CeO2 Grains Research Articles

Temperature and high pressure effects on the structural features of catalytic nanocomposites oxides by Raman spectroscopy

Structural characterizations of nanostructured oxides were studied by X-ray diffraction (XRD), Raman and infrared spectroscopy. The oxides catalysts namely, SnO2, ZrO2, CeO2, MnOx, Al2O3 and TiO2 were prepared by a nanocasting route and the effect of the temperature and pressure on the stability of the solids was evaluated. Raman spectra showed that ZrO2 and TiO2 exhibited phase transitions at moderate temperatures whereas CeO2, SnO2 and MnOx had an effective creation of defects in their structures upon annealing at elevated temperatures. The results suggested also that the effect of the temperature on the particles growth is related to the type of oxide. In this regard, phase transition by up to 600°C accelerated the sintering of ZrO2 and CeO2 grains compared to TiO2, SnO2 and MnOx counterparts. Under hydrostatic pressures lower than 10GPa, rutile TiO2 and tetragonal ZrO2 exhibited pressure induced phase transition whereas CeO2 and SnO2 were stable at pressures close to 15GPa. The experiments revealed that the nanostructured SnO2 oxide exhibited stable performance at relatively high temperatures without phase transition or sintering, being suitable to be used as catalysts in the range of temperature and pressure studied.

Influence of LiMn2O4 modification with CeO2 on electrode performance

In this article we present a low temperature method applied to modify the surface of lithium manganese oxide grains’ by using cerium dioxide. In order to determine the structure, size of particles and morphology of the pristine and surface modified LiMn2O4 powders we used several complementary methods: X-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and Raman spectroscopy. The galvanostatic charge/discharge tests of the pristine and CeO2 modified LiMn2O4 cathode materials were conducted in the potential range: 3.5 to 4.5V vs. Li/Li+ at room temperature. High current rates performance has been evaluated by determination of specific discharge capacity at current rates varied from 1C to 30C.Our work demonstrates that surface modification of LiMn2O4 grains’ by using 1%wt. of cerium dioxide admixture improves cycling stability and capacity retention. Pristine LMO reveals 10% capacity loss after 100 cycles when discharged at 1C, while the sample modified with CeO2 grains reveals only ca. 2% capacity loss.

The Effect of Fabrication Conditions for GDC Buffer Layer on Electrochemical Performance of Solid Oxide Fuel Cells

Abstract A Gd-doped ceria (GDC) buffer layer is required between a conventional yttria-stabilized zirconia (YSZ) electrolyte and a La-Sr-Co-Fe-O3 (LSCF) cathode to prevent their chemical reaction. In this study, the effect of varying the conditions for fabricating the GDC buffer layer, such as sintering temperature and amount of sintering aid, on the solid oxide fuel cell (SOFC) performance was investigated. A finer GDC powder (i.e., ultra-high surface area), a higher sintering temperature (∼1290°C), and a larger amount of sintering aid (∼12%) resulted in improved densification of the buffer layer; however, the electrochemical performance of an anode-supported cell containing this GDC buffer layer was poor. These conflicting results are attributed to the formation of (Zr, Ce)O2 and/or excess cobalt grain boundaries (GBs) at higher sintering temperatures with a large amount of sintering aid (i.e., cobalt oxide). A cell comprising of a cobalt-free GDC buffer layer, which was fabricated using a low-temperature process, had lower cell resistance and higher stability. The results indicate that electrochemical performance and stability of SOFCs strongly depend on fabrication conditions for the GDC buffer layer.

