The decomposition of the oxalate precursor and the stability and reduction of the YBa2Cu4O8 superconductor studied by TG coupled with FTIR and by XRD

The 124 superconductor YBa₂Cu₄O₈ was prepared from the oxalate precursor Y₂(C₂O₄)₃. ·4BaC₂O₄·8CuC₂O₄·xH₂O at one atmosphere oxygen pressure. In O₂ the precursor decomposes in one step at 300°C and more gradually (300°–600°C) in Ar. The stability of the superconductor is strongly dependent on the gas atmosphere: in O₂ and in air there is no significant weight change as long as the temperature does not exceed 800°C, whereas in a 1% O₂-99%N₂ mixture decomposition starts at about 670°C with the formation of CuO and YBa₂Cu₃O_x withx<7. The reduction of YBa₂Cu₄O₈ in a 5% H₂-95% Ar mixture takes place in at least four major steps with formation of products such as Y₂O₃, BaO, Cu₂O, Cu, BaY₂O₄ and Ba₄Y₂O₇.     

Ba2Cu3O5+δ as an intermediate phase during the synthesis of YBa2Cu4O8

We propose a reaction model for the synthesis of YBa₂Cu₄O₈ under normal pressure conditions, which contains 4 partial reaction steps. In a first step bariumnitrate and copperoxide react to Ba₂Cu₃O_5+δ. This substance will be formed for each mixtures Ba:Cu=2∶3...3∶2. The following two partial reaction steps are connected to Ba₂Cu₃O_5+δ, which reacts with Y₂O₃ and CuO to YBa₂Cu₄O₈ or decomposes to BaCuO₂ and CuO. In a last step parts of BaCuO₂ reacts with Y₂O₃ and CuO to YBa₂Cu₄O₈.     

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.     

Existence of the homologous series of Y n Ba m Cu m + n O y (m = 2, 3, 5; n = 1, 2) oxides with the tetragonal and orthorhombic structures of YBa2Cu3O6 + δ

The phase composition of Y _x Ba_1?x CuO _y (x = 0.29?0.40) samples annealed in air (at 930?C990°C) and in an oxygen atmosphere (450?C800°C, P(O₂) = 101 kPa) was studied by X-ray powder diffraction, chemical analysis, electron diffraction, and elemental analysis in a transmission electron microscope. A considerable cation nonstoichiometry was discovered in particles having the tetragonal and orthorhombic structures of YBa₂Cu₃O_{6 + ??}. The variation range of particle compositions comprises matrix oxides of the Ba _m Cu _{m + n} O _y series with (Ba: Cu) 3: 5, 5: 8, 2: 3, and 5: 7, which in the presence of yttrium form the Y _n Ba _m Cu _{m + n} O _y series. Tetragonal oxides Y₂Ba₃Cu₅O _y (235), Y₃Ba₅Cu₈O _y (358), YBa₂Cu₃O _y (123), and Y₂Ba₅Cu₇O _y (257) are formed at the primary synthesis step in air and are preserved in an orthorhombic structure during short-term (1 h) oxygen annealing. Most particles of the 3: 5 and 5: 8 oxides are undersaturated with yttrium relative to the stoichiometry of the Y _n Ba _m Cu _{m + n} O _y series, those of the 2: 3 oxide correspond to this stoichiometry, and those of the 5: 7 oxide are supersaturated with yttrium over the stoichiometry. A trend is observed for the fractions of these oxides to change during long-term (5?C51 h) annealing in an oxygen atmosphere at 450°C and to the alternation of the dominant role of one of the four phases with the superconducting transition temperature T _c = 82, 85, 86, and 91 K. Each orthorhombic oxide undergoes structural transformations during oxygen annealing with a change in T _c. The coexistence of these oxides in the form of nanometer-sized domains does not allow their individual superstructures to be recognized.     

Preparation of YBa2Cu3O y crystals by electrolysis of YO1.5-BaO-CuO x melts

Mass transfer during the electrolysis of melts of Y_0.02Ba_0.30Cu_0.70O _y and Y_0.02Ba_0.25Cu_0.75O _y samples was studied at 950°C (for 0.5 h) and currents of 5–1050 mA. YBa₂Cu₃O_{6 + δ} (123) tetragonal oxide crystal boules were grown, and their cationic composition and structure were studied by X-ray powder diffraction and by electron diffraction and elemental analysis in a transmission electron microscope (ED/TEM and EA/TEM). The 123 oxide was found to have cationic off-stoichiometry and to have a domain structure with domain sizes of 20–50 Å. Magnetic susceptibility versus temperature curves measured in the crystals after oxygen annealing (450°C, 1 h) feature four kinks, which indicate the occurrence of four superconducting phases with T _c = 45, 52, 75, and 86 K. Electrolysis byproducts are platinum-containing oxides Ba₉Pt₄Cu₃O _y and Ba₅₀Pt₁₅Y₁₆Al₁₃Cu₇O _y unknown hitherto; we report structure data for them.     

