Etude de la synthèse sous vide d'une poudre pure d'yBa2Cu3O7−δ

The conditions of synthesis of a pure YBa₂Cu₃O_7−δ powder under very low pressure were investigated. In a first part, the decomposition processes of a (Y₂O₃, Ba(NO₃)₂, CuO) mixture are determined using controlled transformation rate thermal analysis (CRTA). Under a 4,5.10⁻⁶ bar pressure, the total decomposition of all precursors is obtained below 470°C. In a second part, the thermal behaviour of YBa₂Cu₃O_7−δ phase is analysed in order to determine its thermodynamic stability under these conditions of low pressure. A decomposition sequence is proposed based on oxygen release analyses and literature data. Conditions of synthesis of a pure YBa₂Cu₃O_7−δ powder (without any chemical impurities) are finally suggested.     

Der chemische Transport von Ni3Ge,Ni5Ge3, Ni(Ge)-Mischkristall,CoSn, Co3Sn2, Cu41Sn11 (δ-Phase), Cu10Sn3 (ζ-Phase) und Cu(Sn)-Mischkristall

Chemical Vapour Transport of Intermetallic Systems. 7. Chemical Transport of Ni₃Ge, Ni₅Ge₃, Ni(Ge)-mixed Crystal, CoSn, Co₃Sn₂, Cu₄₁Sn₁₁ (δ-phase), Cu₁₀Sn₃ (ζ-phase), and Cu(Sn)-mixed Crystals By means of GaI₃ as transport agent some intermetallics in the Ni/Ge- and Co/Sn-system can be prepared by CVT-methods. Using Iodine Cu–Sn-compounds can be deposited in a similiar way.     

Preparation and characterization of superconducting YBa<Subscript>2</Subscript>(Cu<Subscript>1−x</Subscript>Cr<Subscript>x</Subscript>)<Subscript>4</Subscript>O<Subscript>8</Subscript> oxides by thermal analysis

In this study the formation of chromium substituted YBa₂Cu₄O₈ (Y-124) superconductors has been investigated by TG/DTA measurements. The YBa₂(Cu_1−xCr_x)₄O₈ ceramics with nominal compositions of x=0.01, 0.03, 0.05, 0.10 and 0.20 have been prepared by an aqueous sol-gel method using aqueous mixtures of the corresponding metal acetates and nitrates. Homogeneous precursor gels were obtained by complexing metal ions with tartaric acid. To assist the interpretation of the results obtained the synthesis products were additionally characterized by X-ray powder diffraction (XRD) and resistivity measurements. It was determined that doping the YBa₂Cu₄O₈ phase with chromium has a strong effect on the phase purity and superconducting properties of the synthesis products.     

Preparation and characterization of highly densified YBa2Cu3O7-δ Ceramics used for electrochemical oxidation

An appropriate treatment called “Vacuum-Oxygen Cycle” (V.O.C.) has been carried out in order to obtain highly densified YBa₂Cu₃O_7-δceramics. The obtained materials exhibit a homogeneous oxygen distribution and their feature have been characterized using relevant physical measurements. Preliminary electrochemical experiments have been performed and this treatment seems to improve their superconducting properties.     

Cluster calculations on the electronic structures of the high-temperature superconductors Y1Ba2Cu4OX (x=7.5, 8.0, 8.5) and Y1Ba2Cu5O9

Conclusions Cluster calculations show that there is oxygen stabilization in the copper-oxygen chains in Y₁Ba₂Cu₄O₈ on account of the lower excess negative charge on the oxygen and the presence of an additional Cu–O bond together with geminal O...O and Cu...Cu interactions. No basis is found for possible high T_C for YBa₂Cu₄O_3.5 and YBa₂Cu₅O₉. The mechanisms that raise T_C on compressing YBa₂Cu₄O₈ or doping it with calcium are identical and mount to incrase in the state density at the Fermi level because there is stronger interaction between the copper-oxygen plane and the chain on account of the strengthening of the bonds joining them and the bonding involving barium or calcium. Lattice compression also favors this.This study has been performed within the framework of the ANOKONDA project under the State High-Temperature Superconductivity program.Novomoscow Branch, Mendeleev Moscow Chemical Engineering Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 6, pp. 36–44, November–December, 1992     

Mass distribution of cluster ions produced from laser ablation of metal-composite-oxides Y-M-Cu-O (M=Ba,Sr, Ca,Mg)

Laser ablation of YMCuO metal-composite-oxides (M=Ba, Sr, Ca, Mg) in high vacuum produced cluster ions with various sizes and compositions. Mass spectra of the cluster ions were recorded by a home-built time-of-flight (TOF) mass spectrometer and the mass distributions were analyzed by a statistical model. For YBa₂Cu₃O_7?x (YBCO) high-Tc superconducting samples, six series of cluster ions were observed. Replacement of barium in the metal-composite-oxide sample with other alkaline earth metal, such as strontium, calcium, or magnesium, not only removed the superconductivity, but also changed the compositions and distributions of the laser generated cluster ions. For instance, copper was only found in the compositions of cluster ions generated from YBCO sample. From comparison of the experimental results, interactions among components of the metal-composite-oxides and the function of alkaline earth metal in superconducting material were discussed.     

