Oxidation of Aluminum in Mixtures with Polyethylene after Plastic Deformation at High Pressures

Powder mixtures of low-density polyethylene and aluminum with the 20–80% weight ratio of the species are subjected to plastic deformation at pressures of 1 and 4 GPa in high-pressure devices of the Bridgman anvil type. Mass changes in the deformed mixtures in the temperature range from 30 to 800?C are studied by the method of thermal gravimetry. A mass loss associated with polymer decomposition occurs in the temperature range from 30 to 450?C, which is always smaller than the polymer content in the mixtures. A possible reason is the formation of thermally stable products due to aluminum interaction with polymer decomposition products. In the temperature range from 450 to 800?C, the mass of the specimens increases, which is caused by aluminum oxidation and nitridation. The mass change depends on the deformation magnitude and pressure. To separate the oxidation and nitridation processes, the thermogravimetric measurements are performed in air, nitrogen, and argon. The thermal effects of aluminum interaction both with the polymer decomposition products and with oxygen and nitrogen are analyzed by the method of differential scanning calorimetry.     

Synthesis of AlN nanopowder coated with a thin layer of C,using a thermal plasma reactor

High-purity nanocrystalline aluminum nitride powders were synthesized by using a 12?kW non-transferred arc plasma. The synthesis was conducted in a versatile, new designed, one-chamber thermal plasma reactor (TPR). The novel experimental assembly incorporated better working conditions like: high temperature gradient between the crucible and reactor's wall, and high super-saturation of the system by nitrogen and carbon. Thermodynamic modelling of the synthesis was conducted in order to achieve the best conditions for AlN formation. In this study, aluminum discs of Al 1100 were used as precursor material and pure nitrogen was the only gas used as reagent and plasmogenic gas.Nanopowders collected from reactor's wall were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-Ray diffraction (XRD). Synthesized h-AlN nano-powders were found to be free of oxides and aluminum metal. A thin carbon-layer around the particles was detected. TEM results indicated that the carbon-layer was around 5 and 10?nm. This outcome could make a significant difference with other synthesis reported in the literature since the occurrence of the carbon-layer, could delay AlN oxidation, prevent hydration, and could avoid the agglomeration of the particles.     

Stability of Phases in the Si-C-N-O System

The stability of the phases in equilibrium is calculated for the Si-C-N-O system in order to analyze and predict the reactions in ceramic whisker formation and sintering of silicon nitride composites. Equilibria among SiC, Si₃N₄, Si₂N₂O, SiO₂, Si, and the gas phase are evaluated at different carbon activities, nitrogen pressures, and temperatures. Phase stability diagrams are constructed as a function of nitrogen and oxygen pressures for two levels of carbon activity. Silicon nitride becomes a stable phase with increasing nitrogen pressure or decreasing carbon activity and temperature, whereas silicon carbide becomes a dominant phase at lower nitrogen pressures or at higher temperatures when carbon activity is unity. The maximum sintering temperature of the SiC/Si₃N₄ composite is higher with an elevated nitrogen pressure or a reduced carbon activity.     

Electrochemical formation of AlN in molten LiCl-KCl-Li3N systems

Electrochemical formation of aluminum nitride was investigated in molten LiCl-KCl-Li₃N systems at 723 K. When Al was anodically polarized at 1.0 V (versus Li⁺/Li), oxidation of nitride ions proceeded to form adsorbed nitrogen atoms, which reacted with the surface to form AlN film. The obtained nitrided film had a thickness of sub-micron order. The obtained nitrided layer consisted of two regions; the outer layer involving AlN and aluminum oxynitride and the inner layer involving metallic Al and AlN. When Al electrode was anodically polarized at 2.0 V, anodic dissolution of Al electrode occurred to give aluminum ions, which reacted with nitride ions in the melt to produce AlN particles (1-5 μm of diameter) of wurtzite structure.     

Preparation of nano-AlN powder by sol–gel foaming and its sintering properties

Uniformly dispersed nano-sized aluminum nitride powders were prepared by the sol–gel foaming method using aluminum nitrate as the aluminum source, sucrose as the carbon source, and ammonium chloride as the foaming agent. The effects of ammonium chloride content on the particle size and the sintering properties of aluminum nitride were investigated. The results showed that when the molar ratio of ammonium chloride to aluminum nitrate was .5, the colloidal foams were uniform, large, and fluffy, and amorphous alumina precursors with uniform particles could be prepared. Aluminum nitride powder with a particle size of 22–27 nm can be obtained by calcining these precursors in nitrogen atmosphere at 1400°C for 2 h. At the same time, aluminum nitride bulk material with a relative density of 95% can be obtained by sintering the compact samples in nitrogen atmosphere at 1700°C for 2 h.     

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.     