Estimation of average heat capacities of condensed phase transformation products in the Y−Ba−Cu−O system

A method of calculation of average heat capacities of phase transformation products of complex oxides is suggested. The method takes into account the physical state of products and the increase in the heat capacities of products due to the change of entropy at a phase transformation. Average heat capacities of products formed in a congruous melting of compounds (YCuO₂ and Y₄Ba₃O₉), in an incongruous melting of compounds (Y₂Cu₂O₅, BaCuO₂, BaCu₂O₂, Y₂BaCuO₅, YBa₂Cu₃O₇, YBa₂Cu₃O₆) and in a decomposition in a crystalline state of compounds (Y₂BaO₄, Y₂Ba₂O₅, Y₂Ba₄O₇, Ba₂CuO₃, Ba₃Cu₅O₈, YBa₂Cu_3.5O_7.5, YBa₂Cu₄O₈, YBa₂Cu₅O₉) was estimated by using three methods.     

Estimation of melting (decomposition) heat of some compounds in the Y−Ba−Cu−O system

A thermodynamical method for the estimation of decomposition heat in a crystal state, incongrous and congruous melting of compounds with the use of temperature dependencies of total entropies of compounds was suggested. Entropies and heats of phase transformation of YBa₂Cu₃O₆, YBa₂Cu₃O₇, YBa₂Cu_3.5O_7.5, YBa₂Cu₄O₈, YBa₂Cu₅O₉, YBa₄Cu₃O_8.5, Y₂BaO₄, Y₂Ba₂O₅, Y₂Ba₄O₇, Y₄Ba₃O₉, YCuO₂, Y₂Cu₂O₅, Y₂BaCu₂O₅, Ba₂CuO₃, BaCuO₂, BaCu₂O₂, Ba₃Cu₅O₈ were calculated. Data, obtained by the authors earlier, are discussed.     

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.     

Perovskite solid solutions in the BaO-CuO-Y2O3-Nb2O5 system

Freeze dried complex carboxylates are highly reactive precursors for complex perovskite solid solutions in the system BaO-CuO-Y₂O₃-Nb₂O₅ On thermal decomposition of the amorphous precursors the formation of complex crystalline phases begins at 600 °C. In most cases the themodynamically controlled phase composition is reached after a reaction time of two hours at about 900 °C. Beginning from the perovskite compound Ba₂YNbO₆ a partial substitution of Y by Cu or by a combination 2/3 Cu,1/3 Nb leads to extended fields of solid solutions with cubic perovskite structure. Substitution according to Y_0,5xBa₂(Y_1-0,5xCu_xNb)O_6+x is limited to x ≤ 0,4. A compound LBa₂Cu₂NbO₈ (x=2), well characterized for L=La, does not exist for L=Y. The composition of solid solutions depends on the preparation conditions. There are some signs for an inhomogeneous distribution of B-cations in the cubic perovskites.     

Oxygen reduction mechanism and performance of Y1Ba2Cu3O7−d as a cathode material in a high-temperature solid-oxide fuel cell

The high-Tc Y₁Ba₂Cu₃O_7−δ superconductor with oxygen ion vacancies was employed as the cathode for a high-temperature solid-oxide fuel cell (SOFC). The cathodic current-overpotential characteristics were studied in the temperature range from 500 to 800 °C and the oxygen pressure range from 10⁻⁴ to 0.21 atm. The delocalization of the triple-phase boundary and the oxygen reduction mechanism were identified. The delocalized triple-phase boundary of Y₁Ba₂Cu₃O_7−δ improves the cathodic polarization in SOFCs. By using a mathematical simulation and a particular experimental design, the oxygen adsorption step in the oxygen reduction process was demonstrated to be rate limiting. A layer of strong oxygen-adsorption catalyst such as Pt or Ag coated on the Y₁Ba₂Cu₃O_7−δ electrode was found to be able to largely enhance the activity of oxygen reduction by improving the ability of oxygen to be adsorbed on the electrode surface. Received: 28 October 1997 / Accepted: 16 January 1998     

UTILIZATION OF CROWN ETHER CHEMISTRY TO PREPARE BIMETALLIC COMPOUNDS: PREPARATION AND STRUCTURAL CHARACTERIZATION OF [Ba(15-CROWN-5)2][CuCl4]

Abstract

A solid hetero-bimetallic complex of Ba⁺² and Cu⁺² was prepared by slow evaporation of a 3/1 CH₃CN/CH₃OH mixture containing 15-crown-5 and the chloride salts of Ba⁺², Cu⁺², and Y⁺³. The resulting complex was studied using single crystal X-ray diffraction. [Ba(15-crown-5)₂][CuCl₄] crystallizes in the monoclinic space group Pn with cell parameters (20°C), a=12.119(2), b=9.386(2), c=13.475(3)Å, β=93.81(2)°, and D _calc=1.70 g cm^?3 for Z=2. Ba⁺² is coordinated to all 10 oxygen atoms of two 15-crown-5 molecules in a sandwich geometry. Cu⁺² is coordinated to the four chloride anions and exhibits a distorted tetrahedral geometry. The two shortest Cu…Ba separations are 6.855(2) and 6.902(2)Å.     

