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Organisationseinheit der BAM
Eingeladener Vortrag
- nein (34)
NDE of micro structured materials by X-ray diffraction and refraction topography, Keynote Lecture
(2007)
Erstmals wurden MMCs mit Hilfe der Interface-Kontrast verstärkenden Röntgen-Refraktions-Technik in Kombination mit der 3D-CT-Technik untersucht. Die Methode der Röntgen-Refraktions-Topography sowie der Röntgen-Refraktions-Tomography wurden in den letzten Jahren speziell für die zerstörungsfreie Charakterisierung von Leichtbau-Hochleistungs-Werkstoffen wie Verbund-Werkstoffen und Keramiken entwickelt. Die Technik basiert auf der Refraktion von Röntgenlicht an Grenzflächen zwischen Bereichen unterschiedlicher Elektronendichte wie z.B. Faser- und Matrix-Werkstoff. Auf Grund der kurzen Röntgen-Wellenlänge hat der Brechungsindex einen Wert dicht bei eins, sodass der Streuwinkel nur wenige Bogenminuten beträgt. Der Vorteil der kurzen Wellenlänge besteht jedoch darin, dass Poren und Risse detektiert werden können, deren Größe im Nanometerbereich liegen. Damit kann die Refraktions-Technik zum besseren Verständnis von Mikro- und Submikro-Strukturen in der Materialwissenschaft eingesetzt werden. Mit der hier vorgestellten Refraktions-CT wurden Ermüdungsmechanismen an MMCs, bestehend aus SiC-Faser verstärktem Titan (Ti6242) untersucht. Die Messungen wurden am Röntgen-Messplatz der BAM (BAMline) am Synchrotron-Speicherring-BESSY in Berlin Adlershof durchgeführt.
X-Ray Refraction Techniques
(2005)
Enhanced Spatial Resolution in 2D CT-Reconstruction without Filtered Back Projection: DIRECTT
(2008)
Microstructural aspects of composites such as reinforcement particle size, shape, and distribution play important roles in deformation behavior. In addition, Fe-rich inclusions and porosity also influence the behavior of these composites, particularly under fatigue loading. Three-dimensional (3-D) visualization of porosity and Fe-rich inclusions in three dimensions is critical to a thorough understanding of fatigue resistance of metal matrix composites (MMCs), because cracks often initiate at these defects. In this article, we have used X-ray synchrotron tomography to visualize and quantify the morphology and size distribution of pores and Fe-rich inclusions in a SiC particle-reinforced 2080 Al alloy composite. The 3-D data sets were also used to predict and understand the influence of defects on the deformation behavior by 3-D finite element modeling.
Synchrotron-Refraktions-Computer-Tomographie - EIne neue Methode zur Erkennung von Grenzflächen
(2009)
Rekonstruktion unvollständiger computertomografischer Modelldatensätze mit Directt-Algorithmus
(2010)
Modellrechnungen zur Rekonstruktion unvollständiger computertomographischer Datensätze werden zur Ermittlung der Vorteile des neuen Directt-Algorithmus gegenüber der konventionellen gefilterten Rückprojektion durchgeführt, insbesondere auch zur Vermeidung experimentell begründeter Unwägbarkeiten. Ein entsprechender Bedarf entsteht seitens erweiterter Forschungsaktivitäten und industrieller Bedürfnisse, wo die Rekonstruktion limitierter Messdatensätze von Artefakten dominiert wird. Die bekanntesten Einschränkungen betreffen das Limited-View- und Few-Angles-Problem sowie die ROI-Situation, die jeweils unabhängig voneinander untersucht werden, in der Anwendung jedoch zumeist undefiniert vermischt sind, wie etwa in der Elektronen-Tomografie. Die hochauflösende Rekonstruktion von Einzelpixeln wird durch die Wahl eines kleinen Modells von nur 1012 Pixeln visualisiert und ist ohne Filterung möglich. Rekonstruktionen von 90°-Sektoren oder 50 % -ROI-Querschnitten sind fast ohne Auflösungseinschränkungen realisierbar. Die bisher nicht bekannte Möglichkeit, auch außerhalb der ROI präzise zu rekonstruieren, wurde als "Extended ROI"-Methode erfolgreich erprobt.
