Synthesis of Ternary Titanium Aluminum Carbides Using Self-Propagating High-Temperature Synthesis Technique

Different ternary carbide phases, namely Ti₃AlC₂, Ti₃AlC, and Ti₂AlC, were successfully synthesized in a self-sustaining regime. Direct reactions among elemental powders of titanium, aluminum, and carbon are strongly exothermic, and the resulting reaction products consist of binary carbides and they are partially molten. The use of TiAl, instead of elemental titanium and aluminum, significantly reduces the combustion temperature. As a result, ternary titanium aluminum carbide phases are formed. In addition, the combustion-synthesized products are not sintered and easy to deagglomerate. Reaction conditions and X-ray diffraction patterns of different ternary phases formed in a self-sustaining regime are presented.     

Effect of Dilution and Porosity on Self-Propagating High-Temperature Synthesis of Silicon Nitride

Self-propagating high-temperature synthesis is a very easy and low-cost method to synthesize Si₃N₄. The nitriding of silicon powder takes place in a self-sustained regime under high pressures of nitrogen with dilution of silicon by Si₃N₄. In this work effects of dilution and green-mixture porosity on combustion velocity and phase content of reaction products are studied. Results are compared with previous work of other authors and different behaviors are found. An explanation of these behaviors is given.     

Catalytically Assisted Self-Propagating High-Temperature Synthesis of Tantalum Carbide Powders

The use of gas-phase iodine and carbon dioxide as transport agents in the tantalum/carbon/tantalum carbide combustion synthesis system has been examined to determine the effects of transport agents on product composition and microstructure. Two tantalum reactant particle sizes, a range of transport agent concentrations, and total pressures were studied. The effects of the combustion conditions on product morphology and composition were evaluated using scanning electron microscopy, nitrogen adsorption (specific surface area), and X-ray diffraction analyses. The results of the investigation indicate that the presence of the iodine vapor and carbon dioxide significantly enhances the combustion synthesis process, leading to higher conversion efficiencies and influencing product microstructure. The results are discussed in the context of gas-phase and solid-phase transport models.     

Zirconia-Based Metastable Solid Solutions through Self-Propagating High-Temperature Synthesis: Synthesis, Characterization, and Mechanistic Investigations

Cubic Zr_{1− x} Me _x O _y (Me = Fe, Co, Ni, Cu) metastable solid solutions with metal content significantly higher than equilibrium levels have been synthesized by the self-propagating high-temperature synthesis method based on a thermite reaction between metallic zirconium and the transition-metal oxides CoO, Fe₂O₃, CuO, and NiO. Through in situ XRD analysis, it was determined that when heated to 1100°C, the cubic solid solution transformed to the tetragonal phase with the concomitant formation of iron oxide. When cooled to lower temperatures, the tetragonal phase transformed to the monoclinic phase at or below 500°C. Results of auxiliary experiments strongly suggest that the formation of the solid solution takes place behind the combustion front by a reaction between zirconia and the metal.     

Hercynite-Free Ceramic Liner for Composite Steel Pipe Made Using a Self-Propagating High-Temperature Synthesis Gravitational-Thermite Process

A ceramic-lined steel pipe was fabricated using a self-propagating high-temperature synthesis gravitational-thermite process. Some rare-earth oxides and glass powders were added to the aluminothermite. The experimental results showed that the ceramic liner consisted of three phases: corundum (Al₂O₃), glass, and iron particles. Neither hercynite (FeO·Al₂O₃) nor iron ions existed in the liner of the composite pipe.     

Hot Isostatic Pressing of Chromium Nitrides (Cr2N and CrN) Prepared by Self-Propagating High-Temperature Synthesis

Cr₂N, CrN, and their mixtures (with desired fractions) have been prepared via self-propagating high-temperature synthesis (SHS) under a controlled nitrogen pressure, followed by hot isostatic pressing at 1300°C and 196 MPa under an atmosphere of argon gas. The combustion temperature increased as the nitrogen pressure increased. Single-phase Cr₂N was formed at 1040°C under a pressure of 0.18 MPa, and single-phase CrN was formed at 1730°C under a pressure of 2 MPa. The mechanical properties of the dense nitride ceramics (99.2% of the theoretical density) have been examined; the Vickers hardness (11.2 GPa for CrN and 14.5 GPa for Cr₂N) increased linearly as the fraction of Cr₂N increased, whereas the fracture toughness (∼4.7 MPa·m^1/2) and bending strength (∼355 MPa) are constant, regardless of the fraction of Cr₂N/CrN.     

Effect of an Electrostatic Field on Self-Propagating High-Temperature Synthesis of Manganese Ferrite

Using the synthesis of manganese ferrite as an example, it is shown that an electric field causes a significant change in the parameters of heterogeneous combustion, depending on the field strength and application direction. The possible factors determining the end burning of the starting mixture in the applied electric field are considered.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 4, pp. 67–72, July–August, 2005.     

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.     

Self-Propagating High-Temperature Synthesis of α-SiAlON Doped by RE (RE=Eu,Pr,Ce) and Codoped by RE and Yttrium

Self-propagating high-temperature synthesis (SHS) was applied to synthesize α-SiAlON powders doped by RE (RE = Eu,Pr,Ce) and codoped by RE and yttrium. The results showed that the weight ratio of α-SiAlON to (α-SiAlON +β-SiAlON) decreased from 70%, 55%, and 25% for europium-, praseodymium-, and cerium-doped α-SiAlON compositions, respectively, and the weight percentage of α-SiAlON phase increased to 100% for both (Eu,Y) and (Pr,Y) systems and 94% for the (Ce,Y) system, indicating SHS is a promising approach for synthesizing α-SiAlONs stabilized by the cations that could not be incorporated into the α-SiAlON structure by conventional sintering methods.     

