Ceria Layer Research Articles

All chemical YBa2Cu3O7 superconducting multilayers: Critical role of CeO2 cap layer flatness

New advances toward microstructural improvement of epitaxial CeO2 films grown by chemical solution deposition and their use as buffer layers for YBa2Cu3O7 (YBCO) films are presented. We demonstrate that the degree of epitaxy and the fraction of (001) atomically flat surface area are controlled by the incorporation of tetravalent (Zr4+) or trivalent (Gd3+) cations into the ceria lattice. The degree of epitaxy has been investigated by means of Rutherford backscattering spectroscopy-channeling and reflection high-energy electron diffraction, and a new methodology is also presented to quantify the fraction of (001) atomically flat area from atomic force microscopy images. Results are further correlated with the superconducting properties, microstructure, and texture of YBCO films grown by the trifluoroacetate route. A comparison with pulsed laser deposition and YBCO films grown on the same ceria layers is also presented. This growth procedure has allowed us to obtain all chemical multilayer films with controlled microstructure and critical current densities above 4 MA cm−2 at 77 K.

A ceria layer as diffusion barrier between LAMOX and lanthanum strontium cobalt ferrite along with the impedance analysis

A thin interlayer of samarium doped ceria (SDC) is applied as diffusion barrier between La 1 − x Sr x Co y Fe 1 − y O 3 x = 0.1–0.4, y = 0.2–0.8 (LSCF) cathode and La 1.8Dy 0.2Mo 1.6W 0.4O 9 (LDMW82) electrolyte to obstruct Mo–Sr diffusion and solid state reaction in the intermediate temperature range of SOFC. We demonstrate the effectiveness of the diffusion barrier through contrasting the clearly defined interfaces of LSCF/SDC/LDMW82 against a rugged growing product layer of LSCF/LDMW82 in 800 °C thermal annealing, and analyze the product composition and the probable new phase. In addition, the measured polarization resistance is considerably lower for the half-cell with a diffusion barrier. Therefore, the electrochemical performance of the LSCF cathode is investigated on the SDC-protected LDMW82. The cell with LSCF ( x = 0.4) persistently outperforms the one with x = 0.2 in polarization resistance because of its small low-frequency contribution. The activation energy of polarization resistance is also lower for La 0.6Sr 0.4Co y Fe 1 − y O 3 (112–135 kJ/mol), than that for La 0.8Sr 0.2Co y Fe 1 − y O 3 (156–164 kJ/mol). La 0.6Sr 0.4Co y Fe 1 − y O 3 y = 0.4–0.8 is the proper composition for the cathode interfaced to SDC/LDMW82.

AISI 304 Austenitic stainless steels monoliths for catalytic applications

The thermal treatments of austenitic stainless steels monoliths were studied in order to generate a highly homogeneous and rough oxide scale strongly attached to the base alloy, which will subsequently ensure the good adherence of the catalysts. In this work it has been shown that the morphology, integrity and homogeneity of the scale are strongly influenced by the temperature and time of treatment. Washcoating method was used to deposit on the monolith surface a Au/CeO2 catalyst. The drying procedure turned out to be the most critical variable for the adherence and homogeneity of the catalytic ceria layer, while the ceria colloid concentrations in the starting aqueous dispersion seems to have only a threshold effect. The monolithic reactors containing Au/CeO2 layers are active in the oxidation of CO.

Cerium oxide stoichiometry alteration via Sn deposition: Influence of temperature

Cerium oxide layers grown on Cu(1 1 1) were studied by conventional X-ray and resonant photoelectron spectroscopy with synchrotron radiation. A quantitative method of determining the cerium chemical state from the Ce 3d photoelectron spectra is described in detail. After the preparation of the ceria layer, Sn films of different thickness were evaporated onto the surface at temperatures of 120, 300 and 520 K. In all three cases, the deposited Sn was oxidized, CeO 2 was partially reduced, and a mixed Sn–Ce–O oxide was formed. The quantitative extent of these reactions was found to be determined by limited diffusion of the deposited Sn atoms into the ceria layer at low temperature. The excess of tin formed a metallic overlayer on the sample surface.

