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 of Agglomerated Ultrafine Aluminum Powders in Air

It is shown experimentally that agglomeration of ultrafine aluminum powders decreases their reactivity during nonisothermal oxidation in air. Under normal and low pressures, the concentration of bound nitrogen in combustion products is lower for agglomerated powders than for unagglomerated powders, and under high pressures (>120 kPa), it is higher for agglomerated powders. Key words: ultrafine powder, aluminum, agglomeration, combustion, reactivity, aluminum nitride.     

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 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.     

Liquid-Phase Sintering of Aluminum Nitride by Europium Oxide Additives

Europium oxide has been investigated as a sintering aid and dopant for AIN. Pressureless sintering was carried out with 0 to 9 wt% Eu₂O₃ additives, and dense sintered specimens were obtained using 1 to 4 wt% Eu₂O₃. With increasing Eu₂O₃ content, two additional phases were observed in the X-ray diffraction patterns. The lattice parameters a and c of AIN in the wurtzite structure changed slightly and non-monotonically with Eu₂O₃ content and showed their minimum value in a 4 wt% Eu₂O₃ sample. The Raman and photoluminescence spectra of sintered specimens were measured. These experimental results and the sintering mechanism in the system were discussed from the standpoint of the effects of oxygen, europium, and stress.     

Oxidation and Degradation of Titanium Nitride Ultrafine Powders Exposed to Air

Titanium nitride ultrafine powders were synthesized by an active plasma-metal reaction method. Gas desorption measurements were conducted to estimate the surface chemistry of the powders after exposure to air and storage at room temperature. H₂O, H₂, CO₂, CO, and NH₃ gases were mainly evolved. These gases were considered to be formed by the surface reaction of adsorbed gases on surface oxide of the powders and decomposition of hydroxide-like or ammonialike compounds, which might be produced during a slow oxidation treatment and storage.     

Combustion of Mixtures of Commercial Aluminum Powders and Ultrafine Aluminum Powders and Aluminum Oxide in Air

The paper studies the combustion of mixtures of commercial aluminum powders (ASD-1 and ASD-4) and ultrafine powders of Al and -Al₂O₃ in air. It is shown that the combustion of coarsely dispersed commercial powders is accompanied by binding of air nitrogen with formation of AlN and AlON. The combustion of mixtures proceeds in two stages with the possible formation of intermediate gaseous and liquid products. The processes of sintering and incomplete combustion play an important role in the combustion of mixtures of commercial powders and ultrafine powders of aluminum.     

Effect of Nitrate Salts as Sintering Additives during the Ball-Milling Process of Silicon Nitride Powders

A chemical adsorption method in a Si₃N₄ slurry that contained a nitrate solution was studied during ball milling, with particular interest in increasing the oxide layer in the Si₃N₄ powder and improving the distribution homogeneity of the sintering additives. The nitrate salts Al(NO₃)₃·9H₂O and Y(NO₃)₃·6H₂O were selected as sintering additives. The following characterization techniques were used: oxygen–nitrogen analysis, X-ray photoelectron spectroscopy, high-resolution electron microscopy (coupled with energy-dispersive X-ray spectroscopy), and X-ray imaging (using wavelength-dispersive X-ray spectroscopy). The thickness of the amorphous layer and the oxygen content of the Si₃N₄ powder were greater for samples that were milled with nitrate additives, which were heat-treated at 600°C, than those of powders that were milled with oxide additives. The chemical composition of the oxygen-containing layer—that is, the amorphous layer that formed and/or changed on the Si₃N₄ surface—was similar to Si₂N₂O in heat-treated Si₃N₄ powder with nitrate additives, whereas the composition of heat-treated Si₃N₄ powder with oxide additives was similar to SiO₂. Furthermore, a homogeneous distribution of the additives was achieved via the incorporation of aluminum and yttrium into the amorphous layer on the Si₃N₄ surface. The metal ratio (Y:Al) of the adsorbates was somewhat higher than that of the additives.     

Formation and Characterization of Amorphous Aluminum Nitride Powder and Transparent Aluminum Nitride Film by Chemical Vapor Deposition

Based on thermodynamic predictions that, in the gas-phase system AlCl₃/NH₃, AlN can be produced up to the theoretical yield, the performance of a chemical vapor deposition (CVD) reactor suitable for continuous powder production is investigated. Under the applied reaction conditions and reactor configuration, fine spherical AlN powders and transparent AlN films could be preferentially obtained. The generated amorphous CVD AlN powder is characterized by a BET surface area of 23.5 m²/g. The powder deposition rate was determined to be 3.5 g/h.     

