Influence of nitrogen in aluminum droplet combustion

The influence of nitrogen on the aluminum droplet combustion under forced convection conditions has been studied. An aerodynamic levitation technique of millimetric size liquid droplets heated with a CO₂ laser has been adopted to characterize the combustion of aluminum droplets and, in particular, to observe the surface phenomena. The determination of the burning rate and of the droplet temperature in several atmospheres (H₂O/O₂, H₂O/Ar, H₂O/N₂, and air) has shown that they depend only on the nature and concentration of the oxidizers (O₂ and H₂O); a comparison of experiments in nitrogen and in argon containing mixtures demonstrated that N₂ did not influence the gas phase combustion. However, for nitrogen containing atmospheres we observed the formation of solid aluminum nitride (AlN) at the droplet surface after a latency time depending on the nitrogen pressure. AlN first interacts with the oxide cap producing an aluminum oxynitride, then completely covers the droplet, and finally prevents combustion. The existence of a latency time varying with the nitrogen pressure suggests that the AlN formation is controlled by heterogeneous kinetics. The phenomenon of oxide cap regression during combustion was also observed in all gases, and it is attributed to a chemical decomposition process of alumina by aluminum forming gaseous Al_xO_y species. Therefore, nitrogen effects are significant at the droplet surface rather than in the gas phase, and it is suggested that N₂ is probably one of the main species causing the manifestation of unsteady processes during aluminum droplet burning.     

Nitrogen bonding in aluminum oxynitride films

Assignment of oxidation states of N_1s in XPS spectra of aluminum oxynitride by curve fitting is difficult. The XPS curve fitting was previously discussed in the paper published in J. Non-Cryst. Solids, 224 (1998) 31, in which O_1s photoelectrons from GeO₂ glass were used to illustrate how to fit the XPS spectra. Three different ways were pointed out to eliminate the ambiguity caused by curve fitting such as comparing the data to data from standard samples, investigating the continuous surface modifications caused by slowly sputtering the surface, and monitoring the continuous surface modifications due to gradual increases in surface species under heating, cooling, or irradiation. Our recent work in aluminum oxynitride films provides another example of how to fit the XPS spectra of N_1s by three different oxidation states of N⁺, N²⁺, and N³⁺, by comparison of the measured data to data from previously published results, and by the gradual changes of spectra as functions of the oxygen contents in the films. Three oxidation states in different nitrogen bonding in the aluminum oxynitride, AlO₂N, Al₂O₅N₂, and AlO₃N, were clearly deduced.     

The improvement of Al2O3/AlGaN/GaN MISHEMT performance by N2 plasma pretreatment

This paper discusses the effect of N 2 plasma treatment before dielectric deposition on the electrical performance of a Al2O3 /AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor(MISHEMT),with Al2O3 deposited by atomic layer deposition.The results indicated that the gate leakage was decreased two orders of magnitude after the Al2O3 /AlGaN interface was pretreated by N 2 plasma.Furthermore,effects of N 2 plasma pretreatment on the electrical properties of the AlGaN/Al2O3 interface were investigated by x-ray photoelectron spectroscopy measurements and the interface quality between Al2O3 and AlGaN film was improved.     

X-ray spectral analysis of the interface of a thin Al<Subscript>2</Subscript>O<Subscript>3</Subscript> film prepared on silicon by atomic layer deposition

The extent and phase chemical composition of the interface forming under atomic layer deposition (ALD) of a 6-nm-thick Al₂O₃ film on the surface of crystalline silicon (c-Si) has been studied by depthresolved, ultrasoft x-ray emission spectroscopy. ALD is shown to produce a layer of mixed Al₂O₃ and SiO₂ oxides about 6–8 nm thick, in which silicon dioxide is present even on the sample surface and its concentration increases as one approaches the interface with the substrate. It is assumed that such a complex structure of the layer is the result of interdiffusion of oxygen into the layer and of silicon from the substrate to the surface over grain boundaries of polycrystalline Al₂O₃, followed by silicon oxidation. Neither the formation of clusters of metallic aluminum near the boundary with c-Si nor aluminum diffusion into the substrate was revealed. It was established that ALD-deposited Al₂O₃ layers with a thickness up to 60 nm have similar structure.     

