Synchrotron X-ray Radiography Research Articles

Visualizing Liquid Water Evolution in a PEM Fuel Cell Using Synchrotron X-ray Radiography

Synchrotron X-ray radiography was utilized to study the time evolution of liquid water in an operating polymer electrolyte membrane (PEM) fuel cell. A high aspect ratio fuel cell designed with offset anode and cathode flow field channels was operated at conditions that produced critical water management issues. The X-ray beam was directed along the plane of the fuel cell, and was therefore employed to elucidate the through-plane distribution of liquid water in the porous materials. Due to the offset between the anode and cathode gas channels, the membrane electrode assembly exhibited sinusoidal warping, and liquid water accumulated in possible areas of delamination. Liquid water first appeared near the cathode catalyst layer, and then traveled laterally within the porous gas diffusion layer. The experiment provides a basis for future design considerations, including membrane thickness and attenuation estimates.

Electrode deterioration processes in lithium ion capacitors monitored by in situ X-ray radiography on micrometre scale

Gas bubble formation and the related mechanical and chemical deterioration of electrodes are processes limiting performance and lifetime of lithium ion capacitors and batteries. Moreover, the increase of internal pressure caused by gas formation constitutes a severe safety problem. The authors were able to monitor the degradation process of capacitor cells for the first time by in situ synchrotron and laboratory X-ray radiography in through-plane and in-plane geometry with high optical resolution of 1-5--m. The decomposition is found to be dependent only on the electrolyte system but not on the applied voltage, current or the state of charge. The standard electrolyte system for lithium ion batteries is a solution of LiPF 6 in a mixture of ethylene carbonate and dimethyl carbonate (EC/DMC) and leads to bubble formation because of decarboxylation, increase of internal pressure, material ablation and particle migration on the cathode layer, as could be shown by radiography studies on working capacitor cells. As an alternative, the scarcely studied electrolyte system LiBF 4 in N,N -dimethylformamide (DMF) shows a good performance and high chemical stability even at high charge voltages.

Correlation of Synchrotron X-ray Radiography and Electrochemical Impedance Spectroscopy for the Investigation of HT-PEFCs

Differently conditioned high temperature polymer electrolyte fuel cells (HT-PEFCs) were investigated by electrochemical impedance spectroscopy to study the influence of the break-in procedure on the overall fuel cell performance during the heating-up procedure as well as during different operating conditions. It is shown that a classical 70 h break-in leads to better fuel cell performance compared to a fuel cell which is used without any break-in procedure. A combination of synchrotron X-ray radiographic measurements with in situ time-dependent impedance spectroscopy was used to investigate the de-/hydration processes of the phosphoric acid type electrolyte within the components of the membrane electrode assembly (MEA) of a dynamically operated HT-PEFC. This helps to understand the fuel cell performance under different operating conditions.

Investigation of gravity effects on solidification of binary alloys with in situ X-ray radiography on earth and in microgravity environment

As most of the phenomena involved during solidification are dynamic, in situ and real-time X-ray imaging should be retained as the method of choice for investigating the solidification front evolution of metallic alloys grown from the melt. On Earth, natural convection in the melt is well known to be the major source of various disturbing effects which can significantly modify or mask important physical mechanisms. Microgravity environment is an efficient way to eliminate buoyancy and convection to provide benchmark data for the validation of models and numerical simulations. In addition, a comparative study of solidification experiments carried out on Earth and in space can also enlighten the effects of gravity. In the frame of the ESA - MAP programme entitled XRMON, an experimental set-up has been developed to perform directional solidification in microgravity conditions with in situ X-ray radiography observation. In the first part of this paper, we will present a brief review of some effects induced by gravity on the solidification process and investigated by mean of synchrotron X-ray radiography at ESRF (European Synchrotron Radiation Facility). In the second part of this paper, we will describe some results obtained with a prototype of the XRMON-Gradient Furnace set-up. These preliminary results show the large capabilities of the experimental set-up in terms of thermal behaviour, as well as X-ray observation.