The Effect of Fabrication Conditions for GDC Buffer Layer on Electrochemical Performance of Solid Oxide Fuel Cells

A Gd-doped ceria (GDC) buffer layer is required between a conventional yttria-stabilized zirconia (YSZ) electrolyte and a La-Sr-Co-Fe-O3 (LSCF) cathode to prevent their chemical reaction. In this study, the effect of varying the conditions for fabricating the GDC buffer layer, such as sintering temperature and amount of sintering aid, on the solid oxide fuel cell (SOFC) performance was investigated. A finer GDC powder (i.e., ultra-high surface area), a higher sintering temperature (∼1290°C), and a larger amount of sintering aid (∼12%) resulted in improved densification of the buffer layer; however, the electrochemical performance of an anode-supported cell containing this GDC buffer layer was poor. These conflicting results are attributed to the formation of (Zr, Ce)O2 and/or excess cobalt grain boundaries (GBs) at higher sintering temperatures with a large amount of sintering aid (i.e., cobalt oxide). A cell comprising of a cobalt-free GDC buffer layer, which was fabricated using a low-temperature process, had lower cell resistance and higher stability. The results indicate that electrochemical performance and stability of SOFCs strongly depend on fabrication conditions for the GDC buffer layer.

Sol–gel derived ZnO–CeO2 thin films on glass substrate

The microstructure and optical properties of crystalline ZnO–CeO2 thin films on glass substrate at various preheating temperatures, annealing temperatures and time were investigated. The relative intensity ratio values of the diffraction peaks of ZnO–CeO2 thin films and grain growth behavior were related to the preheating temperature, annealing temperature, and time. A suitable thermal treatment results in higher surface energy, which improves the surface morphology of the grown film. The ZnO–CeO2 thin films present high transparency (over 70%) in the visible region of the optical spectrum.

In Situ HT‐ESEM Observation of CeO2 Grain Growth During Sintering

The CeO2 sintering and grain growth kinetics were determined from in situ experiments performed using an environmental scanning electron microscope (ESEM) equipped with a hot stage in the temperature range 1000°C–1300°C. Videos were recorded during 8 h at various holding temperatures where the grain growth and grain elimination process were clearly observed at the grain scale with a 2–10 nm resolution. Data were extracted at both sample and grain scales to characterize the microstructural evolution (grain growth and grain elimination). The grain growth kinetic was described using a parabolic law. The activation energy determined using this method for pure CeO2 grain growth (290 ± 40 kJ/mol) was in good agreement with that obtained using the Dorn's method based on dilatometric measurements (330 ± 30 kJ/mol). The microstructural modifications observed using the in situ HT‐ESEM experimental method were compared with computed simulations and revealed a good correlation between the experimental data and the models previously developed.

Atomistic Investigation of Oxygen Vacancy Induced Volume Changes in CeO2 Grain Boundaries

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Enhanced oxidation of CO by using a porous biomorphic CuO/CeO2/Al2O3 compound

Self-support porous biomorphic CuO/CeO2/Al2O3 catalysts with different elemental composition were synthesized by using cotton as template. The samples were characterized via SEM, nitrogen gas sorption surface analysis, XRD and Raman scattering. Their performances in catalytic CO oxidation were investigated. The samples consisted of network of mesoporous hollow fibers composing of nanometer-sized CuO, CeO2 and Al2O3 grains. We found that optimal CO oxidation was achieved by using the sample with Cu/Ce/Al weight ratio of 1/5/2. Low Cu content resulted in the formation of the CuCeO solution, the active component for CO oxidation. Small amount of alumina improved the catalytic activity because it promoted the oxygen diffusivity of ceria by reducing the size of ceria crystallites. The catalytic performance of the biomorphic product was compared with a control powder sample. The hollow fibrous structure of the product was found to enhance the catalytic activities by facilitating the passage for the flows of gaseous reactants.

Atomic structure of a Σ3 [110]/(111) grain boundary in CeO2

The atomic structure of a Σ3 [110]/(111) grain boundary in CeO2 was studied by scanning transmission electron microscopy, electron energy loss spectroscopy, and the first-principles calculations. It was revealed that this grain boundary does not promote the formation of oxygen vacancies and keeps oxygen stoichiometry, which is different from that of Σ5 CeO2 grain boundary studied previously [H. Hojo, T. Mizoguchi, H. Ohta, S. D. Findlay, N. Shibata, T. Yamamoto, and Y. Ikuhara, Nano Lett. 10, 4668 (2010)]. It was found that the difference in grain boundary oxygen stoichiometry is correlated with the grain boundary atomic structure.