Die Aufklärung spezifischer Fragestellungen zur Funktionalität von PEM(polymer electrolyte membrane)- und DMFC(Direct Methanol)-Brennstoffzellen bedarf der 3D-Abbildung der spezifischen Komponenten mit unterschiedlichen Strahlungsarten: des Wassergehalts in den Flow Fields mit Neutronen, der Katalysator-Alterung in der MEA (membrane electrode assembly) mit Synchrotronstrahlung und der freien (reaktiven) Katalysator-Oberfläche von Nano-Partikeln mit TEM-Elektronen-Tomografie. Diese Messungen im Bereich der mm- bis nm-Skala unterliegen dabei erheblichen Restriktionen: einer ortsabhängigen Verschmierung der Neutronenradiogramme, Manipulatorinstabilitäten der Synchrotronmessungen sowie einer Kombination von Limited-View-Datensätzen, wenigen Projektionswinkeln und teilweiser Nichtdurchstrahlbarkeit in der Elektronen-CT. Die Vorteile eines fortentwickelten Directt-Algorithmus für die Bewältigung dieser Randbedingungen gegenüber gefilterten Rückprojektionen werden aufgezeigt.
The presence of elongation, streak and blurring artifacts in tomograms recorded under a missing wedge of rotation angles presents a major challenge for the quantitative analysis of tomographic image data. We show that the missing wedge artifacts of standard reconstruction algorithms may be reduced by the innovative reconstruction technique DIRECTT. For the comparison of missing wedge artifacts we apply techniques from spatial statistics, which have been specifically designed to investigate the shape of phase boundaries in tomograms.
In order to demonstrate the challenging tasks in current R&D applications in the field of renewable energy
sources experimental neutron and electron tomographic data sets of PEM fuel cell components are processed by
an advanced version of the DIRECTT algorithm. The neutron measurements of the water distribution suffer from
several intrinsic limitations such as massive position dependent focal smearing due to the large primary neutron
beam aperture. Electron (TEM) tomography allows characterisation of nanometre sized catalyst particles but is
hampered by the following restrictions: partial opacity, a limited sector of projections (missing wedge), very few
projections, samples exceeding the detector size (region of interest) and improper angular alignment. Beyond the
capabilities of other algorithms DIRECTT proves to overcome these essential reconstruction problems, in
comparison to FBP and SIRT. Nevertheless, careful data pre-processing is an inevitable requirement in order to
fully exploit the algorithm's potentials.
Incomplete tomographic data sets such as limited view (missing wedge) data represent a well-known challenge
for reconstruction algorithms, since they unavoidably lead to substantial image artefacts. Such data sets may
occur in industrial computed tomography of limited access (e.g. extended components, fixed objects), directional
opacity, limited sample life time or laminographic set-up. We present strategies to effectively suppress the
typical elongation artefacts (e.g. lemon-like deformed pores) by our iterative algorithm DIRECTT which offers
the opportunity to vary the versatile reconstruction parameters within each cycle. Those strategies are applied to
experimental data obtained from metallic foams as well as model simulations. Comparison is drawn to state-ofthe-
art techniques (filtered backprojection and algebraic techniques). Further reference is made to reconstructions
of complete data sets serving as gold standards. For quantitative assessment of the reconstruction
quality adapted techniques based on spatial statistics are introduced.
A novel approach to strongly suppress artifacts in radiography and computed tomography caused by the effect of
diffuse background signals ('backlighting') of 2D X-ray detectors is suggested. Depending on the detector
geometry the mechanism may be different, either based on the optical scattering of the fluorescent screen
materials into optical detection devices or Compton or X-ray fluorescence scattering by the detector components.
Consequently, these erroneous intensity portions result in locally different violations of Lambert Beers law in
single projections (radiographs) as function of the detector area coverage and the magnitude of the attenuation.
The absorption of multiple metal sheets is investigated by monochromatic synchrotron radiation, thus excluding
beam hardening. The proposed correction procedure simply requires the individual subtraction of one and the
same fraction of the primary and transmitted mean intensity, as a constant (non-local) scattering mechanism is
assumed.