Fracture and debonding behavior of coated brittle alumina layer on ductile aluminum substrate wire

The fracture and debonding behavior of the Al₂O₃ layer coated on a ductile aluminum substrate wire was studied experimentally and analytically. When tensile strain was applied, the brittle Al₂O₃ coating layer showed multiple cracking perpendicular to the tensile axis. After the multiple cracking, compressive fracture of the Al₂O₃ layer arose in the circumferential direction when the layer was thinner than around 30 μm, while interfacial debonding between the Al₂O₃ layer and aluminum substrate arose when it was thicker. Such a difference in behavior between thin and thick layers could be accounted for by the difference in the layer thickness-dependence of the tensile radial stress at the interface and the compressive hoop stress of the Al₂O₃ layer calculated by the finite element method; the former stress increases while the latter one decreases with increasing layer thickness.     

Phase composition and the structure of (1–<Emphasis Type="Italic">x</Emphasis>)KNO<Subscript>3</Subscript> + <Emphasis Type="Italic">x</Emphasis>Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposites by X-ray diffraction

The phase composition and the structural properties of potassium nitrate KNO₃ and its heterogeneous composites with nanometer-sized powder of aluminum oxide Al₂O₃ have been studied by X-ray diffraction at various concentrations of an Al₂O₃ nanopowder. It is found that, in the (1–x)KNO₃ + xAl₂O₃ nanocomposites, additional high-temperature rhombohedral phase of potassium nitrate (phase III) with lattice parameters a = 5.4644 Å and c = 9.0842 Å. With increasing concentration of Al₂O₃ nanopowder, the content of the main potassium nitrate phase (phase II) is found to significantly decrease, and the relative fraction of the phase III in the total content of the nitrate in the composite composition increases. This phase is assumed to be “frozen” in the nanocomposite at the KNO₃–Al₂O₃ interface. The estimated size of KNO₃ crystallites in the phase III is more than 20 nm.     

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.     

Sonochemical synthesis of supersaturated Ga–Al liquid-alloy fine particles and Al3+-doped γ-Ga2O3 nanoparticles by direct oxidation at near room temperature

In this study, we investigated the fabrication of supersaturated gallium (Ga)–aluminum (Al) liquid alloy and Al³⁺-doped γ-Ga₂O₃ nanoparticles (NPs) at near room temperature (60 °C) using sonochemical and sonophysical effects. Supersaturated Ga–Al liquid alloy microparticles (D_av = 1.72 µm) were formed and stabilized at 60 °C by the thermal nonequilibrium field provided by sonochemical hot spots. Compared with liquid Ga, supersaturated Ga–Al liquid alloy was rapidly oxidized to a uniform oxide without Al₂O₃ or Al deposition. Thus, ultrafine Al³⁺-doped γ-Ga₂O₃ NPs were obtained after only 1 h of ultrasonic irradiation at 60 °C. The oxidation of liquid Ga was remarkably accelerated by alloying with metallic Al and ultrasonic irradiation, and the time was shortened. The average diameter and surface area of the γ-Ga₂O₃-based NPs were 59 nm and 181 m²/g, respectively. Compared with γ-Ga₂O₃, the optical bandgap of the Al³⁺-doped γ-Ga₂O₃ NPs was broadened, and the thermal stability improved, indicating Al³⁺-doping into the γ-Ga₂O₃ lattice. However, the lattice constant of γ-Ga₂O₃ was almost unchanged with or without Al³⁺-doping. Al³⁺ was introduced into the defect sites of Ga³⁺, which were massively induced in the defective spinel structure during ultrasonic processing. Therefore, sonochemical processing, which provides nonequilibrium reaction fields, is suitable for the synthesis of supersaturated and metastable materials in metals and ceramics fields.     