Electronic structure and properties of isolectronically substituted compounds Y1Ba2−m M m Cu3O7 and Y1Ba2−m M m Cu4O8 (M=Be,Mg, Ca,Sr, Ba,Ra)

According to cluster calculations, the electronic structures of compounds based on Y₁Ba₂Cu₃O₇ and Y₁Ba₂Cu₄O₈ with isoelectronically substituted barium have some qualitative distinctions. These compounds behave differently upon barium substitution by other elements due to differences in the character of their highest occupied and lowest unoccupied molecular orbitals. Substitution of barium by radium is expected to lead to an increase in oxygen stability without a significant decrease in the critical temperature of superconduction transition T_s. In order to raise T_s, it is of interest to study the systems YBa_2−m(Be or Mg)_mCu₃O_x and YBa_2−m(Ca, Sr)_mCu₃O_x. On partial substitution of barium by calcium in YBa₂Cu₄O₈, the mechanism of T_s elevation may involve contraction of the forbidden band due to oxygen sublattice distortions in the vicinity of Ba centers. D. I. Mendeleev Russian Chemical Engineering University, Novomoskovsk Branch. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 2, pp. 24–31, March–April, 1994. Translated by O. Kharlamova     

Interface analysis of the system Si/YBa2Cu3O7?x

Summary The influence of different preparation conditions and substrate surface orientations on the superconducting properties of thin YBa₂Cu₃O_7–x (YBCO) films on silicon was studied. Comparative electrical and surface spectroscopic measurements were performed. SAM and SIMS depth profile analysis show an enrichment of barium at the interface between the superconductor and silicon for samples with T_c<76 K. Comparison with XPS data obtained for thin silicon films on YBCO indicates the formation of barium and yttrium silicates at the interface under these conditions.     

XPS depth profiling investigations on La2Zr2O7 layers prepared by chemical solution deposition

La₂Zr₂O₇ (LZO) layers have been recently investigated as potential buffer layers for superconducting YBa₂Cu₃O_7–x coated conductors deposited on Ni tapes. Chemical solution deposition was used for LZO layer preparation. X-ray photoelectron spectroscopy (XPS) depth profiling is demonstrated to be an important method for layer characterization in addition to X-ray diffraction techniques. XPS measurements revealed layers that are homogeneous in depth, very smooth, and have no significant impurities. A slight difference to the nominal La:Zr stoichimetry is discussed in combination with structural defects that are suspected from spectral changes during ion sputtering.     

Ein alternativer Weg zur Darstellung von Bi2Sr2CaCu2Ox,YBa2Cu3O7−δ, und YBa2Cu3−xMxO7−δ (M  Ni,Ag und x ≤ 33 Mol%)

An Alternative Method for Preparation of Bi₂Sr₂CaCu₃O_x, YBa₂Cu₃O_7?δ, and YBa₂Cu_3?xM_xO_7?δ (M ? Ni, Ag and x ≤ 33 Mol%) Oxidation of quenched melts of metals in a well defined argon/oxygen atmosphere has been found as an alternative method to prepare hight purity samples of Bi₂Sr₂CaCu₂O_x without Pb-stabilization. A special equipment for the oxidation of powder samples will be described. By our new method it is also possible to prepare YBa₂Cu₃O_7?δ and derivatives of this compound, where Cu is substituted by high amounts of Ni or Ag.     

Size Dependence of Doping by a Vacancy Formation Reaction in Copper Sulfide Nanocrystals

Doping of nanocrystals (NCs) is a key, yet underexplored, approach for tuning of the electronic properties of semiconductors. An important route for doping of NCs is by vacancy formation. The size and concentration dependence of doping was studied in copper(I) sulfide (Cu₂S) NCs through a redox reaction with iodine molecules (I₂), which formed vacancies accompanied by a localized surface plasmon response. X‐ray spectroscopy and diffraction reveal transformation from Cu₂S to Cu‐depleted phases, along with CuI formation. Greater reaction efficiency was observed for larger NCs. This behavior is attributed to interplay of the vacancy formation energy, which decreases for smaller sized NCs, and the growth of CuI on the NC surface, which is favored on well‐defined facets of larger NCs. This doping process allows tuning of the plasmonic properties of a semiconductor across a wide range of plasmonic frequencies by varying the size of NCs and the concentration of iodine. Controlled vacancy doping of NCs may be used to tune and tailor semiconductors for use in optoelectronic applications.     

