New cathode compositions based on La0.84Sr0.16Mn1−xMxO3, where M = Al, Ga for solid oxide fuel cell

In order to identify new cathode compositions for the high temperature solid oxide fuel cell, we have investigated the effect of the trivalent cations Al and Ga at the Mn site of the well-studied cathode composition La_0.84Sr_0.16MnO₃. All the compositions have been prepared by the low temperature citrate-nitrate auto-ignition process and sintered within the temperature range of 1150-1350 °C for 4 h. In order to understand the compatibility of the prepared samples as alternative cathode materials, we compared their electrical conductivity and thermal expansion coefficient with those of La_0.84Sr_0.16MnO₃ and yttria-stabilized zirconia. A 10 mol% Al doped La_0.84Sr_0.16MnO₃ composition exhibited a conductivity of around 122 S cm⁻¹ at 950 °C and a thermal expansion coefficient of 11.04 × 10⁻⁶ K with a minimum reactivity towards yttria-stabilized zirconia. Though the conductivity of the new composition is lower than that of La_0.84Sr_0.16MnO₃ (169 S cm⁻¹ at 950 °C), it is still high enough for use as a cathode material.     

Emulsion processing of SOFC materials Ca0.3La0.7CrO3, Sr0.16La0.84CrO3, and Sr0.2La0.8MnO3

A novel preparation route to the perovskite materials Ca_0.3La_0.7CrO₃, Sr_0.16La_0.84CrO₃, and Sr_0.2La_0.8MnO₃ is described. The method produces the phase pure perovskite phases after calcination at 700°C for 2 hours. The powders produced are unagglomerated, and consist of hollow spherical particles 0.15 m in diameter. EDX has shown that the careful control of reaction conditions is vital to control the phase composition, and that small changes in stoichiometry result in the production of unsinterable powder.     

Synthesis of Enhanced-Conductivity La0.74Sr0.26MnO3

Strontium-doped lanthanum manganite of composition La_0.74Sr_0.26MnO₃ is synthesized via carbonate coprecipitation from aqueous nitrate solutions. Its 1170-K electrical conductivity is 197 S/cm, which far exceeds the conductivity of the material of the same composition and close in porosity but prepared by the conventional ceramic route. The proposed synthesis procedure makes it possible to prepare La_0.74Sr_0.26MnO₃ with a relative density of 66% (needed for fuel-cell cathodes) and 1170-K conductivity as high as 125 S/cm. La_0.74Sr_0.26MnO₃ has a rhombohedral structure (sp. gr. R c) with lattice parametersa = 0.55000 nm and c = 1.33491 nm.     

Influence of the synthesis method on the porosity,microstructure and electrical properties of La0.7Sr0.3MnO3 cathode materials

Lanthanum strontium manganite (La_1 ? xSr_xMnO₃, LSM) has been studied as a promising material for application as a cathode in solid oxide fuel cells. In the present work La_0.7Sr_0.3MnO₃ nanopowders were synthesized by three different methods (combustion, citrate and solid-state) and characterized by thermal analysis, X-ray diffraction, physical adsorption of N₂ and scanning electron microscopy. All powders exhibited single LSM phase formation with crystallite sizes in the range of 12–20 nm. Nanopowders were sintered at 1100 °C to produce porous pellets. The porosity, particle size and microstructure of LSM sintered bodies are strongly dependent on the preparation methodology. The samples synthesized by combustion and citrate methods presented smaller particle sizes and higher porosity after sintering than that derived from solid-state synthesis. However, the electrical conductivity, measured by two-probe technique, was very similar for all three samples.     

