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1.
CeO2 samples doped with 10, 1.0, and 0.1 mol% Y2 O3 and undoped CeO2 samples of high purity were studied by impedance spectroscopy at temperatures <800°C and under various oxygen partial pressures. According to microstructural investigations by SEM and analytical STEM (equipped with EDXS), the grain boundaries were free of any second phase, providing direct grain-to-grain contacts. An amorphous siliceous phase was detected at only a few triple junctions, if at all; as a result, its contribution to the grain-boundary resistance was negligible. Nevertheless, the specific grain-boundary conductivities were still 2–7 orders of magnitude lower than the bulk conductivities, depending on dopant concentration, temperature, and oxygen partial pressure. The charge carrier transport across the grain boundaries occurred only through the grain-to-grain contacts, whose properties were then determined by the space-charge layer. The space-charge potential in acceptor-doped CeO2 was positive, causing the simultaneous depletion of oxygen vacancies and accumulation of electrons in the space-charge layer. The very low grain-boundary conductivities can be accounted for by the oxygen-vacancy depletion; the accumulation of electrons became evident in weakly doped and undoped CeO2 at high temperatures and under low oxygen partial pressures. 相似文献
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The steam reforming of methanol was studied over Cu/Al2O3 catalysts with the addition of yttria-doped ceria (YDC). The YDC-modified catalysts were prepared by impregnating a -Al2O3 support with Y and Ce then with Cu. The addition of YDC drastically enhanced the activity of Cu/Al2O3 in the methanol reforming reaction. The enhanced activity was attributed to the increase of Cu+ species by YDC in the methanol reforming environment. However, the addition of YDC decreased the copper dispersion. The Cu dispersion could be enhanced by adding chromium oxide. The addition of YDC and Cr where Al2O3 was first impregnated with Cr then with YDC showed the most pronounced enhancement of the catalyst activity. At reaction temperatures of 200250 °C, the CO concentration in the products was smaller than 0.1%. 相似文献
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Koji Matsui 《Journal of the American Ceramic Society》2008,91(8):2534-2539
The sintering behavior of 3 mol% Y2 O3 -doped ZrO2 powders with and without a small amount of SiO2 was investigated to clarify the effect of SiO2 addition on the initial sintering stage. The shrinkage behavior of a powder compact was measured under constant rates of heating (CRH). The sintering rate increased remarkably with the addition of a small amount of SiO2 . The apparent activation energy ( nQ ) and apparent frequency factor , where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying the sintering-rate equation to the CRH data. The diffusion mechanism changed from grain-boundary diffusion (GBD) to volume diffusions (VD) on SiO2 addition, and both nQ and increased with the GBD→VD change. It is, therefore, concluded that the sintering rate increases by SiO2 addition because the increase in rather than nQ is predominant. 相似文献
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Sintering Behavior of Fine Aluminum Nitride Powder Synthesized from Aluminum Polynuclear Complexes 总被引:5,自引:0,他引:5
Noboru Hashimoto Hiroyoshi Yoden Shigehito Deki 《Journal of the American Ceramic Society》1992,75(8):2098-2106
