Aluminum Nitride Prepared by Nitridation of Aluminum Oxide Precursors

Aluminum nitride (AlN) powders were prepared from the oxide precursors aluminum nitrate, aluminum hydroxide, aluminum 2-ethyl-hexanoate, and aluminum isopropoxide (i.e., Al(NO₃)₃, Al(OH)₃, Al(OH)(O₂CCH(C₂H₅)(C₄H₉))₂, and Al(OCH(CH₃)₂)₃). Pyrolyses were performed in flowing dry NH₃ and N₂ at 1000°–1500°C. For comparison, the nitride precursors aluminum dimethylamide (Al(N(CH₃)₂)₃) and aluminum trimethylamino alane (AlH₃·N(CH₃)₃) were exposed to the same nitridation conditions. Products were investigated using XRD, TEM, EDX, SEM, and elemental analysis. The results showed that nitridation was primarily controlled by the water:ammonia ratio in the atmosphere. Single-phase AlN powders were obtained from all oxide precursors. Complete nitridation was not obtained using pure N₂, even for the non-oxide precursors.     

Silicon Nitride–Silicon Carbide Nancocomposites Fabricated by Electric-Field-Assisted Sintering

Starting with Si-C-N(-O) amorphous powders, and using the electric field assisted sintering (EFAS) technique, silicon nitride/silicon carbide nanocomposites were fabricated with yttria as an additive. It was found that the material could be sintered in a relatively short time (10 min at 1600°C) to satisfactory densities (2.96–3.09 g/cm³) using 1–8 wt% yttria. With decreasing yttria content, the ratio of SiC to Si₃N₄ increased, whereas the grain size decreased from ∼150 nm to as small as 38 nm. This offers an attractive way to make nano-nanocomposites of silicon nitride and silicon carbide.     

Advances in Manufacturing Boron Carbide-Aluminum Composites

An infiltration method for preparing a boron carbide-aluminum (B₄C-AI) composite was modified so as to reduce the processing temperature and time. Titanium metal and titanium-based compounds were added to B₄C powders to enhance the wettability of the liquid aluminum on boron carbide skeletons. As expected, the time required for infiltration was significantly reduced on using the additives. Of these additives titanium metal was the most effective in facilitating aluminum infiltration. Another method, involving the heat treatment of boron carbide compacts at 1300°3C for 1 h before infiltration, was attempted, and a significant improvement was gained. These findings show that the treatment modified the surface condition of boron carbide powders via the removal of oxides. An additional attempt was made to increase the boron carbide content of the system by using a bimodal powder mixture. A maximum green density of 78% was achieved by mixing fine particle size and coarse particle size powders. The infiltrated boron carbide composites prepared using a bimodal powder with a preinfiltration heat treatment of the compacts exhibited promising mechanical properties, such as a Vickers hardness ( H _V) of 11 Gpa and an indentation toughness ( K _IC) in the range of 5–7.5 MPa·m^1/2.     

Effect of Al Antioxidant on the Rate of Oxidation of Carbon in MgO–C Refractory

Kinetics of air oxidation of MgO–C–Al refractory at 600°–1300°C were investigated using the software based on the modified shrinking core model (KDA). Commercial bricks containing 88.5% MgO, 10% residual carbon, and 1.5% aluminum anti-oxidant were oxidized isothermally with air. Combination of experimental data with model calculations indicated gas diffusion through solid material and pores as a major controlling step. Previously observed chemisorption process was eliminated from the rate-controlling mechanism with addition of aluminum antioxidant. Comprehensive rate equations were devised for MgO–C–Al and MgO–C oxidation reactions. Overall activation energies of Q _id (internal diffusion)=139.15 kJ/mol at T ≤800°C and Q _pd (pore diffusion)=25.48 kJ/mol at T >800°C were obtained for MgO–C–Al oxidation reactions. Corresponding values were determined to be Q _id=134.85 kJ/mol and Q _ca (chemical adsorption)=66.69 kJ/mol at T ≤800°C and Q _pd=18.95 kJ/mol and Q _ca=66.69 kJ/mol at T >800°C for MgO–C oxidation reactions. Addition of aluminum anti-oxidant indicated a reducing effect on oxidation of MgO–C bricks at 800°C≤ T ≤1250°C. Reverse behavior was observed at T ≤700°C.     

