Citrate Route Research Articles

Growth of single phase tetragonal structure iron tantalum oxide thin films grown by sol–gel spin coating technique

For the first time, crack free, and dense tetragonalFe2Ta2O8 thin films have been prepared by sol-gel method using tantalum isopropoxide and iron oxalate. As the annealing temperature increased from 700 to 800 ◦C, we have observed the growth of rhombohedral phase of iron tantalum oxide along with Fe2Ta2O8 tetragonal phase. The complete and single phase of Fe2Ta2O8 tetragonal structure was formed on silicon (100) substrate at an annealing temperature of 900 ◦C for 5 h. Structural, morphological studies of these Fe2Ta2O8 thin films produced by sol-gel synthesis have been characterized by grazing incidence X-ray diffraction (GIXRD), and tapping mode atomic force microscopy. Iron based mixed-type oxides containing metals such as MTaO4 (M = W, Ti, Co, Cr) are well known as functional inorganic materials having a wide range of applications such as electrodes of solid oxide fuels (SOFCs) [1], metal-air barriers [2], gas sensors [3], and as high performance catalysts [4] for the complete oxidation of hydrocarbons or CO and NO2 reduction as well as magnetic oxides for recording medium. Many preparation processes with low cost and low processing temperatures, such as the malate route [5], citrate route [6], polymeric precursor route [7], propionic acid route [8] have been reported to synthesize nanocrystalline powders. These methods can easily synthesize fine powders: however, it is difficult to use them to prepare thin films, as most of these amorphous precursors used in the processes mainly consist of some organic compounds and metal nitrates which easily explode to form powders. So far, oxide thin films structure have only been prepared by dry processes such as sputtering [9] and laser ablation [10] and metal organic chemical vapor deposition [11] methods. The sol-gel technique has many advantages over other fabrication techniques, such as homogeneity, stoichiometry control, purity, ease of processing and controlling the composition and ability to coat large and complex area substrates [12]. The crystallization of sol-gel derived films has followed a development path that is similar to that which has been observed in simple metal oxides that were prepared using sol-gel technique [13]. In the present study, we have selected iron oxalate for iron source and tantalum isopropoxide for tantalum source as starting materials of a sol-gel process to synthesize iron tantalum oxide

Evaluation of La0.8Sr0.2Cu1-xMnxOd Double Perovskite for Use in SOFCs

Compounds with the general formula La0.8Sr0.2Cu1-xMnxO3 were prepared by the citrate route. A solid solution region containing hexagonal perovskite phase, was formed when 0.6{less than or equal to}x{less than or equal to}1, in which oxygen stoichiometry in air changes from oxygen deficiency to oxygen excess with Mn doping. The temperature dependence of electrical conductivity changes from metallic- like behavior in La0.8Sr0.2CuO3-d to semiconducting type in La0.8Sr0.2MnO3+d. Mn doping resulted in a decrease in the conductivity (with the minimum at 40% Mn doping), whereas the thermal expansion coefficient decreased linearly with x. A defect model for the mixed Cu-Mn perovskite is proposed and related to its physical and electrochemical properties.

Phase equilibria and crystal structures of phases in the La-Fe-Ni-O system at 1370 K in air

The phase equilibria in the La-Fe-Ni-O system have been studied at 1370 K in air, and the La-Fe-Ni-O phase diagram at constant temperature and pressure has been constructed. Based on x-ray diffraction results for samples prepared by standard solid-state reactions and via citrate and nitrate routes, the following solid solutions have been shown to exist at 1370 K in air: LaFe1 − xNixO3 − δ (0 < x ≤ 0.4, sp. gr. Pbnm; 0.6 ≤ x ≤ 0.8, sp. gr. R-3 c), La4(Ni1 − yFey)3O10 − δ (0 < y ≤ 0.3), La3(Ni1 − zFez)2O7 − δ (0 < z ≤ 0.05), La2Ni1 − vFevO4 + δ (0 < v ≤ 0.05), NikFe3 − kO4 (0.81 ≤ k ≤ 1.05), Ni1 − mFemO (0 < m ≤ 0.05), and Fe2 − pNipO3 (0 < p ≤ 0.04). The lattice constants and structural parameters of single-phase samples have been refined by the Rietveld profile analysis method.

