Citrate-nitrate Combustion Method Research Articles

Solid state and combustion synthesis of Mn2+-doped scheelites – Their optical and magnetic properties

Abstract Pure and manganese doped calcium molybdates (Ca 1−x Mn x MoO 4 ) as well as molybdato-tungstates (Ca 1−x Mn x (MoO 4 ) 0.50 (WO 4 ) 0.50 ) for 0 x ≤ max. 0.15 were synthesized via high-temperature annealing and citrate-nitrate combustion method. The X-ray diffraction and scanning electron microscopy results confirm the formation of single, tetragonal scheelite-type phases (space group I 4 1 /a ) with an average crystallite size in the range of ~ 100 nm – max. 1 µm (combustion synthesis) or ~ 10 – ~ 40 µm (solid state reaction route). The melting point of each Mn-doped ceramic material is lower than the melting point of adequate scheelite-type matrix. IR analysis exhibited Mo-O or Mo-O as well as W-O stretching and bending bands of MoO 4 or MoO 4 /WO 4 tetrahedra, which related to a scheelite-type structure. The diffused reflectance spectra were used to calculate the optical direct band gap ( E g ) of pure and Mn-doped materials. Their E g value was estimated in the range of 3.65–4.34 eV. EPR measurements confirmed, that Mn 2+ ions exist in the structure as isolated magnetic centres if Mn doping is low. With increasing of manganese concentration the exchange and dipolar interactions lead to developing of the magnetic system, where antiferromagnetic interactions dominate among Mn 2+ ions. Strength of interactions is higher with higher manganese concentration.

Influence of synthesis conditions on the properties of Y2O3–MgO nanopowders and sintered nanocomposites

In this study, a citrate–nitrate combustion method was applied to synthesize composite Y2O3–MgO nanopowders. In order to optimize the synthesis condition to support sufficient combustion, the molar ratio of citric acid to nitrate (c/n molar ratio) used in the reaction mixtures was varied between 0.17 and 0.34. Nanopowders with an average particle size of 17nm were achieved. The properties of these nanopowders indicated that the higher molar ratios decreased the unreacted organic components and increased the amount of carbide on the surface of the oxides, which helped to inhibit the formation of carbonate groups. The amount of carbonate groups was reduced with the increasing c/n molar ratio. Y2O3–MgO nanocomposites fabricated through hot-isostatic-pressing sintering showed a uniform distribution of Y2O3 and MgO grains, which had an average size of ∼180nm. In addition, the absorption peaks at 1410 and 1511cm−1 disappeared until the c/n molar ratio reached 0.28. A high average infrared transmittance of 83% in the range of 4000–1667cm−1 (2.5–6μm) was obtained in the nanocomposites.

New vacancied and Gd3+-doped lead molybdato-tungstates and tungstates prepared via solid state and citrate-nitrate combustion method

New Pb1-3x⌷xGd2x(MoO4)1-3x(WO4)3x (0<x≤0.1774) and Pb1-3x⌷xGd2xWO4 (0<x≤0.1154, ⌷ denotes vacancy) solid solutions were synthesized via solid state reaction route and citrate-nitrate combustion method. XRD and SEM data showed that as-prepared ceramics crystallize in the tetragonal scheelite type symmetry (space group I41/a) with the crystallite size varying between ~10 and ~40µm (solid state method) or ~500nm and 2µm (combustion synthesis). A change in lattice constants (a and c), lattice parameter ratio c/a and progressive deformation of MoO4/WO4 tetrahedra with an increase of Gd content was observed. The melting point of each Gd-doped sample is lower than the melting point of adequate scheelite matrix. Ceramics under study are insulators with indirect band gap (Eg)>3eV. EPR investigation revealed a difference among spectra obtained for varied gadolinium content, whereas synthesis method has no influence on EPR results.

