Electrical Conductivity and Lattice Defects in Nanocrystalline Cerium Oxide Thin Films

The results of the electrical conductivity and Raman scattering measurements of CeO₂ thin films obtained by a polymeric precursor spin-coating technique are presented. The electrical conductivity has been studied as a function of temperature and oxygen activity and correlated with the grain size. When compared with microcrystalline samples, nanocrystalline materials show enhanced electronic conductivity. The transition from extrinsic to intrinsic type of conductivity has been observed as the grain size decreases to <100 nm, which appears to be related to a decrease in the enthalpy of oxygen vacancy formation in CeO₂. Raman spectroscopy has been used to analyze the crystalline quality as a function of grain size. A direct comparison has been made between the defect concentration calculated from coherence length and nonstoichiometry determined from electrical measurements.     

Electrical Conductivities of (CeO2)1−x(Y2O3)x Thin Films

Electrical properties of CeO₂ thin films of different Y₂O₃ dopant concentration as prepared earlier were studied using impedance spectroscopy. The ionic conductivities of the films were found to be dominated by grain boundaries of high conductivity as compared with that of the bulk ceramic of the same dopant concentration sintered at 1500°C. The film grain-boundary conductivities were investigated with regard to grain size, grain-boundary impurity segregation, space charge at grain boundaries, and grain-boundary microstructures. Because of the large grain boundary and surface area in thin films, the impurity concentration is insufficient to form a continuous highly resistive Si-rich glassy phase at grain boundaries, such that the resistivity associated with space-charge layers becomes important. The grain-boundary resistance may originate from oxygen-vacancy-trapping near grain boundaries from space-charge layers. High-resolution transmission electron microscopy coupled with a trans-boundary profile of electron energy loss spectroscopy gives strong credence to the space-charged layers. Since the conductivities of the films were observed to be independent of crystallographic texture, the interface misorientation contribution to the grain-boundary resistance is considered to be negligible with respect to those of the impurity layer and space-charge layers.     

Electrical Behavior of Ceria-Stabilized Zirconia with Rare-Earth Oxide Additives

The electrical properties of rare-earth oxide doped CeO₂-ZrO₂ ceramics are evaluated by impedance spectroscopy. Doping of the trivalent metal oxide decreases both the grain and grain-boundary resistivities of the CeO₂-ZrO₂ system. The grain and grain-boundary resistivities as well as the activation energy for conduction increase with the dopant cation radius. The grain conductivity is fairly constant for 2 mol% YO_1.5-10 mol% CeO₂-88 mol% ZrO₂ samples sintered under different conditions, while lower grain-boundary resistivity is obtained for samples sintered at higher temperature for longer times. Possible mechanisms for the electrical behavior of the rare-earth oxide doped CeO₂-ZrO₂ ceramics are proposed and discussed.     

Raman Spectroscopy of Nanocrystalline Ceria and Zirconia Thin Films

The results of Raman-scattering studies of nanocrystalline CeO₂ and ZrO₂:16% Y (YSZ) thin films are presented. The relationship between the lattice disorder and the form of the Raman spectra is discussed and correlated with the microstructure. It is shown that the Raman line shape results from phonon confinement and spatial correlation effects and yields information about the material nonstoichiometry level.     

Blocking Grain Boundaries in Yttria-Doped and Undoped Ceria Ceramics of High Purity

CeO₂ samples doped with 10, 1.0, and 0.1 mol% Y₂O₃ and undoped CeO₂ samples of high purity were studied by impedance spectroscopy at temperatures <800°C and under various oxygen partial pressures. According to microstructural investigations by SEM and analytical STEM (equipped with EDXS), the grain boundaries were free of any second phase, providing direct grain-to-grain contacts. An amorphous siliceous phase was detected at only a few triple junctions, if at all; as a result, its contribution to the grain-boundary resistance was negligible. Nevertheless, the specific grain-boundary conductivities were still 2–7 orders of magnitude lower than the bulk conductivities, depending on dopant concentration, temperature, and oxygen partial pressure. The charge carrier transport across the grain boundaries occurred only through the grain-to-grain contacts, whose properties were then determined by the space-charge layer. The space-charge potential in acceptor-doped CeO₂ was positive, causing the simultaneous depletion of oxygen vacancies and accumulation of electrons in the space-charge layer. The very low grain-boundary conductivities can be accounted for by the oxygen-vacancy depletion; the accumulation of electrons became evident in weakly doped and undoped CeO₂ at high temperatures and under low oxygen partial pressures.     