Synthesis and Thermal Analysis of the Strontium and Iron-doped Lanthanum Cobaltite Nano-powder Precursors

Powders of perovskite-type oxides, with the general formula La 0.8 Sr 0.2 Co 1-x Fe x O 3 (0.0<x<1.0), were synthesized by the amorphous citrate method, and chemical reactions in such a synthesis process were systematically investigated via thermal analysis. A series of decompositions of citric acid derivatives occurred at 575-750 K, and the subsequent oxidation of metal-organic amorphous compounds to perovskite-type oxides occurred at 650-850 K. The apparent activation energies for those processes in La 0.8 Sr 0.2 Co 0.5 Fe 0.5 O 3 (x=0.5) were determined to be 101-105 and 45-49 kJ/mol, respectively, using a non-isothermal technique based on the Kissinger approach. The exothermic peaks of those reactions were temperature-dependent on Fe content (x), and they increased with increasing x, reaching a maximum at x=0.7-0.8, and then decreased. An X-ray diffraction analysis of samples heat-treated at 873 K for 2 h revealed that they have a single-phase perovskite structure for all compositions, with their unit cell volume increasing with Fe content (x). The obtained oxides were nanoparticles with diameters of 10 to 20 nm, which increased with Fe content (x). The amorphous citrate route was found to give excellent starting gel precursors for the low-temperature synthesis of La 0.8 Sr 0.2 Co 1-x Fe x O 3 nanoparticles.

Thermochemical characteristics of La n+1 Ni nO 3 n+1 oxides

Lanthanum nickelates: La 2NiO 4+ δ , La 3Ni 2O 7− δ , La 4Ni 3O 10− δ and LaNiO 3− δ the members of Ruddlesden–Popper series La n+1 Ni n O 3 n+1 were prepared using citrate route. Dissolution enthalpies of complex oxides as well as a number of subsidiary substances were measured by means of Calvet calorimeter in 1 M solution of hydrochloric acid at 25 °C. The dissolution scheme of complex oxides in hydrochloric acid was proposed and enthalpies of formation of the complex oxides from binary oxides were calculated considering oxygen nonstoichiometry of these substances. Enthalpies of step-by-step oxidation were evaluated. Partial enthalpy contribution of LaO layers was calculated endothermic equals to 30.9 J/mol while partial enthalpy contribution of perovskite LaNiO 3 layers was negative equals to −97.0 J/mol. Enthalpy of formation of any complex oxide of Ruddlesden–Popper series fits very well to the linear regression based on these values.

Novel urea assisted polymeric citrate route for the synthesis of nanocrystalline spinel LiMn 2O 4 powders

Urea assisted polymeric citrate route was investigated for the synthesis of nanocrystalline spinel LiMn 2O 4 powders using metal nitrates/acetates, citric acid and urea under acidic condition at 300 °C. Optimum total metal ions to urea ratio was identified for the synthesis of organic free, phase pure nanocrystalline LiMn 2O 4 powders. Addition of urea changes the microstructure and combustion mechanism of polymeric intermediates as well as the structural coordination and phase formation of the LiMn 2O 4 powders, which are, respectively, confirmed through SEM, TG/DTA, FTIR and XRD analysis. Organic free phase pure nanocrystalline LiMn 2O 4 powder was synthesized by urea assisted polymeric citrate route with the total metal ions to urea ratio of 1:2 at 300 °C and the average crystallite size was found to be 19 nm.

Nanostructured Ceria-Zirconia as a Promising Anode Component for IT-SOFC: Synthesis and Characterization

Ceria-zirconia (CZO) is an attractive candidate as anode material for SOFCs due to its catalytic activity and its electronic and ionic conductivity. In this work, ceria-zirconia powders with different compositions were synthesized by citrate route and characterized using N2 physisorption, SEM, and XRD. TG/DTA curves of the precursors helped to optimize the synthesis method. The reduction at 1000{degree sign}C and the subsequent reoxidation at 600{degree sign}C originated some structural modifications in CZO. The porous powders were impregnated with 50 vol% of NiO by a liquid mixture technique to form model anodes, which were characterized by SEM.

