LaCrO<Subscript>3</Subscript> powder from lanthanum trisoxalatochromate(III) (LTCR) precursor

Lanthanum chromite LaCrO₃, an important catalyst and interconnect material used in solid oxide fuel cell was prepared from lanthanumtrisoxalatochromate(III) hydrate [LaCr(C₂O₄)₃]·9H₂O (LTCR) employing microwave heating technique. The compound LTCR heated in microwave heating system gave pure LaCrO₃ at 500°C within one hour. However LTCR heated in silicon carbide furnace yielded LaCrO₃ at 900°C. BET surface area of LaCrO₃ prepared by microwave and conventional heating techniques were found to be 2.8 and 1.2 m² g⁻¹, respectively. Thermogravimetry, differential thermal analysis and X-ray diffraction techniques were used to optimize the conditions for the microwave processing of the precursor.     

Quantum-chemical modelling of nanotubes of titanium silicocarbides Ti<Subscript>2</Subscript>SiC,Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript>, and Ti<Subscript>4</Subscript>SiC<Subscript>3</Subscript>

Atomic models are proposed for nanotubes of the titanium silicocarbides Ti₂SiC, Ti₃SiC₂, and Ti₄SiC₃, and their electronic structure and interatomic interactions are investigated by the density functional tight-binding method (DFTB) in comparison with the corresponding crystalline phases. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 45, No. 2, pp. 88-92, March-April, 2009.     

Synthesis of LaCoO<Subscript>3</Subscript> from lanthanum trisoxalatocobaltate(III) (LTC) precursor employing microwave heating technique

Lanthanum cobaltite LaCoO₃, an important catalyst and an electronic material used as cathode in solid oxide fuel cells was prepared from lanthanum trisoxalatocobaltate(III) hydrate [LaCo(C₂O₄)₃]·9H₂O (LTC) employing microwave heating technique. It was observed that LTC heated in microwave heating system gives a pure product of LaCoO₃ at 400°C within one hour. Thermogravimetry, differential thermal analysis and X-ray diffraction techniques were used to optimize conditions for microwave processing of the precursor.Authors are thankful to Mr. N. A. Kulkarni and Mr. V. M. Chopade from TIFR, Mumbai for recording the XRD patterns of the sample.     

Effect of polarization on the electrode behavior and microstructure of (La,Sr)MnO<Subscript>3</Subscript> electrodes of solid oxide fuel cells

The electrode behavior and microstructure of freshly prepared (La_0.8Sr_0.2)_0.9MnO₃ (LSM) electrodes were investigated under various polarization conditions. The original, large agglomerates in freshly prepared LSM electrodes were broken down into sphere-shaped grains when exposed to cathodic or anodic current passage of 200 mA cm^–2 at 800 °C in air for 3 h. Microstructural changes under cathodic polarization could be related to the pronounced diffusion and migration of oxygen vacancies and Mn ions on the LSM surface and lattice expansion, while lattice shrinkage under oxidation conditions most likely contributes to the structural changes under anodic polarization. Such morphological changes were irreversible and were found to be beneficial to the performance of freshly prepared LSM electrodes. Freshly prepared LSM electrodes behaved very differently with respect to the cathodic and anodic current passage treatment.     

Preparation of Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript> by polyacrylamide gel processing and its thermoelectric properties

Ca₃Co₄O₉ powder was prepared by a polyacrylamide gel route in this paper. The effect of the processing on microstructure and thermoelectric properties of Ca₃Co₄O₉ ceramics via spark plasma sintering were investigated. Electrical measurement shows that the Seebeck coefficient and conductivity are 170 μV/K and 128 S/cm, respectively, at 700 °C, yielding a power factor value of 3.70 × 10⁻⁴ W m⁻¹ K⁻² at 700 °C, which is larger than that of Ca₃Co₄O₉ ceramics via solid-state reaction processing. The polyacrylamide gel processing is a fast, cheap, reproducible and easily scaled up chemical route to improve the thermoelectric properties of Ca₃Co₄O₉ ceramics by preparing the homogeneous and pure Ca₃Co₄O₉ phase.     

