The Bach equation and the equation of geometrodynamics are based on two quite different physical motivations, but in both approaches, the conformal properties of gravitation plays the key role. In this paper we present an analysis of the relation between these two equations and show that the solutions of the equation of geometrodynamics are of a more general nature. We show the following non-trivial result: there exists a conformally invariant Lagrangian, whose field equation generalizes the Bach equation and has as solutions those Ricci tensors which are solutions to the equation of geometrodynamics.
We combine sensitivity to atomic number, chemical shifts, probing depth, and magnetic order in a field- dependent magnetic circular X-ray dichroism study at the Mn L-edge of the diluted ferromagnetic semiconductor Ga1-xMnxAs and observe different Mn constituents: ferromagnetic Mn with an n(d) > 5 lineshape and paramagnetic Mn with distinct n(d) = 5 lineshape. The paramagnetic Mn is assigned to interstitials with surface segregation tendency. (c) 2005 Elsevier B.V. All rights reserved
Carbon fiber based felt materials are widely used as gas diffusion layer (GDL) in fuel cells. Their transport properties can be adjusted by adding hydrophobic agents such as polytetrafluoroethylene (PTFE). We present a synchrotron X-ray tomographic study on the felt material Freudenberg H2315 with different PIPE finishing. In this study, we analyze changes in microstructure and shape of GDLs at increasing degree of compression which are related to their specific PTFE load. A dedicated compression device mimicking the channel-land pattern of the flowfield is used to reproduce the inhomogeneous compression found in a fuel cell. Transport relevant geometrical parameters such as porosity, pore size distribution and geometric tortuosity are calculated and consequences for media transport discussed. PTFE finishing results in a marked change of shape of compressed GDLs: surface is smoothed and the invasion of GDL fibers into the flow field channel strongly mitigated. Furthermore, the PTFE impacts the microstructure of the compressed GDL. The number of available wide transport paths is significantly increased as compared to the untreated material. These changes improve the transport capacity liquid water through the GDL and promote the discharge of liquid water droplets from the cell. (C) 2016 Elsevier B.V. All rights reserved.
We report observations of dusty clouds in Saturn's rings, which we interpret as resulting from impacts onto the rings that occurred between 1 and 50 hours before the clouds were observed. The largest of these clouds was observed twice; its brightness and cant angle evolved in a manner consistent with this hypothesis. Several arguments suggest that these clouds cannot be due to the primary impact of one solid meteoroid onto the rings, but rather are due to the impact of a compact stream of Saturn-orbiting material derived from previous breakup of a meteoroid. The responsible interplanetary meteoroids were initially between 1 centimeter and several meters in size, and their influx rate is consistent with the sparse prior knowledge of smaller meteoroids in the outer solar system.
Synchrotron based combined in situ x-ray diffractometry and reflectometry is used to investigate the role of vacancies for the relaxation of residual stress in thin metallic Pt films. From the experimentally determined relative changes of the lattice parameter a and of the film thickness L the modification of vacancy concentration and residual strain was derived as a function of annealing time at 130 degrees C. The results indicate that relaxation of strain resulting from compressive stress is accompanied by the creation of vacancies at the free film surface. This proves experimentally the postulated dominant role of vacancies for stress relaxation in thin metal films close to room temperature.
In silicate glasses and melts, water acts according to two main processes. First, it can be dissolved in high temperature/high pressure melts. Second, it constitutes a weathering agent on the glass surface. A number of in-situ x- ray absorption fine structure (XAFS) studies for Fe, Ni, Zr, Th and U show that the more charged cations (Zr, Nb, Mo, Ta, Sn, Th and U) are little affected by the presence of dissolved water in the melt. In contrast, divalent iron and nickel are highly sensitive to the presence of water, which enhance nucleation processes, for example, of phyllosilicates at the angstrom-scale. Such information provides additional constraints on the role of water deep in the Earth, particularly in magmatology. By contrast, the weathering of glass surfaces by water can be studied from a durability perspective. Experimental weathering experiments Of nuclear waste glasses performed in the laboratory show a variety of surface enrichments (carbon, chlorine, alkalis, iron) after exposure to atmospheric fluids and moisture. Mn-, and Fe-surface enrichments of analogous glasses of the XIVth century are related to the formation of Mn and Fe oxy/ hydroxides on the surface. The impact on the glass darkening is considered in terms of urban pollution and mass tourism