Some Properties of Aluminum Carbide Powder Prepared by the Pyrolysis of Alkylaluminu

Ultrafine aluminum carbide (Al₄C₃) powders with crystallite sizes of <40 nm were prepared by the pyrolyses of alkylaluminums, i.e., trimethylaluminum (Al(CH₃)₃: TMAL), triethylaluminum (Al(C₂H₅)₃: TEAL), triisobutylaluminum (Al(i-C₄H₉)₃: TIBAL) at a temperature between 950° and 1100°C. Although the pyrolysis of TMAL produced Al₄C₃ at 950°C, the pyrolysis temperature of TEAL to produce Al₄C₃ was raised up to 1100°C. The pyrolysis of TIBAL at 1100°C produced not only crystalline Al₄C₃ but also amorphous oxycarbide. The TEAL-derived powder had the highest true density (2.89 g^.cm⁻³ or 97% of the theoretical density) among the three kinds of powders.     

Mechanism and Kinetics of Aluminum Nitride Powder Degradation in Moist Air

Aluminum nitride (AlN) powders manufactured via three major commercial processes, namely, chemical vapor deposition from triethyl aluminum, carbothermal reduction and nitridation of alumina, and direct nitridation of aluminum, were exposed to moist air at room temperature to investigate the degradation mechanism and kinetics. In the degradation, the powders were initially hydrolyzed to amorphous aluminum oxyhydroxide, which subsequently transformed into mixtures of crystallized aluminum trihydroxide (Al(OH)₃) polymorphs, i.e., bayerite, nordstrandite, and gibbsite, forming agglomerates around the unreacted AlN. The data were fitted by using the unreacted-core model. Three stages were found in the degradation: the stage of an induction period at the beginning, followed by a stage of fast hydrolysis with the chemical reaction being rate controlling, and finally, with gradual closing of pores in the structure of Al(OH)₃ around AlN, changing to a stage of slow hydrolysis that was controlled by mass diffusion through Al(OH)₃. The existence of an induction period was attributed to slow hydrolysis of the surface oxide/oxyhydroxide layer. The powder produced by the carbothermal process showed the longest induction period, which was attributed to its surface structure being different from other powders.     

Synthesis and Mechanism of Nanosized AlN from an Aluminum Oleic Emulsion Using Carbothermal Reduction at Low Temperatures

Nanosized Al₂O₃ particles homogeneously dispersed in a matrix of amorphous carbon (a-C) were prepared by decomposition of an aluminum oleic emulsion at 600°C in Ar. Nanosized aluminum nitride (AlN) grains were prepared by carbothermal reduction and nitridation (CRN) of this Al₂O₃–a-C mixture in NH₃ using graphite, BN, and alumina crucibles or boats. The phases formed by CRN were identified by X-ray diffraction analysis. The morphology and grain size of the AlN were determined by transmission electron microscopy. The formation of single-phase AlN was achieved at temperatures as low as 1150°–1200°C in NH₃ using a cylindrical graphite crucible with holes in its two flat faces. Mass spectroscopy (MS) showed that a significant amount of HCN and a minor amount of C₂H₂ are formed at 500°C by reaction of NH₃ with carbon at the decomposition temperature of NH₃. A most probable formation mechanism of the AlN from nanosized Al₂O₃ and a-C in NH₃ is discussed on the basis of MS results and thermodynamic considerations.     