Thermal oxide synthesis and characterization of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanorods and Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires

Fe₃O₄ nanorods and Fe₂O₃ nanowires have been synthesized through a simple thermal oxide reaction of Fe with C₂H₂O₄ solution at 200–600°C for 1 h in the air. The morphology and structure of Fe₃O₄ nanorods and Fe₂O₃ nanowires were detected with powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The influence of temperature on the morphology development was experimentally investigated. The results show that the polycrystals Fe₃O₄ nanorods with cubic structure and the average diameter of 0.5–0.8 μm grow after reaction at 200–500°C for 1 h in the air. When the temperature was 600°C, the samples completely became Fe₂O₃ nanowires with hexagonal structure. It was found that C₂H₂O₄ molecules had a significant effect on the formation of Fe₃O₄ nanorods. A possible mechanism was also proposed to account for the growth of these Fe₃O₄ nanorods. Supported by the Fund of Weinan Teacher’s University (Grant No. 08YKZ008), the National Natural Science Foundation of China (Grant No. 20573072) and the Doctoral Fund of Ministry of Education of China (Grant No. 20060718010)     

Determination of the enthalpy of fusion of K<Subscript>3</Subscript>TaO<Subscript>2</Subscript>F<Subscript>4</Subscript> and KTaF<Subscript>6</Subscript>

Areas of fusion and crystallization peaks of K₃TaO₂F₄ and KTaF₆ were measured using the DSC mode of a high-temperature calorimeter (SETARAM 1800 K). On the basis of these quantities, considering the temperature dependence of the calorimeter sensitivity, values of the fusion enthalpy of K₃TaO₂F₄ at the fusion temperature of 1181 K of (43 ± 4) kJ mol⁻¹ and of KTaF₆ at the fusion temperature of 760 K of (8 ± 1) kJ mol⁻¹ were determined.     

Temperature dependence of the polarization resistance of the O<Subscript>2</Subscript>,Au/La<Subscript>0.88</Subscript>Sr<Subscript>0.12</Subscript>Ga<Subscript>0.82</Subscript>Mg<Subscript>0.18</Subscript>O<Subscript>2.85</Subscript> electrode system

The impedance of a porous gold electrode in contact with solid electrolyte La_0.88Sr_0.12Ga_0.82Mg_0.18O_2.85 and the effect of the manufacture conditions on its polarization resistance are studied at 600–800°C in air. The overall oxygen reaction rate on a gold electrode is described as the sum of two partial constituents, namely, the oxygen exchange at the gas/electrolyte interface at the gold/gas/electrolyte triple-phased boundary.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 190–197.Original Russian Text Copyright © 2005 by Shkerin, Sokolova, Khlupin, Beresnev.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Formation of Composites Based on CsHSO<Subscript>4</Subscript> and Cs<Subscript>5</Subscript>H<Subscript>3</Subscript>(SO<Subscript>4</Subscript>)<Subscript>4</Subscript> and Mechanism of Their Protonic Conduction

Vibrational spectra of finely divided amorphous CsHSO₄,Cs₅H₃(SO₄)₄ · H₂O, and composites based on these are measured and analyzed. An analysis of the spectra indicates the occurrence of substantial changes in the system of hydrogen bonds and in the spectral range of the sulfate group of acid sulfates in the composites. Structural dynamics of the SO₄ tetrahedrons is in full conformance with protonic conduction and the data of x-ray diffraction analyses accompanied by differential scanning calorimetry. It is shown that mobility of protons in the composites increases. A mechanism of the formation of the composites and their conduction is proposed.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 640–645.Original Russian Text Copyright © 2005 by Ponomareva, Lavrova, Burgina.     

Electronic structure and chemical bonding in K<Subscript>2</Subscript>V<Subscript>3</Subscript>O<Subscript>8</Subscript>

The calculations of the electronic structure of layered polyvanadate K₂V₃O₈ were made employing the spin-polarized tight-binding LMTO method. Calculated magnetic moment for K₄V₆O₁₆ compound phase equals 1.97 μ_B. V-O interactions were established to be dominating in the chemical bonding generation in this polyvanadate according to the estimated crystal orbital overlap population. The covalent bonds V(2)-V(2) in V(2)₂O₇ groups and electron density localization on vanadium atoms in isolated pyramids V(1)O₅ were found.     