Optical and Electrical Properties of Amorphous and Nanocrystalline (La0.8Sr0.2)0.9MnO3 Thin Films Prepared from Low-Temperature Processing Technique

The results of a study on the optical and electrical properties of (La_0.8Sr_0.2)_0.9MnO₃ (LSM) thin films obtained by a polymeric precursor spin coating technique were presented. This method allowed preparation of optical quality thin films at annealing temperatures around 800°C. Amorphous and crystalline LSM thin films were studied by optical and electrical conductivity measurements. The energy-dependent absorption coefficients for the crystalline specimen were calculated from optical spectra and extra absorption was observed in the range of 1.8–2.5 eV with the exchange-gap excitation behavior in the 3–5 eV range. In comparison to the amorphous specimens, the electrical conductivity of the nanocrystalline specimen increased two to three orders of magnitude with decreasing activation energy. The charge carrier absorption model suggested an increase of the carrier concentration in the nanocrystalline specimen which may be a reason for the change in the electrical conductivity.     

Oxygen Storage Capacity, Specific Surface Area, and Pore-Size Distribution of Ceria–Zirconia Solid Solutions Directly Formed by Thermal Hydrolysis

The oxygen storage capacity (OSC) of CeO₂–ZrO₂ solid solutions that were directly formed as nanocrystals by thermal hydrolysis of acidic aqueous solutions of (NH₄)₂Ce(NO₃)₆ and ZrOCl₂ at 150°C increased from 94 μmol of O₂/g for pure CeO₂ to >400 μmol of O₂/g for compositions of CeO₂/ZrO₂ with molar ratios (C/Z) from 74.1/25.9 to 41.7/58.3 (maximum value of 431 μmol O₂/g was reached at the composition C/Z = 51.7/48.3) and then decreased with increased ZrO₂ content in the solid solutions. As compared with pure CeO₂, the CeO₂–ZrO₂ solid solutions that contained <84.8 mol% ZrO₂ maintained high specific surface area and large pore volume with nanosized pores (pore size at maximum pore volume) <10 nm in diameter after heat treatment at 700°C.     

Ionic Conductivity Enhancement of La2Mo2−xWxO9 Nanocrystalline Films Deposited on Alumina Substrates by the Sol–Gel Method

Dense, crack-free, and uniform La₂Mo_{2− x} W _x O₉ ( x =0, 0.1, and 0.2) nanocrystalline films were successfully synthesized on poly-alumina substrates via a modified sol–gel method, with inorganic salt of La(NO₃)₃·6H₂O, (NH₄)₆Mo₇O₂₄·4H₂O, and (NH₄)₆H₂W₁₂O₂₄ as precursors. Pure La₂Mo₂O₉ phase was confirmed by X-ray diffractometer when the annealing temperature was >500°C. The average grain size of the La₂Mo_{2− x} W _x O₉ films is in the range of 90–400 nm, depending upon the conditions of thermal treatment, and the thickness of films can reach 1 μm by repetitive spin-coating. The electrical conductivity increases with decreasing grain size and reaches 0.074 S/cm at 600°C in the film with a grain size of 90 nm, which is one order of magnitude higher than that in the corresponding bulk materials. W-doping can suppress the phase transition that occurs at 580°C in pure La₂Mo₂O₉ and enhance the low-temperature ionic conductivity. Furthermore, the activation energy of conductivity in the nanocrystalline La₂Mo₂O₉ films decreases to about 0.6 eV in comparison with 1.0 eV in the bulk ones, which implies that the grain resistance prevails in the total resistance, when grain size reduces to nanometer domain.     