The sintering behavior of fine AIN powder synthesized from an aluminum polynuclear complex was investigated. The focus of this work was to investigate the densification behavior of the AIN powder with different particle sizes (specific surface area: 3.2–22.8 m2 /g). The AIN powder was synthesized from basic aluminum chloride and glucose mixed in a water solution. This powder was divided into two groups: one with 2 wt% Y2 O3 added as the sintering aid and the other without such an additive. The AIN powder investigated possessed favorable densification potential. The density of the AIN powder with a surface area of 16.6 m2 /g and without additives attained theoretical density at 1700°C. Adding Y2 O3 further decreased the sintering temperature required for full densification to 1600°C. It is speculated that low-temperature sintering of our fine AIN powder with Y2 O3 proceeds in two steps: in the initial stage, sintering proceeds predominantly through interdiffusion between yttrium aluminates formed on the AIN powder surface; in the second stage, the densification may occur by the interdiffusion between solid phases formed by a reaction between the yttrium aluminates and AIN. To investigate the effect of oxygen on sintering, the content of oxygen in AIN powder was varied while the particle size was kept constant. In this study, the difference in surface oxygen content scarcely affected the sintering behavior of fine AIN powder. 相似文献
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Koji Matsui Takanori Yamakawa Masato Uehara Naoya Enomoto Junichi Hojo 《Journal of the American Ceramic Society》2008,91(6):1888-1897
The isothermal shrinkage behavior of 2.9 mol% Y2 O3 -doped ZrO2 powders with 0–1 mass% Al2 O3 was investigated to clarify the effect of Al2 O3 concentration on the initial sintering stage. The shrinkage of the powder compact was measured at constant temperatures in the range of 950°–1050°C. The Al2 O3 addition increased the densification rate with increasing temperature. The values of apparent activation energy ( nQ ) and apparent frequency-factor term (β0 n ), where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying a sintering-rate equation to the isothermal shrinkage data. The diffusion mechanism changed from grain-boundary diffusion (GBD) to volume diffusion (VD) by Al2 O3 addition and both nQ and β0 n increased with increasing Al2 O3 concentration. The kinetic analysis of the sintering mechanism suggested that the increase of densification rate by Al2 O3 addition largely depends on the increase of β0 n , that is, the increases of n with GBD→VD change and β0 with an increase in Al2 O3 content, although the nQ also increases with Al2 O3 addition. This enhanced sintering mechanism is reasonably interpreted by the segregated dissolution of Al2 O3 at ZrO2 grain boundaries. 相似文献
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Masayuki Imose Akihiro Ohta Yoshihiko Takano Masaru Yoshinaka Ken Hirota Osamu Yamaguchi 《Journal of the American Ceramic Society》1998,81(4):1050-1052
Mullite/Y-TZP composites in the mullite-rich region, using powders of mullite prepared via the hydrazine method and a commercially available zirconia compound (ZrO2 (2Y)), have been fabricated by sintering for 3 h at 1450°C in air. The grain sizes of mullite and ZrO2 (2Y) are changed with increased ZrO2 (2Y) content; the former decreases from 0.25 µm to 0.13 µm, and the latter increases from 0.21 µm to 0.35 µm. High strength (780 MPa) and fracture toughness (5.4 MPam1/2 ) are obtained in the 50/50 (vol%) mullite/ZrO2 (2Y) composite with 99.6% of theoretical density. 相似文献
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Seiji Adachi Takahiro Wada Toshihiro Mihara Yoshinari Miyamoto Mitsue Koizumi Osamu Yamada 《Journal of the American Ceramic Society》1989,72(5):805-809
Titanium carbide ceramics have been fabricated from the reactants titanium powder and carbon fiber by the highpressure self-combustion sintering method (HPCS) under two pressure conditions (65 MPa and 3 GPa). Porous TiC with a density of about 50% of theoretical was obtained under a pressure of 65 MPa. A possible model accounting for the formation mechanism of TiC is proposed on the basis of observation of the microstructure of the products formed from the incomplete reaction. On the other hand, dense TiC (>95% of theoretical) was fabricated under 3 GPa. The mean grain size varied, depending on the mixing molar ratio of the reactants. The influence of differences in the carbon (powder and fiber forms) on the structure of TiC compacts is discussed. 相似文献