Effect of Carbon Precursors on the Microstructure and Bonding State of a Boron–Carbon Compound Grown by LPCVD

Methane (CH₄) and propylene (C₃H₆) were used to fabricate a boron–carbon coating by a low-pressure chemical vapor deposition (LPCVD) technique. The effects of carbon precursors on the phase, microstructure, and bonding state of the deposits have been investigated. X-ray diffraction results show that the 2θ value of the deposit from the C₃H₆ precursor shifts to 25.78° when the coating is deposited at 1223 K, and shifts to 26.1° when deposited at 1273 K, compared with the 2θ value of the pyrocarbon (PyC) peak deposited by LPCVD, which is 25.42°, and no boron–carbon (B–C) compound peak exists. However, the phases of coating deposited from CH₄ include B₂₅C, B₁₃C₂, elemental carbon, and boron. X-ray photoelectron spectroscopy (XPS) results show that the percent contents of boron atom in the coatings from the CH₄ precursor are 61.18% and 67.78% when deposited at 1223 and 1273 K, respectively, much higher than that from the C₃H₆ precursor, 10.85% and 15.30%, respectively. Scanning electron microscopy (SEM) results show that the coatings deposited from CH₄ have a coarse crystal-like morphology; however, the coatings deposited from the C₃H₆ precursor are smooth. The formation of PyC from C₃H₆ is more facile than that from CH₄, which leads to differences in the phase, atom content, and microstructure of coatings from CH₄ and C₃H₆ precursors.     

Synthesis of AlN Nanopowder from γ-Al2O3 by Reduction–Nitridation in a Mixture of NH3–C3H8

Aluminum nitride (AlN) powders were synthesized by gas reduction–nitridation of γ-Al₂O₃ using NH₃ and C₃H₈ as the reactant gases. AlN was identified in the products synthesized at 1100°–1400°C for 120 min in the NH₃–C₃H₈ gas flow confirming that AlN can be formed by the gas reduction–nitridation of γ-Al₂O₃. The products synthesized at 1100°C for 120 min contained unreacted γ-Al₂O₃. The ²⁷A1 MAS NMR spectra show that Al–N bonding in the product increases with increasing reaction temperature, the tetrahedral AlO₄ resonance decreasing prior to the disappearance of the octahedral AlO₆ resonance. This suggests that the tetrahedral AlO₄ sites of the γ-Al₂O₃ are preferentially nitrided than the AlO₆ sites. AlN nanoparticles were directly formed from γ-Al₂O₃ at low temperature because of this preferred nitridation of AlO₄ sites in the reactant. AlN nanoparticles are formed by gas reduction–nitridation of γ-Al₂O₃ not only because the reaction temperature is sufficiently low to restrict grain growth, but also because γ-Al₂O₃ contains both AlO₄ and AlO₆ sites, by contrast with α-Al₂O₃ which contains only AlO₆.     

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.     

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.     

Effects of Transition-Metal Substitution on the Catalytic Properties of Barium Hexaaluminogallate

The effects of the substitution of transition-metal ions and/or reductant gases on the catalytic properties of barium hexaaluminogallate were investigated. Transition-metal-substituted hexaaluminogallates (BaM(Al,Ga)₁₁O₁₉, M = transition metal, Al/Ga = 9/3) were synthesized from aqueous metal nitrates and ammonium carbonate by the coprecipitation followed by crystallization at 1100°C. The direct NO _x reduction was observed over BaM(Al,Ga)₁₁O₁₉ to be around 10%. The NO _x removal activity of BaM(Al,Ga)₁₁O₁₉ powders was improved by addition of C₃H₆ as a reductant gas. Co-, Ni- and Cu-substituted BaM(Al,Ga)₁₁O₁₉ catalysts exhibited about 40% NO _x reduction with C₃H₆ in excess oxygen at a high space velocity of 10 000 h⁻¹. The NO _x reduction on Mn- and Fe-substituted BaM(Al,Ga)₁₁O₁₉ catalysts was less than 10% even in the presence of C₃H₆. The temperature of the effective NO _x reduction on BaM(Al,Ga)₁₁O₁₉ catalysts could be adjusted from 350° to 500°C by the selection of the transition-metal substitution in the catalysts. The catalysts hold high activities for NO _x reduction even at 500°C in water vapor produced in the combustion system of reductant gases.     

