Emilio Bellingeri

Direct observation of the Bi,Pb(2223) phase formation inside Ag-sheathed tapes and quantitative secondary phase analysis by means of in situ high-temperature neutron diffraction

Abstract Reactions occurring in Ag-sheathed Bi,Pb(2223) monofilamentary tapes have been studied in situ using high-temperature neutron diffraction. A neutron compatible furnace enabled us to anneal tapes under the same processing conditions as those used for standard high-performance monofilamentary tapes. Scattering data were collected all the time during the thermal treatment and different cooling procedures were compared. The data were analyzed using a full-pattern profile refinement: seven phases were simultaneously refined so that it was possible to carry out a full quantitative analysis of secondary phases during the reaction thermal treatment. The steady conversion of Bi,Pb(2212) into Bi,Pb(2223) was quantified, and the structural transformations occurring in the Bi(2212) prior to its conversion in to Bi(2223) were carefully analyzed. The role of different Pb-rich phases, such as Ca 2 PbO 4 and Pb 3 (Sr,Bi) 3 Ca 2 CuO y , at this stage of the reaction is discussed. Formation of Sr 8 Ca 6 Cu 24 O 41 and Bi(2201) was observed at high temperature and quantitatively analyzed. During slow cooling, we observed a re-growth of Bi(2212) which did not correspond to any decomposition of Bi,Pb(2223). At the same time, the decomposition of Sr 8 Ca 6 Cu 24 O 41 and Bi(2201) was observed and this phenomenon was likely to be related to the increase of the Bi(2212) amount on cooling. Thanks to the neutron diffraction technique, which allows a direct measurement of the absolute amount of crystalline matter inside the sample, it was possible to estimate for the first time the amount of the amorphous matter present. It was found to vary with time and temperature: its non-monotonic evaluation provides a proof of melting and recrystallization occurring over a wide time window at the early stages of the reaction.

New Bi-based high-Tc superconducting phases obtained by low-temperature fluorination

Abstract A new superconducting phase, Bi 2 Sr 2 Ca 2 Cu 3 O 8 F 4 , was obtained by fluorination of standard Bi(2223) at moderate temperatures (250–300°C), using as fluorine source ammonium hydrogen difluoride, NH 4 HF 2 . The presence of fluorine in this new phase was confirmed by energy-dispersive X-ray analysis. X-ray and neutron diffraction experiments furthermore showed that fluorine atoms replace oxygen atoms in the BiO layers in the ratio 2:1. The additional anions form a square-mesh layer between neighboring BiF layers. The incorporation of fluorine increases the crystallographic c -axis parameter by ∼1.8 A ( a =5.409 A, b =5.407 A, c =38.792 A). The CuO 2 layers remain undistorted but the distance from the Cu atom to the apical oxygen atom of the square pyramids is decreased to 2.27 A, with respect to 2.42 A in the original phase. The superconducting transition temperature, determined from magnetic susceptibility measurements, was found to be 75 K. When applied to Bi(2212), the same fluorination process produced the new phase Bi 2 Sr 2 CaCu 2 O 6 F 4 with similar structural features.

Role of the Grain Oxidation in Improving the In-Field Behavior of

The grain size and the grain boundaries play an important role in determining the pinning properties and the current carrying capability of MgB2. Here we present an experimental study on the effect of grain oxidation on the superconducting properties of MgB2 ex-situ Powder-in-Tube (PIT) tapes. We fabricated Fe-sheathed monofilamentary tapes using a variety of powders synthesized in inert (IA) or oxidizing (OA) atmosphere. In order to understand the role of magnesium oxide (MgO) we studied the effect of the presence of the oxidation layer on the field dependence of the critical current density. To this purpose, we developed a process for the powder synthesis and tube filling and sealing in controlled atmosphere to reduce the oxygen contamination; with this process the connectivity of the grains in the final tapes proved to be increased. Then, we introduced a controlled oxidation step. We will show how the presence of the MgO layer enhances the critical current field dependence. High energy X-ray diffraction measurements were performed on the samples, and the grain size of the unsintered and sintered powders in the tapes was measured by means of Scanning Electron Microscopy (SEM). The pinning force was studied, and different mechanisms were found for the samples prepared in IA and OA. The different behavior shown by the two different typologies of tapes was correlated with the presence or absence of the MgO layer.

{\rm MgB}_{2}

We have shown that the superconducting properties of FeSe_0.5 Te_0.5 thin films are strongly dependent on the growth conditions and, in particular, the in-plane lattice constant of the substrate influences the crystallographic lattice parameters of the films, affecting the strain and is responsible for strong enhancements of the critical temperature <i>T</i>_c and for the introduction of different pinning mechanisms. Here we compare the structural, electrical and transport properties of superconducting Fe(Se_0.5, Te_0.5) epitaxial films deposited through pulsed laser ablation on three different substrates namely lanthanum aluminate (LaAlO₃), strontium titanate (SrTiO₃), and calcium fluoride (CaF₂). We analyze in particular the critical current density <i>J</i>_c as a function of the temperature and magnetic field, and its anisotropy, which is related to the different pinning mechanisms in play. The film grown on SrTiO₃ exhibits a higher critical current when the field is perpendicular to the film surface, opposite to what happens in the sample grown on LaAlO₃ due to the presence of extrinsic pinning along the <i>c</i>-axis, while we observe almost no anisotropy on the thin film grown on CaF₂.

