Assessment of the real contact area of a multi-contact interface from electrical measurements

The electrical supply of moving trains is provided by a sliding contact between the train's pantographs and the catenary. This electromechanical interface is composed of the pantograph's strips ? made mainly of carbon ? and the catenary contact wire. The temperature rising induces the rising of the carbon strip wear. Moreover, the heating produced at the interface depends on the surface quality. Indeed, the smaller is the surface contact, the greater is the electrical resistance which implies more heat production because of the Joule effect. The problem is multi-physical, there is a coupling between mechanical, electrical and thermal states. The objective is to define the real contact area with a simple electrical measurement. In many practical or fundamental situations involving contacting solids, the relevant notion of the real contact area is a very delicate one and especially its experimental assessment. Based on the Drude's classical transport model and within the linear elasticity approximation, a phenomenological model of a metal/metal contact is built up, offering a simplified interpretation framework of experimental data. The model accounts for the influence of the mechanical state of the contacting zone upon its electrical properties, such as its impedance. Interpreting available data within this framework leads to the assessment of the spots' number. The total contact force works on the spots and on the average contact length. In this model, the interface is treated as a new medium with its own conductivity and mean free time between ionic collisions. There are two types of measurement: - Electrical measurements carried out on two copper sheets with dimensions 50X50X1 mm3 acted upon with an external compression force, allowed to check and validate the model. In agreement with the conditions of the model and to avoid the complexification of the model due to the intricacy between thermal and electrical processes at the contact interface, the measurements were operated in alternative current at low voltages. - Additional measurements of surface states have been realized to join the electrical measure with roughness and the nature of the metal.  In the case of contact between train's pantograph and catenary, this method allows predicting electric transfer's quality to control the heating of the interface. Being sensitive to the spots' mechanical solicitation conditions, properly interpreted, electrical impedance measurements should lead to a better understanding of the complex mechanical responses of these interfaces and their ageing process or even to detect a fatigue and prevent a potential failure

Temperature evolution of the structural properties of monodomain ferroelectric thin film

The structural evolution of epitaxial monodomain (only 180° domains) ferroelectric PbTiO3 thin film has been investigated, using high-resolution, temperature-dependent, x-ray diffraction. The full set of lattice parameters was obtained from room temperature up to 850K. It allowed the calculation of the different strains stored in the film at room temperature, underlying the difference between the mechanical strain and the misfit strain. The evolution of the misfit strain as a function of temperature was also calculated and was found to be consistent with the theoretical temperature-misfit strain phase diagram. These data strongly suggest that the film remains ferroelectric and tetragonal up to 940K

Domain structure sequence in ferroelectric Pb(Zr0.2Ti0.8)O-3 thin film on MgO

International audienceThe structural evolution of a polydomain ferroelectric Pb(Zr0.2Ti0.8)O3 film was studied by temperature-dependent x-ray diffraction. Two critical temperatures were evidenced: T∗=740K, corresponding to a change in the domain structure (a∕c∕a∕c to a1∕a2∕a1∕a2), and TfilmC=825K, where the film undergoes a ferroelectric-paraelectric phase transition. The film remains tetragonal on the whole range of temperature investigated. The evolutions of the domain structure and lattice parameters were found to be in very good agreement with the calculated domain stability map and theoretical temperature-misfit strain phase diagram, respectively

Evidence for a first-order transition from monoclinic α to monoclinic β phase in BiFeO₃ thin films

We report a temperature-dependent high-resolution x-ray diffraction investigation of 200-nm epitaxial BiFeO3 thin films grown on (001) SrTiO3. We find that BiFeO3 undergoes two high-temperature transitions: a first-order α−β phase transition between 745 and 780 °C and a more diffuse transition toward the γ phase at 860 °C. Reciprocal space maps reveal that thin films remain monoclinic crossing the α−β phase transition. Linear extrapolation of the in-plane lattice parameters to higher temperatures appears to rule out cubic symmetry for the γ phase

Unexpected formation by pulsed laser deposition of nanostructured Fe/olivine thin films on MgO substrates

International audienceOlivine-type LiFePO(4) thin films were grown on MgO (1 0 0) substrates by pulsed laser deposition (PLD). The formation of an original nanostructure is evidenced by transmission electron microscopy measurements. Indeed, on focused ion beam prepared cross sections of the thin film, we observe, the amazing formation of metallic iron/olivine nanostructures. The appearance of such a structure is explained owing to a topotactic relation between the two phases as well as a strong Mg diffusion from the substrate to the film surface. Magnesium migration is thus concomitant with the creation of metallic iron domains that grow from the core of the film to the surface leading to large protuberances. To the best of our knowledge, this is the first report on iron extrusion from the olivine-type LiFePO(4)

Bipolar resistive switching and substrate effect in GdK2Nb5O15 epitaxial thin films with tetragonal tungsten bronze type structure

