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1.
The flexure creep behaviour of monolithic Al 2O 3 and 10 vol% SiC-particle reinforced Al 2O 3 matrix composites was investigated in air atmosphere at 1160 to 1400 °C and under a stress of 40 to 125 MPa. Two kinds of SiC particles with different particle sizes and oxygen contents were used in the composites, one having an average size of 0.6 μm with 1.7 vol% SiO 2 impurities and the other of average size 2.7 μm with 3.4 vol% SiO 2 impurities. Compared with the creep behaviour of monolithic Al 2O 3 the strain rate of the composites with 0.6 μm SiC particles did not decrease; however, the composites with 2.7 μm SiC particles exhibited excellent creep resistance. Microstructure analysis showed that the Al 2O 3 grains in the composites with 0.6 μm SiC particles were mainly equiaxed with most of the SiC particles lying at the grain boundaries or triplegrain junctions, whereas the grain features of the composites with 2.7 μm SiC particles were irregular and elongated and most of the SiC particles were entrapped into Al 2O 3 matrix grains. It was revealed that the entrapment of 2.7 μm SiC particles into Al 2O 3 matrix grains was related to the high SiO 2 impurity content on SiC particle surfaces, and the change of grain morphology and the good high-temperature oxidation resistance were responsible for the creep resistance increase of the composites with 2.7 μm SiC particles. 相似文献
2.
Al 2O 3–SiC composites containing up to 30 wt.% of dispersed SiC particles (280 nm) were fabricated via hot-pressing and machined as cutting tools. The Al 2O 3–SiC particulate composites exhibit higher hardness than their unreinforced matrix because of the inhibited grain growth by adding SiC and the presence of hard secondary phase (SiC). The fracture toughness of the composites remains constant up to 10 wt.% loading of SiC. For machining heat-treated AISI 4144140 steel, the Al 2O 3–10 wt.% SiC composite tool showed the longest tool life, seven times longer than a commercial tool made of Al 2O 3–TiC composite, while the composite tool with 5 wt.% SiC showed the longest tool life for machining gray cast iron. The improved performance of the Al 2O 3–SiC composite tools attributes to the transformation of fracture mode from intergranular fracture for Al 2O 3 to intragranular fracture for Al 2O 3–SiC composites. 相似文献
3.
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
4.
Nanosized particles dispersed uniformly on Al 2O 3 particles were prepared from the decomposition of precursor Cr(CO) 6 by metal organic chemical vapor deposition (MOCVD) in a fluidized chamber. These nanosized particles consisted of Cr 2O 3, CrC 1−x, and C. A solid solution of Al 2O 3–Cr 2O 3 and an Al 2O 3–Cr 2O 3/Cr 3C 2 nanocomposite were formed when these fluidized powders were pre-sintered at 1000 and 1150 °C before hot-pressing at 1400 °C, respectively. In addition, an Al 2O 3–Cr 2O 3/Cr-carbide (Cr 3C 2 and Cr 7C 3) nanocomposite was formed when the particles were directly hot pressed at 1400 °C. The interface between Cr 3C 2 and Al 2O 3 is non-coherent, while the interface between Cr 7C 3 and Al 2O 3 is semi-coherent. 相似文献
5.
In the present study, both t-phase zirconia and m-phase zirconia particles are incorporated into an alumina matrix. Dense Al 2O 3/(t-ZrO 2+m-ZrO 2) composites were prepared by sintering pressurelessly at 1600 °C. The microstructure of the composites are characterized, the elastic modulus, strength and toughness determined. Because the ZrO 2 inclusions are close to each other in the Al 2O 3 matrix, the yttrium ion originally in t-ZrO 2 particles can diffuse to nearby m-ZrO 2 particles during sintering, and the m-phase zirconia is thus stabilized after sintering. The strength of the Al 2O 3/(t-ZrO 2+m-ZrO 2) composites after surface grinding can reach values as high as 940 MPa, which is roughly three times that of Al 2O 3 alone. The strengthening effect is contributed by microstructural refinement together with the surface compressive stresses induced by grinding. The toughness of alumina is also enhanced by adding both t-phase and m-phase zirconia, which can reach values as high as two times that of Al 2O 3 alone. The toughening effect is attributed mainly to the zirconia t–m phase transformation. 相似文献
6.
