Alessandro Figini Albisetti

Structures from Powders: Polynuclear Hg(II) Complexes Containing the Flexible Bisimidazolylmethane Ligand

Several polynuclear Hg(II) complexes containing the flexible ditopic bisimidazolylmethane ligand (C(7)H(8)N(4), bim) have been prepared by reaction of equimolar quantities of mercury salts (acetate, cyanide, thiocyanate, chloride, and iodide) in EtOH or acetonitrile solution. Their crystal and molecular structures were retrieved from laboratory powder diffraction data, and their thermal properties were fully characterized, including the determination of the thermal expansion coefficients and the related strain tensor using thermodiffractometric methods. [Hg(bim)(CH(3)COO)(2)](2) consists of cyclic dimers with chelating acetates, while the [Hg(bim)X(2)](n) species (X = Cl, CN, SCN, and I) are one-dimensional polymers, with dangling X groups. A further complex of nominal Hg(2)(bim)Cl(2) formulation was also prepared, but the complexity and nonideality of its powder diffraction traces prevented the determination of its main structural features.

Magnetic Field Trapping in MgB

In order to realize superconductive permanent magnets to be used in power applications like the magnetic levitation or electrical motors, we have studied the magnetic field trapping capability of MgB2 discs of different shapes, at temperatures >;10 K. In particular we have compared MgB2 bulk discs with MgB2 Superconductive Inserts in Metallic Substrates (SIMS) of diameters of 70 mm. Both superconductive devices have been produced by the Reactive Mg Liquid Infiltration (Mg-RLI) process. The magnetization was performed by the application and removal of an external magnetic field up to 2 T, produced by a superconducting magnet, or by Field Cooling of the superconductive devices nearby NdFeB permanent magnets. The SIMS devices showed a higher stability of the trapped fields with respect to the bulk discs. Typical trapped fields, measured at 1 mm from the surface of the device, are of the order of 1 T. The density distribution of the supercurrents has been estimated by measuring the trapped field at various temperatures up to Tc and performing magnetic levitation forces measurements.

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Both MgB2 and (RE)BCO bulk materials can provide a highly compact source of magnetic field when magnetized. The properties of these materials when magnetized by a pulsed field are potentially useful for a number of applications, including magnetic levitation. This paper reports on pulsed field magnetization of single 25 mm diameter (RE)BCO bulks using a recently constructed pulse magnetization facility, which allows an automated sequence of pulses to be delivered. The facility allows measurement of force between a magnetized (RE)BCO bulk and a bulk MgB2 hollow cylinder, which is field cooled in the field of the magnetized (RE)BCO bulk. Hysteresis cycling behavior for small displacement is also measured to extract the stiffness value. The levitation forces up to 500 N were obtained, the highest ever measured between two bulks and proves the concept of a bulk-bulk superconducting bearing design.

Bulks and Inserts

Pulsed field magnetization (PFM) was performed for the first time on a large MgB2 bulk 50 mm diameter fabricated by a reactive liquid Mg infiltration (Mg-RLI) method, and the time dependence of the local field BLC(t) and the trapped field profiles were measured. The trapped field of Bz=0.47 T and the total trapped flux of Φ=0.50 mWb were achieved at Ts=23 K and both values decreased with increasing temperature Ts. The experimental results can be qualitatively reproduced by numerical simulation using electromagnetic and thermal fields for PFM. The flux dynamics and the heat generation/propagation in the MgB2 bulk during PFM were in clear contrast with those in REBaCuO superconducting bulks because of the large thermal conductivity, small specific heat, and narrow temperature margin against the transition temperature Tc.

The Use of an

In many superconducting applications the feasibility of a persistent mode operation is a prerequisite of acceptance of the superconducting solution. Regarding the High Temperature Superconducting oxides, this persistency is obtained only for the bulks and, when realized for large samples as for the BSCCO-2212 material, allows the transport of very limited superconducting currents, with densities of the order of some kA/cm2. Instead we were able to maintain in a MgB, ring a persistent current corresponding to several tens of kA/cm2. Designing large superconducting systems, based either on wires or on the bulk materials, as the magnets for MRI or for magnetic separation it will be important to realize persistent superconducting joints between the parts, allowing to sustain the high currents needed for these power applications. In the framework of the MgB2 material development and using the liquid Mg infiltration technique, we have explored the possibility to join wires with bulks and bulks between themselves. The persistency of the joints has been verified by the measurement of the levitation forces of the superconducting system in presence of a permanent magnet and by direct transport current measurements. How to apply the joining technique to the magnet manufacturing is presented.

