Top-Seeded Melt-Growth of YBa2Cus3Ox Crystals for Neutron Diffraction Studies

We have grown cubic centimeter-size crystals of YBa₂Cu₃O _x suitable for neutron studies, by a top-seeded melt-growth technique. Growth conditions were optimized with an eye toward maximizing phase purity. It was found that the addition of 2% Y₂BaCuO₅ and 0.5% Pt (by mass) were required to prevent melt loss and to obtain the highest crystallinity. A neutron diffraction study on a mosaic of six such crystals found that the final Y₂BaCuO₅ concentration was 5%, while other impurity phases comprised less than 1% by volume. The oxygen content was set to x = 6.5, the crystals were detwinned, and then carefully annealed to give the well-ordered ortho-II phase. The neutron study determined that 70% of the mosaic's volume was in the majority orthorhomic domain. The neutron (006) and (110) rocking curve widths were 1° per crystal and 2.2° for the mosaic, and the oxygen chain correlation lengths were >100 Å in the a- and b-directions and 50 Å in the c-direction.     

Phase Transformations in the Y2Cu2O5–BaCuO2 System

The sequence of phase transformations in the Y₂Cu₂O₅–BaCuO₂ pseudobinary system was studied during heating and cooling in the range 1170–1310 K. The results demonstrate that the reaction zone in BaCuO₂/Y₂Cu₂O₅ diffusion couples consists of BaCuO₂/YBa₂Cu₃O_{7 –} /Y₂BaCuO₅/Y₂O₃/Y₂Cu₂O₅ layers, corresponding to the sequence of chemical changes during YBa₂Cu₃O_{7 –} crystallization between 1170 and 1220 K. In the range 1260–1310 K, BaCu₂O₂ is formed. During cooling of a Y₂Cu₂O₅ + 4.3BaCuO₂ mixture, YBa₂Cu₃O_{7 –} crystallizes in a wide temperature range, between 1240 and 1190 K. The process depends on the presence of BaCu₂O₂ on the surface of Y₂BaCuO₅ grains in the high-temperature solution and the oxygen supply from the gas phase.     

Preparation of YBa2Cu3O7–x high-Tcceramic superconductors from melt

Fabrication of high-T _c ceramic superconductor in the system Y₂O₃-BaO-CuO by melting a mixture of component oxides has been investigated. The compositions of the resulting specimens and the effects of heat treatment have been investigated. It was determined that molten material was composed of phases including BaCuO₂, CuO, Y₂O₃, and Y₂BaCuO₅. A subsequent heat treatment in air produced a nominal amount of the high-T _c phase, while heat treatment in an O₂ atmosphere resulted in a significantly large percentage of the superconducting phase.     

Low-temperature specific heat of YBa2Cu3O7, YBa2Cu3O6, Y2BaCuO5, YBa3Cu2O7, BaCuO2, and CuO

We have measured the low-temperature specific heat (1.3T20 K) and the dc magnetic susceptibility (100T250 K) of eight samples of the high-T _c superconductor Y _x Ba_3–x Cu₃O_7– (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa₂Cu₃O₆₊. We have also performed specific heat measurements on the possible impurity phases: YBa₃Cu₂O₇, Y₂BaCuO₅, CuO, and BaCuO_2+x . The superconducting samples all have a nonzero, sample-dependent linear term * and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (1%) of BaCuO_2+x impurity phase in the measured samples. This is evidenced by the correlation between * and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.     

Phase Transformations in the Systems Y2BaCuO5–“Ba3Cu5O8” and Y2BaCuO5–BaCuO2

Data are presented on the sequence of phase transformations leading to the formation of YBa₂Cu₃O_{7 –} textured ceramics and single crystals in the systems Y₂BaCuO₅–Ba₃Cu₅O₈ and Y₂BaCuO₅–BaCuO₂. During cooling in the Y₂BaCuO₅–BaCuO₂ system, YBa₂Cu₃O_{7 –} crystallization in the range 1260–1210 K occurs through the intermediate phase YBa₄Cu₃O_{9 –} , without an additional oxygen source. In the Y₂BaCuO₅–Ba₃Cu₅O₈ system between 1250 and 1210 K, YBa₂Cu₃O_{7 –} crystallization is accompanied by oxygen absorption.     

