Fluorite-type Oxides Research Articles

Study on the Gas Sensitivity of a CeO<sub>2</sub> Based Ceramic Sensing Element: Structure and Characteristic

The sensitivity of CuO dispersed on fluorite-type oxide, namely CeO2 was studied in this work. Mixed oxide sample of nanostructured CuxCe1-xO2-y of various composition were generated by step chemical precipitation method. Distinct copper species were identified as a function of copper content by X-ray photoelectron spectroscopy, X-ray powder diffraction, the special surface areas, transmission electron microscopy, scanning electron microscopy analysis, and sensing properties to CO. It was found that only small amounts of copper are sufficient to promote the sensitivity of CeO2 by several orders of magnitude, which excessive amounts of copper (Cu/(Cu+Ce)>0.12) are detrimental to the sensing properties of nanocompositions. The possible causes for this behavior are also discussed.

Influence of microstructure on oxide ionic conductivity in doped CeO2 electrolytes

Doped ceria (CeO2) compounds are fluorite type oxides, which show oxide ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmospheres. As a consequence of this, considerable interest has been shown in application of these materials for `low (500 degrees-650 degrees C)' temperature operation of solid oxide fuel cells (SOFCs). In this study, some rare earth (eg. Gd, Sm, and Dy) doped CeO2 nano-powders were synthesized via a carbonate co-precipitation method. Fluorite-type solid solution were able to be formed at low temperature, such as 400 degrees C and dense sintered bodies were subsequently fabricated in the temperature ranging from 1000 degrees to 1450 degrees C by conventional sintering (CS) method. To develop high quality solid electrolytes, the microstructure at the atomic level of these doped CeO2 solid electrolytes were examined using transmission electron microscopy (TEM). The specimens obtained by CS had continuous and large micro-domains with a distorted pyrochlore structure or related structure, within each grain. We conclude that the conducting properties in these doped CeO2 systems are strongly influenced by the micro-domain size in the grain. To minimize the micro-domain size, spark plasma sintering (SPS) was examined. SPS has not been used to fabricate dense sintered bodies of doped CeO2 electrolytes, previously; carbon from the graphite dies penetrates the specimens and inhibits densification. To overcome this challenge, and to be able to produce dense sintered bodies of doped CeO2 of a grain size that minimizes the microdomain growth, a combination of SPS and CS methods were examined. Using this combined method we report that we were able to produce fully dense specimens with improved conductivity. This is correlated with a reduction in the size of the micro-domains. Consequently we conclude that the control of micro-domain size within the grain structure is a key component in the successful design of electrolyte materials with improved conductivity.

First-principles study of elastic properties ofCeO2,ThO2 and PoO2

Using first-principles density functional calculations, the structural and elastic properties of fluorite typeoxides CeO2, ThO2 and PoO2 were studied by means of the full-potential linear muffin-tin orbital method.Calculations were performed within the local density approximation (LDA) aswell as generalized gradient approximation (GGA) to the exchange correlationpotential. The calculated equilibrium lattice constants and bulk moduli are in goodagreement with the experimental results, as are the computed elastic constants forCeO2 and ThO2. For PoO2 this is the first quantitative theoretical prediction of the ground stateproperties, and it still awaits experimental confirmation. The calculations findPoO2 tobe a semiconductor with an indirect band gap and elastic constants similar in magnitude to those ofCeO2 and ThO2.

Study of Ce-Zr-Co fluorite-type oxide as catalysts for hydrogen production by steam reforming of bioethanol

Bioethanol, obtained by biomass fermentation, could be an important hydrogen supplier as a renewable source. The development of active, selective and stable catalysts for bioethanol steam reforming is a key point. In this work, a fluorite type Ce-Zr-Co oxide, Ce 2Zr 1.5Co 0.5O 8− δ , is studied for hydrogen production by steam reforming of ethanol/water mixture and bioethanol solution. The catalyst is characterized before and after catalytic test by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM-EDX). The preparation method based on propionate polymerization in solution ensures the insertion of cobalt in the mixed oxide lattice and provides high micro-homogeneity at nanoscale level. The reduction procedure to activate the catalysts is controlled by thermo programmed reduction (TPR). The partially reduced Ce-Zr-Co oxide catalyst presents high ethanol conversion and high hydrogen selectivity. Superior alcohols (fusel oils) arising from the fermentation process do not influence the catalytic behavior compared to a model ethanol/water mixture. Decreasing activity is related to the formation of carbon filaments, evidenced by thermo programmed oxidation (TPO) measurements and transmission electron microscopy.