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Fabrication of Transparent Lead Lanthanum Zirconate Titanate Ceramics from Fine Powders by Two-Stage Sintering 总被引:1,自引:1,他引:1
Transparent lead lanthanum zirconate titanate (PLZT) ceramics were fabricated from fine powders using an inexpensive two-stage sintering technique. The powders were prepared by hydrolysis from low-cost inorganic precursors. In the two-stage sintering method, uniaxially pressed green pellets were densified to nearly theoretical values in an oxygen gas atmosphere during the first-stage sintering, at 1000°C for 1 h, and then residual, free lead oxide in the pellets was removed by second-stage sintering at 1100°C for 12 h. Transparent ceramic with an average grain size of 1.6 μm and a porosity of 1.3% was obtained. The transparency and dielectric characteristics of the present samples were compared with those of hot-pressed samples: The study of the polarization–field hysteresis loops of the present samples yielded a remanent polarization of 6.8 μC/m2 and a coercive field of 1.6 kV/cm. The low coercive field of PLZT ceramics could potentially reduce the driving voltage of electrooptic devices in many applications. 相似文献
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Eva Jud Christoph B. Huwiler Ludwig J. Gauckler 《Journal of the American Ceramic Society》2005,88(11):3013-3019
Pure and CoO1– x -doped Ce0.8 Gd0.2 O1.9 (CGO) have been analyzed with constant heating rate dilatometry. Doping changes the grain boundary structure of CGO resulting in enhanced mass transport and in increased densification rates. Significant changes in activation energies have been detected. While pure CGO exhibits a significant amount of surface diffusion, cobalt oxide doping enhances grain boundary diffusion. The obtained activation energies are 4.70±0.32 eV for CGO and 6.24±0.32 eV for CoO1− x -doped CGO. The higher activation energy of CoO1− x -doped CGO is a result of the formation of intergranular films. It is suggested that sintering of CoO1− x -doped CGO occurs by rearrangement and grain boundary diffusion whereas grain boundary and volume diffusion govern the densification of pure CGO. 相似文献
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概述了混合烧结法的分类、致密化机理、影响因素和在玻璃陶瓷制备中的应用.和传统玻璃陶瓷制备工艺相比,混合烧结法的特点在于玻璃陶瓷中的晶体是直接加入或是通过外加晶体和玻璃反应析出而不是从母相玻璃中直接析出,因而对母相玻璃组分要求不十分严格,在直接利用废玻璃制备玻璃陶瓷和复相玻璃陶瓷方面有着独特的优势.本文重点介绍了近年来该工艺在这方面的应用和进展. 相似文献
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Sintering Kinetics at Isothermal Shrinkage: Effect of Specific Surface Area on the Initial Sintering Stage of Fine Zirconia Powder 总被引:1,自引:0,他引:1
Koji Matsui Akira Matsumoto Masato Uehara Naoya Enomoto Junichi Hojo 《Journal of the American Ceramic Society》2007,90(1):44-49
The isothermal shrinkage behaviors of fine zirconia powders (containing 2.8–2.9 mol% Y2 O3 ) with specific surface areas of about 6 and 16 m2 /g were investigated to clarify the effect of specific surface area on the initial sintering stage. The shrinkage of powder compact was measured under constant temperatures in the range of 1000°–1100°C. The increase in specific surface area enhanced the densification rate with increasing temperature. The values of activation energy ( Q ) and frequency-factor term (β0 ) of diffusion at initial sintering were estimated by applying the sintering-rate equation to the isothermal shrinkage data. The Q of diffusion changes little but the β0 increases with the increase in specific surface area. It is therefore concluded that the increase in the specific surface area of fine zirconia powder enhances the shrinkage rate because of an increase in the β0 at the initial stage of sintering. 相似文献
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Feng-Yun Wang Guo-Bin Jung Ay Su Shih-Hong Chan Xiang-Ai Li Lifen Xiao 《Journal of the American Ceramic Society》2007,90(10):3357-3359
