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.     

聚丙烯酰胺凝胶法制备大孔α-氧化铝及其发光性能研究

采用聚丙烯酰胺凝胶法制备了大孔α-氧化铝材料.利用X射线衍射仪、扫描电镜、荧光分光光度计表征了所制备的氧化铝样品.结果表明,在1150℃烧结温度下可获得高纯的α-氧化铝陶瓷材料,其形貌呈类电路板型(monolithic)大孔结构.荧光光谱测试分析发现,在228 nm波长的光激发下,其荧光光谱在300-400 nm范围内由两个主发射带组成,其峰值分别位于330 nm和365nm.基于实验结果,探讨了多孔氧化铝的形成机理和发光机制.     

Thermoluminescence of α-Al 2O 3 by neutron irradiation at low temperature

Thermoluminescence (TL) mechanisms of neutron-irradiated α-Al₂O₃ at 20 K is reported. The TL glow curves of neutron-irradiated and γ-ray-irradiated α-Al₂O₃ were observed. The TL emission bands near 340, 430, 530 and 694 nm were observed in the neutron-irradiated α-Al₂O₃. The γ-ray-irradiated α-Al₂O₃ only showed the TL emission line nearly at 694 nm, corresponding to the R lines of α-Al₂O₃:Cr³⁺. Therefore, the first three emission bands are related to the atomic displacement defects as F-type centers caused by neutron irradiation.     

Electronic structure of an oxygen vacancy in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> from the results of Ab Initio quantum-chemical calculations and photoluminescence experiments

The electronic structure of an oxygen vacancy in α-Al₂O₃ and γ-Al₂O₃ is calculated. The calculation predicts an absorption peak at an energy of 6.4 and 6.3 eV in α-Al₂O₃ and γ-Al₂O₃, respectively. The luminescence and luminescence excitation spectra of amorphous Al₂O₃ are measured using synchrotron radiation. The presence of a luminescence band at 2.9 eV and a peak at 6.2 eV in the luminescence excitation spectrum indicates the presence of oxygen vacancies in amorphous Al₂O₃.     

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。     

Formation and mechanical properties of alumina ceramics based on Al<Subscript>2</Subscript>O<Subscript>3</Subscript> micro- and nanoparticles

Alumina micro- and nanopowders with the particle size from 200 μm to 40 nm synthesized by the sol-gel method are studied. The particle size dependence of γ-Al₂O₃→α-Al₂O₃ phase transformation is studied by differential thermal analysis, X-ray diffraction method, and transmission electron microscopy. X-ray diffraction data show that for alumina nanoparticles γ-Al₂O₃→θ-Al₂O₃ phase transformation occurs at 900°C, and for micro-particles it occurs in the temperature range 1150–1200°C. The alumina ceramics produced of alumina nanoparticles is shown to have higher flexural strength under three-point bending than the ceramics produced of micro-particles. The obtained results demonstrate that alumina particle size reduction stabilizes the formation of α-Al₂O₃ at lower temperatures, due to which the grain growth rate decreases and the flexural strength of monolithic oxide ceramics increases.     

Influence of annealing on the structure of nanoporous oxide films on the surface of titanium‒aluminum powder alloy

Oxide films obtained during anodization of Ti?40% Al sintered powder samples in fluorine-containing electrolytes are investigated. With scanning electron microscopy and X-ray phase analysis, it is demonstrated that an X-ray amorphous nanoporous anodic oxide film is formed on the surface of the powder microparticles under optimal anodization conditions. After annealing at T = 1093 K in air and vacuum (10^?2 Pa), the oxide films are revealed to crystallize with its regular porous structure retained. The composition of the polycrystalline anodic-oxide films annealed in air is a mixture involving TiO₂ (anatase and rutile) and α- and γ-Al₂O₃ phases and Ti₂O₃ and Al₂TiO₅ traces. The vacuum annealing process makes it possible to identify TiO₂, in which anatase is the main phase, α- and γ-Al₂O₃, and Ti₂O₃ and TiO traces. However, rutile is not revealed. The presented results indicate that the application of the anodic nanostructuring of Ti?40% Al powders is promising for the obtainment of new photocatalytic active nanomaterials.     

Structural and phase transition of α-Al2O3 powders obtained by co-precipitation method

Aluminium oxide has been synthesized by co-precipitation technique at different annealing temperature. Powder XRD confirms the formation of α-Al₂O₃ with rhombohedral crystal structure having lattice constant a = 4.76 Å and b = 12.99 Å by the Scherer formula, the average crystallite size is estimated to be 66 nm. The scanning electron microscope results expose the fact that the α-Al₂O₃ nanomaterials are seemingly porous in nature and highly agglomerated. Chemical composition of aluminium oxide is confirmed by energy dispersive spectroscopy. The molecular functional group is confirmed by FTIR. Optical absorption of α-Al₂O₃ has been studied in the UV–vis region and its direct band gap is estimated to be 5.97 eV. This study involves the structural and phase transition of Al₂O₃ and also indicates that α-Al₂O₃ has considerable properties, deserving further investigation for the energetic materials with excellent properties for the possibility of using thin-layer α-Al₂O₃ as a thermo luminescence material.     

