C Harnois

EBSD study on YBCO textured bulk samples: correlation between crystal growth and 'microtexture'

This work describes an electron backscattered diffraction (EBSD) study of the perovskite-derived structures YBa2Cu3O7−δ .A fter having pointed out the difficulties of EBSD analyses in resolving the orientations of these pseudo-cubic structures, various YBaCuO bulk samples are analysed and the correlation between the microstructure, crystal growth and global texture, determined by neutron diffraction, is carried out. Homogeneous ‘microtexture’ with small subdomain misorientation of 12 ◦ are measured for YBCO top seeding melt textured growth (TSMTG) samples. YBCO perforated samples also exhibit misoriented subdomains, giving rise to a heterogeneous ‘microtexture’ correlated to the YBCO growth front and to the pattern used for the perforating. (Some figures in this article are in colour only in the electronic version)

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.

Attempts and success in texturing large single domains of cerium and silver doped YBa2Cu3Oy

As silver is known to improve the mechanical properties of the brittle superconducting oxide YBa2Cu3O7−δ (Y123), studies are performed to texture large single domains of silver doped Y123. Here we report on the attempts realized to texture silver+cerium doped Y123. It is shown that, as silver decreases the peritectic decomposition temperature of cerium doped Y123 to approximately 960 °C, it becomes possible to texture samples with a maximal temperature of the thermal cycle of 1000 °C, which is interesting in terms of energy saving. However, it is very difficult, even impossible, to grow domains larger than 1 cm in diameter. Several thermal treatments have indeed been used, such as slow cooling ramps and different temperature dwells, without any success. The increase of the melting temperature and the use of a melt textured NdBa2Cu3Oy seed was proven to be efficient in solving this size limitation problem and a 30 mm diameter single domain was textured.

Joining of YBCO textured domains: a comparison between the multi-seeding and the welding techniques

Abstract The multi-seeding and welding techniques are employed to join top seeded melt textured domains of YBa 2 Cu 3 O 7−δ along the a - or b -direction. Silver doped Y123 is used as a welding compound. In the case of the dual seeded sample, micro-structural features and trapped field maps reveal that cracks existence and large non-superconducting agglomerations at the frontier between the two domains lead to a bad junction quality. On the contrary, the results obtained on the welded sample show that the spacer material has effectively grown from the textured domains and that the junction is clean enough to allow superconducting current to flow from one domain to the other.

Ca doping of TSMTG-YBa2Cu3O7-δ/Y2BaCuO5 composites

Abstract Calcium doped YBa 2 Cu 3 O 7− δ /Y 2 BaCuO 5 bulk samples have been synthesised by the top-seeding-melt-texture growth (TSMTG) process up to 1 wt.% of CaCO 3 . Calcium additions up to 0.25 wt.% of CaCO 3 do not change the decomposition temperature whereas additions of 1 wt.% of CaCO 3 lead to an increase of the decomposition temperature of about 10 °C. This difference is not important enough to change the thermal cycle used to process YBa 2 Cu 3 O 7− δ undoped bulk samples. Microstructure studies show that no precursor or secondary phases containing calcium are present in the samples. Energy dispersive spectroscopy analyses have shown that Ca is present both in the YBa 2 Cu 3 O 7− δ matrix and the Y 2 BaCuO 5 particles. The position of the calcium atoms in the superconducting matrix is discussed. The calcium doped samples present slightly depressed critical temperatures but drastically depressed critical current densities.

Enhancement of the Critical Current Densities and Trapped Flux of Gd-Ba-Cu-O Bulk HTS Doped With Magnetic Particles

For bulk HTS rotating machines, enhancement of the trapped flux is a crucial task to achieve practical applications with high torque density. The increase of critical current density Jc using artificial pinning centers is an efficient technique for the enhancement of the flux trapping properties. We attempted to enhance both Jc and the trapped flux in bulk HTS with magnetic/ferromagnetic particles additions. Fe-B-Si-Nb-Cr-Cu amorphous alloy, Fe2O3 and CoO particles were introduced into the Gd123 matrix. The melt growth of the single-domain bulks with different magnetic particles was performed in air. Enhancement of the critical current density Jc at 77 K was derived in the bulks with Fe2O3 and Fe-B-Si-Nb-Cr-Cu additions, while the superconducting transition temperature of 93 K was not degraded significantly. The experiment of the magnetic flux trapping was then conducted under static magnetic field magnetization with liquid nitrogen cooling. In the bulk with 0.4 mol% of Fe-B-Si-Nb-Cr-Cu, the integrated trapped flux exceeds over 35% compared to the one without magnetic particle addition. On the other hand, the addition of CoO particles resulted in a reduction of both Jc and trapped magnetic flux. Present results indicate that the introduction of magnetic particles gives a significant effect to the flux pinning performance.

