Structural studies of the (La,Sr) CrO3 system

Phase relationships in the La_1?xSr_xCrO₃ (0.25 ≤ x ≤ 0) system have been investigated. Substitution of Sr into LaCrO₃ leads to a phase region with rhombohedral symmetry at room temperature which is oxygen-deficient under strongly reducing conditions. The orthorhombic structure of LaCrO₃ and the rhombohedral structure of La_0.75Sr_0.25CrO₃ have been refined by profile fitting of neutron powder diffraction data.     

Synthesis and sintering of Sr- and Ca-doped lanthanum chromite ultrafine powder for SOFC interconnect

In this study, ultrafine powder of mixed Sr- and Ca-doped lanthanum chromite with two different compositions (La_0.7Sr_0.1Ca_0.2CrO₃ and La_0.7Sr_0.2Ca_0.1CrO₃) and average particle size of 150 nm was successfully synthesized by the simple process of glycine nitrate. The samples were characterized by thermal analysis, X-ray diffraction, and nitrogen adsorption–desorption, scanning and transmission electron microscopy. The synthesized ultrafine powders with perovskite type crystal structure and some SrCrO₄ or CaCrO₄ phases were cold isostatically pressed at 200 MPa, and then sintered in air at 1400 °C for several time periods (1–10 h). Relative density measurements were conducted by Archimedes method. The maximum relative density for both samples obtained after sintering in air at 1400 °C for 10 h was 98.2 % of the theoretical density and the average grain size of the sintered pellets was about 2 μm. In addition, the effects of composition on phase transition behavior and electrical properties were studied.     

Reaction characteristics of La0.84Sr0.16CrO3 formation

We studied the kinetics of La_0.84Sr_0.16CrO₃ formation from a precursor consisting of La and Sr chromium oxides and carbonates made by spray roasting. Pure LaCrO₃ becomes cubic at temperatures exceeding 1900 °C. Strontium doping lowers the transition temperature, for example, that of La_0.84Sr_0.16CrO₃ is 1700 °C. This transition is gradual and occurs over a 700 °C range upon heating and cooling. Low temperature (LT) air calcination (450 °C) of the precursor yields a mixture of LaCrO₄ and SrCrO₄, which following 20 h of heating at 1440 °C produces a homogeneous powder. Secondary electron images of this precursor reveal dense spheres with 95% of the theoretical density of La_0.84Sr_0.16CrO₃. High temperature (HT) calcination (800 °C) yields a mixture of LaCrO₃ and SrCrO₄, which following 40 h of heating at 1500 °C produces a uniform product. The LT and HT calcination causes oxygen loss.     

Preparation and electrochemical characterization of Sr- and Mn-doped LaGaO3 as anode materials for LSGM-based SOFCs

Sr- and Mn-doped lanthanum gallate powders (LSGMn, La_0.9Sr_0.1Ga_{1 – x} Mn _x O_{3 –} , x = 0.20, 0.35, 0.43) were prepared by glycine-nitrate combustion synthesis. X-ray diffraction patterns indicate the perovskite structure was formed without any second phase after calcining the powders at 1000°C for 4 h. Compacts of powders synthesized under stoichiometric combustion were sintered to densities over 95% of theoretical values. The electrical conductivity of this material in both air and H₂ were characterized using AC impedance spectroscopy. It showed that the conductivity in H₂ atmosphere is lower than that in air due to p-type electrical conduction in this material, and the electrical conductivity increases remarkably with increasing manganese content. Good chemical stability of La_0.9Sr_0.1Ga_{1 – x} Mn _x O_{3 –} in H₂ atmosphere as well as the relatively high conductivity makes it an appropriate anode material for Sr- and Mg-doped lanthanum gallate (LSGM)-based IT-SOFCs. Preliminary fuel cell performance measurements were performed, showing promising electrochemical properties of such anode materials.     

