Investigation on mechanochemical synthesis of Al2O3/BN nanocomposite by aluminothermic reaction

Alpha-alumina–boron nitride (α-Al₂O₃–BN) nanocomposite was synthesized using mixtures of aluminum nitride, boron oxide and pure aluminum as raw materials via mechanochemical process under a low pressure of nitrogen gas (0.5 MPa). The phase transformation and structural evaluation during mechanochemical process were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential thermal analysis (DTA) techniques. The results indicated that high exothermic reaction of Al–B₂O₃ systems under the nitrogen pressure produced alumina, aluminum nitride (AlN), and aluminum oxynitride (Al₅O₆N) depending on the Al value and milling time, but no trace of boron nitride (BN) phases could be identified. On the other hand, AlN addition as a solid nitrogen source was effective in fabricating in-situ BN phase after 4 h milling process. In Al–B₂O₃–AlN system, the aluminothermic reaction provided sufficient heat for activating reaction between B₂O₃ and AlN to form BN compound. DTA analysis results showed that by increasing the activation time to 3 h, the temperature of both thermite and synthesis reactions significantly decreased and occurred as a one-step reaction. SEM and TEM observations confirmed that the range of particle size was within 100 nm.     

Synthesis and characterization of high-purity aluminum nitride nanopowder by RF induction thermal plasma

High-purity aluminum nitride nanopowder was synthesized using the RF induction thermal plasma technique. The nitrogen gas flow rate, plasma power and reactor pressure were controlled to increase the conversion rate of Al powder to AlN nanoparticles. The compositions of the obtained powders were investigated through XRD and EDS analysis. The synthesized aluminum nitride nanoparticles included polygonal and rod-shaped nanoparticles and ultra-fine particles below 10 nm. The particle sizes generally ranged from 20–60 nm in TEM analysis. The specific surface area, band structure and impurities of aluminum nitride nanoparticles were also evaluated by BET, FTIR and ICP-OES analysis.     

Nanocrystalline Aluminum Nitride: I, Vapor-Phase Synthesis in a Forced-Flow Reactor

A forced-flow reactor has been designed for the synthesis of nanocrystalline AlN via in situ and ex situ nitridation of aluminum. Various reactor parameters, including evaporation temperature, microwave plasma generation, reactor pressure, gas flow rate, nitriding gas, carrier gas, and crucible purge, have been examined and optimized. Fully nitrided powders with crystallite sizes of 10–100 nm and surface areas of 45–370 m²/g were produced using these techniques. These ultrafine AlN particles were highly moisture-sensitive, but they could be processed and handled without exposure to air to achieve fully dense materials with low oxygen content.     

Conditions for growth of AlN single crystals in Al–Sn flux

In this study, we investigated the Al–Sn flux system and its growth conditions to obtain AlN single crystals. AlN single crystals of a size of 50 μm were successfully grown using an Al–Sn melt under nitrogen gas pressure. The growable region of the AlN crystals was established using a pressure‐temperature diagram. The required nitrogen gas pressure for the growth of the AlN crystals was found to decrease with increasing temperature, and AlN was grown at 0.1 MPa nitrogen pressure above 1300°C. By investigating the AlN yield with various Al concentrations, we confirmed that the Al component in the Al–Sn melt facilitated nitrogen dissolution. Finally, scanning electron microscopy analysis showed that the obtained AlN particles showed good morphology.     

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.     

Oxidation of Sintered Aluminum Nitride at Near-Ambient Temperatures

Oxidation of sintered aluminum nitride at low temperatures (20°–200°C) was studied using transmission electron microscopy (TEM). Particles of α-Al₂O₃, about 20–30 Å in size, were found to form within minutes on freshly cleaned surfaces of AlN at room temperature. The oxide was found to grow nearly epitaxially on AlN when the {0001}_AlN planes were exposed to the surface. Limited nonepitaxial oxidation was also observed when the basal planes were inclined to the TEM foil surface. After 10 h in air at 75°C, the particles coarsened to about 50 Å, while after 150 h at 200°C, an oxide film, about 500 Å thick, was observed on some grains.     

Synthesis of Nanocrystalline Aluminum Nitride by Nitridation of δ-Al2O3 Nanoparticles in Flowing Ammonia

Nanocrystalline aluminum nitride (AlN) with surface area more than 30 m²/g was synthesized by nitridation of nanosized δ-Al₂O₃ particles using NH₃ as a reacting gas. The resulting powders were characterized by CHN elemental analysis, X-ray diffraction (XRD), Fourier transform infrared spectra, X-ray photoelectron spectra, field-emission scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller surface area techniques. It was found that nanocrystalline δ-Al₂O₃ was converted into AlN completely (by XRD) at 1350°–1400°C within 5.0 h in a single-step synthesis process. The complete nitridation of nanosized alumina at relatively lower temperatures was attributed to the lack of coarsening of the initial δ-Al₂O₃ powder. The effect of precursor powder types on the conversion was also investigated, and it was found that α-Al₂O₃ was hard to convert to AlN under the same conditions.     

