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.     

Synthesis and Sintering of Cerium(II) Monosulfide

Cerium monosulfide (CeS) powder was synthesized by the reduction of Ce₂S₃ powder with metallic Ce, which was obtained from ceria (CeO₂) powder using carbon disulfide (CS₂) gas. To obtain the maximum amount of CeS from a mixture of Ce₂S₃ and Ce, an excess amount of metallic Ce, a stoichiometric composition, was necessary in the synthesis at 1273 K for 10.8 ks. The preliminary sintering experiments also were performed using a synthetic CeS powder containing a small amount of Ce, Ce₂O₂S, and β-Ce₂S₃ as impurities. It was found that the oxygen content in the sintered compact decreases gradually as the sintering temperature increases, because of the removal of the impurities due to the evaporation of the volatile CeO. Single-phase CeS was formed by sintering at 2173 K. To evaluate the activation energy for densification of single-phase CeS, a CeS powder was prepared by milling an initial sintered compact and was used as an ingredient for hot-press experiments. Densification data during hot-press sintering were analyzed using a kinetic equation, showing that boundary diffusion is a rate-limiting process. The results suggest that this boundary diffusion model can explain well the densification data, with an apparent activation energy of 479 kJ·mol^-1.     

Sintering of Platelike Iron(III) Oxide Powders

Packing and sintering of platelike hematite powders were studied with a special emphasis on pore behavior, and the results were compared with those for acicular powders. Platy particles gave a relatively high green density of 65% of the theoretical value, but did not achieve high fired densities. This is in strong contrast to acicular powders, in which high fired densities were reached from a relatively low green density under the same firing conditions. Features observed with platy particles have been interpreted as being due to the wedgelike pores which tend to close at the early stages of densification.     

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.     

Characterization and Sintering of Reactive Cerium(IV) Oxide Powders Prepared by the Hydrazine Method

Nanocrystalline cerium(IV) oxide (CeO₂) powders have been prepared by adding hydrazine monohydrate to an aqueous solution of hydrous cerium nitrate (Ce(NO₃)₃·6H₂O), followed by washing and drying. The lattice parameter of the as-prepared powder is a = 0.5415 nm. The powder characteristics and sinterability of reactive CeO₂ have been studied. The surface areas of powders that have been heated at low temperatures are high, and these surface areas do not decrease to 10 m²/g until the temperature is >1200°C. Crystallite size and particle size are strongly dependent on the heating temperature. Optimum sintered densities are obtained by calcining in the temperature range of 700°–800°C. Ceramics with almost-full density can be fabricated at a temperature as low as 1150°C.     

Low-Temperature Synthesis and Characterization of Mixed Sodium Cerium(III) Hexa-Aluminate

Two forms of sodium cerium(III) mixed hexa-aluminate were prepared at temperature as low as 1270°C, i.e., 300°C lower than those reported previously, and characterized by X-ray diffraction techniques. These compounds are formed by thermal treatment, under argon, of an amorphous organometallic precursor obtained by the "citric acid method". During the synthesis, the 3+ oxidation state of cerium is retained at all temperatures.     

纳米羟基磷灰石粉体的制备和低温烧结

对沉淀法制备纳米HAp粉体及低温烧结HAp材料进行了研究. 结果表明：陈化对于防止HAp在高温下分解有重要作用；在陈化前加入一定量的聚乙二醇，可以明显增大纳米HAp粉体的比表面积，降低烧结温度；在普通无压烧结条件下，可在950℃的低温下，获得相对密度达98%左右的HAp材料，致密化温度比商业用粉低数百度. 研究还表明：醇洗不能有效提高纳米HAp的烧结性能.     

Synthesis of Calcium Hydroxyapatite-Tricalcium Phosphate (HA-TCP) Composite Bioceramic Powders and Their Sintering Behavior

Composite (biphasic) mixtures of two of the most important inorganic phases of synthetic bone applications-namely, calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂ (HA)) and tricalcium phosphate (Ca₃(PO₄)₂ (TCP))-were prepared as submicrometer-sized, chemically homogeneous, and high-purity ceramic powders by using a novel, one-step chemical precipitation technique. Starting materials of calcium nitrate tetrahydrate and diammonium hydrogen phosphate salts that were dissolved in appropriate amounts in distilled water were used during powder precipitation runs. The composite bioceramic powders were prepared with compositions of 20%-90% HA (the balance being the TCP phase) with increments of 10%. The pellets prepared from the composite powders were sintered to almost full density in a dry air atmosphere at a temperature of ~1200°C. Phase-evolution characteristics of the composite powders were studied via X-ray diffractometry as a function of temperature in the range of 1000°-1300°C. The sintering behavior of the composite bioceramics were observed by using scanning electron microscopy. Chemical analysis of the composite samples was performed by using the inductively coupled plasma-atomic emission spectroscopy technique.     

