Nikolay Kardjilov

Investigation of water evolution and transport in fuel cells with high resolution synchrotron x-ray radiography

The authors report on in situ investigations of liquid water evolution and transport in an undisturbed operating fuel cell at the microscopic level. Synchrotron x-ray radiography enhances the spatial resolution by two orders of magnitude compared to the state-of-the-art techniques in this field. The primary spots of liquid water formation, their growth, and transport inside the porous gas diffusion material were analyzed; correlations between operating conditions and the dynamics of droplet formation are described. Previous findings from modeling and simulation approaches are confirmed and the applicability for the description of in situ processes of a recently proposed model has been proven.

Cross-sectional insight in the water evolution and transport in polymer electrolyte fuel cells

The evolution of liquid water and its transport through the porous gas diffusion media in an operating fuel cell were investigated applying an experimental setup for high spatial resolution of 3μm. Fundamental aspects of cluster formation in hydrophobic/hydrophilic porous materials as well as processes of multiphase flow are addressed. The obtained water distributions provide a detailed insight in the membrane electrode assembly and the porous electrode with regard on the existence and transport of liquid water. In addition, the results approve transport theories used within the framework of percolation theory and demonstrate the need for adapted modeling approaches.

Advances in neutron radiography and tomography

Neutron imaging can provide two- or three-dimensional, spatially resolved images of the internal structure of bulk samples that are not accessible by other techniques, making it a unique tool with many potential applications. The method is now well established and is available at neutron sources worldwide. This review will give a survey of the technique of neutron imaging with a special focus on neutron tomography; the basics of the method as well as the technology of instrumentation will be outlined, and the techniques will be illustrated by representative applications. While the first part of the paper focuses on conventional attenuation contrast imaging, the second part reviews and critically assesses recent methodical developments.

Quasi–in situ neutron tomography on polymer electrolyte membrane fuel cell stacks

Quasi–in situ neutron tomography is applied to polymer electrolyte membrane fuel cell stacks for a cell-by-cell detection of liquid water agglomerates. Water distributions in the corresponding anodic and cathodic flow fields are analyzed separately. The influence of the membrane thickness as well as effects of the electro-osmotic drag and of back-diffusion from the cathode to the anode on the water distribution are investigated. Furthermore, the well-known engineering problem of the anomalous behavior of the outermost cells in long multistacks is addressed. The suitability of neutron tomography to support the development of fuel cells is shown.

Neutron imaging in materials science

Neutron imaging is a non-destructive technique that can reveal the interior of many materials and engineering components and also probe magnetic fields. Within the past few years, several new imaging modes have been introduced that extend the scope of neutron imaging beyond conventional neutron attenuation imaging, yielding both 2- and 3D information about properties and phenomena inaccessible until now. We present an overview of the most important advances in the application of neutron imaging in materials research with a focus on novel techniques such as energy-selective imaging, interferometric imaging with phase gratings, and polarized-neutron imaging. Examples given include the investigation of fluid dynamics in fuel cells, materials phases and structural heterogeneities, distribution of strains, and magnetic structures or phase transitions.

Wavelength tunable device for neutron radiography and tomography

A special double monochromator system was tested for a conventional operating tomography setup in order to use a broad wavelength band of monochromatic neutrons for radiography and tomography. Scanning through the wavelength region of Bragg edges, it is possible to make series of radiographs and tomographs at different wavelengths from 2.0 until 6.5A. So no beam hardening influences the measurements and is not to be corrected. With this instrument for cold neutron radiography and tomography, energy selecting quantitative radiography, stress and strain mapping, and phase radiography were performed.

Detection system for microimaging with neutrons

A new high-resolution detector setup for neutron imaging has been developed based on infinity-corrected optics with high light collection, combined with customized mounting hard- ware. The system can easily be installed, handled and fitted to any existing facility, avoiding the necessity of complex optical systems or further improved electronics (CCD). This is the first time optical magnification higher than 1:1 has been used with scintillator-based neutron detectors, as well as the first implementation of infinity corrected optics for neutron imaging, achieving the smallest yet reported effective pixel size of 3.375 mm. A novel transparent crystal scintillator (GGG crystal) has been implemented with neutrons for the first time to overcome limitations of traditional powder scintillators (Li6/ZnS, Gadox). The standardized procedure for resolution mea- surements with the Modulation Transfer Function (MTF) is summarized to facilitate comparison between instruments and facilities. Using this new detector setup, a resolution of 14.8 mm with a field of view of 6 mm�6 mm has been achieved while maintaining reasonable count times. These advances open a wide range of new possible research applications and allow the potential for additional future developments.

In situ investigation of the discharge of alkaline Zn–MnO2 batteries with synchrotron x-ray and neutron tomographies

Zn–MnO2 alkaline batteries were investigated in situ at different stages of electric discharge by synchrotron tomography with monochromatic x rays and by neutron tomography. The spatial distribution and the changes in the morphology of different components of a battery caused by the reduction of MnO2, the dissolution of Zn, and the nucleation and growth of ZnO are investigated with high spatial resolution around several micrometers with x rays. Neutron tomography is used to monitor the changes in the spatial distribution of hydrogen in the MnO2 matrix and provides complementary information about the process.

Characterization of water exchange and two-phase flow in porous gas diffusion materials by hydrogen-deuterium contrast neutron radiography

Liquid water exchange in two-phase flows within hydrophobic porous gas diffusion materials of polymer electrolyte membrane fuel cells was investigated spatially resolved with H–D contrast neutron radiography. A commonly used one-phase model is sufficient to describe water exchange characteristics at low water production rates. At higher rates, however, a significantly higher exchange velocity is found than predicted by a simple model. A new model for the water transport is derived based on an eruptive mechanism guided by Haines jumps, which is supported by recent experimental findings and leads to a very good agreement with the experiments.

X-ray and neutron imaging – Complementary techniques for materials science and engineering

Abstract Using X-ray and neutron radiography and tomography, images of material and component inhomogeneities and their development with time can be obtained. Due to their non-destructiveness and non-invasive nature both methods give insight into the function of devices and their decay processes. Fundamentals of X-ray and neutron radiography and tomography are briefly outlined, examples for both techniques are given, their complementarities are highlighted and emerging techniques and frontiers are discussed.