Fabrication and microstructure of directionally solidified SrCe1 − xYxO3 − δ (x = 0.1, 0.2) high temperature proton conductors

SrCe_1 − xY_xO_3 − δ (x = 0.1, 0.2) high temperature proton conductors (HTPC) have been fabricated by directional solidification using a laser-heated float zone (LHFZ) method. The resulting microstructures have been studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The LHFZ method produces a cellular microstructure consisting of crystalline cells embedded in an amorphous matrix, showing a strong biaxial texture. EBSD studies show that all the crystalline cells have their <0 0 1> axis as described in the cubic Pm3m prototype unit cell parallel to the growth direction. The observed microstructural features are explained in terms of the fabrication process.     

Sintering and electrical conductivity in fast oxide ion conductors La2−xRxMo2−yWyO9 (R: Nd, Gd, Y)

Attrition and ball milling are used as mechanical means to reduce grain size of optimized fast oxide-ion conductors La_2−xR_xMo_2−yW_yO₉ (R: rare earths). Dilatometry is used to determine the optimal sintering conditions in order to obtain high density samples (greater than 96% of relative density) with help of scanning electron microscopy to characterize their microstructure. The optimal sintering temperatures are highly dependent on the chemical composition, and therefore identical annealing temperatures do not warrant similar relative densities. Complex impedance spectroscopy show that above the transition temperature of La₂Mo₂O₉ at 580 °C, the conductivity of all the studied compounds is lower than that of the parent compound, whereas just below the transition, in most cases the stabilization of the cubic phase increases conductivity. An interesting result is that tungsten substitution, which stabilizes La₂Mo₂O₉ against reduction, does not affect significantly the oxide ion conduction.     

Electrical resistivity and thermopower of (La1−xSrx)MnO3 and (La1−xSrx)CoO3 at elevated temperatures

Electrical resistivity and Seebeck (S) measurements were performed on (La_1−xSr_x)MnO₃ (0.02x0.50) and (La_1−xSr_x)CoO₃ (0x0.15) in air up to 1073 K. (La_1−xSr_x)MnO₃ (x0.35) showed a metal-to-semiconductor transition; the transition temperature almost linearly increased from 250 to 390 K with increasing Sr content. The semiconductor phase above the transition temperature showed negative values of S. (La_1−xSr_x)CoO₃ (0x0.10) showed a semiconductor-to-metal transition at 500 K. Dominant carriers were holes for the samples of x0.02 above room temperature. LaCoO₃ showed large negative values of S below ca. 400 K, indicative of the electron conduction in the semiconductor phase.     

Improving the oxygen permeability of Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranes by a surface-coating layer of GdBaCo2O5+δ

A GdBaCo₂O_5+δ layer was coated on the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membranes to enhance their oxygen permeability by employing the fast oxygen adsorption/desorption surface-exchange properties of the GdBaCo₂O_5+δ material. The oxygen flux of the coated and uncoated Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membranes was measured in the temperature range of 600–850 °C. The results reveal that the oxygen-permeation flux of the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membranes coated by a GdBaCo₂O_5+δ layer shows significant enhancement. The GdBaCo₂O_5+δ layer coated on the oxygen desorption side (He side) has much effect than that coated on the oxygen adsorption side (air side). At 850 °C, the oxygen flux with a single coating layer on the air side can rise 16%, while a single coating on the helium side will result into a rise of 23%.     

Synthesis of La2−xNiO4+δ oxides by polymeric route: non-stoichoimetry control

The synthesis of La_2−xNiO_4+δ oxides has been done via a polymeric route. This method allows the preparation of a wide range of non-stoichoimetry values. Oxides with values as high as 0.25 have been synthesised. Correlations between processing parameters such as sol composition and heat treatment have been done with structural and microstructural properties of the oxides. In our synthesis conditions, the higher the mean grain size, the higher the non-stoichoimetry level.

Transmission electron microscopy on these oxides has shown that whatever the non-stoichoimetry level, the extra-oxygen arranges in the structure according to two superstructures which correspond to δ=0.25 and δ=0.17. This shows that our grains consist of a mixture of these compositions.     