\hbox{MgB}_{2}

This study refers to Rare-Earth elements (RE = Ce, Sm, Gd) doping of MgB2 bulk materials. The synthesis of doped MgB2 has been performed by Reactive Liquid Mg Infiltration process (Mg-RLI): the RE additives, in the form either of pure metallic powder or of borides powder, have been mixed with the starting boron powder. The RE dopants could operate through two different doping mechanisms: the substitution of Mg ions in the MgB2 crystal lattice or the growth of intergrain boride phases of nanometric size that modify the grain boundaries of the MgB2 polycrystals. The structure of the resulting RE-doped MgB2 has been characterized by X-ray diffraction and SEM/EDS analysis. The effects of this kind of doping on the high magnetic field properties of MgB2 have been measured by magnetization loops.

Hollow Cylinder and Pulse Magnetized (RE)BCO Bulk for Magnetic Levitation Applications

The brittleness of the MgB2 wires prevents performing a cable architecture with very small bending radius, similar to NbTi Rutherford cables. Consequently, it is important to reduce the thickness of the superconducting materials in order to increase its bending strain limit. In this context, thin Ni sheathed MgB2 hollow wires, having wall thickness of the order of 20 μm and length of the order of 100 m, have been produced by the Reactive Mg-Liquid Infiltration process (Mg-RLI). The latest hollow monofilament made by Mg-RLI shows very high transport properties, having an engineering critical current density at 4.2 K, 3 T of 730 A/mm2. In this wire the small thickness of the MgB2 corona allows a twisting with pass of the order of a few cm and fulfils the thermal stability criterion for MgB2. We have demonstrated that cables made by such braided or twisted wires may be conveniently reinforced, by embedding the wires in a molten Mg bath, so that the Ni sheath is almost completely dissolved and the wires are clad by an eutectic Mg10%at Ni alloy. This alloyed matrix is structurally well connected to the MgB2 material, with minimal thermal and electrical resistance at the interface. A cable prototype based on this metallic composite has been prepared with 21 mono-core MgB2 wires, twisted around a central hole, which may be useful for cooling purposes. This cable design may be applied as current leads or in short bus-bars, for high current supply.

Pulsed Field Magnetization of Large MgB2 Bulk Fabricated by Reactive Liquid Mg Infiltration

Stacks of commercial high-temperature superconducting tape can be cut and soldered together to form slabs of a large range of shapes and sizes. They are most interesting for magnetic levitation applications due to the flexibility of geometry, allowing them to be created in large thin slabs suitable for planar rotary magnetic bearings and linear maglev bearings. In this paper, the axial levitation force was measured between a field cooled slab of 30 × 30 mm and a 25-mm-diameter rare-earth permanent magnet (PM), which produced a cylindrically symmetric field necessary in the context of rotary bearings. The force results were compared with that achieved between the same PM and a larger 43-mm-diameter bulk MgB2 disk, as well as to FEM modeling using the Perfectly Trapped Flux approximation.

Superconducting Joints Between

MgB2 manufacts sintering requires a special attention in order to promote a good grain connectivity. In case of wires production, where application of pressure during sintering is practically unrealistic, the “in-situ” methods give a higher connectivity with respect to the “ex-situ” methods. In order to further improve the in-situ processing technology for the wires, we perform the Mg-RLI process, in which liquid Mg infiltration proceeds from the inner core of the wires to the surrounding B powders. The detailed microstructure analysis on the resulting MgB2 layer of these wires shows the influence of different B powders and different operative conditions of the process, giving explanations about the relationship between powder packing density and impurity phase abundance. In particular we discuss the use, as starting precursor materials, of low-price boron powders.

{\hbox {MgB}}_{2}

We report on the microwave properties of coaxial cavities built by using bulk MgB2 superconductor prepared by reactive liquid Mg infiltration technology. We have assembled a homogeneous cavity by using an outer MgB2 cylinder and an inner MgB2 rod and a hybrid cavity by using an outer copper cylinder and the same MgB2 rod as inner conductor. By the analysis of the resonance curves, in the different resonant modes, we have determined the microwave surface resistance Rs of the MgB2 materials as a function of the temperature and the frequency, in the absence of dc magnetic fields. At T=4.2 K and f ≈ 2.5 GHz, by an mw pulsed technique, we have determined the quality factor of the homogeneous cavity as a function of the input power up to a maximum level of about 40 dBm (corresponding to a maximum peak magnetic field of about 100 Oe). Contrary to what occurs in many films, Rs of the MgB2 material used does not exhibit visible variations up to an input power level of about 10 dBm and varies less than a factor of 2 on further increasing the input power of 30 dB.