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1.
The rapid sintering of nanostructured Al2O3 and Al2O3 to Al2SiO5 composites was investigated by a high-frequency induction heating sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. Highly dense nanostructured Al2O3 and Al2O3 to Al2SiO5 composites were produced with simultaneous application of a 80 MPa pressure and induced output current of a total power capacity (15 kW) within 3 min. The sintering behavior, grain size and mechanical properties of Al2O3 and Al2O3 to Al2SiO5 composites were investigated.  相似文献   

2.
The combined effect of nano-Al2O3 and TiO2 fillers on residual mechanical and thermal properties of glass fiber-reinforced polymer composites has been evaluated. The results reveal that the addition of 0.1?wt% of Al2O3 and 0.1?wt% of TiO2 into the epoxy matrix reduces the water diffusivity by 12%. The residual flexural and interlaminar shear strength of the nanocomposite have been increased by 19 and 21%, respectively, as compared to those of neat epoxy glass fiber-reinforced polymer composite. In spite of reduction in water diffusivity and increase in strength, there was no improvement in glass transition temperature of the nanocomposites.  相似文献   

3.
The wear behaviour of a monolithic alumina and an alumina-aluminium titanate laminated structure was studied. The laminate, containing surface fine grained alumina layers and internal composite layers with 10 vol.% of aluminium titanate, showed relatively low (≅20 MPa) compressive residual stresses at the surface. Interfaces between layers were constituted by large alumina grains (up to ≅50 μm) that promoted toughening due to crack deflection and branching. Wear tests were performed on square specimens (30 mm × 30 mm × 6 mm) using the pin-on-disc method. The laminates showed higher wear resistance than the monolithic alumina. The analysis of the results together with SEM-EDX observations was performed to identify possible wear mechanisms. The wear resistance improvements are discussed in terms of the residual stresses in the laminate and the properties provided by the special microstructure of the interfaces.  相似文献   

4.
In this study, TiO2, ZnO, TiO2/ZnO (Ti/Zn), and TiO2/ZnO/Sep (Ti/Zn/Sep) catalysts have been synthesized using sol–gel and chemical precipitation method. Their photocatalytic performances have been compared using Flumequine (FLQ) antibiotic. X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), scanning electron microscopy (SEM), N2-adsorption, and the determination of a zero point charge has been used to characterize the synthesized catalysts. The degradation studies showed that the catalytic efficiency of Ti/Zn/Sep is higher than that for other catalysts. The operational parameters such as pH, initial FLQ concentration, and catalyst dosage were evaluated. UV–vis and high-resolution mass spectroscopy (HRMS) analyses were used to determine the degradation efficiency and products. ZnO played a major role in the FLQ degradation process, and sepiolite contributed to adsorption of FLQ on the catalyst surface enormously. The catalysts exhibited 11%, 23%, 63%, and 85% degradation efficiency for ZnO, TiO2, Ti/Zn, and Ti/Zn/Sep in the decomposition of FLQ, respectively.  相似文献   

5.
Atomic scale computer simulation was used to predict the mechanisms and energies associated with the accommodation of aliovalent and isovalent dopants in three host oxides with the corundum structure. Here we consider a much more extensive range of dopant ions than has previously been the case. This enables a rigorous comparison of calculated mechanism energetics. From this we predict that divalent ions are charge compensated by oxygen vacancies and tetravalent ions by cation vacancies over the full range of dopant radii. When defect associations are included in the model these conclusions remain valid. At equilibrium, defects resulting from extrinsic dopant solution dominate intrinsic processes, except for the largest dopant cations. Solution reaction energies increase markedly with increasing dopant radius. The behaviour of cluster binding energies is more complex.  相似文献   

6.
Catalyst Cu/ZnO/Al2O3 was prepared through co-precipitation method and investigated in a stirred slurry autoclave reactor system for methanol synthesis. The structure of the catalysts was studied by derivative thermogravimetry (DTG), X-ray diffraction (XRD) and H2 temperature programmed reduction (H2-TPR). The results show that the average size of CuO crystallite of the catalyst was 3.23 nm when the catalyst was calcined at 623 K for 2.0 h, and the catalyst with this size of CuO crystallite is highly active and stable. Its space time yield and deactivation rate reached 172.2 g/kgcat·h and 0.43%/d, respectively under the reaction conditions of 513 K, 4.0 MPa, H2/CO/CO2 = 68/24/5 and space velocity = 810 ml/gcat·h.  相似文献   

