip of the penetrating nail, respectively. The influence of the nail, as well as how and where it penetrates the cell, on the initiation and propagation of thermal runaway is described and the suitability of this test method for representing in-field failures is discussed.

Synchrotron X-ray Radiography as a Highly Precise and Accurate Method for Measuring the Spatial Distribution of Liquid Water in Operating Polymer Electrolyte Membrane Fuel Cells

For the first time, the precision and the accuracy of liquid water content measurements in operating polymer electrolyte membrane (PEM) fuel cells from synchrotron X-ray radiographic imaging are determined. We define the precision of X-ray radiographic-based water measurements by combining two types of uncertainties. In the uncertainty of type A, the standard deviation of the average through-plane water thickness distribution is determined as a function of time and space. In the uncertainty of type B, the effects of the instrumentation (camera readout noise and X-ray energy level) in the Beer-Lambert law are considered. The volume of liquid water in the gas diffusion layer (GDL) is quantified with the greatest precision (plus or minus 5%) when using a beam energy of 24 keV and a low camera readout noise. The accuracy of our synchrotron X-ray-based liquid water measurements is determined from the direct comparison to neutron radiography measurements reported by Kotaka et al. (Electrochim. Acta. 146, 2014). The GDL through-plane distributions and volumes of liquid water measured by two techniques are in excellent agreement. From this work, synchrotron X-ray radiography is shown to be a powerful technique that is both precise and accurate for measuring the spatial distribution of liquid water in miniature operating PEM fuel cells.