Luis A. Angurel

Growth rate effects on thin Bi2Sr2CaCu2O8+δ textured rods

Abstract Cylindrical polycrystalline textured Bi-2212 samples of lengths up to 10 cm and 1 mm of diameter have been grown using a Laser Float Zone (LFZ) melting technique. In order to improve their transport critical current density ( J c ) at 77 K, the influence of growth rate on their microstructure has been analysed. The final transport properties of the textured materials are determined by the balance between better grain alignment and larger grain size, that takes place at the lowest growth rates, and the simultaneous appearance of cracks. The potential of LFZ grown BSCCO rods for the development of current leads at low fields is suggested. Optimum transport properties correspond to thin rods obtained using intermediate growth rates of 15 to 30 mm/h, with J c (77 K) values up to 5500 A/cm 2 in the self field. These also exhibit minimum resistivity values above T c and irreversibility lines shifted towards higher temperatures and fields.

Study of Ag and Cu/MgB2 powder-in-tube composite wires fabricated by in situ reaction at low temperatures

Superconducting Ag or Cu sheathed MgB2 wires have been fabricated by conventional powder-in-tube techniques. The in situ reactions of Mg and B powders with atomic proportions of 1:2 and 1.33:2 inside Ag or Cu tubes at low temperatures have been explored. An active alloying of Mg with Ag or Cu sheaths at temperatures below 660 °C has been observed. For Ag sheaths, there is a strong diffusion of Mg, which does not allow the formation of large amounts of MgB2 at the core. For Cu sheaths, the diffusion of Mg is stopped by the formation of a MgCu2 layer around the superconducting core and an appropriate excess of Mg leads to superconducting cores with Tc ≈ 39 K and good Jc values. Thus, the development of Cu/MgB2 composite wires is possible.

Correlation of normal and superconducting transport properties on textured Bi-2212 ceramic thin rods

The electric and thermal properties well above and below Tc of Bi-2212 textured ceramics have been correlated through a careful analysis of the microstructure and the transport measurements. Thin rods with the same Bi-2122 stoichiometry and textured by a laser floating zone technique have been studied with that aim. By changing the growth parameters, it has been possible to produce strong changes in microstructure and critical current density, Jc, with small variations in the thermal conductivity. The existence of phase and composition gradients across the thin rods, which explains the variations of Tc, makes the relation difficult between the normal state resistivity and Jc (77 K). A simple qualitative analysis that takes into account the observed microstructure has been developed to correlate the electric transport properties in the normal and in the superconducting states.

Numerical and Experimental Analysis of Normal Zone Propagation on 2G HTS Wires

In this paper we analyse numerically and experimentally the quench behavior of Cu-stabilized second-generation high temperature superconducting (2G HTS) wires at self-field and in adiabatic conditions. The electric field profiles along the superconductor after applying an energy pulse to the conductor have been obtained together with the parameters characterizing the quench: minimum quench energy (MQE) and the normal zone propagation velocity (NZPV). The analysis has been performed at different temperatures, T, between 72 K and 77 K, and at different applied currents, I/Ic(T) <; 1, Ic being the critical current. Our analysis shows that the numerical results obtained by finite element method (FEM) are closer to the experimental MQE values when the contribution of the thermal mass of the heater is taken into account.

Thermal stability analysis of YBCO-coated conductors subject to over-currents

The thermal stability of superconducting YBCO-coated conductors subject to over-currents are analysed. We have studied the effect of DC and AC over-current pulses in Cu-stabilized and non-stabilized coated conductors by measuring the electric field and temperature profiles of these conductors immersed in liquid nitrogen. Current pulses of short duration of about 90 ms and long duration of a few seconds were applied to the samples. Three different cooling regimes of liquid nitrogen—convection, nucleate boiling and film boiling—were observed and their influence on the recovery time of superconductivity in the coated conductors after the over-current pulses has been analysed. We have studied the recovery behaviour under two different conditions, in which the current was set to zero and to the operating current after the current pulses. These experiments simulated the conditions during an over-current situation in different electric power applications with special attention given to the behaviour of these coated conductors acting as in-fault current limiters.