Synthesis of Ultrahigh-Temperature Si–B–C–N Ceramic from Polymeric Waste Gas

A "green" route to ultrahigh-temperature Si–B–C–N ceramic from vacuum-degassing waste gas of polyborosilazane {B[C₂H₄Si(CH₃)NH]₃} _n (T2-1) has been developed. After gas-to-gel transformation, an amorphous ceramic Si_5.3B_1.0C₁₉N_3.7 was derived from the gel by dehydrocoupling and polymer-to-ceramic transformation. The ceramic started to form a nanostructure at 1700°C and resisted thermal degradation up to 2200°C in argon. This suggests that vacuum-degassing waste gases of polymer precursors may be perfect raw materials for various advanced ceramics.     

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.     

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.     

Preparation of Sinterable Cubic Aluminum Oxynitride by the Carbothermal Nitridation of Aluminum Oxide

Sinterable cubic aluminum oxy nitride (ALON) has been prepared by carbon reduction of aluminum oxide inflowing nitrogen. Three different sources of Al₂O₃ (A1₂O₃ from clay, commercial A1₂O₃, and A1₂O₃ derived from AlCl₃.6H₂O) and two different sources of carbon (carbon black and starch) were used. Pressed pellets of ALON powder were sintered in N₂ at 1950°C greater than 95% of theoretical density.     

Synthesis and Properties of Dielectric Ceramics from Metallo-Organic Precursors through the Solid-State Reaction of Mixed Gels

A perovskite structure of 0.4Pb(Mg_1/3Nb_2/3)O₃·0.3Pb(Mg_1/2W_1/2)O₃·0.3PbTiO₃ was prepared from metallo-organic precursors through the solid-state reaction of the mixed gels. Three types of mixed gels were crystallized to obtain PbTiO₃, MgNb₂O₆, and MgWO₄ powders. These powders were calcined at 900°C after mixing with a stoichiometric amount of Pb(CH₃COO)₂·3H₂O. The dielectric constant of the ceramic fired at 900°C was improved by adding an excess of 10 mol% Mg(OC₂H₅)₂, and the ceramic achieved X7T specification of the Electric Industries Association standard. The dielectric loss was reduced by adding an excess of 5 mol% Pb(CH₃COO)₂·3H₂O.     

Growth of Quasi-Aligned AlN Nanofibers by Nitriding Combustion Synthesis

Quasi-aligned AlN nanofibers were formed by the nitriding combustion synthesis according to a unique micro-reactor model. A charge composed of aluminum and aluminum nitride diluent powders (40/60 mol%) with a mixture of yttria and ammonium chloride as additives (5 wt% each) was combusted at low nitrogen gas pressures of 0.25 MPa. The FE-SEM images of as-synthesized AlN product showed the formation of ball-like grains (same shape and size as the original Al reactant) that consisted of a thin surface nitride layer or crust cover quasi-aligned AlN nanofibers grown in the interior. The cross-sectional view is sea anemone like. Formation of this novel morphology is believed to occur through a two-stage process. The first one occurs at the preliminary stage of the combustion outside Al particles. After the ignition, the heat generated causes the sublimation and dissociation of ammonium chloride into various gaseous species. This effectively interrupts the combustion and slows down the increase of reaction temperature. In addition, yttria interacts with the native oxide layer present on the surface of Al particles and forms a stable Al–N–Y–O crust. The second stage begins by the infiltration of various gaseous species such as HCl_(g), NH_3(g), and N_2(g) through the crust into the molten Al cores. The "crust–core" systems function as "micro-reactors" where both the nitridation and growth processes occur inside. The molten Al cores are spontaneously halogenated to AlCl₃ vapors and the nitridation proceeds by the gas–gas reaction of AlCl₃ and NH₃/N₂ vapors. The AlN nanofibers are then grown from the vapor phase quasi-aligned inside the micro-reactors by VLS and VS mechanisms.     