Catalytic oxidation of nitrogen monoxide over La1−xCexCoO3 perovskites

This paper presents an investigation on the NO oxidation properties of perovskite oxides. La_1−xCe_xCoO₃ (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) perovskite-type oxides were synthesized through a citrate method and characterized by XRD, BET and XPS. The catalytic activities were enhanced significantly with Ce substitution, and achieved the best when x was 0.2, but decreased at higher x values. The performed characterizations reveal that the adsorbed oxygen on the surface plays an important role in the oxidation of NO into NO₂. The surface compounds after the co-adsorption of NO and O₂ at room temperature, were investigated by DRIFTS and TPD experiments. Three species: the bridging nitrate, the hyponitrites and the monodentate nitrate, were formed on the surface. The order of thermal stabilities was as follows: monodentate > hyponitrite > bridging. Among them, only the monodentate nitrate which decomposed at above 300 °C, would desorb NO₂ into the gas phase. When Ce was added, the temperature of monodentate nitrate desorption became low and the adsorption of the other two species decreased. This might be related to the oxidation state of Co on the surface. Analysis by synthesizing the characterization results and catalytic activity data shows that large amounts of adsorbed oxygen, small amount of inactive compounds on the surface and low NO₂ desorption temperature are favorable for the oxidation of NO.     

Stability of SrFeO3 based materials in the presence of SiOx species in humid atmosphere at high temperatures and pressures

Strontium silicate was found on the surface of La_0.2Sr_0.8Fe_0.79Cr_0.2Mg_0.01O₃ exposed to hydrogen containing humid atmospheres at 1000 °C and 30 bars. Silica originated from the furnace tube material and was transported via the gas phase as a gaseous silica hydrate. Fe and Sr were initially preferentially expelled from the perovskite grain boundaries to give Sr₂SiO₄ at the surface, along with a secondary Fe-rich phase and a LaCrO₃-rich grain boundary region. Eventually, Fe and Sr were drawn from the grains, leaving a porous structure. This investigation highlights the importance of avoiding Si sources near Sr-rich membranes in humid atmospheres at high temperatures.     

The NO selective reduction on the La1−xSrxMnO3 catalysts

La_1−xSr_xMnO₃ (x=0, 0.1, 0.3, 0.5, 0.7) perovskite-type oxides (PTOs) were prepared by coprecipitation under various calcination temperature, and their performances for the NO reduction were evaluated under a simulated exhaust gas mixture. The X-ray diffraction (XRD) and thermogravimetric analysis were carried out to find the formation process of the perovskite. The NO reduction rate under different reaction temperature, the concentration of oxygen and the presence of hydrocarbon were observed by the input/output analysis. In the presence of 10% excess oxygen, the catalyst La_0.7Sr_0.3MnO₃ calcined at 900 °C showed a NO reduction rate of 61% at 380 °C. The study of the reaction curves showed that C₃H₈ could act as the reducer for the NO reduction below 400 °C. The NO reduction is highly affected by increasing the O₂ concentration from 0.5 to 10%, especially at high temperatures when oxygen becomes more competitive than NO on the oxidation of C₃H₈, leading to a decrease of the NO reduction from 100% to zero at 560 °C.     

Preparation, microstructure and microwave dielectric properties of ZrxTi1−xO4 (x=0.40–0.60) ceramics

Zr_xTi_1−xO₄ (x=0.40–0.60) ceramics sintered without additives were prepared from powders made by the coprecipitation of metal salts from aqueous solutions in order to investigate the existence range of a homogeneous phase and the relationships between composition, microstructure and the dielectric properties. XRD, TEM, SEM, EDS, and the dielectric measurements were used to characterize the products. A homogeneous solid solution was obtained. Its crystal structure was isomorphous with ZrTiO₄. The variation of the lattice parameters with TiO₂ content was discussed. The optimum sintering temperature of samples was dependent of composition. TiO₂ suppressed the densification and acted as a grain growth enhancer during the sintering process. With the increase in TiO₂ content the relative densities of the sintered bodies decrease, while the grain sizes increase. The dielectric properties at microwave frequency (1.8 GHz) in this system, especially Q value, were poor, due to low densification, impurities and lattice defects. The dielectric constant _r and Q value exhibited a significant dependence on the relative density and composition. Both _r and Q increased with the increase in relative density, but they were primarily influenced by the composition and the effect of the relative density could be ignored when the relative density was greater than 90% theoretical. _r increased slightly with increasing TiO₂ content, while Q value decreased.     

