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Morphological and topological quantification of complex pore networks is of great relevance for environmental engineering, earth science and industry. Recent developments of 3D imaging techniques such as X-ray microtomography or X-ray microscopy provide an opportunity to perform a comprehensive analysis of the pore network topology. Such an analysis is crucial to understand how transport or mechanical properties evolve during the growth and/or the aging of a pore network, especially near a percolation threshold. In the first part of this work, we present some properties related to the graph of retraction of a 3D pore network, a powerful way to characterize the topological evolution. In the second part, we analyze the topology of an evolving 3D pore network in the vicinity of a percolation transition. Two distinct scenarii of evolution are presented. The last part is dedicated to an experimental example of evolving pore network: the setting of an ordinary cement paste probed in its early age by synchrotron X-ray microcomputerized tomography.
Synchrotron x-ray μ-tomography has been used to reconstruct the three-dimensional view of a rough surface extracted from a heterogeneous ceramic coating composed of Pr2NiO4+δ. Radiographs with a resolution of 0.7 μm have been recorded at T = 300, 600, and 900 K. The analysis of surface geometry makes use of the geometrical optic approximation up to T = 900 K possible. Subsequently, a large number of rays (105) are impinged onto the numerical surface, as revealed by x-ray tomography, to reproduce the normal emissivity of the coating. This normal emissivity was obtained beforehand by infrared emittance spectroscopy at T = 1000 K. Comparison of the two approaches suggests that the optical contribution of the coating micropores can be integrated into the ray tracing code. The effective medium approximation is used for this purpose. Finally, the applicability of this hybrid approach is discussed.
The formation of magnetic domains in thin epitaxial Co/Au(111) films is investigated by spin-polarized scanning electron microscopy. Three-monolayer films are shown to decay into out-of-plane domains of micrometer size. The transition from out-of-plane to in-plane magnetization at a crossover thickness of 4.5 layers is followed by imaging the domains, and the transition is shown to occur as a continuous rotation of the magnetization. The domain size in field-free-grown perpendicular films depends linearly on film thickness. From high-resolution line scans across magnetization reversals we determine the resolution in magnetic imaging to be better than 40 nm.
The optically induced spin polarization P of the photoelectrons emitted from tin and germanium during ps and ns laser pulses is measured as a function of the pulse energy. P is defined by the lattice symmetry of the sample. It vanishes for amorphous or molten surfaces. The melting of the metal and the semiconductor is found to occur on a time scale which is short compared to the duration of a 70 ps laser pulse. The emission of positive ions starts at the laser intensity for which the drop of the polarization is observed. If Ge is heated with ns laser pulses a temperature dependent shift of the work function causes a decrease of the spin polarization for laser energies far below the energy necessary for melting. With ps laser pulses the polarization only vanishes for pulse energies exceeding Eion- The difference between the ps and ns measurements on Ge points to an unexpected dynamical behavior of the work function shift in semiconductors.
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