Effect of aliovalent doping on the transport properties of lanthanum cuprates

Cuprates with perovskite-related structure and the general formula \({{La}}_{{1 - x}} {{A}}_{x} {{Cu}}_{{1 - y}} {{B}}_{y} {{O}}_{{2.5 \pm d }} \) (A=Ca, Ba, Sr; B=Li; x=0.1, 0.2, 0.3; y=0, 0.1) were prepared by the citrate route. Their structures were studied by X-ray powder diffraction and their electrical conductivities (as a function of temperature) by four-point DC measurements on sintered bars. The highest values of electrical conductivity were observed for the x=0.3 Sr-doped cuprate, which showed a metallike temperature dependence. Ba-doped cuprates showed a transition from semi-conducting to metallic temperature dependence of conductivity. The extent of A-site deficiency is small and had the effect of slightly decreasing the conductivity and thermal expansion coefficient. Li solubility was also found to be small, but it reduced conductivity drastically, due in part to the disruption of the copper–oxygen conductive network. Reactions between cuprates and a typical manganite fuel cell cathode, and Pt and Pd contacts were observed at 1,000 °C.

UV absorption properties of ceria-modified compositions within the fluorite-type solid solution CeO 2–Y 6WO 12

A new fluorite-type solid solution domain has been evidenced in the system (1− x) CeO 2− x/7 Y 6WO 12□ 2 using the amorphous citrate route. All the studied phases (0⩽ x⩽1) crystallize in a cubic-type symmetry. Diffuse reflectance spectra reveal a strong optical absorption between 380 and 400 nm. All substituted compositions spectral selectivities are estimated suitable for application as inorganic UV absorbers. The non linear variation observed in the optical gap values between Y 6WO 12 and CeO 2 is attributed to the presence of the cerium 4f-block band. Additionally, Time Resolved Microwave Conductivity (TRMC) experiment and phenol photodegradation analyses carried out on the Ce 0.81Y 0.16W 0.03O 1.95□ 0.05 ( x=0.19) composition do not indicate any photocalatytic activity for this material.

Optical Properties of the Perovskite Solid Solution LaTiO2N–ATiO3(A = Sr, Ba)

AbstractStarting from the colored oxynitride LaTiO2N, new perovskite‐type solid solutions have been evidenced in the systems La1–xAxTiO2+xN1–x (A = Sr, Ba) by ammonolysis at 950 °C of the corresponding oxide precursors prepared by the molten salt route. The color of the resulting powders varies progressively from the brown of LaTiO2N to the white of the oxide with a decrease in both the nitrogen and lanthanum contents. The optical absorption was characterized by diffuse reflectance. The nonlinear variation of the optical bandgap may result from the competition between the variation of the inductive effect between lanthanum and strontium, structural changes, and the difference in electronegativity between nitrogen and oxygen. For the compounds with a low nitrogen content the observed green shade is attributed to the presence of a nitrogen‐rich secondary phase: TiOxNy in which titanium presents a mixed‐valence state. Taking advantage of a chimie douce process, the amorphous citrate route, the highest oxidation state of titanium was stabilized until x = 0.9 in the La0.1Sr0.9Ti(O,N)3 phase giving the powder a bright yellow color (Eg = 2.49 eV). (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

Magnetic studies on Sr 0.8Ce 0.1Fe 0.7Co 0.3O 3− δ perovskite

The A-site deficient perovskite oxide Sr 0.8Ce 0.1Fe 0.7Co 0.3O 2.66 was prepared using a modified citrate route with the oxygen content determined by TGA analysis and iodometric titration. Physical characterisation of this compound was carried out using electrical conductivity, magnetisation and Mössbauer studies. From the magnetic characterisation of this compound the existence of two different ordered magnetic states below 100 K was deduced, the overall behaviour being antiferromagnetic. The combined results of chemical analysis, Mössbauer spectroscopy and magnetisation allowed the determination of the oxidation states of the Co and Fe ions.