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.     

Combustion Based Synthesis of AlN Nanoparticles Using a Solid Nitrogen Promotion Reaction

Combustion based synthesis of AlN nanoparticles using the “solid nitrogen” promotion reaction was investigated in Al₂O₃ + 3Mg system in nitrogen atmosphere. A controlled amount of Mg + 0.5NH₄Cl mixture as a solid source of nitrogen was blended with the Al₂O₃ + 3Mg starting system and the synthesis reaction of AlN nanoparticles was conducted using the exothermic heat of the entire reaction system. The resulting AlN nanoparticles were characterized by X‐ray diffraction (XRD), Raman and Fourier transform infrared spectroscopy (FTIR), PL spectroscopy, field‐emission scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller surface area techniques. The analysis results confirmed that single phase and crystalline AlN nanoparticles with an average size of 50–500 nm were obtained from the developed approach. Photoluminescence spectra of AlN nanopowders under the excitation of 230–270 nm UV light revealed that AlN emits yellow‐red light having a wavelength near to 590 nm. The chemistry of the combustion process is discussed and the basic reactions that led to the formation of AlN are presented.     

Reactive Laser Ablation Synthesis of Nanosize Aluminum Nitride

An aluminum (Al) target was laser ablated in a nitrogen (N₂) atmosphere, producing aluminum nitride (AlN) powder. These powders were calcined at 900°C for 2 h. Powders were produced at various nitrogen pressures, and the calcined powders were tested for unreacted aluminum content, using differential thermal analysis (DTA). The AlN powder, produced at a laser fluence of 12 J/cm² and a nitrogen pressure of 10.0 kPa (75 torr), showed no evidence of unreacted aluminum by DTA and was phase-pure AlN by X-ray diffraction (XRD). The surface area of this powder is 82 m²/g, corresponding to a particle size of ∼11 nm, which is in good agreement with TEM observations.     

Production of Nanosized YAG Powders with Spherical Morphology and Nonaggregation via a Solvothermal Method

Monodispersed nanosized yttrium aluminum garnet (YAG) powder was synthesized via a mixed-solvent thermal method using stoichiometric amounts of inorganic aluminum and yttrium salts. Pure-phase YAG crystalline powder was obtained at low temperature (290°C) and low pressure (10 MPa). The resulting products were characterized by X-ray powder diffraction (XRD), infrared, and transmission electron microscopy (TEM). XRD results showed that single-phase YAG could be formed directly from an amorphous precursor at 280°C and become fully developed at 290°C. TEM images showed that the YAG powder particles in the study were basically spherical in shape and well-dispersed with a mean grain size of about 60 nm.     

Synthesis of submicron AlN powder using dynamic/thermochemical method

Carbothermal reduction and nitridation (CRN) method, used for the synthesis of nitride-based ceramic powders, is an effective and economic technique that has been widely investigated. In this study, a CRN-based novel approach, denominated as dynamic/thermochemical method (DTM), has been used to synthesize submicron high-purity aluminum nitride (AlN) powders with equiaxed-sized particles. DTM is a modified CRN method in which the reaction takes place in a controlled atmosphere using a rotary tube furnace, allowing the synthesis of fine particle-size powders in a relatively short time. Following the DTM process, homogeneous submicron AlN powders were synthesized from a mixture of aluminum hydroxide (Al(OH)₃) and carbon black at 1450°C for 1.5 h. Furthermore, dynamic synthesis parameters, as well as the use of ammonia (NH₃) and propane (C₃H₈) gas mixtures instead of carbon black and nitrogen, were investigated.     

Synthesis of an AlN Polycrystalline Bulk Layer and Nanotubes by Using NH3 and Bi

A bulk layer of aluminum nitride (AlN) polycrystals was synthesized on a boron nitride crucible surface by heating Al chunks with 5 mol% of bismuth at 1273 K for 3 h under NH₃ gas flow. The fragments of the layer were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The platelet grains of AlN with a size of 0.1–1.0 μm and having preferred orientation of the c -axis perpendicular to the layer were formed at the crucible side. Nanotubes 6–15 μm long and about 20–100 nm thick grew on the gas phase side of the layer.     

