Very fast formation of superconducting MgB2/Fe wires with high Jc

In this paper we have investigated the effects of sintering time and temperature on the formation and critical current densities of Fe-clad MgB2 wires. MgB2 wires were fabricated using the powder-in-tube process and sintered for different periods of time at predetermined temperatures. All the samples were examined using XRD, SEM and magnetisation measurements. In contrast to the common practice of sintering for several hours, the present results show that there is no need for prolonged heat treatment in the fabrication of Fe-clad MgB2 wires. A total time in the furnace of several minutes is more than enough to form nearly pure MgB2 with high performance characteristics. The results from Tc, Jc and Hirr show convincingly that the samples which were sintered for 3 minutes above 800 oC are as good as those sintered for longer times. In fact, the Jc field performance for the most rapidly sintered sample is slightly better than for all other samples. Jc of 4.5 times 10 ^5 A/cm2 in zero field and above 10 ^5 A/cm2 in 2T at 15 K has been achieved for the best Fe-clad MgB2 wires. As a result of such a short sintering there is no need for using high purity argon protection and it is possible to carry out the heat treatment in a much less protective atmosphere or in air. These findings substantially simplify the fabrication process, making it possible to have a continuous process for fabrication and reducing the costs for large-scale production of MgB2 wires.Comment: 15 pages, one table, 9 figures, submitted to Physica C on June 8, 200

Separating hard and soft scales in hard processes in a QCD plasma

We present a picture of hard processes in a hot plasma in terms of the hard scale part of the process, where perturbative QCD should be applicable, and the soft scale part of the process, where we look to the AdS/CFT correspondence for guidance to possible strong effective coupling phenomena. In particular we estimate $\hat{q}$, the transport coefficient, supposing that at soft scales partons in the plasma all cascade to small-$x$-values as indicated by strong coupling SYM theory.Comment: 8 page

Magnetic and Dyonic Black Holes in D=4 Gauged Supergravity

Magnetic and Dyonic solutions are constructed for the theory of abelian gauged N=2 gauged four dimensional supergravity coupled to vector multiplets. The solutions found preserve 1/4 of the supersymmetry.Comment: Latex, 14 page

Some comments on the bi(tri)-Hamiltonian structure of Generalized AKNS and DNLS hierarchies

We give the correct prescriptions for the terms involving the inverse of the derivative of the delta function, in the Hamiltonian structures of the AKNS and DNLS systems, in order for the Jacobi identities to hold. We also establish that the sl(2) AKNS and DNLS systems are tri-Hamiltonians and construct two compatible Hamiltonian structures for the sl(3) AKNS system. We also give a derivation of the recursion operator for the sl(n+1) DNLS system.Comment: 10 pages, LaTe

Une solution semi-analytique améliorée pour le stress aux encoches arrondies

International audienceIn order to investigate the brittle failure of keyhole notched components, the stress distribution at notch tips is studied numerically and theoretically. A semi-analytical formula is developed for the maximum notch-tip-stress, incorporating crack-tip-blunting, stress-concentration and stress-equilibrium. Stress distributions in notched plates are simulated by the finite-element method, showing improved accuracy of the formula relative to established solutions. Application of the developed equation to components containing U-notches and blunt V-notches, is explored, demonstrating its broad applicability. When combined with stress-based failure criteria, the semi-analytical model can be employed to assess brittle failure in notched components with significance toward fracture in heterogeneous materials.Afin d’étudier la défaillance fragile des composants à encoche en trou de serrure, la répartition des contraintes aux extrémités des entailles est étudiée numériquement et théoriquement. Une formule semi-analytique est élaborée pour la contrainte maximale en pointe, intégrant l’atténuation des fissures, la concentration en contrainte et l’équilibre en contrainte. Les distributions de contraintes dans les plaques à encoches sont simulées par la méthode des éléments finis, ce qui montre une précision améliorée de la formule par rapport aux solutions établies. L’application de l’équation développée aux composants contenant des encoches en U et des encoches en V contondantes est explorée, démontrant ainsi sa large applicabilité. Lorsqu'il est combiné à des critères de rupture fondés sur des contraintes, le modèle semi-analytique peut être utilisé pour évaluer la défaillance fragile de composants entaillés présentant une importance significative pour la rupture dans des matériaux hétérogènes

Flux Jumping and a Bulk-to-Granular Transition in the Magnetization of a Compacted and Sintered MgB2 Superconductor

The recent discovery of intermediate-temperature superconductivity (ITC) in MgB2 by Akimitsu et al. and its almost simultaneous explanation in terms of a hole-carrier-based pairing mechanism by Hirsch, has triggered an avalanche of studies of its structural, magnetic and transport properties. As a further contribution to the field we report the results of field (H) and temperature (T) dependent magnetization (M) measurements of a pellet of uniform, large-grain sintered MgB2. We show that at low temperatures the size of the pellet and its critical current density, Jc(H) - i.e. its M(H) - ensure low field flux jumping, which of course ceases when M(H) drops below a critical value. With further increase of H and T the individual grains decouple and the M(H) loops drop to lower lying branches, unresolved in the usual full M(H) representation. After taking into account the sample size and grain size, respectively, the bulk sample and the grains were deduced to exhibit the same magnetically determined Jc s (e.g. 105 A/cm2, 20 K, 0T) and hence that for each temperature of measurement Jc(H) decreased monotonically with H over the entire field range, except for a gap within the grain-decoupling zone.Comment: 7 pages, 6 figures, Changes: Fig 6 Vertical scale an order of magnitude out (changed figure and associated text). Also corrected typo in last sectio