Aluminum Nitride Fibers Synthesized from Alumina Fibers Using Gas-Reduction–Nitridation Method

Aluminum nitride fibers were successfully synthesized from alumina fibers using an NH₃–C₃H₈gas mixture as a reduction–nitridation agent. Observation using SEM clearly demonstrated that the morphology of the nitrided fibers was exactly the same as that of the raw alumina fibers, retaining the original regular shape and smooth surface. Up to 95% of the starting alumina was converted to aluminum nitride at 1400°C within 0.5 h via a single-step synthesis process.     

Preparation of Aluminum Nitride–Silicon Carbide Nanocomposite Powder by the Nitridation of Aluminum Silicon Carbide

Aluminum nitride (AlN)–silicon carbide (SiC) nanocomposite powders were prepared by the nitridation of aluminum-silicon carbide (Al₄SiC₄) with the specific surface area of 15.5 m²·g⁻¹. The powders nitrided at and above 1400°C for 3 h contained the 2H-phases which consisted of AlN-rich and SiC-rich phases. The formation of homogeneous solid solution proceeded with increasing nitridation temperature from 1400° up to 1500°C. The specific surface area of the AlN–SiC powder nitrided at 1500°C for 3 h was 19.5 m²·g⁻¹, whereas the primary particle size (assuming spherical particles) was estimated to be ∼100 nm.     

Preparation of Silicon Carbide/Aluminum Nitride Ceramics Using Organometallic Precursors

Solid solutions of 2H -SiC/AlN can be prepared at temperatures less than 1600°C by rapid pyrolysis ("hot drop") of mixtures of [(Me₃Si)_0.80((CH₂=CH)MeSi)_1.0(MeHSi)_0.35] _n (VPS) or [MeHSiCH₂] _n (MPCS) with [R₂AlNH₂]₃, where R=Et, i -Bu or simply by slow pyrolysis of the precursor mixture in the case of [Et₂AlNH₂]₃. In contrast, slow pyrolysis of mixtures of VPS or MPCS with [ i -Bu₂AlNH₂]₃ yields a composite of 2 H -AlN and 3 C -SiC at 1600°C, which transforms into a single 2 H -SiC/AlN solid solution on heating to 2000°C. The influences of the nature of the precursor and processing conditions on the structure, composition, and purity of the SiC/AlN materials are discussed.     

Preparation of Silicon Nitride-Titanium Nitride and Titanium-Titanium Nitride Composites from (CH3)3SiNHTiCl3-Coated Si3N4 and Ti Particles

[(Trimethylsilyl)amino]titanium trichloride, (CH₃)₃-SiNHTiClj, was isolated as a red-orange crystalline solid in 58% yield from the reaction of TiCl₄ with [(CH₃)₃Si]₂NH in 1:1 molar ratio in dichloromethane at —78°C. Pyrolysis of (CH₃)₃SiNHTiCl₃ at 600°C furnished titanium nitride. This precursor is suitable for the preparation of composites and was employed to prepare Si₃N₄-TiN and Ti-TiN powders by adding Si₃N₄ particles or titanium powders to a solution of (CH₃), SiNHTiCl₃ in dichloromethane, drying and pyrolyzing the resulting solid. This precursor also has been used as a binder to prepare Si₃N₄-TiN and Ti-TiN bodies. High-resolution transmission electron microscopic studies of the Si₃N₄-TiN composite showed that titanium nitride is concentrated on the surface of the Si₃N₄ particles.     

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.     

Deposition Mechanism for Chemical Vapor Deposition of Zirconium Carbide Coatings

Zirconium carbide (ZrC) coatings were fabricated by chemical vapor deposition (CVD) using ZrCl₄, CH₄/C₃H₆, and H₂ as precursors. Both thermodynamic calculation results and the film compositions at different temperatures indicated that zirconium and carbon deposited separately during the CVD process. The ZrC deposition rates were measured for CH₄ or C₃H₆ as carbon sources at different temperatures based on coating thickness. The activation energies for ZrC deposition demonstrated that the CVD ZrC process is controlled by the carbon deposition. This is also proven by the morphologies of ZrC coatings.