Synthesis of cerium hydroxycarbonate powders via a hydrothermal technique

CeOHCO₃ powders have been directly synthesized using a hydrothermal process at temperatures as low as 160°C. The well-dispersed powders are obtained in a short period of reaction time during hydrothermal reaction via the hydrolysis of urea. For synthesizing CeOHCO₃, the concentration of urea is found to be a crucial determinant, which has significant effects on the morphology of the derived powders. When low urea concentrations are provided, the formed particles are rhomboidal platelets. On the other hand, the high urea concentrations cause the shape of the powders to become prismatic. Increasing the concentration of urea tends to increase the particle size as well as the aspect ratio of CeOHCO₃ powders. After further heating at 500°C, a phase transformation from orthorhombic CeOHCO₃ to cubic CeO₂ takes place. The crystallinity and size of CeO₂ strongly depend on the particle size of CeOHCO₃.     

Solution combustion synthesis of CeO2-CeAlO3 nano-composites by mixture-of-fuels approach

Nano-composites of CeO₂-CeAlO₃ are synthesised by solution combustion method employing (a) urea and (b) a mixture of urea and glycine as fuels with corresponding metal nitrates. The as-prepared powders are all nano-sized (5-30 nm) and the same is confirmed by broadening of the X-ray diffraction peaks and transmission electron microscopy. A starting composition of Ce:Al in the atomic ratio 4:6 gives rise to different phases depending on the fuel being used for combustion. When urea alone is used as fuel, nano-crystalline CeO₂ phase is formed with Al₂O₃ being in the amorphous state. When the mixture of fuels is used, a mixture of nano-sized CeO₂ and CeAlO₃ phases is obtained. However, upon sintering at 1400 ° C in air, the stable phases CeO₂ and -Al₂O₃ are formed in both the cases. Combustion synthesis using mixture-of-fuels is proposed to be a route to stabilise low oxidation compounds such as CeAlO₃.     

Self-propagating high temperature synthesis of Ni0.35Zn0.65Fe2O4 ferrite powders

The stoichiometric Ni_0.35Zn_0.65Fe₂O₄ ferrite powders were synthesized by SHS method. In the process of SHS, the effects of the molar ratio Fe/Fe₂O₃ in the starting mixture, oxygen pressure, grain size and relative density of the raw materials on combustion temperature, combustion wave velocity, phase composition and microstructure of the combustion products were investigated. X-ray diffraction, scanning electron microscope, TEM, vibrating sample magnetometry were used to characterize the microstructure and magnetic properties of the products. The results showed that as the molar ratio Fe/Fe₂O₃ increases, the combustion temperature and combustion wave velocity increased. The same results can be observed when the oxygen pressure increased from 0.1 to 0.9 MPa. The increase of grain size and relative density of raw materials resulted in the decrease of combustion temperature and combustion wave velocity. Compared with other methods, SHS process leads to ferrite powders with improved magnetic properties.     

Modification and optimization of nano-crystalline Al2O3 combustion synthesis using Taguchi L16 array

Nanocrystalline Al₂O₃ powders have been synthesized by combustion method using 8 new fuels. The effectiveness of important factors on the production of nanopowders was investigated and optimized using Taguchi L₁₆ array design. The products were characterized by XRD, BET, TGA, EDX, FESEM, and TEM analyses. Results demonstrated that the alumina nanoparticles had crystallite sizes between 8.31 nm and 13.54 nm. The optimized sample had the specific surface area of 72 m²/g and crystallite size of 7.25 nm. The synthesis of γ-alumina was modified in order to achieve higher specific surface area (122.63 m²/g). A nano-network of alumina powders woven by alumina nano-fibers has been fabricated successfully by modified combustion synthesis. The length and diameter of fibers were about 160 nm and 10 nm respectively.     

Complete oxidation of acetaldehyde on Pt/CeO2-ZrO2-Bi2O3 catalysts

Pt/CeO₂-ZrO₂-Bi₂O₃ catalysts for catalytic combustion of acetaldehyde, which is one of volatile organic compounds (VOCs), were prepared by a wet impregnation method in the presence of polyvinylpyrrolidone K25 (PVP). The addition of PVP in the preparation process was effective to enhance the specific surface area and the Pt²⁺ ratio on the surface. Additionally, the pore volume and size of the catalysts were modified by the PVP addition. The Pt/CeO₂-ZrO₂-Bi₂O₃ catalysts are specific for the total acetaldehyde oxidation and CO and any acetaldehyde-derivative compounds were not observed as by-products. The catalytic activity of the Pt/CeO₂-ZrO₂-Bi₂O₃ catalysts was significantly promoted by the PVP addition and the total oxidation temperature decreased. By the optimization of the amount of platinum, the complete oxidation of acetaldehyde was realized at a temperature as low as 140 °C on a 10 wt%Pt/CeO₂-ZrO₂-Bi₂O₃ catalyst.     