Control of Co content and SOFC cathode performance in Y1−ySr2+yCu3−xCoxO7+δ

The electrochemical performance of the layered perovskite YSr₂Cu_3−xCo_xO_7+δ, a potential solid oxide fuel cell (SOFC) cathode, is improved by increasing the Co content from x = 1.00 to a maximum of x = 1.30. Single phase samples with x > 1.00 are obtained by tuning the Y/Sr ratio, yielding the composition Y_1−ySr_2+yCu_3−xCo_xO_7+δ (where y ≤ 0.05). The high temperature structure of Y_0.95Sr_2.05Cu_1.7Co_1.3O_7+δ at 740 °C is characterised by powder neutron diffraction and the potential of this Co-enriched material as a SOFC cathode is investigated by combining AC impedance spectroscopy, four-probe DC conductivity and powder XRD measurements to determine its electrochemical properties along with its thermal stability and compatibility with a range of commercially available electrolytes. The material is shown to be compatible with doped ceria electrolytes at 900 °C.     

Kinetic study on thermal decomposition of the high-temperature superconductor in vacuum microbalance

In this paper the theoretical approach and applications of Cahn ultramicrobalance to kinetic study on the thermal decomposition of the high-temperature Y₁Ba₂Cu₃O_7-x superconductor are presented. Thermogravimetric in situ measurements of oxygen loss from Y₁Ba₂Cu₃O₆ samples heated isothermally in a relatively high dynamic vacuum were performed with a Cahn RG electrobalance. Single-phase orthorhombic samples of composition Y₁Ba₂Cu₃O_7-x (highest oxygen content) were synthesized from stoichiometric (1:2:3) mixtures of high-purity Y₂O₃, BaCO₃ and CuO. The original 1:2:3 mixture was prepared by the two-stage procedure described earlier. The crystal structure of the sample in the original orthorhombic phase was controlled by the X-ray powder method (CuK_α radiation) using a Stadi P Stoe diffractometer with a position-sensitive detector. Activation energy is estimated from appropriate Arrhenius plots. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of garnet structured Li7+x La3Y x Zr2-x O12 (x = 0–0.4) by modified sol–gel method

Preparation of lithium garnet Li₇La₃Zr₂O₁₂ (LLZ) in cubic phase by solid state method requires high temperature sintering around 1,200 °C for 36 h in Al₂O₃ crucible with intermittent grinding. Synthesis of LLZ in cubic phase at lower temperatures by wet chemical methods was reported earlier, however that decompose at high temperature around 850 °C. In this work we report the systematic studies on synthesis of garnet structured electrolytes by modified sol–gel method by the simultaneous substitution of Li⁺ and Y³⁺ for Zr⁴⁺ according to the formulae Li_7+x La₃Y _x Zr_2-x O₁₂ (x = 0, 0.1, 0.2, 0.3 and 0.4). The present investigation revealed that the cubic garnet phase is obtained at much lower temperature for Li₇La₃Zr₂O₁₂ and the simultaneous increase of both Li⁺ and Y³⁺ in Li_7+x La₃Y _x Zr_2-x O₁₂ requires slightly higher sintering temperatures for the formation of cubic garnet phase. SEM micrographs of the Li_7+x La₃Y _x Zr_2-x O₁₂ (x = 0, 0.1, 0.2, 0.3 and 0.4) annealed at minimum sintering temperature required for the formation of cubic garnet phase revealed the increase in grain size and relatively dense structure with increase of x in Li_7+x La₃Y _x Zr_2-x O₁₂.     