Synthesis and characterization of cerium oxide by electrochemical methods

AbstractCeria‐based materials have been synthesized by electrochemical process. Electrodeposition is an interesting cheap method which can be performed at ambient pressure and rather low temperature (less than 100 °C). Moreover, it is easy to control in situ the film thickness. Ceria coatings were obtained by an indirect electrodeposition method. A potentiostatic technique (–0.7 V/SCE) was used to first reduce a hydroxide precursor (O2 or NO3–) before leading to the formation of cerium oxide after 2h of deposition time. This work focused on the characterization of ceria films deposited onto stainless steel in view of high temperature fuel cell applications. The chosen deposition conditions lead to quite adherent, homogenous and covering films. The microstructure and the crystallinity of the ceria thin layers were characterized by SEM, TEM and XRD measurements. Electrochemical microscopy (SECM) was also used to locally study the conductive properties of ceria layers and the homogeneity of the deposited films. Finally, electrochemical characterizations such as impedance spectroscopy were performed under air atmosphere. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Demonstration of the Rapid Start-Up Operation of Cathode-Supported SOFCs Using a Microtubular LSM Support

The performance of a cathode-supported solid oxide fuel cell (SOFC) was improved by using a microtubular (LSM) support. The cell had a laminated structure, with a bilayer of dense gadolinium-doped ceria (GDC) layer and scandia-stabilized zirconia as the electrolyte and a NiO-GDC layer as the anode on the microtubular LSM support. It was fabricated by extrusion of the cathode support and subsequent surface coating by a wet process. Microstructural observations and electrochemical analyses showed that the cell has a stable and durable structure for rapid start-up operation at a heating rate of .

Cerium oxide layers on the Cu(1 1 1) surface: Substrate-mediated redox properties

Ceria submonolayers consisting of nanosized 2D islands on a Cu(1 1 1) surface have been prepared by oxidation of the nucleating Ce submonolayer (0.7 ML) and characterized using XPS and STM. The reducibility of the resulting well-defined CeO 2− x /Cu(1 1 1) model system of the “inverse supported catalyst” type was studied by XPS using CO as reducing agent. In order to investigate the contribution of the substrate in the redox process, the properties of a ceria submonolayer (0.7 ML) on Cu(1 1 1), of an optically dense layer of ceria (1.5 ML) on Cu(1 1 1) and of a ceria submonolayer (0.7 ML) on Pt(1 1 1) have been compared. The direct comparison reveals the (metal substrate mediated) spillover mechanism of the ceria reduction by CO in the present CeO 2− x /Me fcc(1 1 1) model systems.

Fabrication and evaluation of cathode-supported small scale SOFCs

In this study, we have developed a fabrication process for cathode-supported micro-SOFCs via extrusion of micro-tubular cathode supports and subsequent surface coating with electrolyte and anode slurries. The complete micro-tubular cell had a laminated structure, with a dense gadolinium-doped ceria (GDC) layer as an interlayer, a dense scandia-stabilized zirconia (ScSZ) layer as an electrolyte and NiO–GDC layer as an anode on a LaSrMnO 3 (LSM) micro tube as a cathode support. The micro-tubular cell with an electrode length of 7 mm produced a power density above 0.13 W/cm 2 and 0.30 W/cm 2 at 700 and 800 °C, respectively, in a wet H 2 flow.

A resonant photoemission study of the Ce and Ce-oxide/Pd(1 1 1) interfaces

We have investigated the Ce 4f electronic states in the Ce/Pd(1 1 1) and Ce-oxide/Pd(1 1 1) systems, using resonant photoemission (Ce 4d → 4f transitions), and XPS to understand Pd–Ce interactions in ultra thin layers of cerium and ceria deposited on Pd(1 1 1). Cerium deposited on Pd(1 1 1) at room temperature forms surface Ce–Pd alloys with Ce rich character, while a Pd rich Ce–Pd alloy is formed by heating to 700 °C. A modification of the chemical state of Ce can also be seen after oxygen exposure. RPES provides evidence that Ce-oxide layers deposited on Pd(1 1 1) have a CeO 2 (Ce 4+) character, however a net contribution of the Ce 3+ states is also revealed. The Ce 3+ states have surface character and are accompanied by oxygen vacancies. Heating to 600 °C causes Ce-oxide reduction. A significant shift of Pd 4d-derived states, induced by Pd 4d and Ce 4f hybridization, was observed. The resonant features in the valence band corresponding to Ce 4+, Ce 3+ and Ce 0 states have been investigated for various Pd−Ce(CeO x ) coverages.