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,Ion Exchange Properties,and Applications of Amorphous Cerium(III) Tungstosilicate

Abstract

A novel hetropolyacid-based cation exchanger cerium(III) tungstosilicate was synthesized in amorphous form by mixing tungstosilisic acid (TSA) solutions to cerium(III) nitrate solutions at different Ce:TSA ratios. The materials were precipitated from the liquid phase by raising the pH of the solutions using sodium hydroxide. The produced ion exchange powders were characterized using powder X-ray diffractometry, thermogravimetry, infrared spectrometry, inductively coupled plasma and atomic absorption elemental analysis. The materials which were dried at 50°C were found to be stable in water, dilute acids, alkaline solutions, and high temperature up to 1000°C. The Ion exchange properties of the synthesized samples were studied by measuring the distribution coefficients (K_d) for 29 metal ions in demineralized water and nitric acid media. On the basis of K_d values, some quantitative separations such as Co²⁺-Pb²⁺, Cr³⁺-Zr⁴⁺, and Mo⁶⁺- W⁶⁺ are achieved on their columns.     

Stability and Thermodynamics of Glutamic Acid Complexes with Cerium(III) and Yttrium(III)

Stepwise stability constants of the complexes of glutamic acid with cerium(III) and yttrium (III) have been determined by the Calvin-Bjerrum pH titration technique as used by Irving and Rossotti in aqueous solution at 25° and 45°C. The values of log β₂ for cerium complexes are: 9.75 (μ = 0.1), 9.57 (μ = 0.2), 9.38 (μ = 0.3) at 25°C; and 10.90 (μ = 0.1), 10.74 (μ = 0.2), 10.64 (μ = 0.3) at 45°C. The log β₂ values for yttrium complexes at μ = 0.1 are 9.98 and 9.77 at 25° and 45°C respectively. The values of log β₂ (μ = 0.0) for cerium complexes are 10.21 and 11.24 at 25° and 45°C respectively. The increasing ionic strength of the medium decreases the stabilities of cerium complexes which are also more stable at 45° than 25°C whereas yttrium complexes are less stable at 45°C. The values of AH and ΔS are positive for cerium complexes whereas in the case of yttrium complexes, the values of ΔH are negative and those of ΔS are positive.     

Synthesis and Characterization of Zinc Sulfide/Gallium Phosphide Nanocomposite Powders

Nanocomposite powders between zinc sulfide and gallium phosphide were synthesized. Monodisperse, submicrometer, spherical, and porous clusters of ZnS nanocrystallites were used as a host material, into which a solution containing a phosphinogallane, [( t -Bu)₂GaPR₂] _x wherein R = i -Pr, x = 2 or R = t -Bu, x = 1, was impregnated. The pure ZnS powders and the composite powders obtained after flash pyrolysis at 600°C were subsequently heat-treated at various temperatures ranging from 500° to 900°C and characterized using a variety of techniques. While pure ZnS powders undergo significant grain growth and morphology change at temperatures as low as 600°C, the composite Powders maintain their integrity up to 800°C.     

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.     

Sintering of Chemically Synthesized Lead(II) Selenide Fine Powders

PbSe fine powders were synthesized from a liquid-phase reaction of lead-ascorbic acid complex and sodium selenosulfate. The resultant powders were sintered up to nearly theoretical density by controlling the sintering atmosphere and by optimizing the sintering temperature and time. The highest sintered density, 99.7% relative to the theoretical density of 8.27 103 kg/m3, was attained when a powder compact was sintered at 998 K for 6 h under a flow of argon containing 3% H2 gas. Powder compacts could be sintered to higher than 98% when the sintering temperature was 1073 K or lower. When the sintering temperature or time exceeded the optimum range, considerable volatilization of the component occurred, resulting in a decrease in the sintered density. The use of a powder bed was found to be effective in suppressing the volatilization of the component.     

