Solid solutions of metastable tetragonal ZrO2 and Ce3ZrO8 in the system ZrO2-CeO2

In the system ZrO₂-CeO₂, metastable t-ZrO₂ solid solutions containing up to 30 mol% CeO₂ crystallize at temperatures of 385–430 °C from amorphous materials prepared by the hydrazine method. Crystalline Ce₃ZrO₈ solid solutions are formed in as-prepared powders between 30–75 mol % CeO₂. The variation of the lattice parameters of both solid solutions is determined as a function of CeO₂ content. The value of the lattice parameter of pure Ce₃ZrO₈ (cubic) is a = 0.5342 nm. Detailed characterization of the Ce₃ZrO₈ powder has been performed. Crystallite size and particle size are strongly dependent on the heating temperature. Specific surface areas do not drop below 40 m²g^–1 until the heating temperature is above 1000°C.     

Phase equilibria in the HfO2-ZrO2-CeO2 system at 1250°C

The 1250°C phase equilibria in the HfO₂-ZrO₂-CeO₂ system have been studied for the first time over the entire composition triangle, and the 1250°C section of the HfO₂-ZrO₂-CeO₂ phase diagram has been constructed using x-ray diffraction, optical microscopy, and scanning electron microscopy. No new phases have been identified in the system. We have located three regions of solid solutions based on the constituent oxides (M-HfO₂, T-ZrO₂, and F-CeO₂), three two-phase (T + F, T + M, and F + M) regions, and one three-phase (T + F + M) region.     

A new tentative phase equilibrium diagram for the ZrO2-CeO2 system in air

The phase relationships over a wide range of temperature and compositions in the ZrO₂-CeO₂ system have been reinvestigated. From DTA results, thermal expansion measurements andK _IC determinations it was established that additions of CeO₂ to ZrO₂ decreases the monoclinic to tetragonal ZrO₂ transition temperature, from 990 ° C to 150 50 ° C, and an invariant eutectoid point at approximately 15 mol% CeO₂ exists. The extent of the different single- and two-phase fields were determined with precise lattice parameter measurements on quenched samples. Evidence for the existence of a binary compound Ce₂Zr₃O₁₀ (ø-phase) was obtained by X-ray diffraction. The ø-phase was stable below approximately 800 ° C, above which it decomposes into tetragonal zirconia + fluorite ceria solid solutions. Taking into account the polymorphic tetragonal-cubic transition and the narrowness of the two-phase tetragonal zirconia + fluorite ceria field above 2000 ° C, the existence of a new invariant eutectoid point was assumed, in which the metastable fluorite zirconia solid solution decomposes into tetragonal zirconia + fluorite ceria solid solutions. From the results obtained, the phase diagram also incorporates a eutectic point located at approximately 2300 ° C and 24 mol % CeO₂.     

Microstructure and phase composition of ZrO2-CeO2 thermal barrier coatings

The microstructure and phase composition of zirconia plasma-sprayed thermal barrier coatings containing 12 to 25 wt% ceria addition have been investigated. Coatings containing less than 20 wt% CeO₂ are composed of a monoclinic and retained metastable transformable tetragonal phase due to the constraint developed by the small crystal size. This phase transforms readily under moderate thermal stresses. Compositions greater than 20 wt% CeO₂ are composed only of a metastable non-transformable tetragonal, tz′ structure, resistant to transformation under thermal or mechanical stresses. The microstructure of this phase shows microstructural similarities to the high-yttria t′-phase in the ZrO₂-Y₂O₃ system, such as transformation twins and anti-phase boundaries. This suggests that the phase observed in the ZrO₂-CeO₂ system forms by a similar mechanism.     

Cathodoluminescence from crystalline phases in a ZrO2-SiO2-CeO2 ceramic

The chemical composition of crystalline phases in a ZrO₂-SiO₂-CeO₂ ceramic was determined by electron probe x-ray microanalysis, and the luminescent properties of these phases were studied. The valence state of Ce in these phases was determined from cathodoluminescence data.     

Preparation and formation mechanism of nano-sized MnOx-CeO2 hollow spheres via a supercritical anti-solvent technique

Manganese and cerium composite oxide (MnO_x-CeO₂) hollow nanospheres were successfully prepared by precipitating manganese acetylacetonate and cerium acetylacetonate from their mixed methanol solution using supercritical carbon dioxide as an anti-solvent. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) were employed to characterize the precursor and as-prepared MnO_x-CeO₂. XRD analysis reveals the cubic fluorite structure of the MnO_x-CeO₂. HRTEM results indicate that the MnO_x-CeO₂ hollow spheres have an average diameter of about 50 nm, and a wall thickness of 10-20 nm. A new formation mechanism of these nano-sized hollow spheres has also been proposed based on the experimental results.     