Fabrication process of YBCO thin film starting from amorphous film for microstrip line device

Abstract We investigated the process for fabricating YBa2Cu3Oy (YBCO) thin film microstrip lines from amorphous thin films. First, we made amorphous YBCO thin films and then post-annealing was carried out. The amorphous 300-nm-thick YBCO thin films were deposited on a CeO2 buffered r-Al2O3 substrate by pulsed laser deposition. The 50-nm-thick CeO2 buffer layer was made by a facing target sputtering system. We examined the optimal conditions of the post-annealing process by using a two-step crystallization process. The heating rate of the first step was set to 360 K/min until 1023 K, and then the heating rate of the second step was set to 20 K/min until 1073 K. The oxygen pressure was kept to 3000 Pa at 1073 K for 210 min. From the XRD measurement, we observed that the YBCO thin films grew c-axis orientation on the CeO2/r-Al2O3 substrate. Both YBCO and CeO2 φ-scan peaks showed four-fold symmetry and rotated 45 degrees between YBCO (102) grain axis and CeO2 (220) grain axis. The surface roughness, Δφ, and Δφ of YBCO thin films were 35.12 nm, 0.89 degree, and 1.51 degree, respectively. We found these to be the optimal post-annealing conditions in which to make a single phase of YBCO films with Tc of 87 K from amorphous YBCO thin films. Therefore, our process is useful to make high-quality YBCO microstrip lines.

Grain boundary atomic structures and light-element visualization in ceramics: combination of Cs-corrected scanning transmission electron microscopy and first-principles calculations

Grain boundaries and interfaces of crystals have peculiar electronic structures, caused by the disorder in periodicity, providing the functional properties, which cannot be observed in a perfect crystal. In the vicinity of the grain boundaries and interfaces, dopants or impurities are often segregated, and they play a crucial role in deciding the properties of a material. Spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM), allowing the formation of sub-angstrom-sized electron probes, can directly observe grain boundary-segregated dopants. On the other hand, ceramic materials are composed of light elements, and these light elements also play an important role in the properties of ceramic materials. Recently, annular bright-field (ABF)-STEM imaging has been proposed, which is now known to be a very powerful technique in producing images showing both light- and heavy-element columns simultaneously. In this review, the atomic structure determination of ceramic grain boundaries and direct observation of grain boundary-segregated dopants and light elements in ceramics were shown to combine with the theoretical calculations. Examples are demonstrated for well-defined grain boundaries in rare earth-doped Al(2)O(3) and ZnO ceramics, CeO(2) and SrTiO(3) grain boundary, lithium battery materials and metal hydride, which were characterized by Cs-corrected high-angle annular dark-field and ABF-STEM. It is concluded that the combination of STEM characterization and first-principles calculation is very useful in interpreting the structural information and in understanding the origin of the properties in various ceramics.

Erosion–corrosion in a laboratory-scale coal-firing FBC of various aluminized coatings prepared by low-temperature pack cementation

Using a halide-activated pack-cementation method but at a temperature (600°C) noticeably lower than normal, an η-Fe2Al5 coating and two δ-Ni2Al3 coatings with and without dispersions of CeO2 nanoparticles were developed respectively on a low-carbon steel and the steel pretreated with an electrodeposited film of Ni or Ni–CeO2. The erosion–corrosion (E–C) performance of the three aluminide coatings during 100h exposure at ~600°C in a coal-firing laboratory-scale fluidized-bed combustor (FBC) was investigated, by mounting the aluminized samples onto a rig which maintained rotation for accelerating the relative impacting speed of flying solid particles (mainly SiO2 bed materials). The η-Fe2Al5 and the CeO2-free δ-Ni2Al3 coatings experienced an unacceptable recession rate. Compared to the two CeO2-free aluminide coatings, the CeO2-dispersed δ-Ni2Al3 coating offered profoundly improved E–C resistance, because the latter coating was not only strengthened by the CeO2 dispersion and grain refinement, it also could grow a more adherent alumina scale.

