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1.
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. 相似文献
2.
Effect of pH of Medium on Hydrothermal Synthesis of Nanocrystalline Cerium(IV) Oxide Powders 总被引:1,自引:0,他引:1
Nan-Chun Wu Er-Wei Shi Yan-Qing Zheng Wen-Jun Li 《Journal of the American Ceramic Society》2002,85(10):2462-2468
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. 相似文献
3.
Characterization and Sintering of Reactive Cerium(IV) Oxide Powders Prepared by the Hydrazine Method 总被引:1,自引:0,他引:1
Singo Nakane Toshiaki Tachi Masaru Yoshinaka Ken Hirota Osamu Yamaguchi 《Journal of the American Ceramic Society》1997,80(12):3221-3224
Nanocrystalline cerium(IV) oxide (CeO2 ) powders have been prepared by adding hydrazine monohydrate to an aqueous solution of hydrous cerium nitrate (Ce(NO3 )3 ·6H2 O), 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. 相似文献
4.
Shinji Hirai Kazuyoshi Shimakage Yasushi Saitou Toshiyuki Nishimura Yoichiro Uemura Mamoru Mitomo Leo Brewer 《Journal of the American Ceramic Society》1998,81(1):145-151
Cerium(III) sulfide (Ce2 S3 ) powder was synthesized via the sulfurization of ceria (CeO2 ) powder using carbon disulfide gas. Single-phase α-Ce2 S3 could be formed via sulfurization at 973 K for 28.8 ks. The preparation of α-Ce2 S3 became feasible at low temperature, in comparison to sulfurization using hydrogen sulfide gas. According to the fact that the formation of α-Ce2 S3 was accelerated by the addition of carbon black to the CeO2 powder, carbothermic reduction was considered to become a dominant reaction, as the temperature increased. To obtain the activation energy for the densification of β-Ce2 S3 powder, which was prepared by vacuum heating α-Ce2 S3 , the data of densification by hot pressing was analyzed by a kinetic equation that was proposed by other researchers. As a result, the sintering behavior could be best explained by a grain-boundary-diffusion mechanism that had an apparent activation energy of 382 kJ/mol. 相似文献
5.
Hydrothermal Synthesis of Cerium(IV) Oxide 总被引:2,自引:0,他引:2
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. 相似文献
6.
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. 相似文献
7.
Xianghua Yu Feng Li Xiangrong Ye Xinquan Xin Ziling Xue 《Journal of the American Ceramic Society》2000,83(4):964-966
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. 相似文献
8.
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. 相似文献
9.
Shinji Hirai Eiji Sumita Kazuyoshi Shimakage Yoichiro Uemura Toshiyuki Nishimura Mamoru Mitomo 《Journal of the American Ceramic Society》2004,87(1):23-28
Cerium monosulfide (CeS) powder was synthesized by the reduction of Ce2 S3 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 Ce2 S3 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, Ce2 O2 S, and β-Ce2 S3 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 . 相似文献
10.
Jose Alberto Cerri Edson Roberto Leite Douglas Gouvêa Elson Longo Jose Arana Varela 《Journal of the American Ceramic Society》1996,79(3):799-804
Additions of 0. 5 to 2. 0 mol% of CoO or MnO2 onto SnO2 promote densification of this oxide up to 99% of theoretical density. The temperature of the maximum shrinkage rate ( TM ) and the relative density in the maximum densification rate (p*) during constant sintering heating rate depend on the dopant concentration. Thus, dopant concentration controls the densifying and nondensifying mechanisms during sintering. The densification of SnO2 witih addition of CoO or MnO2 is explained in terms of the creation of oxygen vacancies. 相似文献
11.
Nanocrystalline TiO2 with densities higher than 99% of rutile has been deformed in compression without fracture at temperatures between 600° and 800°C. The total strains exceed 0.6 at strain rates as high as 10−3 s−1 . The original average grain size of 40 nm increases during the creep deformation to final values in the range of 120 to 1000 nm depending on the temperature and total deformation. The stress exponent of the strain rate, n , is approximately 3 and the grain size dependence is d − q with q in the range of 1 to 1.5. It is concluded that the creep deformation occurs by an interface reaction controlled mechanism. 相似文献
12.
Solid-state reactions have the potential for direct preparation of ceramic powders and offer a low-temperature and low-cost alternative to conventional techniques for production of oxide powders. This paper describes a simple and effective mechanochemical method based on solid-state reactions during ball milling for synthesis of nanoscaled ceria (CeO2 ) particles. By using an organic base instead of an inorganic base, metal-ion-free nanoscaled CeO2 can also be made. The effects of annealing temperature on particle sizes and lattice strain are investigated. The results show that the average particle sizes of the particles increases and the average crystal lattice distortion decreases with the annealing temperature. Transmission electron microscopy examinations demonstrate that the CeO2 particles synthesised by this method are near-spherical shaped. 相似文献
13.