Electrolytes for Solid-Oxide Fuel Cells

AbstractThree solid-oxide fuel cell (SOFC) electrolytes, yttria-stabilized zirconia (YSZ), rare-earth–doped ceria (REDC), and lanthanum strontium gallium magnesium oxide (LSGM), are reviewed on their electrical properties, materials compatibility, and mass transport properties in relation to their use in SOFCs. For the fluorite-type oxides (zirconia and ceria), electrical properties and thermodynamic stability are discussed in relation to their valence stability and the size of the host and dopant ions. Materials compatibility with electrodes is examined in terms of physicochemical features and their relationship to the electrochemical reactions. The application of secondary ion mass spectrometry (SIMS) to detect interface reactivity is demonstrated. The usefulness of doped ceria is discussed as an interlayer to prevent chemical reactions at the electrode–electrolyte interfaces and also as an oxide component in Ni–cermet anodes to avoid carbon deposition on nickel surfaces. Finally, the importance of cation diffusivity in LSGM is discussed, with an emphasis on the grain-boundary effects.

Influence of particle morphology on nanostructural feature and conducting property in Sm-doped CeO 2 sintered body

Doped ceria (CeO 2) compounds are fluorite type oxides, which show oxide ionic conductivity higher than yttria stabilized zirconia (YSZ), in oxidizing atmospheres. As a consequence of this, considerable interest has been shown in application of these materials for ‘low (500–650 °C)’ or ‘intermediate (650–800 °C)’ temperature operation, solid oxide fuel cells (SOFCs). In this study, the authors prepared two kinds of nanosize Sm-doped CeO 2 particles with different morphologies: one type was round and the other was elongated. Processing these powders with different morphology produced dense materials with very different ionic conducting properties and different nanoscale microstructures. Since both particles are very fine and well dispersed, sintered bodies with high density (relative density >95% of theoretical) could be prepared using both types of powder particles. The electrical conductivity of sintered bodies prepared from these powders with different starting morphologies was very different. Materials prepared from particles having a round shape were much higher than those produced using powders with an elongated morphology. Measured activation energies of the corresponding sintered samples showed a similar trend; round particles (60 kJ/mol), elongated particles (74 kJ/mol). While X-ray diffraction (XRD) profiles of these sintered materials were identical, diffuse scatter was observed in the background of selected area electron diffraction pattern recorded from both sintered bodies. This indicated an underlying structure that appeared to have been influenced by the processing technology. Detailed observation using high-resolution transmission electron microscopy (HR-TEM) revealed that the size of microdomain with ordering of cations in the sintered body made from round shape particles was much smaller than that of the sintered body made from elongated particles. Accordingly, it is concluded that the morphology of doped CeO 2 powders strongly influenced the microdomain size and electrolytic properties in the doped CeO 2 sintered body.

Significant effect of water on surface reaction and related electrochemical properties of mixed conducting oxides

The interaction between water vapor and fluorite-type oxide ion conductors, and its effects on surface and electrochemical properties are investigated. High deuterium ion intensity was observed for rare-earth doped ceria annealed in D 2O-containing atmospheres. It indicates that a pretty amount of deuterium is absorbed in the ceria polycrystalline. The oxygen isotope exchange rate on YSZ surface increased with water vapor pressure, indicating that the interaction between water and oxygen vacancy in YSZ is significant. The interfacial conductivity at platinum/YSZ interface in air at T=968 K was drastically improved by addition of 1 kPa of water vapor pressure. This improvement was clearly observed for YSZ electrolyte, however, only a small effect was observed for Ce 0.8Gd 0.2O 1.9.

