Non-congruence of high-temperature mechanical and structural behaviors of LaCoO3 based perovskites

This paper presents the mechanical behavior of LaCoO₃ and La_0.8Ca_0.2CoO₃ ceramics under four-point bending in which the two cobaltites are subjected to a low stress of ∼8 MPa at temperatures ranging from room temperature to 1000 °C. Unexpected stiffening is observed in pure LaCoO₃ in the 700–900 °C temperature range, leading to a significant increase in the measured Young’s modulus, whereas La_0.8Ca_0.2CoO₃ exhibits softening from 100 °C to 1000 °C, as expected for most materials upon heating. Neutron diffraction, X-ray diffraction and micro-Raman spectroscopy are used to study the crystal structure of the two materials in the RT–1000 °C temperature range. Despite a detailed study, there is no conclusive evidence to explain the stiffening behavior observed in pure LaCoO₃ as opposed to the softening behavior in La_0.8Ca_0.2CoO₃ at high temperatures (above 500 °C).     

Study of strontium ferrites substituted by lanthanum on the structural and magnetic properties

M-type strontium ferrites, Sr_0.8La_0.2Fe₁₂O₁₉ have been synthesized by conventional ceramic process. The effects of lanthanum addition and sintering temperature on microstructures and magnetic properties of SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ samples were investigated. Microstructural analysis of the SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ specimens, sintered at different temperatures revealed that average grain sizes of SrFe₁₂O₁₉ ferrites were larger than that of Sr_0.8La_0.2Fe₁₂O₁₉ ferrite and increased with increasing sintering temperature. The X-ray diffraction (XRD) results confirmed the strontium hexagonal ferrite phase of SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ compounds. A maximum coercivity value of 4850 Oe and maximum saturation magnetization value of 102 emu/g were obtained for the SrFe₁₂O₁₉ ferrite sintered at 1150 °C and for the SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ ferrites sintered at 1300 °C, respectively. The remanence (Mr) of Sr_0.8La_0.2Fe₁₂O₁₉ sample sintered at 1200 °C possesses the maximum value of 60 emu/g.     

Synthesis and characterization of Sr2+ and Mg2+ doped LaGaO3 by co-precipitation method followed by hydrothermal treatment for solid oxide fuel cell applications

In the present study, LSGM (La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8) powder has been synthesized using precipitation route followed by hydrothermal treatment. Quantitative phase analyses of different powders, have been done by Rietveld analyses of the XRD data and they reveal formation of single phase orthorhombic LSGM at 1400 °C, 8 h. Morphology of the calcined powder and microstructure of the sintered pellets are observed by transmission electron microscope (TEM) and scanning electron microscope (SEM), respectively. Thermal analysis has been carried out to find out the thermal expansion co-efficient. Successive electrical characterization of the 99% dense sintered pellet has been done by impedance spectroscopic analysis. The diffused semicircles observed in the Nyquist plots have been simulated as (RQ)(RQ) circuit and the total ionic conductivity obtained is found to be the highest for LSGM synthesized by similar routes.     

Dielectric and magnetic properties of BiFeO3 ceramics prepared by hydrothermal synthesis

Single-phase BiFeO₃ powders were prepared at a temperature of 200 °C by a hydrothermal synthesis. BiFeO₃ ceramics were prepared with the powders by a conventional ceramic process. The BiFeO₃ ceramics with no impurity phase were prepared at the sintering temperature of 650–800 °C. The dense microstructure was observed in the BiFeO₃ ceramics sintered at a temperature of 700 °C and higher. BiFeO₃ ceramics show linear M–H curves in low H, which are antiferromagnetic behaviors. The dielectric dispersion was observed at the frequency range of 10 kHz to 1 MHz in the BiFeO₃ ceramic sintered at 700 °C or lower. The dielectric constant and loss of the BiFeO₃ ceramics sintered at 750 °C or higher were about 85 and 0.4 at 100 kHz, respectively.     

