Effects of processing variables on the Y2BaCuO5 size and magnetic properties of melt-processed YBa2Cu3Ox

Samples of YBa₂Cu₃O_x (123) with excess Y₂BaCuO₅ (211) in the molar ratio of 5:1 ( 123/211) were processed using the “solid liquid melt growth” (SLMG) technique. The effect of hold time above the peritectic on the magnetic properties was examined. Extended hold times above the peritectic during processing degrade the magnetic properties of SLMG processed 123. In SLMG 123, the very fine (>100 nm) 211 particles produced by this processing route are the primarymagnetic flux pinners. Extended hold periods reduce the number and/or coarsen the average size of these fine precipitates, resulting in a reduced magnetization. These results were compared to undoped Y123 processed by the more traditional melt texture growth (MTG). In MTG processing, extended hold times above the peritectic are found to result in improved magnetic behavior because of increased defect densities.     

Dislocations and flux pinning in YBa2Cu3O7

The dislocation structures of bulk textured and epitaxial thin film YBa₂Cu₃O₇ superconductors are examined. Correlations between increases in flux pinning and dislocation densities are noted. A model for flux pinning by individual dislocations is presented. This gives a treatment of strain induced effects and effects of normal state region interactions. It is shown that the values of pinning predicted are in line with experimental observations.     

Y2BaCuO5 particle coarsening during melt processing of YBa2Cu3O7−x

Quenching experiments were carried out to study the coarsening phenomena of Y₂BaCuO₅ (211) particles in liquid (Ba₃Cu₅O₈) which form as a result of the peritectic decomposition of YBa₂Cu₃O_7−x (123) above 1010°C. The morphology of 211 particles was observed to become more acicular as the time of hold increased at 1040°C, showing an anisotropic growth rate of the 211 particles in the liquid. Also, the coarsening rate was found to be increased in the presence of excess added liquid phase (Ba₃Cu₅O₈) in the 123. Externally added 211 particles were found to act as heterogeneous nucleation sites for the peritectically formed 211. The observed cored 211 morphology development in the Er 211 doped, melt textured, Y 123 pellets was explained on the basis of ripening of 211 particles. Reduced coarsening of 211 particles was observed in the presence of 0.5 wt.% Pt doping.     

Factors affecting Y2BaCuO5 precipitate size during melt processing of YBa2Cu3O7−x

In the microstructures of melt processed YBa₂Cu₃O_7−x (123) superconductors, often unconsumed Y₂BaCuO₅ (211) particles are observed. The 211 particle size and distribution depend upon i) processing parameters such as peak temperature, heating rate, residence time above 1010°C, starting 123 grain size, etc., ii) second phase additions, and iii) the processing route employed. 211 particle size control is of primary interest for enhancing 123 flux pinning, and fracture toughness. Factors which determine the 211 particle size are reviewed.     

Magnetic flux pinning in epitaxial YBa2Cu3O7−δ thin films

The influence of microstructure on the critical current density of laser ablated YBa₂Cu₃O_7−δ thin films has been examined. Scanning tunneling microscopy was used to examine the morphologies of YBa₂Cu₃O_7−δ films and the morphology data were then correlated with measurements of the critical current density. The films were found to grow by an island nucleation and growth mechanism. The critical current densities of the films are similar to those of films with screw dislocation growth, indicating that screw dislocation growth is not necessary for good pinning. The data suggest that the critical current density in applied magnetic field may be higher in films with higher densities of growth features.     

Effects of platinum and oxygenation on microstructure in YBa2Cu3O7−δ/Y2BaCuO5 bulk materials

Transmission electron microscopy examinations have been conducted on undoped and PtO₂ doped YBa₂Cu₃O_7−δ, with excess Y₂BaCuO₅ (211) in the molar ratio of 5∶1 (123/211), processing using the solid liquid melt growth technique. Magnetization hysteresis suggests that addition of Pt strongly influences the pinning behavior. Considerable differences in dislocation and stacking fault densities were observed. Dislocation nets and tangles were commonly observed in the Pt doped samples. In both samples, stacking faults were observed near 211 precipitates, interplatelet boundaries, and dislocations. Dislocations appear to be formed during high temperature processing, while stacking faults appear to be generated during the final oxygenation step. The density and distribution of fine precipitates (∼25–100 nm) were comparable in both specimens suggesting that Pt additon affects the size and acicular morphology of only the coarser 211 (∼1–10 μm). It is proposed that the observed increase in J_c due to Pt addtion may be attributed to the increase in defect density rather than fine precipitates.     