Magnetic and catalytic properties of inverse spinel CuFe2O4 nanoparticles

In this research, inverse spinel copper ferrite nanoparticles (CuFe2O4 NPs) were synthesized via citrate-nitrate combustion method. The crystal structure, particle size, morphology and magnetic studies were investigated using various instrumental tools to illustrate the formation of the inverse spinel structure. Mossbauer spectrometry identified Fe is located both in the tetrahedral and octahedral site in the ratio (40:60) and the observed magnetic parameters values such as saturation magnetization (Ms=20.62emug−1), remnant magnetization (Mr=11.66emug−1) and coercivity (Hc=63.1mTesla) revealed that the synthesized CuFe2O4 NPs have a typical ferromagnetic behaviour. Also tested CuFe2O4 nanoparticles as a photocatalyst for the decolourisation of methylene blue (MB) in the presence of peroxydisulphate as the oxidant.

Co-synthesized (La0.8Sr0.2)0.9MnO3[sbnd]Y0.15Zr0.85O2 composite for solid oxide fuel cell cathode

(La0.8Sr0.2)0.9MnO3Y0.15Zr0.85O2 (LSM–YSZ) composite co-synthesized by citrate-nitrate combustion method are investigated for solid oxide fuel cell cathode. The co-synthesized LSM–YSZ composite powders are characterized by XRD, HR-SEM, TEM, HR-TEM and EDX analysis. The phase separation of the precursor into LSM perovskite and YSZ fluorite occurs at temperatures above 900 °C, about 200 °C higher than the formation temperature of pure LSM or pure YSZ. In the nano-sheets of LSM–YSZ composite, nano-particles of LSM and YSZ are well distributed and intimately contacted with each other. The nanostructures remain in the co-synthesized LSM–YSZ cathode. The co-synthesized LSM–YSZ cathode improves oxygen reduction reaction (ORR) activity, especially at reduced operation temperature. The cell with co-synthesized LSM–YSZ cathode delivers current density 150% times of the cell with traditional LSM–YSZ cathode at 600 °C. Electrochemical impedance spectra (EIS) analysis shows that oxygen reduction reaction is accelerated due to high three phase boundary (TPB) density. The co-synthesized LSM–YSZ cathode shows good stability under cell operation at 800 °C.

Investigation on thermophysical properties of RE6UO12(s) (RE = La, Pr, Nd, Sm)

RE6UO12(s) (RE=La, Pr, Nd, Sm) was synthesized by citrate-nitrate combustion method. The synthesis condition for Pr6UO12(s) was optimized. Nonstoichiometry in these rare earth uranates in argon atmosphere was analysed using various techniques like thermogravimetry (TG), X-ray photo electron spectroscopy (XPS), chemical analysis and electrical conductivity measurements. Thermal expansions of RE6UO12(s) (RE=Pr and Sm) was studied in the temperature range 298–1273K by high temperature X-ray powder diffractometry and compared with that of similar rare earth compounds reported in the literature. Heat capacity of RE6UO12(s) (RE=La, Pr, Nd, Sm) was measured by differential scanning calorimetry in the temperature range 300–870K. Enthalpy, entropy and Gibbs energy functions of these compounds were computed from the measured heat capacity data.

Electrochemical performance of Co-containing mixed oxides as oxygen electrode materials for intermediate-temperature solid oxide electrolysis cells

Composite materials Nd0.8Sr1.2CoO4 − δ – Nd0.5Sr0.5CoO3 − δ were prepared and evaluated as oxygen electrode for intermediate temperature reversible solid oxide cells (IT-RSOCs). Nd0.8Sr1.2CoO4 − δ (NSC0812) material with A2BO4-type structure was prepared by citrate–nitrate combustion method, and Nd1 − xSrxCoO3 − δ (x = 0.3, 0.4, 0.5, 0.6, 0.7) materials with perovskite structure were prepared via the glycine nitrate process (GNP). The crystal structure, thermal expansion, electrical conductivity and electrochemical properties were investigated. The lattice volume of Nd1 − xSrxCoO3 − δ increases with increasing Sr content, x. XRD analysis show that no reaction occurs between oxygen materials and Gd0.2Ce0.8O1.9 (GDC). The thermal expansion coefficient increases with increasing x. AC impedance spectroscopy on symmetric cells was used to assess electrochemical performance of the oxygen electrode materials. Nd0.5Sr0.5CoO3 − δ (NSC0505) has the lowest total cathodic polarization resistance for 0.099 Ω cm2 at 750 °C. The composite materials NSC0505 and NSC0812 with the ratio of 7:3 reveals a better performance than either material individually at 700 °C, which is due to introduce of more oxygen vacancies and oxygen interstitials with NSC0812. For an electrolysis voltage of 1.3 V, current densities in a single cell fabricated with Nd0.5Sr0.5CoO3 − δ – Nd0.8Sr1.2CoO4 − δ are 420 mA cm−2 at 800 °C.

