Preparation and properties of a γ-Al2O3 washcoat deposited on a ceramic honeycomb

Washcoat deposited on ceramic honeycomb was prepared using pseudoboehmite, the CeO₂–ZrO₂–La₂O₃ solid solution, pore enlarger and other additives. The microstructures and surface performances of washcoat/honeycomb were investigated by SEM, BET surface area, XRD, ultrasonic vibration and hot shock simulation. The results show that the performance and loading of washcoat are affected obviously by the properties of slurry gel, such as the apparent viscosity, solid content, particle size and its distribution. When the apparent viscosity of slurry is lower, the gel with a narrow particle size distribution and finer particles can be obtained, with which the coating having an excellent performance can be prepared. Adding a small quantity of the CeO₂–ZrO₂–La₂O₃ solid solution can promote the thermal stability of washcoat, such as, after calcined at 1000 °C for 5 h the sample exhibits mainly the γ-Al₂O₃ phases and the θ-Al₂O₃ -Al₂O₃ and κ-Al₂O₃ phases have not been detected in the XRD spectra. It is found also that the washcoat prepared has excellent properties of the vibration-resistant, heat-resistant and its BET surface area reaches 50 m²/g.     

Microstructure and properties of CVD γ-Al2O3 coatings

This work deals with the microstructures and wear properties of chemical vapour deposited γ-Al₂O₃. The γ-Al₂O₃ coatings were deposited at 800 °C on TiN and Ti(C,N) pre-coated cemented carbide substrates. The microstructures developed in the γ-Al₂O₃ coatings and the influence of the nucleation surface on the growth of γ-Al₂O₃ were characterised using transmission electron microscopy, electron energy-loss spectroscopy and X-ray diffraction. The γ-Al₂O₃ coatings were fine-grained with a high density of {1 1 1} growth twins and contained some residual sulphur. γ-Al₂O₃ was found to grow epitaxially on the investigated substrates. The mechanical properties were evaluated in metal cutting and were compared with those of κ-Al₂O₃ coated tools. As compared with the κ-Al₂O₃ coatings, the γ-Al₂O₃ coatings exhibited slightly worse adhesion and tendency for edge chipping. However, the γ-Al₂O₃ coatings showed better crater wear resistance on the rake face than κ-Al₂O₃ coatings.     

Synthesis of crystalline γ-Al2O3 with high purity

Crystalline γ-AlO(OH) was synthesized by the precipitation of sodium aluminate and oxalic acids in aqueous solution. And then γ-AlO(OH) was successfully transferred to γ-Al₂O₃ after subsequent high temperature heat treatment. The effects of reaction conditions on formation of γ-AlO(OH) and γ-Al₂O₃ were further investigated in detail. The XRD analysis shows that the complete formation of crystalline γ-Al₂O₃ is at pH 8–9, reaction temperature of 93–96 °C and calcination temperature of higher than 400 °C. The product of γ-Al₂O₃ contains impurity, including iron, calcium and silicon ion with a low content of about 0.01% and has large specific surface area and high pore volume of 269.9 m²/g and 0.57 mL/g, which can be applied in catalysts and catalyst supports.     

Partial dislocation configurations in a low-angle boundary in α-Al2O3

The dislocation structures of a low-angle tilt grain boundary in alumina bicrystal were investigated by transmission electron microscopy. The grain boundary was found to consist of two regions: an area with pairs of partial dislocations and an area with groups of odd numbered partial dislocations (multiple-partial-structure). Eight kinds of multiple-partial-structures were found in the fabricated grain boundary. The Burgers vectors of each partial dislocation in the grain boundary can be distinguished by dark-field imaging, and thus the arrangement of partial dislocations in the multiple-partial-structures are determined. It is concluded that a slight twist component of the boundary is the origin of the characteristic multiple-partial-structures.     

