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.     

Thermodynamic and Pseudo Binary Phase Diagram of REBa2Cu3O7-x-"Ba2Cu5O7" systems

In this work the effect of oxygen pressure on the primary crystallisation fields for REBa₂Cu₃O_7-x(RE=Nd, Sm, and Eu) has been studied. A DTA apparatus has been modified in order to carry out analyses under gas pressure, so the trend of temperatures of peritectic decomposition of the REBa₂Cu₃O_7-xphases and of the eutectic equilibrium involving REBa₂Cu₃O_7-x phases and flux mixture "Ba₂Cu₅O₇" have been studied at oxygen pressures of 0.21, 1, and 10 atm. This showed that primary crystallisation fields spread at the increase of the oxygen pressure and allowed us to calculate the enthalpies of reactions of REBa₂Cu₃O_7-xphases too. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure and electrical properties of Y,Fe-based perovskite mixed conducting composites fabricated by a modified polymer precursor method

In this work, samples of Y_0.07Sr_0.93Ti_1-xFe_xO_3-δ with 20, 40, 60 and 80 mol% of iron amount were prepared by a low-temperature polymer precursor method. The SEM-EDS analysis proved that analyzed Y_0.07Sr_0.93Ti_1-xFe_xO_3-δ samples were composites of two Ti- and Fe-rich perovskite samples. This kind of composite consists of two phases in which one has a good ionic and the other electronic conductivity, which makes such a composite a potential mixed ionic and electronic conductors (MIECs) material. The total electrical conductivities of analyzed samples were measured in air atmosphere (cathode conditions in Solid Oxide Fuel Cell). The values changed from ∼10⁻³ to 10⁻¹ S cm⁻¹ and depended on the ratio between two observed perovskite phases. The 0.12 S cm⁻¹ conductivity value at 800 °C for sample with the highest amount of Fe-rich perovskite in the structure makes this composite material a candidate for air electrode in electrochemical devices.     

Characterization of phase transition of Ba2-xSrxIn2O5 by thermal analysis and high temperature X-ray diffraction

The phase transitions of Ba_2-xSr_xIn₂O₅ were investigated with various thermal analyses and high-temperature X-ray diffraction. It was clarified that crystal structure of Ba_2-xSr_xIn₂O₅ with x=0.0~0.4 varies from brownmillerite through distorted perovskite to another distorted perovskite with increase of temperature. The phase transition from brownmillerite to distorted perovskite was revealed to be first order, whereas transition from distorted perovskite to another one was second order. The specimen with x≥0.5 showed only one first order phase transition from brownmillerite to distorted perovskite. The phase diagram of Ba_2-xSr_xIn₂O₅ was established and existence of tricritical point at ~1100°C with x=0.4~0.5 was suggested. This revised version was published online in August 2006 with corrections to the Cover Date.     

Enhanced visible light photocatalytic activity in N-doped edge- and corner-truncated octahedral Cu2O

Edge- and corner-truncated octahedral Cu₂O is successfully synthesized using an aqueous mixture of CuCl₂, sodium dodecyl sulfate, NaOH, and NH₂OH₃·HCl. Cu₂O_1-xN_x(150 °C, 30 min) samples are synthesized by nitridation of Cu₂O using an ammonothermal process. Cu retains a formal valence state through and beyond the nitridation process. N concentration in this sample is 1.73 at%, out of which 1.08 at% is substitutional in nature. Photocatalytic activity of Cu₂O_1-xN_x(150 °C, 30 min) sample is investigated and compared to that of pristine edge- and corner-truncated octahedral Cu₂O. Results show that Cu₂O_1-xN_x(150 °C, 30 min) sample with dominant {110} facets has a higher photocatalytic activity than the pristine Cu₂O material. Higher surface energy and a greater density of the “Cu” dangling bonds on {110} facets of edge- and corner-truncated octahedral Cu₂O_1-xN_x is the plausible reason for the observed optimum catalytic activity. Furthermore defect states induced by nitridation results in improved visible light adsorption. And also the band edge states changes which brought about by N doping. This is an interesting result since it bypasses the usual challenge faced by pristine Cu₂O which have band edge states between which transitions are normally forbidden. Selective radical quenching experiments suggest that photocatalytic activity of Cu₂O_1-xN_x is due to formation of hydroxyl radicals in water, subsequent to photogeneration of charge carriers in the photocatalyst.     