Theoretical study on the reaction mechanism of (CH<Subscript>3</Subscript>)<Subscript>3</Subscript>CO<Superscript>.</Superscript> radical with NO

The reaction mechanism of (CH₃)₃CO^. radical with NO is theoretically investigated at the B3LYP/6-31G* level. The results show that the reaction is multi-channel in the single state and triplet state. The potential energy surfaces of reaction paths in the single state are lower than that in the triple state. The balance reaction: (CH₃)₃CONO⇔(CH₃)₃CO^.+NO, whose potential energy surface is the lowest in all the reaction paths, makes the probability of measuring (CH₃)₃CO^. radical increase. So NO may be considered as a stabilizing reagent for the (CH₃)₃CO^. radical.     

The intrinsic origin of the grain-boundary resistance in Sr-doped LaGaO3

Abstract  

In this paper I summarize our recent investigations (Park and Kim, Phys Chem C 111:14903, 2007; Solid State Ionics 179:1329, 2008) on the origin of the grain-boundary resistance in a doped LaGaO₃, a perovskite-structured solid electrolyte. The partial electronic and ionic resistances of the bulk and the grain boundaries, as well as the total resistance, in 1 mol% Sr-doped LaGaO₃ were measured separately by means of a dc-polarization method and ac-impedance spectroscopy. Both of the partial resistances at the grain boundaries were greater than the bulk counterparts, indicating that the grain boundaries impede the ionic as well as the electronic transport in this material. The transference number of the partial electronic conductivity at the grain boundary was however greater than that in the bulk. This fact strongly suggests that both electronic and ionic charge carriers deplete at the grain boundaries to form the space-charge zones and that the grain-boundary cores in this material are positively charged. In light of the fact that the effective charge of the oxygen vacancy (+2) is greater than that of the electron hole (+1), the oxygen vacancies deplete more sharply in the space-charge zones compared to the electron holes such that the grain boundaries become more mixed conducting relative to the bulk. These observations verify that the electrical conduction across the grain-boundaries in 1 mol% Sr-doped LaGaO₃ is governed by the space charge.     

Determination of the enthalpy of fusion of Na<Subscript>3</Subscript>FeF<Subscript>6</Subscript>

The areas of the fusion and crystallization peaks of Na₃FeF₆ and of four calibration substances (KCl, NaCl, Na₂SO₄, and K₂SO₄) were measured using the DSC mode of a high-temperature calorimeter. Using the measured quantities and known values of the enthalpy of fusion of the calibration substances, the enthalpy of fusion of Na₃FeF₆ was determined. Its value at the temperature of fusion 1224 K was 70 ± 4 kJ mol⁻¹.     

Phase relations in the CoO–V<Subscript>2</Subscript>O<Subscript>5</Subscript>–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> system in subsolidus area

Thermal properties of Co₂FeV₃O₁₁ have been reinvestigated. It has been proved that this compound does not exhibit polymorphism. It melts incongruently at the temperature of 770±5°C and the phase with lyonsite type structure is the solid product of this melting. Phase relations in the whole subsolidus area of the CoO–V₂O₅–Fe₂O₃ system have been determined. The solidus area projection onto the component concentration triangle plane of this system has been constructed using the DTA and XRD methods. 15 subsidiary subsystems can be distinguished in this system.     

Synthesis of ammonia from natural gas at atmospheric pressure with doped ceria–Ca<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>–K<Subscript>3</Subscript>PO<Subscript>4</Subscript> composite electrolyte and its proton conductivity at intermediate temperature

A new type of oxide–salt composite electrolyte, yttrium doped ceria YDC–Ca₃(PO₄)₂–K₃PO₄, was developed and demonstrated for its promising use for ammonia synthesis. Using this composite electrolyte, ammonia was synthesized from nitrogen and natural gas at atmospheric pressure in the solid-state proton conducting cell reactor, and the optimal condition for ammonia production was determined . The evolved rate of ammonia is up to 6.95×10⁻⁹ mol s⁻¹ cm⁻².     

