A study of YBa2(Cu1?xMex)3O6 + δ (Me = Al, Co, Fe) electrophysical and electrochemical properties

The structure, thermal expansion coefficient, and electroconductivity of YBa₂(Cu_1−x Al _x )₃O_6+δ (x = 0.0–0.9) were studied at 20 to 900°C in air. The most conducting compositions of YBa₂(Cu_1−x Me _x )₃O_6+δ (Me = Al, Co, Fe) were determined. The electrochemical activity of electrodes with the most conducting compositions of YBa₂(Cu_1−x Me _x )₃O_6+δ (Me = Al, Co, Fe) was studied in a wide polarization range in the contact with 0.9ZrO₂ + 0.1Y₂O₃ solid electrolyte in air at the temperatures of 700 to 900°C. Original Russian Text ? V.K. Gil’derman, I.D. Remez, 2009, published in Elektrokhimiya, 2009, Vol. 45, No. 5, pp. 612–615. Published by report at IX Conference “Fundamental Problems of Solid State Ionics”, Chernogolovka, 2008.     

Heat capacity of GdBaSr(Cu<Subscript>3−x</Subscript><Emphasis Type="Italic">M</Emphasis><Subscript>x</Subscript>)O<Subscript>7−δ</Subscript> superconductor (<Emphasis Type="Italic">M</Emphasis>=Zn and Ni)

In this study, GdBaSr(Cu_3−x M _x)O_7−δ bulk samples (M=Zn and Ni; 0≤x≤0.1) were prepared via solid-state reaction. Specific heat measurement (measured with thermal relaxation technique using PPMS) shows an obvious specific heat jump around the T _c for GdBaSrCu₃O_7−δ sample as observed in most of the high temperature superconductors. It shifts towards lower temperature with increasing of both Zn and Ni doping contents, whose tendency is similar to the decreasing of T _c. Debye temperature, Θ_D (derived from specific heat measurements) calculated at around 10 K is found to be directly proportional to the T _c.     

Reactions between YBa2Cu3O7−δ and La2O3 and SrCO3

Solid state reactions at 925°C between the high-T _c ceramic superconductor YBa₂Cu₃O_7?δ and La₂O₃ and SrCO₃, respectively, mixed in various molar ratiosr=MeO_n/YBa₂Cu₃O_7?δ, were studied using X-ray powder diffraction and scanning electron microscopy. The reaction between YBa₂Cu₃O_7?δ and La₂O₃ yielded (La_1?xBa_x)₂CuO_4?δ, withx≈0.075?0.10. La_2?xBa_1+xCu₂O_6?δ, withx≈0.2?0.25 and La-doped (Y_1?xLa_x)₂BaCuO₅, withx≈0.10?0.15. Forr=3.0, Y-doped La₂BaCuO₅ resulted also. The reaction between YBa₂Cu₃O_7?δ and SrCO₃ yielded (Sr_1?zBa_z)₂CuO₃, withz≈0.1, Y₂(Ba_1?zSr_z)CuO₅, withz=0.1?0.15, and a nonsuperconducting compound with an approximate composition of Y(Ba_0.5Sr_0.5)₅Cu_3.5O_10±δ. At values ofr≤2.0, unsubstituted YBa₂Cu₃O_7?delta was found in the reaction products.     

Influence of partial substitution of zinc for copper on electronic structures of YBa2Cu3O y

Summary By use of an approximate band-structure treatment based on the EHMO approach, the energy band structures for the Zn-doped superconductor YBa₂Cu_3–x Zn _x O _y were calculated in the present paper and the influence of partial substitution of zinc for copper on the electronic structures for orthorhombic YBa₂Cu₃O_y was studied. From analysis of the band structures and the densities of states for YBa₂Cu_3–x Zn _x O _y , it was demonstrated that the 2D Cu-O planes in the Y-Ba-Cu-O superconducting system have a direct and dominant influence on superconductivity, whereas the role of the 1D Cu-O ribbons and the O(4) atoms is also of some importance.     

Electronic structures of superconductors YBa2Cu3−x M x O y (M=Sn,Ni)

Based on the EHMO approach, the energy band structures for superconductors YBa₂Cu_3–x Sn _x O _y (y>7) and YBa₂Cu_3–x Ni _x O_y (y<7) were calulated in the present paper. The influence of the cation doping at the Cu site in the unit cell and the oxygen content on their electronic structures was studied. The results showed that the cation doping at the Cu site resulted in the great decreases in the bandwidths of the broad anisotropic Cu-O bands and the densities of states. In YBa₂Cu_3–x Sn _x O _y , however, these decreases are compensated by the increase in the oxygen content caused by the Sn-doping, which results in a small change in the total densities of states. For YBa₂Cu_3–x Ni _x O _y , the effect of the doping on its electronic structures in dominant. The Ni-doping, therefore, results in a great change in the electronic structures. In addition, the study on the projected densities of states of the Ni-doped system revealed that the 2D Cu-O planes in the Y-Ba-Cu-O system played a dominant role in superconductivity.     

