RESUMEN
TOF-SIMS analyses of state-of-the-art high temperature solid oxide electrolysis cells before and after testing under different operating conditions were performed. The investigated cells consist of an yttria stabilized zirconia (YSZ) electrolyte, a La1-xSrxMnO3-δ composite anode and a Ni-YSZ cermet cathode. The surfaces and cross-sections of the cells were analyzed, and several elemental impurities like Si, Ca and Na were identified and spatially mapped and their enrichment and migration during operation is reported. With advancing operation time, the concentration of these elements, especially Na and Ca, increases. For Si, a concentration gradient is found from the gas inlet to the gas outlet. Additionally, a loss of Ni percolation in the active cathode is observed in the same area where the Si enrichment is found. Based on the obtained TOF-SIMS results, the influence of the operating conditions on degradation is discussed.
RESUMEN
X-ray diffraction (XRD), in situ energy-dispersive X-ray diffraction (EDXRD), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and magnetic measurements were applied to investigate the effects of lithium deintercalation on pseudolayered Li(0.70)CrTi(0.25)Se(2). A detailed picture of structural changes during the deintercalation process was obtained by combining the results of EDXRD and EXAFS. Removal of Li from the host-guest complex leads to anisotropic contraction of the unit cell with stronger impact on the c axis, which is the stacking axis of the layers. The EDXRD experiments evidence that the shrinkage of the lattice parameters with decreasing x(Li) in Li(x)CrTi(0.25)Se(2) is nonlinear in the beginning and then becomes linear. Analysis of the EXAFS spectra clearly shows that the Cr/Ti-Se distances are affected in a different manner by Li removal. The Cr-Se bond lengths decrease, whereas the Ti-Se bonds lengthen when the Li content is reduced, which is consistent with XRD data. Magnetic measurements reveal a change from predominantly antiferromagnetic exchange (theta(p) = -300 K) interactions for the pristine material to ferromagnetic exchange interactions (theta = 25 K) for the fully intercalated material. Thus, the magnetic properties can be altered under ambient conditions by directed adjustment of the dominant magnetic exchange. The unusual magnetic behavior can be explained on the basis of the variation of the metal-metal distances and the Cr-Se-Cr angles with x, which were determined by Rietveld refinements. Owing to competing ferromagnetic and antiferromagnetic exchange interactions and disorder, the magnetic ground state of the intercalated materials is characterized by spin-glass or spin-glass-like behavior.
RESUMEN
A hydrothermal approach employing an amine as reducing agent enables synthesis of an analogue of the arsenato(iii)-oxovanadate {V(15)As(6)}, representing the first systematic variation of this intensely studied magnetic system.
RESUMEN
The intercalation reaction between Cr(4)TiSe(8) and Li was investigated from a kinetic and an electrochemical perspective. The structural phase transition from monoclinic to trigonal symmetry was probed by in situ energy-dispersive X-ray diffraction (in situ EDXRD) for chemical intercalation with butyllithium (BuLi). A change in the kinetic mechanism was detected for the reaction at room temperature; this was interpreted in terms of a trend from phase boundary control to diffusion control. A single diffusion-controlled mechanism is obeyed at 60 degrees C. The electrochemical measurements and the corresponding in situ X-ray diffraction (in situ XRD) data revealed that the monoclinic host is intercalated up to the composition Li(x approximately 0.1)Cr(4)TiSe(8) before the characteristic phase transition starts. The monoclinic phase undergoes complex structural changes in the following two-phase regime. Owing to the co-existence of two phases, the cell potential is constant for 0.1
RESUMEN
The reaction of Cr/Se multilayers when they are annealed occurs in two steps: interdiffusion of the single layers to an amorphous Cr-Se alloy and crystallization of Cr3Se4. Both reaction steps were characterized using various techniques. At approximately 300 degrees C the layers have interdiffused completely to form a homogeneous amorphous Cr-Se alloy. Short-range order in the alloy was probed with X-ray absorption spectroscopy (XAS) and, according to the results of this, is already very similar to Cr3Se4, which crystallizes around 500 degrees C. Crystallization occurs at a well-defined temperature, whereas crystallite growth proceeds in the whole temperature interval above the crystallization temperature and is not finished at 660 degrees C. The reaction yields a polycrystalline thin film of Cr3Se4 in a preferred orientation exhibiting a (00l) texture. In Cr-rich samples amorphous Cr is present as a by-product. A Cr-Se/Se multilayer was observed as an intermediate in the interdiffusion of some Cr-rich samples which is stable between 200 and 250 degrees C.