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Perovskite-related (La1− xSr x)2NiO4− δ ( x=0.5-0.8) phases were explored for possible use as oxygen electrodes in solid electrolyte cells with a main focus on the effect of oxygen deficiency on the electrocatalytic activity. (La1− xSr x)2NiO4− δ solid solutions were demonstrated to preserve the K2NiF4-type tetragonal structure under oxidizing conditions. Acceptor-type substitution by Sr is compensated by the formation of oxygen vacancies and electron holes and progressively increases high-temperature oxygen nonstoichiometry, which reaches as high as δ=0.40 for x=0.8 at 950 °C in air. The electrical conductivity of (La1− xSr x)2NiO4− δ ceramics at 500-1000 °C and p(O2)≥10−3 atm is p-type metallic-like. The highest conductivity, 300 S cm−1 at 800 °C in air, is observed for x=0.6. The average thermal expansion coefficients, (14.0-15.4)×10−6 K−1 at 25-900 °C in air, are sufficiently low to ensure the thermomechanical compatibility with common solid electrolytes. The polarization resistance of porous (La1− xSr x)2NiO4− δ electrodes applied on a Ce0.9Gd0.1O2− δ solid electrolyte decreases with increasing Sr concentration in correlation with the concentration of oxygen vacancies in the nickelate lattice and the anticipated level of mixed ionic-electronic conduction. However, this is accompanied by increasing reactivity between the cell components and necessitates the microstructural optimization of the electrode materials to reduce the electrode fabrication temperature. [ABSTRACT FROM AUTHOR]

In developing elongated α-sialon grains, a nitrogen-rich liquid phase sintering method was introduced by using CaH2 as a sintering aid, so as to vary the N/O ratio of the liquid phase formed in the sintering process while keeping the Si/Al ratios constant. With increasing addition the phase contents changed from single β-sialon to dual β/α-sialons and to single Ca-α-sialon. At low N/O ratios the microstructures contained mainly equiaxed β-sialon grains, and at high N/O ratios well faceted elongated Ca-α-sialon grains. The improved toughness ( KI C=7.8 MPa·m1/2) and hardness ( HV10=17.5 GPa) properties can be attributed to the formation of interlocked microstructures. High-temperature compressive deformation tests indicated that the deformation onset temperature is determined mainly by the Si/Al and N/O ratios, whereas the deformation rate is affected by the microstructure, i.e., the morphology and amounts of elongated α-sialon grains and residual glass phase, especially for the sialons with low N/O ratios. [ABSTRACT FROM AUTHOR]

Nitrogen-rich Ca–α-SiAlON ceramics with nominal compositions Ca xSi12−2 xAl2 xN16 and 0.2≤ x≤2.6, extending along the Si3N4–1/2Ca3N2:3AlN tie line, were prepared from Si3N4, AlN, and CaH2 precursors by hot pressing at 1800°C. The x values attained were determined by energy-dispersive X-ray (EDX) microanalysis and X-ray powder diffraction (XRPD) data using the Rietveld method. The results show that Ca–α-SiAlONs form continuously within the compositional range x=0 to at least x=1.82. Phase assemblages, lattice parameters, Vickers hardness, and fracture toughness were determined and correlated to the calcium content, x. Owing to a high sintering temperature and the use of CaH2 as a precursor, grain growth was kinetically enhanced, resulting in self-reinforced microstructures with elongated grains. The obtained Ca–α-SiAlON ceramics demonstrate a combination of both high hardness ∼21 GPa, and high fracture toughness ∼5.5 MPa·m1/2. [ABSTRACT FROM AUTHOR]

Abstract: Nitrogen-rich Ca-α-sialon ceramics, prepared with CaH2 as one of the starting powders, were compressively deformed in spark plasma sintering equipment. Compared with the oxygen-rich Ca-α-sialons, increasing onset deformation temperatures (about 150K higher) were observed for nitrogen-rich Ca-α-sialons deformed at a rate of 2×10−3 s−1. High hardness (H V10 =18–20GPa) and toughness (K IC =4–7MPam1/2) were maintained after the deformation. Anisotropic grain growth was found to take place during deformation, resulting in anisotropic microstructures, containing coarse and elongated grains. The observed differences in deformation behaviour and properties between nitrogen-rich and oxygen-rich Ca-α-sialons are, as indicated by transmission electron microscopy and electron energy loss spectroscopy analysis, attributed to the presence of reduced amounts of a nitrogen-rich grain-boundary glass phase. [Copyright &y& Elsevier]