High-temperature mechanical properties of La0.7Sr0.3Cr1−yCoyO3 in reducing environments

La_0.7Sr_0.3Cr_1−yCo_yO₃ (y=0–0.2) was synthesized so that it could be sintered at temperatures as low as 1500°C, producing a highly dense system. The materials were fabricated into bars and mechanically tested using a four-point bend test, from room temperature to 1000°C. High mechanical strength was obtained at 1000°C; however, degradation of the mechanical properties was observed in the system at cobalt B-site dopant concentrations as low as 5 mol%, in reducing atmospheres at 1000°C. La_0.7Sr_0.3Cr_0.95Co_0.05O₃, for example, showed degradation after 55 min in H2 and 75 min in H2/3%H2O atmospheres, with total disintegration of the sample soon after. The degradation, and hence loss in mechanical strength, was postulated as being due to reduction of the perovskite, because CoO was observed as a second phase. La_0.7Sr_0.3CrO₃ showed no such degradation in the mechanical strength even after 10 days in H₂ at 1000°C. Hence, the increased sinterability of cobalt addition on the B-site of La(Sr)CrO₃ is offset by the reduction of stability (and mechanical properties) under either H₂ or H₂/3%H₂O environments. This revised version was published online in November 2006 with corrections to the Cover Date.     

The effect of sintering on the mechanical properties of SOFC ceramic interconnect materials

Solid oxide fuel cell (SOFC) interconnect materials, strontium and calcium doped lanthanum chromite, were synthesized to investigate the effect of dopant content and sintering temperature on their sinterability. The results show that approximately 96% of the theoretical density could be achieved when LaCrO₃, doped with 30 mol% Ca, was sintered in air at 1400°C. However, to get the same sintered density for the strontia doped material, a 1700°C sintering temperature had to be used. The effect of sintering temperature on the fracture strength was also investigated. A maximum fracture strength of 234 MPa for La_0.7Sr_0.3CrO_3– and 256 MPa for La_0.7Ca_0.3CrO_3–were obtained for both samples sintered at 1700°C.     

Characterization and electrical conductivity of La<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>CrO<Subscript>3</Subscript> NTC ceramics

The La_1?xSr_xCrO₃ (x?=?0–0.1) negative temperature coefficient (NTC) ceramics have been prepared by the traditional solid state reaction method. X-ray diffraction (XRD) analysis has revealed that the as-sintered ceramics crystallize in a single perovskite structure. Scanning Electron Microscope (SEM) images show that the doped Sr²⁺ contributes to in the decrease in porosity. X-ray photoelectron spectroscopy (XPS) analysis indicates the existence of Cr³⁺ and Cr⁶⁺ ions on lattice sites, which result in hopping conduction. The presence of the Cr⁶⁺ is one of the key factors that affect the electrical conductivity of La_1?xSr_xCrO₃. Resistance–temperature characteristics were studied in the range of ?80 to 10?°C for the ceramic samples, the electrical characterizations show that the electrical resistivity and material constant B decrease with the increase of the strontium content.     

Proton and oxide-ionic conduction in Sr- and Zn-doped LaGaO3

The ionic conduction behaviors in La_0.9Sr_0.1Ga_0.9Zn_0.1O_3−α under different atmospheres at 600–1,000 °C were studied by various electrochemical methods including ac impedance, hydrogen and oxygen concentration cells, electrochemical hydrogen and oxygen pumping, etc. The proton conduction in this oxide was investigated for the first time. The hydrogen concentration cell and oxygen concentration cell showed stable electromotive forces close to the theoretical ones calculated from Nernst’s equation, indicating that the conduction was almost pure ionic under hydrogen atmosphere or dry oxygen atmosphere. The electrochemical hydrogen pumping rates coincided with the theoretical ones calculated from Faraday’s law, confirming that La_0.9Sr_0.1Ga_0.9Zn_0.1O_3−α is a proton conductor under hydrogen atmosphere. A similar result for electrochemical oxygen pumping was obtained, indicating that it is an oxide-ionic conductor under dry oxygen atmosphere. The ionic conductivity was about 0.06 S cm⁻¹ at 1,000 °C.     