Synthesis and characterization of Li ion conducting La2/3−xLi3xTiO3 by a polymerizable complex method

Ultrafine lithium ion conducting La_2/3−xLi_3xTiO₃ (x = 0.11, LLT) powder was synthesized by a simple polymerizable complex method based on the Pechini-type process. The formation mechanism, homogeneity and microstructure of the samples were investigated by thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD analysis indicated the formation of pure perovskite-type phase. The powder synthesized at a temperature as low as 900 °C in a much shorter time than solid-state reaction method was well crystallized. The lithium ion conductivity of the LLT ceramics sintered at 1200 °C was found to be 9 × 10⁻⁴ S/cm at room temperature.     

Crystalline structure and electrical properties of YCuxMn1–xO3 solid solutions

Solid solutions belonging to the Mn-rich region of the YCu_xMn_1–xO₃ system have been studied. The powders were prepared by solid-state reaction between the corresponding oxides. Sintered ceramics were obtained by firing at 1100–1325 °C. The incorporation of 30 at.% Cu to the yttrium manganite induces the formation of a perovskite-type phase, with orthorhombic symmetry. Increase of the Cu amount do not appreciably affects the orthorhombicity factor b/a, up to an amount of 50 at.% Cu. Above this Cu amount, a multiphase system has been observed, with the presence of unreacted-Y₂O₃, YMnO₃ and Y₂Cu₂O₅, along with a perovskite phase. DC electrical conductivity measurements have shown a semiconducting behaviour for all the solid solutions with perovskite-type structure. The room temperature conductivity increases with Cu until 33 at.% Cu, and then decreases. Thermally activated small polaron hopping mechanism, between Mn³⁺ and Mn⁴⁺ cations, controls the conductivity in these ceramics. Results are discussed as a function of the Mn³⁺/Mn⁴⁺ ratio for each composition.     

Nanosized perovskite-type oxides La1−xSrxMO3−δ (M = Co, Mn; x = 0, 0.4) for the catalytic removal of ethylacetate

Nanometer perovskite-type oxides La_1−xSr_xMO_3−δ (M = Co, Mn; x = 0, 0.4) have been prepared using the citric acid complexing-hydrothermal-coupled method and characterized by means of techniques, such as X-ray diffraction (XRD), BET, high-resolution scanning electron microscopy (HRSEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and temperature-programmed reduction (TPR). The catalytic performance of these nanoperovskites in the combustion of ethylacetate (EA) has also been evaluated. The XRD results indicate that all the samples possessed single-phase rhombohedral crystal structures. The surface areas of these nanomaterials ranged from 20 to 33 m² g⁻¹, the achievement of such high surface areas are due to the uniform morphology with the typical particle size of 40–80 nm (as can be clearly seen in their HRSEM images) that were derived with the citric acid complexing-hydrothermally coupled strategy. The XPS results demonstrate the presence of Mn⁴⁺ and Mn³⁺ in La_1−xSr_xMnO_3−δ and Co³⁺ and Co²⁺ in La_1−xSr_xCoO_3−δ, Sr substitution induced the rises in Mn⁴⁺ and Co³⁺ concentrations; adsorbed oxygen species (O⁻, O₂⁻, or O₂²⁻) were detected on the catalyst surfaces. The O₂-TPD profiles indicate that Sr doping increased desorption of the adsorbed oxygen and lattice oxygen species at low temperatures. The H₂-TPR results reveal that the nanoperovskite catalysts could be reduced at much lower temperatures (<240 °C) after Sr doping. It is observed that under the conditions of EA concentration = 1000 ppm, EA/oxygen molar ratio = 1/400, and space velocity = 20,000 h⁻¹, the catalytic activity (as reflected by the temperature (T_100%) for EA complete conversion) increased in the order of LaCoO_2.91 (T_100% = 230 °C) ≈ LaMnO_3.12 (T_100% = 235 °C) < La_0.6Sr_0.4MnO_3.02 (T_100% = 190 °C) < La_0.6Sr_0.4CoO_2.78 (T_100% = 175 °C); furthermore, there were no formation of partially oxidized by-products over these catalysts. Based on the above results, we conclude that the excellent catalytic performance is associated with the high surface areas, good redox properties (derived from higher Mn⁴⁺/Mn³⁺ and Co³⁺/Co²⁺ ratios), and rich lattice defects of the nanostructured La_1−xSr_xMO_3−δ materials.     