Highly porous Ce0.8 Sm0.2 O1.9 (CSO) was prepared by acid leaching MgO from CSO–MgO (CSM) ceramics with acetic acid solution. The effects of leaching conditions and MgO content were studied. Using 60 vol% acetic acid at 85°C for 8.5 h, the MgO in CSM ceramics was effectively leached out but no CSO was dissolved; thus highly porous CSO with an open porosity of 68% was successfully prepared. The open porosity of sufficiently leached pellet increased with the MgO content, but when the vol% of MgO exceeded 60%, the flexural strength of the leached pellets decreased from 25 to 1 MPa. The leaching approach allows the fabrication of porous CSO with the vol% of MgO up to 60%. 相似文献
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Isabel Alvarez-Clemares Gustavo Mata-Osoro Adolfo Fernández Sonia Lopez-Esteban Carlos Pecharroman Ramon Torrecillas Jose S. Moya 《Journal of the European Ceramic Society》2012,32(11):2917-2924
Al2O3–CeO2 ceramics were found to be transparent in the IR range and high-traslucent in the visible range. The surface of nanometric α-alumina particles was modified by deposition–precipitation of small fractions of ceria nanoparticles. The powders were sintered using Spark Plasma Sintering. Values of Real In-line Transmittance up to 70% in the IR-Vis range have been measured. Transparency enhancement has been attributed to Cerium oxide nanoparticles located at the grain boundaries and triple points. These particles are hindering alumina grain growth during SPS at a temperature as high as 1430 °C. This effect is found to be effective under SPS low vacuum conditions and short dwell times. The optimum ceria content was found to be 0.7 wt.%. Diffusion in Al2O3–CeO2 as a function of the atmosphere has been studied in diffusion couples. The results obtained by the proposed route are discussed considering the data reported in the literature for SPSed transparent alumina. 相似文献
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The sintering behavior of compacts of ultrafine silicon powder (0.02 to 0.1 μm particle size) was investigated. Two sintering modes occur: normal sintering associated with densification and subnormal sintering without densification. The micro-structure developed in normal sintering has a fine grain size (0.05 to 0.3 μm) and fine porosity; the grains contain stacking faults and twins. The microstructure developed in subnormal sintering exhibits larger grains (∼1 μm in size) and coarse pores. Green densities >42% of theoretical and temperatures >1100°C are required for densification. Densification follows an exponential time and temperature dependence with an activation energy of 470 kJ/mol, indicating bulk diffusion as the transport mechanism. Grain-boundary diffusion is thought to be inhibited by grain-boundary oxide films. The carbon phase-separates into discrete amorphous regions and is thought to have little effect on sintering behavior. 相似文献
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Hiroyuki Yoshida Kazuhiro Miura Jun-ichi Fujita Toru Inagaki 《Journal of the American Ceramic Society》1999,82(1):219-221
Samaria-doped ceria (SDC) was prepared by using the solid-state reaction method. Sintering of SDC was significantly promoted by adding a small amount of gallium. SDC that had 1% of gallium added, sintered at 1450°C, showed almost the same properties as SDC sintered at 1600°C. Measurements showed that the addition of gallia could reduce the sintering temperature by 150°C without deteriorating the properties of SDC as an electrolyte for solid oxide fuel cells. 相似文献
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Formation and Sintering of Yttria-Doped Tetragonal Zirconia with 50 mol% Alumina Prepared by the Hydrazine Method 总被引:1,自引:0,他引:1
Shinya Kimoto Ken Hirota Osamu Yamaguchi Hideki Kume Suguru Inamura Hiroki Miyamoto 《Journal of the American Ceramic Society》1994,77(6):1694-1696
Al2 O3 /Y2 O3 -doped ZrO2 composite powders with 50 mol% Al2 O3 are prepared by the hydrazine method. As-prepared powders are mixtures of AlO(OH) gel and amorphous ZrO2 solid solutions containing Y2 O3 and Al2 O3 . The formation process leading to α-Al2 O3 - t -ZrO2 composite powders is examined. Hot isostatic pressing is performed for 2 h at 1400°C under 196 MPa using θ-Al2 O3 - t -ZrO2 composite powders. The resulting dense, sintered α-Al2 O3 - t -ZrO2 composites show excellent mechanical strength. 相似文献