氮化硅对烧成铝炭耐火材料的性能研究

通过在铝炭耐火材料中引入氮化硅及很细的铝粉,在埋碳烧成过程中氮化硅与碳或与铝反应生成少量赛隆相及碳化硅。适当引入氮化硅,降低了铝炭砖的氧化失重率,提高了其抗脱磷剂侵蚀的能力及砖的抗氧化性能。     

燃料空气炸药中铝粉与液相的反应特性

为了研究燃料空气炸药中固相组分(铝粉)与液相组分(IPN和烃类燃料MC10)的反应特性,用高压DSC测试铝粉与IPN及MC10在氮气气氛中的分解过程.结果表明,在IPN分解之前存在MC10的氧化过程.与纯MC10液体相比,MC10在IPN存在条件下氧化过程更明显,氧化温度更低,同时铝粉能够改变MC10的高温分解过程.Al/IPN/MC10三组分共存体系在高温条件下具有更稳定的状态,有利于提高燃料空气炸药的性能.     

Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride

This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them. The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O₂, CO, and N₂. Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen and/or carbon monoxide (or nitrogen) diffusion. These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion.     

Thermal Oxidation of Aluminum Nitride Powder

The oxidation kinetics, morphology, and crystallinity of aluminum nitride (AlN) powder thermally oxidized in flowing oxygen were determined from 800° to 1150°C. At 800°C the oxidation became detectable with weight change. AlN powder was almost completely oxidized at 1050°C after only 0.5 h. Amorphous aluminum oxide formed at relatively low temperatures (800°–1000°C), with a linear oxidation rate governed by the oxygen–nitride interfacial reaction. Transmission electron microscopy displayed individual aluminum oxide grains which formed a discontinuous oxide layer at this temperature range. The aluminum oxide was crystalline at higher temperatures (>1000°C), as detected by X-ray diffraction, and the density of oxide grains increased with temperature.     

Kinetics and Mechanisms for Nitridation of Floating Aluminum Powder

Aluminum powder entrained by ammonia-containing nitrogen gas was reacted at various temperatures and time to form aluminum nitride powder. The kinetics of nitride formation were determined by a quantitative X-ray analysis and compared with those determined by a nitrogen analysis of the product. The conversion to aluminum nitride increased with the reaction time following a sigmoidal rate law. The reaction time for full conversion decreased as the temperature increased in the range 1050°–1300°C. The reaction rate constant at a given temperature was evaluated using the Avrami equation. The activation energy for the reaction was 1054 ± 91 kJ/mol in the temperature range of 1050°–1200°C, and decreased to 322 ± 70 kJ/mol above 1200°C. Comparative analysis of powders formed below and above 1200°C suggested strongly that the rate-controlling step changed from chemical reaction to mass transport above 1200°C.     

金属-氮化物结合刚玉质滑板的结构与性能

由80%～90%板状刚玉及20%～10%金属铝组成的坯料经氮化处理(温度1100℃)后,再进行表面氧化处理(温度800℃),可制得显气孔率为2%的Al-AlN-Al2O3滑板材料,其1400℃高温抗折强度高达48.7 MPa. 该滑板材料浇钢的使用寿命是Al2O3-C滑板的2倍. 显微结构分析表明,部分金属铝氮化形成AlN的体积膨胀效应及其对刚玉晶粒的结合作用,提高了材料的结构致密度和强度,赋予材料优良的抗钢液侵蚀性能. 部分金属铝的高温塑性状态、金属铝及氮化铝的高导热性、刚玉与氮化铝的复相热失配等,是材料具有高抗热震性能的主要原因.     

Thermal degradation of urea‐formaldehyde cellulose composites filled with aluminum particles: Kinetic approach to mechanisms

This article reports a study on structural characterization and thermal degradation kinetics of insulating/conducting urea‐formaldehyde cellulose (UFC) composites filled with aluminum particles. Structural characterization of UFC/Al composites carried out by SEM, XRD, and FTIR analyses reveals that composites are fairly homogenous, and the interactions between UFC and aluminum in UFC/Al composites are more probably physical in nature. Measurements of inherent thermal stabilities, probing reaction complexity, and thermal degradation kinetics of UFC and UFC/Al composites have been undertaken by thermogravimetric (TG)/differential thermogravimetric (DTG) analyses under nonisothermal conditions. The integral procedure decompositions temperature (IPDT) elucidates significant thermal stability of UFC, and higher aluminum contents in composites are capable of enhancing the thermal stability of UFC resin. TG/DTG analyses suggest highly complicated thermal degradation profiles of UFC and UFC/Al composites, which consist of various parallel/consecutive reactions. Generalized linear integral isoconversional method has been employed to determine the activation energies of thermal degradation processes. Substantial variations in activation energies of UFC and UFC/Al composites with the advancement of reaction verify their multi‐step reaction pathways. Advanced reaction model determination methodology with the help of a novel kinetic function F(α,T) reveals that the multi‐step thermal degradation of UFC goes to completion by principally following intricate nucleation/growth mechanisms. It is also found that aluminum more likely participates in the thermal degradation of resin and tends to alter its reaction mechanism. Detailed interpretations of the obtained kinetic parameters are given, and their probable physical significances are discussed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44826.     

Oxidation Kinetics of Aluminum Nitride

Thermal oxidation kinetics of aluminum nitride (AlN) powders, having fine- and coarse-particle-size distributions, were studied using thermogravimetric analysis (TGA). The kinetics showed dependence on the particle-size parameter, and the experimental TGA data were curve fitted using empirical mass relations employing both linear and parabolic models. The simulations predicted mixed kinetics in AlN oxidation.     

