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.     

Characteristics of La0.7Ca0.3MnO3 Powders Prepared by the Solution Combustion and Solid State Reaction Methods for Colossal Magnetoresistance Applications

La_0.7Ca_0.3MnO₃ powders were prepared by both the solution combustion method and the solid state reaction method and were calcinated at various calcination temperatures and time intervals in air atmosphere. In the solid state reaction method, single-phase La_0.7Ca_0.3MnO₃ was obtained after heat treatment of the powder at 1000°C for 24 hr. In the solution combustion method, however, single-phase La_0.7Ca_0.3MnO₃ powder could be obtained easily when the powder was heat-treated at 650°C for only 30 min. Polycrystalline La_0.7Ca_0.3MnO₃ powder, using the solution combustion method, showed good powder characteristics, such as an average grain size of 50 nm and a specific surface area of 92 m²/g. The resistance as a function of temperature and the magnetoresistance ratio in La_0.7Ca_0.3MnO₃ thin films were attempted to examine the colossal magnetoresistance characteristics. These thin films also showed excellent colossal magnetoresistance properties in that 96% of the maximum magnetoresistance ratio was obtained at 97K.     

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.     

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.     

Powder prepared by spray pyrolysis as an electrode material for solid oxide fuel cells

A variety of spray pyrolysis (SP) techniques have been developed to directly produce ceramic powders from solutions. Examples that are discussed include the following powders: (La_0.8Sr_0.2)_0.9MnO₃-YSZ (La(Sr)MnO₃-YSZ), La_0.6Sr_0.4CoO₃ (La(Sr)CoO₃), (CeO₂)_0.8(SmO_1.5)_0.2-NiO (SDC-NiO) and La_0.9Sr_0.1Ga_0.8Mg_0.2O₃-NiO (LSGM-NiO). For all these powders, spherical, non-agglomerated, submicrometer particles were obtained from aqueous solution of metal salts into a furnace using an ultrasonic atomizer at 1.7 MHz. After SP some of the particles exhibit a hollow-shell morphology. Subsequent calcination at 1000°C yielded crystalline particles. The electrical performance of Ni-SDC/LSGM/La(Sr)CoO₃ fuel cells operating at 800°C, prepared from the powders obtained by SP, is reported.     

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.     

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.     

Microwave assisted low temperature rapid synthesis of manganite system using La0.67Ce0.03Sr0.3MnO3 mini-cavity furnace

Amorphous La_0.7Sr_0.3MnO₃ (LSMO) and La_0.7Ca_0.3MnO₃ (LCMO) precursor powders synthesized by the citrate gel method at 673 K, have been found to crystallize by microwave irradiation in just 60 s using La_0.67Ce_0.03Sr_0.3MnO₃ (Ce-LSMO) as couplant. This is the lowest temperature treatment and synthesis time so far reported in literature for the formation of manganite systems. Using ceramic route, the same amorphous samples crystallize on heat treatment only at temperatures greater than 1000 K. The microwave heating through this method is novel and has tremendous potential for accelerating the evolution of the product phase in very shorter durations, with just low temperature processing of the precursors, which cannot be realized in normal process.     

Synthesis of nano-particle and highly porous conducting perovskites from simple <Emphasis Type="Italic">in situ</Emphasis> sol-gel derived carbon templating process

Nano-sized La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) and La_0.8Sr_0.2MnO_3−δ (LSM) oxides were synthesized by a simple in situ sol-gel derived carbon templating process. Nano-sized LSCF-carbon and LSM-carbon composites were first obtained with a grain size of 20–30 nm. Further calcination of the obtained composites under air resulted in the nano-sized pure-phase perovskites with crystalline size of as small as 14 nm. Such a decrease in crystalline size of perovskite via the indirect calcination process was ascribed to the suppressing effect of carbon in the grain growth of perovskite. Furthermore, when the in situ created carbon was applied as a template for pore forming, a highly porous perovskite sintering body packing from the nano-sized perovskite oxide was obtained.     

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.     

Synthesis of nanosized perovskite (Ba,Sr)TiO3 powder via a PVA modified sol-precipitation process

Ba_0.55Sr_0.45TiO₃ (BST) precursors were synthesized via a polyvinyl alcohol (PVA) modified sol-precipitation route. And the obtained precursors were then calcined in air at temperatures ranging from 400 to 800 °C. The formation mechanism of BST phase was investigated using X-ray diffraction (XRD), while the effect of PVA on the particle morphology of the BST phase was studied by using scanning electron microscopy (SEM). The results show that the introduction of PVA significantly affects the morphology of the BST powders. High purity nanocrystalline powders were synthesized by calcination of the BST precursors encapsulated by PVA gel, with narrow particle distribution (30-80 nm) and being nearly free of agglomeration. While, large particles (40-170 nm) with evident agglomeration were obtained from the solution without PVA.     

Electrical Resistivity of La0.7Sr0.3MnO3 Ceramics Prepared via Chelate Homogenization

La_0.7Sr_0.3MnO₃ ceramics are prepared from powders produced via gelation and/or microwave processing of solutions of polynuclear chelates (La, Sr, and Mn diethylenetriaminepentaacetates), and their electrical resistivity is measured as a function of temperature. As the sintering temperature is raised from 800 to 1100°C, the average grain size of the ceramics, evaluated by the Debye–Scherrer method, increases by about a factor of 2.5 and their resistivity drops by about two orders of magnitude. The effect of the sintering temperature on the average grain size depends very little on the preparation procedure. For some of the samples, the room-temperature weak-field magnetoresistance is determined.     