The Changes on Microstructure and Electrical Properties of Bi:2223/Ag Composite as a Function of the Granulation and the Content of the Added Silver Powder

The Bi-Sr-Ca-Cu-O system has been one of the most studied superconducting ceramic materials for industry applications. The most of the studies with this aim are on silver/ceramic composites, due to the benefits and great compatibility of this metal with the oxide. In this paper we describe a systematic and comparative study on Ag/BSCCO composite, made by the citrate route, in which the ceramic pellets are sintered in the presence of silver powder using several proportions and having several granulations. It was observed that the introduction of fine (0.5 and 2 μm) silver powder in the proportions of 5 wt. % always implies in a better critical current density compared to the no silver pellet. According to the results, the silver powder in excess of 5 wt.% may not promote best electrical properties, depending on the size of the silver particles.

Synthesis of La&lt;sub&gt;0.8&lt;/sub&gt; Sr&lt;sub&gt;0.2&lt;/sub&gt; MnO&lt;sub&gt;3±δ&lt;/sub&gt; Powders by Amorphous Citrate and Pechini Techniques

La0.8Sr0.2MnO3±d (LSM) powders were prepared by a Pechini-type polymerizable complex process and by amorphous citrate technique. Both processes involve the complexation of the LSM cations from their nitrates or carbonates. The aim of this work is to determinate the differences between the powers obtained by these techniques. The powders synthesized by the Pechini-type process were calcined between 500 °C and 1100 °C. The powders prepared by amorphous citrate technique were calcined at 900 °C. No contamination of either of the powders was observed by X ray fluorescence analysis. X ray diffraction results showed that a perovskitetype structure phase was obtained. BET results showed that the specific surface areas of the powders prepared by the polymerizable complex process and the amorphous citrate route were 6.6 m2g-1 and 5.7 m2g-1, respectively.

Ce&lt;sub&gt;0.8&lt;/sub&gt;Gd&lt;sub&gt;0.2&lt;/sub&gt;O&lt;sub&gt;1.9&lt;/sub&gt; Powders Synthesized by Co-Precipitation and Amorphous Citrate Processes

Powders of Ce0.8Gd0.2O1.9 were synthesized by co-precipitation and amorphous citrate routes and their compositions and structures were compared. Co-precipitation process was carried out at room temperature, while amorphous citrate synthesis was performed at 60-70 °C. The powders obtained were calcined at 700 °C for 1 h. X-ray diffraction analyses showed that a single fluorite structure was formed by both synthesis techniques. Pellets from these powders were prepared by compaction at 30 kN/cm2 and sintered at 1593 °C in air. WDS analysis confirmed the homogeneity of the pellets. The sintered pellets obtained by amorphous citrate and by coprecipitation routes showed relative densities over 97 % of the theoretical density. These results indicated that the citrate amorphous route is an interesting and simple technique to prepare gadolinium-doped ceria powders with high sinterability.

Preparation of an Li0.7Ni0.8Co0.2O2 Electrode Material from a New Li—Co—Ni Mixed‐Citrate Precursor.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Preparation of an Li0.7Ni0.8Co0.2O2 Electrode Material From a New Li–Co–Ni Mixed‐Citrate Precursor

AbstractA new chemical route to prepare the mixed oxide Li0.7Ni0.8Co0.2O2, which is a member of the well‐known Li‐Ni‐Co‐O family that is of interest as an electrode material for rechargeable lithium batteries, from a mixed citrate is reported. In a first step the new citrate (NH4)3LiNi0.8Co0.2(C6H5O7)2, is synthesized, which, after thermal decomposition, gives the layered mixed‐oxide phase Li0.7Ni0.8Co0.2O2. This oxide has been characterized by chemical analysis, XRD, FTIR, SEM, and DTA, revealing an “ideal” trigonal α‐NaFeO2 type structure with a well‐ordered distribution of Li and Ni ions in different lattices. Thermoelectric power and four‐point electrical conductivity measurements indicate a nonactivated conductivity mechanism of a small polaron. The Li0.7Ni0.8Co0.2O2 compound prepared by this so‐called “citrate route” has a better reversibility of Li‐insertion in charge–discharge cycles than related phases reported previously. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2005)