Microwave dielectric properties and co-firing with copper of (Bi1−xCux)(Nb1−xWx)O4 ceramics

Effect of substitution of CuO and WO₃ on the microwave dielectric properties of BiNbO₄ ceramics and the co-firing between ceramics and copper electrode were investigated. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ (x = 0.005, 0.01, 0.015, 0.02) composition can be densified between 900 and 990 °C. The microwave dielectric constants lie between 36 and 45 and the pores in ceramics were found to be the main influence. The Q values changes between 1400 and 2900 with different x values and sintering temperatures while Q_f values lie between 6000 and 16,000 GHz. The microwave dielectric losses, mainly affected by the grain size, pores, and the secondary phase, are discussed. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ ceramics and copper electrode was co-fired under N₂ atmosphere at 850 °C and the EDS analysis showed no reaction between the dielectrics and copper electrodes. This result presented the (Bi_1−xCu_x)(Nb_1−xW_x)O₄ dielectric materials to be good candidates for LTCC applications with copper electrode.     

A stabilization mechanism for the perovskite La2/3TiO3 compound with Fe2O3: A structural and electrical investigation

We have stabilized the perovskite La_2/3TiO₃ by adding LaFeO₃ and shown that in general the stabilization mechanism for the (1 − x)La_2/3TiO_3–xLaFeO₃ mixture involves the formation of a solid solution for compositions with x ≥ 0.04. The crystal structure of the solid solution transforms from orthorhombic to tetragonal at x = 0.2, becomes cubic in the range 0.3 < x < 0.8, and transforms again into orthorhombic (typical for pure LaFeO₃) for values greater than 0.8. Detailed impedance-spectroscopy measurements for various compositions and conditions showed that the limiting step in the conduction mechanism was conduction across the grain boundaries. In the concentration range 0.04 < x < 0.25 the room temperature conductivity increases up to 0.0017 S cm⁻¹, after which it decreases again. Part of the initial increase is probably due to the formation of free electrons in accordance with (Fe_Ti)′ → (Fe_Ti)^x + n′. Other defect-formation mechanisms are also discussed, but are ruled out for a variety of reasons. Another interesting phenomenon that also affected the average conductivity was identified, i.e., the variation of the average particle size with composition.     

Performance of La1−xSrxAl1−y−y′FeyMgy′O3−δ perovskites in methane combustion: effect of aluminum and magnesium content

Perovskites of different La_1−xSr_xAl_1−y−y′Fe_yMg_y′O_3−δ compositions (x=0, 0.1, 0.15, 0.2 and y=0.1, 0.3, 0.5, 0.8) were prepared from a reactive precursor slurry of hydrated oxides. Each sample was aged between 16 and 26 h up to 1473 K. Activity in methane combustion (1%/air) was determined in a plug-flow reactor, with 1 g catalyst and 24 l/h flowrate. Gradual decrease in activity due to thermal aging was observed, the degree of activity loss being composition dependent. Nevertheless, activity of samples aged at 1370 K was nearly independent of composition. The best thermal stability showed LaAl_0.65Fe_0.15Mg_0.2O₃ perovskite. None of the magnesium substituted perovskites performed better than a La_0.85Sr_0.15Al_0.87Fe_0.13O₃ reference sample.     

AMnO3 (A=La, Nd, Sm) and Sm1−xSrxMnO3 perovskites as combustion catalysts: structural, redox and catalytic properties

Catalytic combustion of methane has been investigated over AMnO₃ (A = La, Nd, Sm) and Sm_1−xSr_xMnO₃ (x = 0.1, 0.3, 0.5) perovskites prepared by citrate method. The catalysts were characterized by chemical analysis, XRD and TPR techniques. Catalytic activity measurements were carried out with a fixed bed reactor at T = 623–1023 K, space velocity = 40 000 N cm³ g⁻¹ h⁻¹, CH₄ concentration = 0.4% v/v, O₂ concentration = 10% v/v.

