Development of Al2O3–SiC composite tool for machining application

Al₂O₃–SiC composites containing up to 30 wt.% of dispersed SiC particles (280 nm) were fabricated via hot-pressing and machined as cutting tools. The Al₂O₃–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 Al₂O₃–10 wt.% SiC composite tool showed the longest tool life, seven times longer than a commercial tool made of Al₂O₃–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 Al₂O₃–SiC composite tools attributes to the transformation of fracture mode from intergranular fracture for Al₂O₃ to intragranular fracture for Al₂O₃–SiC composites.     

Microstructure and flexure creep behaviour of SiC-particle reinforced Al2O3 matrix composites

The flexure creep behaviour of monolithic Al₂O₃ and 10 vol% SiC-particle reinforced Al₂O₃ 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% SiO₂ impurities and the other of average size 2.7 μm with 3.4 vol% SiO₂ impurities. Compared with the creep behaviour of monolithic Al₂O₃ 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 Al₂O₃ 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 Al₂O₃ matrix grains. It was revealed that the entrapment of 2.7 μm SiC particles into Al₂O₃ matrix grains was related to the high SiO₂ 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.     

Effect of Al2O3 on mechanical properties of Ti3SiC2/Al2O3 composite

The effect of Al₂O₃ on mechanical properties of Ti₃SiC₂/Al₂O₃ composite fabricated by SPS was studied systematically. The results show that the hardness of the Ti₃SiC₂/Al₂O₃ composite can reach 10.28 GPa, 50% higher than that of pure Ti₃SiC₂. However, slight decrease in the other mechanical properties was observed with Al₂O₃ addition higher than 5–10 vol.%, which is believed to be due to the agglomeration of Al₂O₃ in the composite.     

Processing and microstructure development in Al2O3-SiC ‘nanocomposites’

Composites consisting of Al₂O₃ + 5 vol.% 0·15 μm SiC particles were prepared by pressureless sintering. The optimum conditions for achieving dense and uniform microstructures by conventional ceramic processing are given in detail. The SiC particles were found to strongly inhibit grain growth of the Al₂O₃ matrix. Densification was also significantly retarded by these ultra-fine particles, and possible explanations for this behavior are discussed.     

Gas pressure sintering of Al2O3/TiCN composite

Al₂O₃/TiCN composites have been fabricated by gas pressure sintering, which overcomes the limitations of hot pressing. The densification behavior and mechanical properties of the Al₂O₃ gas pressure sintered with 30 wt.% TiCN at different temperatures have been investigated. The gas pressure sintered Al₂O₃–30 wt.%TiCN composite achieved a relative density of 99.5%, a bending strength of 772 MPa, a hardness of 19.6 GPa, and a fracture toughness of 5.82 MPa·M^1/2.     

Phase diagram of the system: Al2O3–ZrO2

The system Al₂O₃–ZrO₂ was studied by differential thermal analysis in inert atmosphere and in vacuum. The eutectic was located at 1866°C and 40% mass of ZrO₂. Zirconia solid solution at the eutectic temperature is up to 1.1±0.3% mass of Al₂O₃. Enthalpy of melting of this eutectic is 1080±90 J/g. Pure ZrO₂ transforms from monoclinic to tetragonal at 1162±7°C, but the saturated solid solution of ZrO₂, with 0.7±0.2% mass Al₂O₃ at this temperature, transforms at 1085±5°C. Inverse transitions occur with hysteresis correspondingly at 1055±5 and 995±5°C. Enthalpy of transformation of pure ZrO₂ from monoclinic to tetragonal phase is 42±5 J/g (5.2±0.6 J/mol) but only 30±5 J/g for a ZrO₂ saturated solid solution.     

MoSi2–Al2O3 electroconductive ceramic composites

Al₂O₃–MoSi₂ composites with MoSi₂ volume fractions between 16 and 40% were fabricated from commercial ceramic Al₂O₃ and intermetallic MoSi₂ powders by granulation, cold isostatic pressing and vacuum-sintering. The addition of MoSi₂ had only a slight influence on the densification of the composites, with sintered densities of 98% for samples with 16 vol.% MoSi₂ and 94% for samples with 40 vol.% MoSi₂. Composites with MoSi₂ contents of 20 vol.% and higher were electroconductive due to the formation of a three-dimensional percolating network of the conductive MoSi₂ phase.     