Formation of Al2O3–TiC Composite Nano-Particles Synthesized from Carbon-Coated Precursors

The formation of nano-sized alumina–titanium carbide (Al₂O₃–TiC) composite powders from a carbon-coated titanium dioxide–aluminum (TiO₂–Al) mixture was investigated. The carbon-coated TiO₂–Al mixture altered the mechanism of the reaction, compared with standard mixtures, to produce high-quality nano-sized Al₂O₃–TiC powders. Data synthesized from intermediate temperatures indicate that these products form via Ti₂O₃ and Al₃Ti. TEM images of the Al₂O₃–TiC powders showed fine size (50–100 nm), narrow size distribution, and lack of agglomeration. DSC data for the carbon-coated TiO₂–Al mixture showed a single endothermic and four successive weak exothermic reactions as the carbon coating moderated the heat release during the reaction.     

The Evolutionary Process during Pyrolytic Transformation of Poly(N-methylsilazane) from a Preceramic Polymer into an Amorphous Silicon Nitride/Carbon Composite

The pyrolytic evolution of poly(N-methylsilazane), –[H₂SiN-Me] _x –, from preceramic polymer to ceramic product is followed by heating samples of the partially cross-linked polymer, in 200°C increments, from ambient temperature to 1400°C. The intermediate products are characterized by chemical analysis, diffuse reflectance Fourier transform IR spectroscopy (DRIFTS), Raman spectroscopy, and ²⁹Si and ¹³C magic-angle spinning (MAS) solid-state NMR. Spectro-scopic characterization indicates that the 1400°C pyrolysis products are amorphous silicon nitride mixed with amorphous and graphitic carbon (as determined by Raman spectroscopy), rather than silicon carbide nitride, as expected based on the presence of up to 20 mol% retained carbon. Efforts to crystallize the silicon nitride through heat treatments up to 1400°C do not lead to any crystalline phases, as established by transmission electron microscopy (TEM) and small-area electron diffraction (SAD). It appears that the presence of free carbon, along with the absence of oxygen, strongly inhibits crystallization of amorphous silicon nitride. These results contrast with the isostructural poly-(Si-methylsilazane), –[MeHSiNH] _x –, which is reported to form silicon carbide nitride on pyrolysis.     

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.     

Mechanochemical Formation of Metal–Ceramic Composites

A mechanical activation technique has been used to form composites of alumina with titanium carbide, nitride, or carbonitride, both with and without elemental iron. The composites were formed by reacting elemental aluminum with either ilmenite (FeTiO₃) or rutile (TiO₂) concentrates in the presence of carbon and/or nitrogen in a ball-mill at ambient temperature. The reaction was complete for the ilmenite samples after milling but was completed only for rutile under hot pressing conditions. Microhardness measurements indicated that the composites had hardnesses in the range 19–30 GPa (1740–2750 VHN), with only a small variation within each sample. Elemental mapping of the pressed pellets indicated that titanium and aluminum were evenly distributed on a submicrometer level whereas iron tended to coalesce into <20 μm particles in the presence of TiC. The coalescence decreased with the carbon content of the hard material until iron was evenly distributed with TiN. A superstoichiometric amount of aluminum led to the formation of iron–aluminum phases which decreased the iron coalescence. The XRD crystallite size of the alumina was 30–50 nm and was 25–50 nm for the titanium phases, confirming the extremely fine microstructure.     

