P. S. Baldonado

Variation of oxygen content and crystal chemistry of YBa4Cu3O8.5+δ

Abstract All phases discovered so far in the Y–Ba–Cu–O system have layered (2-dimensional) structures with the exception of YBa4Cu3O8.5+δ (Y143), which has a cubic (3-dimensional) structure. In an attempt to understand the crystal chemistry of the Y143 phase and to make it superconductive, the Y143 compound has been treated at 450°C in oxygen and subsequently annealed at several different temperatures in a nitrogen atmosphere. The samples have been characterized by X-ray diffraction, thermogravimetric analysis (TGA), iodometric titration, and SQUID magnetometer measurements. The Y143 phase was found to be nonsuperconductive for all annealing conditions. The oxygen content was found to decrease with increasing annealing temperature in two steps, corresponding to the loss of oxygen from two different lattice sites. The lattice parameter decreased during the first step and increased during the second step with increasing annealing temperature or with decreasing oxygen content. Rietveld refinement of X-ray powder diffraction data showed that the O3 and O4 sites, which are geometrically identical within the crystal lattice, have an equal occupancy of 51.5% in samples not annealed in N2. However, when samples were annealed in N2, an oxygen redistribution from the O4 site to the O3 site occurred during the low temperature (

Crystal structure and chemistry of four new RBa4Cu3O8.5+δ (R=Ho, Er, Tm and Yb) compounds

Abstract Four new RBa 4 Cu 3 O 8.5+ δ (R143) compounds, where R=Ho, Er, Tm, and Yb, were synthesized from precursors Ho 2 O 3 , Er 2 O 3 , Tm 2 O 3 , Yb 2 O 3 , BaO 2 , and CuO at 1000°C in an oxygen atmosphere. The oxygen stoichiometric value δ was found to be 0.50 for Ho143, 0.53 for Er143, 0.79 for Tm 143 and 1.01 for Yb143 by iodometric titration. Rietveld refinement of X-ray powder diffraction data showed that all the compounds belong to the space group Pm3. The compounds have a cubic unit cell with lattice parameters of 8.09485±0.00007 A for Ho143, 8.08236±0.00005 A for Er143, 8.07670±0.00008 A for Tm143, and 8.06198±0.00005 A for Yb143. SQUID measurements indicated that none of the compounds was superconducting above 5 K.

Powder diffraction data of SmBa 4 Cu 3 O 8.5+δ

Rietveld analysis of X-ray powder diffraction data was performed on SmBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}, which was synthesized from precursors Sm{sub 2}O{sub 3}, BaO{sub 2}, and CuO at 1000{degree}C in an oxygen atmosphere. SmBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} has a cubic perovskite-related structure that is isostructural with YBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}, and a doubled perovskite unit cell parameter of 8.17790{plus_minus}0.00004{Angstrom}. {copyright} {ital 1997 International Center for Diffraction Data.}

Powder diffraction data of SmBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}

Rietveld analysis of X-ray powder diffraction data was performed on SmBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}, which was synthesized from precursors Sm{sub 2}O{sub 3}, BaO{sub 2}, and CuO at 1000{degree}C in an oxygen atmosphere. SmBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} has a cubic perovskite-related structure that is isostructural with YBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}, and a doubled perovskite unit cell parameter of 8.17790{plus_minus}0.00004{Angstrom}. {copyright} {ital 1997 International Center for Diffraction Data.}

Compressive anneal processing (CAP) of Bi2223 superconducting tapes

The conventional processing of oxide powder-in-tube Bi2223 superconductor tapes involves iterative cycles of mechanical deformation to develop texture and reduce porosity, followed by thermal annealing to react components, relieve stress and heal cracks. However, texture and deformation hardening and cracking may limit the ability to obtain the desired grain texture through deformation alone. In addition, desintering of the oxide core occurs during annealing. We have developed a compressive anneal processing (CAP) technique in which a continuous uniaxial pressure is applied during the annealing. The new process is found to promote texture formation, enhance reaction kinetics and prevent desintering and bloating. Our initial results show that the CAP technique significantly improves the critical current density.

Isothermal melt processing of Bi-2212 tapes

Isothermal melt processing (IMP) has been used to produce phase-pure, high critical current density (J/sub c/) Bi-2212 tapes at temperatures as low as 780/spl deg/C, Bi-2212 tapes processed by IMP have yielded J/sub c/ values up to 250 kA/cm/sup 2/ (I/sub c/=345 A) at liquid helium temperatures. Small test coils have been produced with J/sub c/ values up to 150 kA/cm/sup 2/. The effects of the oxidation rate, processing temperature, and time spent in the partial melt on the superconducting properties and microstructure were investigated. Optimal superconducting properties result from the interplay of these parameters such that the grain size of the Bi-2212 phase in the polycrystalline core is allowed to coarsen and align itself with the silver sheath to form a well connected superconductor. Deviations from optimal conditions result in either too little coarsening to form a well aligned structure or instabilities in the coarsening process that lead to the formation of large secondary phases which disrupt connectivity.