Microstructure and transport properties of ceria and samaria doped ceria thin films prepared by EBE–IBAD

Ceria and samaria doped ceria thin films were prepared by ion beam assisted electron beam evaporation. Two substrate temperatures were used (200 and 500 °C), together with ion assistance, as preparation parameters to obtain thin films with controlled microstructure. The samaria doped ceria films were grown using a multilayer strategy, alternating deposition of ceria and samaria layers. X-ray diffraction and scanning electron microscopy were used to characterise the evolution of the structure and microstructure of the films. The electrical conductivity of the thin films was studied in pure O 2 atmosphere as a function of temperature by two point dc measurements. The transport characteristics of the films (activation energy and conductivity) exhibited clear grain size dependence. The importance of both ionic and electronic contributions to the total conductivity was identified. In particular, the highest electrical conductivity was achieved for ceria samples prepared at 200 °C under Ar + assistance. These samples had the smallest grain size (∼ 20 nm). Samaria doped ceria films showed the highest conductivity and activation energies consistent with ionic charge transport when they were prepared at 500 °C under Ar + bombardment and doped with 35 wt.% of Sm 2O 3.

First-Principles Thermodynamics of Coherent Interfaces in Samarium-Doped Ceria Nanoscale Superlattices

Nanoscale superlattices of samarium-doped ceria layers with varying doping levels have been recently proposed as a novel fuel cell electrolyte. We calculate the equilibrium composition profile across the coherent {100} interfaces present in this system using lattice-gas Monte Carlo simulations with long-range interactions determined from electrostatics and short-range interactions obtained from ab initio calculations. These simulations reveal the formation of a diffuse, nonmonotonic, and surprisingly wide (11 nm at 400 K) interface composition profile, despite the absence of space charge regions.

Progress on Single Buffer Layered Coated Conductors Prepared by Thermal Evaporation

Coated conductors with a single CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> buffer layer architecture have been developed in recent years by various research groups owing to the simplicity and cost effectiveness. The main challenge of this architecture is the modest thickness (~100 nm) of crack-free CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layers. In this work, we report on the successful deposition of thicker crack-free single CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> buffer layers on biaxially textured Ni-5 at%W substrates by e-beam evaporation thanks to the use of dopants, such as Sm, Yb or Zr, introduced in pure CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> targets. We have minimized the mechanical stress at the CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /substrate interface, that is responsible for the cracking, by optimizing the dopant concentration. X-Ray diffraction rocking curve analyses show that such doped Ceria layers have a strong out-of-plane c-axis orientation with FWHM values of 5deg. Further analysis by scanning electron microscopy shows dense and crack-free layers. These results indicate no sizeable degradation of the high epitaxial quality of the layers induced by the doping. We finally report on the deposition of high-quality superconducting YBCO films on such improved single buffer layer structures using thermal co-evaporation assisted by a novel oxygenation method based on a supersonic oxygen gas beam.

Robust and Reliable Cells for an Easy Stack Operation

Cell failures are often the cause for stopping a running stack. Staxol, a new company active in the SOFC area is developing thick cells that will provide robustness and reliability. Keeping in mind that fuel utilisation has to be high to allow power efficiency, strong efforts will be dedicated to optimise the structure and the proportion of the porosity in the cell. In order to keep as low as possible the ohmic resistance, thin electrolyte (5-20 μm), made of yttria or scandia stabilized zirconia equips the cell. A thin top layer of doped ceria is added when reactive cathodes are used.