Measurement and interpretation of the plasmon energy in alumina

The electron energy loss spectra from well categorized samples of α-Al₂O₃, γ-Al₂O₃ and amorphous alumina films were measured using combined electron microscopy and energy analysis. The plasmon energies were compared using an elementary dielectric theory whereby a method of determining the density of the amorphous films was established.     

Effect of annealing on the phase composition and morphology of Al2O3 formed in a complex electrolyte

The phase composition and morphology of surfaces and cleavages of anodic aluminum oxide (AAO) formed in a complex electrolyte and annealed at 900, 1000, and 1300°C are studied by X-ray diffraction, atomic-force and scanning electron microscopy. It is shown that, depending on the preparation conditions, the following phase states occur: amorphous AAO, the δ phase of Al₂O₃, and the α phase of Al₂O₃. The phase transition from the amorphous to the crystalline state under annealing is accompanied by increasing surface area, and the transition from the tetragonal to rhombohedral phase is accompanied by an abrupt decrease in the value of the specific surface and a change in the morphology of the AAO cleaved facets.     

Luminescence of F + and F centers in YAlO3

In YAlO₃ crystals grown in vacuum or reduced by annealing at low oxygen pressure, the luminescence of F centers in the band at 420 nm, with τ=30 ms at 9 K, excited in the bands at 212 and 242 nm, as well as the luminescence of F ⁺ centers in the band at 355 nm, with τ=2.7 ns, excited in the main band at 220 nm and weaker bands at 190 and 288 nm, was detected. On the basis of the results obtained and data in the literature, the behavior of the emission of F ⁺ and F centers in oxides of the Al₂O₃-Y₂O₃ system is analyzed on the example of the compounds α-Al₂O₃, YAlO₃, and Y₃Al₅O₁₂. The role of antisite defects in the stabilization of F-like luminescence and absorption centers in multisublattice oxides is discussed.     

Investigation of phase transition of γ-alumina to α-alumina via mechanical milling method

In this research, the results obtained from studying the phase transition of γ-alumina (γ-Al₂O₃) to α-alumina (α-Al₂O₃) during intense mechanical activation in high-energy ball milling are presented. The powder samples were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), thermogravimetric and Differential thermal analyses (TG-DTA). With respect to the results achieved from above analyses, the transition of γ → α-alumina through δ- and θ-phases, can be initiated. Also, it was found that the pure γ-alumina phase showed a great stability during high-energy ball milling and there was no transformation to any other phase after a long milling time (30 h). On the other hand, γ-alumina containing a small amount of the α-alumina seed, showed a gradual phase transition from γ-alumina to α-alumina in milling. The phase transition mechanism during milling is nucleation and growth, which is promoted by the α-alumina seed.     

α-Al2O3单晶的热释光和光释光特性

研究了纯α-Al₂O₃单晶的热释光发光曲线和三维发光谱，以及光释光衰变曲线，对它们的发光机理和剂量学特性进行了分析和讨论.实验观察到α-Al₂O₃单晶β射线照射后立即测量的热释光发光曲线，有峰温为76℃和207℃两个发光峰.经γ射线照射数小时后测量的三维发光谱，只有峰温207℃波长为416 nm发光峰，它与α-Al₂O₃:C晶体的发光波长基本相同，是受热激发到导带的电子与F< 关键词： 2O₃')" href="#">α-Al₂O₃ 三维发光谱 TL/OSL剂量响应     

The formation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles in the oxidation of aluminum by water under sub- and supercritical conditions

Massive aluminum samples were oxidized by sub- and supercritical water with the formation of (AlOOH) _n and (Al₂O₃) _n nanoparticles. The release of H₂ began at 523 K when the reagents were heated uniformly to 700 K. The time lag of the beginning of oxidation was 140 s when supercritical water was injected into a reactor with aluminum samples at 665 K and 23.1 MPa. Oxidized aluminum powders were analyzed using a transmission electron microscope. Predominantly large (300–400 nm) α-Al₂O₃ particles were formed when supercritical water was injected into a reactor with aluminum. Smaller (20–50 nm) γ-Al₂O₃ particles were also observed in samples oxidized by water vapor under temperature increase conditions. Kinetic equations describing the rate of H₂ formation in the reaction of H₂O with aluminum were obtained. Possible nanostructuring mechanisms are discussed.     

Deposition of duplex Al2O3/aluminum coatings on steel using a combined technique of arc spraying and plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) is a cost-effective technique that can be used to prepare ceramic coatings on metals such as Ti, Al, Mg, Nb, etc., and their alloys, but this promising technique cannot be used to modify the surface properties of steels, which are the most widely used materials in engineering. In order to prepare metallurgically bonded ceramic coatings on steels, a combined technique of arc spraying and plasma electrolytic oxidation (PEO) was adopted. In this work, metallurgically bonded ceramic coatings on steels were obtained using this method. We firstly prepared aluminum coatings on steels by arc spraying, and then obtained the metallurgically bonded ceramic coatings on aluminum coatings by PEO. The characteristics of duplex coatings were analyzed by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The corrosion and wear resistance of the ceramic coatings were also studied. The results show that, duplex Al₂O₃/aluminum coatings have been deposited on steel substrate after the combined treatment. The ceramic coatings are mainly composed of α-Al₂O₃, γ-Al₂O₃, θ-Al₂O₃ and some amorphous phase. The duplex coatings show favorable corrosion and wear resistance properties. The investigations indicate that the combination of arc spraying and plasma electrolytic oxidation proves a promising technique for surface modification of steels for protective purposes.     