Quantitatively comparing phase-field modeling with direct real time observation by synchrotron X-ray radiography of the initial transient during directional solidification of an Al–Cu alloy

The initial transient during directional solidification of an Al–4 wt.% Cu alloy was simulated by a quantitative phase-field model solved with the adaptive finite element method. The simulated solidification process was compared with the related analytical theory and in situ and real time observations by means of X-ray radiography at the European Synchrotron Radiation Facility. The simulated velocity of the planar interface and solute profile ahead of the solidification front in the liquid are close to the predictions of the Warren–Langer model of the initial planar solidification transient, but in fair quantitative agreement with experimental results only at early stages of planar solidification. After the accelerated flat interface lost its stability a transition to cellular solidification was initiated. The initial cell spacing predicted by the phase-field simulation agreed well with the experimental observations in the region where the cell growth direction was perpendicular to the fluid flow, whereas a discrepancy was obvious in the corners where the fluid flow was parallel to growth. An analytical relation describing the wavelength of the initial solid–liquid interface corrugations under diffusion-limited transport is screened out by comparing the phase-field simulation data with expressions based upon the Mullins–Sekerka linear stability analysis theory or derived for primary spacing. The gravity-driven natural convection in the experiment resulted in misfits between the phase-field predictions and the experimental observations in the late stage of planar solidification, onset and development of morphological instability.

Investigation of HT-PEFCs by Means of Synchrotron X-ray Radiography and Electrochemical Impedance Spectroscopy

Differently conditioned high temperature polymer electrolyte fuel cells (HT-PEFCs) were investigated by electrochemical impedance spectroscopy to study the influence of the break-in procedure on the overall fuel cell performance during the heating-up procedure as well as during different operating conditions. It is shown that a classical 70 h break-in procedure leads to the best fuel cell performance compared to fuel cells which are used after a rapid break-in or rather without any break-in procedure. A combination of synchrotron X ray radiographic measurements with in-situ performed time-dependent impedance spectroscopy was used to investigate the de-/hydration processes within the electrolyte of the membrane electrode assembly (MEA) components of a dynamically operated HT-PEFC. This helps to understand the fuel cell performance under different operating conditions.

Investigation on HT-PEFCs by Means of Synchrotron X-ray Radiography and Electrochemical Impedance Spectroscopy

not Available.

Pressure effect on forsterite dislocation slip systems: Implications for upper-mantle LPO and low viscosity zone

In order to better constrained the effect of pressure ( P) on olivine dislocation slip-system activities, deformation experiments were carried out in a Deformation-DIA apparatus (D-DIA) on pure forsterite (Fo100) single crystals, at P ⩾ 5.7 GPa, temperature T ∼ 1675 K, differential stress σ < 350 MPa and in water-poor conditions. Constant σ and specimen strain rates ( ε ˙ ) were monitored in situ by synchrotron X-ray diffraction and radiography, respectively. Two compression directions were tested, promoting either [1 0 0] slip or [0 0 1] slip in (0 1 0) crystallographic plane. Comparison of the obtained high- P rheological data with room- P data previously reported by Darot and Gueguen (1981) shows that [1 0 0] slip is strongly inhibited by pressure while [0 0 1] slip is virtually P insensitive. This translates in creep power laws into a high activation volume V a ∗ = 15 ± 3 cm 3 / mol for [1 0 0](0 1 0) slip system, and V c ∗ = 0 ± 1.2 cm 3 / mol for [0 0 1](0 1 0) slip system. Using these laws along geotherms at natural σ condition shows that the [1 0 0] slip/[0 0 1] slip transition may occur at ∼200 km depth in the upper mantle, and be responsible for the observed lattice preferred orientation (LPO) transition. A rheological law for polycrystalline forsterite is deduced from the single-crystal rheological laws, assuming that individual grains are randomly oriented in the aggregate. Applying the aggregate law within a 2D geodynamic model of upper-mantle couette flow suggests that the pressure dependence of olivine dislocation-slip activities may partly explain the low viscosity zone (LVZ) observed underneath oceanic plates.

Investigation of 3D water transport paths in gas diffusion layers by combined in-situ synchrotron X-ray radiography and tomography

The three-dimensional water distribution and water transport paths in the gas diffusion layer (GDL) and the adjacent micro-porous layer (MPL) of a polymer electrolyte membrane fuel cell (PEMFC) were analyzed during cell operation. The technique of quasi in-situ X-ray tomography was used for a 3D visualization of the water distribution and the structure of the GDL at different operating conditions. Based on findings from in-situ radiographic measurements water transport paths were detected and subsequently examined by tomography. The combination of these 2D and 3D techniques allows for a fully three-dimensionally resolved visualization of transport paths through the GDL.

Granular deformation mechanisms in semi-solid alloys

Deformation mechanisms in equiaxed, partially solid Al–15 wt.% Cu are studied in situ by coupling shear-cell experiments with synchrotron X-ray radiography. Direct evidence is presented for granular deformation mechanisms in both globular and equiaxed-dendritic samples at solid fractions shortly after crystal impingement. It is demonstrated that dilatancy, arching and jamming occur at the crystal scale, and that these can cause stick–slip flow due to periodic dilation and compaction at low displacement rate. Granular deformation is found to be similar in globular and equiaxed-dendritic samples if length is scaled by the crystal size and packing is considered to occur among crystal envelopes. Rheological differences between the morphologies are discussed in terms of the competition between crystal rearrangement and crystal deformation.