Thermodynamic Stability of Gadolinia-Doped Ceria Thin Film Electrolytes for Micro-Solid Oxide Fuel Cells

Next-generation micro-solid oxide fuel cells for portable devices require nanocrystalline thin-film electrolytes in order to allow fuel cell fabrication on chips at a low operation temperature and with high power outputs. In this study, nanocrystalline gadolinia-doped ceria (Ce_0.8Gd_0.2O_{1.9− x} ) thin-film electrolytes are fabricated and their electrical conductivity and thermodynamic stability are evaluated with respect to microstructure. Nanocrystalline gadolinia-doped ceria thin-film material (Ce_0.8Gd_0.2O_{1.9− x} ) exhibits a larger amount of defects due to strain in the film than state-of-the-art microcrystalline bulk material. This strain in the film decreases the ionic conductivity of this ionic O²⁻ conductor. The thermodynamic stability of a nanocrystalline ceria solid solution with 65 nm grain size is reduced compared with microcrystalline material with 3–5 μm grain size. Nanocrystalline spray-pyrolyzed and PLD Ce_0.8Gd_0.2O_{1.9− x} thin films with average grain sizes larger than 70 nm show predominantly ionic conductivity for temperatures lower than 700°C, which is high enough to be potentially used as electrolytes in low to intermediate-temperature micro-solid oxide fuel cells.     

Effect of Powder Coating on Stabilizer Distribution in CeO2-Stabilized ZrO2 Ceramics

The phase and microstructure relationship of 12 mol% CeO₂-stabilized ZrO₂ ceramics prepared from coated powder was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersed X-ray spectroscopy (EDS). As compared with the sample prepared with co-precipitated method, which exhibited a similar grain size distribution, the EDS analysis revealed that the powder coating induced a wide distribution of CeO₂ solubility, which decreases monotonically with the increase of grain size. This variation of stabilizer content from grain to grain rendered many large grains in the monoclinic phase. Stronger cerium segregation to grain boundaries was observed between large grains, which often form thin amorphous films there. The inhomogeneous CeO₂ distribution keeps more tetragonal ZrO₂ grains close to the phase boundary to facilitate the transforming toughness. Addition of an Al₂O₃ precursor in coated powders effectively raises the overall CeO₂ stabilizer content in the grains and preserves more transformable tetragonal phase in the microstructure, which further enhanced the fracture toughness. The dependence of CeO₂ solubility on grain size may be explained in a simple coating-controlled diffusion and growth process that deserves further investigation.     

Electrochemical Stability of Strontium-Doped Ceria Electrolyte in Solid-Oxide Fuel Cell Applications

SrO-doped CeO₂ electrolyte has been evaluated in single-cell configuration under solid-oxide fuel cell operating conditions. Because of oxygen loss from the crystal lattice, the material experiences a macroscopic expansion of several percent at 1000°C. On extended cell operation, strontium precipitates-out at/near the anode, resulting in irreversible cell degradation in the case of SrO-doped CeO₂. Precipitation and diffusion of SrO causes decreased ionic conductivity and may result in anode delamination. SrO precipitation is attributed to insolubility of the dopant in the reduced CeO₂ phase. The diffusion of strontium seems to be related to the flux of oxygen through the sample, but an exact mechanism is unknown.     