Low-temperature synthesis of fine BaMgAl 10O 17:Eu 2+ phosphor based on the solubility isotherms

A homogeneous and stoichiometric BaMgAl 10O 17:Eu 2+ (BAM) phosphor powder has been prepared by the citrate route. Solubility isotherms have been calculated for metal–citric acid–water system at 25 °C to predict the optimum pH condition, which was found to be pH=7 for preparing pure and stable metal citrate complexes. Well crystallized and sub-micrometer-sized BAM particles could be obtained by thermal decomposition of the optimally prepared citrate precursor at a temperature as low as 1150 °C. Though the luminescent properties of the present samples under UV excitation well reflect the bulk properties, VUV excitation has exhibited the luminescent properties greatly influenced by the surface, which might be due to the fine particle. The maximum luminance of the samples heat treated at 1350 °C was 105% in comparison with that of the commercial BAM under VUV excitation.

Processing of Nickel–Zinc Ferrites Via the Citrate Precursor Route for High‐Frequency Applications

Nickel–zinc ferrites of various compositions, Ni1−xZnxFe2O4wherex=0.2, 0.4, 0.5, and 0.6 investigated in the present work, have been prepared by the citrate precursor method. The complex permittivity (∈′−j∈″) and permeability (μ′−jμ″) of the ferrites were measured at X‐band (8–12 GHz) microwave frequencies. The dielectric constants or the real part of permittivity, ∈′, are observed to lie in the range 5.46–8.95 for ferrites sintered at 1200°C, while those sintered at 1300°C exhibit relatively higher dielectric constants. The permittivity loss, tan δ∈(=∈″/∈′), is observed to be of the order of 10−2–10−3, depending upon the composition and sintering temperature of the ferrite. These losses are lower by at least one to two orders of magnitude compared with those normally reported for ferrites processed by the conventional ceramic method. The presently studied ferrites also exhibit high values of DC‐resistivity, 107–1011Ω·cm. The low dielectric losses and high resistivity can be co related to small grain size and better compositional stoichiometry obtained as a result of processing via the citrate route. Magnetic properties such as the Curie temperature, saturation magnetization, initial permeability, andB–Hhysteresis parameters of the compositions withx=0.5 and 0.6 are also given.

Characterization of perovskite powders for cathode and oxygen membranes made by different synthesis routes

Strontium lanthanum manganite (LSM) and lanthanum ferrite (LSF) perovskite cathode and oxygen membrane materials were synthesized using different techniques: spray pyrolysis, a modified citrate route, oxalate and carbonate co-precipitations. The use of Ca, a cheaper substituent on the A-site, was explored along to the substitution of La by Pr. The differently sourced powders were characterized by TG/DTA, XRD, ICP, TEM, XPS, PSD and BET. The co-precipitation of La, Ca and Fe was also possible using the cyanide route. This complexation method allowed the precipitation of a crystalline phase as evidenced by XRD. Among all methods, the cyanide and carbonate co-precipitation allowed the lowest perovskite phase transformation for LSF and LCF, followed by the nitrate (i.e. ‘ spray pyrolysis’). These phase transformation differences affected much the particle size distribution and the surface areas of these materials, the carbonate and the cyanide routes giving rise to very fine powders in the nm range. XPS and TEM analyses indicated uneven composition distributions. These different powder characteristics are expected to affect the catalytic and electrochemical properties of these materials.

Catalyst based on BaZrO 3 with different elements incorporated in the structure : II. BaZr (1− x) Rh xO 3 systems for the production of syngas by partial oxidation of methane

New catalytic systems, synthesised by a variant of the citrate route, are proposed for the partial oxidation of methane. They consist of solid solutions – barium, zirconium, rhodium and oxygen – with a perovskite structure of formula BaZr (1− x) Rh x O 3. Detailed analysis of the XRD diffractograms and the TGA cycles show that Rh is randomly distributed as Rh IV among the B sites of the perovskite, together with Zr. The activities of the catalysts have been tested for the catalytic partial oxidation of methane at short contact times to evaluate the potential of materials giving promising results in terms of syngas yield at low Rh loading.