Abstract: The underlying mechanisms responsible for the giant magnetic or mechanical field-induced-strains in the Ni-Mn-Ga ferromagnetic shape memory alloys are briefly discussed. The fundamental aspect is illustrated by experimental data related to the lattice instability and composition dependence of magnetization alongside literature results. An implementation of Ni-Mn-Ga single crystal as a strain sensor is described.

Abstract: Preliminary results of measurements performed on two bulk MgB2 cylinders intended for use as toroidal magnetic shields with a novel Cryogenic Current Comparator are reported up to 37 K, showing an attenuation factor greater than 106 at 27 K lowering by a factor of 10 at 34 K.

Abstract: The thermal conductivity (TC) of a superconducting bulk materials is a key property in determining its stability at the cryogenic conditions and the superconducting behaviour in applications as the current leads and the resistive current limiters. The thermal conductivity of bulk MgB2 materials obtained by the Reactive Liquid Mg Infiltration technique has been measured, in the temperature range from 4.2K to 300K, on samples having different grain size and derived from Boron powders of different crystallinity and different granulometry. A wide spread of the TC values has been detected for the different samples and a typical maximum of TC at temperatures around Tc has been detected only in the materials derived by very pure and macro-crystalline Boron. On the contrary, the MgB2 materials derived from commercial so called �amorphous � Boron, in which finer crystalline grains are detectable, show very low values of the thermal conductivity without any maxima in the lower temperatures range. The different behaviour can be phenomenologically explained in term of the dependence of the electronic thermal conductivity from the lattice defects and impurities in the samples, with a larger amount of these defects in the case of the microcrystalline samples.

Abstract: The effects of SiC additions on pinning properties of MgB2 bulk samples, obtained by reactive Mg liquid infiltration (RLI) in Boron powders preforms have been analysed by means of electric transport measurements at different applied magnetic fields. For doped and undoped systems the E vs. J characteristics as well as the pinning energy dependence as a function of the current density has been evaluated and analysed in the framework of thermal creep models. The doping-induced pinning centres, very effective in the pinning property improvements, turned out to be of similar nature than those of the undoped sample and can be mostly attributed to SiC reactive insertions inside the MgB2 grains with an increase of lattice disorder induced by the relative reaction products.

Abstract: The Ni51.4Mn28.3Ga20.3 thin films deposited on the alumina ceramics are studied by X-ray powder diffraction and dynamical mechanical analysis (DMA). The substantial temperature vs. film thickness dependencies of interatomic spacing measured in the direction of the film normal are observed in the range of 25�200 oC and 0.1 � 5 μm, respectively. The thermal expansion coefficient of the film in the paramagnetic cubic phase has been determined to be equal to (15±1)·10�6 K�1 for all the films in agreement with the thermal expansion coeffic ient of the bulk material. The thickness-dependent shrinkage of the pseudo-cubic lattice along the film normal direction is attributed to the thermally-induced tensile stressing in the film plane. The thickness dependence of the elastic modulus of submicron films is obtained. It is shown that the internal stresses result in both the thickness dependence of martensitic transformation temperature and the reversib le thermally-induced shape change of the Ni-Mn- Ga/alumina cantilever actuator.

Abstract: DC magnetic fields have been applied to a superconducting hollow MgB2 cylinder of relatively large dimensions (dext/int = 48/44 mm, h=31 mm), produced by reactive Mg liquid infiltration (RLI) process. Such a kind of full dense cylinder has no discontinuity in its shape, at difference with other HTS textured cylindrical objects, and it represents a prototype of larger one that can be applied in many electromechanical applications, like bearing, or pure magnetic application, like shielding. The aim of the experiments is to verify its shielding ability at low fields and the maximum trapped fields, when higher field are applied. The measurements are performed at variable temperatures from 13 K up to Tc. The magnetic field values, measured by Hall probe located at the center of the cylinder, are dependent on the various cooling regimes (either Zero Field Cooling (ZFC) and Field Cooling (FC)) and on the presence of a background external magnetic field. The trapped fields dependence from the temperature and background field is reproduced by a simple model of the current distribution and with an analytical expression of Jc(B,T). It was possible also to describe the transition of the superconducting cylinder to the normal state, induced by flux jumping, on a base of a phenomenological model with two process having different time scale