We have examined the adsorption of CO and NO on powder Pd/Al 2O 3, Pd–Ce/Al 2O 3 and CeO 2/Al 2O 3 catalysts, using temperature-programmed desorption (TPD). For CO adsorption on oxidized and pre-reduced Pd–Ce/Al 2O 3 TPD profiles are identical to those observed for Pd/Al 2O 3, suggesting that interactions between ceria and Pd have a negligible effect on the adsorption properties of CO. It does, however, affect the oxidation state of the palladium particles. For NO, there are differences between Pd/Al 2O 3 and Pd–Ce/Al 2O 3. On oxidized catalysts, Pd/Al 2O 3 is more efficient for NO dissociation. However, pre-reduction increases the amount of NO that can adsorb on Pd–Ce/Al 2O 3 and react to N 2O and N 2. In comparison with Pd/Al 2O 3, reduced Pd–Ce/Al 2O 3catalysts dissociate NO at relatively high temperatures but they are more reactive and favor N 2 over N 2O. 相似文献
7.
Oxidation of propene and propane to CO 2 and H 2O has been studied over Au/Al 2O 3 and two different Au/CuO/Al 2O 3 (4 wt.% Au and 7.4 wt.% Au) catalysts and compared with the catalytic behaviour of Au/Co 3O 4/Al 2O 3 (4.1 wt.% Au) and Pt/Al 2O 3 (4.8 wt.% Pt) catalysts. The various characterization techniques employed (XRD, HRTEM, TPR and DR-UV–vis) revealed the presence of metallic gold, along with a highly dispersed CuO (6 wt.% CuO), or more crystalline CuO phase (12 wt.% CuO). A higher CuO loading does not significantly influence the catalytic performance of the catalyst in propene oxidation, the gold loading appears to be more important. Moreover, it was found that 7.4Au/CuO/Al2O3 is almost as active as Pt/Al2O3, whereas Au/Co3O4/Al2O3 performs less than any of the CuO-containing gold-based catalysts. The light-off temperature for C3H8 oxidation is significantly higher than for C3H6. For this reaction the particle size effect appears to prevail over the effect of gold loading. The most active catalysts are 4Au/CuO/Al2O3 (gold particles less than 3 nm) and 4Au/Co3O4/Al2O3 (gold particles less than 5 nm). 相似文献
8.
The pyrolised polysilazanes poly(hydridomethyl)silazane NCP 200 and poly(urea)silazane CERASET derived Si–C–N amorphous powders were used for preparation of micro/nano Si 3N 4/SiC composites by hot pressing. Y 2O 3–Al 2O 3 and Y 2O 3–Yb 2O 3 were used, as sintering aids. The resulting ceramic composites of all compositions were dense and polycrystalline with fine microstructure of average grain size <1 μm of both Si 3N 4 and SiC phases. The fine SiC nano-inclusions were identified within the Si 3N 4 micrograins. Phase composition of both composites consist of , β modifications of Si 3N 4 and SiC. High weight loss was observed during the hot pressing cycle, 12 and 19 wt.% for NCP 200 and CERASET precursors, respectively. The fracture toughness of both nanocomposites (NCP 2000 and CERASET derived) was not different. Indentation method measured values are from 5 to 6 MPa m 1/2, with respect to the sintering additive system. Fracture toughness is slightly sensitive to the SiC content of the nanocomposite. Hardness increases with the content of SiC in the nanocomposite. The highest hardness was achieved for pyrolysed CERASET precursor with 2 wt.% Y 2O 3 and 6 wt.% Yb 2O 3, HV 23 GPa. This is a consequence of the highest SiC content as well as the chemical composition of additives. 相似文献
9.
A series of nano-sized Ni/Al 2O 3 and Ni/La–Al 2O 3 catalysts that possess high activities for NH 3 decomposition have been successfully synthesized by a coprecipitation method. The catalytic performance was investigated under the atmospheric conditions and a significant enhancement in the activity after the introduction of La was observed. Aiming to study the influence of La promoter on the physicochemical properties, we characterized the catalysts by N 2 adsorption/desorption, XRD, H 2-TPR, chemisorption and TEM techniques. Physisorption results suggested a high specific surface area and XRD spectra showed that nickel particles are in a highly dispersed state. A combination of XRD, TEM and chemisorption showed that Ni 0 particles with the average size lower than 5.0 nm are always obtained even though the Ni loading ranged widely from 4 to 63%. Compared with the Ni/Al 2O 3 catalysts, the Ni/La–Al 2O 3 ones with an appropriate amount of promoter enjoy a more open mesoporous structure and higher dispersion of Ni. Reduction kinetic studies of prepared catalysts were investigated by temperature-programmed reduction (TPR) method and the fact that La additive partially destroyed the metastable Ni–Al mixed oxide phase was detailed. 相似文献
10.
