Cristina Bormio-Nunes

Volume magnetostriction and structure of copper mold-cast polycrystalline Fe-Ga alloys

The effect of the volume expansion on the total linear magnetostriction of Fe85Ga15, Fe80Ga20, and Fe71Ga29 mold-cast bulk alloys was investigated by measuring the change in length with applied field longitudinal and perpendicular to the temperature gradient during solidification. In the magnetically saturated state, due to the contribution of the volume expansion, the saturation magnetostriction for all three alloys was about 66 % of the total linear magnetostriction. The magnetostriction is strongly dependent on the direction of the temperature gradient, being larger in this direction. The substitution of Fe by Ga atoms increases the lattice constant and causes a change to the A2 crystal structure, which was confirmed by Mossbauer spectroscopy. The thermal-expansion coefficient increases with Ga content at temperatures between 4.2 and 150K.

Improved magnetostriction of Fe72Ga28 boron doped alloys

In order to investigate the effect of boron additions in the magnetostrictive behavior of Fe72Ga28 alloy, this alloy was doped with 0.5, 1.0, 1.5 and 2.0 at.% of boron. The results have shown that boron additions of 0.5 and 1.0% improve significantly the magnetostriction, the values being about two-fold higher compared to the undoped alloy. Microstructural evaluations suggest that boron additions suppress partially the D03 phase ordering, retaining the α-phase. Furthermore, the increase of lattice parameters of the α and D03 phases indicates that boron enters interstitially in the cubic lattice of these phases. Both effects are considered to be responsible for the increased magnetostriction observed in the (Fe0.72Ga0.28)99.5B0.5 and Fe0.72Ga0.28)99.0B1.0 alloys.

Composition gradient as a source of pinning in Nb-Ti and NbTa-Ti superconductors

Superconducting Nb–Ti and NbTa–Ti wires produced by solid-state diffusion were studied, correlating microstructural and transport properties. The most interesting result is that the diffusion layer sharp composition variation is an important source of pinning in these materials. Within this scenario, we suggest that the source of pinning centers in conventional NbTi wires should be re-examined, since the α-Ti precipitation creates a Nb gradient composition around it. We propose that in conventional superconducting wires there are actually two kinds of strong pinning centers. One is the well-known α-Ti normal interface and the new one now identified is the composition gradient around α-Ti precipitates.

Magnetomechanical behavior of a directly solidified Fe-Al-B alloy

Textural analysis of a Fe–Al–B alloy obtained by directional solidification indicates an average direction of solidification of 〈310〉, close to 〈100〉. The magnetomechanical behavior is characterized by the sensing and actuation sensitivities and respectively. The Fe2B phase does not act as a pinning center against domain wall movement, proved by the achieved reversible thermodynamic condition. This phase also does not degrade the saturation magnetization of the alloy, because it has a high saturation magnetization of 1.5 T. Relatively high sensitivities of 13 kA m−1 MPa−1 were obtained for very low fields of ~4 kA m−1, the same magnitude as that obtained in rare earth based materials, but for much lower fields. The good formability and machining properties of the Fe–Al–B alloy are also of benefit compared to rare earth based materials.

Optimization of Heat Treatment Profiles Applied to Nanometric-Scale

Nb3Sn is one of the most used superconducting materials for applications in high magnetic fields. The improvement of the critical current densities (Jc) is important, and must be analyzed together with the optimization of the flux pinning acting in the material. For Nb3Sn, it is known that the grain boundaries are the most effective pinning centers. However, the introduction of artificial pinning centers (APCs) with different superconducting properties has been proved to be beneficial for Jc. As these APCs are normally in the nanometric-scale, the conventional heat treatment profiles used for Nb3Sn wires cannot be directly applied, leading to excessive grain growth and/or increase of the APCs cross sections. In this work, the heat treatment profiles for Nb3Sn superconductor wires with Cu(Sn) artificial pinning centers in nanometric-scale were analyzed in an attempt to improve Jc . It is described a methodology to optimize the heat treatment profiles in respect to diffusion, reaction and formation of the superconducting phases. Microstructural, transport and magnetic characterization were performed in an attempt to find the pinning mechanisms acting in the samples. It was concluded that the maximum current densities were found when normal phases (due to the introduction of the APCs) are acting as main pinning centers in the global behavior of the Nb3Sn superconducting wire.