The relevance of nano sized structures and interfaces in upcoming technologies requires new non-destructive Xray
techniques exploiting attenuation as well as scattering. In the course of driving the resolution and the contrast
of modern radiography into micro and nano metre dimension the inevitable scattering effects require increased
attention. Several radiologists interpret the observed edge artefacts simply by 'phase contrast' due to different
X-ray path lengths within the sample and others refer to Fresnel scattering. The ultra small angle scattering is of
high intensity and may reach the level of the primary beam. We present monochromatic synchrotron
measurements on rather simply shaped objects (such as plates and cylinders) which prove clearly the dominance
of refraction intensity over total reflection and coherent small angle scattering. The refraction conditions hold for
angles of incidence of several degrees to the surface, but typical deflection angles range from several seconds to
minutes of arc. Additionally to the concept of 'phase contrast' of the primary beam its deflection becomes the
essential issue. The appropriate removal of the attenuation contribution enables the imaging of inner surfaces and
interfaces by refraction scanning topography. The presented measurements of reference particles demonstrate the
reliability of laboratory refraction scanners applying X-ray tubes.
Evaluating porosity in cordierite diesel particulate filter materials, part 1 X-ray refraction
(2013)
Bi-continuous porous ceramics for filtration applications possess a particularly complicated microstructure, with porosity and solid matter being intermingled. Mechanical, thermal, and filtration properties can only be precisely estimated if the morphology of both solid matter and porosity can be quantitatively determined. Using x-ray absorption and refraction, we quantitatively evaluate porosity and pore orientation in cordierite diesel particulate filter ceramics. Porosity values turn out to agree with mercury intrusion measurements, while pore orientation factors agree with published crystallographic texture data.
We present a synchrotron X-ray tomographic study on the morphology of carbon fiber-based gas diffusion layer (GDL) material under compression. A dedicated compression device is used to provide well-defined compression conditions. A flat compression punch is employed to study the fiber geometry at different degrees of compression. Transport relevant geometrical parameters such as porosity, pore size and tortuosity distributions are calculated. The geometric properties notably change upon compression which has direct impact on transport conditions for gas and fluid flow. The availability of broad 3D paths, which are most important for the transport of liquid water from the catalyst layer through the GDL, is markedly reduced after compression. In a second experiment, we study the influence of the channel-land-pattern of the flow-field on shape and microstructure of the GDL. A flow-field compression punch is employed to reproduce the inhomogeneous compression conditions found during fuel cell assembly. While homogenously compressed underneath the land the GDL is much less and inhomogeneously compressed under the channel. The GDL material extends far into the channel volume where it can considerably influence gas and fluid flow. Loose fiber endings penetrate deeply into the channel and form obstacles for the discharge of liquid water droplets.
Complementary to Part 1 of this work, the bi-continuous microstructure of porous synthetic cordierite ceramics for filtration applications was investigated using 3D x-ray computed tomography at different resolutions. Applying both Fast Fourier Transform and a newly developed image analysis algorithm, we quantitatively evaluated porosity and pore orientation. The statistical approach allows extraction of spatially resolved or average values. Porosity values based on x-ray absorption agree with mercury intrusion measurements, while pore orientation factors agree with x-ray refraction data (Part 1 of this work), and with published crystallographic texture data.
Typische 'Missing Wedge'(MW)-Artefakte in Rekonstruktionen der Computertomografie können mit dem iterativen DIRECTT-Algorithmus (Direkte iterative Rekonstruktion computertomografischer Trajektorien) unterdrückt werden. Die durch fehlende Projektionswinkel bedingten MW-Restriktionen betreffen bisher nur Sonderapplikationen der Röntgen- und Neutronentomografie, müssen in der Nanometer auflösenden Elektronentomografie jedoch als Standard akzeptiert werden. Die MW-Artefakte werden besonders effektiv mit einer spezifischen DIRECTT-Prozedur partiell diskreter Zwischenlösungen beseitigt. Für die Beurteilung der Abbildungsqualität werden Rekonstruktionen von vollständigen und unvollständigen Mess- und Modell-Datensätzen erzeugt und mit Ergebnissen anderer Algorithmen (gefilterte Rückprojektion (FBP) und ART-Methoden) verglichen. Die Fourier-Transformierten der Rekonstruktionen demonstrieren die erfolgreiche Ergänzung der fehlenden Information im MW-Bereich. Die quantitative Bewertung der lokalen Rekonstruktionsqualität mit Methoden der räumlichen Statistik ergibt auch für unterschiedliche Signal-Rausch-Verhältnisse signifikante Vorteile gegenüber FBP und ART.