Dynamic Features of Combustion of Polydisperse Aluminum Powders in a Gas

The results of a mesh analysis of aluminum powders are considered. It is shown that the particle mass distributions are nearly normal or lognormal. The differences in the combustion dynamics between powders consisting of polydisperse particles and particles of the same sizes are studied. It is established that these differences are manifested only in the late stage of combustion and are due to the presence of a coarse particle fraction in the powder.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 4, pp. 78–83, July–August, 2005.     

Effect of Additives on the Pressureless Sintering of Aluminum Nitride between 1500° and 1800°C

Combined oxide additives (Y₂O₃, CaO, La₂O₃, CeO₂, SiO₂, TiO₂, and Fe₂O₃) were investigated as AIN sintering aids. AIN can be fully sintered at 1600°C to substantial thermal conductivity (92 W/(m·K)) using a multiple sintering aid of Y₂O₃, CaO, SiO₂, La₂O₃, and CeO₂. This lowtemperature material has small grain size (1 to 3 μm).     

Reactivity of Aluminum Nitride Powder in Aqueous Silicon Nitride and Silicon Carbide Slurries

The reactivity of AlN powder with water in supernatants obtained from centrifuged Si₃N₄ and SiC slurries was studied by monitoring the pH versus time. Various Si₃N₄ and SiC powders were used, which were fabricated by different production routes and had surfaces oxidized to different degrees. The reactivity of the AlN powder in the supernatants was found to depend strongly on the concentration of dissolved silica in these slurries relative to the surface area of the AlN powder in the slurry. The hydrolysis of AlN did not occur if the concentration of dissolved silica, with respect to the AlN powder surface, was high enough (1 mg SiO₂/(m² AlN powder)) to form a layer of aluminosilicates on the AlN powder surface. This assumption was verified by measuring the pH of more concentrated (31 vol%) Si₃N₄ and SiC suspensions also including 5 wt% of AlN powder (with respect to the solids).     

Microstructural Evolution during Sintering of Aluminum Nitride Ceramics Doped with Alumina and Yttria

The microstructural evolution of AlN sintered at >1950°C was studied in a specimen doped with 10 wt% Al₂O₃ and 5 wt% Y₂O₃. The constituent phases of the specimen were AlN, YAG, γ-AlON, and AlON polytypoids (compositional polytypes). Transmission and scanning electron microscopy revealed the microstructural characters: platelike 7AlN·Al₂O₃ first crystallized with concurrent formation of a residual liquid, then spherical AlN crystals formed. The liquid itself changed composition with the progress of the crystallization and reached the eutectic composition in the pseudobinary system AlN–YAG, and crystallized to an aggregate of AlN and YAG during cooling. As a product of the reaction of 7AlN·Al₂O₃, γ-AlON was formed.     

Reactivity of Aluminum Nitride Powder in Dilute Inorganic Acids

The reactivity of AlN powder in diluted inorganic acids was studied by measuring the pH and temperature during its hydrolysis. At very low starting pH (pH ∼1) no reaction was observed, regardless of the acid used. In contrast, in a less acidic environment, i.e., at higher pH values (pH ∼3), the reaction was fast enough to reveal the influence of different acids on the hydrolysis reaction. Monoprotonic acids which are completely dissociated (HCl, HF, HNO₃), and form water-soluble salts with aluminum, did not influence the hydrolysis reactions. In the presence of incompletely dissociated diprotonic H₂SO₄ and H₂CO₃ acids which form water-soluble salts with aluminum, the reaction was hindered but not prevented. In the presence of phosphoric acid the hydrolysis was prevented at room temperature, presumably because of the formation of insoluble phosphates on the powder surface. At elevated temperatures their solubility was substantial, and the reactivity of AlN powder in a diluted hot phosphoric acid was reestablished. In the presence of silicic acid the reaction was suppressed at both room and elevated temperatures, which was also ascribed to the formation of insoluble silicates. The adsorption of silicate anions onto the powder surface was confirmed by chemical analysis and zeta potential measurement. Using DRIFT measurements, however, the presence of Si–O bonds on the powder could not be unambiguously confirmed, since the characteristic wavelengths for these bonds are in the region of very strong Al–N stretching frequencies.     

Effect of Porosity on the Combustion Synthesis of Titanium Nitride

The effect of porosity on the self-propagating reaction between porous titanium and gaseous nitrogen was investigated. The relationship between total nitrogen uptake and porosity showed a maximum at about 44% porosity. High initial sample porosities lead to partial melting of the titanium and subsequently to a lower degree of conversion to the nitride. Low initial porosities limit the conversion through lower reaction interfacial areas and lower gas permeation. Wave velocity measurements, thermogravimetric determinations on the progression of the combustion front, and microstructural analyses demonstrated that the passage of the front is associated primarily with the formation of a surface layer of TiN_1-x with x being 0.10 and 0.06 for lowdensity (49%) and high-density (59%) samples. The product of combustion contained titanium nitride and primary and β-transformed α solid solutions. The relative abundance of the latter two phases was dependent on the initial relative density of the samples.     