Atomic Structure of a CeO2 Grain Boundary: The Role of Oxygen Vacancies

Determining both cation and oxygen sublattices of grain boundaries is essential to understand the properties of oxides. Here, with scanning transmission electron microscopy, electron energy-loss spectroscopy, and first-principles calculations, both the Ce and oxygen sublattices of a (210)Σ5 CeO(2) grain boundary were determined. Oxygen vacancies are shown to play a crucial role in the stable grain boundary structure. This finding paves the way for a comprehensive understanding of grain boundaries through the atomic scale determination of atom and defect locations.

Effective thickness of CeO2 buffer layer for YBCO coated conductor by advanced TFA-MOD process

YBCO films were fabricated on PLD-CeO2/IBAD-Gd2Zr2O7/Hastelloy substrates using the advanced TFA-MOD process. The effective thickness of the CeO2 buffer layer for obtaining high Ic was investigated in short samples of YBCO films. The CeO2 buffer layer was epitaxially grown on an IBAD-Gd2Zr2O7 template tape with 18° of Δϕ by a reel-to-reel PLD system. The in-plane grain alignment of PLD-CeO2 buffer layers rapidly improved with the thickness and saturated at a critical thickness of 0.8μm. The size of CeO2 grains was about 1μm at the saturated thickness of Δϕ. YBCO films with the thickness of 1μm were deposited by the TFA-MOD on the CeO2 buffer layer with different thickness films. Improvement of the CeO2 in-plane grain alignment resulted in increase of Ic. The Ic values of 250–290A were obtained with the CeO2 layer thicker than 0.8μm. The CeO2 thickness, at which the intensity ratio of the BaCeO3 was saturated, corresponded to the critical thickness. From the view points of achieving higher production rates and to obtain the CeO2 Δϕ value of 5° as well as considering the reaction between YBCO and CeO2, the optimum thickness of the CeO2 buffer layer on the IBAD-Gd2Zr2O7 with 18° of Δϕ was found to be at least 0.8μm.

Microstructural evolution of nickel nanoparticle catalysts supported on gadolinium-doped ceria during autothermal reforming of iso-octane

The microstructure and composition of a nanoparticle Ni catalyst supported on gadolinium-doped ceria (Ce1−xGdxO(4−x)/2) were studied using transmission electron microscopy (TEM), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). The support of the fresh catalyst exhibits a homogenous aggregation of crystalline grains, with sizes ranging between 20 nm and 50 nm. The crystalline structure of the fresh catalyst support is of the CeO2 phase, in which gadolinium atoms exist in a solid solution of CeO2. Nickel in the fresh catalyst is highly dispersed and forms granular crystals that are 5–30 nm in size on the surface of the ceria support. The support of the used catalyst exhibits a bimodal distribution of grains in which smaller grains have similar structure and morphology as those in the fresh catalyst, while the larger sized grains appear dull and exhibit nonfaceted crystal morphology resulting either from the sintering of a number of CeO2 grains or by the occupation of highly defective crystals of Ce2O3 and CeO phases. A thin amorphous layer of carbon also covers most of the larger grains in the used catalyst. The Ni particles could not be imaged by TEM in the used catalyst, but energy dispersive x-ray spectroscopy (EDX) detected their presence. The XPS analysis of the catalyst samples suggests the participation of lattice O atoms from the ceria support in the catalytic reaction. The XPS data also show the presence of carbonate species and a higher hydrocarbon concentration in the used catalyst.

TEM observations of Au and Ir particles supported on CeO2

Au/CeO2 and Ir/CeO2 catalysts were observed by a transmission electron microscope in order to investigate the nano-structures of Au and Ir particles and CeO2 grains and the interface structure between metallic particles and CeO2. An annular dark field scanning transmission electron microscope (ADF-STEM) and an energy dispersive X-ray spectroscopy (EDS) revealed that the metallic particles smaller than 2 nm in diameter are highly dispersed on CeO2 supports in both catalysts. For model samples of larger CeO2 particles with flat facets of low-index surfaces, high resolution transmission electron microscopy observations of Au/CeO2 and Ir/CeO2 interfaces were made and the epitaxial relationship between Au and CeO2, (111)[110]Au//(111)[110]CeO2 has been found for the first time.

