Marco Stampanoni

Determination of Liquid Water Distribution in Porous Transport Layers

Porous transport layers (PTL), also termed gas diffusion layers (GDL) play an important role for the mass transport on the milliscopic scale and its related losses, especially at high current densities and under condensing conditions. Understanding the mass transport properties of the PTL is therefore essential to understand and improve the water management as well as power density in the high current domain. PTL’s are structured to fulfill a multitude of requirements, such as to allow access of reactants to the catalyst layer and removal of gaseous and liquid product water. Therefore it is highly porous and hydrophobized with polytetrafluorethylene (PTFE). Furthermore electrons need be to collected or distributed at the CLs. PTL’s need therefore to be based on a conducting substrate which is chemically inert (e.g. carbon fibers). Furthermore the structure of the GDL needs to mechanically withstand the clamping pressure of the cell/stack which is necessary to reduce contact resistance and avoid leakages. Assuming no transport bottleneck in the CL, the strongest limitation comes from the flooding of PTL pores with liquid water and a subsequent shortage of oxygen in the cathode CL. Liquid water and transport thereof in the PTL has therefore been widely investigated [1-3]. However to date no systematic data of the structure of water in the PTL with the required micrometer resolution has been available. In this work, x-ray micro tomography at the Tomcat Beamline of the Swiss Light Source has been employed.

A Forward Regridding Method With Minimal Oversampling for Accurate and Efficient Iterative Tomographic Algorithms

Reconstruction of underconstrained tomographic data sets remains a major challenge. Standard analytical techniques frequently lead to unsatisfactory results due to insufficient information. Several iterative algorithms, which can easily integrate a priori knowledge, have been developed to tackle this problem during the last few decades. Most of these iterative algorithms are based on an implementation of the Radon transform that acts as forward projector. This operator and its adjoint, the backprojector, are typically called few times per iteration and represent the computational bottleneck of the reconstruction process. Here, we present a Fourier-based forward projector, founded on the regridding method with minimal oversampling. We show that this implementation of the Radon transform significantly outperforms in efficiency other state-of-the-art operators with O(N2log2N) complexity. Despite its reduced computational cost, this regridding method provides comparable accuracy to more sophisticated projectors and can, therefore, be exploited in iterative algorithms to substantially decrease the time required for the reconstruction of underconstrained tomographic data sets without loss in the quality of the results.

Combining Monte Carlo methods with coherent wave optics for the simulation of phase-sensitive X-ray imaging

A framework combining wave-optics with Monte Carlo methods for numerical simulations of phase-sensitive X-ray imaging has been developed.

Sensitivity in X-ray grating interferometry on compact systems

The optimization of compact X-ray grating interferometry systems is crucial for the progress of this technique in industrial devices. Here, an analytical formulation for the sensitivity of the phase contrast image acquisition is derived using previous results from noise analyses. Furthermore, experimental measurements of the sensitivity for different configurations are compared, providing further insight into the dependence on polychromatic radiation. Finally, strategies for the geometrical optimization are given.

Determination of Local GDL Saturation on the Pore Level by In Situ Synchrotron Based X-ray Tomographic Microscopy

In-situ synchrotron-based tomographic microscopy (SRXTM) with a spatial resolution in the order of 1μm and sensitivity for carbon and liquid water, has the potential to provide fundamental information for the understanding of the wetting properties of gas diffusion layer (GDL) materials on the pore level. This is important for the understanding of the solid-water interactions in the porous structures since water transport in GDLs is considered a key transport mechanism polymer electrolyte fuel cells (PEFC). However SRXTM of PEFC is a major experimental challenge. To obtain quantitative results, a complete cell needs to be operated under realistic conditions in the constrained space of the small field of view on the beamline sample stage without disturbing the sample rotation.