Effect of the Presence of Ammonium Peroxodisulfate on the Direct Precipitation of Ceria and Ceria–Zirconia Solid Solutions from Acidic Aqueous Solutions

Direct precipitation of nanometer-sized particles of ceria–zirconia (CeO₂–ZrO₂) solid solutions with cubic and tetragonal structures was successfully attained from acidic aqueous solutions of cerium(III) nitrate (Ce(NO₃)₃) and zirconium oxychloride (ZrOCl₂) through the addition of ammonium peroxodisulfate ((NH₄)₂S₂O₈), because of promotion of the hydrolysis via the oxidation of Ce³⁺ ions, together with the simultaneous hydrolysis of ZrOCl₂ under hydrothermal conditions. Ultrafine CeO₂ particles also could be formed from relatively concentrated aqueous solutions of the same trivalent cerium salt in the presence of (NH₄)₂S₂O₈ via hydrolysis. The crystallite size and lattice strain of as-precipitated solid solutions varied, depending on the composition within the CeO₂–ZrO₂ system. Creation of a solid solution of ZrO₂ into a fluorite-type CeO₂ lattice clearly introduced lattice strain, as a consequence of the decreasing crystallite size. Both the direct precipitation process and the effectiveness of the presence of (NH₄)₂S₂O₈ for the synthesis of CeO₂–ZrO₂ solid solutions were discussed.     

Effect of pH of Medium on Hydrothermal Synthesis of Nanocrystalline Cerium(IV) Oxide Powders

Well-crystallized cerium(IV) oxide (CeO₂) powders with nanosizes without agglomeration have been synthesized by a hydrothermal method in an acidic medium by using cerium hydroxide gel as a precursor. The relationship between the grain size, the morphology of the CeO₂ crystallites, and the reaction conditions such as temperature, time, and acidity of the medium was studied. The experiments showed that with increasing reaction temperature and time, the CeO₂ crystallites grew larger. The crystallites synthesized in an acidic hydrothermal medium were larger and had a more regular morphology than the ones synthesized in a neutral or alkaline medium when the reaction temperature and time were fixed. The CeO₂ crystallites synthesized in an acidic medium were monodispersed; however, there was vigorous agglomeration among the grains synthesized in a neutral or alkaline medium. It was demonstrated that the hydrothermal treatment was an Ostwald ripening process and the acidity (pH) of the used hydrothermal medium played a key role in the dissolution of smaller grains. It is proposed that the dissolution process can control the kinetics of the growth of larger grains.     

Synthesis of Ultrafine Ceria Powders by Mechanochemical Processing

The synthesis of ultrafine cerium dioxide (CeO₂) powders via mechanochemical reaction and subsequent calcination was studied. Anhydrous CeCl₃ and NaOH powders, along with NaCl diluent, were mechanically milled. A solid-state displacement reaction—CeCl₃+ 3NaOH → Ce(OH)₃+ 3NaCl—was induced during milling in a steady-state manner. Calcination of the as-milled powder in air at 500°C resulted in the formation of CeO₂ nanoparticles in the NaCl matrix. A simple washing process to remove the NaCl yielded CeO₂ particles ∼10 nm in size. The particle size was controlled in the range of ∼10–500 nm by changing the calcination temperature.     

Synthesis of Cerium(IV) Oxide Ultrafine Particles by Solid-State Reactions

CeO₂ ultrafine particles were prepared by solid-state reactions at room temperature. These particles were found to have very fine particle sizes (∼3 nm) with a fluorite structure ( a = 5.42 Å). BET measurements showed that the surface area of the particles was 96.2 m²/g. The use of two different precursors was found to affect the size of the CeO₂ particles. We discuss the effect of calcination at different temperatures on the morphology, size, and BET surface area of CeO₂ particles. A salt byproduct coating prevented agglomeration of the CeO₂ particles.     

Effect of Low-Energy Accelerated Ion Bombardment on the Properties of Metal-Organic Decomposition Derived SrBi2(Ta,Nb)2O9 Thin Films Processed at Low Temperature

Ferroelectric SrBi₂(Ta,Nb)₂O₉ (SBTN) thin films were deposited on Pt (200 nm)/TiO _x (40 nm)/SiO₂ (100 nm)/Si substrates by metal-organic decomposition. The effects of bombardment from accelerated argon and oxygen ions on the properties of SBTN thin films were investigated. It was found that the argon ion bombardment could decrease the crystallization temperature owing to the increase of internal energy of the films. Also, the oxygen vacancies at the interface between the SBTN film and platinum bottom electrode or at grain boundaries in the film were passivated through the oxygen ion treatment, resulting in the improved electrical properties. By optimizing the process parameters and using bombardment of accelerated argon and oxygen ions, SBTN films with good ferroelectric and electrical properties could be obtained, at a temperature as low as 650°C.     