7.
Ionic mobilities of Y3+ and Al3+ are controlled using either amorphous or well-crystallized precursors and enhanced by the addition of molten NaCl flux. Experimental results indicate that phase transformation prefers a relatively slow reaction route via YAM-YAP-YAG because of limited ionic mobility when either Y3+ or Al3+ is confined to a crystallized structure. The selection of intermediate phases can be either Y-rich (YAM) or Al-rich (YAP), depending on the competition of ionic mobility between Y3+ and Al3+ during phase transformation. The YAH-YAG route is selected only when the ionic mobility of Y3+ or Al3+ is high enough to induce a nucleation and growth reaction while a certain degree of stoichiometric inhomogeneity is presented; otherwise YAG is crystallized directly from amorphous precursors without passing through any intermediate phase.  相似文献   

8.
A method for non-destructive detection of microcracks in ceramic composites is described. The method involves the combination of ultrasound characterisation with the application of a three-phase micromechanical model, which considers cracks and pores as void constituents. Four alumina-aluminium titanate materials with different levels of microcracking, from no cracks (monophase alumina) to severely cracked (alumina + 40 vol.% of aluminium titanate) including an alumina + 10 vol.% aluminium titanate material with incipient microcracking have been developed to test the validity of the method. Specimens have been fabricated by colloidal processing and the longitudinal and transverse ultrasound velocities have been determined by the ultrasonic pulse-echo and transmission ultrasound-immersion techniques, employing a digital signal processing. It has been demonstrated that it is possible to differentiate between pores and microcracks, both modelled as void constituents, in terms of the aspect ratio.  相似文献   

9.
The phase diagram of the Al2O3-HfO2-Y2O3 system was first constructed in the temperature range 1200-2800 °C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this system. Four new ternary and three new quasibinary eutectics were found. The minimum melting temperature is 1755 °C and it corresponds to the ternary eutectic Al2O3 + HfO2 + Y3Al5O12. The solidus surface projection, the schematic of the alloy crystallization path and the vertical sections present the complete phase diagram of the Al2O3-HfO2-Y2O3 system.  相似文献   

10.
Titanium was allowed to diffuse into synthetic sapphire (α-Al2O3) at 1773–1923 K for 200 h in air. Specimens were prepared by four different methods. Samples were irradiated with a 10 MeV electron beam to fluencies of 2×1017 cm−2 for 1 h to induce vacancy formation. A 1-μm layer of titanium was sputtered onto sapphire samples to provide intimate contact with the diffusing elements. Ti diffusion was performed using TiO2 powder or a mixture of TiO2 and BeO powders in a ratio of 95:5 to take advantage of the beryllium activity. Ti diffusion was profiled using scanning electron microscope-energy dispersive X-ray spectrometry (SEM–EDX). The diffusion coefficients of Ti were as follows:  相似文献   

11.
In order to prepare ceramic nanoparticles by CO2 laser vaporization (LAVA) coarse ceramic powders (e.g. ZrO2, Al2O3, and TiO2) were used as raw materials. The raw powder was vaporized into a flowing process gas under normal pressure. Expanding into the gas, the vapor instantly cools down. Gas-phase condensation leads to the formation of nanoscale particles with a composition that corresponds to that of the raw powder. LAVA nanoparticles are chemically pure, spherical, crystalline, exhibit a narrow size distribution, and form merely soft agglomerates. The co-laser vaporization of mixtures of ceramic raw powders allows for the preparation of nanoparticles of multi-phase (e.g. Fe2O3-SiO2) or single-phase (e.g. CaTiO3) mixed-oxides and dispersion ceramics (e.g. ZrO2-Al2O3). In order to modify the surface of the nanoparticles they can be coated in-process with an organic additive. Thus, the LAVA method allows for the targeted development of a wide range of ceramic nanopowders with tailored properties.  相似文献   