Dielectric properties and electrical conductivity of ZrO<Subscript>2</Subscript>-CeO<Subscript>2</Subscript> ceramics

ZrO₂-CeO₂ (10, 18, and 23 mol % CeO₂) solid solutions have been synthesized via coprecipitation. The powders have been sintered at 1875 K, and the electrical conductivity and dielectric properties of the resultant ceramics have been studied. The dielectric relaxation observed in the ceramics can be understood in terms of ionic transport accompanied by the formation of dipoles relaxing in an ac electric field.     

Effect of the precipitation sequence on phase formation in the ZrO<Subscript>2</Subscript>-CeO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system

We have studied phase formation processes during heat treatment of precipitates in the ZrO₂-Al₂O₃ and ZrO₂-CeO₂-Al₂O₃ systems. During heat treatment of powders prepared by coprecipitation of precursors to ZrO₂, CeO₂, and Al₂O₃, α-Al₂O₃ is formed at higher temperatures, which is due to the formation and decomposition of T-ZrO₂ and metastable Al₂O₃ phases. The precipitation sequence in the ZrO₂-CeO₂-Al₂O₃ system influences the lattice parameters of the forming T-ZrO₂-based solid solutions because of the different degrees of Ce⁴⁺ and Al³⁺ substitutions for Zr⁴⁺.     

Subsolidus phase equilibria in the Al2O3-CeO2-PbO and Al2O3-CeO2-RuO2 systems

The subsolidus phase equilibria in air for the Al₂O₃-CeO₂-PbO and Al₂O₃-CeO₂-RuO₂ systems were studied with the aim of obtaining information on possible interactions between a CeO₂-based solid electrolyte in solid-oxide fuel cells (SOFCs) and other oxides. No ternary compound was found in either of the systems. The tie line in the Al₂O₃-PbO-CeO₂ system is between Al₂Pb₂O₅ and the CeO₂.     

Formation and hot isostatic pressing of ZrO2 solid solution in the system ZrO2-Al2O3

In the system of ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminium alkoxides. At higher temperatures, they transform into tetragonal solid solutions. Metastable ZrO₂ solid solution powders containing 25 mol% Al₂O₃ have been sintered at 1000–1150 °C under 196 M Pausing the hot isostatic pressing technique. The solid solution ceramics consisting of homogeneous microstructure with an average grain size of 50 nm exhibited a very high fracture toughness of 23 MN m ^–1.5. They have been characterized by X-ray diffraction and electron probe surface analyses.     

Structure and scintillation of Eu-activated solid solutions in the BaBr2-BaI2 system

We report the structure and scintillation of Eu²⁺-activated solid solutions in the BaBr₂-BaI₂ system. Samples were synthesized in the form of ∼1 mm size crystals by melting the reactants in a sealed quartz tube followed by slow cooling. The solid solutions form an orthorhombic PbCl₂-type crystal structure with an ordered arrangement of the anions. Upon optical and X-ray excitation, the Eu²⁺-activated samples show an intense emission centered between 410 and 423 nm. The samples exhibit a fast decay characteristic of Eu²⁺, with the primary decay time between 315 and 600 ns for ∼75% of the total emitted light. Light yields for the compositions are compared to a newly discovered scintillator, BaBrI:Eu²⁺, measured under identical conditions.     

Infrared spectra of mixed oxide thin films containing Group IV oxides with CeO2 -comparative study

The infrared spectra of a series of vacuum co-evaporated SnO₂-CeO₂ thin amorphous films are presented and compared with those of similar series of SiO-CeO₂ and GeO₂-CeO₂ films reported earlier. It is noted that although they are chemically similar, each series of spectra contains unique features. The differences are attributed to structural variations related to the size and mass differences among the Group IV cations. The picture of a modified random network is invoked to explain the possible structure of the films. Some preliminary extended X-ray absorption fine structure results are quoted to support the idea that Ce⁴⁺ exists as six-fold coordinated modified cations amongst the silicate-like anions formed by the network-forming Si⁴⁺ ions.     