Ex-Situ Tapes

Investigations on the Bi,Pb(2223) phase formation confirm a mechanism based on nucleation and growth. The same mechanism was found to hold under various external conditions: a) in pressed samples, b) in Ag sheathed tapes, c) in air or in reduced oxygen partial pressure and d) with or without the presence of Pb. A high temperature neutron diffraction investigation on monofilamentary Bi,Pb(2223) tapes shows that this phase remains stable during the cooling process after reaction. On cooling, an enhancement of the Bi(2212) phase is observed, which occurs at the expense of the other phases, (Sr,Ca)/sub 14/Cu/sub 24/O/sub 41/ and Bi(2201). New tape configurations are presented, in view of a) the reduction of anisotropy (on both, tapes or wires) and b) the reduction of AC losses. These configurations require the use of nonconventional techniques, e.g. two-axis rolling and/or periodic pressing. Using periodic pressing, j/sub c/(77K,0T) values of 35,000 A/cm/sup 2/ have been obtained in Bi,Pb(2223) multifilamentary tapes of lengths >2 m.

Comparison of Superconducting Properties of

Fluorine-doped Tl(1223) superconductors were prepared starting from mixtures of oxides (carbonates) and fluorides (in particular TlF) under moderately high gas pressure (50 bar). The thermodynamics of the phase formation were found to be modified so that the melting point was lowered and the region where a liquid phase coexists enlarged. 90% phase purity was obtained for bulk materials with the composition , prepared by a two-step synthesis including TlF among the starting materials. The critical temperature remained approximately the same as for F-free samples , but a remarkable increase of the irreversibility line was observed below 75 K. The samples showed superconducting properties even for the highest amount of fluorine introduced (about one atom per formula unit). Ag-sheathed tapes were prepared by the conventional PIT method, using ex situ reacted Tl(1223) powders or producing the reaction in situ. Transport critical current densities of about were measured. This value is slightly lower than generally observed for F-free samples, but the magnetic-field dependence was slightly improved. The tape anisotropy could be increased to a factor of 2.5 by performing cycles of mechanical pressing and heating above the formation temperature. The results (in particular neutron scattering diffraction) indicate that, with high-pressure synthesis, F remains in the sample and substitutes for O in the TlO layers in the Tl(1223) phase.

\hbox{FeSe}_{0.5}\hbox{Te}_{0.5}

Fluorine substitution in the Bi(2223), Bi(2212) and Tl(1223) superconducting phases was studied. We obtained superconducting structures, never observed before, of the Bi-based superconductors by a low temperature (200-400 °C) fluorination process. Fluorine substitutes completely the oxygen sites in the Bi layers and additional F atoms are inserted in the structure. As a consequence, changes in the arrangements of cation and anions were induced, especially in the Bi and partially in the Sr layer. F-doped Tl(1223) has been prepared in the same way as Bi(2223) and Bi(2212) (low temperature fluorination), but also starting from precursor containing fluorides of different elements. No significant differences in the crystal structure have been observed between the Tl-based samples with F inclusions and without. The critical temperature (116 K) remains unchanged but a significant increase of the irreversibility field at low temperature was found.

Thin Films Grown on Different Substrates

This paper reports on a method for estimating the average grain size of superconducting nanoparticles through their magnetic properties. The use of SQUID magnetometry to determine the average MgB2 particle size was investigated and the results were compared with those of several different techniques. In particular, the data obtained from zero field cooled magnetization measurements as a function of the temperature were compared with the results obtained by the scanning electron microscopy and Brunauer?Emmett?Teller techniques.The particle magnetization was measured by a commercial SQUID magnetometer in a magnetic field (10?G) and at temperatures ranging from 5 to 50?K by dispersing the powders in a grease medium. The grain size was obtained by fitting the data taking into account the Ginzburg?Landau temperature dependence of the London penetration depth. Variations in typical modeling parameters were explored in order to gain a better picture of the average grain size and the effectiveness of various measurement techniques.We find that it is possible to use the magnetization measurements to determine the average grain size, even though an SEM analysis coupled to the image analysis allows the extraction of more information about the grain-size distribution. Furthermore, a Matlab routine has been developed in order to automatically analyze several SEM images.

Observation of the Bi,Pb(2223) reaction mechanism and alternative ways of producing tapes with new filament configurations

Critical current densities of multifilamentary Ag-sheathed Bi(2223) tapes up to about 35 000 A/cm/sup 2/ have been achieved at 77 K and self field for lengths of several meters using an improved route: periodic pressing. This corresponds to an increase by 30-40% compared to the values obtained for conventionally rolled tapes starting from the same powders. Several pressing steps have been introduced during the anneal instead of the only standard rolling step (based on previous studies performed on both mono- and multifilamentary tapes). In contrast to earlier attempts by pressing techniques, periodic pressing is a practical and scaleable process for the fabrication of long lengths of Bi(2223) conductor as the standard intermediate rolling step. Engineering critical current densities of 8 000 A/cm/sup 2/ have successfully been obtained for tapes with high filling factor (35%).

Synthesis and properties of fluorine-doped Tl(1223): bulk materials and Ag-sheathed tapes

The influence of changes in the nominal composition and preparation conditions on the formation of Tl(1223) under a high isostatic gas pressure was studied. Optimization of the cation substitution allowed to increase the phase purity, whereas substitution of small amounts of oxygen by fluorine decreased the formation temperature of the Tl(1223) phase and extended the temperature range favorable for grain growth. A two-step reaction involving substantial melting produced well shaped, plate-like grains.