12.
The addition of B2O3 to a Cu/ZnO/Al2O3 catalyst increased the activity of the catalyst for methanol synthesis after an induction period during the reaction. The stability of the B2O3-containing Cu/ZnO/Al2O3 catalyst was greatly improved by the addition of a small amount of colloidal silica to the catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

13.
A method to evaluate the elastic constants of aluminium titanate in alumina/aluminium titanate composites is described. Results are derived from a three-phase micromechanical model proposed to relate the velocity of propagation of ultrasounds in the materials with their microstructural characteristics and the elastic behaviour of the constituents. Dense and un-cracked alumina and alumina + 10 vol.% aluminium titanate specimens have been fabricated by colloidal processing and the longitudinal and transverse ultrasound velocities have been determined by the ultrasonic jointly pulse-echo and transmission ultrasound-immersion techniques, employing a digital signal processing. In order to assure the adequacy of the proposed method, results for monophase alumina have been compared to those obtained from the resonance frequencies of high density alumina plates tested in flexure and shear. The values of elastic moduli obtained using the two methods were coincident, which assured the validity of the non-destructive proposed method.  相似文献   

14.
In order to elucidate the effect of sodium on the activity of ZSM-5 supported metal oxides catalysts (ZnO–Al2O3/ZSM-5 and SnO–Al2O3/ZSM-5) for the transesterification of soybean oil with methanol, ZSM-5 supported metal oxides were prepared with and without sodium hydroxide by impregnation. The metal compositions of the ZSM-5 supported metal oxide catalysts and the metal concentrations dissolved from the catalysts to the methylester phase were measured by SEM-EDS and inductive coupled plasma spectroscopy, respectively. The catalytic activity of ZnO–Al2O3/ZSM-5 and SnO–Al2O3/ZSM-5 containing sodium did not originate from surface metal oxides sites, but from surface sodium sites or dissolved sodium leached from the catalyst surface.  相似文献   

15.
In this work results on dynamic corrosion studies of fused cast Al2O3-SiO2-ZrO2 and isostatically pressed and sintered Cr2O3-based refractories by two crystalline (transparent) frits are described. Experiments have been performed using the “Merry Go Round” test at ≅1500 °C.Microstructural and mineralogical analyses of selected areas from the corroded regions of the studied refractories were performed by reflected light optical microscopy and scanning electron microscopy with analysis by X-ray dispersive energy.Significant differences between the corrosion mechanisms acting in the two types of materials were found. In the fused cast Al2O3-SiO2-ZrO2 specimens corrosion took place by the dissolution of alumina and zirconia in the frit and in the glass formed by the reaction between the frit and the refractory. In the Cr2O3-based materials the corrosion process was controlled by the capillar penetration of the molten frit through the open pores. The reaction between the ZnO from the frits and Cr2O3 led to the formation of spinel (ZnCr2O4), a high-melting point bonding phase that retarded the frit penetration. Results are discussed using the relevant phase equilibrium diagrams.  相似文献   

16.
Interactions between a poly(vinyl)silazane and Al2O3 or Y2O3-stabilised ZrO2 fillers were studied during the fabrication of polysilazane-derived bulk ceramics in order to investigate the influence of oxide fillers on resulting properties. Specimens were produced by coating of the filler powders with the polysilazane, warm-pressing of the resulting composite powders, and pyrolytic conversion in flowing N2 at various temperatures between 1000 °C and 1400 °C. Significant differences in densification were observed, depending on the filler used. Reactions between the polysilazane-derived matrix and Al2O3 or ZrO2 at temperatures ≥1300 °C resulted in the formation of Si5AlON7 or ZrSiO4, respectively. Reactivity in the polysilazane-derived component was a result of SiO2 contamination caused primarily by adsorbed species on the filler particle surface. Knowledge of polysilazane/filler interface processes is found to be decisive for the prediction of properties such as shrinkage and porosity, which heavily influence performance of a material.  相似文献   

