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1.
A suspension stabilizer-coating technique was employed to prepare x mol% Yb 2O 3 ( x = 1.0, 2.0, 3.0 and 4.0) and 1.0 mol% Y 2O 3 co-doped ZrO 2 powder. A systematic study was conducted on the sintering behaviour, phase assemblage, microstructural development and mechanical properties of Yb 2O 3 and Y 2O 3 co-doped zirconia ceramics. Fully dense ZrO 2 ceramics were obtained by means of pressureless sintering in air for 1 h at 1450 °C. The phase composition of the ceramics could be controlled by tuning the Yb 2O 3 content and the sintering parameters. Polycrystalline tetragonal ZrO 2 (TZP) and fully stabilised cubic ZrO 2 (FSZ) were achieved in the 1.0 mol% Y 2O 3 stabilised ceramic, co-doped with 1.0 mol% Yb 2O 3 and 4.0 mol% Yb 2O 3, respectively. The amount of stabilizer needed to form cubic ZrO 2 phase in the Yb 2O 3 and Y 2O 3 co-doped ZrO 2 ceramics was lower than that of single phase Y 2O 3-doped materials. The indentation fracture toughness could be tailored up to 8.5 MPa m 1/2 in combination with a hardness of 12 GPa by sintering a 1.0 mol% Yb 2O 3 and 1.0 mol% Y 2O 3 ceramic at 1450 °C for 1 h. 相似文献
2.
The sintering of lime doped with up to 2 mol% La 2O 3 as well as CeO 2 was followed on firing at 1400, 1500 and 1600°C. X-ray diffraction and reflected-light microscopy techniques have been used to investigate the phase composition and texture of the fired samples. The load bearing capacity and the rate of hydration of the obtained dense lime samples have also been determined to assess its quality. It is found that doping lime with 0·5–1·0 mol% of either La 2O 3 or CeO 2 and firing up to 1600°C leads to dense and hydration-resistant lime with uniform texture. At this level of the dopant oxides, normal grain growth occurred with the increase in the degree of direct bonding. Doping with less than 0·5 mol% La 2O 3 or CeO 2 is insufficient for inhibiting the discontinuous grain growth of the lime grains. On the other hand, increase in the dopant content above 1·0 mol% showed finer grain sizes at the expense of the degree of direct bonding. 相似文献
3.
The influence of catalyst pre-treatment temperature (650 and 750 °C) and oxygen concentration ( λ = 8 and 1) on the light-off temperature of methane combustion has been investigated over two composite oxides, Co 3O 4/CeO 2 and Co 3O 4/CeO 2–ZrO 2 containing 30 wt.% of Co 3O 4. The catalytic materials prepared by the co-precipitation method were calcined at 650 °C for 5 h (fresh samples); a portion of them was further treated at 750 °C for 7 h, in a furnace in static air (aged samples). Tests of methane combustion were carried out on fresh and aged catalysts at two different WHSV values (12 000 and 60 000 mL g−1 h−1). The catalytic performance of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 were compared with those of two pure Co3O4 oxides, a sample obtained by the precipitation method and a commercial reference. Characterization studies by X-ray diffraction (XRD), BET and temperature-programmed reduction (TPR) show that the catalytic activity is related to the dispersion of crystalline phases, Co3O4/CeO2 and Co3O4/CeO2–ZrO2 as well as to their reducibility. Particular attention was paid to the thermal stability of the Co3O4 phase in the temperature range of 750–800 °C, in both static (in a furnace) and dynamic conditions (continuous flow). The results indicate that the thermal stability of the phase Co3O4 heated up to 800 °C depends on the size of the cobalt oxide crystallites (fresh or aged samples) and on the oxygen content (excess λ = 8, stoichiometric λ = 1) in the reaction mixture. A stabilizing effect due to the presence of ceria or ceria–zirconia against Co3O4 decomposition into CoO was observed. Moreover, the role of ceria and ceria–zirconia is to maintain a good combustion activity of the cobalt composite oxides by dispersing the active phase Co3O4 and by promoting the reduction at low temperature. 相似文献
4.