Single step reactive sintering and chemical compatibility between La9Sr1Si6O26.5 and selected cathode materials

Apatite-type silicates are considered as promising electrolytes for solid oxide fuel cells (SOFC). However more studies on the chemical compatibility of these materials with common SOFC electrodes are required. Here, we report the synthesis of single phase La₉Sr₁Si₆O_26.5 composition by reactive sintering at 1650 °C for 10 h. Fully dense pellets showed very high oxide-anion conductivity, 25 mS cm^?1 at 700 °C. Furthermore, the chemical compatibility of La₉Sr₁Si₆O_26.5 with some selected cathode materials has also been investigated. The lowest reaction temperatures were determined to be 1100 °C, 1000 °C and 900 °C for La_0.8Sr_0.2MnO_3?δ, La₂Ni_0.8Cu_0.2O₄ and La_0.6Sr_0.4Co_0.8Fe_0.2O₃, respectively. The segregation of minor amounts of SiO₂ seems to be a key limiting factor that must be overcome. Finally, these cathode materials were deposited over dense oxy-apatite pellets and the area specific resistances in symmetrical cells were determined. These values, at 700 °C, were 14.4 and 2.6 Ω cm² for La_0.8Sr_0.2MnO_3?δ and La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ, respectively. Furthermore, the area specific resistances are notably improved 0.6 Ω cm² when a 50 wt.% composite of La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ and Ce_0.8Gd_0.2O_1.9 is used.     

Preparation of ultra-fine Sm0.2Ce0.8O1.9 powder by a novel solid state reaction and fabrication of dense Sm0.2Ce0.8O1.9 electrolyte film

A novel solid state reaction was adopted to prepare Sm_0.2Ce_0.8O_1.9 (SDC) powder. A mixed oxalate Sm_0.2Ce_0.8(C₂O₄)_1.5·2H₂O was synthesized by milling a mixture of cerium acetate hydrate, samarium acetate hydrate, and oxalic acid for 5 h at room temperature. An ultra-fine SDC powder with the primary particle size of 5.5 nm was obtained at 300 °C. The ultra-low temperature for the formation of SDC phase was due to the atomic level mixture of the Sm³⁺ and Ce⁴⁺ ions. The crystal sizes of SDC powders at 300 °C, 550 °C, 800 °C, and 1050 °C were 5.5 nm, 11.4 nm, 24.1 nm and 37.5 nm, respectively. The sintering curves showed that the powder calcined at lower temperature was easier to be sintered owning to its smaller particle size. A solid oxide electrolytic cell (SOEC), comprising porous La_0.8Sr_0.2Cu_0.1Fe_0.9O_3−δ (LSCF) for substrate, LSCF–SDC for active electrode, SDC for electrolyte, and LSCF–SDC for symmetric electrode, was fabricated by dip-coating and co-sintering techniques. An extremely dense SDC film with the thickness of 20 μm was obtained at only 1200 °C, which was about 100–300 °C lower than the literatures׳ reports. The designed SOEC was proved to work effectively for decomposing NO (3500 ppm, balanced in N₂), 80% NO can be decomposed at 600 °C.     

Cellulose templating method for the preparation of La0.8Sr0.2Ga0.83Mg0.17O2.815 (LSGM) solid oxide electrolyte

La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM) materials are synthesized with a fast and facile cellulose templating method for the first time and characterized by XRD, EIS, Archimedes method and SEM–EDS. LSGM powders with a phase purity of 91.7 mol% are obtained after the calcination at 1300 °C for 12 h. SEM–EDS results indicate possible decomposition and reconstruction of the LSGM phase due to the diffusion of Sr-rich species to the grain boundaries for the sample sintered at 1500 °C for 6 h. Maximum conductivity value is found to be 4.2 × 10^?2 S cm^?1 at 800 °C for the sample calcined at 1300 °C for 12 h and sintered at 1400 °C for 6 h. Phase purity, stability and relative density are the important factors for obtaining high performance LSGM electrolytes. Therefore, cellulose templating method is a promising candidate for the preparation of LSGM electrolytes.     

Microstructures and microwave dielectric properties of (1 − y)La1−xSmx(Mg0.5Sn0.5)O3–yCa0.8Sm0.4/3TiO3 ceramics

The (1 ? y)La_1?xSm_x(Mg_0.5Sn_0.5)O₃–yCa_0.8Sm_0.4/3TiO₃ ceramics were prepared by the conventional solid-state method. The X-ray diffraction patterns of the La_1?xSm_x(Mg_0.5Sn_0.5)O₃ ceramics revealed that La_1?xSm_x(Mg_0.5Sn_0.5)O₃ is the main crystalline phase, which is accompanied by a little La₂Sn₂O₇ as the second phase. An apparent density of 6.59 g/cm³, a dielectric constant (?_r) of 19.9, a quality factor (Q × f) of 70,200 GHz, and a temperature coefficient of resonant frequency (τ_f) of ?77 ppm/°C were obtained when the La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics were sintered at 1500 °C for 4 h. The temperature coefficient of resonant frequency (τ_f) increased from ?77 to +6 ppm/°C as y increased from 0 to 0.6 when the (1 ? y)La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃–yCa_0.8Sm_0.4/3TiO₃ ceramics were sintered at 1500 °C for 4 h. 0.425La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃–0.575Ca_0.8Sm_0.4/3TiO₃ ceramic that was sintered at 1500 °C for 4 h had a τ_f of ?3 ppm/ °C.     

