Hydrothermal Synthesis of Cerium(IV) Oxide

CeO₂ powders have been prepared from cerium(III) nitrate, cerium(IV) sulfate, and cerium(IV) ammonium sulfate under hydrothermal conditions at 120° to 200°C for 5 to 40 h. The effects of the starting cerium compounds, hydrothermal treatment temperature, and the concentration of the solutions on the crystal growth of CeO₂ were investigated. CeO₂ powders hydrothermally synthesized at 180°C for 5 h from cerium(IV) salts had very fine particle sizes (30 Å); on the other hand, the powder from the cerium(III) salt had a relatively coarse particle size (160 Å). Although the crystallite size of the powder synthesized from the cerium(IV) compounds depended on the treatment temperature, that from the cerium(III) compound was insensitive to the treatment temperature. The mechanisms for the growth of CeO₂ particles under hydrothermal conditions are discussed.     

Reactive Cerium(IV) Oxide Powders by the Homogeneous Precipitation Method

CeO₂ powders have been prepared by aging a cerium(III) nitrate solution in the presence of hexamethylenetetramine. Oxidation of Ce³⁺ occurs in the precipitate and the wet precipitate is identified as crystallized CeO₂ before any heat treatment. The cold-pressed powders can be sintered to full density at temperatures as low as 1250°C in just 6 min. Moreover, the sinterability of the powders is insensitive to the calcination temperatures, particle size, or green density. The powders calcined at 850°C with a crystallite size of 600 Å have a sinterability as good as the powders calcined at 450°C with a crystallite size of 145 Å. The mechanisms for direct CeO₂ precipitation and its relation to the excellent sinterability are discussed.     

Reactive Ceria Nanopowders via Carbonate Precipitation

Nanocrystalline CeO₂ powders have been successfully synthesized via a carbonate precipitation method, using ammonium carbonate (AC) as the precipitant and cerium nitrate hexahydrate as the cerium source. The AC/Ce³⁺ molar ratio ( R ) affects significantly precursor properties, and spherical nanoparticles can be produced only in a narrow range of 2 < R ≤ 3. The precursor, having an approximate composition of Ce(OH)CO₃·2.5H₂O, decomposes to CeO₂ at temperatures ≥300°C. The CeO₂ powder calcined at 700°C exhibits high reactivity and can be densified to >99% of theoretical at 1000°C.     

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.     

Electrochemical Synthesis and Sintering of Nanocrystalline Cerium(IV) Oxide Powders

Nanocrystalline CeO₂ powders were prepared electrochemically by the cathodic electrogeneration of base, and their sintering behavior was investigated. X-ray diffraction and transmission electron microscopy revealed that the as-prepared powders were crystalline cerium(IV) oxide with the cubic fluorite structure. The lattice parameter of the electrogenerated material was 0.5419 nm. The powders consisted of nonaggregated, faceted particles. The average crystallite size was a function of the solution temperature. It increased from 10 nm at 29°C to 14 nm at 80°C. Consolidated powders were sintered in air at both a constant heating rate of 10°C/min and under isothermal conditions. The temperature at which sintering started (750°C) for nanocrystalline CeO₂ powders was only about 100°C lower than that of coarser-grained powders (850°C). However, the sintering rate was enhanced. The temperature at which shrinkage stopped was 200°-300°C lower with the nanoscale powder than with micrometer-sized powders. A sintered specimen with 99.8% of theoretical density and a grain size of about 350 nm was obtained by sintering at 1300°C for 2 h.     

Hydrothermal Synthesis of Nanocrystalline Cerium(IV) Oxide Powders

Nanocrystalline cerium(IV) oxide (CeO₂) powders were prepared by heating solutions of cerium(IV) salts in the presence of urea under hydrothermal conditions at 120° to 180°C. The effects of the concentration of urea and hydrothermal treatment temperature on the morphology and crystallite size of the synthesized particles were investigated. The synthesized particles were angular, ultrafine CeO₂, with a cubic fluorite structure. Their crystallite size decreased from 20 to 10 nm with increasing urea concentration from 2 times to 8 times that of the Ce⁴⁺ ion. The size only slightly changed by calcining at temperatures below 600°C.     

Preparation of Cerium Dioxide Powders for Catalyst Supports

Studies of the mechanisms by which cerium dioxide enhances automotive catalyst performance would be facilitated by the use of pure, high-surface-area forms of cerium dioxide. A cerium dioxide powder with a BET area of 50 m²·g⁻¹ has been prepared by spray drying an aqueous solution of cerium nitrate, citric acid, and nitric acid. The thermal stability of the BET area at temperatures from 500° to 1100°C has been determined and compares favorably to other CeO₂ powders. Microstructure evolution during thermal treatments is discussed and pelleting procedures are reported.     

