G Desgardin

Microstructure and transport properties of YBCO zone melted samples processed in a microwave cavity and infra-red furnace

Abstract The unidirectional solidification method was used to prepare well textured YBa 2 Cu 3 O 7− δ (Y123) bars for current lead applications using two different processes. The first one consists in using the Melt Textured Growth method (MTG) in a classical furnace, applying an axial thermal gradient ( G ) of about 60°C/cm. In the second process, a microwave cavity working at 2.45 GHz is used, allowing to obtain larger G (about 260°C/cm). The different values of G corresponding to the two processes were considered to study the effect of G on the Y123 growth rate, microstructure, crystal orientation and transport properties. Thus, for both processes, the maximum pulling rate which permits a continuous growth of Y123 was determined. Microstructure of samples was studied, using polarised optical microscopy for each process. Moreover, the microstructure was correlated with the transport properties of textured bars, using pulse current with the four-point technique. Although the crystal orientation was not perfect ( a – b planes are not aligned with the bar axis), high values of self field J c (higher than 20 kA/cm 2 ) along several centimetres were measured with a high reproducibility for samples presenting single-grain features independently on the texturing process. However, when the pulling rate increases ( R >2 mm/h), the measured J c values were higher for samples obtained using the microwave melt process than for samples processed under lower thermal gradient. The difference in the term of J c is well correlated with the different microstructure.

Texturing of high-Tc superconductors

Most of the potential applications of the high-temperature superconductors require large critical currents, much higher than those obtained using a classical sintering process. This paper reviews different processes and corresponding recent results in the field of texturing high-Tc superconductors in order to align and develop the planes where the current flows easily. Different melt-textured growth methods are discussed from both theoretical and practical viewpoints, and the limitations associated with the diffusion and interface phenomena are introduced. Influence of the process parameters, as well as the microstructure of the precursors and the use of additives in order to enlarge the single domain size and to optimise the superconducting characteristics is discussed. Mechanical texturing and texturing under a magnetic field, as well as combination of both processes, are also considered. Recent results, which appear to be an important breakthrough for the industrial development because of the large crystallization rates are also mentioned.

Optimization of the texture formation and characterization of large size top-seeded-melt-grown YBCO pellets

Abstract The thermal texture cycle for the top seeding of Ce doped YBCO pellets has been studied and optimised. Several quenches have proved the importance of a presintering step to have a homogeneous distribution of the Y 2 BaCuO 5 particles in the melt. The temperature window, in which a heterogeneous nucleated grain can grow, without being hindered by homogeneous nucleated grains, has been determined. By adjusting the process rate to the crystal growth rate in this window, large well textured single domains of YBa 2 Cu 3 O 7− δ have been obtained, with a high degree of reproducibility. Microstructure and texture homogeneity, as well as superconducting properties have been investigated as a function of the position in the bulk pellet. The results have shown the high quality of the textured sample, and its high homogeneity.

High performance in a bulk melt-textured YBa2Cu3O7- delta ceramic

In the present work a YBa2Cu3O7- delta melt-textured growth (MTG) sample was used to achieve grain orientation in a thermal gradient over a large domain using 211 bars as support. Starting from a YBa2Cu3O7- delta precursor sintered at 950 degrees C, the authors were able to obtain unidirectional orientation over a large area, 22*8*7 mm, using a 20 degrees C cm-1 gradient and a very slow cooling rate, 1 degrees C h-1. The so-obtained microstructure has been characterized. The study of cleaved surfaces using diffraction and Laue patterns shows widespread development of a-b planes parallel to the thermal gradient and to the bar axis. Moreover, microstructural observations point out that the texturization may extend to the 211 support. Polarized optical microscopy observations show large twinning domains all over the sample which confirm orthorhombicity and thus good reoxygenation of the sample. Magnetic and electrical characteristic were investigated. Large magnetic hystereses were obtained, even at 77 K, revealing the presence of strong pinning centres. Transport critical current density higher than 3.5*104 A cm-2 at 77 K in zero field and 2.6*104 A cm-2 at 77 K in a 7 T magnetic field were measured.

