Preparation of alumina nanowires, nanorods, and nanowalls by chemical etching

Alumina (Al₂O₃) nanowires, nanorods, and nanowalls have been prepared from anodic aluminum oxide (AAO) templates by chemical etching in NaOH solution. Heating the template prior to etching is crucial to the morphology of subsequent prepared alumina nanostructures, which greatly depend on the phases of the AAO templates. It has been found that the templates with amorphous Al₂O₃, γ-Al₂O₃, and α+γ-Al₂O₃ phases will grow nanowires, nanorods, and nanowalls, respectively. A possible mechanism for forming different alumina nanostructures is proposed.     

高质量多孔氧化铝模板的制备

在草酸电解液中研究了阳极氧化法制备多孔氧化铝模板(AAO模板)。采用场发射扫描电子显微镜对多孔模板的形貌进行表征,结果表明:模板孔的分布均匀有序,孔径在40~70 nm;电解液浓度、氧化电压、氧化温度和氧化时间都会影响模板的形态;不经过高温退火及抛光也可以制得规则排布的多孔AAO模板。X射线衍射分析表明:氧化铝膜的主要成分为非晶态Al2O3。     

Optical properties of anodic aluminum oxide produced in a complex electrolyte

Features of the absorption, transmission, photoluminescence, and infrared (IR) spectra of anodic aluminum oxide (AAO) formed in a complex electrolyte and annealed at 800, 900, 1000, and 1300°C are investigated. The variations in the phase composition changes of the anodic aluminum oxide are reflected in the respective features of its optical properties. A decrease in the transmission coefficient in the visible range of the spectrum is shown for the phase series: amorphous AOA → γ-Al₂O₃, γ-, θ-, δ-Al₂O₃ mixture → α-Al₂O₃. It is established that the highest absorption coefficient is characteristic of α-Al₂O₃, and the amorphous samples are the most transparent in the visible range. An intensive luminescence band in the red region of the spectrum with maxima at 678 and 694 nm is found for α-Al₂O₃. The emergence of this band is explained by the presence of octahedrally coordinated Mn₊₄ and Cr₊₃ impurity ions in the structure. A luminescence band at 700–800 nm is observed for the mixture of low-temperature phases. Intense luminescence in the region 350–500 nm is found for amorphous AAO and γ-Al₂O₃.     

Study of the KNO<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system by differential scanning calorimetry

The structural and the thermodynamic properties of potassium nitrate KNO₃ and its composites with nanosized aluminum oxide Al₂O₃ have been studied by differential scanning calorimetry. It has been found that an amorphous phase forms in composites (1–x)KNO_3–xAl₂O₃. The thermal effect corresponding to this phase has been observed at 316°C. It has been found that the phase transition heats of potassium nitrate decreased as the aluminum oxide fraction increased.     

SEM/TEM characterization of periodical novel amorphous/nano-crystalline micro-composites obtained by laser interference structuring: The system HAlO-Al·Al2O3

Layers of the metastable, amorphous HAlO are synthesized by chemical vapor deposition from the molecular compound tert-butoxyalane ([^tBu-O-AlH₂]₂). At temperatures above 500 °C, these layers transform to biphasic Al·Al₂O₃ due to the elimination of di-hydrogen. The interaction of HAlO films with short laser pulses causes partial transformation of amorphous HAlO into nano-crystalline Al·Al₂O₃. Using an interference pattern of two coherent high-power Nd:YAG laser beams produces local and periodic heating, inducing crystallization at equally distant lines in the HAlO layer. Depending on the laser fluence, different morphologies and different amounts of crystalline phases are obtained. In this study, the surface morphology and the distribution of crystalline phases of the structured samples are analyzed using SEM, FIB and TEM. The two-dimensional structures consist of periodic variations of morphology, chemical composition, and phase identity with a well-defined long-range order. When bio-functionalized, the structured samples may be used as carriers for structurally controlled cell-cultivation.     

New polymorphs of alumina

Abstract

We present the results of high-pressure, high-temperature studies on xerogel gamma alumina which is made up of nanocrystalline alumina and an amorphous phase. Not many studies of this type have been reported on xerogel materials. At about 5 GPa and 1400°C, the xerogel alumina transforms into a polymorphic mixture containing α Al₂O₃, B Al₂O₃ and C Al₂O₃ where the last two phases have B Ln₂O₃ and C Ln₂O₃ type structures respectively. Here Ln stands for the rare earths. The xerogel alumina containing 1 wt.% Cr₂O₃ under these conditions transforms into a polymorphic mixture containing Al₂O₃, k' Al₂O₃ and H Al₂O₃ where H Al₂O₃ has the H Ln₂O₃ type structure. For the first time, the observation of rare earth sesquioxide structures of alumina is reported. At about 5 GPa at 27°C, the xerogel aluminal transforms to an amorphous phase. The results are discussed using the free energy diagram for xerogels proposed by R. Roy (J. Amer. Cer. Soc., 52, 344, 1969; 67, 468, 1984).     