Control of particle size and shape of precursors for ceria using ammonium carbonate as a precipitant

Spherical nanocrystalline precursors of ceria (CeO₂) and CeO₂ powder with different size distributions were prepared by a reflux method using cerium nitrate hexahydrate (CN) as the cerium source and ammonium carbonate (AC) as the precipitant. The crystalline phases of the synthesized CeO₂ precursors were identified as orthorhombic Ce₂O(CO₃)₂·H₂O and hexagonal CeCO₃OH. The particle size and shape could be easily controlled by the CN concentration and the ratio of [AC]/[CN]. The CeO₂ precursors were calcined at 400-700 °C to obtain CeO₂. The particle size distribution and morphology of the synthesized CeO₂ powders were unaffected by the calcination. The specific surface area of the CeO₂ powders was increased by the release of CO₂ and H₂O during the calcination. The calcination temperature is an important factor for the preparation of CeO₂ powder with a high surface area.     

Novel wet-chemical synthesis and characterization of nanocrystalline CeO2 and Ce0.8Gd0.2O1.9 as solid electrolyte for intermediate temperature solid oxide fuel cell (IT-SOFC) applications

Novel wet-chemical methods of synthesis have been adopted to synthesize nano-crystalline CeO₂ and Gd-substituted compositions aiming to explore an efficient oxide ion conducting solid electrolyte for intermediate temperature solid oxide fuel cell (IT-SOFC) applications. Nano-crystalline CeO₂ powders were synthesized by combustion method using redox mixture of cerric ammonium nitrate or cerium nitrate and maleic acid or 1,3-dimethylurea and compared with high surface area CeO₂ powders prepared by hydrothermal technique with microwave precipitated precursor from aqueous solutions of (NH₄)₂Ce(NO₃)₆ and urea. The grain size achieved by the hydrothermal technique is ∼7 nm which is smaller than that of commercial nano CeO₂ powders. Conventional or microwave sintering was used to prepare dense Ce_0.8Gd_0.2O_1.9 pellets from the ceria powders made of redox mixture of cerium nitrate, 1,3-dimethylurea (DMU) and Gd₂O₃ as the starting ingredients. The samples were characterized by X-ray diffractometry (XRD), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and ac impedance spectroscopy. The ionic conductivity measured for the pellet sintered at 1400 °C is 1 × 10⁻² and 2.4 × 10⁻² S/cm at 700 °C and 800 °C respectively.     

Nanostructured cobalt oxides (Co3O4 and CoO) and metallic Co powders synthesized by the solution combustion method

The combustion synthesis technique using glycine and urea as fuels and cobalt nitrate as an oxidizer is capable of producing well-crystallized Co₃O₄, CoO, as well as metallic Co powders. An interpretation based on the thermodynamic viewpoint and the measurement of the combustion temperatures during the reactions occurring for various fuel-to-oxidant ratios was proposed for a study of the nature of combustion and its correlation with the characteristics of as-synthesized powders. The largest measured specific surface area of the powders was 36 m²/g at a 0.14 glycine-to-nitrate ratio. The crystallites were nano-sized ranging from approximately 23 to 90 nm.     

Combustion of TiO2-Mg and TiO2-Mg-C systems in the presence of NaCl to synthesize nanocrystalline Ti and TiC powders

The combustion process of TiO₂-Mg and TiO₂-Mg-C systems with sodium chloride as an inert diluent was investigated. The values of combustion parameters and temperature distribution on a high-temperature wave according to the amount of sodium chloride were obtained by the thermocoupling technique. The leading stages of combustion processes are found and the sizes of reactionary zones were estimated. It is shown that the introduction of NaCl in an initial mixture promotes the formation of a nanocrystalline structure of the final products. As a result, nanosized titanium, and titanium carbide powders have been successfully obtained.     