Preparation and topotactical oxidation of ScVO3with bixbyte structure: a low-temperature route to stabilize the new defect fluorite ScVO3.5metastable phase

ScVO3 has been prepared by controlled reduction of a ScVO4 precursor under an H2/N2 flow at 1250 degrees C. The crystal structure of this material has been studied at room temperature by Rietveld refinement of high-resolution neutron powder diffraction (NPD) data. Sc3+ and V3+ are distributed at random over the metal sites of a C-M2O3 bixbyite-type structure, space group Ia3, a = 9.6182(1) Angstroms. The thermal analysis of ScVO3 in an air flow shows two subsequent oxidation processes, with a final reversal to ScVO4 above 600 degrees C. An intermediate phase of composition ScVO(3.5), containing V4+ cations, can be isolated by isothermal annealing at 350 degrees C in air. This metastable phase has been identified by X-ray diffraction (XRD) as a fluorite-type oxide (space group Fm3m, a = 4.947(2) Angstroms), also showing a random distribution of Sc and V cations over the metal positions. The Rietveld refinement of the ScVO(3.5) structure from powder XRD data in a fluorite structural model yields abnormally high thermal factors for the oxygen atoms, suggesting oxygen mobility in this metastable material.

Nonstoichiometry of Ce 1− XY XO 2−0.5 X− δ ( X=0.1, 0.2)

Nonstoichiometry of the fluorite-type oxide solid solutions Ce 1− X Y X O 2−0.5 X− δ ( X=0.1, 0.2) was measured as a function of temperature ( T=973–1373 K) and oxygen partial pressure ( P(O 2)=10 −2–10 −24 atm) by means of thermogravimetry. The result shows that the nonstoichiometry of Ce 1− X Y X O 2−0.5 X− δ ( X=0.1, 0.2) is not explained by a simple point defect model, therefore, defect association models are suggested. From the calculation, it is found that (Ce Ce′V O Ce Ce′) x is the major defect association not only in Ce 1− X Y X O 2−0.5 X− δ ( X=0.1, 0.2), but also in pure CeO 2 for nonstoichiometries less than 0.10. It is also found that the defect association (Ce Ce′V O Ce Ce′) x is dominating at lower temperature and smaller X composition.

Electrolytic Properties and Nanostructural Features in the La2 O 3 ­ CeO2 System

Doped ceria compounds are fluorite-type oxides which show oxide ionic conductivity higher than yttria-stabilized zirconia in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in applications of these materials for low or intermediate temperature operation of solid-oxide fuel cells (SOFCs). In this study, the effective index was suggested to maximize the ionic conductivity in based oxides. The index considers the fluorite structure, and combines the expected oxygen vacancy level with the ionic radius mismatch between host and dopant cations. Using this approach, the ionic conductivity of this system has been optimized and tested under operating conditions of SOFCs. 0.15, 0.175, and 0.20), 0.2, and 0.4), and 0.05, and 0.07) were prepared and characterized as the specimens with low, intermediate, and high index, respectively. The ionic conductivity was increased with increasing suggested index. The transmission electron microscopy analysis suggested that partial substitution of alkaline earth elements in place of La into Ce site contributes to a decrease of microdomain size and an improvement of conductivity. with high index and small microdomains exhibited the highest conductivity, wide ionic domain, and good performance in SOFCs. © 2003 The Electrochemical Society. All rights reserved.

Influence of nano-structural feature on electrolytic properties in Y2O3 doped CeO2 system

Doped ceria (CeO2) compounds are fluorite type oxides which show oxide ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in application of these materials for ‘low temperature operation (500–650 8C)’ of solid oxide fuel cells (SOFCs). In this study, YxCe12xO22d (x ¼ 0:05; 0:1; 0:15; 0:2 and 0.25) fine powders were prepared using a carbonate co-precipitation method. The relationship between electrolytic properties and nano-structural features in the sintered bodies was examined. The micro-structures of Y0.05Ce0.95O1.975, Y0.15Ce0.85O1.925 and Y0.25Ce0.75O1.875 as representative three specimens have been investigated in more detail with transmission electron microscopy (TEM). The big diffuse scattering was observed in the background of electron diffraction pattern recorded from Y0.15Ce0.85O1.925 and Y0.25Ce0.75O1.875 sintered bodies. This means that the coherent micro-domain with ordered structure is in the micro-structure. While Y0.25Ce0.75O1.875 sintered body with low conductivity and high activation energy has big micro-domains, the micro-domain size in Y0.15Ce0.85O1.925 with high conductivity and low activation energy was much smaller than that of Y0.25Ce0.75O1.875. TEM observation gives us message that the size of coherent micro-domain with ordered structure would closely relate to the electrolytic properties such as conductivity and activation energy in the specimens. It was concluded that a control of micro-domain size in nano-scale in Y2O3 doped CeO2 system was a key for development of high quality solid electrolyte in fuel cell application.© 2003 Elsevier Ltd. All rights reserved.