Shaping of melt textured samples for fault current limiters

Resistive fault current limiters require long length of superconducting materials with relatively low sections. Meander is now a frequently used geometry for this type of application. Nevertheless, the shaping of a textured sample is a quite delicate operation because of its brittle character. Cracks are often observed in the final meander and reduce the life time of these samples. Recently, we have presented a new process that allows overcoming this problem. Sintered pellets are drilled to adopt the chosen geometry. Then, the texturing process is performed. By this way, no large mechanical stresses are applied to the fragile textured samples. Here, the idea is to reproduce the meander pattern. Holes performed do not cross the whole sample thickness but alternatively emerge from top or bottom of the pellet. Slices extracted from the textured samples face like meanders. Different geometries have been tested with holes emerging perpendicular to the (ab) planes or parallel to the (ab) planes. The annealing time under oxygen flow has been studied to fit the required critical current densities values, which were evaluated through transport measurements at 77 K. SEM studies allow comparing both configurations and in particular the crack existence. The quality of the textured material around the holes was found to be not deteriorated compared to the bulk microstructure. Finally, the influence of the holes on the texture quality was studied by neutron diffraction.

Relations between microstructure and superconducting properties of Ce+Sn doped YBCO

Abstract Pellets of cerium plus tin doped YBa 2 Cu 3 O 7–δ (Y123) were textured using the top seeding melt texturing growth method. Amounts of SnO 2 varying from 0.25 to 1.00 wt.% were added to the 0.5 wt.% CeO 2 doped compound. The critical transition temperature, which is 90.5 K for cerium alone doping, recovers gradually the pure Y123 value of 92 K with tin addition. For the lowest tin amount studied, the bean critical current density is enhanced compared to the cerium alone doping, reaching more than 90 kA/cm 2 in self field and 50 kA/cm 2 under 1 T at 77 K. SEM studies have revealed the existence of sub micron size particles containing Y, Ba, Cu, Ce, Sn, and O. Their number increases with tin addition. TEM observations coupled with EDX analyses have shown that their composition largely varies from one particle to another and include impurities such as Sr or Zr. Twin density was also calculated and related to the observed properties of the doped materials.

Bismuth doping of top-seeding melt-texture-grown YBa2Cu3O7−δ ceramics

We have investigated the effects of Bi2O3 additions, up to 1 wt% of Bi2O3, on the superconducting properties and microstructures of top-seeding-melt-texture-growth-processed YBa2Cu3O7−δ ceramics. Differential thermal analysis (DTA) measurements show that these additions do not change the decomposition temperature of the mixtures. Energy dispersive x-ray spectroscopy (EDS) studies of the microstructures after texturation have shown that bismuth additions have no effect on the size of the Y211 particles and do not hinder the action of CeO2 on the inhibition of the coarsening of these particles. However, secondary phase particles, presenting no particular shape, and containing both cerium and bismuth, have been pointed out even for samples with the lowest content of bismuth. The higher the added amount of bismuth, the more numerous the particles are. The composition of these small particles (about 2 µm) evolves with the amount of bismuth added to the starting composition; as the added bismuth content increases, the amount of bismuth increases in the secondary phase particles whereas the amount of cerium is lowered. The critical current densities (77 K, self-field) present an optimum value (67 000 A cm−2) for an addition of 0.125 wt% of Bi2O3. A subsequent annealing treatment (430–300 °C, 100 h) leads to self-field values of 88 000 A cm−2. Bi-doped samples synthesized without the usual pre-sintering step exhibit an optimum value (81 000 A cm−2) for a lower doping rate of 0.0325 wt% of Bi2O3 and the subsequent annealing treatment (430–300 °C, 100 h) leads to self-field values of 95 000 A cm−2.