P. Núñez

The effect of cobalt oxide sintering aid on electronic transport in Ce0.80Gd0.20O2−δ electrolyte

Abstract Additions of 2 mol% CoO 1.333 into gadolinia-doped ceria (CGO) solid electrolyte considerably improve sinterability and make it possible to obtain Ce 0.8 Gd 0.2 O 2− δ ceramics with 95–99% density at 1173–1373 K. The effect of cobalt oxide on the total electrical conductivity in air is negligible if the sintering is performed at 1173 K, although p-type electronic conduction measured at 900–1200 K increases with doping by 10–30 times. When increasing the sintering temperature up to 1773 K, grain growth in Co-containing CGO ceramics is accompanied with a decrease in both ionic and electron-hole transport. The oxygen ion transference numbers under oxygen/air gradient vary in the range 0.89–0.99. The n-type conductivity measured by the ion-blocking technique is lower for Co-containing materials than for undoped CGO, suggesting that the electrolytic domain can, to some extent, be enlarged by cobalt oxide additions. The relative role of both p- and n-type electronic contributions to the total conductivity of CGO increases with increasing temperature. The results show that Co-doped materials can still be used as solid electrolyte for intermediate-temperature electrochemical applications, when the operation temperature is 770–970 K.

New crystal structure and characterization of lanthanum tungstate “La6WO12” prepared by freeze-drying synthesis

Lanthanum tungstates with a La/W atomic ratio between 6 and 4.8 have been synthesized as polycrystalline materials using the freeze-drying wet-chemical precursor method. Our results show that a single phase material is obtained when the La/W ratio is between 5.3 and 5.7 (T = 1500 degrees C). Outside this compositional range, segregation of either La(2)O(3) (La/W > or = 5.8) or La(6)W(2)O(15) (La/W < or = 5.2) are found. We have solved the crystal structure for the composition with a La/W nominal atomic ratio of 5.6 by combining powder X-ray and powder neutron diffraction techniques. This structure substantially differs from that previously reported for Ln(6)WO(12) (Ln = Y, Ho). The main differences between the two structure types are the crystal symmetry, the different coordination environment of the cations and the formula unit. The formula unit can be written as La(6.63)W(1.17)O(13.43) (Z = 4; calculated density = 6.395 g/cm(3)), well in accordance with the diffraction techniques, He-pycnometry and electron probe microanalysis. These materials can be described as a face centred cubic structure with space group F43m. Lattice parameters vary between 11.173 and 11.188 A, depending on composition. Dense ceramic materials are obtained at 1400 degrees C, and microanalyses measurements indicate that no significant tungsten evaporation occurs compared to the nominal values. Compositions with La(2)O(3) segregation show similar conductivity values as the single phase ones, but those containing segregation of W-rich phases show a considerable drop in conductivity with increasing content of the secondary phase.

Optical properties of Er3+ ions in transparent glass ceramics

Abstract A study of optical properties and upconversion processes among Er 3+ ions in oxyfluoride glass and glass ceramic matrix has been carried out. From optical absorption spectra, the oscillator strengths have been obtained for several transitions and they have been used to calculate the Judd–Ofelt parameters. Experimental lifetime values are compared with those obtained with the Judd–Ofelt theory. Different upconversion emissions at 545, 660 and 800 nm have been obtained in Er 3+ doped glass and glass ceramics by exciting at 975 nm. A systematic investigation of the green upconversion is reported with the purpose of determining the involved upconversion mechanisms.

Optical properties of Nd3+ ions in oxyfluoride glasses and glass ceramics comparing different preparation methods

A study of optical properties of Nd3+ doped oxyfluoride glasses and glass ceramics prepared by three different methods has been carried out. These methods start from NdF3, Nd2O3, or a Nd3+ ion solution as doping agent. The alternative preparation method based on a preliminary dissolution of the Nd3+ ions is proposed in order to avoid nonhomogeneous dopant distribution and spontaneous devitrification during glass elaboration. In the frame of the Judd–Ofelt theory, main radiative parameters have been studied: transition probabilities, lifetimes, and stimulated emission cross sections. Fluorescence decay curves have been also analyzed in order to study the final distribution of the Nd3+ ions after the ceramming process, discerning between ions that reside in the fluoride nanocrystals precipitated during heat treatment and those remaining in the glassy phase. The NdF3 based glass ceramics present the best values for spectroscopic parameters as the stimulated cross section of the 4F3/2→4I11/2 laser transition.

