Characterization of ferroelectric and superconducting ceramics prepared from precursor carbonates

The advantages of a new semi-wet method for the synthesis of (Ba, Ca)TiO₃ and YBa₂Cu₃O_{7 − y} powders from precursor carbonates are described. The precursor carbonates are prepared using chemical coprecipitation technique under controlled pH conditions to ensure uniform distribution of Ba⁺²/Ca⁺² and Y⁺³/Ba⁺² ions. It is shown that the powders synthesized by this route possess better chemical homogeneity as compared to those obtained by the conventional ceramic technique involving calcination of a mixture of BaCO₃, CaCO₃, TiO₂ and Y₂O₃, BaCO₃, CuO powders. The behaviour of the ferroelectric (Ba, Ca)TiO₃ and the superconducting YBa₂Cu₃O_{7 − y} ceramics prepared from powders obtained by our method are found to be markedly different from those known for the conventionally prepared ceramics.     

Role of Calcium in the Evolution of Superconductivity in a (La2?xRx)Ba2(CayCu4+y)Oz (R = Y, Er, Gd) System

It has been reported that, by adding equal amounts of CaO and CuO to non superconducting La₃Ba₃Cu₆O _z (La-336), a series of superconductors with nominal compositions La₃Ca _y Ba₃Cu_6+y O _z were prepared with maximum T^on _c 80K. Similar studies on addition of CaO and CuO in nonsuperconducting LaBaCu₂O _z (La-112) resulted into superconducting LaCaBaCu₃O _z (La-1113). To date no attempt has been made to synthesize La₂CaBa₂Cu₅O _z (La-2125) superconducting phase by addition of CaO and CuO to non superconducting La₂Ba₂Cu₄O _z (La-224) system. Also no reports are published to study the effect of replacing larger La³⁺-ions (1.01Å) by smaller rare earth ions viz Y³⁺(0.89Å), Er³⁺(0.91Å), Gd³⁺(0.91Å) on the structural and superconducting properties of (La_2–x R _x )Ba₂(Ca _y Cu_4+y )O _z (LRBCaC); 0.0 x 0.5; y=2x system. In this paper, we report the method of synthesis, structural and superconducting property characterization using X-ray diffraction, oxygen content measurements using iodometry, resistivity measurements using d.c. four probe technique and a.c. susceptibility measurements in the temperature range RT to 15K. Also a comparative study, on the evolution of superconducting phase with Ca-concentration for different rare earth substitutions for LRBCaC system in the context of hole doping mechanism, is carried out.     

Enhancement of Pinning Role by Nonstoichiometry in YBa2Cu3O7?y Superconductors

Considerably improved flux pinning and critical current density J c values have been achieved in Y-deficient Y-123 superconductors by directional solidification in air. In comparison with the regular Y-123 composition, Y-deficient one also has an orthorhombic structure and Y-123 main crystal phase remains in it. Whereas with the shortage of Y, Y_1–x Ba₂Cu₃O_7–y can be regarded as (YBa₂Cu₃)_1–x O_7–t(Ba₂Cu₃) _x O _t–y or (YBa₂Cu₃O_7–z )( _x ^YO _z–y ), so there may develop several kinds of microstructure defects as pinning sites in the system, such as highly dense, fine-scale, and faultlike defects, as well as localized superstructure, which are able to induce the increasing in flux pinning and J c values in higher external magnetic fields. This kind of simple nonstoichiometric route could lead to a commercial technique for flux-pinning enhancement in Y-123 bulk materials.     

Structure and Superconducting Properties of Cd0.8Ba2(Y0.7Ca0.4)Cu3.5Oy

A new Cd-containing superconductor with nominal composition ofCd_0.8Ba₂(Y_0.7Ca_0.4)Cu_3.5O _y was synthesized and investigated. The obtained Cd and Ca-doped 123 phase exhibits an orthorhombic (T _c=80 K) or tetragonal (T _c=65 K) modification depending on the reaction atmosphere. It was shown that the combined Cd and Ca substitution facilitates the 123 phase formation. The results of the EDX analysis, as well as the comparison of the obtained lattice parameters with those of undoped, Cd-doped, and Ca-doped 123 have shown that both Cd and Ca enter the 123 phase and form a new Cd–Ba–Y–Ca–Cu–O superconducting compound.     

Effects of substituting mercury for yttrium,barium and copper on the superconducting transition temperature of YBa2Cu3Oδ

Several superconducting oxide samples such as YBa₂Cu₃O (denoted AO), Y_1–xHg_xBa₂Cu₃O (x=0.5–0.8, Al), YBa_2–xHg_xCu₃O (x=0.3–1.5, A2) and YBa₂Cu_3–xHg_xO (x=0.5, A3) were prepared by mixing Y₂0₃, BaCO₃, copper acetate (instead of CuO) and HgO. The superconducting transition temperatures,T _c, in all the mercury-doped samples were found to be more than 90 K, with the highestT _c = 100 K observed in A2. Results of electrical, magnetic and structural studies of some of these superconducting samples have been reported.     