The effect of calcination conditions on the superconducting properties of Y−Ba−Cu−O powders

Calcination conditions of the precursor powders, i.e. temperature, type of atmosphere and duration, were determined with a view to obtain superconducting powders with the most advantageous physico-chemical properties. Investigated were powders in the Y?Ba?Cu?O system prepared by the sol-gel method. Thermogravimetric examinations of the powders have revealed that the decomposition kinetics of BaCO₃ determines the formation rate of the superconducting YBa₂Cu₃O_7?x (‘123’) phase. It follows from the decomposition kinetics of BaCO₃ that the process is the most intensive in argon, whereas in static air and oxygen it is the slowest. The phase composition analysis (XRD) and low-temperature magnetic susceptibility measurements of the calcinated powders, confirm the above mentioned changes in the decomposition kinetics. The reaction of barium carbonate can be completed if the calcination process is conducted at the temperature of 850°C for 25 h, yielding easily sinterable powders for obtaining single-phase superconducting bulk samples with advantageous functional parameters.     

Copper oxide and barium yttrium cuprate reduction in a methanolic solution of titanium tetrachloride: Conditions and kinetics

The possibility of quantitative CuO reduction to Cu₂O and of oxygen stoichiometry control in YBa₂Cu₃O _x in the nonaqueous medium CH₃OH-TiCl₄-Al at room temperature is demonstrated, and the conditions required for these processes are reported. In this reduction system (with Al as the electron donor), which affords a marked decrease in the reaction temperature, solvated TiCl₄ ions transfer electrons to the oxide surface and CH₃OH provides ligands for titanium chloro complexes. The rate-limiting step of the reaction is diffusion through the product layer. The diffusion coefficients determining the rate of the reaction are estimated to be D = (1.8 ± 0.5) × 10⁻² × exp[−(91.5 ± 3.0)/RT] m²/s for CuO reduction and D = (8.1 ± 3.6) × 10⁻¹⁴ × exp[−(23.6 ± 2.5)/RT] m²/s for YBa₂Cu₃O_6.96 reduction (the activation energies are in kJ/mol). A comparison of these diffusion coefficients with experimental data suggests that the rate of the reaction is determined by copper ion transfer through the Cu₂O layer via rapid diffusion pathways and by oxygen transfer in the basal planes (001) of the YBa₂Cu₃O _x crystallites.     

Application of the van der Pauw Methodto Conductivity Relaxation Experimentson YBa2Cu3O6+δ

Summary.  The van der Pauw method has been applied to conductivity relaxation experiments on YBa₂Cu₃O_6+δ at 600°C in order to determine the chemical diffusion coefficient as a function of the oxygen partial pressure in the surrounding atmosphere (10⁰ > p _{O 2}/bar > 10⁻³). It is shown that the van der Pauw technique is suitable for monitoring the conductivity relaxation when the oxygen diffusion is perpendicular to the direct current flowing through the sample in accordance with the van der Pauw geometry using thin tablets as samples. The oxygen partial pressure is changed stepwise (generally Δlogp _{O 2} ≤ 0.5) by employing appropriate gas mixtures as well as an electrochemical oxygen pump device. An evaluation formula is given for the determination of the chemical diffusion coefficient neglecting slow surface processes. In addition, the electronic conductivity of YBa₂Cu₃O_6+δ has been measured at 600°C as a function of oxygen partial pressure of the ambient atmosphere (10⁰ > p _{O 2}/bar > 10⁻⁵) by means of the van der Pauw method applying the same experimental set-up. Typical values of the chemical diffusion coefficient are in the range of 10⁻⁶ cm²·s⁻¹; the results of the conductivity measurements are interpreted in terms of an appropriate defect model. Received May 30, 2000. Accepted June 8, 2000     

Sol–gel Al2O3/ZrO2 duplex films for protection of a γ-TiAl based alloy against high temperature oxidation

Al₂O₃/ZrO₂ duplex films were deposited on a γ-TiAl based alloy by sol–gel processing starting from aluminum isopropoxide (Al(OC₃H₇)₃) and zirconium (IV) oxychloride octahydrate (ZrOCl₂ · 8H₂O) as raw materials. Isothermal oxidation at 900 and 1,000 °C in 0.1 MPa O₂ and cyclic oxidation at 1,000 °C in air of the coated and uncoated specimens were performed to investigate the effect of the duplex films on the oxidation behavior of the γ-TiAl alloy. The results of the isothermal oxidation tests indicated that the parabolic rate constants of the alloy were decreased due to the applied thin film. Additionally, the present film exhibited a beneficial effect on the cyclic oxidation resistance of the alloy in air. The duplex film could restrain the growth of TiO₂, causing an increase of the Al₂O₃ content in the oxide mixture and thus decreased the oxidation rate.