Comparison of Segregation Behaviors for Special and General Boundaries in Polycrystalline Al2O3 with SiO2-TiO2 Impurities

A systematic study of grain boundaries with an elongated grain morphology in TiO₂-doped polycrystalline Al₂O₃ materials with SiO₂ impurities is reported in this paper. Over 20 grain boundaries, all having common (0001) basal plane surface on one side, were investigated by using HRTEM and analytical TEM. A few of them were basal plane twin boundaries or other non-wetting boundaries, and the majority of grain boundaries were covered by amorphous phases, either as continuous films or with small pockets bounded by surface facets. Si and Ti were segregated to all boundaries, however, two categories of segregation were observed. Excesses of both segregants were between 1.0–4.0 atom/nm² at special and non-wetting boundaries, while they were in the range of 7.0–30 atom/nm² at boundaries with amorphous phases. A variation of amorphous film thickness was also observed, which has a clear relation with Si excess level while Ti excess remained at the same level. This observation suggests that the amorphous phases were primarily made of SiO₂ but Ti segregants were attached to grain surfaces. The variation in thickness of SiO₂-based film is strongly related to the surface structure of anisotropic Al₂O₃ grains.     

Effect of thermal annealing on the structure of ZnSe/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposite films

The ZnSe/Al₂O₃ nanocomposite films synthesized by laser evaporation followed by heat treatment are studied. X-ray diffraction and electron-microscopic investigations of the as-deposited films demonstrate the presence of ZnSe crystallites in an Al₂O₃ amorphous matrix. Annealing changes the structures of ZnSe and Al₂O₃, increases the ZnSe crystallite size, and causes the appearance of the ZnSeO₄ phase. The presence of aluminum oxide layers decreases the phase transformation temperature of zinc selenide.     

Microstructure and mechanical properties of Al2O3-Al composite coatings deposited by plasma spraying

Al₂O₃ and Al₂O₃-Al composite coatings were prepared by plasma spraying. Phase composition of powders and as-sprayed coatings was determined by X-ray diffraction (XRD), while optical microscopy (OM) and scanning electron microscopy (SEM) were employed to investigate the morphology of impacted droplets, polished and fractured surface, and the element distribution in terms of wavelength-dispersive spectrometer (WDS). Mechanical properties including microhardness, adhesion and bending strength, fracture toughness and sliding wear rate were evaluated. The results indicated that the addition of Al into Al₂O₃ was beneficial to decrease the splashing of impinging droplets and to increase the deposition efficiency. The Al₂O₃-Al composite coating exhibited homogeneously dispersed pores and the co-sprayed Al particles were considered to be distributed in the splat boundary. Compared with Al₂O₃ coating, the composite coating showed slightly lower hardness, whereas the coexistence of metal Al phase and Al₂O₃ ceramic phase effectively improved the toughness, strength and wear resistance of coatings.     

Mechanical stresses in vapour quenched films of Al and Al rich compounds

The measurement of mechanical stresses in vapour quenched metal films can give additional structural information. Especially the near absence of stresses yields a sufficient condition for the formation of amorphous structures, since stress contributions due to the crystallite boundary mismatch are present in every micro crystalline film. From this point of view we find that Al films containing more than 25 at.% Ag grow in an amorphous phase whereas films with less than 25 at.% Ag are micro crystalline. The influence of other impurities, especially oxygen, is also investigated. Further evidence is presented for the high superconducting transition temperatures of Al films evaporated from Al₂O₃ crucibles, which films are micro crystalline, to be due to the presence of Al₂O impurities in these films.     

非晶态锂离子导体B2O3-0.7Li2O-0.7LiCl-xAl2O3

木文研究了Al₂O₃对B₂O₃-0.7Li₂O-0.7LiCl非晶态的形成和电学性能的影响,我们发现:加入适量的Al₂O₃后,无需借助液氮骤冷技术,直接将熔体倾倒在室温下的紫铜板上就很容易形成大块非晶锂离子导体B₂O₃-0.7Li₂O-0.7LiCl-xAl₂O₃。Al₂O₃的加入使B₂O₃-0.7Li₂O-0.7LiCl的电导率有所降低,但在高温下不太明显,电导激活能略微升高,实验发现:Al₂O₃含量x=0.03是较合适的剂量,较容易形成大块非晶态,对电导率的影响也不大。 关键词：     