Cerium(III)-Catalyzed One-Pot Synthesis of Quinazolinones from 2-Aminobenzamide and Aldehydes in Dimethyl Carbonate

A simple and efficient cerium(III) chloride -catalyzed procedure for the synthesis of 2-aryl or alkyl substituted quinazolinones from the condensation and oxidative cyclization of 2-aminobenzamide with aldehydes was reported. The reaction was carried out in the green solvent dimethyl carbonate in the presence of 5 mol% of CeCl₃at 100°C under air and gave the corresponding quinazolinone derivatives in high yield.

Supplemental materials are available for this article. Go to the publisher's online edition of Polycyclic Aromatic Compounds to view the supplemental file.     

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.     

Sintering of Zinc Sulfide

The mechanisms of the sintering of ZnS were determined by measurement of the rate of growth of the necks between polycrystalline spheres. In vacuum (10⁻⁶ mm Hg) at temperatures higher than 600° C the mechanism of sintering is that of volume diffusion with coefficient D_v, = 1.06 × 10⁻² exp (-26,400/RT); below 600°C surface diffusion predominates, with coefficient D₃, = 9.14 × 10_-3 exp (-5700/RT). In Zn vapor (10⁻² mm Hg) between 550° and 650°C the predominating mechanism of sintering is surf ace diffusion, D₃, = 7.06 × 10⁻² exp (-8600/RT). It has been found that in an argon atmosphere the diffusion coefficient is much lower.     

Synthesis of Nanocrystalline Cubic Tantalum(III) Nitride Powders by Nitridation–Thermal Decomposition

Tantalum(V) nitride, prepared by nitridation of nanosized Ta₂O₅ at 800°C for 8 h under ammonia flow, was thermally decomposed to cubic nanocrystalline TaN at a temperature of 1000°C for 3 h under argon atmosphere. The resulting powders have been characterized using X-ray diffraction (XRD) and transmission electron microscopy. XRD-pure cubic TaN nanoparticles with a diameter of 50–100 nm can be obtained by the process. The decomposition process was found to depend on the temperature. Mechanisms that account for the decomposition of Tantalum(V) nitride are discussed. The results indicate that the method can permit formation of cubic-phase Tantalum(III) nitride under ambient pressure and moderate temperatures.     

Effective Removal of Arsenic with Lanthanum(III)- and Cerium(III)-loaded Orange Waste Gels

Abstract

Orange waste has been chemically modified and loaded with lanthanum(III) and/or cerium(III) to examine its adsorption behavior to both As(V) and As(III). Arsenate removal was found to be favored over a pH range of 6 ～ 9.5 while arsenite removal took place at pH values ranging from 9 to 11. The maximum sorption capacity of the gel for As(III) removal was evaluated as 43 mg/g, while that for As(V) was 42 mg/g. Column-mode tests using the La(III)-loaded gel confirmed a complete removal of As(V). A reasonably high adsorption potential within the design criteria makes the present gel an alternative choice for arsenic removal.     

Synthesis and Crystal Structures of a Lanthanum(III) 1D Polymer and a Mixed-Ligand Cerium(III) Binuclear Complex Derived from Pyridine-2,6-dicaboxylic Acid

Lanthanum(III) and cerium(III) complexes of pyridine-2,6-dicaboxylic acid (H₂Pydc) have been prepared and their crystal structures were determined by X-ray crystallography. The single crystal analysis reveals that the lanthanum(III) complex, 1 is polymeric consisting of {[La(Pydc)₂(H₂O)₂]·4H₂O} _n units linked through carboxylate oxygen atoms and exhibiting nine coordination number. Intermolecular O–H···O hydrogen bonds produce R ₁ ¹ (6), R ₄ ⁴ (16) and R ₄ ⁴ (20) rings, which lead to three-dimensional polymeric chains. The crystal structure of the cerium(III) complex, 2 [{Ce(Pydc)₃}{Ce(Pydc)(HO–CH₂CH₂–OH)(H₂O)₃}·6H₂O)] shows that the complex is a mixed-ligand binuclear system in which one cerium coordinated to three Pydc molecules, while the other cerium is bound to one Pydc, one oxygen atom of the other Pydc, one ethylene glycol and three water molecules. Each of the two Ce(III) ions is nine coordinated. Intermolecular O–H···O hydrogen bonds produce R ₂ ² (8) and R ₂ ² (20) rings, which lead to three-dimensional polymeric chains. The complexes were further investigated using elemental analysis, FTIR spectroscopy and thermogravimetric analysis.