Abstract: The possibility to manufacture superconducting ${rm MgB}_{2}$ wires, applying the Reactive Liquid Mg Infiltration (RLI) process, was demonstrated several years ago, obtaining wires characterized by hollow cores. The short samples of these wires gave transport critical currents in line with the best Powder in Tube (PIT) wires produced by �in situ� technique. Nevertheless the extension of the RLI technology to long superconducting wires initially was prevented owing to difficulties in controlling the liquid Magnesium dynamics during the reaction. Now we succeed in controlling the reaction and in the manufacturing wires or tubular long manufacts. For tubular manufacts having large diameter, of several millimetres, useful for current leads or relatively short resistive current limiters, the liquid Mg is also supplied continuously during the reaction, to refill completely the core of the tube. For thin wires, with diameter of the order of a millimetre or less, useful for magnets and racetrack winding and other filamentary manufacts, the movement of liquid Mg is counteracted by the liquid metal surface tension. Concerning the superconducting characteristics of the wires, we have explored some key variables effective on the quality of the crystalline ${rm MgB}_{2}$ : they include the choice of the original Boron powders and the addition of doping elements.

Abstract: The issues in the conventional sintering of the MgB2 superconductors have conducted to the discovery of a new way to densify this material. The new process is an �in situ� method that relies on the reactive liquid infiltration (RLI) of liquid Magnesium into Boron powders packed preform. The RLI process allows to obtain highly dense manufacts without the use of hot pressing apparatus and can be applied to the manufacture of large superconducting pieces. One of the peculiarities of the MgB2 superconductivity, that withstand up to 39 K, is represented by the relative insensitiveness of the supercurrent percolation to the orientation of the grain boundaries. This property allows to use polycrystalline material without loosing superconducting performance, granted that a good connectivity between the crystalline grains must be realized, as the RLI process allows to do. The microstructure of the bulk material obtained by RLI shows a variety of morphologies, according to the kind of the used Boron powders and to the process variables. A detailed analysis of the microstructure of the MgB2 obtained by RLI will be presented, as well as its analytical description and the correlation with the superconducting characteristics.

Abstract: The magnetic shielding produced by superconducting tubes was of applicative interest in high energy physics apparatus. Except several attempts with LTS materials, up to now no HTS material has been used for this application, partly due to the low current capability and partly to the poor mechanical properties of the conventional bulk cuprates. An alternative, among the Medium Temperature Superconductor (MTS) material is represented by MgB2 in bulk form. This material can be quite easily produced at high density by the reactive liquid Mg infiltration route, generating tubular bodies of various shape and good mechanical strength. The shielding efficiency in several tubular MgB2 objects has been tested at 4.2 K, applying DC and AC fields and the criticality of the thermal instability due to flux jumps of the superconducting manufacts has been evidentiated. The possibility to shield field larger than 1 T has been demonstrated.

Abstract: Superconducting MgB2 wires, of different diameter and length, have been produced by the Reactive Liquid Mg Infiltration (RLI) process, also implemented with the addition of extra Mg, to completely fill the internal hole created from the normal infiltration process. The actual wires have an Iron external lining and the metallic Mg internally added has a twofold effect: to avoid Mg deficiency during the reaction and to increase the quenching resistance of the wire. The superconducting characteristics of the wires have been measured either at the liquid He and at the liquid Ne temperatures, on small samples cut from meter long wires. The typical critical current density in self field, at 27.1 K, is about 1000 A/mm2, a value of interest for many power applications. The extension of the manufacturing process to longer wire will be discussed as well as the effect of the metallic lining of the wires on the contact resistance and on the current injection length.

Abstract: An alternative and simple manufacturing process is presented to produce high density bulk MgB2 superconducting objects of large dimensions. The process avoids the use of high pressure apparatus and consists in the reactive infiltration of liquid Mg in B powders preforms. With an appropriately designed stainless steel container for the reactants, several manufacts of different shape have been obtained, including tubes, cylinders, rings or disks, of dimensions of the order of the tens of centimeters. The MgB2 material, unlike high temperature oxide superconductors, allows an easy percolation of the supercurrents across the grains boundaries, even if it is in the polycrystalline form. Due to this property, the large MgB2 manufacts obtained by the reactive liquid infiltration present high superconducting characteristics, as demonstrated by transport and magnetic measurements up to 35 K. In particular the magnetic levitation and the magnetic shielding capability appear as the most promising applicative fields in which there is a need for large and homogeneously superconducting bulk pieces. For space applications, the use of MgB2 superconductors may enable a substantial improvement in the compactness and weight of cryogenic systems, with respect to the actual systems based on the liquid He temperatures. Furthermore, with respect to other high temperature oxide superconductors, MgB2, besides the disadvantages of the need of a 20�30 K cryogenic system, presents the advantages of a relative lower density (2.4 g/cm3), higher mechanical strength and easier processability, like that here described. 2005 Elsevier Ltd. All rights reserved.