Enhanced sintering behavior of LSGM electrolyte and its performance for solid oxide fuel cells deposited by vacuum cold spray

How to obtain dense La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) electrolyte at low sintering temperature (<1300 °C) is a challenge to improve solid oxide fuel cell (SOFC) performance at intermediate operation temperature. In this study, a double-layer design method for vacuum cold spray (VCS) prepared-LSGM electrolyte assisted with two-step sintering at a low temperature was proposed. The sintering behavior of VCS deposited LSGM layers at 1200 °C was investigated. The LSGM layers became denser in most regions except the appearance of some cracks. Subsequently, the effect of a second LSGM layer on the sintered top layer was studied to block cracks. Results showed that the co-sintered layer with a thickness of approximately 5 μm presented a maximum open circuit voltage of ∼0.956 V at 650 °C and a maximum power density of 592 mW/cm² at 750 °C. Result indicates that the sintering assisted VCS is a promising method to prepare the LSGM electrolyte applied in intermediate temperature SOFCs.     

Sintering behavior of La2O3–B2O3–TiO2 glass powders prepared by spray pyrolysis for low temperature co-fired ceramics

Fine-sized La₂O₃–B₂O₃–TiO₂ glass powders with spherical shape were directly prepared by spray pyrolysis at a temperature of 1500 °C. The optimum flow rate of the carrier gas to prepare the glass powders with dense inner structure and fine size by complete melting was 10 L/min. The ratio of La/Ti was identified to be 2.06:1, which was close to the original starting ratio of La/Ti in mixture of the spray solution. The T_g and T_c of the powders were 614 and 718 °C. The crystal structures within the powders were observed from the sintered disc at 630 °C. The mean sizes of the powders changed from 0.24 to 0.71 μm when the concentrations of the spray solution were changed from 0.025 to 0.5 M. The BET surface areas of the powders changed from 4.4 to 1.6 m²/g. The grain sizes of the sintered discs increased with increasing the sintering temperatures. The main crystal structure of the sintered discs was LaBO₃.     

Conductivity and electrochemical performance of (Ba0.5Sr0.5)0.8La0.2CoO3−δ cathode for intermediate-temperature solid oxide fuel cell

This study reports the successful preparation of a single-phase cubic (Ba_0.5Sr_0.5)_0.8La_0.2CoO_3?δ perovskite by the citrate–EDTA complexing method. Its crystal structure, thermogravimetry, coefficient of thermal expansion, electric conductivity, and electrochemical performance were investigated to determine its suitability as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Its coefficient of thermal expansion shows abnormal expansion at 300 °C, which is associated with the loss of lattice oxygen. The maximum conductivity of a (Ba_0.5Sr_0.5)_0.8La_0.2CoO_3?δ electrode is 689 S/cm at 300 °C. Above 300 °C, the electronic conductivity of (Ba_0.5Sr_0.5)_0.8La_0.2CoO_3?δ decreases due to the formation of oxygen vacancies. The charge-transfer resistance and gas phase diffusion resistance of a (Ba_0.5Sr_0.5)_0.8La_0.2CoO_3?δ–Ce_0.8Sm_0.2O_1.9 composite cathode are 0.045 Ω cm² and 0.28 Ω cm², respectively, at 750 °C.     

Studies on the redox properties of chromite perovskite catalysts for soot combustion

This paper deals with the preparation (by combustion synthesis), the characterization (by XRD, AAS, BET, SEM, TEM, TPD/R, and XPS analyses), the catalytic activity testing (in a temperature-programmed combustion microreactor and in a DSC analyzer), and the assessment of the reaction mechanism of a series of nanostructured soot combustion catalysts based on La–Cr substoichiometric or alkali-metal-substituted perovskites (La_0.9CrO₃, La_0.8CrO₃, La_0.9Na_0.1CrO₃, La_0.9K_0.1CrO₃, La_0.9Rb_0.1CrO₃, La_0.8Cr_0.9Li_0.1O₃), whose performance is compared with that of the standard LaCrO₃. Some conclusions are drawn concerning the role of each single constituting element on the activity of the most promising catalyst, La_0.8Cr_0.9Li_0.1O₃, which is already active well below 400 °C. The role of weakly chemisorbed O⁻ surface species in particular is pointed out as crucial for the soot combustion process. This indicates the way for the development of new, more active catalysts, possibly capable of delivering amounts of these oxygen species even higher than those obtained (about 700 μmol / g) for the most active Li-substituted lanthanum chromite catalyst developed.     