Growth of α-Al2O3:C crystal with highly sensitive optically stimulated luminescence

In this work, an α-Al₂O₃:C crystal was directly grown by the temperature gradient technique (TGT) using Al₂O₃ and graphite powders as the raw materials. The optical, optically stimulated luminescence (OSL) properties and dosimetric characteristics of as-grown crystal were investigated. As-grown α-Al₂O₃:C crystal shows strong absorption band at 205, 230 and 256 nm. Three-dimensional thermoluminescence (TL) emission spectrum of the crystal shows a single emission peak at ∼415 nm. The OSL decay curve can be fitted to two exponentials, the faster component and the slower component. The OSL response of the crystal shows a linear-sublinear-saturation characteristic. As-grown α-Al₂O₃:C crystal shows excellent linearity in the dose range from 5×10⁻⁶ to 50 Gy. For doses higher than the saturation dose (100 Gy), the OSL sensitivity decreases as the dose increases.     

XRD and Mössbauer spectroscopy investigation of Fe2O3–Al2O3 nano-composite

Fe₂O₃–Al₂O₃ nano-composites were synthesized by sol–gel means. The properties of samples sintered at various thermal treatment temperatures were investigated by X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). The experimental results show that the γ- to α-Al₂O₃ transformation occurs at lower temperature after iron oxide doping. The samples obtained at 1173 K contained poorly crystallized γ-Al₂O₃ phase and an amorphous iron oxide. When the temperature of heating was increased to 1373 K, the sample was composed of α-Fe₂O₃/α-Al₂O₃ nano-composite and some solid solution. A superparamagnetic phenomenon was observed until the thermal treatment temperature reached 1373 K.     

Synthesis of nanoparticles and preparation and characterization of nanopolymer matrix composites

In this study, nano α-alumina particles were synthesized by a sol–gel method using aqueous solutions of aluminum isopropoxide and 0.5?M aluminum nitrate. 1/3-benzene disoulfonic acid disodium salt (SDBS) and fluoride were used as surfactant stabilizing agent and additive, respectively. Results indicated that the finest size for nonagglomerated nanoalumina particles (15–20?nm) was achieved at 950?°C. The next part was about preparing PP nanocomposite containing nano α-Al₂O₃ particles. Mechanical tests, such as tensile, flexural, and impact tests showed that mechanical properties of the composite were enhanced by addition of nano α-Al₂O₃ particles and dispersant to the polymer. However, higher concentration of nano α-Al₂O₃ loading resulted in reduction of those mechanical properties, which could be due to agglomeration of nano α-Al₂O₃ particles. Transmission and scanning electron microscopic observations of the nanocomposites also showed that fracture surface became more roughened by increasing the content of filler loading from 1 to 4% wt.     

Influence of treating frequency on microstructure and properties of Al2O3 coating on 304 stainless steel by cathodic plasma electrolytic deposition

Alumina ceramic coatings were fabricated on 304 stainless steel by cathodic plasma electrolytic deposition (CPED). Influence of treating frequency of the power supply on the microstructure and properties of the coatings were studied. The results indicated that coatings obtained at various frequencies on 304 stainless steels were all composed of α-Al₂O₃ and γ-Al₂O₃, and α-Al₂O₃ was the dominant phase. The contents of α-Al₂O₃ decreased gradually in a very small rate with increasing the frequency and γ-Al₂O₃ gradually increased. The surface of alumina ceramic coating was porous. With increasing the frequency, the coating surface gradually became less rough and more compact, resulting in low surface roughness. The bonding strength of Al₂O₃ coating was higher than 22 MPa and was not strongly affected by treating frequency. With increasing the frequency, the alumina coated steels showed better and gradually increasing corrosion resistance than the uncoated one in 3.5% NaCl solution. The coating steel with desirable corrosion resistance was obtained at 800 Hz whose corrosion current potential and corrosion density were −0.237 V and 7.367 × 10⁻⁸ A/cm², respectively.     

Mechanism of <Emphasis Type="Italic">F</Emphasis>-center luminescence in anion-defective aluminum oxide single crystals

New experimental data illustrating the effect of deep traps on the luminescence properties of anion-defective α-Al₂O₃ single crystals are presented. It was established that deep traps have electronic nature and their filling occurs through photoionization of F centers and is accompanied by F → F⁺-center conversion. Model concepts were developed that describe the luminescence mechanism in anion-defective aluminum oxide single crystals with inclusion of thermal ionization of the excited F-center states. The validity of the model was supported by experimental data obtained in a study of thermoluminescence, thermally stimulated exoelectron emission, and thermally stimulated electrical conductivity.