Microstructure and NO decomposition behavior of sol-gel derived (La0.8Sr0.2)0.95MnO3/yttria-stabilized zirconia nanocomposite thin film

(La_0.8Sr_0.2)_0.95MnO₃ and (La_0.8Sr_0.2)_0.95MnO₃/YSZ gel films were deposited by a spin-coating technique on scandium-doped zirconia (ScSZ) substrate using the precursor solution prepared from La(Oi-C₃H₇)₃, Sr(Oi-C₃H₇)₂, Mn(Oi-C₃H₇)₂ and 2-methoxyethanol. By heat-treating the gel films, the membrane reactors, (La_0.8Sr_0.2)_0.95MnO₃|ScSZ|Pt and (La_0.8Sr_0.2)_0.95MnO₃/YSZ|ScSZ|Pt were fabricated. It was found that the pre-firing temperature affected the microstructure evolution of (La_0.8Sr_0.2)_0.95MnO₃ and (La_0.8Sr_0.2)_0.95MnO₃/YSZ thin films. Pre-firing at low temperature resulted in high porosity and large grain size of the thin films. NO decomposition characteristics of the obtained membrane reactors were investigated at 600 °C in reactant gas, 1000 ppm of NO and 2% of oxygen. By applying a direct current to the membrane reactors, NO can be decomposed at the (La_0.8Sr_0.2)_0.95MnO₃ and (La_0.8Sr_0.2)_0.95MnO₃/YSZ composite cathode. By incorporating YSZ into (La_0.8Sr_0.2)_0.95MnO₃, the required consuming power to decompose NO could be reduced.     

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.     

Synthesis of Ultrafine-Particle La0.7Sr0.3MnO3 via Chelate Homogenization and Microwave Processing

Ultrafine La_0.7Sr_0.3MnO₃ powders were prepared via homogenization in chelate solutions, followed by microwave dehydration, using polynuclear heterometallic diethylenetriaminepentaacetates as precursors. To assess the effect of the dehydration procedure on the phase composition and grain size of La_0.7Sr_0.3MnO₃ ceramics, three routes were tested: concentration of chelate solutions by evaporation until the formation of a glassy precursor, microwave dehydration of chelate solutions, and a combination of gelation and microwave dehydration. Phase-pure La_0.7Sr_0.3MnO₃ with a crystallite size of 30–40 nm (as determined by transmission electron microscopy) could be obtained via microwave dehydration of heterobimetallic precursor solutions, followed by calcination at a temperature as low as 800°C.     

Electrical properties of BaTiO3-based NTC thermistors doped by BaBiO3 and La2O3

In our research it has been firstly found that BaTiO₃ materials doped with BaBiO₃ only show negative temperature coefficient effect over a wide temperature range. Major phases present in the sintered bodies are BaTiO₃ compounds with a perovskite structure and BaBiO₃ compounds with a monoclinic structure. Also, at a given BaTiO₃ and BaBiO₃ content, the influence of La₂O₃ content on the microstructure and electrical properties has been investigated. The mean grain size of samples decreases with an increase in La₂O₃ content. However the mean grain size remains unchanged with a further increase in La₂O₃ content when the La₂O₃ content in material is more than 0.20. As the amount of La₂O₃ in BaTiO₃-based ceramics thermistors increases, the resistivity decreases to a minimum value and then slowly increases again.     

Electrical conductivity of yttrium-doped SrTiO3: influence of transition metal additives

The electrical conductivity of yttrium-doped SrTiO₃ with transition metals added as acceptor dopants (V, Mn, Fe, Co, Ni, Cu, Zn, Mo, Mg, Zr, Al, or Ga) was measured by the dc four-probe method at 600-900°C in reducing atmospheres. The replacement of 5 mol% titanium by acceptors leads to a decrease of conductivity of Sr_1−1.5xY_xTiO_3−δ. The degree of the decrease depends strongly on the type of dopant. Of the 5 mol% acceptor-doped compositions, the system Sr_0.85Y_0.10Ti_0.95Co_0.05O_3−δ had the highest conductivity of 45 S/cm at 800°C and oxygen partial pressure of 10⁻¹⁹ atm. The oxidation kinetics of yttrium-doped SrTiO₃ was significantly retarded by the addition of cobalt or manganese dopants. The ionic conductivity of SrTiO₃ doped with 5 mol% acceptors at Ti-sites was estimated from the total conductivity to lie in the vicinity of 10⁻⁴ S/cm, depending on oxygen partial pressure and temperature.     