Effect of aluminum substitution on the structural and magnetic properties of cobalt ferrite synthesized by sol-gel auto combustion process

Aluminum substituted cobalt ferrite powders (CoFe_2−xAl_xO₄) with varying composition from 0.0 to 1.0 in the step of 0.2 have been obtained by sol-gel auto combustion technique using citric acid as a fuel. The metal nitrate to fuel ratio was maintained 1:4 throughout the synthesis of CoFe_2−xAl_xO₄. The thermal analysis of as prepared samples is done by TGA technique. The compositional stoichiometry of the prepared samples is confirmed by Energy dispersive X-ray analysis technique. Single phase cubic spinel structure and nano phase structure of the synthesized powders were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallite size of 16-26 nm was obtained using Scherrer formula. SEM analysis shows the formation of uniform grain growth. The grain size obtained from SEM results is of the order of 30 nm. Maximum specific surface area was observed to be of the order of 52 m²/gm. The highest value of saturation magnetization and coercivity was observed for pure cobalt ferrite sample and it decreases as the aluminum content x increases. A strong co-relation between the saturation magnetization and aluminum content was observed. The decrease in magnetic properties is due to the substitution of aluminum ions in place of Fe³⁺.     

钛铝合金高温氧化机理电子理论研究

为了从电子层面揭示钛铝合金高温氧化的物理本质,采用递归法与Castep相结合的方式, 计算了原子埋置能、亲和能、结合能等电子结构参数,探索合金氧化机理.研究表明: 氧在钛中有较大固溶度,氧原子可以在钛表面的基体内聚集,逐步向深层扩散. 氧与钛具备较强的亲和力,能形成钛的氧化膜.钛基体中铝原子间具有相互吸引力, 能形成铝的原子团簇.铝原子团簇中的钛原子间相互排斥与铝形成化合物. 铝、钛与氧的亲和能相近,不易发生铝的优先氧化,而是同时生成钛的氧化物和铝的氧化物. Al₂O₃比TiO₂的结合能略低,因而更加稳定,铝在TiO₂中有较大的固溶度, 能替换其中的钛形成更稳定的Al₂O₃氧化物.     

Comparative effects of macro-sized aluminum oxide and aluminum oxide nanoparticles on erythrocyte hemolysis: influence of cell source,temperature, and size

Al₂O₃ is the most abundantly produced nanomaterial and has been used in diverse fields, including the medical, military, and industrial sectors. As there are concerns about the health effects of nanoparticles, it is important to understand how they interact with cells, and specifically with red blood cells. The hemolysis induced by three commercial nano-sized aluminum oxide particles (nanopowder 13 nm, nanopowder <50 nm, and nanowire 2–6 × 200–400 nm) was compared to aluminum oxide and has been studied on erythrocytes from humans, rats, and rabbits, in order to elucidate the mechanism of action and the influence of size and shape on hemolytic behavior. The concentrations inducing 50 % hemolysis (HC₅₀) were calculated for each compound studied. The most hemolytic aluminum oxide particles were of nanopowder 13, followed by nanowire and nanopowder 50. The addition of albumin to PBS induced a protective effect on hemolysis in all the nano-forms of Al₂O₃, but not on Al₂O₃. The drop in HC₅₀ correlated to a decrease in nanomaterial size, which was induced by a reduction of aggregation. Aluminum oxide nanoparticles are less hemolytic than other oxide nanoparticles and behave differently depending on the size and shape of the nanoparticles. The hemolytic behavior of aluminum oxide nanoparticles differs from that of aluminum oxide.     

EPR of Transition-Metal Ions in Microplasma Coatings on the Aluminum Alloy D16

The influence of microplasma oxidation (MPO) conditions on the creation of the Al₂O₃ coatings on aluminum alloy D16 was studied by the electron paramagnetic resonance (EPR) technique. Aluminum alloy D16 contains alloyed transition metals such as Cu, Mn, Fe, Ni, and Ti incorporated into the coating structure during MPO. EPR data on transition-metal ions allow estimating the appearance of a high-temperature phase of Al₂O₃ due to the difference in incorporation coefficients of transition-metal ions in low- and high-temperature Al₂O₃ phases of MPO coatings. These data show that no high-temperature Al₂O₃ phases were created during anodic MPO. Authors' address: Vladimir A. Nadolinny, Institute of Inorganic Chemistry, Lavrentyev prospect 3, Novosibirsk 630090, Russian Federation     

