Thermogravimetric studies of Y1Ba2Cu3O7−x superconductors

The synthesis conditions and oxygen stoichiometry were determined using a thermogravimetric analyser. The kinetics of oxygenation at different temperatures and at different atmospheres were studied. The oxygenation kinetics were found to be sensitive to porosity of the material. The appropriate oxygenation temperature was found to be 400 °C. For oxygenation temperatures below 400 °C, the oxygenation occurs by a nucleation and growth process, whereas for higher temperatures it is diffusion limited.     

The mechanical and structural properties of Y1Ba2Cu3O7−x superconductors

Mechanical losses spectroscopy of two superconducting Y₁Ba₂Cu₃O_7–x samples (T _c=93 K) measured at 5, 10, 20 and 30 MHz revealed four peaks. As the frequency of measurement was increased, the peaks shifted to higher temperatures. The relaxation process was found to have an activation enthalpy of 0.188, 0.092, 0.209 and 0.314 eV with an attempt frequency,f _o, equal to 2.46 × 10¹⁵, 5.95 × 10¹⁰, 1.64 × 10¹⁴, and 7.82 × 10¹³ Hz for P₁, P₂, P₃ and P₄ peaks, respectively. The mechanism responsible for these relaxation peaks is discussed. The infrared spectra of some of these compounds provide evidence for oxygen rearrangement on going from tetragonal to orthorhombic phase. X-ray diffraction analysis shows that these compounds correspond to a single phase which is an orthorhombic perovskite structure.     

Resistive and magnetic properties of Y1−x Zr x Ba2Cu3O7−δ and Y1−2x Zr x Eu x Ba2Cu3O7−δ superconductive compounds

The results of X-rays, electrical resistivities and magnetic studies performed on Y_−x Zr _x Ba₂Cu₃O_t7−δ and Y_1−2x Zr _x Eu _x Ba₂Cu₃O_7−δ superconducting compounds, in the temperature range 4.2–500 K and external fields up to 50 kOe are reported. The presence of zirconium decreases the superconducting transition temperatures,T _c. BelowT _c, the hysteresis loops narrow when the zirconium content increases. From the magnetization curves, the critical current densities were determined. A time dependence of the magnetizations of logarithmic form was found. The flux-pinning energy was estimated to beU ≃0.03 eV. Above the transition temperatures, Y_−x Zr _x Ba₂Cu₃O_t7−δ samples show a Pauli-type paramagnetism, while Y_1−2x Zr _x Eu _x Ba₂Cu₃O_7−δ compounds have, in addition, temperature-dependent contributions to the magnetic susceptibilities. The effective europium moments are ∼3.40 μ_B suggesting that the ion is in (+3) valence state.     

Superconductivity in compound Y1-2x Ce x Ba2+x Cu3O6·5+δ system

Partial substitution of Ce in place of Y has been achieved by taking care of charge compensation and compounds with the general formula Y_1-2 xCe_xBa₂₊ xCu₃O_6·5+δ have been successfully prepared. X-ray powder diffraction analysis reveals that this substitution gives orthorhombic single phase for 0⩽x⩽0·3. TheT _c remains nearly unchanged between 94-90 K.     

Effect of silver in Y1Ba2Cu3O7−x samples

Two techniques for preparing Y-Ba-Cu-O/Ag specimens are presented: (a) solid-state reaction, and (b) normal precursors dissolved in hot concentrated nitric acid and dried in air. The samples are characterized by scanning electron microscopy (SEM), scanning Auger, X-ray diffraction, optical microscopy and electrical resistivity measurements. In comparison with a normal 1-2-3 superconductor it was found that the normalized resistance [R(T)/R(280 K)] decreases for both preparation methods. The material retained a high initial temperature of superconductivity, T _c, for a maximum of 6% w/o Ag₂O during solid-state reaction and 6.5% w/o Ag₂O for samples dissolved in hot concentrated nitric acid and dried in air.     

High-capacity superconducting current leads of Y1Ba2Cu3O7−x

We have fabricated and measured a high-capacity superconducting current lead composed of a Y₁Ba₂Cu₃O_7?x cylinder, 20 cm long and 0.9 cm² cross section. A steady-state, d.c., critical current of 225 A at a temperature of 77 K was measured in this sample, using a voltage criterion of 2×10^?7 V/cm (p = 8×10^?10 ohm-cm). This current was limited by the currentinduced, self magnetic field. To our knowledge this is the largest d.c. critical current so far reported in a Y₁Ba₂Cu₃O_7?x sample and demonstrates the possibility of using hightemperature superconducting HTS materials for current leads to low-temperature superconducting LTS magnets or in power distribution systems.     

Phononic and spin excitations in Y1−x Pr x Ba2Cu3O6.9 crystals

The Y_1?x Pr _x Ba₂Cu₃O_6.9 system has been investigated by quantitativeμ-Raman measurements. The magnetic response of the antiferromagnetic ordered Cu spins in the CuO₂ plane has been observed via the two-magnon-Raman scattering. The in-plane exchange energy is determined to 780 cm^?1 in the system withx=1. Phonon behavior as well as magnetic scattering indicate that the band structure of the CuO₂ planes in PrBa₂Cu₃O_6.9 is similar to that in insulating YBa₂Cu₃O₆.     