Characterization of proton-conducting electrolyte based on La0.9Sr0.1YO3 – δ and its application in a hydrogen amperometric sensor

In the present study, a Sr-doped LaYO3 proton-conducting material is successfully synthesized by the citrate–nitrate combustion method and its phase features, microstructure, thermal behavior and transport properties are investigated by the aid of X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), dilatometry and four-probe dc electrical measurements. The set of the data obtained (single-phase structure, nonporous ceramic, low thermal expansion such as 10.5×10–6K–1 and predominant ionic transport in oxidizing and reducing atmospheres even at 900°C), allows one to propose this material as an electrolyte component for hydrogen amperometric sensors. To this purpose, a sensor, containing two electrochemical cells based on oxygen anionic YSZ and protonic La0.9Sr0.1YO3–δ electrolytes, is constructed and its electrochemical properties are investigated under different conditions, including different H2+N2 gas mixtures and temperatures. The possibility of direct measurement of hydrogen content in nitrogen on the base of the limiting current value is shown. It is found that the developed sensor exhibits good operation reproducibility, clear response and precision for the detection of trace hydrogen content (0.1–3.3 vol.%) in process gases at 500–600°C.

Preparation of Y-Doped BaZrO&lt;sub&gt;3&lt;/sub&gt; Proton Conducting Solid Electrolyte via Modified Low Temperature Pechini Method

One of the promising material for proton-conducting solid electrolyte operating at intermediate temperature range (400-600 °C) is the Yttrium-doped BaZrO3 (BZY) due to its high conductivity and chemical stability. In this study, a modified citrate-nitrate combustion method (Pechini method) has been employed for BZY powder preparation. A stoichiometric amounts of starting nitrates and oxide raw materials with nitric acid, citric acid and ethylene glycol for the synthesis of 20 mol% Y-doped BaZrO3 (BZY20) were prepared, then calcined and sintered at 1000 °C for two heat treatment durations of 24 hours and 48 hours. The obtained BZY20 powder samples have been fully characterized for its structure, morphology, and thermal properties. From the X-ray diffraction (XRD) results, the sample sintered for 48 hours showed a cubic phase of BZY20 which can be indexed to a Pm3m cubic structure which is also supported by Raman analysis. The calculated lattice parameter is 4.2067 Å which is higher than the reported lattice parameter of a pure BaZrO3 (BZ) of 4.1930 Å which indicates a successful doping due to higher ionic radius of Y3+ dopant as compared to Zr4+ in the B-site ABO3 perovskite sub-lattice. In addition, SEM-EDX analyses of the sintered pellet revealed a uniform distribution of Yttrium dopant in the BZY20 prepared solid electrolyte.

Charge-Coupled Substituted Garnets (Y3–xCa0.5xM0.5x)Fe5O12 (M = Ce, Th): Structure and Stability as Crystalline Nuclear Waste Forms