CeO2 nanocrystals: Seed-mediated synthesis and size control

This work reports on seed-mediated synthesis and size control of monodispersed CeO₂ nanoparticles. CeO₂ nanoparticles of mean size smaller than 2 nm were first prepared by a simple mixing of aqueous solution of cerium (IV) sulfate and ammonia solution at ambient conditions. Using these as-prepared fine particles as the tiny seeds, tunable sizes of CeO₂ nanoparticles were achieved via a facile hydrothermal treatment. All samples were characterized by X-ray diffraction (XRD), infrared (IR) spectroscopy, UV-vis spectroscopy, and thermogravimetric analysis (TGA). It is shown that in comparison with other inorganic cerium salts such as cerium (III) nitrates, cerium (IV) sulfate appears more suitable for forming CeO₂ nanoparticles at room temperature. Sulfate groups are strongly thermodynamically adsorbed on CeO₂ nanoparticle surfaces. The formation mechanism, surface hydration and sulfation characteristics of the resulting CeO₂ nanoparticles are also discussed.     

Study of local environment of Ag in Ag/CeO2 catalyst by EXAFS

The combustion synthesized Ag/CeO₂ catalysts have been characterized by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy at the Ag K-edge. It has been found that Ag⁺ like species is present in 1% Ag/CeO₂ catalyst, whereas mostly Ag metal clusters are found in 3% Ag/CeO₂. The analysis of EXAFS spectra indicates that about one oxygen atom is coordinated to Ag central atom at a distance of 2.19 Å in 1% Ag/CeO₂ catalyst along with eight coordinated AgAg bond at 2.86 Å. The AgO bond is absent in 3% Ag/CeO₂.     

Structural, morphological and optical investigations on BaMgAl10O17:Eu elaborated by a microwave induced solution combustion synthesis

Blue-emitting Eu²⁺-doped barium magnesium aluminate (BaMgAl₁₀O₁₇:Eu²⁺) for advanced displays and lighting devices was prepared by a microwave induced solution combustion synthesis using urea as combustion fuel and nitrates as oxidizer. Purity control of as-synthesized blue phosphor particles was undertaken by modifying the fuel to oxidizer molar ratio. X-ray diffraction, scanning electron microscopy and photoluminescence were used to investigate powders crystallinity, particles size, morphology and luminescent properties, respectively. Fuel-rich urea reactions preferentially lead to pure phases compared to the powders synthesized with a stoichiometric fuel to oxidizer ratio. In both cases, we produce a nearly pure well-crystallized and nanostructured BaMgAl₁₀O₁₇:Eu²⁺. Photoluminescence measurements exhibit the characteristic blue emission of Eu²⁺ under UV light excitation however a weak red emission associated to Eu³⁺ is also detected.     

Dopant induced variations in microstructure and optical properties of CeO2 nanoparticles

Nanocrystalline CeO₂ particles doped in the range of 0-20% of Ca²⁺, La³⁺, and Zr⁴⁺ have been prepared from hydrothermal synthesis of nitrate solutions at 200 °C and the influences of the dopants on microstructure and optical properties of the nanoparticles have been investigated. The unit cell parameter is found to be modified by −0.39, +0.83 and +0.16% for doping of 20% Zr⁴⁺, La³⁺, and Ca²⁺, respectively. For each batch prepared, nanoparticles with a narrow size distribution of 5-15 nm have been obtained. A high-resolution transmission electron microscopy investigation reveals that these particles are single crystals mostly having hexagonal, square or circular two-dimensional projections. UV-visible spectra of doped powders exhibit shift of the absorption edge and absorption peak with respect to those of the undoped CeO₂ particles and has been attributed to compensation of Ce³⁺ and decreasing crystallite size as result of doping.     

Mechanical-activation-assisted combustion synthesis of SiC powders with polytetrafluoroethylene as promoter

Mechanical-activation-assisted combustion synthesis of SiC has been conducted with polytetrafluoroethylene (PTFE) as promoters. The mechanical activation of the Si-C reactants with the addition of NH₄Cl as process controlling agent during the high-energy attrition milling could substantially decrease the content of PTFE promoter, which is needed for inducing the Si-C combustion reaction. Ultra-fine β-SiC powders with spherical particles were synthesized at PTFE content as low as 6 wt.%. The combustion synthesized SiC powders have a narrow particle size distribution in the range of 0.5-1 μm.     