Co-evaporated YBCO/doped-CeO2/Ni–W coated conductors oxygen improved using a supersonic nozzle

A novel process for the coated conductors (CC) deposition, characterized by a single CeO2 buffer layer architecture, and a new oxygenation device for the YBCO layer has been developed.In CC technology, usually the ceria layer thickness must be less than 100nm to avoid the formation of cracks; in order to ensure an efficient barrier effect, complex and costly multi-buffer layers architectures must be grown. In this work, we describe the way to increase the thickness of crack-free single buffer layer.Single CeO2 buffer layer is grown on biaxially textured Ni–5at.%W substrates (RABiTS) by e-beam evaporation technique, introducing different doping elements at various percentages in pure Ceria targets. Since the crack formation in cerium oxide layers mainly depends on the stresses introduced by the lattice mismatch with Ni alloy substrate and on the arrangement of the oxygen vacancies in the lattice, it is possible to reduce these stresses by optimizing the doping in the Ceria layer, thus obtaining very thick and crack-free CeO2 buffer layers.The texture quality of our samples is tested by XRD, giving a strong in-plane and out-of-plane orientation (FWHM values of about 5°). Optical and electron microscopy shows dense and crack-free thick layers (up to 450nm) and confirms excellent insulating properties.A novel technique has been developed to enhance the oxygen pressure on the YBCO film during the deposition as the oxygen content is known to be critical for the growth of YBCO films with high Tc. A supersonic oxygen nozzle (SNEO) incorporated into the vacuum chamber enables a ×103 increasing of the local oxygen pressure close to the YBCO layer keeping a 10−4mbar background pressure.The work demonstrates that the single thick doped-Ceria buffer layer could be a viable way to reduce CC complexity and production costs and that SNEO apparatus can be used to make the oxygenation process of the YBCO films easier and more reliable.

Large-area quantification of BaCeO3 formation during processing of metalorganic-deposition-derived YBCO films

A method is described for the quantification of BaCeO3 formation during the growth of YBa2Cu3O7–x (YBCO) films on CeO2 buffer layers. The method is based on the selective etching of BaCeO3 followed by inductively coupled plasma (ICP) excitation spectroscopy. A 10% HNO3 solution, at room temperature, dissolved BaCeO3 and YBCO in minutes but did not significantly etch CeO2 films. ICP excitation spectroscopy was used to quantify the extent of the reaction over macroscopic areas of film (∼1 cm2). BaCeO3 peak areas were measured by x-ray diffraction (XRD) and calibrated to the ICP excitation spectroscopy results. XRD and ICP excitation spectroscopy results indicated that BaCeO3 formation through a metalorganic deposition (MOD)-derived CeO2 layer follows the parabolic growth law. Almost the entire ceria cap layer was consumed by the growth of BaCeO3 after 2 h at 760 °C in the MOD process examined. BaCeO3 growth was substantially slower at 725 °C; only 25 ± 3% of the ceria layer reacted.

Thermochromic CeO2–VO2 bilayers: Role of ceria coating in optical switching properties

Bilayers of thermochromic vanadium dioxide (VO2) and cerium dioxide (CeO2) were elaborated by RF sputtering. The VO2 phase was first deposited on an amorphous substrate. Then a ceria layer was deposited on the VO2 film with variable thicknesses (from 20 to 160nm). These systems of CeO2–VO2 bilayers present interesting optoelectronic properties, with a thermochromic transition at 68°C due to the insulator–metal transition of VO2 phase, active in the infrared wavelength range.The bilayers were then characterized by X-ray diffraction, atomic force microscopy and Fourier Transform Infrared (FTIR) spectroscopy. For infrared radiations having long wavelengths (10μm and above), the optical properties (reflectance, emissivity) of CeO2–VO2 bilayers are independent of ceria thickness up to 160nm. For shorter wavelengths (5μm), the optical reflectance and emissivity of VO2 are strongly attenuated by ceria layer, and this attenuation increases with thickness.For a fixed temperature, the apparent temperature determined by a thermal camera depends on ceria thickness. This shows that ceria can be used to control the apparent surface temperature of VO2. In the case of thin ceria layers, we show that ceria insures a good chemical protection of VO2.