Coprecipitation and Hydrothermal Synthesis of Ultrafine 5.5 mol% CeO2-2 mol% YO1.5ZrO2 Powders

Ultrafine 5.5 mol% CeO₂—2 mol% YO_1.5ZrO₂ powders with controllable crystallite size were synthesized by two kinds of coprecipitation methods and subsequent crystallization treatment. The amorphous gel produced by ammonia coprecipitation and hydrothermal treatment at 200°C for 3.5 h results in an ultrafine powder with a surface area of 206 m²/g and a crystallite size of 4.8 nm. The powder produced by urea hydrolysis and calcination exhibits a purely tetragonal phase. In addition, the powders crystallized by hydrothermal treatment exhibit high packing density and can be sintered at lower temperature (,1400°C) with nearly 100% tetragonal phase achieved.     

Sintering, Microstructure, Hardness, and Fracture Toughness Behavior of Y2O3-CeO2-ZrO2

A wet-chemical approach is applied to derive fine powders with compositions 11 mol% CeO₂-ZrO₂, 1 mol% YO_1.5-10 mol% CeO₂-ZrO₂, 12 mol% CeO₂-ZrO₂, and 2 mol% YO_1.5-10 mol% CeO₂-ZrO₂ by the coprecipitation method. The characteristics of the as-derived powders are evaluated through thermal analysis and electron microscopy. The sintering behavior of the calcined powders is carried out at 1400° and 1500°C for 1 to 10 h. Sintered density higher than 98% of theoretical is achieved for sintering at 1400°C for several hours. The as-sintered density dependence on the sintering condition is related to the extent of tetragonal-to-monoclinic phase transformation as well as the associated microcracks. Partial substitution by Y₂O₃ in CeO₂-ZrO₂ results in reduced grain size and tends to stabilize the tetragonal structure. Y₂O₃ is more effective than CeO₂ with respect to the grain size refinement and tetragonal stability. In addition, Y₂O₃ substitution in CeO₂-ZrO₂ increases the hardness, while it decreases the fracture toughness.     

Preparation of an Alkali-Element-Doped CeO2-Sm2O3 System and Its Operation Properties as the Electrolyte in Planar Solid Oxide Fuel Cells

The ionic conductivity of the ceria-samaria (CeO₂-Sm₂O₃) system is higher than that of yttria-stabilized zirconia and other CeO₂-based oxides. In this study, a small amount of alkali-element-doped CeO₂-Sm₂O₃ solid solution was prepared. This solid solution was characterized by measuring the powder density and the chemical composition. Moreover, its electrochemical properties were investigated in the temperature range from 700° to 1000°C. It was found that a small amount of alkali-element-doped CeO₂ solid solution enhanced the ionic conductivity. The power density of an oxygen-hydrogen fuel cell for alkali-element-doped CeO₂-Sm₂O₃ ceramics exhibited high values at low temperatures such as 700° to 800°C. It is concluded that the improved fuel cell performance can be attributed to the high stability of this composition in the fuel atmosphere.     

Properties of the Solid Electrolyte Gadolinia-Doped Ceria Prepared by Thermal Decomposition of Mixed Cerium-Gadolinium Oxalate

Fine-grained powder of the mixed oxide (CeO₂)_0.9(Gd₂O₃)_0.1, which is an ionic conductor for oxygen ions, was prepared by coprecipitation of the corresponding oxalates followed by calcination. The powder was used to prepare pellets sintered at a relatively low temperature of 1000°C compared with the usual sintering temperature of 1700° to 1800°C. The size of the powder grains was determined from BET surface area (S_BET) measurements. The effect of precipitation conditions and calcination temperature on S_bet was examined. The largest surface area measured was 88 m²/g. Decomposition of the oxalate powder was followed using an optical dilatometer. The decomposition was indicated by a large shrinkage and it was completed below 300°C (for a heating rate of 3.3°C/min). The formation of the oxide was verified by X–ray diffraction analysis. It shows that the product of decomposition is the oxide and that decomposition can be carried to completion at 250°C if the heating lasts for 1 h. The pellets had a density of 83% of theoretical, small grains (0.5 μm), and a conductivity which, at 900°C, is two–thirds of the conductivity of dense samples obtained from the same raw material, but calcined and fired at much higher temperatures.     

Synthesis of Bundle- and Flake-Like CeO2 Powders Via a Precursor-Pyrolysis Approach

Micrometer-sized CeO₂ powders with bundle- and flake-like morphologies have been successfully synthesized by a precursor-pyrolysis approach, in which the precursors are first synthesized by a hydrothermal method and CeO₂ powders are finally obtained through a pyrolysis process. Results reveal that the stoichiometric concentration of cerium ion plays an important role in the formation of the final products with different morphologies. The CeO₂ powders obtained by pyrolyzing the precursors prepared from the solutions with high concentration (0.05 M ) and low concentration (0.0125 M ) exhibit bundle- and flake-like morphology, respectively. The possible growth process is proposed by preparing a series of intermediate morphologies during the shape evolution of bundle-like CeO₂ based on the scanning electron microscopy image observation.     