Simple cubic perovskites as components in R1−yNayBa2Cu3Ox (R=Ce,Pr, Tb)

Simple cubic perovskites derived from RBaO₃ appear in the X-ray diagrams of materials with nominal composition R_1−yNa_yBa₂Cu₃O_x with R = Ce, Pr, Tb. Y_1−yBi_yBa₂Cu₃O_x also contains this structure type. Materials are not superconducting at 77 K. The absence of YBa₂Cu₃O_x type structures in these cases is explained on thermodynamic arguments considering the high stability of phases R⁴⁺Ba²⁺O₃ which dominate the phase diagram.     

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.     

Peculiar Hole Doping by Ca Substitution in Cuprate Superconductors

The doping effect of Ca substitution in Y_0.7Ca_0.3Ba₂Cu₃O _y has been studied for y=6.9 and 6 by the x-ray absorption near edge spectroscopy (XANES). While Ca doping leads to an obvious increase in hole concentration in the CuO₂ planes in Y_0.7Ca_0.3Ba₂Cu₃O_6.9, XANES indicates absence of this doping effect in Y_0.7Ca_0.3Ba₂Cu₃O₆. These two contradict results suggest that the doping effect by Ca substitution could be sensitive to the oxygen content when y6.0. Comparisons with others' works are made.     

Effect of Ca Substitution on the Superconductivity of La2.5Y0.5CaBa3(Cu0.88Fe0.12)7Oz

The structural and superconducting properties of single-phase La_2.5–y Y_0.5Ca_1+y Ba₃ (Cu_0.88Fe_0.12)₇O _z (LYCaBCuFe) (y= 0.0–1.0) compounds with triple perovskite structure are investigated using X-ray diffraction, resistivity, a.c. susceptibility, and oxygen content measurements. Increasing Ca substitution for La resulted in a decrease in unit cell axes and volume. T _c ^R=0 shows a marginal increase from 31 K to 37 K for y = 0.0–0.21 and thereafter it decreases with increasing y leading to zero T _c ^R=0 at y 0.84. This shows that the suppression of T _c from 80 K to 31 K by Fe doping at x = 0.12 La_2.5Y_0.5CaBa₃(Cu_1–x Fe _x )₇O _z cannot be compensated by appropriate hole doping with Ca in LaYCaBCuFe.     

Synthesis, composition, and structure of superconducting (Y, Ca)Sr, Ba)2Cu2GaO7?δ

The superconducting properties of Y_1–y Ca _y Sr₂Cu₂GaO_7– have been examined and related to the Ca content,y, and the use of annealing treatments at 350 bar oxygen. Superconductivity withT _c up to 41 K was found only for high-pressure-annealed samples, and the structural effects of Ca substitution and high-pressure treatment were examined using powder neutron diffraction. Small but significant changes in Cu-Cu and Cu-O distances were found and suggest that the Cu ions are more highly charged in superconducting samples. Partial substitution of Ba for Sr was found to be possible (up to 20%) to give samples which, after annealing in high-pressure oxygen, were superconducting at temperatures up to 68 K.     

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.     

Superconducting gap in PrxY1?xBa2Cu4O8 and YBa2?ySryCu4O8 probed by infrared phonon self-energies

We report a reflectivity study of thez-polarized TO-phonons of Pr _x Y_1–x Ba₂Cu₄O₈ and YBa_2–y Sr _y Cu₄O₈ alloys in the temperature range 10–300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibration at300 cm^–1 due to the opening of the superconducting gap are found to occur upon crossing the superconducting transition temperatureT _c . Phonon self-energy effects are strongly dependent onT _c , providing evidence for a relative shift of the gap with respect to the energy of phonon.On leave from the Institute for Semiconductor Physics, Ukrainian Academy of Sciences, 252650 Kiev-28, Ukraine.     

Structure and Superconducting Properties of Cd0.8Ba2(Y0.7Ca0.4)Cu3.5Oy

A new Cd-containing superconductor with nominal composition ofCd_0.8Ba₂(Y_0.7Ca_0.4)Cu_3.5O _y was synthesized and investigated. The obtained Cd and Ca-doped 123 phase exhibits an orthorhombic (T _c=80 K) or tetragonal (T _c=65 K) modification depending on the reaction atmosphere. It was shown that the combined Cd and Ca substitution facilitates the 123 phase formation. The results of the EDX analysis, as well as the comparison of the obtained lattice parameters with those of undoped, Cd-doped, and Ca-doped 123 have shown that both Cd and Ca enter the 123 phase and form a new Cd–Ba–Y–Ca–Cu–O superconducting compound.     