Synthesis and crystal chemistry of the new compounds GdBa4Cu3O8.5+δ and DyBa4Cu3O8.5+δ

Two new compounds, GdBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Gd143) and DyBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Dy143), were synthesized from precursors Gd{sub 2}O{sub 3}, Dy{sub 2}O{sub 3}, BaO{sub 2}, and CuO at 1000h{degree}C in an oxygen atmosphere. The oxygen stoichiometric value {delta} was found to be 0.68 for Gd143 and 0.6 for Dy143 by iodometric titration. Rietveld refinement of x-ray powder diffraction data showed that Gd143 belongs to the space group {ital Pm}3 while Dy143 belongs to the space group {ital P}23. The two space groups, {ital Pm}3 and {ital P}23, are very similar. Their main difference is that {ital P}23 does not have the inversion symmetry of {ital Pm}3. Both compounds have a cubic unit cell with a lattice parameter of 8.16528thinsp{plus_minus}thinsp0.00006thinsp{Angstrom} for Gd143 and 8.10807thinsp{plus_minus}thinsp0.00010thinsp{Angstrom} for Dy143. Superconducting quantum interference device (SQUID) measurement indicated that neither compound was superconductive down to 5 K. {copyright} {ital 1999 Materials Research Society.}

Isothermal melt processed Bi-2212/Ag tapes containing MgO and Al2O3 additions

The development of the superconducting phase in melt-processed Bi-2212 tapes depends on controlling the development of solid phases in the partial melt and the conversion of the partial melt into a highly-aligned superconducting phase. MgO and Al2O3 additions have been used with isothermal melt processing (IMP) for the grain refinement of phases in the partial melt and the prevention of secondary phase growth during solidification. Controlled solidification with reduced oxidation rates also significantly increased the transport properties of the tapes with or without the oxide additions. The optimal oxidation rate was found to depend upon the processing temperature. Critical current densities in excess of 100 kA cm−2 were obtained in tapes melt-processed below 800 °C.

Synthesis and crystal chemistry of the new compounds GdBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} and DyBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}

Two new compounds, GdBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Gd143) and DyBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Dy143), were synthesized from precursors Gd{sub 2}O{sub 3}, Dy{sub 2}O{sub 3}, BaO{sub 2}, and CuO at 1000h{degree}C in an oxygen atmosphere. The oxygen stoichiometric value {delta} was found to be 0.68 for Gd143 and 0.6 for Dy143 by iodometric titration. Rietveld refinement of x-ray powder diffraction data showed that Gd143 belongs to the space group {ital Pm}3 while Dy143 belongs to the space group {ital P}23. The two space groups, {ital Pm}3 and {ital P}23, are very similar. Their main difference is that {ital P}23 does not have the inversion symmetry of {ital Pm}3. Both compounds have a cubic unit cell with a lattice parameter of 8.16528thinsp{plus_minus}thinsp0.00006thinsp{Angstrom} for Gd143 and 8.10807thinsp{plus_minus}thinsp0.00010thinsp{Angstrom} for Dy143. Superconducting quantum interference device (SQUID) measurement indicated that neither compound was superconductive down to 5 K. {copyright} {ital 1999 Materials Research Society.}

Synthesis and crystal chemistry of the new compounds GdBa 4 Cu 3 O 8.5+δ and DyBa 4 Cu 3 O 8.5+δ

Two new compounds, GdBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Gd143) and DyBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Dy143), were synthesized from precursors Gd{sub 2}O{sub 3}, Dy{sub 2}O{sub 3}, BaO{sub 2}, and CuO at 1000h{degree}C in an oxygen atmosphere. The oxygen stoichiometric value {delta} was found to be 0.68 for Gd143 and 0.6 for Dy143 by iodometric titration. Rietveld refinement of x-ray powder diffraction data showed that Gd143 belongs to the space group {ital Pm}3 while Dy143 belongs to the space group {ital P}23. The two space groups, {ital Pm}3 and {ital P}23, are very similar. Their main difference is that {ital P}23 does not have the inversion symmetry of {ital Pm}3. Both compounds have a cubic unit cell with a lattice parameter of 8.16528thinsp{plus_minus}thinsp0.00006thinsp{Angstrom} for Gd143 and 8.10807thinsp{plus_minus}thinsp0.00010thinsp{Angstrom} for Dy143. Superconducting quantum interference device (SQUID) measurement indicated that neither compound was superconductive down to 5 K. {copyright} {ital 1999 Materials Research Society.}