Processing of Composite Thin Film Solid Oxide Fuel Cell Structures

A solid oxide fuel cell (SOFC) structure is proposed in which a composite thin film cathode substrate supports a dense thin film electrolyte with a thickness of less than 1 μm. The cathode substrate has a graded porosity achieved through the partial sintering of a spin-coated CeO₂ colloidal suspension. The resulting surface has a pore size and surface roughness which allowed a fully dense ZrO₂:16%Y (YSZ) electrolyte to be spin-coated directly from a polymeric precursor without capillary forces removing the precursor from the surface of the porous substrate. Using this process, fuel cell structures were constructed with temperatures not exceeding 800°C. The porous CeO₂ interlayer should allow for decreased ohmic losses, as well as decreased reactions between the YSZ and the cathode substrate. In addition, the nanocrystalline grain sizes should allow for increased catalytic activity on the cathode. Calculated ohmic losses indicated the resistance of the CeO₂ interlayer limited the power of the structure, which was minimized by impregnating the porous layer with a mixed-conducting perovskite. The final structure shows significantly reduced ohmic losses as calculated at 400°C.     

Microstructure and Grain-Boundary Effect on Electrical Properties of Gadolinium-Doped Ceria

The microstructural evolution and grain-boundary influence on electrical properties of Ce_0.90Gd_0.10O_1.95 were studied. The nanoscale powders synthesized from a semibatch reactor exhibited 50% green density and 92% sintering density at 1200°C (∼200°C lower than previous studies). Impedance spectra as a function of temperature and grain size were analyzed. The Ce_0.90Gd_0.10O_1.95 with finest grain size possessed highest overall grain-boundary resistance; this contribution was eliminated at temperatures >600°C, regardless of grain size. The grain conductivity was independent of grain size and was dependent on temperature with two distinct regimes, indicative of the presence of Gd'_Ce− V _o^∘∘ complexes that dissociated at a critical temperature of ∼580°C. The activation energy for complex dissociation was ∼0.1 eV; the value for the grain-boundary was ∼1.2eV, which was size independent.     

Charge-Compensating Defect in Ceria-Doped Strontium Barium Niobate by Electron Energy-Loss Spectroscopy and X-ray Photoelectron Spectroscopy

Undoped and CeO₂-doped Sr_0.5Ba_0.5Nb₂O₆ (SBN) powders were synthesized using the solid-state reaction method. The lattice parameters of undoped and CeO₂-doped SBN were evaluated using X-ray diffractometry. The valence state of the cerium ion in Ce-doped SBN was identified using X-ray photoelectron spectroscopy (XPS), and the valence of 3+ was confirmed. The charge-compensated defect of the cerium doping in SBN ceramics, i.e., excessive oxygen ions occupying the vacant O(4) or O(5) site, was further evidenced using electron energy-loss spectroscopy (EELS) and XPS. The relationships of charge-compensated defect, structure, binding energy, and temperature of the maximum of the relative permittivity ( T _m) were discussed.     

Effect of Additives on the Pressureless Sintering of Aluminum Nitride between 1500° and 1800°C

Combined oxide additives (Y₂O₃, CaO, La₂O₃, CeO₂, SiO₂, TiO₂, and Fe₂O₃) were investigated as AIN sintering aids. AIN can be fully sintered at 1600°C to substantial thermal conductivity (92 W/(m·K)) using a multiple sintering aid of Y₂O₃, CaO, SiO₂, La₂O₃, and CeO₂. This lowtemperature material has small grain size (1 to 3 μm).