Deposition, microstructure and properties of texture-controlled CVD α-Al2O3 coatings

The influence of nucleation on the microstructure and properties of CVD Al₂O₃ was investigated. The experimental α-Al₂O₃ layers were deposited (a) without nucleation control and (b) with nucleation steps resulting in pronounced , and growth textures. The experimental layers were characterised using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Wear properties of the textured coatings were evaluated in turning. The chemistry of the nucleation surface appeared to be an important factor in pre-determining the phase content and growth textures of the Al₂O₃ layers. Optimised nucleation resulted in substantially improved wear properties and these kinds of α-Al₂O₃ layers were typically composed of relatively small, defect-free grains exhibiting no porosity. The textured α-Al₂O₃ layer showed the best wear resistance.     

Microstructural investigation of heat-treated CVD κ-Al2O3 multilayer coatings

In this work, the κ-Al₂O₃ to -Al₂O₃ phase transformation in CVD multilayer coatings has been investigated. Coatings with different degrees of transformation have been prepared by isothermal heat-treatments of as-deposited κ-Al₂O₃ multilayer coatings. These coatings have then been examined by means of transmission electron microscopy of cross-section specimens. The aim of this work was to examine the microstructural evolution during the κ-Al₂O₃ to -Al₂O₃ phase transformation and to describe how the transformation growth is affected by the κ-Al₂O₃ microstructure. The microstructure of the non-transformed κ-Al₂O₃ was not affected by the heat-treatments. During the phase transformation, a linkage of voids took place at the -Al₂O₃/TiC interfaces, resulting in large separated voids and continuous cracks. A large number of small faceted voids appeared within the -Al₂O₃ grains. It was found that during the transformation c_κ → c, i.e. the close packed planes are preserved, but rotated slightly around the c-axis. Three modes for the κ-Al₂O₃ to -Al₂O₃ phase transformation were found: (i) a columnar transformation; (ii) a lateral transformation; and (iii) a mixed columnar-lateral transformation. The predominance of any of the transformation modes is attributed to the nucleation/growth ratio. A high ratio yields a columnar transformation, while a low ratio yields a lateral transformation. The nucleation sites in all transformation modes are suggested to be at voids at the κ-Al₂O₃/TiC interfaces.     

Growth and microstructural stability of epitaxial Al films on (0001) α-Al2O3 substrates

Al films were grown epitaxially on single-crystal α-Al₂O₃ substrates by magnetron sputtering and molecular beam epitaxy, respectively. The microstructure and thermal stability of these films were analysed in detail using X-ray diffraction methods and electron microscopy techniques. The films consist of two twin-related growth variants, related by a 180° rotation around the <111> film normal resulting in a {111} Al || (0001) α-Al₂O₃, and ± < > Al || < > α-Al₂O₃ orientation relationship. The Al variants are separated by Σ3 { } Al twin boundaries possessing a rigid body translation of the {111} Al planes across the boundary plane in order to reduce their energy. Motion of the twin boundaries was observed by annealing plan-view samples in situ in a transmission electron microscope. The twin boundaries advance in jerky motion at velocities of several μm/s at temperatures of ˜400 °C, resulting in grain coarsening. In all cases, heat treatments resulted in increased area fraction of one twin variant, which finally will result in single-crystal films upon further annealing.     

Oxygen partial pressure dependence of memory effect of sputtered nc-Al/α-Al2O3 thin films

Nanocrystalline aluminum embedded in amorphous dielectric alumina matrix thin films (nc-Al/α-Al₂O₃) was synthesized via reactive magnetron sputtering. The nc-Al/α-Al₂O₃ films at different oxygen partial pressures were sputtered on p-type Si substrates from a pure Al target in the mixed ambient of Ar and O₂. Both deposition rate and aluminum concentration increase as the oxygen partial pressure decreases. X-ray photoelectron spectroscopy and high-resolution transmission electron microscope studies give the confirmation of nanocrystalline Al embedded in amorphous Al₂O₃ matrix. This nanocomposite thin film exhibits memory effect as a result of charge trapping. The flat band voltage value depends on the Al nanocrystal concentration which is related to oxygen partial pressure.     