Investigation of strontium-doped La(Cr,Mn)O3 for solid oxide fuel cells

The (La, Sr) (Cr, Mn)O₃ system was investigated in an effort to develop an interconnect and cathode materials for solid oxide fuel cells. Sintering studies were done in air at temperatures below 1500°C. Significant improvements in densification were observed with substitution of 50 mol% Mn for chromium and a density of 95% theoretical was achieved with the substitution of 70 mol% Mn for chromium in the La(Cr, Mn)O₃ system. Electrical conductivity (d.c.) measurements were made as a function of temperature and oxygen activity. At 1000°C and 1 atm oxygen, the electrical conductivity ranged from 2.2–20 S cm^–1 for LaCr_0.8Mn_0.4O₃ and La_0.9Sr_0.1Cr_0.2Mn_0.8O₃, respectively. All of the compositions showed similar dependence of electrical conductivity on the oxygen activity. Dependence was small at high oxygen activities; as the oxygen activity decreased, a break in electrical conductivity at 10^–12 atm and 1000°C was observed, and then the electrical conductivity decreased asP _O2 ^1/4 . Sintering and electrical conductivity studies indicate that La_0.9Sr_0.1Cr_0.2Mn_0.8O₃ appears to be a candidate for solid oxide fuel cell applications.     

Proton conduction in La0.9Sr0.1Ga0.8Mg0.2O3−α ceramic prepared via microemulsion method and its application in ammonia synthesis at atmospheric pressure

The popular electrolyte of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−α was prepared via microemulsion method by using (NH₄)₂CO₃–NH₄OH as the co-precipitation reagent, followed by calcination and sinteration in air. The sintered ceramic was characterized by scanning electron microscopy (SEM) and powder X-ray diffraction (XRD) methods. The conduction behaviors in La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−α were investigated by using the electrochemical methods including gas concentration cells and electrochemical hydrogen permeation. The results indicate that the charge carriers in La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−α are protons under hydrogen atmosphere. Ammonia was synthesized at atmospheric pressure successfully by using La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−α as solid electrolyte for the first time.     

Structural, thermal, and electrical properties of La0.7Sr0.3Mn1−y FeyO3±δ

The crystal structure, thermal expansion, and electrical conductivity of the doped manganites La_0.7Sr_0.3Mn_1?y Fe_yO_3±δ (y = 0, 0.1, 0.2, 0.3) are studied. The manganites are shown to crystallize in rhombohedral symmetry (sp. gr. $R\bar 3c$ ). Their lattice parameters are determined, and their thermal expansion coefficients are measured in the range 600–1223 K in air. The conductivity of the manganites is measured in broad ranges of oxygen partial pressures (p _{O 2} = 0.1 to 10^?15 MPa) and temperatures (T = 1073–1273 K). The effects of Fe content, temperature, and p _{O 2} on the dominant defect species and the nature of electrical transport in La_0.7Sr_0.3Mn_1?y Fe_yO_3±δ are analyzed.     

Studies on Sr substituted lanthanum indate as mixed ionic conductor

Sr substituted LaInO₃ compounds have been prepared by the conventional solid state reaction method. The as-prepared samples are characterized by X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), differential thermal analysis (DTA), thermal expansion coefficient (TEC), and impedance spectroscopy. The XRD analysis of LSI 20 shows the formation of single phase La_0.9Sr_0.1InO_2.95 compound. However, the higher substitution of Sr²⁺ for La³⁺ leads to the formation of secondary insulating phase, i.e., SrIn₂O₄. Sr substitution leads to anionic vacancy which is the possible mechanism for ionic conduction in the given system. Reitveld refinement confirms orthorhombic phase formation in all the samples except LSI 23 (La_0.77Sr_0.23InO_2.89) which is well fitted monoclinic phase. TEC is nearly 9.1 × 10⁻⁶ °C⁻¹ in the range of 500–1000 °C in LSI 20 sample. The ionic conductivity is 0.11 mS cm⁻¹ at 800 °C in single phase LSI 20 sample. The solid solubility limit is x = 0.20 which is higher than earlier reported values for similar systems.     