Preparation of Fine Spherical Particles of Precursors for Yttrium Barium Cuprate Superconductors via thermal decomposition of triethylene glycol solution

A new synthesis route of fine spherical precursor powders for YBaCuO superconducters via thermal decomposition of triethylene complexes has been developed. YBa₂Cu₃O_7–δ and YBa₂Cu₃O₈ ceramics with sharp superconducting transitions can be obtained in one step from the 1 μm grained precursor powder.     

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.     

Characterization of sol–gel process in the Y–Ba–Cu–O acetate–tartrate system using IR spectroscopy

Systematic investigation of sol–gel process in the Y–Ba–Cu–O acetate–tartrate system was performed using IR spectroscopy. Different values of synthesis parameters (temperature and duration of inorganic polymerization reactions, evaporation temperature and evaporation time during gelation) were used, and significant influence in some cases on superconducting characteristics of the oxide ceramics of composition YBa₂Cu₄O₈ was observed. Characterization by IR spectroscopy and thermogravimetric analysis, revealed the difference of local homogeneity in the precursor gels prepared under different evaporation regimes during gelation process. In addition, the explanation of the possible hydrolysis and condensation reactions in the sol state has been suggested.     

Normal pressure synthesis of (Tl1−yCy)Ba2Ca3Cu4O12−δ superconductor

Single phase (Tl_1−yC_y)Ba₂Ca₃Cu₄O_12−δ (Tl_1−yC_y-1234) (y = 0, 0.25, 0.5 and 0.75) superconductor samples have been prepared by solid state reaction method. The FTIR absorption measurements have confirmed the substitution of carbon at thallium site in the charge reservoir layer, (Tl_1−yC_y)Ba₂O_4−δ. The electron micrographs of these samples have shown that the carbon substitution has improved the grain morphology of Tl_0.75C_0.25-1234 sample. The y = 0.25 was found to be the optimum carbon concentration to achieve higher superconducting transition temperature T_c[0] and improved grain morphology. The superconducting transition temperature of Tl_0.75C_0.25-1234 sample has been increased to 100 K whereas a decrease in the superconducting transition temperature of Tl_1−yC_y-1234 (y = 0.5 and 0.75) samples was observed. However, the magnitude of diamagnetism has been decreased in all the carbon substituted samples.     

Fabrication of YBCO/CeO2/YBCO multilayer films through an all chemical solution deposition approach

In this paper, the growth characteristics and superconducting properties of YBCO/CeO₂/YBCO multilayer films derived from an all chemical solution deposition process layer by layer are investigated. X-ray diffraction results show that the c-axis epitaxial growth feature of the bottom YBa₂Cu₃O_7-δ (YBCO) film was perfectly inherited. Alternating epitaxial growth of YBCO/CeO₂/YBCO multilayer films was confirmed by high resolution transmission electron microscopy in combination with selected area electron diffraction, which will lay the groundwork for preparation of SIS-type Josephson junctions. In addition, nanosized BaCeO₃ particle rather than BaCeO₃ transition layer was observed in the interface between YBCO and CeO₂. The superconductivity measurement results show that the proposal method gives better superconducting properties only for high magnetic field and at low magnetic field it deteriorates the superconducting properties.     

The composition analysis of YBa2Cu3O7-δ or PrBa2Cu3O7-δ thin films and (YBa2Cu3O7-δ/PrBa2Cu3O7-δ)n heterostructures prepared by CVD

Some YBa₂Cu₃O_7-δ films and heterostructures prepared by Chemical Vapor Deposition (CVD) were analyzed in our laboratories by EPMA-EDX or WDX, RBS, SNMS and AES. It was found that in some cases the results of composition analysis can significantly deviate from each other. At least two main reasons for these deviations exist: the different lateral resolution and the application of different reference samples for the calibration. Received: 3 June 1996 / Revised: 14 October 1996 / Accepted: 16 October 1996     

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.     

Copper‐Based Ternary and Quaternary Semiconductor Nanoplates: Templated Synthesis,Characterization, and Photoelectrochemical Properties

Two‐dimensional (2D) copper‐based ternary and quaternary semiconductors are promising building blocks for the construction of efficient solution‐processed photovoltaic devices at low cost. However, the facile synthesis of such 2D nanoplates with well‐defined shape and uniform size remains a challenge. Reported herein is a universal template‐mediated method for preparing copper‐based ternary and quaternary chalcogenide nanoplates, that is, CuInS₂, CuIn_xGa_1−xS₂, and Cu₂ZnSnS₄, by using a pre‐synthesized CuS nanoplate as the starting template. The various synthesized nanoplates are monophasic with uniform thickness and lateral size. As a proof of concept, the Cu₂ZnSnS₄ nanoplates were immobilized on a Mo/glass substrate and used as semiconductor photoelectrode, thus showing stable photoelectrochemical response. The method is general and provides future opportunities for fabrication of cost‐effective photovoltaic devices based on 2D semiconductors.