Catherine Villard

Effect of adding MgO to bulk Bi2212 melt textured in a high magnetic field

Bulk samples of Bi2212 pure and doped with MgO or were prepared by melt processing in a high magnetic field. The structure of the samples was analysed using XRD and SEM and their transport properties were studied. The results show that when a magnetic field is applied during melt processing the bulk Bi2212 is textured with the c-axis parallel to the field direction. The addition of MgO induces an improvement in the material properties. This is not observed using . A significant improvement is observed at 77 K in the critical current density in self-field and in an external magnetic field. This improvement can be related to the better texturation obtained when MgO is added: it maintains high viscosity during melt processing and thus the texturation induced by the magnetic field is not weakened by liquid-phase spreading. The addition of MgO also induces a decrease in volume of residual phases. The improvement in properties in an external magnetic field can be attributed to a better indirect vortex pinning due to submicron MgO-rich inclusions in the superconducting matrix.

How morphological constraints affect axonal polarity in mouse neurons.

Neuronal differentiation is under the tight control of both biochemical and physical information arising from neighboring cells and micro-environment. Here we wished to assay how external geometrical constraints applied to the cell body and/or the neurites of hippocampal neurons may modulate axonal polarization in vitro. Through the use of a panel of non-specific poly-L-lysine micropatterns, we manipulated the neuronal shape. By applying geometrical constraints on the cell body we provided evidence that centrosome location was not predictive of axonal polarization but rather follows axonal fate. When the geometrical constraints were applied to the neurites trajectories we demonstrated that axonal specification was inhibited by curved lines. Altogether these results indicated that intrinsic mechanical tensions occur during neuritic growth and that maximal tension was developed by the axon and expressed on straight trajectories. The strong inhibitory effect of curved lines on axon specification was further demonstrated by their ability to prevent formation of multiple axons normally induced by cytochalasin or taxol treatments. Finally we provided evidence that microtubules were involved in the tension-mediated axonal polarization, acting as curvature sensors during neuronal differentiation. Thus, biomechanics coupled to physical constraints might be the first level of regulation during neuronal development, primary to biochemical and guidance regulations.

Nanoscale surface topography reshapes neuronal growth in culture.

Neurons are sensitive to topographical cues provided either by in vivo or in vitro environments on the micrometric scale. We have explored the role of randomly distributed silicon nanopillars on primary hippocampal neurite elongation and axonal differentiation. We observed that neurons adhere on the upper part of nanopillars with a typical distance between adhesion points of about 500 nm. These neurons produce fewer neurites, elongate faster, and differentiate an axon earlier than those grown on flat silicon surfaces. Moreover, when confronted with a differential surface topography, neurons specify an axon preferentially on nanopillars. As a whole, these results highlight the influence of the physical environment in many aspects of neuronal growth.

Micro-magnetic imprinting of high field gradient magnetic flux sources

We report here on the fabrication of hard magnetic powder based micro-flux sources using micro-patterned hard magnetic films as templates or master structures. The micro-magnetic imprinting (μMI) process is simple and the constituent materials of the final structures, commercial hard magnetic powders and polymer, are inexpensive. The structures may be transparent, and either flexible or rigid, depending on the choice of polymer matrix used. The peak-to-peak intensity of the z-component of the stray magnetic field measured above a test μMI structure made with spherical NdFeB particles of average particle size 16 μm is in good agreement with simulated field values (150 mT at 5 μm). Simulations indicate magnetic field gradients of up to 5 × 105 T/m at the surface of such μMI structures. The trapping of cells functionalised with superparamagnetic beads by these structures has been demonstrated. The μMI fabrication technique has much potential for the development of high field gradient magnetic flux sources for applications in biology and beyond.

Neuronal Architectures with Axo‐dendritic Polarity above Silicon Nanowires

An approach is developped to gain control over the polarity of neuronal networks at the cellular level by physically constraining cell development by the use of micropatterns. It is demonstrated that the position and path of individual axons, the cell extension that propagates the neuron output signal, can be chosen with a success rate higher than 85%. This allows the design of small living computational blocks above silicon nanowires.