International audienceFerroelectric compound GdK2Nb5O15 with tetragonal tungsten bronze type structure thin films were grown by pulsed laser deposition on (001) SrRuO3/SrTiO3 and (001) SrRuO3/LSCO/MgO substrates respectively, in the same experimental conditions. Structural properties were investigated using X-ray diffraction in different modes theta-2 theta diffraction shows a single crystalline SRO and GKN phases, with c-axis perpendicular to the substrate. Phi-scans show for both SRO and GKN an epitaxial cube-on-cube growth. Reciprocal space mapping (RSM) was performed on GKN to determine the in-plane lattice parameters, and showed that GKN grown on MgO is more constrained comparing to STO. Room temperature electrical properties were investigated using platinum (Pt) top electrodes. Results show that GKN thin films deposited on MgO and STO substrates are resistively switchable. It was found that the nature of the substrate affects strongly the resistance ratio: GKN on SRO/LSCO/MgO showed a large hysteresis comparing to GKN on SRO/STO. Resistance switching in GKN is attributed to the oxygen vacancies migration which can be controlled by the substrate or the frequency sweep. (C) 2016 Elsevier Ltd. All rights reserved

The Leech Nervous System: A Valuable Model to Study the Microglia Involvement in Regenerative Processes

Microglia are intrinsic components of the central nervous system (CNS). During pathologies in mammals, inflammatory processes implicate the resident microglia and the infiltration of blood cells including macrophages. Functions of microglia appear to be complex as they exhibit both neuroprotective and neurotoxic effects during neuropathological conditions in vivo and in vitro. The medicinal leech Hirudo medicinalis is a well-known model in neurobiology due to its ability to naturally repair its CNS following injury. Considering the low infiltration of blood cells in this process, the leech CNS is studied to specify the activation mechanisms of only resident microglial cells. The microglia recruitment is known to be essential for the usual sprouting of injured axons and does not require any other glial cells. The present review will describe the questions which are addressed to understand the nerve repair. They will discuss the implication of leech factors in the microglial accumulation, the identification of nerve cells producing these molecules, and the study of different microglial subsets. Those questions aim to better understand the mechanisms of microglial cell recruitment and their crosstalk with damaged neurons. The study of this dialog is necessary to elucidate the balance of the inflammation leading to the leech CNS repair

High-Temperature Energy Storage Properties of Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ Thin Films

Bi0.5Na0.5TiO3-0.06BaTiO3 (BNT-BT) thin films were prepared via both chemical solution (CSD) and pulsed laser deposition (PLD). The structural, dielectric, and ferroelectric properties were investigated. High stability of the dielectric permittivity or TCC (∆ε/ε (150 °C) ≤ ±15%) over a wide temperature range from room temperature to 300 °C was obtained. Distinctly, the CSD film showed high TCC stability with variation of ±5% up to 250 °C. Furthermore, the CSD film showed an unsaturated ferroelectric hysteresis loop characteristic of the ergodic relaxor phase. However, the PLD one exhibited an almost saturated loop characteristic of the coexistence of both ergodic and non-ergodic states. The energy storage properties of the prepared films were determined using P–E loops obtained at different temperatures. The results show that these films exhibited a stable and improved energy storage density comparable to ceramic capacitors. Moreover, the CSD film exhibited more rigidity and better energy storage density, which exceeded 1.3 J/cm3 under a weak applied field of 317 kV/cm, as well as interesting efficiency in a large temperature range. The obtained results are very promising for energy storage capacitors operating at high temperatures

Stress and orientation in the relaxor/ferroelectric superlattices (PbMg1/3Nb2/3O3)((1-x)Lambda)/(PbTiO3)(x Lambda)

International audienceWe have grown a series of [PbMg1/3Nb2/3O3]((1-x)Lambda)/[PbTiO3](xLambda)(PMN1-x/PTx) superlattices for which the composition varies, 0.1less than or equal toxless than or equal to0.8, but the modulation period Lambda is kept constant at about 130 A. X-ray diffraction indicates that the polar c axis of the PT layers lies in the plane of the films. The Raman spectra, which reflect the sum of the spectra of PMN and the PT layers, confirm the a-domain orientation of the PT layers. A downshift of the E(1TO) soft mode of the PT layers was attributed to tensile stress, estimated to be in the range of 2.5-3.6 GPa, induced by the lattice mismatch of PMN/PT layers. We observe the splitting of the "silent" mode in PT into distinct modes B-1 and E and attribute this to the strains present in the layers

Structural studies of relaxor/ferroelectric Pb(Mg1/3Nb2/3)O-3/PbTiO3 superlattices

We have synthesized [Pb(Mg1/3Nb2/3)O-3]((1-x)Lambda)/[PbTiO3](x Lambda) (PMN(1-x)LambdaPT(x)Lambda) super-lattices by pulsed laser ablation onto MgO substrates buffered with La1/2Sr1/2CoO3 (LSCO). We varied the composition (0.1 <= x <= 0.8), and kept the modulation period A constant at about 140 angstrom. X-ray diffraction indicates that the polar c-axis of the PT layers lies in the plane of the films. This is confirmed by the Raman spectra. There is a shift to lower frequencies of the PT soft mode due to stress but we observe no indications of stress dependence in the Raman spectra of the PMN layers