Yttria-ceria-doped tetragonal zirconia (Y,Ce)-TZP)/alumina (Al 2O 3) composites were fabricated by hot isostatic pressing at 1400° to 1450°C and 196 MPa in an Ar–O 2 atmosphere using the fine powders prepared by hydrolysis of ZrOCl 2 solution. The composites consisting of 25 wt% Al 2O 3 and tetragonal zirconia with compositions 4 mol% YO 1.5–4 mol% CeO 2–ZrO 2 and 2.5 mol% YO 1.5–5.5 mol% CeO 2–ZrO 2 exhibited mean fracture strength as high as 2000 MPa and were resistant to phase transformation under saturated water vapor pressure at 180°C (1 MPa). Postsintering hot isostatic pressing of (4Y, 4Ce)-TZP/Al 2O 3 and (2.5Y, 5.5Ce)-TZP/Al 2O 3 composites was useful to enhance the phase stability under hydrothermal conditions and strength. 相似文献
6.
Microstructural development associated with diffusionless phase transformation was investigated in sintered ZrO 2-10–60 mol% CeO 2 ceramics cooled rapidly from a high temperature, using TEM and XRD techniques. The results show that (112) reflections appeared and a domain structure was found in ZrO 2-20–40 mol% CeO 2 samples, which is a result of c-t′ diffusionless transition, while the structure of the ZrO 2-60 mol% CeO 2 sample was fully stabilized zirconia, in which no forbidden reflections of c-ZrO 2 appeared. Finally, plate martensite and lath martensite structure were found in the ZrO 2-10 mol% CeO 2 sample; the former is the tetragonal phase with internal twins and the latter is the twinned monoclinic phase. 相似文献
7.
The microstructure and mechanical properties of ZrO 2-2 mol% Y 2O 3 ceramics were studied on samples prepared by cold isostatic pressing and pressureless sintering. It was shown that the density of the alloy increases with increasing sintering temperature. The Vickers hardness decreases with the appearance of the monoclinic phase and increasing its content. Compared with the single tetragonal phase, the (t + m) dual-phase structure with microcracks has a much higher fracture toughness (16·5 MPa√m) because of a complex mechanism of toughening (transformation, microcracks and residual stresses). 相似文献
8.
Mullite ceramics were fabricated at relatively low temperatures from powder mixtures of -Al 2O 3 and quartz, with an Y 2O 3 addition. The mullitization process was analyzed by X-ray diffraction. The densification behavior was investigated as a function of the Y 2O 3 content, sintering temperature and holding time as well as mullite seeds. It has been shown that mullitization occurs via a nucleation and growth mechanism within an yttrious aluminosilicate glass, but lattice and grain-boundary diffusion becomes important during the densification process. Moreover, the incorporation of mullite seeds was observed to enhance both mullitization and densification. At 1400°C for 5 h or 1450°C for 2 h, 15 mol% Y 2O 3-doped and 5 mol% mullite-seeded specimens can be sintered to almost full density. 相似文献
9.
This paper reports results of studies on structure and activity in soot combustion of nanocrystalline CeO 2 and CeLnOx mixed oxides (Ln = Pr, Tb, Lu, Ce/Ln atomic ratios 5/1). Nano-sized (4–5 nm) oxides with narrow size distribution were prepared by a microemulsion method W/O. Microstructure, morphology and reductivity of the oxides annealed up to 950 °C in O 2 and H 2 were analyzed by HRTEM, XRD, FT-IR, Raman spectroscopy and H 2-TPR. Obtained mixed oxides had fluorite structure of CeO 2 and all exhibited improved resistance against crystal growth in O 2, but only CeLuOx behaved better than CeO 2 in hydrogen. The catalytic activity of CeO2, CeLnOx and physical mixtures of CeO2 + Ln2O3 in a model soot oxidation by air was studied in “tight contact” mode by using thermogravimetry. Half oxidation temperature T1/2 for soot oxidation catalysed by nano-sized CeO2 and CeLnOx was similar and ca. 100 °C lower than non-catalysed oxidation. However, the mixed oxides were much more active during successive catalytic cycles, due to better resistance to sintering. Physical mixtures of nanooxides (CeO2 + Ln2O3) showed exceptionally high initial activity in soot oxidation (decrease in T1/2 by ca. 200 °C) but degraded strongly in successive oxidation cycles. The high initial activity was due to the synergetic effect of nitrate groups present in highly disordered surface of nanocrystalline Ln2O3 and enhanced reductivity of nanocrystalline CeO2. 相似文献
10.