载体中ZrO2/Al2O3比对Pt/ZrO2-Al2O3催化剂性能的影响

相比汽油车而言,柴油车具有高效、低油耗的优势已得到广泛应用。本实验以ZrO₂作为改性剂,探究了ZrO₂与Al₂O₃的质量比对催化剂的影响。研究结果表明：随着ZrO₂的加入,Pt粒子先减小后增大;Pt粒子与载体的交互作用先增大后减小。活性实验数据分析表明,ZrO₂的最佳添加量为40 wt%,CO和C₃H₆完全氧化温度分别降低20 _oC 、25 _oC。贵金属在催化剂的分散度以及贵金属与载体的相互作用随着ZrO₂与Al₂O₃质量比的变化而变化。Pt粒子越小,其与载体的交互作用越强,这表明催化剂性能越强。     

Influence of micro-blasting on the microstructure and residual stresses of CVD κ-Al2O3 coatings

Cemented carbides coated with CVD multilayers are commonly used in metal cutting turning and milling operations. For many applications, a micro-blasting finishing procedure based on an impact treatment of the surfaces is carried out in order to smooth the coated surface and reduce sharp cutting edges. In this work, micro-blasting with corundum in aqueous solution at pressures between 0.05 and 0.3 MPa was applied to CVD TiN/Ti(C,N)/κ-Al₂O₃ multilayer coatings deposited onto cemented carbides in order to investigate its influence on the micro-topography, microstructure and residual stresses. The results showed that the micro-blasting reduces the surface roughness and affects the coating thickness. TEM investigations revealed no significant changes on the microstructure of the κ-Al₂O₃ top layer. Synchrotron X-ray investigations showed that the residual stress state of the as-deposited κ-Al₂O₃ top layer is not affected by the micro-blasting treatment under the conditions investigated.     

TiN/Cr/Al2O3 and TiN/Al2O3 hybrid coatings structure features and properties resulting from combined treatment

New experimental results are presented on the structure and the elemental and phase composition of hybrid coatings, which were deposited on a substrate of AISI 321 stainless steel using a combination of plasma-detonation, vacuum-arc and subsequent High-Current Electron Beam (HCEB) treatment. We found that an increase in energy density intensified mass transfer processes and resulted in changes in aluminum oxide phase composition (γ → α and β → α). Also we observed the formation of a nanocrystalline structure in Al₂O₃ coatings. Electron beam treatment of a hybrid coating surface induced higher adhesion, decreased the intensity of surface wear and increased corrosion resistance in a sulphuric acid solution. The corrosion resistance of the coatings was studied in several electrolytic solutions (0.5 M H₂SO₄, 1 M HCl, 0.75 M NaCl) using electrochemical techniques. In most cases the corrosion resistance was improved, except those in NaCl solutions. The nano-hardness of the protecting coating was 13 GPa before electron beam melting and 9 GPa after it (as a result of TiN and Al₂O₃ sub-layers mixing).     

Growth and characterization of Al2O3 thin films for the buffer insulator in Pt/SrBi2Nb2O9/Al2O3/Si ferroelectric gate oxide structure

Pt/SrBi₂Nb₂O₉ (SBN)/Al₂O₃/Si (MFIS) ferroelectric gate oxide structures were prepared with the rf (radio frequency) magnetron sputtering method for the application of non-destructive read-out ferroelectric RAM (NDRO-FRAM) devices. An Al₂O₃ intermediate layer between the perovskite SrBi₂Nb₂O₉ film and Si substrate prevents the serious inter-diffusion of the SrBi₂Nb₂O₉ (SBN) into the Si substrate. The coercive field that decisively affects the memory window was increased by inserting the Al₂O₃ insulator between the SBN and Si, and thus the memory window also increased with the increase in the electric field to the SBN. The memory windows of the metal-ferroelectric-insulator-semiconductors (MFIS) structures were in the range of 0.7–3.4 V when the gate voltage varied from 3 to 9 V. The memory windows of the MFIS structures were found to be dependent on the thickness and stoichiometry of the buffer layer. We obtained the maximum memory window in an MFIS with an insulator of 11.4 nm in thickness deposited in the deposition condition of a 15∶5 flow ratio (Ar:O₂) during sputtering.     

