Reactive Laser Ablation Synthesis of Nanosize Alumina Powder

An aluminum (Al) target was laser ablated in an oxygen (O₂) atmosphere, producing nanosize alumina (Al₂O₃) powder. The powder surface area decreased (and the particle size increased) with both increasing oxygen pressure and laser fluence. All powders produced had surface areas between 135 and 250 m²/g, corresponding to primary particle sizes ranging from 7 to 3 nm in radius. Phase evolution with temperature was studied via X-ray diffraction. These powders showed a direct transformation from γ- to α-alumina at approximately 1200°C, bypassing other transition alumina phases, while still maintaining small particle size ( 30 nm). Despite the nanosize particles, green densities equal to 54% of the skeletal density (i.e., true density of the solid phase) were obtained by uniaxial pressing at 40 MPa.     

Synthesis of Nanosized Aluminum Nitride Powders by Pulsed Laser Ablation

The influence of the target material, fluence, laser wavelength, and nitrogen pressure on the synthesis of AlN nanosized powders via reactive laser ablation has been investigated. Using infrared laser radiation and fluences of ≥11 J/cm², pure AlN nanosized powders were produced at nitrogen pressures of ≥1.3 kPa via ablation of an AlN target and ≥13.3 kPa via ablation of an aluminum target. With ultraviolet laser radiation, AlN powders could be synthesized at a lower fluence (9 J/cm² at a pressure of 8 kPa). The mean powder size was 7.5−15 nm.     

Nanosize Powders of Aluminum Nitride Synthesized by Pulsed Wire Discharge

Nanosize particles of aluminum nitride have been successfully synthesized by a pulsed wire discharge (PWD). Intense pulsed current through an aluminum wire evaporated the wire to produce a high-density plasma. The plasma was then cooled by an ambient gas mixture of NH₃/N₂, resulting in nitridation. As a result, nanosize particles of aluminum nitride were formed. The average particle diameter was found to be ∼28 nm with a geometric standard deviation of 1.29. The maximum AlN content of 97% in the powders was achieved by optimizing various parameters: the gas pressure, the ratio of NH₃ and N₂, the wire diameter, the pulse width, and the input electrical energy. The ratio of the AlN powder production to the electrical energy consumption was evaluated as ∼40 g/(kW·h). Thus, PWD is a very efficient and promising method to synthesize nanosize powders of AlN.     

Synthesis of Aluminum Nitride/Boron Nitride Composite Materials

Aluminum nitride/boron nitride composite was synthesized by using boric acid, urea, and aluminum chloride (or aluminum lactate) as the starting compounds. The starting materials were dissolved in water and mixed homogeneously. Ammonolysis of this aqueous solution resulted in the formation of a precomposite gel, which converted into the aluminum nitride/boron nitride composite on further heat treatment. Characterization of both the precomposite and the composite powders included powder X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. Analysis of the composite revealed that the aluminum nitride phase had a hexagonal structure, and the boron nitride phase a turbostratic structures.     

Carbothermal Synthesis of Aluminum Nitride Using Sucrose

Several aluminum oxides (α-Al₂O₃, θ-Al₂O₃, and AIOOH) were examined to study the differences in reaction behavior and powder characteristics during carbothermal nitrida-tion to AIN using sucrose and carbon black. The reaction conditions investigated were carbon-to-alumina ratio, reaction temperature, and time. Carburized sucrose resulted in Full conversion to AIN and produced a uniform powder morphology using a near-istoichiometric ratio of C:Al₂O₃ while carbon black required higher C:Al₂O₃ ratios (i.e., >4:1) for full conversion and led to agglomeration of the AIN powder. The most favorable reaction temperature was 1600°C, with the reaction time to full conversion being dependent on the type of Al₂O₃. The particle and agglomerate size of the AIN powders did not change significantly with reaction time. However, the particle size and morphology were strongly dependent on that of the initial AI₂O₃ with sucrose, whereas agglomeration of the AIN occurs when using carbon black. A solid–solid reaction mechanism is proposed.     

Carbothermal Synthesis of Nanocrystalline Aluminum Nitride Powders

A new precursor technique for the carbothermal synthesis of nanocrystalline AlN powder has been developed. A precursor that contains an intimate mixture of nanocrystalline Al₂O₃ and carbon has been synthesized by using a chemical pyrophoric reaction. The formation of AlN starts at 1473 K, and complete conversion has been observed at temperatures >1673 K. The synthesized AlN particles are nanocrystalline (<100 nm) in size.     

Aluminum Nitride Whisker Formation during Combustion Synthesis

In this study, the microstructural development of AlN during combustion synthesis (CS) in a nitrogen atmosphere was investigated. CS using an aluminum–50 wt% AlN–3 wt% MgCl₂ powder compact yielded a mixture of AlN whiskers and powder. The microstructural development during the combustion reaction was studied by heating the compact to various temperatures. Based on the experimental results, the formation mechanisms of the AlN with a whisker morphology were discussed.     