Optimization of calcination conditions on the Bi-2223 kinetic formation and grain size

The effects of the calcination conditions (time, temperature, intermediate milling) on the kinetics of formation of Bi-2223 and on the grain size were studied using a powder precursor synthesized by the polymer matrix method. The samples were characterized by XRD and SEM analysis. The grain size and the kinetics of formation of Bi-2223 strongly depend on the calcination conditions and thus on the phase assemblages formed at the end of the calcination that determines the reactivity during sintering. The higher the calcination temperature is the larger the grain size. We have observed that 24 h of calcination at 820 °C without intermediate milling allows one to obtain 79% of Bi-2223 phase in 60 h of sintering at 835 °C.

Effect of the oxygenation time and of the sample thickness on the levitation force of top seeding melt-processed YBCO

The effect of the oxygenation time on the levitation force of top-seeded melt-textured YBCO domains has been investigated. The existence of an optimal annealing time, depending on the sample volume, has been brought to the fore. Longer annealings lead to a definitive deterioration of samples. A study of the levitation force versus thickness has been performed. The force has shown a monotonic dependence below a critical thickness as a function of the experimental conditions. When the sample thickness exceeds this critical value, the force has reached its maximal value and saturates.

Kinetics study of the Bi-2223 grain growth thickness

Abstract A kinetic study of the Bi-2223 phase formation mechanism has been done at temperatures ranging from 830°C to 865°C and for long sintering times (25–100 h), since the formation of Bi-2223 phase slows down. Above a sintering temperature of 850°C, the Bi-2223 decomposition begins to occur. The reaction order, n, depends on the sintering temperature; from 830°C to 850°C, n is roughly equal to 0.45 and the related mechanism corresponds to the thickening of plates after their edges have impinged. For temperatures higher than 850°C, n decreases to reach 0.13 at 865°C. Using the Johnson–Mehl–Avrami equation, an activation energy of 1437 kJ/mol has been found for temperatures between 835°C and 850°C where the partial melting becomes more significant. This activation energy has been shown to correspond to the thickening of the Bi-2223 grain.

High quality weld of melt textured YBCO using Ag doped YBCO junctions

Abstract The addition of silver to YBa2Cu3O7−δ (Y123) is well known to decrease its decomposition temperature. This study demonstrates the success in welding large Y123 textured samples with Ag doped Y123 junctions. Attempts to weld two melt textured domains of Y123 by a 1 mm thick layer of Ag doped Y123 are reported. The crucial importance of the initial Y211 excess content in the spacer material is pointed out. Microstructural observations, trapped field measurements and transport critical current density measurements prove that for a given Y211 amount a very high joint quality can be obtained. A transport critical current density as high as 10 kA/cm2 is registered through the junction at 77 K and 5 T when the applied field is parallel to the (a,b) planes.

A new way of welding YBa2Cu3O7-δ bulk textured domains

A welding technique has been employed to join two textured bulk domains of YBa2Cu3O7-δ (Y123) by a spacer layer of textured silver-doped Y123. The influence of the composition of the textured domains and that of the spacer layer thickness on the quality of the junction have been studied. Trapped field measurements have shown that, by controlling these two parameters, good welding can be reached between the different parts and confirmed the high potential of this welding method.

Optimization of sintering conditions on the Bi-2223 formation and grain size

Phases equilibrium between Bi-2201, Bi-2212 and Bi-2223 phases has been established depending on the sintering time and the sintering temperature. Starting with calcined powder (820°C/24 h), optimization of sintering conditions (temperature, time and intermediate milling) has been achieved. Three sintering steps are necessary to obtain high Bi-2223 phase content. The sintering conditions can be described as follows: 100 h at 850°C, first intermediate milling, 25 h at 850°C, second intermediate milling and 25 h at 835°C. The final Bi-2223 phase content reaches 93% with a resulting grain size between 2 and 3 μm.