Charge Carrier Maps for (La0.9Sr0.1)MIIIO3−δ (MIII = Sc and In) Perovskites and (Ce0.8Sm0.2)O2−δ Fluorite

Total (oxide ion, proton, hole and electronic) conductivities, σ t , were investigated for (La 0.9 Sr 0.1 )M III O 3-δ (M III = Sc and In) perovskite-type oxides (LSM) and (Ce 0.8 Sm 0.2 )O 2-δ fluorite-type oxide (SDC) under different temperatures, T, oxygen partial pressures, P(O 2 ), and hydrogen partial pressures, P(H 2 ). The electrical conduction properties have been depicted uniformly as charge carrier maps that show predominant charge carrier (i.e., oxide ion, proton, hole, and electron) domains as functions of T, P(O 2 ), and P(H 2 ). The LSM showed the oxide ion, proton, and hole conduction domains; the SDC the oxide ion and electronic domains. The charge carrier maps are useful to predict the T, P(O 2 ), and P(H 2 ) regions where each oxide can be used as the electrolyte or electrode material of solid oxide fuel cells (SOFCs).

Influence of nano-structure on electrolytic properties in CeO2 based system

Doped ceria (CeO2) compounds are fluorite type oxides that show oxygen ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmosphere. In order to improve the conductivity, the effective index was suggested to maximize the oxygen ionic conductivity in doped CeO2 based oxides. In addition, the true microstructure of doped CeO2 was observed at atomic scale for conclusion of conduction mechanism. Doped CeO2 had small domains (10-50 nm) with ordered structure in a grain. It is found that the electrolytic properties strongly depended on the nano-structural feature at atomic scale in doped CeO2 electrolyte.

Bi0.85Ln0.15(1−n)V0.15nO1.5+0.15n Fluorite Type Oxide Conductors: Stability, Conductivity, and Powder Crystal Structure Investigations

To extend the study of Bi3O3–Ln2O3–V2O5 systems, an investigation of Bi0.85Ln0.15(1−n)V0.15nO1.5+0.15n materials (Ln=La, Pr, Nd, Sm, Tb, Ho, Yb), which adopt a fluorite-related structure for specific composition ranges, has been realized. The composition dependence of thermal stability of the structure evidenced using thermodiffractometry, and of conductivity properties determined from impedance spectroscopy, has been investigated. The pure anionic oxide conductor behavior evidenced by emf measurements of an oxygen concentration cell for Bi0.85Pr0.105V0.045O1.545 mixed oxide can be considered for other samples containing other rare earths. X-ray powder crystal structure refinements have been realized for praseodymium-containing terms Bi0.85Pr0.15(1−n)V0.15nO1.5+0.15n; n=0.1, 0.3, 0.5) and for a selection of the other lanthanide elements (Ln=La, Sm, Tb, Ho, and Yb) with n=0.3, on the basis of the Willis model. The stability and modification of the crystal structure are discussed.

Evolution of microstructure and impedance upon the sintering of a Bi–Pr–V-based fluorite-type oxide conductor

Samples of Bi 0.85Pr 0.105V 0.045O 1.545, a fluorite-type oxide conductor, with varying porosities and different average grain sizes determined by the sintering temperature (500–900 °C), were investigated using scanning electron microscopy (SEM) and impedance spectroscopy. Sintering treatment leads to increased density of ceramic materials between 500 and 800 °C and predominantly to grain growth at higher temperatures, in dense samples (porosity <4%). The impedance diagram for these materials is composed of a high-frequency domain characteristic of the electrical properties of the ceramic itself and a low-frequency domain attributed strictly to electrode response. The impedance diagram of the ceramic is a single semicircle. Grain boundaries and porosity do not lead to the appearance of a separate semicircle. The influence of sintering temperature on electrical properties may be parameterized by total electrical resistance, depression angle and relaxation frequency as seen by curve fitting of the impedance diagram. The variation of electrical conductivity and dielectric constant as a function of sample porosity are reported and discussed.