Combustion Synthesis of Aluminum Nitride Particles and Whiskers

Aluminum nitride (AlN) was produced via the combustion synthesis of loosely packed aluminum powder (pore fraction of ∼0.8) in a graphite reactor that was lined with permeable carbon felt. Almost-complete conversion was achieved with a forced flow of nitrogen for beds 50-150 mm deep (mass of 200-650 g). The product was in the form of a loosely aggregated bed, with regions of distinct morphology, and had a predominantly whisker morphology. Some control over the microstructure was possible by changing the processing parameters. The addition of 5% of ammonia to the nitrogen resulted in a uniform production of whiskers, whereas a 50% solid-phase dilution with AlN favored the production of particles.     

Fabrication of Transparent Silicon Nitride from Nanosize Particles

Compaction of ultrafine silicon nitride (Si₃N₄) powder at high pressures and various temperatures followed by pressureless sintering was investigated. The powder, consisting of nearly spherical particles (16 nm in diameter) of amorphous stoichiometric Si₃N₄, was pressed in a diamond anvil cell under pressures up to 5 GPa and temperatures ranging from liquid nitrogen to 500°C. Quality of compaction, evaluated by visual transparency and hardness of the produced compacts, depended on the amount of adsorbed gases on the surface of the particles and on the temperature of compaction. Visually transparent compacts were produced by pressing the starting powder without outgassing in liquid nitrogen under 5 GPa. The transparent compacts exhibited a hardness of 1200 kg/mm² after pressing in the diamond anvil cell at 500°C for 3 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 1400°C in a tube furnace under nitrogen, the hardness of these samples increased to over 2000 kg/mm² and the visual transparency was maintained. The results demonstrated that transparency was maintained. The results demonstrated that transparent compacts of nanosize amorphous Si₃N₄ particles could be sintered to high hardness at relatively low temperatures without using sintering aids or applying pressure during sintering.     

AlN陶瓷粉末的主要制备方法及展望

论述了国内外氮化铝陶瓷粉末的主要制备方法：铝粉直接氮化法、Al2O3碳热还原法、化学气相沉积法、溶胶-凝胶法、自蔓延高温合成法和等离子化学合成法,分析了这几种制备方法的特点和研究进展,为氮化铝粉末的制备指明了方向。     

Hydrothermal Synthesis of Nanosize alpha-Al2O3 from Seeded Aluminum Hydroxide

alpha-Alumina and boehmite particles were synthesized by coprecipitation followed by a hydrothermal treatment. X-ray diffraction (XRD) indicated that alpha-Al₂O₃ was the major phase and coexisted with 4% of boehmite in the presence of the alpha-Al₂O₃ seeds. On the other hand, a single boehmite phase was obtained in the absence of the alpha-Al₂O₃ seed particles. The powder densified in the temperature range from 1050° to 1350°C. High-resolution transmission electron microscopy (HRTEM) showed that the particle size of the synthesized alpha-Al₂O₃ was 60 nm. The surface area was 245 m²/g.     

Aerosol Synthesis of Aluminum Nitride Powder Using Metalorganic Reactants

We describe a new laboratory-scale aerosol process for making submicrometer AIN powder by reaction of gas phase Al(C₂H₅)₃ and NH₃. The process can produce spherical, near-stoichiometric AIN powder with mean particle diameter ranging from 0.07 to 0.20 μm and polydispersity index between 0.2 and 0.5. The inlet TEA concentration is found to have the greatest effect on particle size, with the furnace temperature and carrier gas flow rate having lesser influence. Our results appear to favor a LaMer-type mechanism (particle nucleation, followed by condensation growth) over fusion-coalescence to explain particle growth.     

Self-Propagating High-Temperature Synthesis of Aluminum Nitride under Lower Nitrogen Pressures

The synthesis of aluminum nitride (AlN) via self-propagating high-temperature synthesis (SHS) was attempted, using aluminum powder that was mixed with AlN powder as a diluent. The AlN content in the reactant was varied over a range of 30%–70%, and the nitrogen pressure was varied over a range of 0.1–1.0 MPa. The SHS reaction that was performed using a reactant that contained 50% AlN diluent, under a nitrogen-gas pressure of 0.8 MPa, yielded the highest conversion ratio of aluminum powder to AlN powder. A mechanism for the reaction of aluminum with nitrogen gas during the SHS process was discussed, based on observations of the microstructures of the reaction zone and products.     