Hydrothermal synthesis and thermodynamic properties of 2CaO·3B<Subscript>2</Subscript>O<Subscript>3</Subscript>·H<Subscript>2</Subscript>O

2CaO·3B₂O₃·H₂O which has non-linear optical (NLO) property was synthesized under hydrothermal condition and identified by XRD, FTIR and TG as well as by chemical analysis. The molar enthalpy of solution of 2CaO·3B₂O₃·H₂O in HCl·54.572H₂O was determined. From a combination of this result with measured enthalpies of solution of H₃BO₃ in HCl·54.501H₂O and of CaO in (HCl+H₃BO₃) solution, together with the standard molar enthalpies of formation of CaO(s), H₃BO₃(s), and H₂O(l), the standard molar enthalpy of formation of −(5733.7±5.2) kJ mol⁻¹ of 2CaO·3B₂O₃·H₂O was obtained. Thermodynamic properties of this compound were also calculated by a group contribution method.     

Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> powders by the complex polymerization method: synthesis,characterization and morphology

Lanthanum-modified bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂, BLTO) powders were prepared by the complex polymerization method. The structure and morphology of BLTO powders were investigated by X-ray diffraction and scanning electron microscopy. The complexation of citric acid with the metallic cations was detected by Fourier transformed infrared (FT-IR). The thermal analyses of obtained gels were investigated by differential thermal gravimetric (DTG). The pure and normally stoichiometric phase of BLTO powders could be obtained at relatively low temperature of 550–700 °C even if the bismuth content is not excess in the starting precursors, while the secondary phase could be detected at lower and higher calcination temperatures. The shape of the BLTO grains is similarly to platelet in Bi-layer structure and stoichiometry BLTO was detected by the analysis of energy dispersive spectrometry.     

Magnetorheological characteristics of aqueous suspensions that contain Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles

This investigation examines the magnetorheological (MR) characteristics of Fe₃O₄ aqueous suspensions. Magnetite particles (Fe₃O₄) were synthesized using a colloidal process and their sizes were determined to be normally distributed with an average of 10 nm by TEM. Experimental results reveal that the MR effect increases with the magnetic field and suspension concentration. The yield stress increases by up to two orders of magnitude when the sample is subjected to a magnetic field of 146 Oe/mm. In comparison with other published results, concerning a concentration of approximately 10–15% v/v, this study demonstrates that the same increase can be obtained with a concentration of nano-scale particles as low as 0.04% by volume. The viscosity was increased by an order of magnitude while the shear rate remained low; however, the increase decayed rapidly as the shear rate was raised. Finally, the MR effect caused by DC outperformed that caused by AC at the same current.     

Cu<Subscript>4</Subscript>RbCl<Subscript>3</Subscript>I<Subscript>2</Subscript> solid superionic conductor in thermodynamic study of three-component copper chalcogenides

The Cu-B-Se (B = In, As, Sb, Bi) systems are studied by measurement of EMF for concentration circuits vs. a copper electrode in the temperature range of 300–430 K. A solid superionic Cu₄RbCl₃I₂ conductor is used as an electrolyte. Diagrams of solid-phase equilibriums in the studied systems are constructed. Partial molar functions of alloyed copper are calculated on the basis of the equations of the temperature dependences of EMF. Potential-forming reactions corresponding to the measured EMF values are determined on the basis of the phase diagrams and standard thermodynamic formation functions and standard entropies of ternary compounds are calculated.     

X-ray photoelectron study of the interaction of H<Subscript>2</Subscript> and H<Subscript>2</Subscript>+O<Subscript>2</Subscript> mixtures on the Pt/MoO<Subscript>3</Subscript> model catalyst

The effects of H₂ and H₂ + O₂ gas mixtures of varying composition on the state of the surface of the Pt/MoO₃ model catalyst prepared by vacuum deposition of platinum on oxidized molybdenum foil were investigated by X-ray photoelectron spectroscopy (XPS) at room temperature and a pressure of 5–150 Torr. For samples with a large Pt/Mo ratio, the XP spectrum of large platinum particles showed that the effect of hydrogen-containing mixtures on the catalyst was accompanied by the reduction of molybdenum oxide. This effect results from the activation of molecular hydrogen due to the dissociation on platinum particles and subsequent spill-over of hydrogen atoms on the support. The effect was not observed at low platinum contents in the model catalyst (i.e., for small Pt particles). It is assumed for the catalyst that the loss of its hydrogen-activating ability is a consequence of the formation of platinum hydride. Possible participation of platinum hydride as intermediate in hydrogen oxidation to H₂O₂ is discussed.