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.     

自蔓延高温合成制备单相氮化硅镁粉体

通过自蔓延高温合成技术制备了以MgSiN2为主相的粉体,然后利用酸洗工艺除去杂质得到单相MgSiN2粉体.研究了原料配比、稀释剂MgSiN2的添加量和N2压力对燃烧合成产物相组成的影响,并探讨了酸洗条件对洗除MgO杂质的影响.研究结果表明,以化学计量比配制的原料,难以通过自蔓延高温合成法直接合成单相的MgSiN2粉体;...     

Microstructure and Mechanical Properties of Silicon Nitride Materials Fabricated from SHS Powders

Two different silicon nitride powders, one produced by self-propagating high-temperature synthesis (SHS) and one commercial, have been equally processed to obtain dense materials. Y₂O₃ powders as a sintering aid and hot-pressing techniques were used to process the materials. The fracture toughness and strength of the materials prepared have been evaluated and the results comparatively discussed as a function of the microstructure, with special emphasis on the characteristics of the initial powders.     

合成无机颜料的新技术--自蔓延高温合成

利用自蔓延高温合成技术合成无机颜料是制备颜料的一种新技术,优于传统的固相烧结法,是一项具有发展前途的技术.     

Effect of Heating Rates on the Synthesis of Al2O3–SiC Composites by the Self-Propagating High-Temperature Synthesis (SHS) Technique

Various aspects of in situ formation of Al₂O₃–SiC composites by the self-propagating high-temperature synthesis (SHS) technique have been investigated using thermal analyses (TG/DTA) of a powder mixture (4Al, 3SiO₂, 3C) and pellets in an argon atmosphere at different heating rates. Both the reaction initiation and peak temperatures are found to increase with the heating rates. At lower heating rates, the powder samples do not reveal any exothermic peak possibly because of poor reactivity and sluggish exothermic reaction. The appearance of exothermic peaks in the DTA plots after melting of aluminum indicates reduction of silica by liquid aluminum. Conversion of aluminum is found to decrease marginally with an increase in heating rates. The apparent activation energy of the process compares well with the interdiffusion activation energy of silicon and oxygen, indicating that oxygen diffusion in Si formed at the reaction front may be the rate-controlling factor for this SHS process. From SEM studies it appears that the formation of SiC whiskers is through liquid-phase mass transfer.     

High-Temperature Creep and Cavitation of Polycrystalline Aluminum Nitride

Dense, polycrystalline AlN samples of grain size between 1.8 and 19 μm were fabricated by hot-pressing. Bar-shaped samples were subjected to creep in four-point bending under static loads in nitrogen atmosphere. The outer fiber stress was varied between 20 and 140 MPa and the temperature between 1650 and 1940 K. Steady-state creep rate, dɛ/d t was proportional to σ ⁿ d ^{− m} where the stress exponent, n , was between 1.27 and 1.43 and grain-size exponent, m , between ∼ 2.2 and ∼ 2.4. The activation energy for creep ranged between 529 and 625 kJ/mol. Both round (r type) and wedgeshaped (w type) cavities were observed in electron micrographs of the deformed samples. No noticeable change in the dislocation density was observed. Contribution of cavitation to the creep rate was estimated using an unconstrained cavity model. Based on this study it is concluded that the dominant mechanism of creep in polycrystalline AlN is diffusional.     

Effect of Fe and Cr Addition on the Sintering Behavior of ZrB2 Produced by Self-Propagating High-Temperature Synthesis

An investigation of the sintering behavior of ZrB₂ powder with Fe and Cr (0 to 20 wt%) addition was conducted. It was observed that Fe addition helps to enhance the density of ZrB₂ only up to 10 wt%. Further addition of Fe degrades the sintering by segregation of Fe-rich phases. Formation of a eutectic phase containing a Fe:Zr ratio of 92.57:7.43 was also found in Fe-added samples. The addition of Cr to a ZrB₂ matrix was found to result in swelling of the samples, leading to several cracks.     

Preparation of Aluminum Nitride Powder from Aluminum Polynuclear Complexes

AIN powder was synthesized from aluminum polynuclear complexes. Basic aluminum chloride and basic aluminum lactate were used as the aluminum polynuclear complexes. These starting materials and glucose were dissolved in water and mixed homogeneously. AIN powder was obtained by calcining after drying and precalcining at 800°C under nitrogen gas flow. Then excess carbon was removed by firing in air. Nitridation in the system was investigated and compared with that in the alumina–carbon black system. It was found that in our reaction system nitridation began and proceeded at lower calcination temperatures above 1200°C than in the alumina–carbon black system. Using aluminum polynuclear complexes, AIN was synthesized through the nitridation of γ-alumina and produced in a very fine and sharp particle size distribution.     

氮化铝陶瓷的研制及应用

氮化铝 (AlN)因具有高热导率、低介电常数、与硅相匹配的热膨胀系数及其他优良的物理特性 ,在新材料领域越来越引起人们的关注。此文主要介绍并分析了AlN粉体合成、烧结、性能结构、AlN陶瓷的应用与前景     

Microwave-Assisted Combustion Synthesis of Tantalum Nitride in a Fluidized Bed

Combustion synthesis experiments in a fluidized bed have been conducted using nitrogen as the fluidizing gas for the formation of transition-metal nitrides that are potential replacements for traditional hydrodenitrogenation and hydrodesulfurization catalysts. The microwave-assisted ignition of reaction has been investigated for its potential to produce nitride overlayers on two different sizes of tantalum particle substrates. Various characterization techniques—X-ray diffractometry, energy dispersive spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy—have been used to study the presence of nitride overlayers. The results indicate that microwave assistance can permit controlled formation of tantalum nitride (Ta₂N) overlayers.     

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.