Synthesis of Aluminum Nitride Nanopowder by Gas-Reduction–Nitridation Method

Aluminum nitride (AlN) nanopowder was successfully synthesized from transition alumina nanopowder using an NH₃–C₃H₈ gas mixture as a reduction–nitridation agent. Phase-pure, nanocrystalline AlN powder with a specific surface area of 36.4 m²/g and a mean particle size of 51 nm was prepared under typical reaction conditions. The resulting AlN nanopowders possessed excellent sinterability, allowing full densification in conventional processing, even without the addition of sintering aids.     

Composition and Microstructure of Chemically Vapor-Deposited Boron Nitride, Aluminum Nitride, and Boron Nitride + Aluminum Nitride Composites

The composition and microstructure of dispersed-phase ceramic composites containing BN and AIN as well as BN and AIN single-phase ceramics prepared by chemical vapor deposition have been characterized using X-ray diffraction, scanning electron microscopy, electron microprobe, and transmission electron microscopy techniques. Under certain processing conditions, the codeposited coating microstructure consists of small single-crystal AIN fibers (whiskers) surrounded by a turbostratic BN matrix. Other processing conditions resulted in single-phase films of AIN with a fibrous structure. The compositions of the codeposits range from 2 to 50 mol% BN, 50 to 80 mol% AIN with 7% to 25% oxygen impurity as determined by electron microprobe analysis.     

脉冲激光溅射合成八-(甲基硅倍半氧烷)

以脉冲浙江溅射作用于聚甲基硅氧烷,经真空升华,直接得到了一种产物的单晶。经Ｘ－射线晶体衍射测定确定其为具有（ＣＨ３ＳｉＯ１．５）８组成的硅氧烷。该种产物具有多环立体空间构型。结果表明,在脉冲激光的作用下,反应物的链状结构发生解离,并在等离子体条件下进行结构重组,生成了具有立体空间构型的产物。     

Effect of Polynuclear Complex Content on the Synthesis of Aluminum Nitride from Aluminum Polynuclear Complex

Aluminum nitride powders were synthesized from an aluminum polynuclear complex and glucose. Basic aluminum chloride (BAC) was used as the aluminum polynuclear complex. The effect of the polynuclear complex content on the nitridation reaction and particle size of the AIN powder synthesized was investigated. BAC solutions with various polynuclear complex contents were synthesized using an aqueous solution prepared from AlCl₃ and Al metal with various compositions. In this system, AlN was synthesized through γ-Al₂O₃ as an intermediate regardless of the polynuclear complex content. Polynuclear complex content affects the reactivity of nitridation and the particle size of the synthesized AIN powder. The reactivity of nitridation and specific surface area of the products increased with the polynuclear complex content. When the precursor with a polynuclear complex content of 94% was calcined at 1400°C, the AlN content reached 95%. Crystallite size by X-ray diffraction measurement and particle size calculated from the specific surface area for the products were in good agreement, indicating that AlN particles formed in the synthesis process are in the form of single crystals. AlN powder synthesized from the precursor with a polynuclear complex content above 80% had a fine particle size and narrow size distribution.     

Reaction Synthesis of Aluminum Nitride–Boron Nitride Composites Based on the Nitridation of Aluminum Boride

Aluminum nitride–boron nitride (AlN–BN) composites were prepared based on the nitridation of aluminum boride (AlB₂). AlN powder was added to change the BN volume fraction in the obtained composites. Thermogravimetry–differential thermal analysis (TG-DTA), X-ray diffractometry, and the nitridation ratio were used to investigate the nitridation process of AlB₂. At ∼1000°C, a sharp exothermic peak occurred in the DTA curve, corresponding to the rapid nitridation of aluminum in AlB₂. On the other hand, the nitridation of the transient phase, Al_1.67B₂₂, was very slow when the temperature was <1400°C. However, the nitridation speed obviously accelerated at temperatures >1600°C. The pressure of the nitrogen atmosphere was also an important factor; high nitrogen pressure remarkably promoted nitridation. Treatment at 2000°C was disadvantageous for nitridation, because of the rapid formation of a dense surface layer that inhibited nitrogen diffusion into the specimen interior. Three specimens, with 5 wt% Y₂O₃ additive and different BN contents, were prepared by pressureless reactive sintering, according to the determined sintering schedule. Electron microscopy (scanning and transmission) observations revealed that the in-situ -formed BN flakes were homogeneously and isotropically distributed in the AlN matrix. A schematic mechanism for microstructural formation was developed, based on the results of nitridation and the microstructural features of the obtained composites. The obtained composites, with a low BN content, exhibited a high bending strength, comparable to that of reported hot-pressed AlN–BN composites.     

Oxidation Kinetics of Aluminum Nitride

Thermal oxidation kinetics of aluminum nitride (AlN) powders, having fine- and coarse-particle-size distributions, were studied using thermogravimetric analysis (TGA). The kinetics showed dependence on the particle-size parameter, and the experimental TGA data were curve fitted using empirical mass relations employing both linear and parabolic models. The simulations predicted mixed kinetics in AlN oxidation.