Preparation, properties, and reactivity of lanthanum strontium ferrite as an intermediate temperature SOFC cathode

Lanthanum strontium ferrite is a promising cathode material especially because of its chemical stability and thermal expansion compatibility with respect to zirconia-based electrolyte materials. La_0.80Sr_0.20FeO₃ is synthesized by the gel-combustion technique using citric acid as the fuel. A phase-pure powder belonging to the perovskite structure is obtained at lower calcination temperature of 850 °C. The powders are tape-cast into tapes and detailed sintering studies are carried out between 1100 and 1450 °C and for different sintering durations. A thermal expansion coefficient of 11.65 ppm/°C from RT-1000 °C is measured for the sample sintered at 1450 °C for 6 h. The DC electrical conductivity is also measured for a number of samples having different sintered densities. Further, investigations on the chemical stability are demonstrated between this cathode and a scandia-doped zirconia electrolyte prepared in our laboratory.     

Effects of Co and Fe addition on the properties of lanthanum strontium manganite

The effects of Co and Fe dopants with the amount of 20 and 40 mol% on the properties of La_0.84Sr_0.16MnO₃ were investigated. All compositions were prepared by conventional mixed oxide process and sintered at 1450 °C. The structure of undoped and Co-doped compositions was found to be monoclinic. In addition, the second phase was observed in these sintered compositions. The conductivity of doped materials decreased as compared to that of La_0.84Sr_0.16MnO₃. The SEM microstructure showed the decrease of grain size as Co content increased. The thermal expansion coefficient (TEC) tended to increase as Co content increased. In contrast, the monoclinic and orthorhombic structures were found in 20 and 40 mol% Fe-doped La_0.84Sr_0.16MnO₃. The amount of second phase in sintered composition depends on the amount of Fe content. The conductivity at 1000 °C decreased, but the grain size increased as Fe content increased. The thermal expansion coefficient slightly changed with Fe addition.     

Enhancement of thermoelectric figure of merit by doping Dy in La0.1Sr0.9TiO3 ceramic

Ceramic samples of La_0.1Sr_0.9−xDy_xTiO₃ (x = 0.01, 0.03, 0.07, 0.10) have been prepared by the solid-state reaction method. Characterization from the powder X-ray diffraction indicates that their crystal structure changes from cubic to tetragonal phase. Their electrical and thermal transport properties are measured in the temperature range of 300-1100 K. n-Type thermoelectric is obtained with large Seebeck coefficient. The figure of merit is markedly improved, due to relatively lower electrical resistivity and thermal conductivity by Dy doping effect. A much lower electrical resistivity of 0.8 mΩ cm at room temperature is obtained in La_0.1Sr_0.8Dy_0.1TiO₃, and with a relatively lower thermal conductivity of 2.5 W/m K at 1075 K. The maximum figure of merit reaches ∼0.36 at 1045 K for La_0.1Sr_0.83Dy_0.07TiO₃, which is the largest value among n-type oxide thermoelectric ceramics.     

Improvement of the Thermoelectric Properties of (Sr0.9La0.1)3Ti2O7 by Ag Addition

Ruddlesden-Popper (RP) phase (Sr_0.9La_0.1)₃Ti₂O₇/xAg (x=0, 0.05, 0.10, 0.15) are systematically investigated with regard to their phase composition and thermoelectric transport properties. The XRD results show that all oxide samples are of two phases and they have layered microstructure. The electrical conductivity is found to be increased simultaneously. And the absolute Seebeck coefficient decreases firstly and then increases Ag addition, which leads to (Sr_0.9La_0.1)₃Ti₂O₇/xAg (x=0.10) sample possesses optimum power factor. The total thermal conductivity lowers with Ag addition. The dimensionless figure of merit, ZT, reaches 0.12 for x=0.10. These results suggest that a proper of metal element Ag addition would be an effective way improving thermoelectric performance of (Sr_0.9La_0.1)₃Ti₂O₇ system.     