Analysis by synchrotron X-ray radiography of convection effects on the dynamic evolution of the solid–liquid interface and on solute distribution during the initial transient of solidification

In situ monitoring of the initial transient of directional solidification was carried out by means of synchrotron X-ray radiography. Experiments with Al–4 wt.% Cu alloy samples were performed on beamline ID19 of the European Synchrotron Radiation Facility (ESRF) in a dedicated Bridgman-type furnace. X-ray radiography enabled a detailed analysis of the evolution over time of the solid–liquid interface macroscopic shape in interaction with convection in the melt. Lateral solute segregation induced by fluid flow resulted in a significant deformation of the solid–liquid interface. The time-dependent velocity of the solidification front was determined at different abscissa values along the curved interface during the solidification process, from the growth phase with a smooth interface to the onset of morphological instability. Further, using a novel quantitative image analysis technique we were able to measure longitudinal solute profiles in the melt during the initial transient. Solutal length was then deduced as well as concentration in the melt, both at the interface and far away from it. The influence of convection on growth velocity and the characteristic parameters of the solute boundary layer are discussed, and a comparison with the Warren and Langer model is also presented.

In situ investigation of unidirectional solidification in Sn–0.7Cu and Sn–0.7Cu–0.06Ni

The dynamics of off-eutectic solidification are studied in two Pb-free solders using synchrotron X-ray radiography and post mortem scanning electron microscopy. The early stages of growth revealed the onset of coupled growth and phase competition. In Sn–0.7 wt.% Cu, the βSn–Cu 6Sn 5 eutectic then grew with Cu 6Sn 5 rods leading the βSn phase, with a range of rod spacings and local interface curvatures. However, these nonfaceted–faceted eutectic features were exhibited much more weakly than in the Al–Si and Fe–C eutectics, despite primary Cu 6Sn 5 crystals exhibiting marked faceting. In Sn–0.7 wt.% Cu–0.06 wt.% Ni, primary intermetallic laths grew ahead of a rod-like βSn–(Cu,Ni) 6Sn 5 univariant eutectic front. The univariant eutectic had a narrow freezing range and a similar spacing to the invariant eutectic at the same velocity. Composition measurements suggest that the univariant eutectic groove descends in the direction of the Sn + Cu 6Sn 5 binary eutectic point.

In situ X-ray observation of semi-solid deformation and failure in Al–Cu alloys

Semi-solid deformation has been directly observed in an Al–12 wt.% Cu alloy through the combination of real-time synchrotron X-ray radiography and a bespoke high-temperature tensile tester over a range of fraction solid from 0.35 to 0.98. During deformation at low and moderate fraction solids, the X-ray radiographs indicate that there is significant feeding of interdendritic liquid in the region of strain localization prior to crack formation. Furthermore, the measured load required to initiate localized tensile deformation was found to be similar over the range of fraction solid 0.35 to 0.66. At higher fraction solids, the radiographic observations are consistent with classical hot tearing behaviour: limited liquid flow due to low permeability; void nucleation and coalescence; and final failure. Based on these results, a three-stage mechanism for semi-solid failure is proposed which includes the effects of liquid flow and micro-neck formation.

In situ analysis of the influence of convection during the initial transient of planar solidification

We report on in situ study of the initial transient during planar solidification of an Al–4wt%Cu alloy by means of synchrotron X-ray radiography. Based on the recorded X-ray images, we first analysed in detail the time evolution of the macroscopic deformation of the solid-liquid interface. This deformation is due to the convective flow in the melt initiated by a residual transverse temperature gradient, and amplified by the solute rejection during solidification process. Additionally the growth rates were measured at three different positions along the deformed interface all along the solidification process. The experimental curves showed the increasing influence of convection as solidification proceeded. Longitudinal composition profiles in the melt were then determined during the solidification using a novel image analysis technique. The evolutions of solutal length, concentration at the interface and far away from the interface were then deduced. A comparison with Warren–Langer predictions revealed quantitative differences due to the convecto-diffusive transport in the melt.

Quantitative visualization of temporal water evolution in an operating polymer electrolyte fuel cell

Synchrotron X-ray radiography is employed to visualize the temporal evolution of water inside the gas diffusion layer (GDL) of an operating ( in situ) polymer electrolyte fuel cell (PEFC). A single-cell PEFC test kit is specially designed for the convenient capture of X-ray images. X-ray images of water in the PEFC components, such as the polymer membrane, GDL, and end plate, are captured consecutively. The synchrotron X-ray radiography of high-spatial and high-temporal resolution is suitable for observing the transport of a liquid layer and for visualizing water distribution inside the PEFC. As a result, the spatial distribution of water in the PEFC components is clearly and quantitatively visualized. The temporal evolution of water in the anode GDL due to back diffusion effect is clearly observed by adopting the image normalization method. The water-saturation characteristics at the cathode GDL, including saturation time and speed, are quite different from those at the anode GDL.