Combined DTA and XRD Study of Sintering Steps Towards YAl3(BO3)4

Sintering processes in the Y₂O₃–Al₂O₃–B₂ O₃ system and its subsystems (Y₂O₃–B₂O₃ and Al₂ O₃–B₂O₃) have been investigated by using combined DTA and XRD measurements to get a better understanding of solid state chemical changes resulting in the formation of yttrium aluminum borate (YAl₃(BO₃)₄, YAB) phase and to study the possible role and contribution of various simple borates formed also in the former processes. Two new exothermic heat effects of YBO₃ formation have been detected by DTA in the Y₂O₃–B₂O₃ system between 720 and 980°C. In the Al₂O₃–B₂O₃ system a new experimental XRD profile of Al₄B₂O₉ was observed. Formation of these borates seems to promote the nucleation of double borate YAB below 1000°C. Conversion of Al₄B₂O₉ to Al₁₈B₄ O₃₃ was observed after a long term (10 h) sintering at 1050°C. Similarly, an increased formation of YAB has been observed as a product of the sintering reaction between YBO₃ and Al₁₈B₄O₃₃ at 1150°C. The two latter single borates are found to be identical with the high temperature decomposition products of YAB. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermogravimetric study of the nonstoichiometric regions in the Y-Ba-Cu-O system

The paper presents the results of studies on thermal reduction and oxidation of the nonstoichiometric phases from the Y-Ba-Cu-O in air.The thermogravimetric (TG, DTA) experiments were performed in air in order to establish the ranges of stoichiometry and temperature of oxidation and reduction of YBa₂Cu₃O_x, YBa₄Cu₃O_x, YBa₅Cu₂O_x and Y₃Ba₈CusO_x.It has been found, that at 950°C in air there are four oxygen deficient ternary cuprates: YBa₂Cu₃O_6.02, YBa₄Cu₃O_8.01, YBa₅Cu₂O_8.35, Y₃Ba₈Cu₅O_16.45 and stoichiometric Y₂BaCuO₅. When these nonstoichiometric cuprates are cooled slowly to room temperature in air they oxidize to the following compositions: YBa₂Cu₃O_6.98, YBa₄Cu₃O_8.97, Y₃Ba₈Cu₅O₁₈ and YBa₅Cu₂O_8.97.This work has been supported by the CPBR 6.6.64 research programme.     

High resolution electron microscopy studies of Ba4Nb14O23

The new compound, Ba₄Nb₁₄O₂₃, has been prepared by heating mixtures of Ba₅Nb₄O₁₅, Nb₂O₅ and Nb at 1 450°C under Ar. Ba₄Nb₁₄O₂₃ has been studied by means of high resolution electron microscopy and X-ray powder diffraction techniques. It has a C-centered orthorhombic unit cell with a=20.782(4), b=12.448(3), c=4.148(1) Å and Z=2. The structure of Ba₄Nb₁₄O₂₃ can be considered as being an intergrowth between BaNbO₃ and NbO. Characteristic building units are triple chains of corner sharing Nb₆ octahedra which are connected via columns of the perovskite type structure to a three dimensional network.     

Synthesis of PrBa2Cu4O8 by Solution Method under Ambient Oxygen Pressure

A highly homogeneous and nearly monophasic PrBa₂Cu₄O₈ compound has been synthesized at 1 atm of oxygen via a route employing polymeric ethylenediamine/citric acid precursors. The PrBa₂Cu₄O₈ compound with a trace amount of BaCuO₂ was synthesized for the first time by sintering the pellets at the temperature of 875 °C under ambient oxygen pressure. The temperature coefficient of the resistivity is positive below 190 K while above 190 K is negative. The superconducting transition was not detected down to 4.2 K. Samples were characterized by X-ray diffraction, scanning electron microscopy, thermal analysis, and transport and magnetic measurements.     

Ein neues quaternäres Oxotitanat: Ba4Ti10Al2O27

A New Quaternary Oxotitanate: Ba₄Ti₁₀Al₂O₂₇ Single crystal of Ba₄Ti₁₀Al₂O₂₇ were prepared by heating oxide mixtures with NaOH flux for 20 days to 1300°C. X-ray investigations show monoclinic symmetry: a = 1973.7; b = 1134.9; c = 983.7 pm; β = 109.4°; space group C_2h³—C2/m. Ti⁴⁺/Al³⁺ occupy statistically more or less distorted octahedra, Ba²⁺ has an coordination number of 11 or 12, respectively. The complex frame work of octahedra is discussed.     

Ba8Cu16P30 – eine neue ternäre Variante des Clathrat I-Strukturtyps

Ba₈Cu₁₆P₃₀ – a New Ternary Variant of the Clathrate I Type Structure Ba₈Cu₁₆P₃₀ (a = 14.117(1) Å, b = 10.093(1) Å, c = 28.022(2) Å) was prepared by heating a mixture of the elements (800°C; excess of Ba and P; removal of the by-products by acetic acid/H₂O₂). The compound crystallizes orthorhombically (Pbcn; Z = 4) in a new superstructure of the cubic clathrate I type structure with an ordered distribution of the atoms. The structure is characterized by a three-dimensional framework of CuP₄ tetrahedra with cavities in the form of pentagonal dodecahedra and tetrakaidecahedra, which are occupied by the Ba atoms. The compound shows semiconduction, therefore the composition should be Ba₈Cu_15.5P_30.5.