Catalytic properties of supported gallium oxides have been examined for the selective reduction of NO by CH 4 in excess oxygen. The activity was greatly affected by the support; Ga 2O 3/Al 2O 3 (Al 2O 3 supported Ga 2O 3) and Ga 2O 3–Al 2O 3 mixed oxide exhibited high activity and selectivity as comparable to Ga-ZSM-5, while unsupported Ga 2O 3 and the other supported Ga 2O 3 were ineffective. For Ga 2O 3/Al 2O 3, the activity changed with Ga 2O 3 content, and was highest at about 30 wt% Ga 2O 3, which corresponds to a theoretical monolayer coverage. Gallium oxide highly dispersed on Al 2O 3 is considered to be responsible for the high activity and selectivity. The reaction characteristics of Ga 2O 3/Al 2O 3 were studied and compared with Ga-ZSM-5 and Co-ZSM-5. Ga 2O 3/Al 2O 3 exhibited the highest activity and selectivity at high temperature. In addition, Ga 2O 3/Al 2O 3 showed higher tolerance against water than Ga-ZSM-5. C 3H 8 and C 3H 6 were also evaluated as reducing agents, and Ga 2O 3/Al 2O 3 showed higher activity than Ga-ZSM-5 above 723 K achieving almost complete reduction of NO to N 2. 相似文献
11.
The effectiveness of Ag/Al 2O 3 catalyst depends greatly on the alumina source used for preparation. A series of alumina-supported catalysts derived from AlOOH, Al 2O 3, and Al(OH) 3 was studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible (UV–vis) spectroscopy, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, O 2, NO + O 2-temperature programmed desorption (TPD), H 2-temperature programmed reduction (TPR), thermal gravimetric analysis (TGA) and activity test, with a focus on the correlation between their redox properties and catalytic behavior towards C 3H 6-selective catalytic reduction (SCR) of NO reaction. The best SCR activity along with a moderated C 3H 6 conversion was achieved over Ag/Al 2O 3 (I) employing AlOOH source. The high density of Ag–O–Al species in Ag/Al 2O 3 (I) is deemed to be crucial for NO selective reduction into N 2. By contrast, a high C 3H 6 conversion simultaneously with a moderate N 2 yield was observed over Ag/Al 2O 3 (II) prepared from a γ-Al 2O 3 source. The larger particles of Ag mO ( m > 2) crystallites were believed to facilitate the propene oxidation therefore leading to a scarcity of reductant for SCR of NO. An amorphous Ag/Al 2O 3 (III) was obtained via employing a Al(OH) 3 source and 500 °C calcination exhibiting a poor SCR performance similar to that for Ag-free Al 2O 3 (I). A subsequent calcination of Ag/Al 2O 3 (III) at 800 °C led to the generation of Ag/Al 2O 3 (IV) catalyst yielding a significant enhancement in both N 2 yield and C 3H 6 conversion, which was attributed to the appearance of γ-phase structure and an increase in surface area. Further thermo treatment at 950 °C for the preparation of Ag/Al 2O 3 (V) accelerated the sintering of Ag clusters resulting in a severe unselective combustion, which competes with SCR of NO reaction. In view of the transient studies, the redox properties of the prepared catalysts were investigated showing an oxidation capability of Ag/Al 2O 3 (II and V) > Ag/Al 2O 3 (IV) > Ag/Al 2O 3 (I) > Ag/Al 2O 3 (III) and Al 2O 3 (I). The formation of nitrate species is an important step for the deNO x process, which can be promoted by increasing O 2 feed concentration as evidenced by NO + O 2-TPD study for Ag/Al 2O 3 (I), achieving a better catalytic performance. 相似文献
12.