In this paper, a strategy is described to develop high toughness yttria-stabilised tetragonal zirconia polycrystalline (Y-TZP) composites reinforced with hard TiB 2 particles. The experimental results revealed that fully dense Y-TZP composites with 30 vol.% TiB 2 can be obtained with a moderate hardness of 13 GPa, a high strength up to 1280 MPa and an excellent indentation toughness up to 10 MPa m 1/2 by hot pressing in vacuum at 1450 °C. The toughness of the composites can be tailored between 4 and 10 MPa m 1/2 by varying the yttria stabiliser content of the ZrO 2 matrix between 3 and 2 mol%. An optimum composite toughness was achieved for a ZrO 2 matrix with an overall yttria content of 2.5 mol%, obtained by mixing pure monoclinic and 3 mol% Y 2O 3 co-precipitated ZrO 2 starting powders. An important observation is that the thermal residual tensile stress in the ZrO 2 matrix due to the TiB 2 addition, needs to be taken into account when optimising the transformability of the ZrO 2 matrix in order to develop high toughness Y-TZP composites. 相似文献
11.
采用柠檬酸络合法制备一系列不同铜铈比的Cu-Ce-O/γ-Al_2O_3催化剂,用XRD、H2-TPR对其进行表征,采用连续固定床微反装置对Cu-Ce-O/γ-Al_2O_3催化剂CO催化氧化活性进行评价。结果表明,Cu-Ce-O/γ-Al_2O_3催化剂的XRD图谱中除归属于γ-Al_2O_3的晶相峰外,还出现CuO和CeO_2的晶相峰。高温水热引起活性组分CeO_2的晶粒聚集、长大和尖晶石结构CuAl2O4物质的生成;CuO-CeO_2之间的共生共存与相互作用,使得Cu-Ce-O/γ-Al_2O_3催化剂中具有非完整结构的[Cu2+1-xCu+x][O1-12x12x]增多,Cu+离子和氧空位增多,有利于其H2-TPR还原峰温度向低温区偏移,有利于提高其CO的催化氧化活性,使得Cu-Ce-O/γ-Al_2O_3催化剂的TCO50和TCO90降低。Cu与Ce物质的量比为5∶5制备的Cu-Ce-O/γ-Al_2O_3-55催化剂的TCO50和TCO90分别降至最低的162℃和199℃,表明此时的Cu-Ce-O协同效应最佳;CuO-CeO_2二相的共生共存与相互作用有利于减少高温水热环境下活性组分的聚集和晶粒长大,有利于Cu-Ce-O/γ-Al_2O_3催化剂能够保持较高的CO催化氧化活性。 相似文献
12.
Ni catalysts supported on γ-Al 2O 3, CeO 2 and CeO 2–Al 2O 3 systems were tested for catalytic CO 2 reforming of methane into synthesis gas. Ni/CeO 2–Al 2O 3 catalysts showed much better catalytic performance than either CeO 2- or γ-Al 2O 3-supported Ni catalysts. CeO 2 as a support for Ni catalysts produced a strong metal–support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO 2 had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/γ-Al 2O 3 catalysts for this reaction. A weight loading of 1–5 wt% CeO 2 was found to be the optimum. Ni catalysts with CeO 2 promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO 2-promoted catalysts are attributed to the oxidative properties of CeO 2. 相似文献
13.
A novel CeO 2–Y 2O 3 (CY) washcoat on cordierite honeycomb was prepared by an impregnation method, which was used as a support to prepare a Pd catalyst. A model reaction of the complete combustion of toluene was conducted to evaluate the performance of the developed Pd/CY catalyst. The CY washcoat support and the Pd/CY catalyst were characterized by XRD, Raman spectroscopy, H 2-TPR and SEM techniques. The results show that compared with conventional washcoat the CY washcoat has better adhesion and higher vibration- and heat-resistance. The CY washcoat can anchor well Pd onto the cordierite honeycomb substrate. The formation of a CeO 2–Y 2O 3 solid solution and the steady present of PdO occur at high calcination temperatures, resulting in a better thermal stability. On a Pd/CY catalyst calcined at 500 °C, a 99% of toluene conversion was obtained at 210 °C, and it was stable for reaction time up to 30 h. 相似文献
14.