A new route to synthesize La1?xSrxMnO3

Self-Propagating High-temperature Synthesis (SHS) was used to producecomplex oxides (La_1–x Sr _x MnO₃ with x = 0, 0.1 and 0.2), which are used as the cathode in solid oxide fuel cells (SOFCs). Thermodynamic predications and experiments show that La_1–x Sr _x MnO₃ can be prepared via SHS under moderate conditions from a mixture of La₂O₃ + SrO₂ + Mn, using either gaseous oxygen or solid NaClO₄ as the oxidant. Partial melting at the high combustion temperature increased product homogeneity. The electrical conductivity of the product was 180 S·cm^–1 at 1000°C in air, matching that of sample made by other synthesis processes. SHS enables a more economical production of La_1–x Sr _x MnO₃ than existing commercial processes.     

Grain boundary analysis and secondary phases in LaCoO3-based perovskites

Secondary phases and grain boundaries in LaCoO₃ and La_0.7Sr_0.3CoO_3−δ ceramics have been studied by transmission electron microscopy. Both materials contained small amounts of grains of a secondary phase; the face centered cubic spinel structure Co₃O₄, located at triple junctions. These grains were agglomerates of several smaller grains. High resolution electron microscopy combined with annular dark field scanning transmission electron microscopy and electron energy loss spectroscopy were used to analyze the grain boundaries. In LaCoO₃, the grain boundaries were sharp with stable compositions of La, Co and O across the boundaries. In La_0.7Sr_0.3CoO_3−δ, a 1–2 nm thick intergranular layer between the grains was observed. This layer was rich in O and Co and deficient in Sr and La, compared to the nominal composition of the material.     

Synthesis of La0.5A0.5MnO3 (A = Sr, Ba) by a hydrothermal method at low temperature

A mild hydrothermal method has been adopted to prepare La_0.5Sr_0.5MnO₃ and La_0.5Ba_0.5MnO₃, which is of interest for a number of possible applications. The results from X-ray diffraction (XRD) indicate that in the present work the temperature of 200 and 240 °C are sufficient to prepare phase pure La_0.5Sr_0.5MnO₃ and La_0.5Ba_0.5MnO₃ crystals. At 200 °C, La_0.5Sr_0.5MnO₃ nanowires are obtained. The average width and length of the nanowires are 40 nm and 4 μm, respectively. At 240 °C, La_0.5Ba_0.5MnO₃ powders obtained have a cubic structure with the average size of 3-5 μm.     

Transformation of yttria partially stabilized zirconia by low temperature annealing in air

The tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia by low temperature annealing in air was investigated in the temperature range 100 to 650° C using a sintered body of zirconia containing 2 to 4 mol% Y₂O₃. The amount of monoclinic phase formed was maximum at about 200° C. Both the decrease in grain size and increase in the yttria concentration were effective in decreasing the critical temperature below which the monoclinic phase was formed. The relationship between the critical temperature (T _c) and the grain size was experimentally determined.     

The effect of Cu-sites doping on electrical properties of La2CuO4 as thermoelectric material

The electrical conductivity and thermoelectric power for solid solutions of La₂Cu_0.9M_0.1O₄ (M = Mn, Fe, Co and Ni) prepared by polymeric precursor synthesis were measured between 300 and 1173 K to test its suitability as a thermoelectric material. Fe-, Mn-and Ni-doped compositions exhibited a metal-semiconductor transition with decreasing temperature, whereas Co doping showed a semiconducting behaviour in the entire temperature range of measurement. With Cu-sites doping in La₂CuO₄, power factors were not enhanced.     

Combined platinum/perovskite catalyst for an efficient nanostructured air electrode

This work shows the stepwise improvement of air electrodes by the right combination of catalysts. In all electrodes carbon nanotubes serve as carbon support. The electrodes are produced by ultrasonic mixing of the carbon nanotubes and the catalysts. Their catalytic activity towards oxygen reduction in alkaline solution is evaluated by polarisation curves and electrochemical impedance spectroscopy. In a first step La_1?xSr_xMnO₃ perovskites are investigated, as well as La_0.65Sr_0.35MnO₃ and La_0.6Sr_0.4CoO₃ are compared. It is found that La_0.65Sr_0.35MnO₃ and La_0.6Sr_0.4CoO₃ have a positive impact on different parts of the current–potential curve. In a second step the influence of small amounts of platinum as an additional catalyst besides the perovskite is analyzed with the result that platinum lowers significantly the activation polarisation. Finally, the optimum composition of the electrode is found by using the synergetic effect of platinum, La_0.65Sr_0.35MnO₃ and La_0.6Sr_0.4CoO₃.     