Mechanochemical Synthesis of ZrO2–CeO2 Solid Solutions

ZrO₂–CeO₂ solid solutions (12 and 13 mol % CeO₂) were prepared by mechanochemical synthesis in different media. The mechanical activation medium was shown to have a significant effect on the phase composition and particle size of the resultant powders. The conditions for sintering the powders were optimized.     

Preparation and characterization of CeO2 highly dispersed on activated carbon

A new material constituted by cerium dioxide highly dispersed on activated carbon (CeO₂/AC) was prepared by an impregnation method using cerium(III) nitrate as CeO₂ precursor. In order to evaluate the degree of ceria dispersion on the carbon support, CeO₂/AC was characterized by a number of techniques: thermogravimetry coupled with a mass spectrometer (TG-MS), N₂ adsorption at 77 K, temperature-programmed desorption (TPD), temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). The analysis of the decomposition process under inert atmosphere indicated that cerium nitrate decomposes at 440-460 K, with the evolution of NO. Furthermore, this process produces an additional oxidation of the carbon surface (with evolution of N₂O) and the subsequent onset of new oxygen surface groups, detected by means of temperature-programmed desorption. The ceria deposition process takes place with a decrease in the N₂ adsorption capacity of the starting carbon support, and the analysis of the pore size distribution showed that the majority of ceria particles are situated at the most internal part of the carbon porosity. The temperature-programmed reduction profile of CeO₂/AC was very different to that shown by unsupported CeO₂, with only one continuous reduction process at low temperatures (800-900 K). Finally, TEM pictures gave direct evidence that ceria is highly dispersed on the carbon surface, with a narrow CeO₂ particle distribution centred around 3 nm.     

Microwave-assisted synthesis of ceria nanoparticles

Nanocrystalline ceria (CeO₂) particles have been successfully prepared by microwave-assisted heating technique from an aqueous solution containing ammonium Ce(IV) nitrate and sodium hydroxide. Further thermal treatment of the as-prepared powder at 500 °C resulted in the formation of the well-crystallized CeO₂ nanoparticles with an average crystal size of about 8 nm, varying with the heating temperature. The as-prepared powder and the CeO₂ nanoparticles were examined using X-ray diffraction (XRD) and transmission electron microscope (TEM) techniques. It was found that the morphologies of the synthesized powder show from rod-like for the as-prepared sample to sphere-like for the heat-treated nanoparticles. Mechanism of CeO₂ nanocrystallite growth during annealing is primarily investigated.     

Mixture of fuels approach for the solution combustion synthesis of Al2O3-ZrO2 nanocomposite

A modified solution combustion approach was used for the first time in the preparation of nanosize zirconia toughened alumina (ZTA) composite. ZTA-1 with an average particle size of ∼37 nm was prepared using corresponding metal nitrates and urea. ZTA-2 with an average particle size of <10 nm was prepared by using mixture of fuels such as ammonium acetate, urea and glycine. The products formed were characterised by powder X-ray diffractometry, Transmission electron microscopy and BET surface area analysis. By using mixture of fuels, the energetics of the combustion reaction and eventually the properties of the combustion product have been changed. A series of combustion reactions were carried out to optimise the fuel ratio combinations required to obtain <10 nm ZTA particles. The microstructure of ZTA consisted of crystallites of Al₂O₃ and ZrO₂ both of which were nanocrystalline as evident from TEM.     

Synthesis, microstructure and mechanical properties of ceria stabilized tetragonal zirconia prepared by spray drying technique

Ceria stabilized zirconia powders with ceria concentration varying from 6 to 16 mol% were synthesized using spray drying technique. Powders were characterized for their particle size distribution and specific surface area. The dense sintered ceramics fabricated using these powders were characterized for their microstructure, crystallite size and phase composition. The flexural strength, fracture toughness and microhardness of sintered ceramics were measured. High fracture toughness and flexural strength were obtained for sintered bodies with 12 mol% of CeO₂. Flexural strength and fracture toughness were dependent on CeO₂ concentration, crystallite size and phase composition of sintered bodies. Correlation of data has indicated that the transformable tetragonal phase is the key factor in controlling the fracture toughness and strength of ceramics. It has been demonstrated that the synthesis method is effective to prepare nanocrystalline tetragonal ceria stabilized zirconia powders with improved mechanical properties. Ce-ZrO₂ with 20 wt% alumina was also prepared with flexural strength, 1200 MPa and fracture toughness, 9.2 MPa√m.     

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.