Carbon nanopowders from the continuous-wave CO2 laser-induced pyrolysis of ethylene

Carbon nanopowders were obtained by the laser pyrolysis of ethylene. The high-temperature gradients and very rapid reaction times characteristic of the process lead to the formation of very fine powders. Carbon powders obtained in runs with different laser power values (400–900 W), pressures (250–950 mbar), and gas flows (100–300 sccm) were characterised by transmission electron microscopy (TEM), including high-resolution mode (HREM), electron energy loss spectroscopy (EELS), X-ray diffraction (XRD) and Raman spectroscopy. The carbon particles were found to be approximately spherical in shape, with diameters around 45 nm, which may coalesce into larger agglomerates. The particles were found to be made up of layers forming a turbostratic structure. The experimental parameters influence the soot morphology and particle microstructure. Increasing the laser power and gas pressure leads to less coalescence and increased order. Structural parameters are presented for particles produced under different conditions.     

Microstructure observation of β-sialon-15R ceramics synthesized from aluminum dross

Fully dense β-sialon-15R multiphase ceramics were synthesized by hot pressing sintering using aluminum dross as raw material. Transmission electron microscopy (TEM) was used to study the microstructure, notably for the impurity and glass phase. The results show that β-sialon grains are generally equiaxed, and 15R AlN polytypoid grains show a fibre-like morphology. The main impurity phase is Fe₅Si₃. High resolution electron microscopy (HREM) results confirm the interface between β-sialon grains and/or 15R grains to be clean in the sample synthesized at 1750 °C, and the glassy phase only exist in triple junctions and pockets. AlN polytypoids in the multiphase ceramics provide a path to reduce the glass phase formed from the oxide impurity in aluminum dross.     

The Influences of H2 Plasma Pretreatment on the Growth of Vertically Aligned Carbon Nanotubes by Microwave Plasma Chemical Vapor Deposition

The effects of H₂ flow rate during plasma pretreatment on synthesizing the multiwalled carbon nanotubes (MWCNTs) by using the microwave plasma chemical vapor deposition are investigated in this study. A H₂ and CH₄ gas mixture with a 9:1 ratio was used as a precursor for the synthesis of MWCNT on Ni-coated TaN/Si(100) substrates. The structure and composition of Ni catalyst nanoparticles were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The present findings showed that denser Ni catalyst nanoparticles and more vertically aligned MWCNTs could be effectively achieved at higher flow rates. From Raman results, we found that the intensity ratio of G and D bands (I _D/I _G) decreases with an increasing flow rate. In addition, TEM results suggest that H₂ plasma pretreatment can effectively reduce the amorphous carbon and carbonaceous particles. As a result, the pretreatment plays a crucial role in modifying the obtained MWCNTs structures.     

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.     

Hollow cathode synthesis of crystalline CN films

We describe a novel carbon hollow-cathode RF plasma reactor which has been used to prepare deposits of carbon nitride. Results of the characterization of the deposits by Fourier transform infrared (FTIR) microscopy, Raman microscopy, transmission electron microscopy (TEM), energy-dispersive X-ray analysis and X-ray diffraction are presented. The variation of the properties of the deposits as a function of the deposition conditions is discussed. The inclusion of small quantities of methane in the gas mixture was found to enhance the formation of the CN deposit, but for conditions of maximum enhancement C–H and N–H groups were observed in the deposit. Elemental analysis of the deposit showed that the nitrogen content was 57 at.%. A crystalline deposit was obtainable at low substrate temperatures, and the crystals were seen to grow preferentially on defects on the substrate surface.     

纳米氮化铝的氨热合成及其光致发光

采用金属铝为前驱物,在450℃的氨热条件下,获得了高质量的氮化铝纳米粉体,测量了合成粉末的X射线衍射谱、光致发光谱和高分辨透射电子显微镜（HRTEM）,发现合成的粉末是平均粒度为32nm的纤锌矿结构的氮化铝,此AlN中存在蓝光发光带,另外,讨论了AlN在高压釜内的低温氨热合成机制。     

An Efficient Route for the Synthesis of Aluminum Nitride/Graphene Nanohybrids

A facile method for the synthesis of aluminum nitride/graphene (AlN/GE) nanohybrids by a urea route was demonstrated. The structures, morphologies, and microstructures of the hybrids were examined by X‐ray diffraction, scanning electron microscope, and transmission electron microscope. Results showed that the content of GE played an important role in the phase composition and could even affect the morphology of AlN/GE nanohybrids. The c ‐AlN nanoparticles occured at high amount of graphene. In addition, the introduction of GE into the AlN/GE hybrids suppressed the aggregation of AlN nanoparticles. Furthermore, the formation mechanism of AlN/GE nanohybrids was proposed.