Specific surface areas of perovskites were in the range 13–20 m² g⁻¹. XRD analysis showed that LaMnO₃, NdMnO₃, SmMnO₃ and Sm_1−xSr_xMnO₃ (x = 0.1) are single phase perovskite type oxides. Traces of Sm₂O₃ besides the perovskite phase were detected in the Sm_1−xSr_xMnO₃ catalysts for x = 0.3, 0.5. Chemical analysis gave evidence of the presence of a significant fraction of Mn(IV) in AMnO₃. The fraction of Mn(IV) in the Sm_1−xSr_xMnO₃ samples increased with x. TPR measurements on AMnO₃ showed that the perovskites were reduced in two steps at low and high temperature, related to Mn(IV) → Mn(III) and Mn(III) → Mn(II) reductions, respectively. The onset temperatures were in the order LaMnO₃ > NdMnO₃ > SmMnO₃. In Sm_1−xSr_xMnO₃ the Sr substitution for Sm caused the formation of Mn(IV) easily reducible to Mn(II) even at low temperature. Catalytic activity tests showed that all samples gave methane complete conversion with 100% selectivity to CO₂ below 1023 K. The activation energies of the AMnO₃ perovskites varied in the same order as the onset temperatures in TPR experiments suggesting that the catalytic activity is affected by the reducibility of manganese. Sr substitution for Sm in SmMnO₃ perovskites resulted in a reduction of activity with respect to the unsubstituted perovskite. This behaviour was related to the reduction of Mn(IV) to Mn(II), occurring under reaction conditions, hindering the redox mechanism.     

Surface properties and catalytic performance of La1−xSrxCrO3 perovskite-type oxides for CO and C3H6 combustion

Catalytic CO oxidation and C₃H₆ combustion have been studied over La_1−xSr_xCrO₃ (x = 0.0–0.3) oxides prepared by solid-state reaction and characterised by X-ray diffraction (XRD), nitrogen adsorption (BET analysis) and X-ray photoelectron spectroscopy (XPS). The expected orthorhombic perovskite structure of the chromite is observed for all levels of substitution. However, surface segregation of strontium along with a chromium oxidation process, leading to formation of Cr⁶⁺-containing phases, is produced upon increasing x and shown to be detrimental to the catalytic activity. Maximum activity is achieved for the catalyst with x = 0.1 in which mixed oxide formation upon substitution of lanthanum by strontium in the chromite becomes maximised.     

Synthesis, structural analysis and electrochemical performance of low-copper content La2Ni1−xCuxO4+δ materials as new cathodes for solid oxide fuel cells

In order to enhance the electrochemical performance of solid oxide fuel cells (SOFCs), La₂Ni_1−xCuO_4+δ (x = 0, 0.01, 0.02, 0.05 and 0.1) doped with copper in percentages, varying between 1% and 10%, were prepared following the modified Pechini method. The microstructure and morphology of the samples were analyzed by XRD and SEM. The electrochemical performance was followed by impedance spectroscopy. La₂Ni_0.99Cu_0.01O_4+δ samples showed good electrochemical and physicochemical properties with respect to the undoped material and is potentially a promising cathode. Indeed, doping with such small amounts of copper (1%) into the nickel site led to the formation of pure phases and stabilized the material before and after use at high temperature under air. In contrast, doping with higher amounts of copper (2%, 5% and 10%) led, after heating at 1000 °C for 48 h, to the formation of another phase resulting from the diffusion of copper into the YSZ electrolyte, limiting the interest to these materials as SOFC cathodes.     

Catalytic combustion of CH4 and CO on La1−xMxMnO3 perovskites

The activities of perovskites depend on compositions and preparation methods. Various perovskites, La_1−xM_xMnO₃ (M=Ag, Sr, Ce, La), have been prepared by two different methods (co-precipitation and spray decomposition). The new preparation method, spray decomposition, produced perovskites of a high surface area of over 10 m²/g. The catalytic activities for CH₄ and CO oxidation have been studied on a series of catalysts, La_1−xM_xMnO₃. The perovskite-type oxide, La_0.7Ag_0.3MnO₃, shows the highest catalytic activity: the complete conversion of CO and CH₄ at 370 and 825 K, respectively.     

Sol-gel synthesis of BaTiO3 and Ba1−x−yCaySrx(ZryTi1−y)O3 perovskite powders

A BaTiO₃ powder has been prepared by the sol-gel process from the hydrolysis of a solution of barium acetate and titanium ethylate in the presence of acetic acid as a catalyst. Supplementary constituents in the form of Ca(CH₃COO)₂, Zr(OC₃H₇)₄, Sr(NO₃)₂ also can be used. Intermediate phases of barium acetate and barium carbonate have been identified by differential thermal analysis, X-ray diffraction, infra-red and scanning electron microscopy. BaTiO₃ with perovskite structure synthesizes in the temperature range from 600 to 1000°C.