A comparative study of Ag/Al2O3 and Cu/Al2O3 catalysts for the selective catalytic reduction of NO by C3H6

The selective catalytic reduction (SCR) of NO by C₃H₆ in excess oxygen was evaluated and compared over Ag/Al₂O₃ and Cu/Al₂O₃ catalysts. Ag/Al₂O₃ showed a high activity for NO reduction. However, Cu/Al₂O₃ showed a high activity for C₃H₆ oxidation. The partial oxidation of C₃H₆ gave surface enolic species and acetate species on the Ag/Al₂O₃, but only an acetate species was clearly observed on the Cu/Al₂O₃. The enolic species is a more active intermediate towards NO + O₂ to yield—NCO species than the acetate species on the Ag/Al₂O₃ catalyst. The Ag and Cu metal loadings and phase changes on Al₂O₃ support can affect the activity and selectivity of Ag/Al₂O₃ and Cu/Al₂O₃ catalysts, but the formation of enolic species is the main reason why the activity of the Ag/Al₂O₃ catalyst for NO reduction is higher than that of the Cu/Al₂O₃ catalyst.     

Mechanical properties of Al2O3 + ZrO2 + nano-SiCp ceramic composites

The mechanical properties of Al₂O₃ matrix composites reinforced by ZrO₂(2 mol% Y₂O₃) and nanometre scale SiC dispersions have been investigated. It is shown that the Al₂O₃ matrix is simultaneously strengthened and toughened by both ZrO₂(2 mol% Y₂O₃) 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 ZrO₂ (2 mol% Y₂O₃) and nano-SiC particles appears to be synergetic.     

Polysilazane derived micro/nano Si3N4/SiC composites

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 Si₃N₄/SiC composites by hot pressing. Y₂O₃–Al₂O₃ and Y₂O₃–Yb₂O₃ 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 Si₃N₄ and SiC phases. The fine SiC nano-inclusions were identified within the Si₃N₄ micrograins. Phase composition of both composites consist of , β modifications of Si₃N₄ 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.% Y₂O₃ and 6 wt.% Yb₂O₃, HV 23 GPa. This is a consequence of the highest SiC content as well as the chemical composition of additives.     

Selective catalytic reduction of NO by hydrocarbons on Ga2O3/Al2O3 catalysts

Catalytic properties of supported gallium oxides have been examined for the selective reduction of NO by CH₄ in excess oxygen. The activity was greatly affected by the support; Ga₂O₃/Al₂O₃ (Al₂O₃ supported Ga₂O₃) and Ga₂O₃–Al₂O₃ mixed oxide exhibited high activity and selectivity as comparable to Ga-ZSM-5, while unsupported Ga₂O₃ and the other supported Ga₂O₃ were ineffective. For Ga₂O₃/Al₂O₃, the activity changed with Ga₂O₃ content, and was highest at about 30 wt% Ga₂O₃, which corresponds to a theoretical monolayer coverage. Gallium oxide highly dispersed on Al₂O₃ is considered to be responsible for the high activity and selectivity. The reaction characteristics of Ga₂O₃/Al₂O₃ were studied and compared with Ga-ZSM-5 and Co-ZSM-5. Ga₂O₃/Al₂O₃ exhibited the highest activity and selectivity at high temperature. In addition, Ga₂O₃/Al₂O₃ showed higher tolerance against water than Ga-ZSM-5. C₃H₈ and C₃H₆ were also evaluated as reducing agents, and Ga₂O₃/Al₂O₃ showed higher activity than Ga-ZSM-5 above 723 K achieving almost complete reduction of NO to N₂.     

Microstructure and mechanical properties of gas pressure sintered Al2O3/TiCN composite

Al₂O₃–30 wt.%TiCN composites have been fabricated successfully by a two-stage gas pressure sintering schedule. The gas pressure sintered Al₂O₃–30 wt.%TiCN composite achieved a relative density of 99.5%, a bending strength of 772 MPa, a hardness of 19.6 GPa, and a fracture toughness of 5.82 MPa m^1/2. The fabrication procedure involves solid state sintering of two phases without solubility to prepare Al₂O₃–TiCN composite. Little grain growth occurred for TiCN during sintering while Al₂O₃ grains grew about three times to an average size of 3–5 μm. The interface microstress arising during cooling from the processing temperature because of the thermal and/or mechanical properties mismatch between the Al₂O₃ and TiCN phase is about 50 MPa. Such a compressive microstress is not high enough to cause grain boundary cracking that may weaken the composite but it can introduce dislocations within grains, which is very good to enhance the composite properties.     

