Christian Tötzke

Visualization of embolism formation in the xylem of liana stems using neutron radiography

BACKGROUND AND AIMS Cold neutron radiography was applied to directly observe embolism in conduits of liana stems with the aim to evaluate the suitability of this method for studying embolism formation and repair. Potential advantages of this method are a principally non-invasive imaging approach with low energy dose compared with synchrotron X-ray radiation, a good spatial and temporal resolution, and the possibility to observe the entire volume of stem portions with a length of several centimetres at one time. METHODS Complete and cut stems of Adenia lobata, Aristolochia macrophylla and Parthenocissus tricuspidata were radiographed at the neutron imaging facility CONRAD at the Helmholtz-Zentrum Berlin für Materialien und Energie, with each measurement cycle lasting several hours. Low attenuation gas spaces were separated from the high attenuation (water-containing) plant tissue using image processing. KEY RESULTS Severe cuts into the stem were necessary to induce embolism. The formation and temporal course of an embolism event could then be successfully observed in individual conduits. It was found that complete emptying of a vessel with a diameter of 100 µm required a time interval of 4 min. Furthermore, dehydration of the whole stem section could be monitored via decreasing attenuation of the neutrons. CONCLUSIONS The results suggest that cold neutron radiography represents a useful tool for studying water relations in plant stems that has the potential to complement other non-invasive methods.

A dedicated compression device for high resolution X-ray tomography of compressed gas diffusion layers

We present an experimental approach to study the three-dimensional microstructure of gas diffusion layer (GDL) materials under realistic compression conditions. A dedicated compression device was designed that allows for synchrotron-tomographic investigation of circular samples under well-defined compression conditions. The tomographic data provide the experimental basis for stochastic modeling of nonwoven GDL materials. A plain compression tool is used to study the fiber courses in the material at different compression stages. Transport relevant geometrical parameters, such as porosity, pore size, and tortuosity distributions, are exemplarily evaluated for a GDL sample in the uncompressed state and for a compression of 30 vol.%. To mimic the geometry of the flow-field, we employed a compression punch with an integrated channel-rib-profile. It turned out that the GDL material is homogeneously compressed under the ribs, however, much less compressed underneath the channel. GDL fibers extend far into the channel volume where they might interfere with the convective gas transport and the removal of liquid water from the cell.

Capturing 3D Water Flow in Rooted Soil by Ultra-fast Neutron Tomography

Water infiltration in soil is not only affected by the inherent heterogeneities of soil, but even more by the interaction with plant roots and their water uptake. Neutron tomography is a unique non-invasive 3D tool to visualize plant root systems together with the soil water distribution in situ. So far, acquisition times in the range of hours have been the major limitation for imaging 3D water dynamics. Implementing an alternative acquisition procedure we boosted the speed of acquisition capturing an entire tomogram within 10 s. This allows, for the first time, tracking of a water front ascending in a rooted soil column upon infiltration of deuterated water time-resolved in 3D. Image quality and resolution could be sustained to a level allowing for capturing the root system in high detail. Good signal-to-noise ratio and contrast were the key to visualize dynamic changes in water content and to localize the root uptake. We demonstrated the ability of ultra-fast tomography to quantitatively image quick changes of water content in the rhizosphere and outlined the value of such imaging data for 3D water uptake modelling. The presented method paves the way for time-resolved studies of various 3D flow and transport phenomena in porous systems.

Echinoderms: Hierarchically Organized Light Weight Skeletons

Echinoderm skeletons are described within a hierarchical framework ranging from complete organisms to the ultrastructural level. They consist of numerous elements which can be isolated, connected by soft tissue or locked together in rigid structures. The top level considers skeletons as a whole with all associated elements and the basic symmetry of the echinoderms. The next level deals with the structural analysis and modeling of echinoids with respect to the growth parameters and stress resistance of the corona. The flexibility and movement has also been studied for the stalks and arms of recent and fossil crinoids. The next level deals with the elaborate morphology and types of symmetry found in single skeletal elements. The numerous types of stereom architectures found within the elements of all echinoderms are highly correlated to specific functions. A high number of recent studies concern the last hierarchical level on ultrastructure and biomineralization. Lightweight aspects of the skeleton are especially present at the level of conjoined plates, single elements and the stereom.

