C. Varanasi

The effect of PtO2·H2O additions on the Y2BaCuO5 morphology in melt textured YBa2Cu3O7−χ

Abstract YBa 2 Cu 3 O 7−χ (123) pellets doped with 0.5 wt.% of PtO 2 ·H 2 O were melt textured to determine the effects of PtO 2 additions on the microstructure of melt textured 123. The addition of PtO 2 ·H 2 O resulted in both a refinement and a change in the morphology of the Y 2 BaCuO 5 (211) phase. The 211 particles are more acicular in melt textured PtO 2 -doped 123 than in undoped melt textured 123. The 211 particles were found to segregate and form tracks in the melt textured PtO 2 -doped 123 samples.

Microstructure and critical current density of zone melt textured YBa2Cu3O6+x/Y2BaCuO5 with BaSnO3 additions

Additions of BaSnO3 have been made to YBa2Cu3O6+x/Y2BaCuO5 composite wires to examine its potential for improving solidification processing and enhancing pinning. Zone melting has been used to produce texture in extruded YBa2Cu3O6+x (Y‐123) wires with Y2BaCuO5(211) and BaSnO3 additions. BaSnO3 additions markedly improve the magnetization properties over similarly textured Y‐123 or Y‐123+211 wires in low fields, but do not show a noticeable improvement at higher fields (≳2 T). Microstructural effects of BaSnO3 include the growth of much larger Y‐123 grains, and a substantial reduction in size of 211 precipitates. Magnetic critical current densities (Jc) in excess of 1.2×105 A/cm2 in zero field, and 3.0×104 A/cm2 at 1 T are observed.

Extremely fine precipitates and flux pinning in melt-processed YBa2Cu3Ox

Abstract Extremely fine Y 2 BaCuO 5 (211) precipitates (∼ 100 A) have been observed by transmission electron microscopy in melt-processed YBa 2 Cu 3 O x (123). Magnetization measurements have been performed on samples with and without these ultrafine 211 particles. Our results show that there is a strong correlation between the microstructure and the flux pinning behavior. The J c values have significantly increased in a wide range of temperature and field, indicating that these fine 211 precipitates can act as strong flux pinning centers in the 123 system.

Critical current density and microstructure of melt-processed YBa2Cu3Ox with PtO2 additions

Abstract YBa 2 Cu 3 O x (123) with excess Y 2 BaCuO 5 (211) in the molar ratio of 5:1 (123/211) were processed using the SLMG (“solid-liquid melt growth”) technique. The effect of PtO 2 on the microstructure and properties of samples with coarse (∼ 1–10 μm) and fine 211 precipitates (on the order of 100 nm) was studied. PtO 2 leads to a decrease in the total volume of the coarser 211 precipitates and segregation of these 211 particles, forming “tracks” of the precipitates within a domain. Also, the morphology of the coarse 211 phase changed to a long acicular shape with PtO 2 additions, as compared with approximately spherical particles in undoped samples. TEM studies revealed the presence of 211 particles of ∼ 100 nm size in both doped and undoped samples with a density of 10 13 / cm 2 along with the coarser 1–10 μm 211 particles. The defect density was found to be higher in the case of PtO 2 doped samples. The intragrain J c measured on powdered textured specimens was found to be enhanced by the PtO 2 additions, especially at low temperatures, despite the fact that the coarser 211 precipitates were inhomogeneously distributed. Magnetization measurements on textured domains also showed an improvement of J c ( H ‖ c at 1 T) with PtO 2 doping, corroborating the measurements on the powdered samples.

Microstructure and magnetic properties of zone melt textured YBa2Cu3O6+x with BaSnO3 additions

Abstract The effect of BaSnO 3 additions on the microstructure and magnetic properties of zone melt textured YBa 2 Cu 3 O 6+ x (YBCO) wires has been investigated. BaSnO 3 additions improve the magnetization properties over similarly textured YBCO wires in low fields, but do not show a noticeable improvement at higher fields (> 2 T). Microstructural effects of BaSnO 3 include a reduction in size of 211 precipitates and a decrease in the YBCO platelet width. Magnetic critical current densities ( J c ) in excess of 8 × 10 4 A/cm 2 in zero field, and 2.0 × 10 4 A/cm 2 at 1 T are observed. The improvement in low field properties are observed to peak at a YBCO:BaSnO 3 molar ratio of 3:1, and then degrade with additional BaSnO 3 additions.

Factors affecting Y 2 BaCuO 5 precipitate size during melt processing of YBa 2 Cu 3 O 7−x

In the microstructures of melt processed YBa2Cu3O7−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.

Y 2 BaCuO 5 particle coarsening during melt processing of YBa 2 Cu 3 O 7-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 YBa2Cu3O7−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.

Nd substitution in Y/Ba sites in melt processed YBa 2 Cu 3 O 7-d through Nd 2 O 3 additions

YBa2Cu3O7− δ (Y123) samples with excess Nd2O3 and Y2O3 additions in the same molar ratios were melt textured in air. In the Nd-doped samples, in addition to Y ion site substitution, partial substitution into the Ba2+ sites is anticipated because of the similar ionic sizes of Nd3+ and Ba2+. The microstructure, Tc, and magnetic properties of Nd-doped samples were analyzed and compared with undoped Y123 and samples with excess Y2O3. The Nd2O3 additions lead to significant magnetization improvements, likely due to both rare earth- and Ba-site substitution by the doped Nd3+ ions, and to increases in Tc. Y2O3 additions resulted in no marked property enhancement.

Effect of YBa2Cu3O7−x grain size on the nucleation of Y2BaCuO5 during melt texturing

Abstract The effect of YBa 2 Cu 3 O 7− x grain size on the nucleation of Y 2 BaCuO 5 was investigated by melt texturing of pellets with varying 1-2-3 grain size. In the absence of more effective heterogeneous nucleation sites, it was found that the sintered 1-2-3 grain size affects the 2-1-1 particle size in subsequent melt-textured YBa 2 Cu 3 O 7− x , with finer 1-2-3 grain sizes yielding finer 2-1-1.

The chemical stability of BaSnO3 in the melt of YBa2Cu3O6+x during solidification

Abstract The chemical interactions between YBa 2 Cu 3 O 6+ x (YBCO) and BaSnO 3 additions during solidification have been investigated. It is found that a Y-Ba-Sn-O phase forms upon solidification. The formation of this phase removes some yttrium from the YBCO that would otherwise be produced, and results in the formation of additional liquid.