Matteo Leoni Rosa Di Maggio Stefano Polizzi Paolo Scardi 《Journal of the American Ceramic Society》2004,87(6):1133-1140
The microstructural evolution of nanocrystalline ceria produced by sol–gel has been analyzed as a function of the calcination temperature employing a novel nondestructive method based on the modeling of the whole X-ray diffraction pattern. The results have been thoroughly verified by transmission electron microscopy. A variation both in the average size and in the distribution of the crystalline domains is evidenced. In addition, information concerning lattice defects can be inferred on a larger scale than that normally accessible by microscopy techniques. 相似文献
14.
Kyung-Han Seo Joon-Hyung Lee Jeong-Joo Kim M. I. Bertoni B. J. Ingram T. O. Mason 《Journal of the American Ceramic Society》2006,89(11):3431-3436
Nanocrystalline powders of two indium tin oxide (ITO) polymorphs—rhombohedral and cubic—were prepared by a co-precipitation process. The temperature and aging time of precipitates after co-precipitation were controlled to obtain selectively the two different crystal structures. X-ray diffraction and transmission electron microscopy were used to characterize the powders. The electrical conductivity of the two ITO powders was determined by the powder solution composite method. The conductivities obtained were 0.26±0.04 and 0.65±0.17 S/cm for the rhombohedral and cubic ITO samples, respectively, the first such report for the rhombohedral phase. 相似文献
15.
Nanocrystalline niobium nitride (NbN) powders were sintered by spark plasma sintering under a nitrogen atmosphere at temperatures from 1040° to 1230°C. Fully dense bulk NbN ceramic with grain sizes of 0.5–1.0 μm was obtained at 1130°C. The effects of sintering temperature on the density, phase content, electrical conductivity, Vickers hardness, and microstructure of the NbN ceramic were discussed. 相似文献
16.
Jingguo Li Lian Gao Jing Sun Qinghong Zhang Jingkun Guo Dongsheng Yan 《Journal of the American Ceramic Society》2001,84(12):3045-3047
Nanocrystalline TiN powder has been synthesized by the direct nitridation of nanocrystalline TiO2 powder. Powder XRD patterns indicated that the TiN nanocrystalline powder could be obtained by nitridation at 800°C for 5 h. TEM micrographs showed that the synthesized TiN powders consisted of uniform spherical particles with an average diameter of ∼20 nm. The effect of the nitridation temperature and holding time on the powder properties is discussed. 相似文献
17.
Synthesis of Nanocrystalline Chromium Nitride Powders by Direct Nitridation of Chromium Oxide 总被引:2,自引:0,他引:2
Yaogang Li Lian Gao Jingguo Li Dongsheng Yan 《Journal of the American Ceramic Society》2002,85(5):1294-1296
Nanocrystalline CrN powder was synthesized by the direct nitridation of nanosized Cr2 O3 powder. Powder X-ray diffractometry patterns indicated that pure cubic-phase CrN powder could be obtained by nitridation at 800°C for 8 h. Transmission electron microscopy images showed that the particle sizes were 40–80 nm. The effect of the nitridation temperature and holding time on the powder properties was studied. 相似文献
18.
19.
Jagadish C. Ray Asit B. Panda Chitta R. Saha Panchanan Pramanik 《Journal of the American Ceramic Society》2003,86(3):514-516
The polymer precursor method is very useful to prepare Nb5+ -stabilized nanocrystalline powders of t -ZrO2 . The precursor solution is composed of zirconium oxalate, niobium tartrate, and poly(vinyl alcohol), which help to form a network matrix to disperse the metal ions homogeneously. Nb5+ is an effective agent to stabilize t -ZrO2 , and ease of formation of the tetragonal phase increases with increasing dopant concentration. Thermal stability of t -phase is found up to 1700°C having 15 mol% Nb5+ , prepared at 600°C with particle sizes of 35 ± 5 nm. 相似文献
20.
Toshio Suzuki Igor Kosacki Harlan U. Anderson 《Journal of the American Ceramic Society》2002,85(6):1492-1498
The results obtained from a study on the microstructure and the electrical properties of Gd-doped CeO2 thin films were reported. Dense, nanocrystalline films on sapphire substrates are prepared using a polymeric precursor spin coating technique. The electrical conductivity was studied as a function of temperature and oxygen activity and correlated with the grain size. For nanocrystalline Gd-doped CeO2 thin films, the ionic conductivity increased with decreasing activation energy as the grain size decreased. A conductivity model was developed to analyze P O2 behavior of the electrical conductivity. Using the conductivity model, the hopping energy of electron conduction and the enthalpy of oxygen vacancy formation were determined for different microstructures. 相似文献