Sputter-deposited Al/CuO multilayers exhibit fast combustion reactions in which an exothermic chemical reaction wavecontrolled by the migration of oxygen atoms from the oxide matrix toward the aluminum layers through interfacial layersmoves throughout the multilayer at subsonic rates (meters per second to tens of meters per second). We directly observed the structural and chemical evolution of Al/CuO/Al multilayers upon heating to 700 °C using high-magnification transmission electron microscopy (TEM) and scanning TEM, providing simultaneous subnanometrer imaging resolution and detailed chemical analysis. Interestingly, as deposited, the trilayer is characterized by two distinct interfacial layers: 4.1 ± 0.2 nm thick amorphous alumina and a 15 ± 5 nm thick mixture of AlO x and Cu x Al y O z , at the bottom interface and top interface, respectively. Upon heating, we accurately characterized the evolving nature and structure of these interfaces, which are rapidly replaced by the reaction terminal oxide (Al 2 O 3 ). For the first time, we unraveled the release of gaseous O from the sparse columnar and defective CuO well below reaction onset (at ∼200 °C) which accumulates at interfaces and contributes to initiate the Al oxidation process at the vicinity of native interfaces. The oxidation process is demonstrated to be accompanied by a continuous densification and modification of the CuO layer. Between 300 and 350 °C, we observed a brutal shrinkage of the CuO layer (14% loss of its initial thickness) leading to the mechanical fracture in the top alumina growing layer. Consequently, this latter becomes highly permeable to oxygens leading to a brutal enhancement of the oxidation rate (×4). We also characterized stressed-induced interfacial delamination at 500 °C pointing clearly to the mechanical fragility of the top interface after the CuO transformation. Altogether, these results permit one to establish a multistep reaction scenario in Al/CuO sputter-deposited films supporting to an unprecedented level a mechanistic assignation of differential scanning calorimetry peaks. This study offers potential benefits for the development of aging models enabling the virtual prediction of the calorimetric response of exothermic Al/CuO thin-film reactions.

International audienc

The complex Al/CuO self-propagating reaction involving multi-phase and multi-species dynamics was studied in order to investigate the very high flame temperature around the vaporization temperature of alumina, even under a neutral environment. Experiments were performed on different sputter-deposited Al/CuO multilayers coupling optical spectroscopy with high speed camera measurements. The clear presence of both AlO and Al signatures in gas phase suggests that the redox reaction starts in the bulk nanolaminate, which then rapidly tear off the substrate to continue burning in a heterogeneous (condensed and gas) phase in the environment. The flame temperature increases with the stoichiometry but is independent of the bilayer thickness. In addition to the confirmation of the effects of stoichiometry and the bilayer thickness on the characteristics of the self-propagating reaction, the predominant role of

In this work, we report magnetoelectric coefficient measurements in BiFeO3 (BFO) compounds using dynamic lock-in technique to evidence magnetoelectric coupling behavior. The response of the magnetoelectric coefficient shows rapid increase from low frequencies (0.1kHz) to around 5kHz, reaching a maximum, and then decreasing monotonously to 100kHz. For a constant frequency at 7kHz, the maximum magnetoelectric coefficient was close to 7mV∕cmOe, obtained at a bias magnetic field of H=120Oe. Thus, we demonstrated the feasibility of this technique for characterizing multiferroic materials.

We have developed a new method to study the oxygen surface exchange kinetics in oxide materials in the form of epitaxial thin films by analyzing subtle cell parameter variations induced by changes in the oxygen stoichiometry of the material. The method consists of continuously analyzing the X-ray diffraction pattern of particular film reflections with a linear X-ray fast detector in a static position, while exposing the sample to sudden changes in the pO 2 of the atmosphere at elevated temperatures. With this method, we have been able to follow cell parameter changes as small as 2.10 −4 Å in time intervals as short as 10 s in La 2 NiO 4+δ epitaxial films and La 2 NiO 4+δ /LaNiO 3−δ bilayers. This method provides a simpler and contactless tool for dynamically analyzing oxygen surface exchange kinetics and diffusion in transition metal oxide compounds, and complements other currently used techniques such as Electric Conductivity Relaxation (ECR) and Isotopic Exchange depth profiling (IEDP). In addition, this method is a unique tool to address oxygen transport across solid−solid interfaces in thin film heterostructures.

sotopic 18 O tracer diffusion he layered-structure degree of anisotropy in the oxygen diffusion coefficient, being arger along the engineering high Fuel Cells.

In all cases the values correspond to their particular defect equilibrium and degree of charge localization. The oxygen surface exchange kinetics was also evaluated from in-situ time-resolved analyses of the cell parameter variations. LSC, LNO and GBCO films show fast oxygen reduction kinetics, k chem = 5 · 10 −6 , 3 · 10 −6 , and 2 · 10 −7 cm/s at 700 • C, respectively, in relative agreement with reported values, while BSCF films show much slower kinetics than expected, below k chem = 10 −7 cm/s at 650 • C, related to the degradation process observed in the films.

Epitaxial thin films of GdBaCo2O5.5+δ (GBCO) grown by pulsed laser deposition have been studied as a function of deposition conditions. The variation in film structure, domain orientation, and microstructure upon deviations in the cation composition have been correlated with the charge transport properties of the films. The epitaxial GBCO films mainly consist of single- and double-perovskite regions that are oriented in different directions depending on the deposition temperature. Additionally, cobalt depletion induces the formation of a high density of stacking defects in the films, consisting of supplementary GdO planes along the c-axis of the material. The presence of such defects progressively reduces the electrical conductivity. The films closer to the stoichiometric composition have shown p-type electronic conductivity at high pO2 with values as high as 800 S/cm at 330 °C in 1 atm O2, and with a pO2 power dependence with an exponent as low as 1/25, consistent with the behavior reported for bulk GBCO. These values place GBCO thin films as a very promising material to be applied as cathodes in intermediate temperature solid oxide fuel cells.