Structural properties of Al2O3-La2O3 binary oxides prepared by sol-gel

The structural properties of La₂O₃ and Al₂O₃-La₂O₃ binary oxides prepared by sol-gel were studied by XRD, HRTEM and UV-vis. The binary oxides with high lanthana contents show an amorphous structure after calcination at 650 °C. At calcination temperatures higher than 1000 °C there is a phase transformation from the amorphous state to the crystalline LaAlO₃ with a perovskite structure. The structure of La₂O₃ is consistent with the hexagonal system; however, some crystalline microdomains with a monoclinic structure were detected by HRTEM. Islands of La₂O₃ and LaAl₁₁O₁₈ phases were detected at high lanthana concentration in the binary oxide. The modification in the coordination shell of the Al³⁺ cations due to the interaction with La³⁺ cations confirms the formation of phases with a perovskite structure and the presence of islands of the LaAl₁₁O₁₈ phase.     

Machinable Al2O3/BN composite ceramics with strong mechanical properties

Al₂O₃/BN composite ceramics with nano-sized BN dispersions ranging from 0 to 30 vol.% were successfully fabricated by hot-pressing α-Al₂O₃ powders with turbostratic BN (t-BN) coating, which was prepared through chemical processes using boric acid and urea. SEM observations revealed that the nano-sized hexagonal BN (h-BN) particulates were homogeneously dispersed within Al₂O₃ grains as well as at grain boundaries. Vickers hardness of materials decreased with an increase in BN content. The fracture toughness was improved but the fracture strength had a small decrease, in comparison to Al₂O₃ monolithic ceramics. The nanocomposite ceramics with BN content more than 20 vol.% exhibited excellent machinability, which could be drilled using conventional hard metal alloy drills. Drilling rates and normal forces demonstrate the ease of machining of these materials. The preliminary information on the relationship between microstructures and properties are provided. The mechanism of material removal is also discussed.     

Effect of Al2O3 particle size on vacuum breakdown behavior of Al2O3/Cu composite

In order to clarify the effect of Al₂O₃ particle size on the arc erosion behavior of the ceramic-reinforced Al₂O₃/Cu composite, Al₂O₃/Cu composites with different sizes of Al₂O₃ particles were prepared by powder metallurgy, the effect of Al₂O₃ particle size on the characteristics of arc motion was studied, and the mechanism of arc erosion of Al₂O₃/Cu composites was discussed as well. The results show that with decrease in the size of Al₂O₃ particles, the erosion area increases significantly and the erosion pits become shallower. The vacuum breakdown is preferred to appear in the area between Al₂O₃ particle and the copper matrix. Based on the experimental results and theoretical analysis, a particle partition arc model is proposed.     

Nanorod alumina-supported Ni-Zr-Fe/Al2O3 catalysts for hydrogen production in auto-thermal reforming of ethanol

Nanorod alumina-supported Ni-Zr-Fe/Al₂O₃ catalysts were prepared by co-impregnation, characterized by TEM, TPR, XRD, XPS, and TPD-pyridine, and tested in auto-thermal reforming of ethanol. The characterization results indicate that, with iron and zirconia promotion, the Ni_xFe_1−xAl₂O₄ mixture spinel forms, the valence of the surface Ni species is modified, and the acidity decreases. As a result, during a 30-h test over the Ni-Zr-Fe/Al₂O₃ catalyst, sintering is restrained, and the selectivity to hydrogen remains around 85.79% without obvious loss, while the un-promoted Ni/Al₂O₃ shows poor stability and selectivity.     

Phases ε obtenues par decomposition des spinelles non-stoechiometriques dans les systemes Ga2O3-MgO,Al2O3-NiO,Al2O3-AlN et Al2O3-Li2O

In the systems Al₂O₃-AlN, Al₂O₃-NiO, Al₂O₃-Li₂O and Ga₂O₃-MgO, non-stoechiometric spinels, when decomposed at high temperatures, form an intermediate metastable phase ε. This phase has a one-dimensional periodic antiphase-domain structure. The antiphase boundaries are parallel to the (310) plane and the displacement vector is $\text{1}\text{4}$ [110] when referred to the spinel structure. Across each antiphase boundary, some octahedral and tetrahedral sites share faces instead of corners. In the case of ε_AlN, cationic vacancies occupy these sites; in the other cases, this is likely to go along with a segregation of the more charged ions.     