Ca2+‐Doped LaCrO3: A Novel Energy‐Saving Material with High Infrared Emissivity

The present study describes the successful synthesis of a Ca²⁺‐doped LaCrO₃ ceramic with high infrared (IR) emissivity, which is important for high‐temperature applications for significant energy saving. It is demonstrated that 20 mol% Ca²⁺‐doped LaCrO₃, i.e., La_0.8Ca_0.2CrO₃, exhibited an IR emissivity as high as 0.95 in the spectral region of 3–5 μm, which was 33.8% higher than that of LaCrO₃. By using La_0.8Ca_0.2CrO₃ as IR radiation agent in surface coating of heating unit, the radiative heat transfer could be enhanced significantly. The mechanism of the high IR emissivity of La_0.8Ca_0.2CrO₃ was attributed to the following aspects: Ca²⁺ doping introduced an impurity energy level of Cr⁴⁺ into LaCrO₃ and increased the hole carrier concentration, enhancing both impurity absorption and hole carrier absorption in the IR region; moreover, the doping caused lattice distortion enhanced the lattice vibration absorption. This novel high IR emissivity ceramic shows a promising future in high‐temperature applications for the purpose of energy‐saving.     

Preparation,characterization and electrical properties of Ca and Sr doped LaCrO3

Perovskite structure chromites are one of the most important family of functional materials, especially useful as interconnect materials in solid oxide fuel cells (SOFCs). In this paper, the relationship of the structure, valence states of Cr, and electrical conductivity of La_0.8Ca_0.2CrO₃ and La_0.7Ca_0.2Sr_0.1CrO₃ were studied. The samples were synthesized by solid state reaction under air or nitrogen condition, respectively. It is found that the relationship of AC conductivity with frequency is in consistent with Jonscher power law when the frequency is between 2 × 10²–2 × 10⁶ Hz. The electrical resistivity is in line with temperature change, which means that the conduction mechanism is small polaron theory and meets Arrhenius formula. Binding energy of the Cr_2p in the compounds synthesized in air corresponded to 576.4 eV, 577.6 eV and 579.7 eV, which means the valence states of Cr are + 3, + 5 and + 6. But for the compounds synthesized in N₂ atmosphere the valence states of Cr are + 3 and + 6.     

Effect of strontium and cerium doping on the structural characteristics and catalytic activity for C3H6 combustion of perovskite LaCrO3 prepared by sol–gel

Two series of Sr- or Ce-doped La_1−xM_xCrO₃ (x = 0.0, 0.1, 0.2 and 0.3) catalysts were prepared by thermal decomposition of amorphous citrate precursors followed by annealing at 800 °C in air atmosphere. The effect of Ce and Sr on the morphological/structural properties of LaCrO₃ was investigated by means of thermogravimetric/differential thermal analysis (TG/DTA) of the precursors decomposition under air, X-ray diffraction (XRD), electron paramagnetic resonance (EPR), transmission electron microscopy–X-ray energy dispersive spectroscopy (TEM–XEDS), S_BET determination, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques. The characterization results are employed to explain catalytic activity results for C₃H₆ combustion. It is shown that the lanthanum chromite perovskite structure is obtained upon thermal treatment of the sol–gel derived precursors at T > ca. 800 °C. The presence of the dopant generally induces the formation of segregated oxide phases in the samples calcined at 800 °C although some introduction of the Sr in the perovskite structure is inferred from EPR measurements. The oxidation activity becomes maximised upon formation of such doped perovskite structure.     