The selective catalytic reduction (SCR) of NO by C 3H 6 in excess oxygen was evaluated and compared over Ag/Al 2O 3 and Cu/Al 2O 3 catalysts. Ag/Al 2O 3 showed a high activity for NO reduction. However, Cu/Al 2O 3 showed a high activity for C 3H 6 oxidation. The partial oxidation of C 3H 6 gave surface enolic species and acetate species on the Ag/Al 2O 3, but only an acetate species was clearly observed on the Cu/Al 2O 3. The enolic species is a more active intermediate towards NO + O 2 to yield—NCO species than the acetate species on the Ag/Al 2O 3 catalyst. The Ag and Cu metal loadings and phase changes on Al 2O 3 support can affect the activity and selectivity of Ag/Al 2O 3 and Cu/Al 2O 3 catalysts, but the formation of enolic species is the main reason why the activity of the Ag/Al 2O 3 catalyst for NO reduction is higher than that of the Cu/Al 2O 3 catalyst. 相似文献
13.
The effect of Al 2O 3 levels on the properties of NiO in coprecipitated NiO–Al 2O 3 samples were investigated, using samples with up to 60.7 wt.% Al 2O 3 that had been calcined in the range 300–700°C. Characterization techniques included BET surface area of fresh and reduced catalysts, X-ray diffraction analysis of structure and crystallite size, magnetic susceptibility measurements, oxidizing power, and reducibility in H 2. Only NiO was detected in samples with up to 4.1 wt.% Al 2O 3 for all sample calcination temperatures. Surface areas were similar for all fresh samples but decreased rapidly after calcination at high temperatures. The surface area loss was less for the higher Al 2O 3-containing samples. Nickel oxide crystallite sizes increased at higher calcination temperatures, but remained approximately the same for each Al 2O 3 level. The NiO was nonstoichiometric (NiO1+x), with x decreasing at higher calcination temperatures and increasing with small amounts of added Al2O3 through a maximum at about 3 wt.% Al2O3. However, this did not correlate well with microstrain in the NiO crystallites nor with reducibility, which decreased with Al2O3 addition. At higher levels of Al2O3 (13.6 wt.% and above), surface areas increased with higher Al2O3 loadings, but NiO crystallite sizes remained approximately the same, independent of both Al2O3 content and calcination temperature. X-ray diffraction patterns were very diffuse, and it was not possible to rule out the presence of pseudo-spinel combinations of NiO and Al2O3. Reducibility was more difficult than with low Al2O3 levels, and nonstoichiometry was low and independent of Al2O3 content. Reducibilities of all samples calcined at 300°C correlated well with the final BET surface area of the reduced samples, indicating that more dispersed NiO crystallites are more difficult to reduce, a conclusion that supports a model for reduction proposed previously. 相似文献
14.
Composite types of TiO 2–Al 2O 3 supports, which are γ-aluminas coated by titania, have been prepared by chemical vapor deposition (CVD), using TiCl 4 as a precursor. Then supported molybdenum catalysts have been prepared by an impregnation method. As supports, we employed γ-alumina, anatase types of titania, and composite types of TiO 2–Al 2O 3 with different loadings of TiO 2. We studied the conversion of Mo from oxidic to sulfidic state through sulfurization by X-ray photoelectron spectroscopy (XPS). The obtained spectra unambiguously revealed the higher reducibility from oxidic to sulfidic molybdenum species on the TiO 2 and TiO 2–Al 2O 3 supports compared to that on the Al 2O 3 support. Higher TiO 2 loadings of the TiO 2–Al 2O 3 composite support led to higher reducibility for molybdenum species. Furthermore, the catalytic behavior of supported molybdenum catalysts has been investigated for hydrodesulfurization (HDS) of dibenzothiophene (DBT) and methyl-substituted DBT derivatives. The conversion over the TiO 2–Al 2O 3 supported Mo catalysts, in particular for the 4,6-dimethyl-DBT, is much higher than that obtained over Al 2O 3 supported Mo catalyst. The ratio of the corresponding cyclohexylbenzene (CHB)/biphenyl (BP) derivatives is increased over the Mo/TiO 2–Al 2O 3. This indicates that the prehydrogenation of an aromatic ring plays an important role in the HDS of DBT derivatives over TiO 2–Al 2O 3 supported catalysts. 相似文献
15.
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
16.