Standard Gibbs' energies of formation of BaCuO2, Y2Cu2O5 and Y2BaCuO5

The Gibbs' energies of formation of BaCuO₂, Y₂Cu₂O₅ and Y₂BaCuO₅ from component oxides have been measured using solid state galvanic cells incorporating CaF₂ as the solid electrolyte under pure oxygen at a pressure of 1.01×10⁵ Pa BaO + CuO BaCuO₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=–63.4–0.0525T(K) Y₂O₃ + 2CuO Y₂Cu₂O₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=18.47–0.0219T(K) Y₂O₃ + BaO + CuO Y₂BaCuO₅ G _f,ox ^o (± 0.7) (kJ mol^–1)=–72.5–0.0793T(K) Because the superconducting compound YBa₂Cu₃O_7– coexists with any two of the phases CuO, BaCuO₂ and Y₂BaCuO₅, the data on BaCuO₂ and Y₂BaCuO₅ obtained in this study provide the basis for the evaluation of the Gibbs' energy of formation of the 1-2-3 compound at high temperatures.     

Microstructural properties of Pt-doped YBa2Cu3O7−x high-T c superconductor prepared by melting method

We have studied the effect of platinum addition on the superconducting properties of YBa₂Cu₃O_7–x (123) compound and elucidated from the metallurgical point of view the mechanism of formation of the fine dispersion of Y₂BaCuO₅ (211) particles in YBa₂Cu₃O_7–x superconductor prepared by a melting method. In this study, the amounts of BaCuO₂ and CuO-rich phases unreacted during the peritectic reaction were markedly decreased by the 211 powder addition. The 211 particles of Pt-free sintered samples were 8–10 m in size, but in 1 wt% Pt-added samples 211 particles were finely dispersed in the 123 matrix and the size of 211 particle was about 1–2 m. The critical temperature (T _c,zero) of Pt-doped samples was 91.5 K and the transport critical current density (J _c) of Pt-doped samples was much more than 10⁴A cm^–2. The high J _c and fine dispersion of 211 particles of Pt-doped YBa₂Cu₃O_7–x superconductor are attributed to Ba₄CuPt₂O₉ compounds formed during the partial melting, which were considered as nucleation sites of 211 particles, rather than Pt itself.     

BaWO4–BaCuO2–Y2Cu2O5–“1010” Section of the Y2O3–BaO–WO3–CuO System

The phase region of cubic perovskite-like solid solutions (a = 8.28–8.40 Å) in the Y₂O₃–BaO–WO₃–CuO system is outlined, and the phase compatibility diagram of the BaWO₄–BaCuO₂–Y₂Cu₂O₅–1010 (1010 = Y₂WO₆ + Y₂W₃O₁₂) is constructed.     

Preparation of YBa2Cu3O7–x high-T c ceramic superconductors from melt

Fabrication of high-T _c ceramic superconductor in the system Y₂O₃-BaO-CuO by melting a mixture of component oxides has been investigated. The compositions of the resulting specimens and the effects of heat treatment have been investigated. It was determined that molten material was composed of phases including BaCuO₂, CuO, Y₂O₃, and Y₂BaCuO₅. A subsequent heat treatment in air produced a nominal amount of the high-T _c phase, while heat treatment in an O₂ atmosphere resulted in a significantly large percentage of the superconducting phase.     

Electron spin-lattice relaxation in oxide superconductors and fullerides

Using the original technique of measuring very short spin-lattice relaxation times T₁, the electron-spin relaxation has been investigated in a number of HTSC materials and related phases. The relaxation of Gd³⁺ ions in GdBa₂Cu₃O_6+x is shown to be affected by both quasiparticle and antiferromagnetic fluctuations, the latter being increased at lower oxigen content. The pseudo-spin gap of about 200 K is found for both contributions. For Cu²⁺ EPR in YBaCuO, the T₁ values reveal several types of paramagnetic centers including the green phase Y₂BaCuO₅. In the fulleride RbC₆₀, the measuring of T₁ and T₂ yielded additional support for 1D conductivity and allowed to monitor the metal-insulator phase.     