The garnet structure has been proposed as a potential crystalline nuclear waste form for accommodation of actinide elements, especially uranium (U). In this study, yttrium iron garnet (YIG) as a model garnet host was studied for the incorporation of U analogs, cerium (Ce) and thorium (Th), incorporated by a charge-coupled substitution with calcium (Ca) for yttrium (Y) in YIG, namely, 2Y(3+) = Ca(2+) + M(4+), where M(4+) = Ce(4+) or Th(4+). Single-phase garnets Y3-xCa0.5xM0.5xFe5O12 (x = 0.1-0.7) were synthesized by the citrate-nitrate combustion method. Ce was confirmed to be tetravalent by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. X-ray diffraction and (57)Fe-Mössbauer spectroscopy indicated that M(4+) and Ca(2+) cations are restricted to the c site, and the local environments of both the tetrahedral and the octahedral Fe(3+) are systematically affected by the extent of substitution. The charge-coupled substitution has advantages in incorporating Ce/Th and in stabilizing the substituted phases compared to a single substitution strategy. Enthalpies of formation of garnets were obtained by high temperature oxide melt solution calorimetry, and the enthalpies of substitution of Ce and Th were determined. The thermodynamic analysis demonstrates that the substituted garnets are entropically rather than energetically stabilized. This suggests that such garnets may form and persist in repositories at high temperature but might decompose near room temperature.

Innovative Coating Method of MgAl2O4 or ZnAl2O4 on α-Al2O3 Spheric Nuclei

Structured materials composed of α-Al2O3 spheres covered with a porous and thin layer of MgAl2O4 or ZnAl2O4 were obtained. The innovative coating method developed consists in the formation of a precursor gel of MgAl2O4 or ZnAl2O4 by citrate-nitrate combustion method. The alumina spheres are submerged in this gel and then submitted to a drying process in vacuum and to calcinations with N2 and N2/O2. SEM results showed that a very rough and porous layer uniformly covers the spheres. The thickness of the layer reaches the maximum value of 16 µm in the sample coated with two layers of ZnAl2O4. XRD results confirmed the MgAl2O4 or ZnAl2O4 spinel formation in the layered material. The structured materials were used as Pt catalysts supports, showing a good catalytic activity in the n-butane dehydrogenation reaction.

Structural Evolution and Multiferroics in Sr‐Doped Bi7Fe1.5Co1.5Ti3O21 Ceramics

The ceramics with the composition of SrxBi7−xFe1.5Co1.5Ti3O21−δ (0 ≤ x ≤ 1, SBFCT) were prepared by a citrate–nitrate combustion method, and the phase evolution with an increasing Sr content was investigated. Pure Aurivillius phase SBFCT with a layer number of n = 6 was obtained when x ≤ 0.25, and then the structure collapsed to 5 layers for x = 0.50, then alternating 4 and 5 layers for x = 0.75, and finally 4 layers for x = 1.00. Meanwhile, secondary phase Sr1−mBimFe1−iCoiO3−γ appeared when x &gt; 0.25, which is antiferromagnetic (AFM) and with low resistivity. Enhanced ferromagnetic and ferroelectric properties were observed from single phase SBFCTs at the room temperature, and the ferromagnetic transition temperature (Tc) increases with the Sr doping level x in the single phase range. The remnant magnetization (2Mr) is 2.27 emu/g and the remnant polarization (2Pr) is 2.89 μC/cm2 at an applied electric field of 100 kV/cm for the x = 0.25 sample.

Insights on thermal and transport features of BaCe0.8−xZrxY0.2O3−δ proton-conducting materials

BaCe0.8−xZrxY0.2O3−δ-based materials (BCZYx, 0 ≤ x ≤ 0.8) are obtained through a modified citrate-nitrate combustion method at 1150 °C with following sintering at 1450 °C. Then their thermomechanical and electrical properties are thoroughly investigated. Using of wet-chemical route it is allowed to obtain single-phase and dense ceramic samples at a wide range of Ce:Zr ratio. It is found that the relative linear expansion of ceramics decreased with gradual increasing x, which is reflected in a corresponding drop of thermal expansion coefficient (TEC) values, from 11.6 × 10−6 to 8.3 × 10−6 K−1 (∼30%). It is also found that, the decrease in pO2 does not significantly change the ceramics' linear expansion, whereas the increase in pH2O resulted in marked variation of both linear expansion and TEC values. The electrical conductivity of BCZYx decreases from 0.85 to 0.02 mS cm−1 at 300 °C (∼98%) and from 69.9 to 17.9 mS cm−1 at 900 °C (∼75%) with the increase of x. At the same time, hole contribution on total conductivity increases with increasing x. Taking into account the phase stability in BaCe0.8−xZrxY0.2O3−δ system, the material with optimal properties should be sought at close concentration of Ce and Zr.

