Axel Lange

Quantitative structural assessment of heterogeneous catalysts by electron tomography.

We present transmission electron microscope (TEM) tomography investigations of ruthenium-based fuel cell catalyst materials as employed in direct methanol fuel cells (DMFC). The digital three-dimensional representation of the samples not only enables detailed studies on number, size, and shape but also on the local orientation of the ruthenium particles to their support and their freely accessible surface area. The shape analysis shows the ruthenium particles deviate significantly from spherical symmetry which increases their surface to volume ratio. The morphological studies help to understand the structure formation mechanisms during the fabrication as well as the high effectiveness of these catalysts in the oxygen reduction reaction at the cathode side of fuel cells.

Nondestructive evaluation of single fibre debonding in composites by X-ray refraction

Abstract Regarding the basic relevance of single fibre bonding for the mechanical properties of composites, relevant test methods are required. A new nondestructive method provides a fast and precise tool to determine the fraction of debonded fibres by refraction of X-rays at the interface of fibres in composites. The systematic analysis of thin walled unidirectional tapes of carbon fibres in thermoplastics results in specific characteristics of different lots. These were successfully exploited for the improvement of the processing parameters. Damage distributoon and flaw detection are imaged by two-dimensional scans of X-ray refraction topography.

Computertomographische Rekonstruktion mit DIRECTT

Kurzfassung Das neuartige Rekonstruktionsverfahren DIRECTT (Direkte iterative Rekonstruktion computertomographischer Trajektorien) verarbeitet die Objektprojektionen im Unterschied zur bekannten gefilterten Rückprojektion als selektiv gefilterte inverse Radontransformation in einer iterativen Prozedur. Damit kann die Ortsauflösung des Detektors weit übertroffen werden, weil die Limitierungen des Abtasttheorems durch Überabtastung umgangen werden. Zweidimensionale Modellrechnungen demonstrieren auch die Vorzüge für lokale (ROI-) Rekonstruktionen und fehlende Projektionen (Limited-View-Problem) im Vergleich zur gefilterten Rückprojektion.

Improved Computed Tomography by Variable Desmearing

Abstract In order to improve the spatial resolution of computed tomography reconstructions which suffer from un-sharpness due to an extended source size a new procedure of de-smearing has been developed. As the width of the source related smearing function of the projections may even double between the position near and far from the detector the desmearing procedure has to be variable. In contrast to the essentially wrong approach of de-convolution by a constant smear kernel for all voxel positions an implicit desmearing is established. The pre-known position dependent smearing is applied to the iteratively repeated projections of partial reconstructions obtained from the DIRECTT reconstruction algorithm. This approach of variable desmearing from sinograms of a pixel model with smearing from one to five pixel FWHM achieves a spatial resolution near one pixel.

Numerical correction of X-ray detector backlighting

Abstract 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 by 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). When used as input data for computed tomography these violations are directly observed via modulation of the projected mass as a function of the rotation phase and the samples aspect ratio (dynamics). The magnitude of the diffuse background signal depends on the detector area covered by the projected sample. They are more pronounced the smaller the shadowed area and the stronger the total attenuation. This implies that the reconstruction suffers from additional anisotropic artifacts caused by elongated sample structures. This issue is studied simply by absorption of flat plates in a conventional laboratory radiography set-up and at a synchrotron radiation facility. In the latter case beam hardening artifacts can be excluded due to the monochromatic radiation. The proposed correction procedure requires simple integral intensity offsets as a constant (non-local) light scattering mechanism is assumed.

Refraction computed tomography

Abstract For the first time metal matrix composites have been investigated by 3D computed tomography combined with enhanced interface contrast due to X-ray refraction. The related techniques of refraction topography and refraction computed tomography have been developed and applied during the last decade to meet the actual demand for improved non-destructive characterization of high performance composites, ceramics and other low-density materials and components. X-ray refraction is an optical effect that can be observed at small scattering angles of a few minutes of arc as the refractive index n of X-rays is nearly unity (n = 1 − 10−6). Due to the short X-ray wavelength, the technique determines the amount of inner surfaces and interfaces of nanometer dimensions. The technique can solve many problems in understanding micro and sub microstructures in materials science. Applying 3D refraction computed tomography, some questions could be clarified for a better understanding of fatigue failure mechanisms under cyclic loading conditions.

Wasserverteilung in PEM-Brennstoffzellen

Kurzfassung In-situ-Neutronenradiografie wird erfolgreich eingesetzt, um flüssiges Wasser in Niedertemperatur-PEM-Brennstoffzellen während des Betriebs zerstörungsfrei abzubilden und das sogenannte Wassermanagement — eine der größten Herausforderungen der Entwicklung — durch Anpassung der Materialeigenschaften zu optimieren. Dabei werden die besonderen Eigenschaften der Neutronen genutzt, die metallische Bauteile relativ leicht durchdringen, während selbst kleine Wassermengen durch ihre starke Streuung leicht nachweisbar sind. Die relativ langen Messzeiten für die Neutronentomografie werden durch den Betrieb der Zelle im „angehaltenen‟ Zustand realisierbar, sodass die dreidimensionale Wasserverteilung darstellbar wird.

Analyser-based tomography images of cartilage.

Analyser-based imaging expands the performance of X-ray imaging by utilizing not only the absorption properties of X-rays but also the refraction and scatter rejection (extinction) properties. In this study, analyser-based computed tomography has been implemented on imaging an articular cartilage sample, depicting substructural variations, without overlay, at a pixel resolution of 3.6 microm.

Charakterisierung von Katalysatormaterialien für Brennstoffzellen mittels Elektronentomografie

Kurzfassung Zur Optimierung moderner Katalysatoren für Brennstoffzellen werden diese elektronen-tomografisch charakterisiert. Die Elektronentomografie ermöglicht einzigartige Einblicke in die Nanometer-Strukturen der metallischen Katalysatorpartikel, die auf einem elektrisch leitenden, inerten Kohlenstoffträger abgeschieden werden. Die dreidimensional bildgebende Methode ermöglicht über qualitative Untersuchungen hinaus detaillierte quantitative Form- und Strukturanalysen der Katalysatormaterialien. So werden beispielsweise die Positionen der Katalysatorpartikel relativ zum Trägermaterial analysiert. Ihre Form und Einbettung in den Träger, welche die für die katalytische Reaktion maßgebliche „freie Oberfläche“ definieren, werden bestimmt. Die Elektronentomografie ermöglicht somit quantitative Vergleiche zwischen verschiedenen Katalysatormaterialien und Herstellungsverfahren. Sie erweitert die Möglichkeiten der Korrelation gewünschter elektrochemischer Eigenschaften mit der Nanostruktur dieser Materialien und macht so weitere Optimierungen der Katalysatormaterialien möglich.

Terahertz spectral computed tomography

The novel technique of spectral THz computed tomography is demonstrated at an example of a small plastic figure. The data are reconstructed by conventional filtered back-projection. The reconstruction are discussed.