Low-Temperature Synthesis of Praseodymium-Doped Ceria Nanopowders

Praseodymium-doped ceria (CeO₂) nanopowders have been synthesized via a simple but effective carbonate-coprecipitation method, using nitrates as the starting salts and ammonium carbonate as the precipitant. The precursors produced in this work are ammonium rare-earth double carbonates, with a general formula of (NH₄)_0.16Ce_{1− x} Pr _x (CO₃)_1.58·H₂O (0 < x ≤ 0.20), which directly yield oxide solid solutions on thermal decomposition at a very low temperature of ∼400°C. Praseodymium doping causes a gradual contraction of the CeO₂ lattice, because of the oxidation of Pr³⁺ to smaller Pr⁴⁺, and suppresses crystallite coarsening of the oxides during calcination. Dense ceramics have been fabricated from the thus-prepared nanopowders via pressureless sintering for 4 h at a low temperature of 1200°C.     

Sintering of Ceria-Doped Tetragonal Zirconia Crystallized in Organic Solvents, Water, and Air

Amorphous CeO₂–ZrO₂ gels were prepared by coprecipitation in ammonia solutions. The onset of crystallization of the gels, from calcining in air, was 420°C, while 200° to 250°C in the presence of water and organic solvents such as methanol and ethanol. The sintering behaviors of CeO₂–ZrO₂ powders were sensitive to the crystallizing conditions, since hard agglomerates formed when the precipitated gels were crystallized by normal calcination in air, whereas soft agglomerates formed when they were crystallized in water or organic solvents. CeO₂–ZrO₂ powders crystallized in methanol and water at 250°C were sintered to full theoretical density at 1150° and 1400°C, respectively, whereas that crystallized by calcination in air at 450°C was sintered to only 95.2% of theoretical density, even at 1500°C.     

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.     

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.     

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.     

Fabrication of Transparent Cerium-Doped Lutetium Aluminum Garnet (LuAG:Ce) Ceramics by a Solid-State Reaction Method

Polycrystalline, transparent LuAG:Ce ceramics were fabricated by a solid-state reaction method using high-purity Lu₂O₃, Al₂O₃ and CeO₂ powders. The mixed powder compacts were sintered at 1770°C for 10 h under vacuum and annealed at 1450°C for 20 h in air. The transmittance in the visible light region reaches 70%. The emission spectra at 470–650 nm wavelengths are consistent with that of LuAG:Ce single crystals, and satisfy the property requirements of a scintillator.     

Long-Term Behavior and Application Limits of Plasma-Sprayed Zirconia Thermal Barrier Coatings

Investigations of changes in phase composition, mechanical properties, and microstructure of ZrO₂-based plasma-sprayed thermal barrier coatings (TBCs) with 8 mol% CeO₂, 19.5 mol% CeO₂/1.5 mol% Y₂O₃, 35 mol% CeO₂, and 4.5 mol% Y₂O₃ after long-term heat treatments at typical operation temperatures (1000°–1400°C) are presented. Experimental studies include X-ray diffractometry, mechanical testing, and scanning electron microscopy. Thermal cycling experiments also have been performed. TBCs with 8 mol% CeO₂ contain mainly the tetragonal equilibrium phase and, therefore, show rapid failure because of the high amount of tetragonal → monoclinic phase transformation, even after relatively short heat treatments (1250°C/1 h). In the case of the other systems that consist mainly of the tetragonal or cubic nonequilibrium phases, TBCs with 19.5 mol% CeO₂/1.5 mol% Y₂O₃ or 35 mol% CeO₂ reveal a smaller amount of monoclinic phase after long-term heat treatments (1250°C/1000 h) compared with TBCs containing 4.5 mol% Y₂O₃. TBCs containing 35 mol% CeO₂ show a higher degree of sintering than the TBCs with 19.5 mol% CeO₂/1.5 mol% Y₂O₃ and, therefore, a greater increase of the elastic modulus. Among the systems investigated, TBCs containing 4.5 mol% Y₂O₃ exhibit the highest resistance to failure in thermal-cycling experiments.     

Phase Transformation of Hydrous-Zirconia Fine Particles Containing Cerium Hydroxide

Monoclinic hydrous-zirconia fine particles that contained cerium(IV) hydroxide (Ce(OH)₄) were heated from 200°C to 600°C, to investigate the phase transformation to CeO₂-doped tetragonal ZrO₂. Both ZrOCl₂·8H₂O and CeCl₃·7H₂O were dissolved in aqueous solutions and then boiled to prepare the hydrous-zirconia particles. The Ce(OH)₄-containing hydrous-zirconia particles were prepared by adding aqueous ammonia into the boiled solutions. The monoclinic-to-tetragonal ( m right arrow t ) phase transformation of the Ce(OH)₄-containing hydrous zirconias was observed at 300°C using X-ray diffraction (XRD). XRD and Brunauer-Emmett-Teller (BET) specific surface area measurements revealed that the Ce(OH)₄-containing hydrous zirconias had a tendency to transform from the monoclinic phase to the tetragonal phase at lower temperatures as the primary particle size of the hydrous zirconia decreased and the Ce(OH)₄ content increased. These tendencies for the m right arrow t phase transformation agree with the conclusions that have been derived from thermodynamic and kinetic considerations.