Specific features of preparation of dense ceramic based on barium zirconate

In the present work the BaZr0.8Y0.2O3 − δ samples were synthesized by traditional ceramic (solid-state method), citrate-nitrate methods as well as co-precipitation of hydroxides. Among them, the citrate-nitrate combustion method was found to be the optimal one because the obtained powder calcined at 1150°C is single-phase and characterized by high dispersion. It is shown that the transition from solid-state method to wet chemical ones promotes the decrease of the powder particle size from ∼680 nm to 75 nm, respectively. However, at the same time the relative densities of the samples sintered at 1450°C do not exceed the 67%. The dense ceramic (90.2%) was obtained by combination of the citrate-nitrate method and the addition of 1 wt % Co3O4 as a sintering aid to the precursors.

Samarium and Yttrium Codoped BaCeO3 Proton Conductor with Improved Sinterability and Higher Electrical Conductivity

Acceptor-doped barium cerate is considered as one of the state-of-the-art high temperature proton conductors (HTPCs), and the proton conductivity of such HTPCs is heavily dependent on the dopant. In this work, a codoping strategy is employed to improve the electrical conductivity and sinterability of BaCeO3-based HTPC. BaCe0.8Sm(x)Y(0.2-x)O(3-δ) (0 ≤ x ≤ 0.2) powders are synthesized by a typical citrate-nitrate combustion method. The XRD and Raman spectra reveal all the compounds have an orthorhombic perovskite structure. The effects of Sm and/or Y doping on the sinterability and electrical conductivity under different atmospheres are carefully investigated. The SEM results of the sintered BaCe0.8Sm(x)Y(0.2-x)O(3-δ) pellets indicate a significant sintering enhancement with increasing Sm concentration. BaCe0.8Sm0.1Y0.1O(3-δ) exhibits the highest electrical conductivity in hydrogen among the BaCe0.8Sm(x)Y(0.2-x)O(3-δ) pellets. Anode-supported BaCe0.8Sm0.1Y0.1O(3-δ) electrolyte membranes are also fabricated via a drop-coating process, and the corresponding single cell exhibits desirable power performance and durability at low temperatures. The results demonstrate that BaCe0.8Sm0.1Y0.1O(3-δ) is a promising proton conductor with high conductivity and sufficient sinterability for proton-conducting solid oxide fuel cells operating at reduced temperatures.

Cerium Substitution in Yttrium Iron Garnet: Valence State, Structure, and Energetics

The garnet structure is a promising nuclear waste form because it can accommodate various actinide elements. Yttrium iron garnet, Y3Fe5O12 (YIG), is a model composition for such substitutions. Since cerium (Ce) can be considered an analogue of actinide elements such as thorium (Th), plutonium (Pu), and uranium (U), studying the local structure and thermodynamic stability of Ce-substituted YIG (Ce:YIG) can provide insights into the structural and energetic aspects of large ion substitution in garnets. Single phases of YIG with Ce substitution up to 20 mol % (Y3–xCexFe5O12 with 0 ≤ x ≤ 0.2) were synthesized through a citrate–nitrate combustion method. The oxidation state of Ce was examined by X-ray absorption near edge structure spectroscopy (XANES); the oxidation state and site occupancy of iron (Fe) as a function of Ce loading also was monitored by 57Fe–Mossbauer spectroscopy. These measurements establish that Ce is predominantly in the trivalent state at low substitution levels, while a mixture of trival...