Synthesis of Sr- and Fe-doped LaGaO3 perovskites by the modified citrate method

Fine particle strontium and iron substituted lanthanum gallates La_1–x Sr _x Ga_1–y Fe _y O_3–, where x = 0.2, 0.4, and 0.6; y = 0.2, 0.4, 0.6, and 0.8, have been synthesized by a modified citrate method. The formation of these powders was confirmed by the X-ray powder diffraction (XRD) and the fine particle of La_0.6Sr_0.4Ga_0.2Fe_0.8O_3– was investigated by scanning electron microscopy (SEM), and particle size analysis. The single phase of La_0.8Sr_0.2Ga_0.4Fe_0.6O_3–, La_0.6Sr_0.4Ga_0.2Fe_0.8O_3–, and La_0.4Sr_0.6Ga_0.2Fe_0.8O_3– powders could be obtained both with and without calcination. The amount of the secondary phase increased when the amount of Sr in La_1–x Sr _x Fe_0.6Ga_0.4O_3– was more than 0.2 (x > 0.2) and the amount of Fe in La_0.6Sr_0.4Ga_1–y Fe _y O_3– and La_0.4Sr_0.6Ga_0.2Fe_0.8O_3– was less than 0.8 (y < 0.8). The results indicated that in the pH range of 1.36–9.27, the single phase of La_0.6Sr_0.4Ga_0.2Fe_0.8O_3– was formed without calcination and the pH had negligible effects on the structure and lattice parameter. The fine particle of these calcined powders (<4 m) was obtained with the average particle size 1.70 m at pH = 1.36 and with the average particle size between 0.56–0.60 m at pH range between 3.39–9.27, and with a lattice parameter about 3.9 Å.     

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.     

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.     

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 synthesis conditions on NO oxidation and NOx storage performances of La0.7Sr0.3MnO3 perovskite-type catalyst in lean-burn atmospheres

Synthesis conditions of catalysts can significantly affect catalytic activities for a certain reaction. Here, a series of the La_0.7Sr_0.3MnO₃ perovskite-type catalysts was prepared by the sol–gel method under the different synthesis conditions. The faster calefactive velocities during calcination of the xerogel precursors would produce a lot of the impurities and cause the dropped amount of the excessive oxygen in perovskite, as well as the aggregated particles and the decreased surface areas; the higher calcination temperature would sinter the perovskite phases seriously; and the initial pH value of the precursor solution would greatly affect the morphology of the catalysts including the shape and the size, which directly linked to their NO_x storage capacity. Moreover, our findings revealed that the NO oxidation ability was determined by the amount of the excessive oxygen species in the perovskite. Here, the optimum synthesis conditions were achieved with the calcination temperature of 700 °C, the calefactive velocity of 2 °C min⁻¹, and the precursor solution of pH = 8. This catalyst presented the best performances for the NO oxidization and NO_x storage, i.e. the NO-to-NO₂ conversion of 70.2% and the NO_x storage capacity of 170.4 μmol g⁻¹.     

Effect of high temperature post-annealing of La0.7Sr0.3MnO3 films deposited by radio frequency magnetron sputtering on SiO2/Si substrates heated at low temperature

La_0.7Sr_0.3MnO₃ thin films were deposited on SiO₂/Si substrates by RF magnetron sputtering under different oxygen gas flow rates with a sputtering power of 100 W. During deposition, the substrate was heated at 623 K. To investigate post-annealing effects, the as-deposited La_0.7Sr_0.3MnO₃ thin films were thermal-treated at 973 K for 1 h. The effects of oxygen gas flow rate and post-annealing treatment on the physical properties of the films were systematically studied. X-ray diffraction results show that the growth orientation and crystallinity of the films were greatly affected by the oxygen gas flow rate and substrate heating during deposition. The sheet resistance of the films gradually decreased with increasing oxygen gas flow rate, while the post-annealed films showed the opposite behavior. The temperature coefficient of resistance at 300 K of La_0.7Sr_0.3MnO₃ thin films deposited at an oxygen gas flow rate of 40 sccm decreased from − 2.40%/K to − 1.73%/K after post annealing. The crystalline state of the La_0.7Sr_0.3MnO₃ thin films also affected its electrical properties.     

Synthesis of LSMO at low temperature by novel hydroxide precursor technique

A novel technique of mixing individual hydroxide is employed to prepare La_0.65Sr_0.35MnO₃ (LSMO) at low temperature. Freshly prepared lanthanum and manganese hydroxides are mixed thoroughly with strontium hydroxide in stoichiometric ratio and heated at different temperatures ranging from 100 to 500 °C for 6 h. At 500 °C, formation of La_0.65Sr_0.35MnO₃ was confirmed by the X-ray diffraction studies (XRD). This is the lowest temperature so far reported in the literature. The particle size and morphology were investigated by scanning electron microscopy (SEM).     

A simple sol–gel technique for preparing lanthanum oxide nanopowders

Nanosized lanthanum oxide powders have been prepared by a simple sol–gel technique using commercial lanthanum oxide, nitrate acid and polyethylene glycol (PEG) as the starting materials. The decomposition process of dried gel powders were investigated by differential thermal and thermogravimetric analysis(TG-DSC). The crystalline structures and morphologies of the powders were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). The effects of calcination temperature, calcination time, stirring and the concentration of PEG on the particle size were studied. The results show that the calcination temperature and concentration of PEG have obvious influence on the La₂O₃ particle size and agglomeration tendency. At lower calcination temperature the prolongation of calcination time has slight influence on the particle size. Stirring will promote the growth of particles. The weak-agglomerated nanosized powders with the mean particle size less than 40 nm could be obtained by controlling the process condition.