Réglage de la pénétration capillaire dans les milieux poreux: combinaison des effets géométriques et d'évaporation

International audienceCapillary penetration of liquids in porous media is of great importance in many applications and the ability to tune such penetration processes is increasingly sought after. In general, liquid penetration can be retarded or restricted by the evaporation of volatile liquid at the surface of the porous media. Moreover, when capillary penetration occurs in a porous layer with non-uniform cross section, the penetration process can be accelerated or impeded by adjusting the section geometry. In this work, on the basis of Darcy's Law and mass conservation, a theoretical model of capillary penetration combining evaporation effects in two-dimensional homogeneous porous media of varying cross-section is developed and further examined by numerical simulations. The effects of sample geometry and liquid evaporation on capillary penetration are quantitatively analyzed. Results show that the penetration velocity is sensitive to the geometry of the porous layer, and can be tuned by varying the evaporation rate for a given geometry. Under given evaporation conditions, penetration is restricted to a limited region with a predictable boundary. Furthermore, we find that the inhibition of liquid penetration by evaporation can be offset by varying the geometry of the porous layer. In addition, the theoretical model is further extended to model the capillary flow in three-dimensional porous media, and the interplay of geometry and evaporation during the capillary flow process in 3D conditions is also investigated. The results obtained can be used for facilitating the design of porous structures, achieving tunable capillary penetration for practical applications in various fields.La pénétration capillaire de liquides dans des milieux poreux revêt une grande importance dans de nombreuses applications et la capacité de réglage de tels processus de pénétration est de plus en plus recherchée. En général, la pénétration de liquide peut être retardée ou limitée par l'évaporation de liquide volatil à la surface du support poreux. De plus, lorsque la pénétration capillaire se produit dans une couche poreuse de section transversale non uniforme, le processus de pénétration peut être accéléré ou empêché en ajustant la géométrie de la section. Dans ce travail, sur la base de la loi de Darcy et de la conservation de masse, un modèle théorique de pénétration capillaire combinant les effets d'évaporation dans des milieux poreux homogènes à deux dimensions de sections différentes est développé et examiné plus en détail par des simulations numériques. Les effets de la géométrie de l'échantillon et de l'évaporation du liquide sur la pénétration capillaire sont analysés de manière quantitative. Les résultats montrent que la vitesse de pénétration est sensible à la géométrie de la couche poreuse et peut être ajustée en faisant varier le taux d'évaporation pour une géométrie donnée. Dans des conditions d'évaporation données, la pénétration est limitée à une région limitée avec une limite prévisible. En outre, nous trouvons que l'inhibition de la pénétration du liquide par évaporation peut être compensée en faisant varier la géométrie de la couche poreuse. En outre, le modèle théorique est élargi pour modéliser l'écoulement capillaire dans des milieux poreux tridimensionnels, et les interactions entre la géométrie et l'évaporation pendant le processus d'écoulement capillaire dans des conditions 3D sont également étudiées. Les résultats obtenus peuvent être utilisés pour faciliter la conception de structures poreuses, en réalisant une pénétration capillaire ajustable pour des applications pratiques dans divers domaines

Network Slicing Based 5G and Future Mobile Networks: Mobility, Resource Management, and Challenges

5G networks are expected to be able to satisfy users' different QoS requirements. Network slicing is a promising technology for 5G networks to provide services tailored for users' specific QoS demands. Driven by the increased massive wireless data traffic from different application scenarios, efficient resource allocation schemes should be exploited to improve the flexibility of network resource allocation and capacity of 5G networks based on network slicing. Due to the diversity of 5G application scenarios, new mobility management schemes are greatly needed to guarantee seamless handover in network-slicing-based 5G systems. In this article, we introduce a logical architecture for network-slicing-based 5G systems, and present a scheme for managing mobility between different access networks, as well as a joint power and subchannel allocation scheme in spectrum-sharing two-tier systems based on network slicing, where both the co-tier interference and cross-tier interference are taken into account. Simulation results demonstrate that the proposed resource allocation scheme can flexibly allocate network resources between different slices in 5G systems. Finally, several open issues and challenges in network-slicing-based 5G networks are discussed, including network reconstruction, network slicing management, and cooperation with other 5G technologies

Optimal packetisation of MPEG-4 using RTP over mobile networks

The introduction of third-generation wireless networks should result in real-time mobile video communications becoming a reality. Delivery of such video is likely to be facilitated by the realtime transport protocol (RTP). Careful packetisation of the video data is necessary to ensure the optimal trade-off between channel utilisation and error robustness. Theoretical analyses for two basic schemes of MPEG-4 data encapsulation within RTP packets are presented. Simulations over a GPRS (general packet radio service) network are used to validate the analysis of the most efficient scheme. Finally, a motion adaptive system for deriving MPEG-4 video packet sizes is presented. Further simulations demonstrate the benefits of the adaptive system