Interfacial structure of sub-micron Al2O3 particles in aluminum matrix

The surfaces of ellipsoidal Al₂O₃ particles with average size of 0.15 μm and the interfaces between the Al₂O₃ particles and 1070Al were investigated by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM).The results show that the surfaces of Al₂O₃ particles appear to be polyhedrons consisting of crystal planes with small angle, while every plane of the polyhedrons could be considered as a stepped structure composed of close-packed planes along the close-packed direction. The interfaces of the 0.15 μm Al₂O₃p/1070Al composite bond well, without any interfacial reaction products. It is proposed that there are several kinds of crystallographic orientation relationships between the aluminum matrix and Al₂O₃particles due to the polyhedral structure. In our study, such orientation relationships are found to be {110} _Al ||{1100} _Al2O3 and ?110? _Al ||?1126? _Al2O3 .     

Nanocomposites of Poly(vinyl alcohol) Reinforced with Chemically Modified AL2O3: Synthesis and Characterization

The surface of α-alumina (Al₂O₃) nanoparticles was first modified with γ-aminopropyltriethoxy silane as a coupling agent. Then a series of poly(vinyl alcohol)/ surface modified Al₂O₃ nanocomposite suspensions were prepared in ethanol by a simple ultrasonic irradiation process. Composite films with 5, 10, and 15 wt % of inorganic Al₂O₃ nanoparticles were achieved after solvent evaporation. The formation of the composite materials were confirmed by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and optical transparencies. The FE-SEM and TEM results showed a homogenous dispersion of nanoscale inorganic particles in the poly(vinyl alcohol) matrix. TGA thermographs showed that the thermal stability of the prepared Al₂O₃-reinforced nanocomposites was improved, increasing with increasing content of the nanoparticles. According to the optical transparencies, the optical clarity of poly(vinyl alcohol)/Al₂O₃ nanocomposite films was only slightly affected by the presence of the Al₂O₃ content.     

Fabrication of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al structure by nitric acid oxidation at room temperature

A thick Al₂O₃/aluminum (Al) structure has been fabricated by oxidation of Al with 68wt% and 98wt% nitric acid (HNO₃) aqueous solutions at room temperature. Measurements of the Al₂O₃ thickness vs. the oxidation time show that reaction and diffusion are the rate-determining steps for oxidation with 68wt% and 98wt% HNO₃ solutions, respectively. Observation of transmission electron micrographs shows that the Al₂O₃ layer formed with 68wt% HNO₃ has a structure with cylindrically shaped pores vertically aligned from the Al₂O₃ surface to the Al₂O₃/Al interface. Due to the porous structure, diffusion of HNO₃ proceeds easily, resulting in the reaction-limited oxidation mechanism. In this case, the Al₂O₃/Al structure is considerably rough. The Al₂O₃ layer formed with 98wt% HNO₃ solutions, on the other hand, possesses a denser structure without pores, and the Al₂O₃/Al interface is much smoother, but the thickness of the Al₂O₃ layer formed on crystalline Al regions is much smaller than that on amorphous Al regions. Due to the relatively uniform Al₂O₃ thickness, the leakage current density flowing through the Al₂O₃ layer formed with 68wt% HNO₃ is lower than that formed with 98wt% HNO₃.     

Defect formation and transport in SrFe1-xAlxO3-δ

Perovskite-related phases derived from SrFeO_3-δ are among known mixed conductors with highest oxygen permeability and are thus of interest as the ceramic membrane materials for oxygen separation and partial oxidation of light hydrocarbons. Dense ceramics of SrFe_1-xAl_xO_3-δ (x=0.1–0.5) were prepared via the glycine-nitrate process. The cubic solid solution formation was found to occur in the concentration range x=0–0.35. Increasing aluminum content leads to decreasing thermal expansion coefficients (TECs), relative fraction of Fe⁴⁺ under oxidizing conditions, and also the total conductivity, predominantly p-type electronic at oxygen pressures close to atmospheric. The TECs vary in the range (13.5–16.4)×10^?6 K^?1 at 373–923 K and increase up to (18.6–31.9)×10^?6 K^?1 at 923–1273 K. The oxygen permeation fluxes decrease moderately with aluminum additions. The Mössbauer spectroscopy data and p(O₂) dependencies of electrical properties indicate a small-polaron mechanism of electronic transport in SrFe_1-xAl_xO_3-δ. Reducing oxygen partial pressure results in transition from dominant p- to n-type electronic conduction. The low-p(O₂) stability limit of SrFe_1-xAl_xO_3-δ perovskites lies between that of LaFeO_3-δ and Fe/Fe_1-γO boundary.     