Y-Ba-Cu-O mixed oxides for production of diphenols in liquid-solid phase

Superconductor mixed oxides were often used as catalysts at higher temperature in gas phase oxidations, and considered not suitable for lower temperature reactions in the liquid-solid phase; here the catalysis of YBa2Cu3O7+x and Y2BaCuO5+x in the phenol hydroxylation at lower temperature with H2O2 as oxygen donor was studied, and found that the superconductor YBaCu3O7±x has no catalytic activity for phenol hydroxylation, but Y2BaCuO5±x does, even has better catalytic activity and stability than most previously reported ones. With the studies of catalysis of other simple metal oxides and perovskite-like mixed oxides, a radical substitution mechanism is proposed and the experimental facts are explained clearly, and draw a conclusion that the perovskite-like mixed oxides with (AO)(ABO3) and (AO)2(ABO3) structure have better catalytic activity than the simple per-ovskite oxides with (ABO3)3 structure alone, and (AO) structure unit is the key for the mixed oxides to have the phenol hydroxylation activity.     

Positron annihilation studies in YBa2Cu3O x (6<x≤7)

Positron lifetime and Doppler broadening of the annihilation line measurements, as a function of temperature between 14 and 300 K, have been performed on four YBa₂Cu₃O _x (6<x7) samples with different oxygen deficiency. It was found that the positron lifetime and theS parameter values increase as the oxygen deficiency increases in the YBa₂Cu₃O _x (6<x7) samples. It was also observed that the positron annihilation parameers show different temperature dependence in the YBa₂Cu₃O _x (6<x7) samples associated with their different oxygen deficiency.     

Positron annihilation studies in highT c superconductor YBa2Cu3Cd x O y (x=0, 0.05)

Positron lifetime and Doppler broadening of the annihilation line measurements were performed in highT _c superconducting samples YBa₂Cu₃Cd _x O _y ,x=0, 0.05 and 6.9<y<7, as a function of temperature in the region of 14–300 K. It was found that the positron lifetime and theS parameter values are lower in the Cd doped sample than those in the undoped one. It was also observed that the positron annihilation parameters show similar temperature dependence for the undoped and Cd doped samples. We conclude that the Cd doping in highT _c superconductor YBa₂Cu₃Cd _x O _y , 6.9<y<7 fills defects associated with oxygen vacancies probably in oxygen deficient regions which can trap positrons.     

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.     

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.     

High-pressure defect phase Ce x Cu3V4O12

Nonstoichiometric perovskite oxide Ce _x Cu₃V₄O₁₂ (space group Im $ \bar 3 Nonstoichiometric perovskite oxide Ce _x Cu₃V₄O₁₂ (space group Im Z = 2, a = 7.264–7.328 ?) with cationic vacancies was prepared barothermally. This compound has metal-type conductivity, paramagnetic properties, and a phase transition. Original Russian Text ? N.I. Kadyrova, Yu.G. Zainulin, V.L. Volkov, G.S. Zakharova, A.V. Korolev, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 10, pp. 1650–1654.     

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     

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.     

Synthesis of the Cu2-x Mgx P4 O12 and Ni2-x Mgx P4 O12

Abstract

Some binary cyclo-tetraphosphates have been synthetized by means of thermal dehydration of the starting binary dihydrogenphosphate. The mechanism of dehydration and condensation reactions in dependence on various special conditions of thermal preparation (temp. rate, water vapour pressure, using nuclei) has been studied. The methods of TA at quasi-isothermal and quasi-isobaric conditions with combination of calcination experiments have been used for these purposes. The reaction products obtained were analyzed by chromatography, IR-spectrosco-py, X-ray diffraction analysis, electron microscopy and AAS. The course, the rate and the yields of the condensation reactions of formation of the main products considered Cu_2-xMg_xP₄O₁₂ and Ni_2-xMg_XP₄)₁₂, have been investigated. These coloured products (green or yellow-green) crystallize in the monoclinic system, C2c group (where x ε (0; 2)). Their structural parameters have the values for Cu_2-xMg_xP₄O₁₂ or Ni_2-xMg_xP₄O₁₂ (X = 2 to x = 0) : a = 11.749(5) to 12.546(7) or 11.644(5) Å, b = 8.278(4) to 8.092(5) or 8.236(4) Å, c = 9.905(4) to 9.565(5) Or 9.813 Å and β = 118.92(2)° to 118.63(3)° or 118.53(2)°, respectively.     

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.     

Valence and coordination state of cobalt atoms in bulk and deposited lanthanum cobaltates

The valence and coordination state of paramagnetic cobalt atoms in bulk phases of perovskite cobaltate LaCoO₃ and cobalt oxide Co₃O₄ and in nanosized LaCoO₃ deposited inside the mesoporous molecular sieve MCM-41 matrix has been studied using ESR. Cobalt(II) cations in deposited cobaltates have octahedral coordination, which is characteristic of perovskite-like structures.     