Co 3O 4/CeO 2 composite oxides with different cobalt loading (5, 15, 30, 50, 70 wt.% as Co 3O 4) were prepared by co-precipitation method and investigated for the oxidation of methane under stoichiometric conditions. Pure oxides, Co 3O 4 and CeO 2 were used as reference. Characterization studies by X-ray diffraction (XRD), BET, temperature programmed reduction/oxidation (TPR/TPO) and X-ray photoelectron spectroscopy (XPS) were carried out. An improvement of the catalytic activity and thermal stability of the composite oxides was observed with respect to pure Co3O4 in correspondence of Co3O4–CeO2 containing 30% by weight of Co3O4. The combined effect of cobalt oxide and ceria, at this composition, strongly influences the morphological and redox properties of the composite oxides, by dispersing the Co3O4 phase and promoting the efficiency of the Co3+–Co2+ redox couple. The presence in the sample Co3O4(30 wt.%)–CeO2 of a high relative amount of Ce3+/(Ce4+ + Ce3+) as detected by XPS confirms the enhanced oxygen mobility. The catalysts stability under reaction conditions was investigated by XRD and XPS analysis of the used samples, paying particular attention to the Co3O4 phase decomposition. Methane oxidation tests were performed over fresh (as prepared) and thermal aged samples (after ageing at 750 °C for 7 h, in furnace). The resistance to water vapour poisoning was evaluated for pure Co3O4 and Co3O4(30 wt.%)–CeO2, performing the tests in the presence of 5 vol.% H2O. A methane oxidation test upon hydrothermal ageing (flowing at 600 °C for 16 h a mixture 5 vol.% H2O + 5 vol.%O2 in He) of the Co3O4(30 wt.%)–CeO2 sample was also performed. All the results confirm the superiority of this composite oxide. 相似文献
15.
The mechanical properties of Al 2O 3 matrix composites reinforced by ZrO 2(2 mol% Y 2O 3) and nanometre scale SiC dispersions have been investigated. It is shown that the Al 2O 3 matrix is simultaneously strengthened and toughened by both ZrO 2(2 mol% Y 2O 3) and nano-SiC particles. The maximum flexural strength and fracture toughness of the composites are 945 MPa and 7.3 MPam 1/2, respectively. The reinforcing effect of both t-m phase transformation of ZrO 2 (2 mol% Y 2O 3) and nano-SiC particles appears to be synergetic. 相似文献
16.
Four different β-Si 3N 4 ceramics with silicon oxynitrides [Y 10(SiO 4) 6N 2, Yb 4Si 2N 2O 7, Er 2Si 3N 4O 3, and La 10(SiO 4) 6N 2, respectively] as secondary phases have been fabricated by hot-pressing the Si 3N 4–Re 4Si 2N 2O 7 (Re=Y, Yb, Er, and La) compositions at 1820°C for 2 h under a pressure of 25 MPa. The oxidation behavior of the hot-pressed ceramics was characterized and compared with that of the ceramics fabricated from Si 3N 4–Re 2Si 2O 7 compositions. All Si 3N 4 ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C and the oxidation products of the ceramics were SiO 2 and Re 2Si 2O 7. The Si 3N 4–Re 4Si 2N 2O 7 compositions showed inferior oxidation resistance to those from Si 3N 4–Re 2Si 2O 7 compositions, owing to the incompatibility of the secondary phases of those ceramics with SiO 2, the oxidation product of Si 3N 4. Si 3N 4 ceramics from a Si 3N 4–Er 4Si 2N 2O 7 composition showed the best oxidation resistance of 0·198 mg cm −2 after oxidation at 1400°C for 192 h in air among the compositions investigated herein. 相似文献
17.