通过浸渍法制备了Al_2O_3负载的Pd和Pt催化剂,考察催化剂的甲烷、乙烷和丙烷催化燃烧活性,以及助剂Ba对催化性能的影响。对于Pd/Al_2O_3催化剂,加入Ba使活性物种PdO颗粒变大和还原温度升高,形成更稳定的PdO活性物种,是Pd-Ba/Al_2O_3催化剂活性提升的主要原因。对于Pt/Al_2O_3催化剂,加入Ba助剂使活性物种Pt0含量降低,PtO_x与Al_2O_3载体相互作用增强,使PtO_x物种更难被还原为Pt~0,导致Pt-Ba/Al_2O_3催化剂活性降低。Pd和Pt催化剂催化烷烃氧化反应活性规律一致:丙烷乙烷甲烷。Pd/Al_2O_3催化剂有利于C—H键活化,Pt/Al_2O_3催化剂有利于C—C键活化。Pt/Al_2O_3催化剂对C1-C3烷烃氧化活性的差别明显大于Pd/Al_2O_3催化剂。Pt/Al_2O_3催化剂对碳比例高的烷烃活性更高。 相似文献
17.
Damage resistance and R-curve behavior of multilayer Al 2O 3/SiC ceramics were evaluated in bending by the indentation-strength and the single-edge-notched-beam methods. Due to the crack deflection at the Al 2O 3/SiC interfaces, a plateau indentation strength response was achieved, suggesting an exceptional resistance to contact-induced damage. Moreover, fracture toughness was observed to increase from 8.0 to 15.5 MPa m 1/2 with increasing notch depth from 0.5 to 2.0 mm, indicative of a strong R-curve behavior. 相似文献
18.
The surface of as-prepared LiMn 2O 4 was modified with Al 2O 3 by a melting impregnation method. X-ray diffraction, field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) studies indicated that Al 2O 3 nano-particles are distributed around the spinel. X ray absorption fine structure analysis (XAFS) further demonstrated that Al atoms were also doped to the surface of LiMn 2O 4 particles. The nano-Al 2O 3 particle coating improves the capacity retention of spinel LiMn 2O 4 efficiently at both room temperature and 55 °C. The mechanism of improvement for surface modified LiMn 2O 4 can be attributed to the inhibition of a surface Jahn-Teller distortion and the decrease of manganese dissolution, leading to good electric contact among particles. 相似文献
19.
Surface-phase ZrO 2 on SiO 2 (SZrOs) and surface-phase La 2O 3 on Al 2O 3 (SLaOs) were prepared with various loadings of ZrO 2 and La 2O 3, characterized and used as supports for preparing Pt/SZrOs and Pt/SLaOs catalysts. CH 4/CO 2 reforming over the Pt/SZrOs and Pt/SLaOs catalysts was examined and compared with Pt/Al 2O 3 and Pt/SiO 2 catalysts. CO 2 or CH 4 pulse reaction/adsorption analysis was employed to elucidate the effects of these surface-phase oxides. The zirconia can be homogeneously dispersed on SiO2 to form a stable surface-phase oxide. The lanthana cannot be spread well on Al2O3, but it forms a stable amorphous oxide with Al2O3. The Pt/SZrOs and Pt/SLaOs catalysts showed higher steady activity than did Pt/SiO2 and Pt/Al2O3 by a factor of three to four. The Pt/SZrOs and Pt/SLaOs catalysts were also much more stable than the Pt/SiO2 and Pt/Al2O3 catalysts for long stream time and for reforming temperatures above 700 °C. These findings were attributed to the activation of CO2 adsorbed on the basic sites of SZrOs and SLaOs. 相似文献
20.
Catalytic reduction of NO by propene in the presence of oxygen was studied over SnO 2-doped Ga 2O 3–Al 2O 3 prepared by sol–gel method. Although SnO 2-doped Ga 2O 3–Al 2O 3 gave lower NO conversion than Ga 2O 3–Al 2O 3 in the absence of H 2O, the activity was enhanced considerably by the presence of H 2O and much higher than that of Ga 2O 3–Al 2O 3. The presence of SnO 2 and Ga 2O 3–Al 2O 3 species having intimate Ga–O–Al bondings was found to be essential for the promotional effect of H 2O. The promotional effect of H 2O was interpreted by the following two reasons. The first one is the removal of carbonaceous materials deposited on the catalyst surface by H 2O. The other is the selective inhibition by H 2O of the reaction steps resulting in propene oxidation to CO x without reducing NO. 相似文献
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