Research on the electrochemistry of oxygen ion conductors in the former Soviet Union

This review is focused on the analysis of experimental results on oxygen ion-conducting ceramic materials based on HfO2, CeO2, and ThO2, published in the former Soviet Union. In particular, the physicochemical and transport properties of fluorite-related oxides and the characteristics of electronic conduction in these solid electrolytes are briefly reviewed. Emphasis is given to electrocatalytic and electrochemical properties of cerium-containing oxides, which are promising materials for electrodes of electrochemical cells operating in reducing atmospheres, and mixed-conducting membranes. A comparative analysis of specific features of the solid-electrolyte ceramics based on hafnia, zirconia, ceria, and thoria is performed in order to reveal basic tendencies of oxygen ionic transport in fluorite-type oxides, and to identify the potential applicability of these materials in various high-temperature electrochemical devices.

Ion beam assisted growth of fluorite type oxide template films for biaxially textured HTSC coated conductors

Biaxially aligned growth was studied by a dual-ion-beam sputtering method for fluorite type (Zr/sub 0.85/Y/sub 0.15/O/sub 1.93/ (YSZ), Hf/sub 0.74/Yb/sub 0.26/O/sub 1.87/, CeO/sub 2/), pyrochlore type (Zr/sub 2/Sm/sub 2/O/sub 7/), and rare earth C type (Y/sub 2/O/sub 3/, Sm/sub 2/O/sub 3/) oxide films on polycrystalline Ni-based alloy substrates. Cube-textured (all axes aligned with a <100> axis substrate normal) films were obtained by low energy (<300 eV) ion bombardment at low temperatures (<300/spl deg/C). The assisting ion energy dependence of crystalline alignment was discussed in connection with lattice energies for these oxide crystals. Furthermore, a fluorite-like type oxide (Zr-X-O) was found to have a shorter time constant of texture evolution than YSZ. I/sub c/=98A (J/sub c/=0.65 MA/cm/sup 2/) was obtained for a 1.5 /spl mu/m thick Y-123 film on Zr-X-O.

Thermodynamic Considerations on Ceria-Based Fluorite-type Oxides with Emphases on Miscibility Gap, Electron Concentration and Proton Solubility

Thermodynamic Considerations on Ceria-Based Fluorite-type Oxides with Emphases on Miscibility Gap, Electron Concentration and Proton Solubility

Lattice Oxygen Transfer in Fluorite-Type Oxides Containing Ce, Pr, and/or Tb

Oxygen transfer capacity (OTC) is needed, for example, in three-way catalysts (TWC) for treatment of exhaust gases, extraction of pure oxygen from gaseous mixtures such as air, or the extraction of hydrogen from hydrogen-containing compounds such as water or methane. This property is explored in fluorite-related materials of the formula Ce1−x−y−z−pPrxTbyMzRpO2−x (x+y+z>0.5) in which M is a metal, especially Zr, Ti, V, Mo, W, Nb, Mn, Cu, Cr, Fe, Ni, Co, Ta, and Bi, and in which R is another rare-earth element. The OTC of these oxides is inherent in the characteristics of fluorite-related structures and the variable valency of Ce, Pr, and/or Tb. Such oxides can have the OTC property under reducing and oxidizing conditions without any reconstructive phase transitions. OTC data of some binary, ternary, and quaternary oxides containing Ce, Pr, or Tb are discussed.

Ion Beam Induced Growth Structure of Fluorite Type Oxide Films for Biaxially Textured HYSC Coated Conductors

AbstractBiaxially aligned film growth by dual-ion-beam sputtering methods were studied for fluorite type (Zr0.85Y0.15O1.93(YSZ), Hf0.74Yb0.26O1.87, CeO2), pyrochlore type (Zr2Sm2O7), and rare-earth C type (Y2O3, Sm2O3) oxides on polycrystalline Ni-based alloy substrates. Cubetextured (all axes aligned with a &lt;100&gt; axis substrate normal) films were obtained for fluorite and pyrochlore ones by low energy (&lt;300 eV) ion bombardment at low temperatures (&lt; 300 °C). Besides, cube textured Y2O3 films were obtained in far narrower conditions with a quite low energy (150 eV)-ion bombardment at the temperature of 300 °C. The assisting ion energy dependence was discussed in connection with lattice energies for these oxide crystals.