Carbon deposition on Ni/YSZ composites exposed to humidified methane

Abstract Carbon formation on Ni/YSZ cermets in high-temperature processes involving hydrocarbons can be a severe problem. Understanding the mechanisms of carbon formation is necessary for developing strategies to avoid or minimize the problem. In this study, Ni/YSZ pellets (70/30 NiO/YSZ) have been exposed to humidified methane at temperatures between 773 K and 1073 K. The carbon formed on these pellets and/or any structural changes in the pellets was studied with X-ray diffraction (XRD), scanning electron microscopy (SEM) and temperature-programmed oxidation (TPO). At temperatures below 873 K, carbon fibres dominated. At higher temperatures, the majority of the carbon dissolved into the Ni particles. After TPO and removal of the carbon, the Ni structure was damaged to various extents depending on the exposure temperature. The amount of carbon deposited was significantly reduced by placing zirconia-doped ceria pellets on each side of the Ni/YSZ pellet. The addition of a ceria layer did not completely eliminate the carbon formation but the carbon that was formed was weakly attached to the Ni/YSZ such that the structure was not irreversibly changed. The role of the ceria is beyond simply reforming the methane.

Low temperature solid oxide fuel cells with pulsed laser deposited bi-layer electrolyte

Solid oxide fuel cells (SOFC) using a pulsed laser deposited bi-layer electrolyte have been successfully fabricated and have shown very good performance at low operating temperatures. The cell reaches power densities of 0.5 W cm −2 at 550 °C and 0.9 W cm −2 at 600 °C, with open circuit voltage (OCV) values larger than 1.04 V. The bi-layer electrolyte contains a 6–7 μm thick samarium-doped ceria (SDC) layer deposited over a ∼1 μm thick scandium-stabilized zirconia (ScSZ) layer. The electrical leaking between the anode and cathode through the SDC electrolyte, which due to the reduction of Ce 4+ to Ce 3+ in reducing environment when using a single layer SDC electrolyte, has been eliminated by adopting the bi-layer electrolyte concept. Both ScSZ and SDC layers in the bi-layer electrolyte prepared by the pulsed laser deposition (PLD) technique are the highly conductive cubic phases. Poor conductive (Zr, Ce)O 2-based solid solutions or β-phase ScSZ were not found in the bi-layer electrolyte prepared by the PLD due to low processing temperatures of the technique. Excellent reliability and flexibility of the PLD technique makes it a very promising technique for the fabrication of thin electrolyte layer for SOFCs operating at reduced temperatures.

Interfacial Structure and Point Defects in Ceria/Zirconia Superlattices

AbstractWe report the results of a molecular dynamics simulation study aiming to understand the interfacial structure in ceria/zirconia superlattices and the impact of the interfaces on the energies of oxygen vacancy formation and Gd ion substitution in ceria and zirconia layers of the superlattice structure. It is found that the semi-coherent interface is characterized by misfit dislocations, paired at approximately 3-4 nm, with stacking-fault-like region in between, which agrees with the TEM observations. It is also found that the vacancy formation energy and the Gd substitution energy vary as a function of distance from the interface in the individual layers, and that these energies depend on the layer thickness. In addition, the simulations showed that the defect energy variations across the thickness of the ceria and zirconia layers are consistent with the XPS data for composition profile in the superlattice structure. Finally, in the semi-coherent superlattice structure, the formation energy of oxygen vacancies and the Gd substitution energy are found to depend on the position of these defects relative to the interfacial dislocation core. In particular, the oxygen vacancy formation energy is found to be negative close to the dislocation core, indicating that vacancy concentration will increase in such regions allowing for high conduction parallel to the interface.

Nitrate-based metalorganic deposition of CeO2 on yttria-stabilized zirconia

An aqueous nitrate metalorganic deposition (MOD) process was used to form a 30-nm-thick c-axis-textured ceria (CeO2) layer on a yttria-stabilized zirconia (YSZ) single-crystal substrate. X-ray diffraction showed the CeO2 layer was an (001) oriented film with a ω-scan full width at half-maximum of 1.47°. The average roughness of the ceria films, measured by atomic force microscopy, was about 5 nm. Critical current densities of up to 3.6 × 106 A/cm2 were measured on a 0.35-μm Ba2YCu3O7−x (YBCO) layer deposited over this cap layer using a trifluoroacetate (TFA)-based MOD ex situ process. The physical vapor deposition-derived ceria cap layer used in the most common coated conductor buffer layer stacks may be replaced by such a MOD processed film.