17.
D. Ferdous  J. Adjaye 《Fuel》2006,85(9):1286-1297
A detailed experimental study was performed in a trickle-bed reactor using bitumen derived gas oil. The objective of this work was to compare the activity of NiMo/Al2O3 catalyst containing boron or phosphorus for the hydrotreating and mild hydrocracking of bitumen derived gas oil. Experiments were performed at the temperature and LHSV of 340-420 °C and 0.5-2 h−1, respectively, using NiMo/Al2O3 catalysts containing 1.7 wt% boron or 2.7 wt% phosphorus. In the temperature range of 340-390 °C, higher nitrogen conversion was observed from boron containing catalyst than that from phosphorus containing catalyst whereas in the same temperature range, phosphorus containing catalyst gave higher relative removal of sulfur than boron containing catalyst. Phosphorus containing catalyst showed excellent hydrocracking and mild hydrocracking activities at all operating conditions. Higher naphtha yield and selectivity were obtained using phosphorus containing catalyst at all operating conditions. Maximum gasoline selectivity of ∼45 wt% was obtained at the temperature, pressure, and LHSV of 400 °C, 9.4 MPa and 0.5 h−1, respectively, using catalyst containing 2.7 wt% phosphorus.  相似文献   

18.
This study focuses on the effects of powder size and Ni–Al bonding layer on the electrochemical behaviour of plasma-sprayed Al2O3-13% TiO2 coating in fresh tropical seawater. The presence of the metallic bonding layer reduces the coating porosity and increases the surface roughness for both microparticle and nanoparticle coatings. The nanoparticle exhibits better corrosion rate of 1.9×10−6 mmpy compared to the microparticle coating, with a corrosion rate of 3.05×10−6 mmpy. However, the presence of the metallic bonding layer increases the corrosion rate for both micro and nanoparticle coatings. The corrosion mechanism for the coating with and without the metallic bonding layer is discussed in detail.  相似文献   

19.
Vermiculites intercalated with alumina pillars and modified with transition metals (Cu, Fe) were studied as catalysts of selective reduction of NO with ammonia. Prior to the pillaring process, a raw vermiculite was treated with a solution of nitric acid and then citric or oxalic acid in order to reduce the overall charge of layers. This modification was necessary for a successful pillaring of the clay. Transition metals (Fe, Cu) were deposited on the surface of the modified vermiculites by an ion-exchange method. The obtained samples were characterized with respect to composition (EPMA), structure (XRD), texture (BET) and chemical nature of deposited transition metal species (UV–vis–DRS). The vermiculite based materials have been found to be active and selective catalysts of the DeNOx process. The Cu-containing samples were catalytically active at lower temperatures than the pillared clays modified with iron. A side reaction of ammonia oxidation by oxygen decreased the effectiveness of the DeNOx process in the high temperature range.  相似文献   

20.
Ag/Al2O3 catalysts with 1 wt% SiO2 or TiO2 doping in alumina support have been prepared by wet impregnation method and tested for sulphur tolerance during the selective catalytic reduction (SCR) of NOx using propene under lean conditions. Ag/Al2O3 showed 44% NOx conversion at 623 K, which was drastically reduced to 21% when exposed to 20 ppm SO2. When Al2O3 support in Ag/Al2O3 was doped with 1 wt% SiO2 or TiO2 the NOx conversion remained constant in presence of SO2 showing the improved sulphur tolerance of these catalysts. Subsequent water addition does not induce significant deactivation. On the contrary, a slight promotional effect on the activity of NO conversion to nitrogen is observed after Si and Ti incorporation. FTIR study showed the sulphation of silver and aluminum sites of Ag/Al2O3 catalysts resulting in the decrease in the formation of reactive intermediate species such as –NCO, which in turn decreases NOx conversion to N2. In the case of Ag/Al2O3 doped with SiO2 or TiO2, formation of silver sulphate and aluminum sulphate was drastically reduced, which was evident in FTIR resulting in remarkable improvement in the sulphur tolerance of Ag/Al2O3 catalyst. These catalysts before and after the reaction have been characterized with various techniques (XRD, BET surface area, transmittance FTIR and pyridine adsorption) for physico-chemical properties.  相似文献   

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