Sinterable La0.8Sr0.2CrO3 and La0.7Ca0.3CrO3 powders by sucrose combustion synthesis

Air atmosphere sinterable La_0.8Sr_0.2CrO₃ [LSC] and La_0.7Ca_0.3CrO₃ [LCC] powders have been prepared by sucrose combustion synthesis. Aqueous solution containing stoichiometric quantities of the metal nitrates and sucrose (3 moles/mole of the metal ion) at pH ∼1 was concentrated by heating on a hot plate into a viscous resin which on drying at 120°C produced a foam with interconnected pore structure. This foam ignited with a matchstick in a combustion set up fabricated in the laboratory produced ashes consisting of loose aggregates of LSC and LCC particles. The loose aggregate of LSC and LCC were powdered by planetary ball milling to submicron size particles with D₅₀ value 0.19 and 0.60 μm, respectively. The surface area of the LSC and LCC powders was 23 and 19 m²/g, respectively. Pellets prepared by cold compaction and sintering of LSC and LCC powders in air atmosphere showed density 96.8 and 98.8% of theoretical value respectively. Sintered LCC sample showed finer grains compared to the LSC sample under identical processing conditions.     

Novel Multifunctional Properties Induced by Interface Effects in Perovskite Oxide Heterostructures

Multilayer structures have emerged as a leading research topic and researchers expect that multilayers may lead to interesting artificial materials with novel properties. In this Research News we show that the introduction of interfaces into perovskite oxides can induce a series of novel properties including an unusual positive magnetoresistance, great enhancement of lateral photovoltage in La_0.9Sr_0.1MnO₃/SrNb_0.01Ti_0.99O₃, and an electrical modulation of the magnetoresistance in multi‐p‐n heterostructures of SrTiO_3?δ/La_0.9Sr_0.1MnO₃/SrTiO_3?δ/La_0.9Sr_0.1MnO₃/Si. This novel positive magnetoresistance is attributed to the creation of a space charge region at the interface where the spin of the carriers is anti‐parallel to that of the carriers in the region far from the interface of manganese oxide in the heterostructures.     

High-temperature oxygen non-stoichiometry, conductivity and structure in strontium-rich nickelates La2−xSrxNiO4−δ (x = 1 and 1.4)

Oxygen non-stoichiometry, electrical conductivity and thermal expansion of La_2−xSr_xNiO_4−δ phases with high levels of strontium-substitution (1 ≤ x ≤ 1.4) have been investigated in air and oxygen atmosphere in the temperature range 20–1050 °C. These phases retain the K₂NiF₄-type structure of La₂NiO₄ (tetragonal, space group I4/mmm). The oxygen vacancy fraction was determined independently from thermogravimetric and neutron diffraction experiments, and is found to increase considerably on heating. The electrical resistivity, thermal expansion and cell parameters with temperature show peculiar variations with temperature, and differ notably from La₂NiO_4±δ in this respect. These variations are tentatively correlated with the evolution of nickel oxidation state, which crosses from a Ni³⁺/Ni⁴⁺ to a Ni²⁺/Ni³⁺ equilibrium on heating.     

Sintering mechanisms in strontium doped lanthanum chromite

The sintering behavior of (La_0.7Sr_0.3)_xCrO₃ (0.95 x 1.05) is investigated to compare liquid phase sintering phenomena occuring in stoichiometric and non-stoichiometric compositions. Shrinkage analysis revealed marked contrast between the densification characteristics of the A-site enriched (x > 1.00) and A-site depleted (x < 1.00) materials. A-site depleted samples typically exhibited a single liquid phase sintering event at 1250 °C attributed to the melting of an exsoluted SrCrO₄ phase. A-site enriched samples indicated two rapid shrinkage events due to the melting of SrCrO₄, and a Sr_2.67(CrO₄)₂ phase with a melting temperature of 1450 °C. Sr_2.67(CrO₄)₂ was shown to evolve from a decomposition reaction between SrCrO₄ and La₂CrO₆, detected together in A-site enriched samples from 800–1000 °C. Maximum densities (93% theoretical density) were achieved for (La_0.7Sr_0.3)_xCrO₃ x = 1.00 after sintering at 1700 °C for two hours.