In view of the development of superconducting fault current limiters, the properties of switching and recovering of YBa2Cu3O7−δ/Au (YBCO/Au) thin films are studied at 77 K and 50 Hz for overcurrents. The bilayers present an abrupt transition to a high dissipative state leading to a current limitation at a maximum value of about 2.5 times the critical current Ic, and allow the development of electric fields of 3 kV m−1 without any damage. After the overcurrent regime, the recovery of the superconducting state under the rated current In is studied as a function of overcurrent parameters. These results show clearly the strong potential of YBCO/Au thin films which can recover their superconducting state under nominal mode. This last point is crucial for transformer connection, as experimentally shown in this paper.
We report measurements and analysis of the voltage noise due the to vortex motion, performed in superconducting Niobium micro-bridges. Noise in such small systems exhibits important changes from the behavior commonly reported in macroscopic samples. In the low biasing current regime, the voltage fluctuations are shown to deviate substantially from the Gaussian behavior which is systematically observed at higher currents in the so called flux-flow regime. The responsibility of the spatial inhomogeneities of the critical current in this deviation from Gaussian behavior is emphasized. We also report on the first investigation of the effect of an artificial pinning array on the voltage noise statistics. It is shown that the fluctuations can lose their stationarity, and exhibit a Levy flight-like behavior.
In situ direct deposition at approximately 700 degrees C of thin YBa/sub 2/Cu/sub 3/O/sub 7-x/ superconductive films and multilayers has been done by three techniques using stoichiometric YBa/sub 2/Cu/sub 3/O/sub 7-x/ sintered targets. Excimer laser ablation in a DC magnetron system with hollow and planar targets leads to 0.5-, 1.2-, and 2.5-in diameter uniformly superconductive layers under static conditions. High critical current densities (>10/sup 6/ A cm/sup 2/ at 77 K) associated with low resistivity and good epitaxial behavior are achieved on top of MgO, SrTiO/sub 3/, LaAlO/sub 3/, and YSZ single-crystal wafers. High-quality c-oriented films (T/sub c/>80 K) are routinely obtained by means of a DC magnetron on large sapphire (>2 in) substrates covered by a YSZ RF sputtered buffer layer. The infrared properties of such films have been checked at 1.15- mu m wavelength. In order to achieve active devices (such as optical detectors and high-frequency mixers), small (<10- mu m/sup 2/) YBa/sub 2/Cu/sub 3/O/sub 7/-YSZ-Ag tunnel junctions and arrays have been successfully patterned in the superconductor/insulator/normal-metal trilayers using SNOP (selective niobium overlap process).
Within the context of the electric power market liberalization, DC networks have many interests compared to AC ones. New energy landscapes open the way of a diversified production. Innovative interconnection diagrams, in particular using DC buses, are under development. In this case it is not possible to defer the fault current interruption in the AC side. DC fault current cutting remains a difficult problem. FCLs (Fault Current Limiters) enable to limit the current to a preset value, lower than the theoretical short-circuit current. For this application Coated Conductors (CC) offer an excellent opportunity. Due to these promising characteristics we build a test bench and work on the implementation of these materials. The test bench is composed by 10 power amplifiers, to reach 4 kVA in many configurations of current and voltage. We carried out limiting experiments on DyBaCuO CC from EHTS, samples are about five centimeters long and many potential measuring points are pasted on the shunt to estimate the quench homogeneity. Thermal phenomena in FCLs are essential, numerical models are important to calculate the maximum temperatures. To validate these models we measure the CC temperature by depositing thermal sensors (Cu resistance) above the shunt layer and the substrate. An electrical insulation with a low thermal resistivity between the CC and the sensors is necessary. We use a thin layer of Parylene because of its good mechanical and electrical insulation properties at low temperature. The better quench behaviour of CC for temperatures close to the critical temperature has been confirmed. The measurements are in good agreement with simulations, this validates the thermal models.
We present imaging of biological cells by a fibered high-resolution optical microscope, which we call interferometric Scanning Optical Microscope (iSOM). The topography and surface structure of mouse fibroblasts and living hippocampal neurons are clearly revealed by iSOM. Its working principle is based on the interference of the internally reflected light at an optical fiber tip with the light reflected by the surface facing the tip. iSOM is working in the optical far field, i.e. at tip-sample distances of the order of a few microns, thus avoiding any mechanical contact with the sample. This feature is of particular importance for the observation of living cells. Samples with topography in the micron range can thus be imaged at high resolution without any closed-loop tracking of the fiber tip height as done in AFM or NSOM. This feature allows to use less complex electronic control units and much faster recording speeds. In addition, iSOM can be developed at reduced costs and is well suited for integration with other measurement techniques, as only an optical fiber has to be positioned near the sample surface.
When bicuculline an antagonist of inhibitory connections is introduced in the nutritive solution of an in vivo preparation containing neural networks, a synchronous activity of the neural network is observed. Observation of this activity with micro electrodes arrays presents signals containing periodic bursts of activity. The aim of this study is to obtain the same behavior with simulations. Models of excitatory neural networks coupled to models of measures with micro electrode are tested and the parameters are adjusted. It is shown that the level of noise on synaptic transmission triggers a periodic bursting activity of the neural network like the one observed in experimental conditions. Introduction of delays and durations for the synaptic transmission, and adjustments of the parameters enable to obtain different patterns of bursting.