Synthesis and characterization of Ca and Sr co-doped ceria electrolytes

A series of Ce0.85Ca0.15−x Sr x O2–δ (x = 0, 0.03 and 0.06) were synthesized via citrate–nitrate combustion method. Samples were first characterized by the X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD and SEM results showed that a complete solid solution formed in fluorite structure and Ce0.85Ca0.15−x Sr x O2−δ had homogeneous distribution of particle with grain size in the range of 2.5 to 3 μm. The electrical conductivities of Ce0.85Ca0.15−x Sr x O2−δ were evaluated for its use as a solid electrolyte in the intermediate-temperature solid oxide fuel cells by complex plane impedance measurements. Impedance measurements were made in the frequency range 1 MHz–0.1 Hz and temperature range 300–700 °C. It was found that Ce0.85Ca0.12Sr0.03O2−δ showed highest conductivity.

Synthesis and characterization of BaZr0.3Ce0.5Y0.2−xYbxO3−δ proton conductor for solid oxide fuel cells

The acceptor-doped BaCeO3–BaZrO3 solid solution shows a good compromise between conductivity and chemical stability. Y and/or Yb doped BaCeO3–BaZrO3 solid solution BaZr0.3Ce0.5Y0.2−xYbxO3−δ (x = 0, 0.05, 0.1, 0.15, 0.2) powders are synthesized via a typical citrate–nitrate combustion method in this work. The crystal structure and electrical conductivity of BaZr0.3Ce0.5Y0.2−xYbxO3−δ are investigated. The XRD results reveal all the powders possess orthorhombic perovskite structure. The electrical conductivity decreases monotonously with increasing the proportion of Yb, and BaZr0.3Ce0.5Y0.2O3−δ exhibits the highest electrical conductivity. Single cells with BaZr0.3Ce0.5Y0.2O3−δ as the electrolyte are fabricated and tested, and the cell outputs excellent power density and stability. The peak power density of the cell reaches as high as 513 and 396 mW cm−2 at 650 and 600 °C, suggesting that BaZr0.3Ce0.5Y0.2O3−δ-based fuel cells are promising solid oxide fuel cells (SOFCs) working at low temperatures.

Ag modified LSCF as cathode material for protonic conducting SOFCs

BaCe0·7Zr0·1Y0·2O3−δ (BCZY) electrolyte and La0·6Sr0·4Co0·2Fe0·8O3−δ (LSCF) cathode perovskite materials were synthesised using the citrate–nitrate combustion method. NiO–BCZY anode supported thin film sold oxide fuel cells (SOFCs) was fabricated using spin coating and co-sintering process. LSCF exhibited good chemical stability in H2O containing atmosphere and chemical compatibility with BCZY proton conducting material. The electrochemical performance of the Ag–LSCF composite cathode was dependent on the Ag content. The thin film proton conducting SOFC displayed highest maximum power density of 563 mW cm−2 at 700°C for 10 wt-%Ag–(LSCF+BCZY) composite cathode.

La2−xSrxCoO4−δ (x = 0.9, 1.0, 1.1) Ruddlesden-Popper-type layered cobaltites as cathode materials for IT-SOFC application

La2−xSrxCoO4−δ (x = 0.9, 1.0, 1.1) compounds with Ruddlesden-Popper K2NiF4-type structure have been investigated as potential cathode materials for IT-SOFC application. Materials have been prepared by citrate-nitrate combustion method. Structural evolution analysed by XRD shows a shortened Co–O–Co bond length within the perovskite layer as Sr substitution increases, while the interlayer distance at the same time increases. An oxygen stoichiometry close to 4 has been found for all compositions at room temperature. Thermal expansion coefficients have been obtained from temperature-dependent XRD analysis and show large values (>20 × 10−6 K−1) compared to the currently utilized electrolyte materials. Electrochemical characterisation has been performed by means of impedance spectroscopy on symmetric cells with CGO electrolyte. Pure electrodes have a high Area Specific Resistance, probably due to limited oxygen ion diffusion. By using composite electrode (50 wt.% CGO), an Area Specific Resistance of 0.25 Ω cm2 is obtained at approximately 700 °C for all the three compounds suggesting promising electrochemical properties for IT-SOFCs.