Thermal Stability, Phase Evolution, and Crystallization in Si-B-C-N Ceramics Derived from a Polyborosilazane Precursor

Amorphous Si-B-C-N ceramic powder samples obtained by thermolysis of boron-modified polysilazane, {B[C₂H₄Si(H)NH]₃} _n , were isothermally annealed at different temperatures (1400–1800°C) and hold times (3, 10, 30, and 100 h). A qualitative and semiquantitative analysis of the crystallization behavior of the materials was performed using X-ray diffraction (XRD). The phase evolution was additionally followed by ¹¹B and ²⁹Si MAS NMR as well as by FT-IR spectroscopy in transmission and diffuse reflection (DRIFTS) modes. Bulk chemical analyses of selected samples were performed to determine changes in the chemistry/phase composition of the materials. It was observed that silicon carbide is the first phase to nucleate around 1400–1500°C, whereas silicon nitride nucleates at and above 1700°C. Crystallization accelerates with increasing annealing temperature and proceeds with increasing annealing time. Furthermore, the surface area of the powders strongly influences the thermal stability of silicon nitride and thus controls overall chemical and phase composition of the materials on thermal treatment.     

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.     

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.     

The Design of Crystalline Precursors For the Synthesis of Mn−1AXn Phases and Their Application to Ti3AlC2

A methodology to allow the deliberate design of solid precursors to affect the solid-state synthesis of materials has proven elusive. We have designed a conceptual synthesis route for M _{n +1}AX _n phases that does not involve the usual intermediate phases. Instead, it is proposed that the common structural units within a solid-state precursor M _{n +1}X _n containing vacancy ordering should be the basis for direct synthesis of the desired M _{n +1}AX _n phase. The method is demonstrated to be successful in producing titanium aluminum carbide (Ti₃AlC₂) by the rapid intercalation of Al into TiC_0.67 at 400°–600°C below the conventional synthesis temperature. Time-resolved neutron diffraction at 1 min time-resolution has confirmed the reaction sequence. The vacancy ordering in TiC_0.67 occurred simultaneously to, and appeared to be greatly facilitated by, the ingress of aluminum. There is considerable scope for adaptation of the method to other M _{n +1}AX _n phases.     

High-Temperature Mixed Oxidation of Nitride-Bonded Silicon Carbide in Oxidizing Gas Mixtures Containing 2% Cl2

Nitride-bonded silicon carbide ceramics have lower processing costs than many other SiC-based ceramics and adequate properties for use as high-temperature heat exchangers in oxidizing environments. Silicon nitride has much better resistance to attack by chlorine at temperatures above 900°C than silicon carbide. When nitride-bonded silicon carbide ceramics are exposed to gas mixtures containing 2% Cl₂ and small amounts of oxygen in this temperature range, the SiC is selectively chlorinated, leaving behind a porous matrix of silicon nitride. The rate of corrosion is controlled by a combination of interfacial kinetics at the surfaces of the SiC grains and transport of volatile species through the silicon nitride skeleton. In more oxidizing environments, the rate of chlorination is suppressed by the formation of a protective SiO₂ film. In highly oxidizing environments at temperatures in excess of 1200°C, the formation of volatile chloride reaction products at the interface between the SiC and the passivating SiO₂ layer causes bubbles to form in the SiO₂, which accelerates the oxidation.     