Processing of textured BSCCO superconductors by laser-induced directional solidification

High-temperature superconductors based on BSSCO are interesting for applications in current lead systems required for superconducting magnets working at cryogenic temperatures as well as to facilitate the development of cryocooled turnkey units. The two BSCCO phases considered for applications at or below 77 K, the 110 K 2223 and the 90 K 2212, differ substantially in processing conditions. Some of these differences are presented and discussed in this paper, especially in connection with melt processing methods. Results obtained with the preparation of these phases using the laser float zone melting method are presented in order to analyse some of the most important differences between 2223 and 2212. Their melting and solidification behaviour is studied on the basis of the resultant microstructure and its relation to the critical currents obtained. The 2212 phase is shown to exhibit improved properties and processing conditions that make it more interesting from the fabrication point of view and for the applications considered here.

Experimental and numerical analysis of quench propagation on MgB2 tapes and pancake coils

In the design and feasibility of electric power applications with MgB2 conductors, thermal conductivity plays an important role. In composite wires and tapes it is mainly determined by the amount of stabilizer (usually copper), while for coils the electric insulation and epoxy binder characteristics are added. In this paper we present results on quench development and propagation on isolated superconducting tapes and small single pancake coils cooled by thermal conduction, for three types of MgB2 tape with different stabilizations. The quench parameters at different temperatures, such as minimum quench energy (MQE), minimum propagation zone (MPZ) and quench propagation velocity (vp), are reported. A complete electrical and thermal characterization of the superconducting tapes and of non-superconducting materials used in the coil manufacture has been made. With these data, numerical computational models using finite element simulations have been performed for isolated wires and single pancake coils, and the results of the relevant quench parameters have been compared with the measured values.

Laser zone melted Bi2Sr2CaCu2O8+δ thick films on (100) MgO substrate

Bi2Sr2CaCu2Ox (Bi-2212) thick films were prepared on (100) MgO substrates by combining both the tape cast and laser zone melting techniques. The final thickness of the textured samples is of the order of 350?400??m. The effect of translation rates on the microstructure and, consequently, on the critical current densities (Jc) of the films has been analysed. Jc values measured in zero magnetic field were 1200?A?cm?2 (Ic = 42?A) at 77?K for samples grown at 15?mm?h?1, using a total laser power of 12?W and a power density of 2?W?mm?2. These are similar to the best results obtained in Bi-2212 thick films processed with other methods.

Stoichiometry variation effect on the superconducting properties of polymer-processed (Bi1-xPbx)2Sr2Ca2Cu3O10 ceramics

Abstract Bismuth family high-temperature superconducting samples with varying initial Pb, Sr and Ca stoichiometries have been prepared using a polymer solution synthesis method. These samples have been characterized by powder X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy and magnetic ac susceptibility. The samples which yield the highest percentage of 2223 phase have in common an excess amount of Pb and a deficiency in Sr. The sample with best intragranular properties has a stoichiometry represented by Bi 1.8 Pb 0.35 Sr 1.87 Ca 2.1 Cu 3 O 10+δ . It is also shown that, in contrast with the intragranular properties, the intergranular properties are irreproducible in almost all the samples studied.

Floating zone Ag doped (Bi1·6Pb0·4)Sr2CaCu2O8+δ textured rods

Abstract In this work, superconducting samples of (Bi1.6Pb0.4)Sr2CaCu2O8+δ +3 wt.%Ag have been studied and compared with Ag free samples, (Bi1.6Pb0.4)Sr2CaCu2O8+δ , and with Ag and Pb free ones, Bi2Sr2CaCu2O8+δ . It has been found that Pb doped samples present a worse texture when compared with the undoped ones, resulting in a drastic change on the microstructure. The results showed that electrical resistivity at room temperature, critical current as well as flexural strength are decreasing for the Pb doped samples. When Ag is added to these Pb doped samples, all the properties are improved even if they do not reach the same values as the undoped ones. On the other hand, it has been found that Pb doped samples showed E–I curves with very high sharpness values on the zone of the superconducting to normal transition, reaching n values (E∼In) as high as 45 at 65 K, for samples with simultaneous Pb and Ag additions.