YBa2Cu3O7−x superconductive tapes and wires: a peritectic sintering procedure to improve the quality

YBa₂Cu₃O_7–x tapes were made by a powder-in-binder technique, using polysulphone (PSF) as the organic polymer andN-methyl-2-pyrrolidone (NMP) as the solvent. The suspension is cast onto a glass substrate plate to form the tape (Doctor Blade method) and is consequently immersed into a non-solvent to remove the solvent by phase inversion. In this paper we describe the different steps in an improved peritectic thermal treatment that are necessary to make the green product into a superconductive tape. Three steps are important in this heat treatment. First the polysulphone binder has to be removed as much as possible without reacting with the YBa₂Cu₃O_7–x material. Next the sample has to be sintered into a dense ceramic material. In order to improve the intergranular connections, the sample is partially melted at relative low temperature in vacuum. The sample is subsequently heated up in oxygen to the normal sinter temperature. At this high temperature the YBa₂Cu₃O_7–x phase will be restored by a peritectic reaction of the Y₂BaCuO₅ with a liquid phase. The final step is a two step anneal to ensure the full oxidation of the superconductor.     

Phase relationships in the system Si3N4-Y2O3-SiO2

Examination of compositions in the system Si₃N₄-Y₂O₃-SiO₂ using sintered samples revealed the existence of two regions of melting and three silicon yttrium oxynitride phases. The regions of melting occur at 1600° C at high SiO₂ concentrations (13 mol% Si₃N₄ + 19 mol% Y₂O₃ + 68 mol% SiO₂) and at 1650° C at high Y₂O₃ concentrations (25 mol % Si₃N₄ + 75 mol % Y₂O₃). Two ternary phases 4Y₂O₃ ·SiO₂ ·Si₃N₄ and 10Y₂O₃ ·9SiO₂ ·Si₃N₄ and one binary phase Si₃N₄ ·Y₂O₃ were observed. The 4Y₂O₃ ·SiO₂ ·Si₃N₄ phase has a monoclinic structure (a= 11.038 Å, b=10.076 Å, c=7.552 Å, =108° 40) and appears to be isostructural with silicates of the wohlerite cuspidine series. The 10Y₂O₃ ·9SiO₂ ·Si₃N₄ phase has a hexagonal unit cell (a=7.598 Å c=4.908 Å). Features of the Si₃N₄-Y₂O₃-SiO₂ systems are discussed in terms of the role of Y₂O₃ in the hot-pressing of Si₃N₄, and it is suggested that Y₂O₃ promotes a liquid-phase sintering process which incorporates dissolution and precipitation of Si₃N₄ at the solid-liquid interface.Visiting Research Associate at Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 45433, under Contract No. F33615-73-C-4155 when this work was carried out.     

Preparation of Y0.5[Yb1?xNdx]0.5Ba2Cu3Oz Superconductors Using Nitrate Routes

The chemical solution nitrate route was used to prepare Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase with x=0.1, 0.2 and 0.3. Different calcination temperatures from 870 to 900 °C were used. Both XRD and DSC techniques were utilized in monitoring the Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase and the non-superconducting impurity phases such as BaCuO₅, CuO and RE₂BaCuO₅. These two techniques were used to study the effect of impurity phases on the decomposition temperature of Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase. The value of x, the Nd amount, and the calcination temperature both influenced the type and number of impurity phases, which resulted in changing the eutectic and peritectic reaction temperatures.     

Formation of metastable phases in flame- and plasma-prepared alumina

The formation of metastable phases in plasma- and flame-prepared alumina particles is examined in terms of the classical nucleation theory, rate of transformation of metastable to stable forms, and the thermal history of the particles during solidification. It is suggested that homogeneous nucleation of the solidification of liquid droplets at considerable undercooling results in the formation of-Al₂O₃ rather than-Al₂O₃ because of its lower critical free energy for nucleation. The phase finally observed depends upon the thermal history of the particles during evolution of the heat of fusion and upon the kinetics of the transformation of the nucleating phase to the stable phase. This means that the cooling rate of the particles is relatively unimportant and under the conditions existing in flames and plasmas, metastable alumina will be formed on solidification. The metastable form will be retained on cooling particles less than approximately 10 m diameter, but particles larger than this may transform to-Al₂O₃ during the solidification exotherm     

Growth of YBa2Cu3O7 – δ Single Crystals in “Ba3Cu5O8”/Y2BaCuO5 and (“Ba3Cu5O8” + 0.2BaCuO2)/Y2BaCuO5 Diffusion Couples

YBa₂Cu₃O_{7 –} single crystals were grown in Ba₃Cu₅O₈/Y₂BaCuO₅ and (Ba₃Cu₅O₈ + 0.2BaCuO₂)/Y₂BaCuO₅ diffusion couples at temperatures between 1170 and 1270 K under optimized conditions. Copper nonstoichiometry was shown to have a significant effect on the superconducting properties of YBa₂Cu₃O_{7 –} crystals subjected to thermal cycling.     