Abstract: Equal Channel Angular Extrusion (ECAE) was tested on as-cast Bi0.85 Sb0.15 alloys of different purity (4N and 5N), with the aim of eliminating microstructure inhomogeneities (Sb segregation on dendrite arms). The homogeneity degree promoted by hot plastic deformation was monitored by optical microscopy, SEM, X-ray diffraction, differential thermal analysis and electron microprobe analysis. After 6 ECAE passes at 523 K no trace of dendrites was observable at optical microscope; the micro-homogeneity, however, improved with further increasing the number of passes. High density material was obtained, with an average grain size of about 20 μm. In the temperature range of 90 to 300 K, the variation of the Seebeck coefficient of the ECAE processed alloys, prepared from 5N purity starting elements, approaches the behaviour of single crystals of similar composition; at temperatures above 150 K, the figure of merit is higher than in single crystals.

Abstract: The Reactive Liquid Infiltration technique allows to produce very dense MgB2 bulk material, useful as superconductor in many electro-technical devices. The resulting MgB2 product presents a composite structure characterized by large grains, reminiscent of the grain size of the precursor Boron powders, embedded in a matrix of the same material but with smaller grains, of the order of a micron in size. This composite structure of the product presents peculiar mechanical properties characterized by high flexural strength and fracture toughness, as far as a ceramic materials is concerned, and very high hardness. We have measured the room temperature mechanical properties as a function of the main RLI manufacturing parameters of the materials and we attempted a correlation with the grain size of the precursor B powders and the residual content of impurities in the materials. The obtained values compare favourably with that of the same material produced by other manufacturing techniques and with that of the other HTS bulk materials.

Abstract: High stable trapped fields in bulk superconductors can be useful applied in many electrotechnical applications, as i.e. for motor/generators, for MRI systems or for magnetic separation devices. The HTS bulk materials appear as the matter of choice to this purpose and many attempts have been done on melt textured cuprates tiles, at temperatures up to 77K, applying constant magnetic fields or pulsed magnetization methods. Even if the pulse magnetization is far less effective than the DC magnetization in the trapping capability, it appears as the most friendly technique in practical devices. So we have studied the effects of the pulsed technique, applied to bulk MgB2 cylinders that are very promising devices in view of their easy manufacturing in large dimensions. To this purpose we have used a copper solenoid to magnetize the MgB2 cylinder, with both cylinder and solenoid conductively cooled by a cryorefrigerator, in the range of temperatures between 5 and 20 K. This simple cryogenic arrangement can be representative of many more elaborate practical devices. Tailoring the applied pulsed field waveforms, we successfully magnetize cylinders of diameter of the order of 50 mm, reaching more that 0.5 T trapped flux density, in their centre, and the persistent magnetization is not affected by external perturbations. The parameters which regulate the trapping field performances are mainly related to the heat management of the MgB2 cylinder.

Abstract: The behaviour of the bulk superconductors as levitators of Permanent Magnets (PM) has been extensively studied for the textured YBCO HTS material, in the temperature range lower than 77K, obtaining extremely high trapped fields but also experiencing limitations on the mechanical characteristics of the material and on the possibility to produce large objects. Alternatively the bulk MgB2, even if it is superconducting at lower temperatures, has less mechanical problems, when fully densified, and presents stable magnetization in the temperature range between 10 and 30 K. With the Reactive Infiltration technique we have produced dense MgB2 bulk cylinders up e diameter of 60 mm and height of 100 mm. This kind of cylinders can be consider as prototypes of passive magnetic bearing for flywheels or other rotating electrical machines. We have conductively cooled these superconducting cylinder inside an ad hoc constructed cryostat and their levitation forces and stiffness, respect to various arrangements of the PM, have been measured as a function of the temperature below Tc. The very stable characteristics of the induced magnetization have been also recorded and a model of the superconducting behaviour of the cylinders will be presented