DC superconducting fault current limiter

There is a lack of satisfying solutions for fault currents using conventional technologies, especially in DC networks, where a superconducting fault current limiter could play a very important part. DC networks bring a lot of advantages when compared to traditional AC ones, in particular within the context of the liberalization of the electric market. Under normal operation in a DC network, the losses in the superconducting element are nearly zero and only a small, i.e. a low cost, refrigeration system is then required. The absence of zero crossing of a DC fault current favourably accelerates the normal zone propagation. The very high current slope at the time of the short circuit in a DC grid is another favourable parameter. The material used for the experiments is YBCO deposited on Al2O3 as well as YBCO coated conductors. The DC limitation experiments are compared to AC ones at different frequencies (50–2000 Hz). Careful attention is paid to the quench homogenization, which is one of the key issues for an SC FCL. The University of Geneva has proposed constrictions. We have investigated an operating temperature higher than 77 K. As for YBCO bulk, an operation closer to the critical temperature brings a highly improved homogeneity in the electric field development. The material can then absorb large energies without degradation. We present tests at various temperatures. These promising results are to be confirmed over long lengths.

Tuning the adhesive geometry of neurons: length and polarity control

Neurons acquire their functional and morphological axo-dendritic polarity by extending, from competing minor processes (neurites), one long axon among numerous dendrites. We employed complementary sets of micropatterns built from 2 and 6 μm wide stripes of various lengths to constrain hippocampal neuron shapes. Using these geometries, we have (i) limited the number of neuronal extensions to obtain a minimal in vitro system of bipolar neurons and (ii) controlled the neurite width during growth by the generation of a progressive cell shape asymmetry on either side of the cellular body. From this geometrical approach, we gained a high level of control of each neurite length and of the localization of axonal specification. To analyze these results, we developed a model based on a width and polarization dependent neurite elongation rate and on the existence of a critical neurite length that sets the axonal fate. Our data on the four series of micro-patterns developed for this study are described by a single set of growth parameters, well supported by experiments. The control of neuronal shapes by adhesive micro-patterns thereby offers a novel paradigm to follow the dynamical process of neurite lengthening and competition through the process of axonal polarization.

Critical current density of 165 kA/cm2 at 4 K in bulk Bi2212/MgO textured by solidification in a high magnetic field and hot forging

Abstract Bulk Bi2212 superconductors with MgO addition are highly textured by solidification in a high magnetic field. Hot forging following this first step improves the texture degree and enhances the critical current density. Critical current density J c determined by transport and magnetic measurements for temperatures between 77 and 4 K are presented. Transport J c reaches 1900 A/cm 2 at 77 K in self-field, 4600 A/cm 2 at 65 K and 9500 A/cm 2 at 40 K. At 4 K, a J c of 165 kA/cm 2 is deduced from magnetization measurements. This value is comparable with those of multifilamentary Bi2212 conductors. An increase of the irreversibility line induced by hot forging after magnetic melt processing is also observed.

Quantitative analysis of the critical current due to vortex pinning by surface corrugation

The transport critical current of a Niobium (Nb) thick film has been measured for a large range of magnetic field. Its value and variation are quantitatively described in the framework of the pinning of vortices due to boundary conditions at the rough surface, with a contact angle well explained by the spectral analysis of the surface roughness. Increasing the surface roughness using a Focused Ion Beam results also in an increase of the superficial critical current.

Origin of the irreversibility line in thin YBa2Cu3O7- delta films with and without columnar defects.

We report on measurements of the angular dependence of the irreversibility temperature Tirr (�) in Y Ba2Cu3O7 � thin films, defined by the onset of a third harmonic signal and measured by a miniature Hall probe. From the functional form of Tirr (�) we conclude that the origin of the irreversibility line in unirradiated films is a dynamic crossover from an unpinned to a pinned vortex liquid. In irradiated films the irreversibility temperature is determined by the trapping angle.