Active Stainless Steel/SnO2-CeO2 Anodes for Pollutants Oxidation Prepared by Thermal Decomposition

Stainless steel plates were successfully coated with SnO₂-CeO₂ films (SS/SnO₂-CeO₂) by brush coating with a solution of stannous chloride and cerium trichloride followed by thermal decomposition. It is proven that the properties of SnO₂ films can be evidently improved by Ce doping, and 600°C is the optimum temperature to prepare SS/SnO₂-CeO₂ anodes. The physicochemical and electrochemical properties as well as the electrocatalytic activity of the electrodes were investigated. It is found that the novel electrodes have compact microstructure, high overpotential for oxygen evolution (1.60 V vs SCE), excellent electrochemical stability, relatively low cost and excellent catalytic activity for oxidizing pollutants. An industrial dye wastewater, which is hard to be purified by using conventional chemical flocculation methods, was oxidated by employing the SS/SnO₂-CeO₂ anodes, and 83.00% of color and 48.62% of chemical oxygen demand (COD) removal was achieved under the cell voltage of 5 V within only 2 min, and the electricity consumption is only 1.83 kWh for oxidizing 1 m³ of dye wastewater.     

Microstructure and corrosion resistance of Ni-based alloy laser coatings with nanosize CeO2 addition

Abstract

Micron-size Ni-base alloy (NBA) powders were mixed with both 1.5 wt.% (hereinafter %) micron-size CeO₂ (m-CeO₂) and also 1.5% and 3.0% nano-size CeO₂ (n- CeO₂) powders. These mixtures were coated on low-carbon steel (Q235) by 2.0 kW CO₂ laser cladding. The effects on the microstructures, phases and electrochemical corrosion of the coatings upon the addition of m- and n- CeO₂ powders to NBA (m- and n- CeO₂ /NBA) have been investigated. The results showed that a smooth coating was prepared under suitable processing parameters (P= 2.0 kW, V= 180 mm min^{- 1}) by adding 1.5% n- CeO₂. In addition to the primary phases of γ-Ni, Cr₂₃ C₆ and Ni₃ B in the Ni-base alloy coating, CeNi₃ was formed in Ni-base alloy coatings with both n- CeO₂ and m-CeO₂ particles, and CeNi₅ appeared in the coating upon decreasing the size of CeO₂ particles. Well-developed dendrites were observed in the Ni-base alloy coating; directional dendrites grew at the interface in the coating upon the addition of m-CeO₂, whereas fine and multioriented dendrites grew upon decreasing the size of CeO₂ particles to the nanoscale. Actinomorphic dendrites and compact equiaxed dendrites grew from the interface to near the surface upon increasing the content of n- CeO₂ from 1.5 to 3.0%. In strongly acidic HNO₃ solution, the severe corrosion of dendrites occurred and there were many corrosion pits in the Ni-base alloy coating; intercrystalline corrosion also has a dominant role upon the addition of m-CeO₂, whereas uniform corrosion occurs in the coating as the size of CeO₂ particles is decreased to nanoscale.     

Microstructure and phase composition of ZrO2-CeO2-Al2O3 materials modified with MgO and Y2O3

This paper presents our findings on phase formation processes during heat treatment of sol-gel synthesis products with the composition (mol %) 65(88ZrO₂ + 12CeO₂) + 35Al₂O₃ modified with 1 mol % MgO or Y₂O₃. The composites modified with 1 mol % MgO have been shown to differ significantly in phase composition from the parent nanopowders. Sintering is accompanied by partial decomposition of the tetragonal zirconia (T-ZrO₂) based solid solution and the formation of a monoclinic zirconia (M-ZrO₂) based solid solution and two aluminum-containing phases: corundum and the mixed oxide MgAl₁₁CeO₁₉. The addition of 1 mol % Y₂O₃ leads to successive formation of T-ZrO₂ and corundum and improvement of their structural perfection. The observed differences in phase formation during heat treatment result in different grain size compositions in the microstructure of the composites.     

Microstructure and corrosion resistance of Ni-based alloy laser coatings with nanosize CeO2 addition

Micron-size Ni-base alloy (NBA) powders were mixed with both 1.5 wt.% (hereinafter %) micron-size CeO₂ (m-CeO₂) and also 1.5% and 3.0% nano-size CeO₂ (n- CeO₂) powders. These mixtures were coated on low-carbon steel (Q235) by 2.0 kW CO₂ laser cladding. The effects on the microstructures, phases and electrochemical corrosion of the coatings upon the addition of m- and n- CeO₂ powders to NBA (m- and n- CeO₂ /NBA) have been investigated. The results showed that a smooth coating was prepared under suitable processing parameters (P= 2.0 kW, V= 180 mm min^{- 1}) by adding 1.5% n- CeO₂. In addition to the primary phases of γ-Ni, Cr₂₃C₆ and Ni₃B in the Ni-base alloy coating, CeNi₃ was formed in Ni-base alloy coatings with both n- CeO₂ and m-CeO₂ particles, and CeNi₅ appeared in the coating upon decreasing the size of CeO₂ particles. Well-developed dendrites were observed in the Ni-base alloy coating; directional dendrites grew at the interface in the coating upon the addition of m-CeO₂, whereas fine and multioriented dendrites grew upon decreasing the size of CeO₂ particles to the nanoscale. Actinomorphic dendrites and compact equiaxed dendrites grew from the interface to near the surface upon increasing the content of n- CeO₂ from 1.5 to 3.0%. In strongly acidic HNO₃ solution, the severe corrosion of dendrites occurred and there were many corrosion pits in the Ni-base alloy coating; intercrystalline corrosion also has a dominant role upon the addition of m-CeO₂, whereas uniform corrosion occurs in the coating as the size of CeO₂ particles is decreased to nanoscale.     