Processing and properties of ZrO2(3Y2O3)–Al2O3 nanocomposites

Nanocomposites of yttria-stabilized zirconia (YSZ), containing 20 and 40 wt% alumina, were prepared by a two-step process: (1) fine-particle aggregates of the constituent phases were melted and homogenized in a high enthalpy plasma, prior to rapid quenching in water to obtain metastable starting powders, and (2) the metastable powders were consolidated by hot isostatic pressing (HIP), under conditions designed to ensure the formation of nanocomposites by controlling the metastable-to-stable phase transformation during sintering. In both cases, the resulting nanocomposites had completely uniform structures, comprising 27 and 50 vol% of -Al₂O₃ in a tetragonal YSZ matrix phase. Measurements of hardness and indentation toughness were correlated with observed structures.     

Thermodynamic properties and phase diagrams of the binary systems B2O3–Ga2O3, B2O3–Al2O3 and B2O3–In2O3

We calculated the binary phase diagrams B₂O₃–Ga₂O₃, B₂O₃–In₂O₃ and B₂O₃–Al₂O₃, and the Gibbs energy of formation of the binary compounds, using experimental liquidus data. The B₂O₃–Ga₂O₃ system is of industrial importance, because liquid B₂O₃, in which Ga₂O₃ is not very soluble, is used to protect GaAs during growth of single crystals of GaAs. During recovery of noble metals B₂O₃ is added to slags containing Al₂O₃ to lower the melting point and the viscosity. The B₂O₃–In₂O₃ system is of much less importance to industry. In all three systems we have a liquid miscibility gap, and also solid binary compounds, none of which melt congruently. The miscibility gaps are not surprising, because even in the B₂O₃–Bi₂O₃ system where four congruently melting compounds are present, a liquid miscibility gap exists close to B₂O₃.     

Formation and properties of BaxFe3−xO4 with spinel structure by mechanochemical reaction of α-Fe2O3 and BaCO3

Magnetic Ba_xFe_3−xO₄ (x  0.23) with spinel structure was fabricated by ball milling of mixture of BaCO₃ and nonmagnetic α-Fe₂O₃ powders, and the molar ratio of BaCO₃ and α-Fe₂O₃ is 1:6. In the milling process, a mechanochemical reaction took place between BaCO₃ and α-Fe₂O₃, and Ba cation incorporated into α-Fe₂O₃ with rhombohedral structure to form a α-(Fe,Ba)₂O₃ solid solution. The Ba content in the α-(Fe,Ba)₂O₃ increased with increasing milling time, when the Ba content exceeded a limited solubility, the α-(Fe,Ba)₂O₃ transformed into a phase of Ba_xFe_3−xO₄ with spinel structure, where the Ba cation occupied an octahedral site or tetrahedral site. The product obtained in the balling process was different from that prepared in the annealing process at atmospheric pressure, which was BaFe₂O₄ with orthorhombic structure. Accompanying the crystal structure transition from α-(Fe,Ba)₂O₃ to Ba_xFe_3−xO₄, the magnetic properties also changed from nonmagnetism into ferromagnetism. The saturation magnetization was 53.3 emu/g and coercivity was 113.7 Oe. The mechanism of transitions of the crystal structure was discussed in the present work.     

