Corrosion of E/TBCs: Chemical Interactions of Ceramic Materials with CMAS

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Size and morphology control of ultrafine refractory complex oxide crystals

High-temperature complex oxides are of considerable interest as their applications cover a broad spectrum from catalytic to optical technology. Indeed, new exciting opportunities might emerge if these high-temperature complex oxides, in which structure crystallization is achieved at temperatures T > 1000 °C, could be synthesized as nonaggregated, ultrafine building blocks. In general, such refractory complex oxide particles are difficult to synthesize as ultrafine crystals because of the strong driving force available for sintering and coarsening in this high-temperature range. This paper reports a new synthetic process for the preparation of nonaggregated, ultrafine barium hexa-aluminate, BaO, 6Al2O3, (BHA), and Ba0.9Eu0.1MgAl10O17, (BAM) crystals in which structure crystallization occurs around 1300 °C. Our process is based on the Ba2+ and Al3+ ions high-temperature controlled diffusion from carbon−inorganic hybrid compounds prepared from soft chemistry routes. Control of morphology of these refractory complex aluminates displaying nanoplatelets morphology was achieved via the tailoring of high-temperature diffusion lengths of the various cations involved in the formation of these ultrafine refractory crystals

A new route to prepare anodic coatings on dense and porous metallic supports for SOFC application

Metallic cell supports have been developed for the new generation of fuel cells. Sol–gel process has been used to prepare anodic coatings on these supports at moderate thermal treatment temperature, in order to keep a good support mechanical behavior and limit metallic corrosion. Indeed, we take advantage of the numerous reaction routes that sol–gel method can offer to first synthesize NiO–YSZ (yttria-stabilized zirconia) homogeneous composites, and then to process films of different thicknesses on metallic supports by dipcoating. In this work, the metallic supports could be either dense or porous. To begin with, duplex microstructured anodes were prepared from both thin and thick layers, directly deposited on dense metallic supports. The interfacial anodic layer, around 100 nm thick, improves adhesion and accommodates stresses between metallic interconnect and active thick anode. Moreover, by dipping the substrate into an optimized slurry containing sol–gel composite powders, films of a few microns thick have been obtained and constituted the active anodic part. A heat treatment at only 800 °C leads to a coherent anodic duplex stacking which is continuous, homogeneous and adherent. Subsequently, thick anodic films have also been deposited on two different porous supports, with both dip-coating process and slurries routes. These thick anodic coatings were characterized after thermal treatment at 800 °C

Elaboration of metallic compacts with high porosity for mechanical supports of SOFC

The development of third generation Solid Oxide Fuel Cells (SOFC) with metallic mechanical supports presents several advantages over that of ceramic stacks by offering a lower cost and longer lifetime of the stacks. As a consequence, it is necessary to prepare metallic porous compacts that remain stable at the operating temperature of the SOFC (700–800 C) under reductive atmosphere. This paper presents an innovative process to elaborate iron, nickel and cobalt porous compacts. The process is based on the thermal decomposition of metal oxalate precursors with controlled morphology into metallic powders with coralline shape. Uniaxial compaction of such powders (without binder addition to the powders) under low uniaxial pressures (rising from 20 to 100 MPa) gave rise to green compacts with high porosity and good mechanical properties. After annealing at 800 C under H2 atmosphere, the compacts still present interconnected porosity high enough to allow sufficient gas flow to feed a SOFC single cell in hydrogen: the porosity rises from 25 to 50% for iron compacts, from 20 to 50% for cobalt compacts, and is higher than 40% for nickel compacts. Results from physicochemical characterization (XRD, SEM, gas permeation, Hg porosimetry) corroborated the process for SOFC application

Influences of the Co content and of the level of high temperature on the microstructure and oxidation of cast {Ni, Co}-based Cr-rich TaC- containing cast alloys

International audienceA series of six alloys derived from a Ni-25Cr-0.4C-6Ta (wt.%) base one was developed by substituting nickel by cobalt. They were synthesized by casting and exposed to oxidative environment at two high temperatures. Their bulk microstructures were studied in as-cast condition and in two high temperature aged states. Their surfaces after oxidation during aging were characterized. The cobalt enrichment succeeded in avoiding chromium carbides formation and in stabilizing the TaC carbides at high temperature. As the high temperature morphologic stability of TaC was not perfect, it was much better than the one of the chromium carbides, but can be improved by the total removal of nickel. Unfortunately, at the same time, the oxidation behavior, initially good, shows increased rate of the oxides formation. The room temperature hardness was also significantly increased by the substitution of Ni by Co, and decreased after aging when carbides became rounder or fragmented