Strengthening and Prevention of Oxidation of Aluminum Nitride by Formation of a Silica Layer on the Surface

A silica (SiO₂) layer was deposited on the surface of an AlN ceramic in order to increase the strength and to prevent the high-temperature oxidation of the material. The layer was formed on the surface by exposing coupons to the atmosphere downstream of a bed of SiC powder in a flowing H₂–0.1% H₂O atmosphere at 1450°C. A reaction between the SiC powder and H₂O in the H₂ gas resulted in the generation of SiO₂"smoke" in the product gas stream. Part of the SiO₂ smoke was subsequently deposited on the surface of the AlN specimen to form a dense and uniform SiO₂ layer. The strength of AlN was improved by about 20% apparently because of blunting of surface defects by SiO₂. More importantly, the layer was very effective in protecting the AlN from the oxidation at elevated temperatures, through the inhibition of transport of oxidants to the sample surface.     

Improved Oxidation Resistance of Silicon Nitride by Aluminum Implantation: I, Kinetics and Oxide Characteristics

Hot-isostatically-pressed, additive-free Si₃N₄ ceramics were implanted with aluminum at multi-energies and multidoses to achieve uniform implant concentrations at 1, 5, and 10 at.% to a depth of about 200 nm. The oxidation behavior of unimplanted and aluminum-implanted Si₃N₄ samples was investigated in 1 atm flowing oxygen entrained with 100 and 220 ppm NaNO₃ vapor at 900–1100°C. Unimplanted Si₃N₄ exhibits a rapid, linear oxidation rate with an apparent activation energy of about 70 kJ/mol, independent of the sodium content in the gas phase. Oxides formed on the unimplanted samples are rough and are populated with cracks and pores. In contrast, aluminum-implanted Si₃N₄ shows a significantly reduced, parabolic oxidation rate with apparent activation energies in the range of 90–140 kJ/mol, depending on the sodium content as well as the implant concentration. The oxides formed on the implanted samples are glassy and mostly free from surface flaws. The alteration of the oxidation kinetics and mechanism of Si₃N₄ in a sodium-containing environment by aluminum implantation is a consequence of the effective modification of the properties of the sodium silicates through aluminum incorporation.     

Thermodynamic and Experimental Study of High-Purity Aluminum Nitride Formation from Aluminum Chloride by Chemical Vapor Deposition

Thermodynamic calculations in the systems Al-Cl, Al-Cl-N, and H-Al-Cl-N were used to assess the capabilities of AlCl₃ or mixtures of AlCl₃ with Al to produce AlN by chemical vapor deposition (CVD) techniques. Direct nitridation (N₂ as reaction agent) is possible only at high temperatures (≥1500 K), using AlCl₃–Al mixtures. Reaction with NH₃ at equilibrium gives low yields but the suppression of NH₃ dissociation yields near 100%, which makes the method suitable for powder production, coating, and single-crystal growth. AlN with less than 1 wt% oxygen was obtained from technical grade AlCl₃ by this process. The formation of both amorphous AlN powder and crystalline AlN coatings was observed. It is assumed that the formation of AlCl₃· x NH₃ adducts by mixing of Al-Cl vapor and NH₃ at temperatures ≤1273 K prevents NH₃ dissociation and favors the production of amorphous AlN.     

Effect of Nitrogen Pressure and Diluent Content on the Combustion Synthesis of Titanium Nitride

The effects of gas pressure and diluent content on the combustion synthesis of titanium nitride was investigated. Combustion of titanium powder samples containing TiN as a diluent in the concentration range 0 to 60 wt% was carried out in a nitrogen atmosphere at pressures in the range 0.1 to 1.4 MPa. Analysis of the dependence of combustion wave velocity on temperature gave an activation energy of 342 ± 50 kJ. mol⁻¹, which is in good agreement with reported values for the diffusion of nitrogen in titanium nitride.     

Temperature Dependence of the Thermal Diffusivity/Conductivity of Aluminum Nitride

The temperature dependence of the thermal conductivity of a polycrystalline AIN was measured using the flash-diffusivity technique over a range of experimental conditions. Thermal conductivity data from room temperature to 300°C, obtained with attenuated laser pulse energies to minimize the specimen temperature increase, were found to be inversely proportional to the absolute temperature, as expected from theory. For high pulse energies, the experimental data deviated significantly from expected behavior. This latter effect is offered as an explanation for the anomalously low temperature dependence for the thermal conductivity reported in the literature.