Influence of cobalt phase on thermal shock resistance of Al2O3-TiC composites evaluated by indentation technique

Cobalt-coated Al₂O₃ and TiC powders were prepared using an electroless method to improve resistance to thermal shock. The mixture of cobalt-coated Al₂O₃ and TiC powders (about 70 wt.% Al₂O₃-Co + 30 wt.% TiC-Co) was hot-pressed into an Al₂O₃-TiC-Co composite. The thermal shock properties of the composite were evaluated by indentation technique and compared with the traditional Al₂O₃-TiC composite. The composites containing 3.96 vol.% cobalt exhibited better resistance to crack propagation, cyclic thermal shock and higher critical temperature difference (ΔT_c). The calculation of thermal shock resistance parameters (R parameters) shows that the incorporation of cobalt improves the resistance to thermal shock fracture and thermal shock damage. The thermal physic parameters are changed very little but the flexure strength and fracture toughness of the composites are improved greatly by introducing cobalt into Al₂O₃-TiC (AT) composites. The better thermal shock resistance of the composites should be attributed to the higher flexure strength and fracture toughness.     

Sintering and dielectric properties of 0.88Al2O3-0.12TiO2 microwave ceramics by glass addition

The microwave characteristics and the microstructures of 0.88Al₂O₃-0.12TiO₂ with various amounts of MgO-CaO-SiO₂-Al₂O₃ (MCAS) glass sintered at different temperatures have been investigated. The sintering temperature can be lowered to 1300 °C by the addition of MCAS glass. The densities, dielectric constants (ε_r) and quality values (Q×f) of the MCAS-added 0.88Al₂O₃-0.12TiO₂ ceramics decrease with the increase of MCAS glass content. The temperature coefficients of the resonant frequency (τ_f) are shifted to more negative values as the MCAS content or the sintering temperatures increase. The change of the crystalline phases of Al₂TiO₅ phase and rutile-TiO₂ phase has profound effects on the microwave dielectric properties of the MCAS-added Al₂O₃-TiO₂ ceramics. As sintered at 1250 °C, 0.88Al₂O₃-0.12TiO₂ ceramics with 2 wt.% MCAS glass addition exists a ε_r value of 8.63, a Q×f value of 9578 and a τ_f value of +5 ppm/°C.     

Preparation and photoluminescence of Er-doped Al2O3 films by sol-gel method

The 0-1.5 mol% Er³⁺-doped Al₂O₃ films have been prepared on the thermally oxidized SiO₂/Si(100) substrate in the dip-coating process by the sol-gel method, using the aluminium isopropoxide [Al(OC₃H₇)₃]-derived γ-AlOOH sols with the addition of erbium nitrate [Er(NO₃)₃·5H₂O]. The continuous Er³⁺-doped Al₂O₃ films with the thickness of about 1.2 μm were obtained for nine coating cycles at a sintering temperature of 900 °C. The aggregate size for the Er³⁺-doped Al₂O₃ films increased with increasing the Er³⁺ doping concentration from 0 to 1.5 mol%. The root-mean-square roughness of the films was independent on the Er³⁺ doping, which was about 1.8 nm for the 0-1.5 mol% Er³⁺-doped Al₂O₃ films. The γ-Al₂O₃ phase with a (110) preferred orientation was produced for the Al₂O₃ film. The photoluminescence (PL) spectra of 0.1-1.5 mol% Er³⁺-doped Al₂O₃ films were observed at the measurement temperature of 10 K. There was no significant change for the PL peak intensity with the increase of Er³⁺ doping concentration from 0.1 to 1.5 mol%, and similar full width at half maximum of about 40 nm was detected for the 0.1-1.5 mol% Er³⁺-doped Al₂O₃ thin films. The Er³⁺-doped Al₂O₃ films possess the available PL properties for use in planar optical waveguides.     

Complete oxidation of acetaldehyde on Pt/CeO2-ZrO2-Bi2O3 catalysts

Pt/CeO₂-ZrO₂-Bi₂O₃ catalysts for catalytic combustion of acetaldehyde, which is one of volatile organic compounds (VOCs), were prepared by a wet impregnation method in the presence of polyvinylpyrrolidone K25 (PVP). The addition of PVP in the preparation process was effective to enhance the specific surface area and the Pt²⁺ ratio on the surface. Additionally, the pore volume and size of the catalysts were modified by the PVP addition. The Pt/CeO₂-ZrO₂-Bi₂O₃ catalysts are specific for the total acetaldehyde oxidation and CO and any acetaldehyde-derivative compounds were not observed as by-products. The catalytic activity of the Pt/CeO₂-ZrO₂-Bi₂O₃ catalysts was significantly promoted by the PVP addition and the total oxidation temperature decreased. By the optimization of the amount of platinum, the complete oxidation of acetaldehyde was realized at a temperature as low as 140 °C on a 10 wt%Pt/CeO₂-ZrO₂-Bi₂O₃ catalyst.     