Lean reduction of NO by C3H6 over Ag/alumina derived from Al2O3, AlOOH and Al(OH)3

The effectiveness of Ag/Al₂O₃ catalyst depends greatly on the alumina source used for preparation. A series of alumina-supported catalysts derived from AlOOH, Al₂O₃, 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, O₂, NO + O₂-temperature programmed desorption (TPD), H₂-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 C₃H₆-selective catalytic reduction (SCR) of NO reaction. The best SCR activity along with a moderated C₃H₆ conversion was achieved over Ag/Al₂O₃ (I) employing AlOOH source. The high density of Ag–O–Al species in Ag/Al₂O₃ (I) is deemed to be crucial for NO selective reduction into N₂. By contrast, a high C₃H₆ conversion simultaneously with a moderate N₂ yield was observed over Ag/Al₂O₃ (II) prepared from a γ-Al₂O₃ 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/Al₂O₃ (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 Al₂O₃ (I). A subsequent calcination of Ag/Al₂O₃ (III) at 800 °C led to the generation of Ag/Al₂O₃ (IV) catalyst yielding a significant enhancement in both N₂ yield and C₃H₆ 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/Al₂O₃ (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/Al₂O₃ (II and V) > Ag/Al₂O₃ (IV) > Ag/Al₂O₃ (I) > Ag/Al₂O₃ (III) and Al₂O₃ (I). The formation of nitrate species is an important step for the deNO_x process, which can be promoted by increasing O₂ feed concentration as evidenced by NO + O₂-TPD study for Ag/Al₂O₃ (I), achieving a better catalytic performance.     

Ru-Fe/Al_2O_3氧化-还原性能探究

以Al_2O_3为载体,RuCl_3·xH_2O和FeCl_3·6H_2O为活性组分前驱体,采用吸附-沉淀法制备了Ru-Fe/Al_2O_3和Ru/Al_2O_3催化剂,以马来酸二甲酯加氢合成丁二酸二甲酯为探针反应,结合H_2-TPR和XRD表征技术,考察Fe改性Ru基催化剂的氧化-还原性能及催化活性。经氧化-还原循环处理后,催化剂Ru-Fe/Al_2O_3上马来酸二甲酯加氢活性高于Ru/Al_2O_3。XRD结果显示,经处理的Ru-Fe/Al_2O_3上未见金属Ru的特征衍射峰,而Ru/Al_2O_3上出现了金属Ru的特征衍射峰。结合H_2-TPR结果推断,Ru与Fe之间发生了相互作用,这种协同作用可以改善Ru/Al_2O_3催化剂的热稳定性。     

Hydrodesulfurization of dibenzothiophenes over molybdenum catalyst supported on TiO2–Al2O3

Composite types of TiO₂–Al₂O₃ supports, which are γ-aluminas coated by titania, have been prepared by chemical vapor deposition (CVD), using TiCl₄ 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 TiO₂–Al₂O₃ with different loadings of TiO₂. 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 TiO₂ and TiO₂–Al₂O₃ supports compared to that on the Al₂O₃ support. Higher TiO₂ loadings of the TiO₂–Al₂O₃ 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 TiO₂–Al₂O₃ supported Mo catalysts, in particular for the 4,6-dimethyl-DBT, is much higher than that obtained over Al₂O₃ supported Mo catalyst. The ratio of the corresponding cyclohexylbenzene (CHB)/biphenyl (BP) derivatives is increased over the Mo/TiO₂–Al₂O₃. This indicates that the prehydrogenation of an aromatic ring plays an important role in the HDS of DBT derivatives over TiO₂–Al₂O₃ supported catalysts.     

Mean field modelling of NOx storage on Pt/BaO/Al2O3

A mean field model, for storage and desorption of NO_x in a Pt/BaO/Al₂O₃ catalyst is developed using data from flow reactor experiments. This relatively complex system is divided into five smaller sub-systems and the model is divided into the following steps: (i) NO oxidation on Pt/Al₂O₃; (ii) NO oxidation on Pt/BaO/Al₂O₃; (iii) NO_x storage on BaO/Al₂O₃; (iv) NO_x storage on Pt/BaO/Al₂O₃ with thermal regeneration and (v) NO_x storage on Pt/BaO/Al₂O₃ with regeneration using C₃H₆. In this paper, we focus on the last sub-system. The kinetic model for NO_x storage on Pt/BaO/Al₂O₃ was constructed with kinetic parameters obtained from the NO oxidation model together with a NO_x storage model on BaO/Al₂O₃. This model was not sufficient to describe the NO_x storage experiments for the Pt/BaO/Al₂O₃, because the NO_x desorption in TPD experiments was larger for Pt/BaO/Al₂O₃, compared to BaO/Al₂O₃. The model was therefore modified by adding a reversible spill-over step. Further, the model was validated with additional experiments, which showed that NO significantly promoted desorption of NO_x from Pt/BaO/Al₂O₃. To this NO_x storage model, additional steps were added to describe the reduction by hydrocarbon in experiments with NO₂ and C₃H₆. The main reactions for continuous reduction of NO_x occurs on Pt by reactions between hydrocarbon species and NO in the model. The model is also able to describe the reduction phase, the storage and NO breakthrough peaks, observed in experiments.     