等温热处理过程中一种非晶态Li+导体电导行为的研究

本文在玻璃转变温度以下,采用交流伏安法连续自动观测了非晶态Li⁺导体B₂O₃-0.7Li₂O-0.7LiCl-0.1Al₂O₃在等温热处理条件下的电导行为。结果表明:材料的电导率先随时间升高,出现峰值后单调下降,并含有两个平台部分。与此相配合的DSC与XRD研究证实:形成电导率峰值的基本原因是由于发生了非晶态相分离,而电导率曲线下降部分的平台则是由于非晶态的晶化。最后,从相界 关键词：     

Surface sensitive spectroscopic study of the interaction of oxygen with Al(111) - low temperature chemisorption and oxidation

The interaction of an atomically clean Al(111) surface with O₂ has been studied using a combination of electron energy loss spectroscopy (EELS) and Auger spectroscopy (AES). Oxygen dissociatively adsorbs and occupies both surface and subsurface binding sites under all exposure conditions in the temperature range 122–700 K. Surface sites are preferentially occupied at low exposures, while higher exposures increasingly favor population of subsurface sites. Studies of O₂ adsorption at temperatures as low as 131 K have shown that formation of Al₂O₃ occurs at high oxygen exposures. The Al₂O₃ produced exhibits a 54 eV Auger transition and a characteristic vibrational spectrum with loss features at 430, 645, and 880 cm⁻¹. Argon ion bombardment of thin monolayer level Al₂O₃ layers leads to preferential loss of Al₂O₃ and a reduction in the subsurface-to-surface oxygen ratio. Electron bombardment of similar, thin Al₂O₃ layers is ineffective in inducing desorption of surface species, whereas thick Al₂O₃ layers are strongly influenced by electron bombardment, as judged from AES behavior. Qualitative models for O₂ adsorption, oxidative annealing, and damage by ion and electron bombardment are given.     

Properties of an Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Si interface

The phase chemical composition of an Al₂O₃/Si interface formed upon molecular deposition of a 100-nm-thick Al₂O₃ layer on the Si(100) (c-Si) surface is investigated by depth-resolved ultrasoft x-ray emission spectroscopy. Analysis is performed using Al and Si L_{2, 3} emission bands. It is found that the thickness of the interface separating the c-Si substrate and the Al₂O₃ layer is approximately equal to 60 nm and the interface has a complex structure. The upper layer of the interface contains Al₂O₃ molecules and Al atoms, whose coordination is characteristic of metallic aluminum (most likely, these atoms form sufficiently large-sized Al clusters). The shape of the Si bands indicates that the interface layer (no more than 10-nm thick) adjacent to the substrate involves Si atoms in an unusual chemical state. This state is not typical of amorphous Si, c-Si, SiO₂, or SiO_x (it is assumed that these Si atoms form small-sized Si clusters). It is revealed that SiO₂ is contained in the vicinity of the substrate. The properties of thicker coatings are similar to those of the 100-nm-thick Al₂O₃ layer and differ significantly from the properties of the interfaces of Al₂O₃ thin layers.     

Improvement of surface porosity and properties of alumina films by incorporation of Fe micrograins in micro-arc oxidation

Micro-arc oxidation (MAO) is an effective approach to improve the properties of aluminum and its alloy by forming ceramic films on the surface. However, the oxide layers often have a porous surface structure, which exhibits relatively high friction coefficients. In this work, in order to enhance the surface and mechanical properties of the films produced by micro-arc oxidation, Al₂O₃ coatings embedded with Fe micrograins of different thicknesses were produced on aluminum alloys by adding Fe micrograins into the electrolyte during MAO. Compared to the Al₂O₃ coatings without Fe micrograins, the MAO Al₂O₃ coatings with Fe micrograins are much denser and harder, and the wear resistance is also improved significantly. The enhancement can be attributed to the enhancement of the surface structure and morphology of the MAO Al₂O₃ coatings with embedded Fe micrograins.     