Relationship between the Nanostructures and the Optical Properties of CeO2 Thin Films

CeO2 nanostructured thin films have been deposited on Al2O3(1−102) and TiO2(001) single-crystal substrates through metal−organic chemical vapor deposition (MOCVD) using the Ce(III) 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato diglyme adduct (Ce(hfa)3·diglyme). Structural and morphological characterizations have been carried out by X-ray diffraction pattern (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) measurements. It has been found that the deposition temperature and the lattice mismatch with the substrate have a strong impact on the film nanostructure. X-ray diffraction patterns of samples grown at low deposition temperature point to the formation of 〈100〉 oriented CeO2 films, while films deposited at higher deposition temperatures have 〈111〉 texture. Furthermore, SEM cross section images clearly point to relevant effects of deposition temperatures both on the structural alignment of CeO2 grains as well as on the film/substrate interfaces. Spectroscopic ellipsometric measurements provide a suitable rationalization of correlations between nanostructures and optical properties of the CeO2 films. Moreover, combination of SEM and spectroscopic ellipsometric measurements provide an in-depth interpretation of growth modes.

Optical characterization of ceramic thin films: Applications in low-temperature solid oxide fuel-cell materials research

Characterization of thin film solid oxide fuel-cell materials can be difficult due to the range of porosities in electrodes and electrolytes as well as the nano-sized pores and particles. In this study, optical characterization techniques such as ultraviolet–visible transmission and reflection spectrophotometry are illustrated as methods for achieving information about the film density from the film refractive index as well as the film thickness. These techniques were used to investigate the sintering process of colloidal CeO2 on sapphire substrates and polymeric precursor-derived ZrO2:16%Y (YSZ) thin films on silicon over the temperature range 400–1000 °C, and the results were compared with traditional characterization techniques such as electron microscopy, profilometry, ellipsometry, and x-ray diffraction line broadening analyses. Most of the techniques were in good agreement with the CeO2 grain size changing from 5–65 nm and the film thickness changing from 0.8–0.5 μm. Comparisons of transmission and reflection spectrophotometry with ellipsometry illustrated that scattering effects from the porous CeO2 films caused an overestimation of the refractive index from ellipsometry, but allowed for accurate grain size measurements from transmission and reflection data. Both techniques were in good agreement during the sintering of the YSZ thin films, with the density changing from 90–100% theoretical after heating between 400 and 800 °C.

Growth of CeO2 thin films deposited on biaxially textured nickel substrates

CeO2 films are technologically important as buffer layers for the integration of superconducting YBa2Cu3O7−δ films on {100}-biaxially textured Ni substrates, yielding a Ni–CeO2–YBa2Cu3O7−δ layer sequence. The Ni–CeO2 interface is a metal–oxide interface, and the misfit between substrate and film is about 9%. An epitaxial growth model was suggested for this system in the literature. The investigated films were deposited by a reactive thermal evaporation process at substrate temperatures of 650–670 °C with a thickness of 100 nm after deposition. The CeO2 films were characterized by plan-view and cross-section transmission electron microscopy, atomic force microscopy, and scanning electron microscopy. The CeO2 films had a strong {100} biaxial texture with a roughness of approximately 90 nm. No intermediate layer could be found by cross-section transmission electron microscopy at the Ni–CeO2 interface. The films had columnar grains with diameters of 20–50 nm, much smaller than the grain size of the Ni substrate, which was larger than 1 μm. Small-angle grain boundaries and small amounts of 〈111〉-oriented grains were evidenced in plan-view samples by diffraction patterns. The Moiré fringes technique was applied and was ideally suited to image the small rotations (≤3°) of the small CeO2 grains with respect to the Ni substrate. These small rotations of small grains showed that the growth was nonepitaxial, however, biaxially textured. In the CeO2 film samples, nanovoids 5–10 nm in size were observed and were mostly located close to the film surface. A model for the growth of CeO2 thin films on nickel substrates can be proposed on the basis of our results.