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 and properties of superconducting YBa2Cu3O7−x powder by single-step calcining in air

The preparation of orthorhombic YBa₂Cu₃O_7–x powder using only one calcination step is described. Yttrium and copper nitrates and barium carbonate were used as precursor materials. The use of an appropriate precipitant agent leading to a complete coprecipitation of the Y³⁺ and Cu²⁺ cations, promoted the rapid low temperature formation of YBa₂Cu₃O_7–x (900C for 4 h) with no secondary phases. X-ray diffraction patterns of the obtained powder showed the presence of the only one phase with the orthorhombic structure, and lattice parameters ofa=0.3824 nm,b=0.3893 nm,c=1.1676 nm and =1.775 which correspond to a superconducting YBa₂Cu₃O_6.95 phase. Sintering compacted samples at 940C several times led to highly dense ( 96% theoretical density) superconductor bodies, and a value ofT _c as high as 94 K was measured. Although AES depth profiles in fracture surfaces evidenced good grain boundaries of the superconducting YBa₂Cu₃O_6.95 samples, however, theJ _c measured was never greater than 15 A cm^–2, which could be due to a weak-link region between highJ _c grains. Slow degradation of the YBa₂Cu₃O_6.95 ceramics seems to take place by the formation of Ba(OH)₂ and YO(OH) at the superconductor surface in contact with the moisture of the air.     

Investigations of chemical interaction between Bi-based 2212 and (RE)Ba2Cu3O7 high Tc superconducting materials

Composite materials have been synthesized by mixing 90% (or 95%) YBa₂Cu₃O₇ and 10% (or 5%) Bi₂Sr₂Ca₁Cu₂O₈ by weight, and firing at 900°C to promote grain growth by inducing a liquid phase (Bi₂Sr₂Ca₁Cu₂O₈) in the system. The influence of the amount of liquid phase on the X-ray diffraction data and electrical properties is reported. Energy dispersive X-ray (EDX) analyses are also reported. The YBiBa₂O₆ phase is formed during the heat treatment and introduces additional chemical heterogeneities at the grain boundaries. A previously reported 2212-related superconducting phase, Bi₂(Sr,Ba)₂(Ca,Y)Cu₂O_8+y, could also be formed during the synthesis process, and its effect on the electrical resistance versus temperature measurements is discussed. Attempts to substitute RE ions (Dy³⁺, Er³⁺, Ho³⁺) for Y³⁺ in YBiBa₂O₆ have been successful and are reported in an appendix section. X-ray diffraction data are also reported. EDX analyses have been performed specifically for a typical ErBiBa₂O₆ compound and reveal the presence of a new Er₂Ba₄O₇ phase.     

Structural variation and recovery of superconductivity by Ca substitution in Y0.4Pr0.6Bs2?xCaxCu3

Y_0.4Pr_0.6Ba_2–xCa_xCu₃O_7– polycrystalline samples were prepared and investigated. The Ca substitution led to a structural variation toward tetragonal symmetry, possibly due to the structural modification within the Cu(1)O layer. Superconductivity and metallic conduction were recovered whenx0.2, giving one more evidence for the existence of the ion-size effect at the Ba site.     

Long range coupling in YBa2Cu3O7−δ/Y1−xPrxBa2Cu3O7−δ multilayers

YBa₂Cu₃O_7–/(Y_1–xPr_x)Ba₂Cu₃O_7– multilayers have been used to probe coupling through (Y_1–x:Pr_x)Ba₂Cu₃O_7– alloys. We observe that the coupling between ultrathin YBa₂Cu₃O_7– layers, 12 or 24 Å thick, survives through several hundred Å of (Y_1–xPr_x)Ba₂Cu₃O_7– with x=0.4 and 0.55. T_c versus the thickness of the spacer-alloy, and activation energies for flux motion, with fields parallel and perpendicular to the c-axis, have been used to probe this long range coupling. All these experiments point to an unusually large coupling length for these two alloy compositions. In the x=0.55 case this result is particularly surprising since the alloy material display a semiconducting behaviour for this composition. T_c measurements, activation energies, and a study of the vortex dynamics in these coupled multilayers is presented along with new results obtained on a series of multilayers built with a more insulating alloy, x=0.7.     

Dip-Coating of YBa2Cu3O7?δ and YBa2Cu3O7?δ-Ag Films on Polycrystalline Rare-Earth Barium Niobate Substrates

Dip-coating and partial melting technique have been used to fabricate high quality superconducting YBa₂Cu₃O_7– and YBa₂Cu₃O_7–-Ag thick films with T _c(0)=92 K on polycrystalline REBa₂NbO₆ (RE=Pr, Nd, Sm, and Eu) substrates. The superconducting films showed excellent adhesion to the REBa₂NbO₆ substrate. The effect of Ag addition in YBa₂Cu₃O_7– on the current density, microstructure, and crystal orientation of the superconducting films developed on the above substrates have been discussed in detail. Dip-coating technique was found to be one of the easiest method for obtaining good quality superconducting YBa₂Cu₃O_7– thick films with thickness as low as 3 m even on polycrystalline substrates.