Ethanol steam reforming over Ni/La–Al2O3 catalysts: Influence of lanthanum loading

Hydrogen production from ethanol reforming over nickel catalysts supported on lanthanum loaded Al₂O₃ substrates was studied. Activity results revealed the enhancement in the reforming stability of the Ni catalysts with the increase in the lanthanum loading on Al₂O₃ substrates. Catalytic behavior of Ni/La–Al₂O₃ catalysts in the ethanol steam reforming was found to be the contribution of the activity of the La–Al₂O₃ supports for the ethanol dehydration reaction and the activity of the nickel metallic phase that catalyzes both dehydrogenation and CC bond rupture. Physicochemical characterization of catalysts revealed that acidity, nickel dispersion and nickel-support interaction depend on the La-loading on Al₂O₃. The better reforming stability of catalysts with the increase in La content was explained in terms of the ability of nickel surface and/or La–Ni interactions to prevent the formation of carbon filaments.     

ZrO2/SiO2- and La2O3/Al2O3-supported platinum catalysts for CH4/CO2 reforming

Surface-phase ZrO₂ on SiO₂ (SZrOs) and surface-phase La₂O₃ on Al₂O₃ (SLaOs) were prepared with various loadings of ZrO₂ and La₂O₃, characterized and used as supports for preparing Pt/SZrOs and Pt/SLaOs catalysts. CH₄/CO₂ reforming over the Pt/SZrOs and Pt/SLaOs catalysts was examined and compared with Pt/Al₂O₃ and Pt/SiO₂ catalysts. CO₂ or CH₄ 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.     

Effect of the addition of transition metals to Pt/Ba/Al2O3 catalyst on the NOx storage-reduction catalysis under oxidizing conditions in the presence of SO2

The NO_x storage-reduction catalysis under oxidizing conditions in the presence of SO₂ has been investigated on Pt/Ba/Fe/Al₂O₃, Pt/Ba/Co/Al₂O₃, Pt/Ba/Ni/Al₂O₃, and Pt/Ba/Cu/Al₂O₃ catalysts compared with Pt/Ba/Al₂O₃, Pt/Fe/Al₂O₃, Pt/Co/Al₂O₃, Pt/Ni/Al₂O₃, Pt/Cu/Al₂O₃ and Pt/Al₂O₃ catalysts. The NO_x purification activity of Pt/Ba/Fe/Al₂O₃ catalyst was the highest of all the catalysts investigated in this paper after an aging treatment. That of the aged Pt/Ba/Co/Al₂O₃ and Pt/Ba/Ni/Al₂O₃ catalysts was essentially the same as that of the aged Pt/Ba/Al₂O₃ catalyst, while that of the aged Pt/Ba/Cu/Al₂O₃ and Pt/Cu/Al₂O₃ catalysts was substantially lower than the others.

The Fe-compound on the aged Pt/Ba/Fe/Al₂O₃ catalyst has played a role in decreasing the sulfur content on the catalyst after exposure to simulated reducing gas compared with the Pt/Ba/Al₂O₃ catalyst without the Fe-compound. XRD and EDX show that the Fe-compound inhibits the growth in the size of BaSO₄ particles formed on the Pt/Ba/Fe/Al₂O₃ catalyst under oxidizing conditions in the presence of SO₂ and promotes the decomposition of BaSO₄ and desorption of the sulfur compound under reducing conditions.     

Processing of hot pressed Al2O3–Cr2O3/Cr-carbide nanocomposite prepared by MOCVD in fluidized bed

Nanosized particles dispersed uniformly on Al₂O₃ 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 Cr₂O₃, CrC_1−x, and C. A solid solution of Al₂O₃–Cr₂O₃ and an Al₂O₃–Cr₂O₃/Cr₃C₂ 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 Al₂O₃–Cr₂O₃/Cr-carbide (Cr₃C₂ and Cr₇C₃) nanocomposite was formed when the particles were directly hot pressed at 1400 °C. The interface between Cr₃C₂ and Al₂O₃ is non-coherent, while the interface between Cr₇C₃ and Al₂O₃ is semi-coherent.