Influence of porosity on the electrical properties of La-9.33(SiO4)(6)O-2 oxyapatite

Oxyapatites are very promising materials in terms of ionic conductivity. They can be considered as a potential electrolyte for fuel cells as SOFC. The influence of porosity on the electrical properties of La-9.33(SiO4)(6)O-2 oxyapatite is reported here. Hot pressed pellets with various densification ratios have been characterized by the complex impedance method. The high frequency response associated with the bulk contribution is much more affected by the porosity than the grain boundaries contribution: as a consequence, the electrical behaviour of the samples has been considered in assimilating the porous ceramics to composite materials made of apatite with various amounts of air inclusions. Thus, the porosity dependence of conductivity, activation energy and permittivity are reported here. A percolation threshold has been highlighted for porosity values greater than 30%, involving great lowering of the electrical performances

Influence of cationic vacancies on the ionic conductivity of oxyapatites

Oxyapatites are very promising materials in terms of ionic conductivity. They can be considered as a potential electrolyte for fuel cells as SOFC. Substituted silicated rare earth oxyapatites with formula La9.33+z/3−xMex□0.67−z/3(SiO4)6O2+z/2−x/2 (z < x < z + 4) have been prepared by solid-state reaction at high temperature. Two series have been synthesized: a first one is oxygen stoichiometric with formula La9.33−2x/3Mex□0.67−x/3(SiO4)6O2, and a second one is anion deficient with formula La9.17−2x/3Mex□0.83−x/3(SiO4)6O1.75□0.25. In both cases, cationic vacancies are similarly controlled and vary from 0.67 to 0 per unit cell: the aim is to study the influence of cationic vacancies on the ionic conductivity with two distinct oxygen stoichiometries. Cell parameters of the high-purity oxyapatites have been refined in order to check the strontium incorporation. Discontinuous evolution of the a parameter underlined the strong electrostatic interactions between the defects of the most highly substituted samples. Electrical properties of the samples have also been studied by the complex impedance method between 280 and 620 °C. The evolution of conductivity and activation energy with the cationic vacancies content gives information on the conductivity mechanism, highlighting the importance of the global stoichiometry of the material

Synthesis and characterization of oxide ions conductors with the apatite structure for intermediate temperature SOFC

Silicated rare earth apatite with formula La9.33(SiO4)6O2 has been prepared by solid state reaction at high temperature. The reagents (La2O3 and SiO2) have first been characterized. Then the synthesis process was studied and optimized. Samples with various densities (from 67 to 92% of the theoretical value) have been obtained by hot pressing at 1400 ◦C or by sintering at 1500 and 1550 ◦C. The presence of secondary phases leads to the formation of a liquid phase at temperatures above 1600 ◦C. A microstructural study has been performed on these samples. Electrical properties of all the samples have been characterized between 400 and 900 ◦C by the complex impedance method. Conductivity values of about 2×10−4 S cm−1 have been measured at 700 ◦C. The samples present activation energies of less than 1 eV. The influences of the densification ratio and of the microstructure on the electrical properties of the material have been underscored

Influence of anionic vacancies on the ionic conductivity of silicated rare earth apatites

Oxyapatites are very promising materials in terms of ionic conductivity. They can be considered as a potential electrolyte for Solid Oxide Fuel Cells (SOFC). Doped silicated rare earth apatites with formula La9.33−xCax(SiO4)6O2−x/2 (0 ≤ x ≤ 1) have been prepared by solid state reaction at high temperature in order to determine the influence of anionic vacancies on the electrical properties of the material. The incorporation of calcium in the structure has been checked by characterizations of the powders (X-ray diffraction, helium pycnometry). The cell parameters of the hexagonal apatite were refined. Samples were sintered at 1550 °C. Electrical properties of each composition have been studied between 280 and 620 °C by the complex impedance method. The evolution of the bulk conductivity and of the activation energy with the substituting ratio gives information on the conductivity mechanism in these materials. An improvement of ionic conductivity about one order of magnitude has been observed for low calcium substitution ratios