Preparation of modified polymer blend and electrical performance

In this study, a modified binary polymer blend made up of polycarbonate and polystyrene blend has been prepared by loading of aluminum oxide (Al₂O₃) as a dopant. The role of alumina with polymer blend system was addressed in view of interfacing criteria. The filler concentration of modified blend was taken as 5, 10, and 15%. The morphological, thermal, and electrical properties were characterized by various techniques. Optical microscopy confirms the homogenous dispersion of Al₂O₃ in blend. The presence of alumina was detected by subatomic level using atomic force microscope (both two and three dimensional approach). The differential scanning calorimetric thermographs demonstrate decreasing softing point as function of alumina loading. The dielectric properties such as dielectric constant, loss, and electrical modulus were studied under DC bias. The effect of DC bias exhibits significant changes at low amount of Al₂O₃. The dielectric polarization supports Maxwell Wagner (MW) theory due to low frequency response. 15% Al₂O₃ gives the highest dielectric constant (ε′) value (3.5?×?10⁵) at 10 Hz. The polymer modified blend with Al₂O₃ may be used as a one of the best dielectric medium.     

Structure of Al2O3 thin layers synthesized on the silicon surface by atomic layer deposition

The distribution of the phase and chemical composition at an Al₂O₃/Si interface is studied by depth-resolved ultrasoft x-ray emission spectroscopy. The interface is formed by atomic layer deposition of Al₂O₃ films of various thicknesses (from several to several nanometers to several hundreds of nanometers) on the Si(100) surface (c-Si) or on a 50-nm-thick SiO₂ buffer layer on Si. L _2,3 bands of Al and Si are used for analysis. It is found that the properties of coatings and Al₂O₃/Si interfaces substantially depend on the thickness of the Al₂O₃ layer, which is explained by the complicated character of the process kinetics. At a small thickness of coatings (up to 10–30 nm), the Al₂O₃ layer contains inclusions of oxidized Si atoms, whose concentration increases as the interface is approached. As the thickness increases, a layer containing inclusions of metallic Al clusters forms. A thin interlayer of Si atoms occurring in an unconventional chemical state is found. When the SiO₂ buffer layer is used (Al₂O₃/SiO₂/Si), the structure of the interface and the coating becomes more perfect. The Al₂O₃ layer does not contain inclusions of metallic aluminum, does not vary with the sample thickness, and has a distinguished boundary with silicon.     

On the synthesis of AlPO4-21 molecular sieve by vapor phase transport method and its phase transformation to AlPO4-15 molecular sieve

An experimental design was applied to the synthesis of AlPO₄-21 molecular sieve (AWO structure) by vapor phase transport (VPT) method, using tetramethylguanidine (TMG) as the template. In this study, the effects of crystallization time, crystallization temperature, phosphor content, template content and water content in the synthesis gel were investigated. The materials obtained were characterized by X-ray diffraction, scanning electron microscopy and fourier transform infrared spectroscopy (FT-IR). Microstructural analysis of the crystal growth in vapor synthetic conditions revealed a revised crystal growth route from zeolite AlPO₄-21 to AlPO₄-15 in the presence of the TMG. Homogenous hexagonal prism AlPO₄-21 crystals with size of 7 × 3 μm were synthesized at a lower temperature (120 °C), which were completely different from the typical tabular parallelogram crystallization microstructure of AlPO₄-21 phase. The crystals were transformed into AlPO₄-21 phase with higher crystallization temperature, longer crystallization time, higher P₂O₅/Al₂O₃ ratio and higher TMG/Al₂O₃ ratio.