Gilles Caboche

A comparative study of the surface and bulk properties of lanthanum-strontium-manganese oxides La1-xSrxMnO3±δ as a function of Sr-content, oxygen potential and temperature

Abstract The properties of the perovskite-type La 1− x Sr x MnO 3± δ oxides (0≤ x ≤1) are related to the reversible redox reaction of manganese ions Mn 3+ ↔Mn 4+ . This reaction plays an essential role both in the interfacial transfers and transport properties and depends on relative cation concentrations and experimental conditions. In this paper we present experimental investigations aiming to determine, to compare and to control some of the surface and bulk properties of these oxides. The bulk and surface Mn 4+ contents were investigated by thermogravimetry analysis (TGA) in hydrogen and X-ray photo-electron spectroscopy (XPS) respectively. With a finely divided La 0.8 Sr 0.2 MnO 3 powder, the chemical composition in the surface layers was found to be practically independent of the conditions of a high temperature treatment in oxygen. The same is true for the mean oxidation state of manganese always close to Mn 4+ . With pellets of various compositions ( x =0, 0.2, 0.5 and 1), enrichment in strontium with respect to the bulk composition was detected in oxygen at high temperature and found to depend on the strontium concentration in the bulk. Moreover, for any chemical composition, deficit in manganese were systematically evidenced in the surface layers. Surface strontium concentration and mean oxidation state of manganese was correlated as expected. By comparing the results observed with very finely divided powder and pellets respectively, it was deduced that the grain boundaries have an important effect on the surface composition. Data on the structural evolution of the initial compounds during their hydrogen reduction are also presented.

Low-pressure-MOCVD LaMnO3±δ very thin films on YSZ (100) optimized for studies of the triple phase boundary

Abstract This paper deals with the preparation of LaMnO 3± δ (LM) layers by low pressure-metal organic chemical vapor deposition (LP-MOCVD) using La(tmhd) 3 and Mn(acac) 3 as organometallic precursors. By thermogravimetric analysis, these precursors were found to be suitable for LP-MOCVD in a well-defined range of total pressure and temperature of sublimation. The activation energies of the sublimation process were found to be independent of the pressure within the appropriate range (0.06–3 kPa) and their values were 177 and 100.5 kJ mol −1 for La(tmhd) 3 and Mn(acac) 3 , respectively. LM layers of various thickness ranging between a few and a few hundred nanometers with a controlled La/Mn (L/M) ratio between 0.87 and 1.40 were grown by changing the deposition time and composition of the vapor phase. The influence of the deposition conditions and of the post-deposition treatment on the properties of the films was also studied.

A dilatometric study of the Lao.8Sr0.2MnO3 sintering behaviour

Abstract The sintering behaviour of La0.8Sr0.2MnO3 has been studied by dilatometry between 1100 and 1800 K in various oxygen potentials [pure oxygen, air and nitrogen (PO2 = 1 × 10−5 bar)]. The starting material was prepared by spray pyrolysis of aqueous solutions of nitrates. Two classical sinter stages, neck formation and grain growth respectively, were evidenced from dilatometric curves and this result was corroborated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses. The rate of densification was found to be directly dependent on oxygen partial pressure, the best conditions being in nitrogen. These results are discussed in terms of vacancy diffusion and oxygen stoichiometry linked to two factors: the initial (La + Sr) Mn ratio and the Mn 3+ Mn 4+ ratio determined both by temperature and oxygen partial pressure.

Fundamental and technological aspects of the surface properties and reactivity of some metal oxides

Abstract As is usually accepted for many inorganic compounds, the surface properties often play a fundamental role in the overall properties of metal oxides, in particular when the oxide materials are used in the form of fine grains. One difficulty in characterizing these surface properties is to apply the results of studies performed on ideal surfaces to the actual materials in use. A way to accept this challenge can be to successively consider initial powders, polycrystalline pellets formed after thermal or mechanical treatment of these powders and, finally, monocrystalline surfaces prepared from these powders or pellets. This principle can be applied to the study of the surface properties and reactivity of various metal oxides either used as models or having a technological interest in catalysis, electronics or energetics: doped nickel oxide, barium titanate, lanthanum-strontium manganites, etc. Some selected examples of experimental results for three kinds of investigations are presented: (i) mechanism and kinetics of nucleation and growth of ultrathin, metallic films by surface reduction of NiO and influence of the chromium segregation on the surface reactivity of Cr 2 O 3 NiO solid solutions, (ii) effect of a high temperature treatment on the chemical surface composition of industrial BaTiO 3 samples, (iii) compared properties of bulk and surface layers in lanthanum-strontium manganites as a function of strontium content and biography of the materials. These analyses were performed by electron spectroscopies and diffraction associated with electron microscopy and ultrathermogravimetry.