The influence of the molar ratio of Al2O3 to Y2O3 on sintering behavior and the mechanical properties of a SiC–Al2O3–Y2O3 ceramic composite

The influence of the molar ratio of Al₂O₃ to Y₂O₃ (i.e. M_Al2O3/M_Y2O3) on sintering densification, microstructure and the mechanical properties of a SiC–Al₂O₃–Y₂O₃ ceramic composite were studied. It was shown that the optimal value of M_Al2O3/M_Y2O3 was 3/2, not 5/3, which is customarily considered the optimal molar ratio for the formation of YAG (Y₃Al₅O₁₂) phase. When M_Al2O3/M_Y2O3 is 5/3, materials existed in two phases of YAG and very little YAM phases. The sintering mechanism of the solid phase occurred at 1850 °C. When M_Al2O3/M_Y2O3 was 3/2, materials existed in the two phases YAG (Y₃Al₅O₁₂) and YAM (Y₄Al₂O₉). The formation of the low melting point eutectic liquid phase (YAG + YAM) increased sintering densification. Flexure strength, hardness and relative density were all higher.     

Lamellar Fe/Al2O3 catalyst for high-yield production of multi-walled carbon nanotubes bundles

The lamellar Fe/Al₂O₃ catalysts were prepared by sol-gel method, and then with these prepared catalysts, carbon nanotubes (CNTs) were synthesized by catalytic chemical vapor deposition (CCVD) method using C₂H₂ as precursor. The as-prepared CNTs and Fe/Al₂O₃ catalysts were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectrum. The results proved that the as-prepared CNTs actually existed in bundles. And the growth of CNTs bundles should be attributed to the lamellar catalysts, which supported the bottom growth mechanism of CNTs. The transition metal of Mo was not introduced in catalysts to produce CNTs bundles, which was different with others’ results.     

Low-temperature performance of Al4B2O9 nanowires

In this paper we report on the synthetic investigation of single-crystalline aluminum borate (Al₄B₂O₉) nanowires in large scale by a direct calcination of a precursor powder made of Na₂B₄O₇·10H₂O and Al (NO₃)₃·9H₂O at a low temperature of 850 °C. The nanowires, with the diameter of 20-40 nm and the length up to several micrometers, possess smooth surfaces and uniform sizes along the entire wire. The growth mechanism of the nanowires is attributed to a solid-liquid-solid process, which controls the nanowire morphology.     

Multi-walled carbon nanotubes reinforced Al2O3 nanocomposites: Mechanical properties and interfacial investigations

Well-dispersed multi-walled carbon nanotubes (CNTs) reinforced Al₂O₃ nanocomposites were successfully fabricated by hot-pressing. The resulting promising improvements in fracture toughness, by 94% and 65% with 2 and 5 wt.% CNTs addition respectively, compared with monolithic Al₂O₃, were attributed to the good dispersion of CNTs within the matrix, crack-bridging by CNTs and strong interfacial connections between the CNTs and the matrix. The interfacial phase characteristics between CNTs and Al₂O₃ were investigated via combined techniques. It is believed that a possible aluminium oxy-carbide as the primary interfacial phase was produced via a localized carbothermal reduction process. This interface phase presumably has good chemical compatibility and strong connections with both CNTs and the matrix and led nanocomposites to higher fracture toughness.     

Charge trapping characteristics of Al2O3/Al-rich Al2O3/SiO2 stacked films fabricated by radio-frequency magnetron co-sputtering

A thin-film structure comprising Al₂O₃/Al-rich Al₂O₃/SiO₂ was fabricated on Si substrate. We used radio-frequency magnetron co-sputtering with Al metal plates set on an Al₂O₃ target to fabricate the Al-rich Al₂O₃ thin film, which is used as a charge storage layer for nonvolatile Al₂O₃ memory. We investigated the charge trapping characteristics of the film. When the applied voltage between the gate and the substrate is increased, the hysteresis window of capacitance-voltage (C-V) characteristics becomes larger, which is caused by the charge trapping in the film. For a fabricated Al-O capacitor structure, we clarified experimentally that the maximum capacitance in the C-V hysteresis agrees well with the series capacitance of insulators and that the minimum capacitance agrees well with the series capacitance of the semiconductor depletion layer and stacked insulator. When the Al content in the Al-rich Al₂O₃ is increased, a large charge trap density is obtained. When the Al content in the Al-O is changed from 40 to 58%, the charge trap density increases from 0 to 18 × 10¹⁸ cm^− 3, which is 2.6 times larger than that of the trap memory using SiN as the charge storage layer. The device structure would be promising for low-cost nonvolatile memory.     