Homogeneous-eutectic microstructure of Y 3Al 5O 12–Al 2O 3 system without coarse primary crystals was formed at an off-eutectic composition. This method utilizes a low migration rate in an amorphous phase. A mixture of Y 2O 3 and Al 2O 3 having the off-eutectic composition was melted and quenched rapidly to form an amorphous phase. A heat-treatment of the amorphous phase at 1000 °C and 1300 °C for 30 min formed Y 3Al 5O 12 and Al 2O 3 phases. SEM observation of this material, which was formed from the amorphous phase at 1300 °C for 30 min, showed homogeneous eutectic-like microstructure. The formation of the primary crystals (coarse Al 2O 3), which are always observed in the off-eutectic compositions by ordinary method, was completely suppressed. 相似文献
18.
A 3 mol% Y 2O 3 zirconia stabilized powder has been synthesized by destabilization of an aqueous zirconia sol prepared by the alkoxide hydrolysis method. The powder calcined at 500°C is ultrafine with tetragonal crystallites of about 8 nm, slightly agglomerated and with a narrow pore size distribution having an average pore size of 5.2 nm. Zirconia ceramics with density higher than 92%TD and grain size on the order of 100 nm have been obtained by uniaxial pressing at 500 MPa and vacuum sintering at 1000°C. Electrical conductivity of sintered samples, evaluated by complex impedance spectroscopy measurements, indicated that the zirconia stabilized with 3 mol% Y 2O 3 can potentially be used as an oxygen semipermeable dense membrane, but only at a relatively high temperature. 相似文献
19.
A series of CeO 2 promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N 2O). Addition of CeO 2 to Co 3O 4 led to an improvement in the catalytic activity for N 2O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N 2O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O 2, H 2O or NO. Methods of XRD, FE-SEM, BET, XPS, H 2-TPR and O 2-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO 2 could increase the surface area of Co 3O 4, and then improve the reduction of Co 3+ to Co 2+ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N 2O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO 2, are responsible for the enhancement of catalytic activity of Co 3O 4. 相似文献
20.
The selective catalytic reduction of NO by H 2 under strongly oxidizing conditions (H 2-SCR) in the low-temperature range of 100–200 °C has been studied over Pt supported on a series of metal oxides (e.g., La 2O 3, MgO, Y 2O 3, CaO, CeO 2, TiO 2, SiO 2 and MgO-CeO 2). The Pt/MgO and Pt/CeO 2 solids showed the best catalytic behavior with respect to N 2 yield and the widest temperature window of operation compared with the other single metal oxide-supported Pt solids. An optimum 50 wt% MgO-50wt% CeO 2 support composition and 0.3 wt% Pt loading (in the 0.1–2.0 wt% range) were found in terms of specific reaction rate of N 2 production (mols N 2/g cat s). High NO conversions (70–95%) and N 2 selectivities (80–85%) were also obtained in the 100–200 °C range at a GHSV of 80,000 h −1 with the lowest 0.1 wt% Pt loading and using a feed stream of 0.25 vol% NO, 1 vol% H 2, 5 vol% O 2 and He as balance gas. Addition of 5 vol% H 2O in the latter feed stream had a positive influence on the catalytic performance and practically no effect on the stability of the 0.1 wt% Pt/MgO-CeO 2 during 24 h on reaction stream. Moreover, the latter catalytic system exhibited a high stability in the presence of 25–40 ppm SO 2 in the feed stream following a given support pretreatment. N 2 selectivity values in the 80–85% range were obtained over the 0.1 wt% Pt/MgO-CeO 2 catalyst in the 100–200 °C range in the presence of water and SO 2 in the feed stream. The above-mentioned results led to the obtainment of patents for the commercial exploitation of Pt/MgO-CeO 2 catalyst towards a new NO x control technology in the low-temperature range of 100–200 °C using H 2 as reducing agent. Temperature-programmed desorption (TPD) of NO, and transient titration of the adsorbed surface intermediate NO x species with H 2 experiments, following reaction, have revealed important information towards the understanding of basic mechanistic issues of the present catalytic system (e.g., surface coverage, number and location of active NO x intermediate species, NO x spillover). 相似文献
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