Conductivity of CGO and CSO ceramics obtained from freeze-dried precursors

Ce0.8Gd0.2O1.9 (CGO) and Ce0.8Sm0.2O1.9 (CSO) have been prepared as polycrystalline materials using a freeze-dried precursor. This method yields amorphous nanometric powders. Crystallization of the fluorite phase occurred on heating at 600 8C or higher temperatures. The grain size of freeze-dried powders increases to about 100 nm after calcination at 800 8C, or about 200 nm after firing at 1000 8C. Freeze-dried powders were used to prepare dense ceramic disks by sintering at 1400 8C. Some disks were sintered at 1000 8C by adding small amounts of cobalt nitrate solution to assist the densification. The electrical conductivity results obtained for these gadolinia-doped ceria and samaria-doped ceria ceramics are similar to those obtained for CGO pellets obtained from commercial nanopowders (Rhodia). Though the bulk conductivity of CSO is probably higher than that of CGO, its grain boundary conductivity is inferior, and tends to control the overall behaviour, at least at relatively low temperatures. # 2003 Elsevier Science Ltd. All rights reserved.

Microstructural optimisation of materials for SOFC applications using PMMA microspheres

A novel and facile route to control the porosity of materials for Solid Oxide Fuel Cells (SOFCs) applications has been developed through the simple combination of oxide powders, polyvinyl alcohol and poly(methyl methacrylate) (PMMA) microspheres. This method allows the microstructure to be optimised improving the performances of SOFC electrode materials.

Site selective study of Eu3+-doped transparent oxyfluoride glass ceramics

Optical properties of Eu3+ ions in oxyfluoride glasses and glass ceramics doped with two different concentrations, 0.1 and 1 mol %, have been analyzed and compared with previous results for higher concentrated samples, 2.5 mol %. The Eu3+ ions in the 0.1 mol % doped glass ceramics are diluted into like crystalline environments with higher symmetry and lower coupled phonon energy than in the precursor glasses; meanwhile in the 1 mol % doped glass ceramics the presence of EuF3 clusters has been observed in addition to diluted ions. Fluorescence line narrowing measurements indicate the presence of two main fluoride site distributions for the diluted Eu3+ ions in both glass ceramics.

Role of the Eu3+ ions in the formation of transparent oxyfluoride glass ceramics

Optical properties of Eu3+ ions in oxyfluoride glass ceramics and in the precursor glasses (30 SiO2, 15 Al2O3, 29 CdF2, 22 PbF2, 1.5 YF3, and 2.5 EuF3, in mol %) have been analyzed and compared. The Eu3+ ions in the glass ceramics are incorporated into a crystalline environment with higher symmetry and lower energy coupled phonons. Emission measurements indicate that this crystalline phase is EuF3, instead of PbxCd1−xF2 as generally considered.

Improved Conductivity of Ce1 − x Sm x O2 − δ Ceramics with Submicrometer Grain Sizes

Freeze-drying was used to prepare nanocrystalline powders of Ce 1-x Sm x O 2-δ , x = 0.05-0.3, with high-purity and crystallite sizes in the range 10-15 nm. These powders were used to obtain ceramic samples with 4-7 μm in average diameter by firing at 1873 K, without sintering additive, and < 1 μm for samples sintered at 1423 K, with previous addition of cobalt nitrate solution to the freeze-dried powders. Impedance spectroscopy was used to deconvolute components of the spectra related to grain interiors and grain boundaries. For samples with grain sizes in the range 4-7 μm, the bulk conductivity increases with decreasing content of Sm up to 10%, whereas the grain boundary conductivity shows the opposite trend. However, samples with relatively low content of Sm and submicrometer grain sizes, obtained with addition of Co, retain the highest conductivity for both the bulk and grain boundary conductivities. The differences in grain boundary conductivities between samples with small and large grain sizes were about 1 order of magnitude for 20% Sm and 2 orders of magnitude for 5% Sm and 10% Sm. Differences in grain boundary behavior were interpreted on the basis of a space-charge layer adjacent to the boundary core. The estimated values of grain boundary thickness were in the order of 1 nm for samples prepared with and without addition of cobalt nitrate.

Proton Conduction and Long-Range Ferrimagnetic Ordering in Two Isostructural Copper(II) Mesoxalate Metal–Organic Frameworks

Two compounds of formula {(H3O)[Cu7(Hmesox)5(H2O)7]·9H2O}n (1a) and {(NH4)0.6(H3O)0.4[Cu7(Hmesox)5(H2O)7]·11H2O}n (1b) were prepared and structurally characterized by single-crystal X-ray diffraction (H4mesox = mesoxalic acid, 2-dihydroxymalonic acid). The compounds are crystalline functional metal-organic frameworks exhibiting proton conduction and magnetic ordering. Variable-temperature magnetic susceptibility measurements reveal that the copper(II) ions are strongly ferro- and antiferromagnetically coupled by the alkoxide and carboxylate bridges of the mesoxalate linker to yield long-range magnetic ordering with a Tc of 17.6 K, which is reached by a rare mechanism known as topologic ferrimagnetism. Electric conductivity, measured by impedance methods, shows values as high as 6.5 × 10(-5) S cm(-1) and occurs by proton exchange among the hydronium/ammonium and water molecules of crystallization, which fill the voids left by the three-dimensional copper(II) mesoxalate anionic network.