Preparation and oxidation of an Al2O3-modified aluminide coating

An Al₂O₃-modified aluminide coating was produced by aluminizing an as-electrodeposited Ni-Al₂O₃ nanocomposite film using pack cementation method at 1100  C for 4 h. For comparison, aluminizing was also performed in the same condition on an as-deposited Ni film without Al₂O₃ nanoparticles. SEM/EDAX and TEM results indicated that the co-deposited Al₂O₃ nanoparticles were homogeneously dispersed in the finer-grain nanocrystalline Ni grains. The isothermal and cyclic oxidation in air at 900 °C indicated that the Al₂O₃-modified aluminide coatings were profoundly oxidation resistant as compared to the Al₂O₃-free aluminide coatings due to the faster formation of a continuous adherent α-Al₂O₃ scale. The effect of Al₂O₃ on the microstructure and the oxidation of the aluminide coating are discussed in the detail.     

Synthesis and characterization of Al2O3 hollow spheres

This paper presents a novel technique to create Al₂O₃ hollow spherical nanoparticles. It used Al(OH)₃ which was synthesized with Al₂(SO₄)₃ and NaOH, and the C-Al(OH)₃ core-shell nanoparticle as intermediate phases. The Al₂O₃ hollow spheres were achieved by the calcination of the carbon cores and the dehydration of Al(OH)₃. The chemical composition, morphology, size and superficial crystal structure of the nanoparticles were characterized with TEM, XRD, TGA, FTIR and BET. The result shows that the average diameter of the C-Al(OH)₃ core-shell nanoparticles is about 25 nm, the thickness of the Al₂O₃ shell is about 5 nm and the surface area is 215.2 m²/g. The procedure for the formation of Al₂O₃ hollow nanoparticles is discussed in details.     

On structure and mechanical properties of ultrasonically cast Al-2% Al2O3 nanocomposite

An investigation on the structure of an ultrasonically cast nanocomposite of Al with 2 wt.% nano-sized Al₂O₃ (average size ∼10 nm) dispersoids showed that the nanocomposite was consisting of nearly continuous nano-alumina dispersed zones (NDZs) in the vicinity of the grain boundaries encapsulating Al₂O₃ depleted zones (ADZs). The mechanical properties were investigated by nanoindentation and tensile tests. The nano-sized dispersoids caused a marginal increase in the elastic modulus, and a significant increase in the hardness (∼92%), and tensile strength (∼48%). Subsequent cold rolling to achieve a reduction ratio of 2 resulted in an appreciable increase in the hardness due to change in morphology of the microstructure. Estimation of the strength on the basis of inter-particle spacing, which was measured by transmission electron microscopy, could not be accounted for on the basis of Orowan mechanism, and therefore, strengthening mechanisms like local climb and/or cross slip might have a role in this room temperature (0.32T_M) deformation process.     

Bond strength and microstructure investigation of Al2O3/Al/Al2O3 joints with surface modification of alumina by titanium

Joints of Al₂O₃/Al/Al₂O₃ are formed by liquid-state bonding of alumina substrates covered with thin titanium film of 800 nm thickness using an Al interlayer of 30 or 300 μm at 973 K under a vacuum of 0.2 mPa for 5 min and an applied pressure of 0.01 MPa. The bond strength of the joints is examined by a four-point bend testing at room temperature coupled with optical, scanning and transmission electron microscopy. Results show that: (i) bonding occurs due to the formation of a reactive interface on the metal side of the joint with the presence of Al₃Ti precipitates (ii) a decrease in Al layer thickness leads to stronger Al₂O₃/Al/Al₂O₃ bonds accompanied by a change of both the distribution of reaction products (Al₃Ti) in the region of the interface and the failure surface characteristics.     

Using a slit doser to probe gas dynamics during Al2O3 atomic layer deposition and to fabricate laterally graded Al2O3 layers

A special slit doser is used to form near unit steps in the spatial profile of an Al₂O₃ ALD film thickness. The unit step is formed as the Al₂O₃ ALD occurs mainly downstream from the slit doser because the trimethylaluminum and H₂O reactants are entrained in a viscous flow carrier gas. Spectroscopic ellipsometry measurements yielded thickness profiles of Al₂O₃ ALD on samples placed at different locations relative to the exit of the slit doser and the ALD growth zone. The effects of carrier gas flow rate, reactor pressure, and reactant dose and purge times on the Al₂O₃ ALD film profile provided details about the gas dynamics around the slit doser. Experimental indications of gas turbulence were observed at the exit of the slit doser. Lateral gradients in the Al₂O₃ ALD film thickness were also formed by linear translation of the sample relative to the slit doser during ALD. Lateral gradients of various desired pitches ranging from 119 Å/in to 444 Å/in were achieved as a result of accurate control of the Al₂O₃ ALD film thickness and small sample translation steps relative to the slit doser.