Manfred P. Hentschel

Polypropylene-graft-maleic anhydride-nanocomposites: I—Characterization and thermal stability of nanocomposites produced under nitrogen and in air

Abstract The morphology and thermal behaviour of polypropylene–graft–maleic anhydride (PP–g–MA) layered silicate (montmorillonite) nanocomposites were investigated using X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and thermogravimetry. The study focuses on the influence of the presence of oxygen during the preparation of PP–g–MA–nanocomposite using two different modified clays. The nanocomposites show tactoid, intercalated and exfoliated structures side by side with different dominant states depending on the clay used and on the processing conditions. The systems are described as multi-component blends rather than binary blends since the organic ions do not only change the mixing behaviour, but also influence material properties. Beside the physical barrier property of the clay layers also chemical processes were found to play an important role.

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.

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.

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.

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.