Magnetoimpedance effect in semiconducting La0.4Sr0.6MnO3

In the present work, it was found that for La_0.4Sr_0.6MnO₃, the dc resistance decreases with increasing temperature, from 77 to 280 K. Different from the case of metallic La_0.65Sr_0.35MnO₃, the ac impedance of the semiconducting La_0.4Sr_0.6MnO₃ at room temperature decreases with increasing frequency, from 100 kHz to 12 MHz. The magnetoimpedance effect was observed in La_0.4Sr_0.6MnO₃ at room temperature. The value of impedance ratio (Z(0)−Z(H))/Z(0) at H=0.8 kOe reaches 5% at a frequency of 500 kHz.     

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.     

Substitution induced B-cation ordering in solid oxide fuel cell ceramics: (La0.8Sr0.2)(M0.9Ni0.1)O3 (M = Mn, Cr)

Perovskites are important materials in a number of important technological applications, including solid oxide fuel cells, catalysis, and giant magneto-resistance materials. For many of these purposes, a mixture of B-cations can be used to tune the desired properties, e.g., oxygen reduction, ionic conductivity. For a solid oxide fuel cell, two particular ceramic components are of critical importance and have been extensively studied, the cathode (La_0.8Sr_0.2)MnO_3−x and the interconnect material (La_0.8Sr_0.2)CrO₃. In this study, we examined the mixed B-cation perovskites (La_0.8Sr_0.2)(M_0.9Ni_0.1)O₃ (M = Mn, Cr). All materials were synthesized using the glycine-nitrate method, followed by air annealing. The structures were determined using powder neutron diffraction methods. Refinement of the data showed that even at this low concentration, the compounds have monoclinic symmetry (P2₁/n) and that the nickel had a strong preference for the smaller of the two octahedral sites. This small amount of nickel substituted on the B-site resulted in a symmetry reduction when compared to the unsubstituted (LaSr)MnO₃ or (LaSr)CrO₃ materials. Although this structural type has been seen previously in heavily substituted perovskites, these materials show that even at this low level of substitution a segregation of the metals in a manner similar to the double perovskites A₂BB′O_6−x can be detected. This may have implications involving material stresses on cycling that may result as the temperature is raised or lowered through this crystallographic transition.     

Analytical transmission electron microscopy of La2O3-doped Y2O3

Analytical electron microscopy has been used to study the precipitation reactions in sintered samples of 9 mol% La₂O₃-Y₂O₃ samples upquenched from the single phase cubic region into the cubic and hexagonal phase field. Samples annealed just inside the two-phase cubic-cubic and hexagonal solvus exhibited predominantly grain boundary precipitation. Small La₂O₃ rich second phases formed within the first ten minutes and developed into strained, facetted precipitates after 300 min. Intergranular and intragranular precipitation occurred in samples annealed further into the two-phase field. Strained, lathlike La₂O₃-rich monoclinic precipitates, exhibiting a preferrred orientation in the matrix, appeared as the dominant morphology for long times at temperature. Chemical microanalyses of the strained structures obtained in samples annealed for 300 min revealed La₂O₃ matrix concentrations in agreement with phase diagram predictions. However, the La₂O₃ concentrations in the second-phase precipitates were found to be far in excess of the cubic and hexagonal-hexagonal solvus. This discrepancy is believed to arise from a re-equilibration of the second phase in the cubic and monoclinic phase field during quenching.     

Hydrothermal processing and characterisation of doped lanthanum chromite for use in SOFCs

The perovskite powders Ca_0.3La_0.7CrO₃ and Sr_0.16La_0.84CrO₃ have been prepared using hydrothermal processing. The solid solutions were not formed directly in the autoclave, but the hydrothermally produced powders required calcination at a greatly reduced temperature to form the perovskite phase, reducing the tendency to produce hard agglomerates. Pellets with densities in excess of 95% TD were produced.