Development of a “Built-in” Scanning Near Field Microscope Head for an Atomic Force Microscope System and Stress Mapping of an Al2O3/ZrO2 Eutectic Composite

We constructed a scanning near-field optical microscope (SNOM) on a commercially available atomic force microscopy (AFM) apparatus (SPM-9500J2; Shimadzu Corp.) to measure the stress distribution in ceramic composite materials. Features of our SNOM system are: (1) a compact SNOM head substituted for the original AFM head; (2) a wide scanning range (125 × 125 μm²) inherited from the original scanner; (3) use of conventional shear-force regulation; (4) an optical system for the illumination-collection (I-C) mode; (5) excitation by a 488 nm line of an Ar-ion laser, and (6) light detection by photon counting or a polychromator equipped with an electronically cooled charge coupled device (CCD). This SNOM system was used to measure the surface structure and stress distribution of an Al₂O₃/ZrO₂ eutectic composite. We simultaneously measured topographic images and fluorescence spectra of an Al₂O₃/ZrO₂ eutectic composite. We estimated its peak intensity, peak position, and peak width from the fluorescence spectrum during scanning, which respectively correspond to the abundance of Al₂O₃, stress in the grain, and the anisotropy of that stress. Mapping images showed that the stress and its anisotropy were weaker in the center of the Al₂O₃ grain than its boundary between Al₂O₃ and ZrO₂. That observation suggests that Al₂O₃ underwent intense anisotropic stress induced by volume expansion in the phase transition of ZrO₂ from the cubic phase to the monoclinic phase during preparation.     

Multiple surface conduction channels via topological insulator and amorphous insulator thin film multi-stacks

Multi-channel Bi₂Se₃ thin films were grown by combining molecular beam epitaxy and atomic layer deposition. High-resolution transmission electron microscope images showed that c-axis oriented Bi₂Se₃ grew on amorphous Al₂O₃ even after multiple stacking. While the surface morphology degraded for the upper layers, each layer was electrically similar. The electrical transport measurements showed that the weak anti-localization effect was quantitatively enhanced upon increasing the number of Bi₂Se₃ channels. Our results provide a promising approach to exploit diverse combinations of layered topological insulator films vertically stacked with amorphous insulator films.     

Deposition and characterization of binary Al2O3/SiO2 coating layers on the surfaces of rutile TiO2 and the pigmentary properties

Binary Al₂O₃/SiO₂-coated rutile TiO₂ composites were prepared by a liquid-phase deposition method starting from Na₂SiO₃·9H₂O and NaAlO₂. The chemical structure and morphology of binary Al₂O₃/SiO₂ coating layers were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, TG-DSC, Zeta potential, powder X-ray diffraction, and transmission electron microscopy techniques. Binary Al₂O₃/SiO₂ coating layers both in amorphous phase were formed at TiO₂ surfaces. The silica coating layers were anchored at TiO₂ surfaces via Si-O-Ti bonds and the alumina coating layers were probably anchored at the SiO₂-coated TiO₂ surfaces via Al-O-Si bonds. The formation of continuous and dense binary Al₂O₃/SiO₂ coating layers depended on the pH value of reaction solution and the alumina loading. The binary Al₂O₃/SiO₂-coated TiO₂ composites had a high dispersibility in water. The whiteness and brightness of the binary Al₂O₃/SiO₂-coated TiO₂ composites were higher than those of the naked rutile TiO₂ and the SiO₂-coated TiO₂ samples. The relative light scattering index was found to depend on the composition of coating layers.     

Study of the physical properties of amorphous Zr doped Al2O3 powders

Alumina (Al₂O₃) powders doped with different amounts of zirconium (Zr) ions were synthesized by the sol-gel process. The Zr concentration was changed from 0 to 3%. Here we attempted to fabricate Zr doped Al₂O₃ samples and characterized them for their optical and structural properties. Ultraviolet-Visible analysis (UV-Vis), infrared spectroscopy (FT-IR) and powder X-ray diffraction (XRD) have been used to characterize the optical properties, phase evolution and crystallinity of the obtained samples. X-ray diffraction patterns revealed that all the Al₂O₃ powders obtained were completely amorphous. An optical study was employed to determine the band gap of the samples. The transmittance had decreased from 90 to 86% and the band gap of pure Al₂O₃ was found to be 4.116 eV, and it was shifted to 4.038 eV for 3% Zr doped Al₂O₃. The results obtained in this study are discussed comparatively with those cited in the literature.     

Els investigations of the beginning oxidation on aluminum thin films

ELS and simultaneous quartz microbalance investigations have been carried out on clean aluminum and its surface oxide layer. The loss spectrum of clean Al is interpreted by the collective features of the conduction electrons: volume and surface plasmons, the latter being extremely sensitive to a small oxygen uptake. In the very beginning the oxidation is characterized by a loss peak at 7.3 eV which is attributed to a single electron transition from the O(2p) level to an unfilled state near the Fermi level of the metal. The decreasing intensity of the 7.3 eV loss and the increasing of a second loss at 19.2 eV with further oxygen uptake are tentatively explained by the formation of Al₂O₃ and interband transitions of amorphous Al oxide. The formation of A1₂O₃ is supported by the gravimetric measurements of oxygen mass gain.