Mechanosynthesis of nanopowders of the proton-conducting electrolyte material Ba(Zr, Y)O3−δ

The formation of perovskite nanopowders of the common proton-conducting, electrolyte material Ba(Zr_1−xY_x)O_3−δ is demonstrated by room temperature mechanosynthesis for the compositional range x=0, 0.058 and 0.148. This is achieved with a planetary ball mill at 650 rpm in zirconia vials, starting from BaO₂ with ZrO₂, (ZrO₂)_0.97(Y₂O₃)_0.03 or (ZrO₂)_0.92(Y₂O₃)_0.08 precursors, respectively. Powder X-ray diffraction (XRD) reveals the formation of the perovskite phase in the early stages of milling with phase purity being achieved after milling times of 240 min for composition x=0.058 whereas 420 min is necessary for composition x=0.148. In contrast, traces of ZrO₂ are apparent in composition x=0 even after milling times of 420 min. The use of BaCO₃ as precursor does not allow the formation of the perovskite phase for any composition. The perovskite crystallites are spherical in shape with an average size determined from XRD of ca. 30 nm in agreement with transmission electron microscopy observations. FTIR spectra demonstrate that contamination levels of BaCO₃ in the mechanosynthesized powders are very low. The spherical shape and nanoscale of the crystallites allow densification levels that are highly competitive when compared to BaZrO₃-based materials formed by alternative synthesis techniques documented in the literature.     

Crystal Structure Investigations and Thermal Behavior of the Five‐Leg Spin Ladder Compound La8Cu7O19

La₈Cu₇O₁₉ was synthesized by solid state reaction of the oxides La₂O₃ and CuO at 1288 K in air. The crystal structure was determined by a joint Rietveld refinement of X‐ray and neutron powder diffraction data. La₈Cu₇O₁₉ crystallizes in the monoclinic space group C2/c (No. 15) with the lattice parameters a = 13.8310(4)Å, b = 3.75827(9)Å, c = 34.5917(8)Å and β = 99.332(2)°. La₈Cu₇O₁₉ is the n = 3 member of the homologous series La_4+4nCu_8+2nO_14+8n. The Cu—O sub‐structure in La₈Cu₇O₁₉ contains infinite ribbons, which can be described as perovskite type layers with a width of n = 3 Jahn‐Teller‐elongated octahedra, and Cu—O planes of complex geometry. DSC/TG‐measurements in different gas atmospheres show peritectic decomposition of La₈Cu₇O₁₉. The anisotropic thermal expansion of the lattice parameters was investigated using synchrotron radiation. The Madelung part of lattice energy was calculated and compared with the corresponding values of other lanthanum cuprates.     

Influence of Ce substitution in LaMO3 (M = Co/Ni) perovskites for COx-free hydrogen production from ammonia decomposition

The La based perovskite type LaMO₃ (M = Ni, Co) oxides were prepared by combustion synthesis method using citric acid as organic fuel. These catalyst precursors tested for ammonia decomposition. The LaNiO₃ and LaCoO₃ catalysts showed good activity for NH₃ decomposition. The LaNiO₃ catalyst displayed greater activity than LaCoO₃. This due to high surface area and easily reducibility of Ni species. A 50% of La was substituted by Ce in both LaNiO₃ and LaCoO₃ catalysts. A remarkable effect on catalytic performance was observed with the partial substitution of La by Ce in perovskite catalyst especially at lower temperatures. The La_0.5Ce_0.5NiO₃ catalyst exhibited highest activity among all prepared samples. The achieved superior activity is due to boost in surface area, reducibility and suitable basicity. The SEM elemental mapping of La_0.5Ce_0.5NiO₃ catalyst concluded that metal oxide constituents dispersed homogeneously. The La_0.5Ce_0.5NiO₃ catalyst showed excellent stable catalytic performance during 50 h time on study at 550 °C.     

Mechanochemical synthesis and thermal stability of phases in the Y2O3–Yb2O3 system

Substitutional, continuous solid solution of the general formula Y_2–xYb_xO₃ was obtained from the mixture of Y₂O₃ and Yb₂O₃ oxides, for the first time by the mechanochemical method in a high-energy ball milling. The monophasic samples of nanocrystalline solid solution for x?>?0.00 and x?<?2.00 were examined by the methods: XRD, DTA, SEM, IR and UV–Vis–DR. As follows from the results, the solid solution crystallizes in cubic system and is isostructural with Y₂O₃ and Yb₂O₃. The solution is stable in the air atmosphere up to at least 900°C, and its decomposition temperature decreases with the increase in x, that is, with decreasing number of Yb³⁺ ions replacing Y³⁺ ions in the crystal lattice of Y₂O₃. The energy band gap estimated for the solid solution varies from?~?5.30 eV for x?=?0.50 to?~?4.90 eV for x?=?1.50, which means that it is an insulator.