Imaging samples larger than the field of view: the SLS experience

Volumetric datasets with micrometer spatial and sub-second temporal resolutions are nowadays routinely acquired using synchrotron X-ray tomographic microscopy (SRXTM). Although SRXTM technology allows the examination of multiple samples with short scan times, many specimens are larger than the field-of-view (FOV) provided by the detector. The extension of the FOV in the direction perpendicular to the rotation axis remains non-trivial. We present a method that can efficiently increase the FOV merging volumetric datasets obtained by region-of interest tomographies in different 3D positions of the sample with a minimal amount of artefacts and with the ability to handle large amounts of data. The method has been successfully applied for the three-dimensional imaging of a small number of mouse lung acini of intact animals, where pixel sizes down to the micrometer range and short exposure times are required.

A systematic error in X-ray grating interferometry due to asymmetric scattering distributions

The high sensitivity provided by x-ray grating interferometry is one of the distinct characteristics of this phase contrast imaging technique. Up to now, several contributions to the uncertainty of the determined phase signal are known, which can be addressed by adapting total acquisition time or improving the mechanics of the experimental setup. Here, we demonstrate the occurrence of an additional systematic error, which is intrinsic to the physics of the contrast formation process. Based on a recently established scattering-based description of grating interferometry, we demonstrate analytically that asymmetric scattering distributions lead to a systematic error in the determined phase signal.

Phase-contrast Enhanced Mammography: A New Diagnostic Tool for Breast Imaging

Phase contrast and scattering-based X-ray imaging can potentially revolutionize the radiological approach to breast imaging by providing additional and complementary information to conventional, absorption-based methods. We investigated native, non-fixed whole breast samples using a grating interferometer with an X-ray tube-based configuration. Our approach simultaneously recorded absorption, differential phase contrast and small-angle scattering signals. The results show that this novel technique - combined with a dedicated image fusion algorithm - has the potential to deliver enhanced breast imaging with complementary information for an improved diagnostic process.

Electrochemically Controlled Corrosion Initiation and Propagation in AlMgSi alloys In-situ Monitored Using X-ray Microtomography

An X-ray microtomography monitored, electrochemically controlled corrosion experiment with high spatial resolution was performed. A modified electrochemical microcell setup was mounted on the sample holder of the synchrotron beamline. This setup enabled in-situ corrosion tomography at a small-defined volume combined with well-defined electrochemical control. Hence high spatial submicrometer resolution and early detection of single corrosion events were possible. Three heat treatments are investigated. I) 45min at 180°C showing high intergranular corrosion susceptibility. Preferential initiation at intermetallics lying on grain boundaries was found with subsequent penetration into the material. Correlation of intermetallics and propagation path could not be found. II) 16h at 180°C showing susceptibility to pitting. An exfoliation-like attack was found which attacks straight into the bulk with a width of approximate 2 μm. III) 111h at 350°C showing a high amount of large MgSi phase. intermetallics and MgSi are not initiation sites for corrosion propagation even though dissolution can occur.

Effective segmentation of fresh post-mortem murine lung parenchyma in phase contrast X-ray tomographic microscopy images

The acinus represents the functional unit of the mammalian lung. It is defined as the small tree of gas-exchanging airways, which is fed by the most distal purely conducting airway. Different hypotheses exist on how the fine structure of the acinus changes during ventilation and development. Since in classical 2-dimensional (2D) sections of the lung the borders of the acini are not detectable, every study of acini requires 3-dimensional (3D) datasets. As a basis for further studies of pulmonary acini we imaged rodent lungs as close to life as possible using phase contrast synchrotron radiation-based X-ray tomographic microscopy (SRXTM), and developed a protocol for the segmentation of the alveolar septa. The method is based on a combined multilevel filtering approach. Seeds are automatically defined for separate regions of tissue and airspace during each 2D filtering level and then given as input to a 3D random walk segmentation. Thus, the different types of artifacts present in the images are treated separately, taking into account the samples structural complexity. The proposed procedure yields high quality 3D segmentations of acinar microstructure that can be used for a reliable morphological analysis.