{\rm Nb}_{3}{\rm Sn}

The formation of nanostructures projected to act as pinning centers is presented as a highly promising technique for the transport properties optimization of superconductors. However, due to the necessity of nanometric dimensions of these pinning centers, the heat treatment (HT) profiles must be carefully analyzed. The present work describes a methodology to optimize the HT profiles in respect to diffusion, reaction and formation of the superconducting phases. After the HT, samples were removed for microstructural characterization. Measurements of transport properties were performed to analyze the influence of the introduction of artificial pinning centers (APC) on the superconducting phase and to find the flux pinning mechanism acting in these wires. Fitting the volumetric pinning force vs. applied magnetic field (Fp vs. μ0H) curves of transport properties, we could determine the type and influence of flux pinning mechanism acting in the global behavior of the samples. It was concluded that the maximum current densities were obtained when normal phases (due to the introduction of the APCs) are the most efficient pinning centers in the global behavior of the samples. The use of HT with profile 220oC/100h+575oC/50h+650oC/100h was found as the best treatment for these nanostructured superconducting wires.

Wires With Cu-Sn Artificial Pinning Centers

The diffusion between Nb–20%Ta (wt %) and pure Ti is studied at temperatures of 973, 1023, and 1073K, for duration times among 25 and 121h in an artificial pinning center (APC) wire composed of a Ti core surrounded by a Nb–20%Ta layer. The produced diffusion layer is a ternary alloy with superconducting properties, such as critical field Bc2 and critical current density JC, which intrinsically depend on the layer composition. Measurements of layer morphology and composition were performed, and the results show a preferential diffusion of Nb and Ta into Ti. There is a slight diffusion of Ti into Nb through grain boundaries. The presence of Ta also slows down the diffusion of Nb in Ti if compared to the couple formed by pure Nb and Ti. Regarding the mechanical properties of the composite wire, the use of lower temperatures to form the ternary phase is desirable in order to avoid a larger portion of the diffusion layer rich in Ti that favorites α‐Ti precipitations that are detrimental to the wire ductility. ...

Influence of the introduction and formation of artificial pinning centers on the transport properties of nanostructured Nb3Sn superconducting wires

Since the discovery of Nb3Sn superconductors many efforts have been expended to improve the transport properties in these materials. In this work, the heat treatment profiles for Nb3Sn superconductor wires with Cu(Sn) artificial pinning centers (APCs) with nanometric-scale sizes were analyzed in an attempt to improve the critical current densities and upper critical magnetic field. The methodology to optimize the heat treatment profiles in respect to the diffusion, reaction and formation of the superconducting phases is described. Microstructural characterization, transport and magnetic measurements were performed in an attempt to relate the microstructure to the pinning mechanisms acting in the samples. It was concluded that the maximum current densities occur due to normal phases (APCs) that act as the main pinning centers in the global behavior of the Nb3Sn superconducting wire. The APC technique was shown to be very powerful because it permitted mixing of the pinning mechanism. This achievement was not possible in other studies in Nb3Sn wires reported up to now.

Diffusion studies and critical current in superconducting Nb–Ti–Ta artificial pinning center wire

This paper reports magnetic, magnetostrictive, and piezomagnetic experimental results performed on a pure iron and a Fe-B alloy, and associated modeling. Results allow a better understanding of the role of Fe2B phase in Fe-Al-B alloys.

Heat treatment influence on the superconducting properties of nanometric-scale Nb3Sn wires with Cu–Sn artificial pinning centers

Fe100?xTix alloys (x = 10, 15, 20) were studied with respect to their microstructure and magnetostriction. Depending on heat treatment temperature and composition, the sample retained either the ?-phase (A2 structure) or the ?-phase plus the TiFe2 Laves phase (C14 structure). The saturation magnetostriction measured at 238?K is negative, about ?11?ppm. However, for fields up to 0.4?T the magnetostriction is barely zero, a very interesting result. High values of magnetostriction are of interest for applications mainly in sensors and actuators, but zero magnetostriction is also a remarkable property, desirable for many applications such as electric transformers and fluxgate sensor cores. Therefore, the Fe100?xTix (x < 20?at%) are an attractive option to be considered for these applications.