Combustion Synthesis of AlON–BN Composites under Low Nitrogen Pressure

Hexagonal boron nitride (hBN) and aluminum oxinitride (AlON) composites were synthesized by combustion reaction of powder mixtures of Al–B₂O₃–AlN systems under a low pressure of nitrogen gas (0.5 MPa). Explosive combustion reaction of Al–B₂O₃ systems under the same nitrogen pressure produced alumina, aluminum borate, AlN, and AlON depending on the binary mixing ratio, but no trace of BN phases could be identified. Most of the elemental boron product remained unreacted and amorphous. On the other hand, AlN addition as a diluent in the range of 15–30 wt% was effective in producing hBN phase and forming AlON–BN composites. In the composition range of the ternary mixture of Al, B₂O₃, and AlN, where significant BN formation was identified, the primary role of AlN was to react with B₂O₃ to produce BN and α-Al₂O₃. The temperature profile obtained during the combustion reaction by a thermocouple imbedded in the middle of the powder bed revealed that the initial nitridation reaction of aluminum metal provides the heat required for the combustion reaction, creating a state of a "chemical oven." The reaction product, α-Al₂O₃, reacted subsequently with AlN to produce AlON phases to give final AlON–BN composites. The combustion reaction was highly unstable and followed a mixed mode with a regularly reversing spinning mode for aluminum nitridation reaction in the surface region and an oscillatory mode for the BN formation reaction in the subsurface region.     

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.     

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.     

Reactive Hot-Pressed Alumina–Boron Nitride Composites with Y2O3 Sintering Additive

The effect of Y₂O₃ addition (0–5 wt%) on the densification and properties of reactive hot-pressed alumina (Al₂O₃)–boron nitride composites based on the reaction between aluminum borate (2Al₂O₃·B₂O₃) and aluminum nitride (AlN) was investigated. The densification process was very sensitive to the amount of Y₂O₃. Compared with a low relative density of 79.3 theoretical density (TD)% for material with no Y₂O₃ addition, the material density reached 98.6 TD% with 0.25% Y₂O₃ addition. High Y₂O₃ additions resulted in the formation of a new phase Al₅Y₃O₁₂. The grain growth of the Al₂O₃ matrix was promoted by the Y₂O₃ addition. Owing to the high density and the small Al₂O₃ particle size the sample with 0.25% Y₂O₃ addition demonstrated the highest bending strength of 540 MPa.     

Reaction Mechanisms of Silicon Carbide Fiber Synthesis by Heat Treatment of Polycarbosilane Fibers Cured by Radiation: I, Evolved Gas Analysis.

SiC fibers were synthesized from polycarbosilane (PCS) fibers by heat treatment after electron beam irradiation curing. The pyrolysis reaction mechanisms from the organic PCS to ceramic SiC were investigated by the analysis of gases evolved during heat treatment. There were two steps in the major reaction: the first step was at 800–1200 K where H₂ and CH₄ evolved by scission of Si-CH₃ and Si-H and by rearrangement reactions, and the second step was at 1000–1700 K where H₂ evolved by reactions related to C atoms in the PCS main chain. H₂ evolution in the first step was reduced with increasing oxygen content in the cured PCS fibers.     

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.     

High-Thermal-Conductivity Aluminum Nitride Ceramics: The Effect of Thermodynamic, Kinetic, and Microstructural Factors

Improvement in the thermal conductivity of aluminum nitride (AlN) can be realized by additives that have a high thermodynamic affinity toward alumina (Al₂O₃), as is clearly demonstrated in the aluminum nitride-yttria (AlN-Y₂O₃) system. A wide variety of lanthanide dopants are compared at equimolar lanthanide oxide:alumina (Ln₂O₃: Al₂O₃, where Ln is a lanthanide element) ratios, with samaria (Sm₂O₃) and lutetia (Lu₂O₃) being the dopants that give the highest- and lowest-thermal-conductivity AlN composites, respectively. The choice of the sintering aid and the dopant level is much more important than the microstructure that evolves during sintering. A contiguous AlN phase provides rapid heat conduction paths, even at short sintering times. AlN contiguity decreases slightly as the annealing times increase in the range of 1–1000 min at 1850°C. However, a substantial increase in thermal conductivity results, because of purification of AlN grains by dissolution-reprecipitation and bulk diffusion. Removal of grain-boundary phases, with a concurrent increase in AlN contiguity, occurs at high annealing temperatures or at long times and is a natural consequence of high dihedral angles (poor wetting) in liquidphase-sintered AlN ceramics.