Stability of calcium substituted lanthanum chromites used as SOFC anodes for methane oxidation

La_0·7Ca_0·32CrO₃, La_0·75Ca_0·15Sr_0·1Cr_0·95Mg_0·05O₃, La_0·85Ca_0·15CrO₃ and La_0·85Ca_0·15Cr_0·9Mg_0·1O₃ compounds were studied in reducing atmospheres containing hydrogen, methane and CO/CO₂. They were found to exhibit liquid phase sintering except for La_0·85Ca_0·15CrO₃. Mg enhanced the densification of stoichiometric LaCrO₃. Current was found to affect the stability of the heavily doped samples, whereas the reducing atmospheres did not have a detectable impact. Calcium and or strontium and chromium enriched secondary phases were detected on current treated La_0·7Ca_0·32CrO₃ and La_0·75Ca_0·15Sr_0·1Cr_0·95Mg_0·05O₃ samples. La_0·85Ca_0·15Cr_0·9Mg_0·1O₃ did not degrade under current, at least for 100 h. A current enhanced demixing could be responsable for the segregation. The solubility limit of calcium in LaCrO₃ is thought to be low at 800°C (around 15%).     

Enhanced sinterability of mechanical alloyed La9.33Si2Ge4O26 oxyapatite powders for IT-SOFC electrolytes

Dense oxyapatite-based La_9.33Si₂Ge₄O₂₆ electrolytes have been successfully prepared by electrical sintering at 1400 °C in static air for 1 h from dry milling La₂O₃, SiO₂ and GeO₂ powders, in adequate atomic proportions, at 350 rpm for 15 h, under controlled environmental conditions, in a planetary ball mill. The densification behaviour of apatite-type phase La_9.33Si₂Ge₄O₂₆ powders synthesized by mechanical alloying was investigated through microstructural evolution with sintering temperature by means of XRD and SEM/EDS analyses. The content of germanium in the sintered samples remained almost constant, suggesting that its incorporation in the apatite phase hinders the high temperature (>1250 °C) volatilization process.     

Tuning the microwave dielectric properties of Zn2SnO4 ceramics by adding Ca0.8Sr0.2TiO3

The influence of sintering temperature on the microwave dielectric properties and microstructure of the (1−y)Zn₂SnO₄–yCa_0.8Sr_0.2TiO₃ ceramic system were investigated with a view to their application in microwave devices. A (1−y)Zn₂SnO₄–yCa_0.8Sr_0.2TiO₃ ceramic system was prepared by the conventional solid-state method. The X-ray diffraction patterns of the 0.85Zn₂SnO₄–0.15Ca_0.8Sr_0.2TiO₃ ceramic system did not significantly vary with sintering temperature. A dielectric constant of 9.6, a quality factor (Q×f) of 15,900 GHz, and a temperature coefficient of resonant frequency of −4 ppm/°C were obtained when the 0.85Zn₂SnO₄–0.15Ca_0.8Sr_0.2TiO₃ ceramic system was sintered at 1175 °C for 4 h.     

Characteristics of BaNd2Ti5O14 powders directly prepared by high-temperature spray pyrolysis

BaNd₂Ti₅O₁₄ powders were directly prepared by high-temperature spray pyrolysis. The powders prepared at temperatures of 1300 and 1500 °C exhibited a pure BaNd₂Ti₅O₁₄ phase. The powders prepared at 1300 °C were spherical in shape. However, the powders prepared at 1500 °C showed non-spherical shapes. The BaNd₂Ti₅O₁₄ powders had a composition similar to that of the spray solution. The mean sizes of the BaNd₂Ti₅O₁₄ powders increased from 0.23 to 0.60 μm when the concentration of the spray solution was increased from 0.01 to 0.2 M. At a sintering temperature of 1100 °C, bridge-like structures were formed between the powders. Pellets sintered at 1300 °C exhibited a dense structure comprising rod-like crystals.     

Synthesis of La0.9Sr0.1Ga0.8Mg0.2O2.85 by successive freeze-drying and self-ignition of a hydroxypropylmethyl cellulose solution

The present paper reports the synthesis of La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 perovskite powders by a method combining freeze-drying and self-ignition of an aqueous solution of metallic nitrates containing hydroxypropylmethyl cellulose. The precursor powder obtained after self-ignition was submitted to various thermal treatments and the resulting powders were characterized by X-ray diffraction, electron microscopy, nitrogen adsorption–desorption isotherm analysis, mercury porosimetry and laser granulometry. It turns out that this synthesis method yields single-phase powders with good homogeneity and sinterability properties. The precursor powder treated at 1200 °C presents a coral-like structure which collapses under application of low uniaxial pressure, resulting in a narrow grain size distribution suitable for sintering (98.8% relative density for a pellet sintered at 1400 °C during 1 h). The fact that no milling step is necessary is an additional advantage of this method, which shows promising prospects for the synthesis of other multicationic oxides.