HOPG上ALD沉积Al2O3介电薄膜的生长行为研究

本文采用原子层沉积(ALD)的方法,选择三甲基铝(TMA)和H₂O₂作为反应前驱体,在高定向热解石墨(HOPG)基体上沉积Al₂O₃。系统研究了反应温度和生长周次对Al₂O₃生长行为的影响。研究表明：受HOPG表面饱和成键的影响,Al₂O₃在衬底表面处形核困难,在生长初期主要表现为台阶处择优生长,其形态为线状结构。当沉积100周次Al₂O₃时,其中在沉积温度为50 °C、150 °C和200 °C时呈现为纳米线状结构,而在100 °C时呈现为非连续薄膜。随着生长周次的增加,不同温度下沉积态Al₂O₃都趋于形成连续薄膜,表明其生长行为发生了由三维岛状生长模式向二维平面生长模式的转变。分析认为,生长模式的转变是由纳米线状结构横向生长造成的;横向生长速率主要受生长温度影响。拉曼结果表明：沉积后的石墨烯层结构未受影响,可保留其原有的优越性能。     

Investigation of the defect density in ultra-thin Al2O3 films grown using atomic layer deposition

The film perfection in terms of pinhole defect densities of ultra-thin Al₂O₃ grown by atomic layer deposition (ALD) has been quantitatively characterized. A significant defect density reduction from ~ 1.2 × 10⁵/cm² to ~ 90/cm² was demonstrated for 2 nm-thick Al₂O₃ by using an ALD tungsten (W) buffer layer on the nickel (Ni) substrate. The reason for the defect reduction was attributed to efficient nucleation of ALD Al₂O₃ on ALD W. The effect of the buffer layer becomes less essential as the Al₂O₃ thickness increases, where the substrate surface physical conditions such as particle contamination become the main cause for defects.     

一种可内生Al2O3扩散障的δ-Ni2Al3-Cr2O3/ Ni-Cr2O3涂层体系

通过对Ni-Cr₂O₃复合镀层620 ℃部分渗铝制备了δ-Ni₂Al₃-Cr₂O₃/Ni-Cr₂O₃涂层体系。Cr₂O₃颗粒在渗铝的过程中和Al反应生成更为稳定的Al₂O₃。1000 ℃恒温氧化20 h后发现,铝化物涂层和复合镀层内掺杂的Cr₂O₃颗粒完全转化为Al₂O₃,并在铝化物涂层/Ni镀层界面自发形成了一层Al₂O₃富集层,该富集层起扩散障作用,阻碍铝化物涂层因互扩散所致的退化。     

Effects of annealing and interlayers on the adhesion energy of copper thin films to SiO2/Si substrates

The adhesion of sputtered copper thin films was measured with a quantitative indentation technique utilizing refractory superlayers to trigger and promote delamination. Adhesion energies of the indentation-induced delaminations were analyzed in terms of the critical strain energy release rate. Two groups of films were investigated. Group I ranged in thickness from 225 to 1000 nm in both the as-deposited and annealed condition. Adhesion energies ranged from 2 to 15 J/m², with higher adhesion for the annealed condition. Group II films had nominal thicknesses of 430 and 1100 nm, some with 7–10 nm interlayers of either titanium or chromium. Adhesion energies of these films ranged from 4 to 30 J/m², increasing by a factor from 1.3 to 7.5, as a function of the interlayer presence and type, with the increase in energy due to a chromium interlayer exceeding that of titanium. Adhesion energies increased with film thickness for all films, with interlayers and annealing producing a larger increase. These quantitative results were compared to previous semi-quantitative and quantitative results, and shown to have comparable magnitudes and trends.     

On the mechanism of selective CO oxidation on nanosized Auγ-Al2O3 catalysts

New findings give further information on the mechanism of carbon monoxide selective oxidation over γ-alumina supported nanoparticle sized gold catalysts. a) CO₂ formation, increasing with rising temperature, is observed in the absence of hydrogen and oxygen pointing to a model of active sites consisting of an ensemble of metallic Au atoms and a cationic Au with a hydroxyl group, b) At high temperatures (>200 °C) in excess of H₂, reversed water gas shift (RWGS) reaction results in CO₂ consumption towards CO and H₂O formation. c) Hydrogen strongly influences the interaction of CO on Auγ-Al₂O₃, by weakening the CO adsorption. The presence of hydrogen plays an important role both decreasing the strength of CO bonding and in the prevention of deactivation and regeneration.