Processing of bulk YBa2Cu3O7−δ ceramics prior to peritectic solidification

The influence of the physical and chemical properties of precursor green bodies on the properties of fully melt-processed YBa₂Cu₃O_7– ceramic (YBCO) have been investigated. Cold isostatic pressing has been found to allow better control of the size and distribution of Y₂BaCuO₅ phase inclusions in the melt-processed ceramic than a combination of die pressing and sintering. Astudy of the loss of liquid from YBCO during partial melting has revealed that the total percentage weight loss is sensitive to both the heating rate and proportion of excess 211 phase and is maximum at a temperature corresponding to the peak of the differential thermal analysis partial melting endotherm. Densification and expansion processes in the sample, which compete during melting, have been investigated in detail. The temperature where the expansion rate is maximum has been found to coincide with the maximum rate of oxygen desorption by the sample. The expansion process terminates on completion of partial melting whereas the densification process, which is dominated by the volume proportion of liquid, continues but at a reduced rate. The use of dense green bodies and an optimum heat-treatment process has been found to be essential for the fabrication of large-grain melt-processed YBCO if a fine distribution of 211 phase inclusions and the homogeneity and shape of the sample are to be retained.     

HIP-sinteredβ- and mixedα-β sialons densified with Y2O3 and La2O3 additions

Various fully dense sialon materials sintered by the glass-encapsulated hot isostatic pressing technique were synthesized using Y₂O₃ and/or La₂O₃ as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y₂O₃/La₂O₃:100/0, 75/25, 50/50, 25/75, 0/100. The samples were sintered at two different temperatures, 1550 and 1825° C. At the lower temperature, unreacted-Si₃N₄ was present in the samples in addition to-sialon and secondary phases. The samples sintered at 1825° C showed that yttrium but not lanthanum favoured-sialon formation. The amount of intergranular phase increased by about 50% when Y₂O₃ was replaced by La₂O₃. The La-sialon ceramics have as good an indentation fracture toughness as the Y-sialon ceramics, about 5 MPam^–1/2, but the Vicker's hardness is slightly lower, being 1400 kg mm^–2 at a 98 N load.     

Effects of Barium Titanate and Titania Additions on the Microstructure and Critical Current Density of Yttrium Barium Cuprate Ceramics

Fine-particle BaTiO₃ and TiO₂ additions are shown to have a significant effect on the structure, microstructure, and superconducting properties of YBa₂Cu₃O_{7 –} ceramics prepared via partial melting. The ranges of solid solutions are determined. The additions are found to enhance the critical current density by a factor of 1.5–2.5. X-ray microanalysis of individual grains in the doped ceramics indicates an increased concentration of small inclusions, presumably Y₂BaCuO₅, embedded in superconducting grains and acting as pinning sites.     

Preparation of YBCO Superconducting Thick Films on MgO Substrates by Modified Melt Growth Process

Superconducting thick films (200–300 m) of YBCO have been fabricated successfully on MgO substrates by a new approach. The precursor powders (YBa₂Cu₃O_7– (Y123) + 0.1 mol Y₂BaCuO₅(Y211)) are placed between two MgO (10 mm×10 mm) slices to form sandwich structure. The YBCO thick films have been obtained from the precursors by modified melt growth process. Resistance measurements of YBCO thick films show T _con of 87.3 K and T of 4.5 K. Estimates using hysteresis loops and the Bean model give a value of 2.78×10³ A/cm² (77 K, 0T) for the critical current density. The observations of scanning electron microscopy (SEM) and X-ray diffraction (XRD) patterns show the supercoducting Y123 phase matrix containing discrete Y211 inclusions.