Investigation of Carbon Fiber Gas Diffusion Layers by Means of Synchrotron X-ray Tomography

The 3-dimensional spatial distribution of liquid water in different gas diffusion layer (GDL) materials was analyzed using synchrotron X-ray tomography. The capability of the method was demonstrated by virtually separating the GDL components in order to facilitate individual analysis of fiber material, liquid water and gas filled pore spaces. The influence of hydrophobic surface treatment on the water distribution in the GDL was illustrated by analyzing three GDL materials with different degrees of hydrophobicity. In the least hydrophobic sample, liquid water tends to form larger clusters which stretch out about several hundred µm inside the porous GDL. In contrast, only small water clusters were found in the strongly hydrophobic material with high Polytetrafluoroethylene (PTFE)content as the liquid is partially pressed out of the GDL. Additionally, the influence of fiber orientation on the water distribution in the felt material was demonstrated.

Hochortsauflösendes, großflächiges Neutronen-Detektorsystem für die Brennstoffzellenforschung

Kurzfassung Die speziellen Eigenschaften von Neutronen, insbesondere die hohe Sensitivität für Wasser sowie die Fähigkeit, viele metallische Komponenten zu durchdringen, macht die Neutronenradiografie zum attraktiven Werkzeug für die Brennstoffzellenforschung. Die detaillierte Untersuchung des Wasserhaushalts erfordert eine hohe Ortsauflösung des Detektors sowie ein flexibles und großes Abbildungsfeld. Bislang schränkte die geringe räumliche Ortsauflösung konventioneller Detektorsysteme die Detailtiefe bei der Wiedergabe von Wasserverteilungen in Zellen stark ein. Spezielle hochauflösende Detektorsysteme hingegen ermöglichen nur die Abbildung kleinster Teilbereiche der Zelle. Am Helmholtz-Zentrum Berlin wurde ein Detektorsystem entwickelt, das beide Grundforderungen erfüllt: Ein neuartiger Gadox-Szintillator ermöglicht eine deutlich verbesserte Ortsauflösung von max. 25 μm. Das verwendete Kamerasystem mit einem 4096×4097-Pixel-CCD-Sensor ermöglicht ein großes und flexibles Abbildungsfeld, z.B. 61,44×61,44 mm2 beim 1:1-Abbildungsmaßstab. Mit diesem System lassen sich Brennstoffzellen großflächig und mit hoher Auflösung untersuchen.

X-ray Compton line scan tomography*

Abstract The potentials of incoherent X-ray scattering (Compton) computed tomography (CT) are investigated. The imaging of materials of very different atomic number or density at once is generally a perpetual challenge for X-ray tomography or radiography. In a basic laboratory set-up for simultaneous perpendicular Compton scattering and direct beam attenuation tomography are conducted by single channel photon counting line scans. This results in asymmetric distortions of the projection profiles of the scattering CT data set. In a first approach, corrections of Compton scattering data by taking advantage of rotational symmetry yield tomograms without major geometric artefacts. A cylindrical sample composed of PE, PA, PVC, glass and wood demonstrates similar Compton contrast for all the substances, while the conventional absorption tomogram only reveals the two high order materials. Comparison to neutron tomography reveals astonishing similarities except for the glass component (without hydrogen). Therefore, Compton CT offers the potential to replace neutron tomography, which requires much more efforts.

Röntgentomografische Untersuchung eines kommerziellen Lithium-Ionen-Kondensators*

Kurzfassung Gegenstand dieser Studie ist ein kommerzieller Lithium-Ionen-Kondensator (LIC), welcher einer intensiven Alterung in Form von sukzessive durchgeführten Lade- und Entladeprozessen unterzogen wurde. Die dabei auftretenden dreidimensionalen Veränderungen der inneren Struktur wurden mittels Röntgen-Tomografie analysiert. Mit ansteigender Zyklenzahl finden wir eine Ansammlung von Elektrolytflüssigkeit im Bodenbereich des Kondensators vor. Unter Berücksichtigung der beim Zyklieren aufgezeichneten elektrischen Daten ergibt sich ein klarer Zusammenhang zwischen der Elektrolytverteilung im Bodenbereich und der Leistung des LIC.