An X-ray powder diffraction study of lanthanum–strontium ferromanganites

Abstract Various lanthanum–strontium ferromanganites (La 1− x Sr x )(Mn 1− y Fe y )O 3± δ (LSMF) with x =0.2, 0.5 and 0.7 and y =0.2, 0.5 and 0.8 were prepared by a glycine–nitrate combustion route and conditioned into two different oxygenation states which are likely to be encountered in cathode materials for solid oxide fuel cells (SOFC). Crystal symmetries for both the low and the fully oxygenated states were determined by X-ray diffractometry. Most perovskite compositions crystallized in either cubic or rhombohedral symmetry with the exception of La 0.5 Sr 0.5 Mn 0.2 Fe 0.8 O 3± δ , where a transition from rhombohedral to cubic occurred in going from the full to the low oxygenation state. In addition, two strontium-poor compositions, La 0.8 Sr 0.2 Mn 0.5 Fe 0.5 O 3± δ and La 0.8 Sr 0.2 Mn 0.2 Fe 0.8 O 3± δ , remained diphasic and consisted of a combination of both the rhombohedral and the orthorhombic phases. Changes in unit cell volume were analyzed as a function of chemical composition and oxygenated state.

On the Effect of Surface Treatment to Improve Oxidation Resistance and Conductivity of Metallic Interconnects for SOFC in Operating Conditions

Due to the reduction of operating temperature from 1000°C to 800°C, chromia forming alloys are the best candidates for interconnects in Solid Oxide Fuel Cells (SOFCs). These interconnects have to be operational in service conditions, at 800°C in air (cathode side) and in humidified hydrogen (anode side). The performance of the interconnect stainless steels is limited by the oxide scale formation (chromia), the low electronic conductivity of this scale and the possible volatility of chromium oxides. In the field of high temperature oxidation of metals, it is well known that the addition of a nanometric layer made of reactive element oxide such as, La2O3, Nd2O3 and Y2O3 by MOCVD (Metal Organic Chemical Vapor Deposition) on alloy surface resulted in an important improvement in the high temperature oxidation resistance. These coatings are made on metallic alloys in order to form perovskite oxides such as LaCrO3, NdCrO3 and YCrO3, which are expected to present a good conductivity at 800°C in air. However, this temperature looks somewhat too low to guarantee the formation of perovskite oxides and thus to improve the oxidation resistance and electrical conductivity. In fact, XRD analyses revealed that for Y2O3 coatings, perovskite oxides were not formed after oxidation in air at 800°C for 100 hours. The goal of this study is to perform pre-oxidation at 1000°C for 2 hours in air under atmospheric pressure on coated Crofer22APU to pre-form perovskite phases. The so-prepared perovskite were tested in a thermobalance in air. Experiments performed in H2/10%H2O under 150 mbar at 800°C validated the coating influence from the anode side as well as the cathode side. The corrosion products were analyzed after 100 hours ageing at 800°C by SEM, EDX, and XRD. ASR (Area Specific Resistance) was measured for the same times and temperature in air.

Fabrication and Electrochemical Performance of Unit Anode Supported Intermediate Temperature Solid Oxide Fuel Cells by Single Step Process

In this work micron sized particles of NiO, Gd 0.1Ce0.9O1.9 (GDC10), and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF48) were used in the preparation unit anode supported planar Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC). The laboratories of ICB in Dijon and FCLAB in Belfort France were aimed at preparation of large active area planar cells by low cost effective and efficient fabrication process with high performance and characterizing them in real time stack for use in APU application. The cells were prepared by novel single step and in expensive, process. These cells were characterized electrochemically to have I-V and I-P curves. The study of microstructure was carried on the cells. The initial results obtained were reported in this paper with still large possible room for optimization to further improve the performances of the cells. The maximum power density obtained was 466mW.cm -2 at 648°C. At 500°C 1.002V were obtained.