Hydrothermal synthesis and properties of solid solutions and composite nanoparticles in the TiO2-SnO2 system

Solid solutions and composite nanoparticles in the TiO₂-SnO₂ system were directly formed via the hydrothermal treatment of precursor solutions of TiCl₄ and SnCl₄ under weakly basic conditions in the presence of urea. The rutile-type (Ti, Sn)O₂ solid solutions were formed in the composition range of Ti 0-70 mol%. The composite nanoparticles consisting of anatase- and rutile-type phases were formed at the composition of Ti 80 and Ti 90 mol%. The change in the lattice parameters a₀ and c₀ of the rutile-type solid solutions followed the Vegard Law. The crystallite size of the rutile-type solid solutions was in the range of 5-10 nm. The diffuse reflectance spectra varied with changing Ti content in the precipitates. The photocatalytic activity of composite nanoparticles synthesized at 240 °C was higher than that synthesized at 180 °C. The composite nanoparticles consisting of anatase- and rutile-type phases with compositions Ti_0.90Sn_0.10O₂ and Ti_0.80Sn_0.20O₂ showed improved photocatalytic activity.     

Synthesis,microstructure and oxidation of Co-MgAl2O4 and Ni-MgAl2O4 nanocomposite powders

MgAl₂O₄-CoAl₂O₄ and MgAl₂O₄-NiAl₂O₄ solid solutions are synthesized by combustion in urea of the appropriate metal nitrates. The selective reduction in hydrogen of these oxides gives rise to a dispersion of nanometric Co or Ni particles in the spinel matrix. The reduction of the Co²⁺ and Ni²⁺ ions is complete at 900 and 1000 °C respectively. Owing to the homogeneity of the starting oxide solid solution, the size distribution of the metal particles is narrow and their average size is about 10 nm. During a thermal treatment in air, the nanometric metal particles located in the open porosity of the matrix are totally oxidized at temperatures lower than 700 °C, whereas those dispersed inside the matrix grains are stable up to 900 °C. These latter particles account for more than half of the total metal content.     

Synthesis and sintering of ZrO2-CeO2 powder by use of polymeric precursor based on Pechini process

Nanocrystalline ZrO₂-12 mol % CeO₂powders were synthesized using a polymeric precursor method based on the Pechini process. X-ray diffraction (XRD) patterns showed that the method was effective to synthesize tetragonal zirconia single-phase. The mean crystallite size attained ranges from 6 to 15 nm. The BET surface areas were relatively high reaching 97 m²/g. Studies by nitrogen adsorption/desorption on powders, dilatometry of the compacts, and transmission electron microscopy (TEM) of the powders, were also developed to verify the particles agglomeration state. Both citric acid : ethylene glycol ratio and calcination temperature affected the powder morphology, which influenced the sinterability and microstructure of the sintered material, as showed by scanning electron microscopy (SEM).     

Preparation and Characterization of Nanoceramics for Solid Oxide Fuel Cells

Precursor powders in the ZrO₂–HfO₂–Y₂O₃–CeO₂, In₂O₃–ZrO₂, and NiO–Nd₂O₃ systems for components of solid oxide fuel cells have been prepared by liquid-phase synthesis. We have determined formation conditions and the particle size of ZrO_2- and In₂O_3-based solid solutions and neodymium nickelate (Nd₂NiO₄), demonstrated the feasibility of producing nanocrystalline powders (10–30 nm) of tailored chemical composition in the temperature range 500–900°C, and optimized powder consolidation conditions. Nanoceramics with a crystallite size from 60 to 90 nm have been obtained and their microstructure and phase composition have been investigated. We have studied the electrical properties of the ZrO_2- and In₂O_3-based solid solutions and the Nd₂NiO₄ compound and established the range of their electrical conductivity at temperatures from 300 to 1000°C: 2.27 × 10^–3 to 2.51 S/cm for the ZrO₂-based solid solution, 8.91 × 10¹ to 6.59 × 10³ S/cm for the In₂O₃-based solid solution, and 3.98 × 10² to 5.02 × 10² S/cm for Nd₂NiO₄.