In-situ synchrotron X-ray radiography on high temperature polymer electrolyte fuel cells

In contrast to classical low temperature polymer electrolyte fuel cells (LT-PEFCs), the membrane conductivity in high temperature polymer electrolyte fuel cells (HT-PEFCs) (operating temperature ~ 160 °C) is based on proton transport within phosphorus-oxygen acids at different levels of hydration, orthophosphoric acid (H 3PO 4) being the simplest example. We present for the first time in-situ synchrotron X-ray radiography measurements applied to a HT-PEFC to gain insight into the local composition of the membrane electrode assembly (MEA) under dynamic operating conditions. Transmission changes during the radiographic measurements exhibit a clear influence of the formation of product water on the membrane composition.

Measurement of Solute Profiles by Means of Synchrotron X-Ray Radiography during Directional Solidification of Al-4 wt% Cu Alloys

In the present study, we report on an image analysis procedure, which enables to extract from synchrotron radiographs the long range solute profiles in the whole sample and in both phases (solid and liquid). This image analysis is based on the measurement of local density differences, and is applied to study the directional solidification of Al - 4wt% Cu alloy, from planar to onset of the initial instability. Dedicated experiments were carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). In order to validate this analysis the value of a key solidification parameter, namely the partition coefficient, was experimentally determined during the planar solidification, and a very good agreement was found with value found usually in the literature. On a further step, the evolution of the microstructure and solute profile during the initial transient of solidification was analysed in detail.

In situ Synchrotron X‐ray Radiography Investigations of Water Transport in PEM Fuel Cells

AbstractWater transport in an operating PEM fuel cell was investigated with synchrotron X‐ray radiography with a spatial resolution of 3 μm and a temporal resolution of 5 s. This method allows for the detection of water accumulations with less than 10 μm diameter. We demonstrate that synchrotron X‐ray imaging can dramatically expand the possibilities of imaging with high spatial and time resolution, especially as a complement to neutron radiography. Water transport processes from the first appearance of small water accumulations in the gas diffusion layer to their transport into the channel system were analysed in situ. Correlations between local effects such as water formation and operating conditions of the whole system, e.g. power variations, were found. A recently described eruptive water transport mechanism is analysed in detail.

In situ analysis of dendritic equiaxed microstructure formation in Al-Cu alloys by synchrotron X-ray radiography

This paper reports on experiments dedicated to equiaxed solidification carried out on Al — 10 wt% Cu alloy at the European Synchrotron Radiation Facility (ESRF) in Grenoble-France. Equiaxed growth was achieved in nearly isothermal conditions and observed continuously in real time from the early stages of solidification to the developed grain structure by X — ray radiography. The length of primary dendrite arms was measured on several growing equiaxed grains as a function of time and it was found that dendrite arm evolution can be well fitted by the Kolmogorov — Johnson — Mehl — Avrami (KJMA) function. The growth of four primary dendrite arms of a single grain was then characterized. The dendrite arms behaved differently depending on their growth directions and the presence of grains in their vicinities. At the beginning, the dendrite arm directed upwards was growing faster than the ones growing below because of “self — poisoning” due to a gravity driven fluid flow. In the late stage of solidification, analysis on several couples of grains growing towards each other confirmed that solutal interaction was the main cause of growth being stopped.

In siturheological measurements at extreme pressure and temperature using synchrotron X-ray diffraction and radiography

Dramatic technical progress seen over the past decade now allows the plastic properties of materials to be investigated under extreme pressure and temperature conditions. Coupling of high-pressure apparatuses with synchrotron radiation significantly improves the quantification of differential stress and specimen textures from X-ray diffraction data, as well as specimen strains and strain rates by radiography. This contribution briefly reviews the recent developments in the field and describes state-of-the-art extreme-pressure deformation devices and analytical techniques available today. The focus here is on apparatuses promoting deformation at pressures largely in excess of 3 GPa, namely the diamond anvil cell, the deformation-DIA apparatus and the rotational Drickamer apparatus, as well as on the methods used to carry out controlled deformation experiments while quantifying X-ray data in terms of materials rheological parameters. It is shown that these new techniques open the new field of in situ investigation of materials rheology at extreme conditions, which already finds multiple fundamental applications in the understanding of the dynamics of Earth-like planet interior.