Extraordinary behaviour of the Y1−x Pr x Ba2Cu3O7−δ system

Investigations of Y_1?x M _x Ba₂Cu₃O_7?δ (M=Ce, Th)c-axis oriented thin film specimens show that the rate of depression ofT _c withx is larger for M=Th, than for M=Ce and Pr, and suggest that Ce, like Th, is tetravalent in this compound. Hall effect measurements on Y_1?x Pr _x Ba₂Cu₃O_7?δ single crystals reveal aT ² dependence of the cotangent of the Hall angle in the normal state and a “negative Hall anomaly” belowT _c in the superconducting state, in agreement with recent reports. Our research shows that the depth, Δ, of the negative Hall signal scales withT/T _c and that the maximum value of Δ decreases linearly withx and vanishes atx≈0.24. Magnetoresistance measurements on Y_1?x Pr _x Ba₂Cu₃O_7?δ single crystals indicate that the irreversibility lineH(T ^*) obeys a universal scaling relation characterized by anm=3/2 power law nearT _c, with a crossover to a more rapid temperature dependence of belowT/T _c ≈0.6, similar to that observed for polycrystalline specimens.     

High energy crystal field excitations in Pr x Y1−x Ba2Cu3O7−δ

We have measured the high energy crystalline electric field transitions in samples of Pr _x Y_1−x Ba₂Cu₃O_7−δ withx ≈ 0·5 andx ≈ 1. The fully Pr-doped samples exhibits three strong transitions with excitation energies of about 65, 85 and 105 meV, plus three more weaker ones at 113, 123 and 132meV. Magnetic intensity is observed in the same energy range in thex=0.5 sample but is largely structureless in character, with only one clear peak at about 109meV. The large number of transitions would seem to indicate a Pr valence of 3+, rather than 4+, but the intrinsically broad character of the excitations is evidence for a dynamically mixed valence state.     

Investigation of Y1Ba2Cu3O6.9 and Er1Ba2Cu3O6.8 during cyclic temperature variations

Gravimetric investigations of the superconductive ceramics Y₁Ba₂Cu₃O_6.9, Y₁Ba₂Cu_2.7Al_0.3O_7.1 and Er₁ Ba₂Cu₃O_6.8 during cycling heating cooling in the interval 293–1323 K at partial pressure of the oxygen 0.02 and 0.2 × 10⁵ Nm^–2 are presented. Intervals of intense weight changes during heating and cooling are determined. Oxygen loss was shown to be the main reason for the weight changes. Structural characteristics and resistivity of the yttrium ceramics in the above-mentioned temperature interval are investigated. Experimental data are discussed within the framework of the models of the quasi-equilibrium and essentially nonequilibrium processes. Binding energy and diffusional parameters of the evolving gas are estimated.     

Effect of orientation of c-axis on Ba4CuPt2O9 compounds of fluorine-doped YBa2Cu3Ox superconductors

An experimental study has been carried out to investigate a synergistic effect of Ba₄CuPt₂O₉ compounds on the critical current density to fluorine-doped YBCO superconductors. The Ba₄CuPt₂O₉ compounds were synthesized by a solid state reaction. It was found that the synthesis temperature of the Ba₄CuPt₂O₉ compounds was over 800 °C, which was higher than that of a paper ever reported. In addition, the influence of the Ba₄CuPt₂O₉ compounds on the critical current density to the YBCO superconductors has been investigated to use two types of Ba₄CuPt₂O₉ compound. One is not heat-treated, i.e., source materials, and the other is heat-treated at 950 °C. It was found that the former affected the orientation of c-axes, but the latter did not have an influence on the orientation to the c-axes, and molested the superconductivity to the samples quenched at temperatures above 850 °C.     

Processing of Y1Ba2Cu3O x Films by Solution Techniques Using Metal-Organic Decomposition

Processing of YBCO films by decomposition of metal trifluoroacetate precursors and its application to the development of coated conductors is investigated. The technique involves preparation of the solution, deposition, and a two-stage heat treatment. A stoichiometric mixture of the acetates of Y, Ba, and Cu and trifluoroacetic acid are used as the starting materials. The glassy residue of the trifluoroacetates of Y, Ba, and Cu formed by drying the above mixture is redispersed in a fixed amount of methanol to provide the starting solution of a given concentration. This solution is then deposited on the substrate surface by the spin coating technique and heat treated in two stages. X-Ray characterization shows that the transformation to 123 is complete in less than 1 h. Pole figure analysis of films deposited on (100) SrTiO₃ and LaAlO₃ single crystal substrates show that these films have a very high degreee of alignment with the substrate both out-of-plane (<0.5°) and in-plane (<1.5°). These chemically derived films were also found to have critical current densities well above 5 × 10⁵ at 77 K for 0.5 μm thick films. The applicability of this technique to the production of thick films of YBCO for the development of coated conductors will be discussed.     

DC magnetization studies of (Pr x Y1−x )Ba2Cu3O7−y

Magnetization was measured on Pr-doped YBa₂Cu₃O_7−y for fields up to 5·5T. The paramagnetic behaviour is correlated to the free ion values of Pr³⁺ and Pr⁴⁺. Hysteresis experiments were done for superconducting composition for fields up to 1100 Gauss. The intragrain critical current density is calculated using Bena’s formula.