Influence of excess yttrium and platinum on the growth behavior of YBa2Cu3O7−δ and Y2BaCuO5

The high temperature (1100°C) coarsening of Y₂BaCuO₅ (211) and subsequent YBa₂Cu₃O_7−δ (123) growth kinetics using melt quenched and 123 precursor powders were examined via quenching. Fine scale excess yttrium addition by sol gel coating was employed up to 20 mol percent. X-ray diffraction identified 211 fraction between 123 (∼30 wt.%) and melt quenched (∼10 wt.%) in the precursor powders. The addition of yttrium was seen to shift the 211 weight fraction to higher levels for the MQ powders. Refinement of the 211 particle size was seen in the presence of Pt but not with yttrium addition. The coarsening behavior of 211 in either powder did not appear to significantly change with excess yttrium addition at 1100°C. Differential thermal analysis showed that the 123 phase solidification temperature increased in the presence of Pt and reduced with yttrium addition up to 10 mol %. Dilatometric measurements showed the influence of yttrium addition on the densification behavior of 123 due to the presence of low temperature liquid phase formation. Directly inserting samples at soak teperatures were seen to significantly alter the 211 weight fraction for 123 powder in contrast to slower thermal heating to temperature. However, this effect was not seen in the case of the melt quenched precursor powder.     

Effects of shock-induced defect density on flux pinning in melt-textured YBa2Cu3O7−δ

To introduce a high density of homogeneously distributed defects in YBa₂Cu₃O_7−δ (Y123), melt textured samples were shock-compacted at pressures up to 12.6 GPa (126 Kbar) at orientations favorable to slip along the basal planes.¹ Shock compressing melt-textured Y123 results in a nearly uniform detect density which is two to three orders of magnitude higher than in unshocked melt-textured material. However, the intergranular critical current density in bulk samples (J_c ^b) decreases by two orders of magnitude in the as-shocked state. This decrease in J_c ^b is attributed to microfractures. However, if the shocked disk is annealed in O₂ then ground, sieved, and magnetically aligned, J_c for H τ c-axis (J_c ^ab) is enhanced two to three times over the unshocked value. This indicates that the increase in dislocations density does increase flux-pinning.     

A new process with a potential to rapidly texture bulk YBa2Cu3Ox superconductor

A new process has been developed to texture bulk YBa₂Cu₃O_x superconductors at temperatures about 100°C below the peritectic temperature. The texture is achieved in this process by directional recrystallization of compacts fabricated from quenched YBCO powders. Isothermal recrystallization of quenched precursors at 890°C for 3 min is found to result in the formation of more than 75% of YBa₂Cu₃O_x phase without any Y₂BaCuO₅. Recrystallization at higher temperatures leads to rapid formation of fine Y₂BaCuO₅ precipitates in addition to YBa₂Cu₃O_x. The formation of YBa₂Cu₃O_x increases with increasing heating rates. Directional recrystallization at 10 mm/h results in a well-textured microstructure with the YBa₂Cu₃Ox grains oriented parallel to the sample length. The magnetic field dependence of the critical current density at 77K of the directionally recrystallized material compares well with that of melt-textured YBCO and is superior to that of magnetically aligned and sintered YBCO.     

Microstructural study of the effect of an excess of Y2BaCuO5 and BaSnO3 doping on the texturing process of YBa2Cu3O7−x bulk superconductors

Melt texture process of YBCO leads to a YBa₂Cu₃O_7−x matrix where Y₂BaCuO₅ particles are observed. The Y₂BaCuO₅ inclusions size and distribution depend upon several parameters: YBa₂Cu₃O_7-x grain size in the presintered, heating rate, dopants. The influence of an excess of Y₂BaCuO₅ and/or BaSnO₃ on these Y₂BaCuO₅ particles are observed in the liquid phase and in the texture domain. According to the dopant used, two kinds of coarsening of Y₂BaCuO₅ can be observed: an isotropic and an anisotropic. The control of the distribution of Y₂BaCuO₅ particle size is of primary interest to improve the efficiency of the MTG process. In particular large cooling rate (5°C/h) during the texture formation could be used by adding Y₂BaCuO₅ + BaSnO₃ to YBa₂Cu₃O_7−x composition.