Oxidation behavior of amorphous silicon nitride nanoparticles

Silicon oxynitride is a promising structural/functional material for high temperature applications. Silicon oxynitride can be synthesized through oxidation of amorphous silicon nitride (ASN) nanoparticles followed by a crystallization process. Oxidation of the ASN plays an important role during the synthesis. Here we investigated its oxidation mechanism in an atomic scale using experimental and modelling method. The results of Nitrogen-Oxygen analyzer and X-ray photoelectron spectroscopy indicate that a large amount of nitrogen vacancies exist in ASN, thus oxidation of ASN may include vacancy oxidation ( oxygen atoms move into the nitrogen vacancies) and replacement oxidation ( oxygen atoms replace nitrogen atoms). A model has been made to describe these two oxidation processes, from which the activation energy (E_a) of the vacancy oxidation and replacement oxidation is calculated to be 9.09 kJ/mol and 118.25 kJ/mol, respectively. These values agree well with E_a calculated from well-designed experiments, confirming the existence of the two different mechanisms during oxidation of ASN.     

Kinetics and Mechanisms of Oxidation of 2D Woven C/SiC Composites: I, Experimental Approach

The oxidation behavior of a 2D woven C/SiC composite partly protected with a SiC seal coating and heat-treated (stabilized) at 1600°C in inert gas has been investigated through an experimental approach based on thermogravimetric analyses and optical/electron microscopy. Results of the tests, performed under flowing oxygen, have shown that the oxidation behavior of the composite material in terms of oxidation kinetics and morphological evolutions is related to the presence of thermal microcracks in the seal coating as well as in the matrix. Three different temperature domains exist. At low temperatures (<800°C), the mechanisms of reaction between carbon and oxygen control the oxidation kinetics and are associated with a uniform degradation of the carbon reinforcement. At intermediate temperatures, (between 800° and 1100°C), the oxidation kinetics are controlled by the gas-phase diffusion through a network of microcracks in the SiC coating, resulting in a nonuniform degradation of the carbon phases. At high temperatures (>1100°C), such diffusion mechanisms are limited by sealing of the microcracks by silica; therefore, the degradation of the composite remains superficial. The study of the oxidation behavior of (i) the heat-treated composite in a lower oxygen content environment (dry air) and (ii) the as-processed (unstabilized) composite in dry oxygen confirms the different mechanisms proposed to explain the oxidation behavior of the composite material.     

The Reaction-Bonded Aluminum Oxide Process: I, The Effect of Attrition Milling on the Solid-State Oxidation of Aluminum Powder

The effect of attrition milling on the solid-state oxidation of aluminum powder is important for the reaction-bonded aluminum oxide process. Attrition milling increased the surface area to 14.4 and 20.2 m²/g versus 1.2 m²/g for unmilled powder and smeared the Al particles, and the surface was hydrolyzed to form bayerite and boehmite. Upon heating the hydroxides decompose to form an 11–13 nm thick amorphous plus γ-Al₂O₃ layer which subsequently retards oxidation kinetics. The oxidation per unit area decreases for the higher surface area powders at temperatures below the critical temperature but the total oxidation of the milled powder is ∼70% versus ∼9% for the as-received powder because of the higher surface area. The critical temperature depends on Al particle surface characteristics and is defined as the transition temperature above which the initial rate of oxidation is linear, not parabolic. Above the critical temperature the oxidation per unit area decreases significantly. In addition, linear oxidation occurs faster than parabolic oxidation and thus the initial fast oxidation kinetics (i.e., linear) can cause thermal runaway during oxidation. Therefore, oxidation below the critical temperature is essential to maximize solid-state oxidation and to prevent thermal runaway. The critical temperatures for the as-received (1.24 m²/g), the 6 h (14.4 m²/g), and 8 h (20.2 m²/g) attrition-milled Al powders were 500°, 475°, and 500°C, respectively. A model for oxidation during the parabolic and linear oxidation stages is presented.     

Oxidation of sintered aluminium nitride

The oxidation of sintered aluminium nitride samples having porosity of 12–16% has been studied at temperatures of 900–1100°C and 98.66 kPa oxygen pressure. It has been established that the reaction of AIN with oxygen obeys the parabolic law. The main products of AIN oxidation are α-Al₂O₃ and nitrogen with nitrogen oxides traces. The corresponding rate constants and apparent activation energy (61 kcal/mol) were calculated from the experimental data. It has been demonstrated that sintered aluminium nitride is resistant enough to high-temperature oxidation and can be used as a refractory material up to 1100°C.     

Al5O6N纳米粉体的制备与表征

在水介质保护下,以三硝基甲苯和铝粉为原料用冲击波等离子体技术由铝化炸药爆炸生成了碳/Al5O6N混合粉体,经过高温氧化除碳,获得了纯立方Al5O6N粉体.利用X射线衍射、Raman光谱、高分辨率透射电镜等对碳/Al5O6N混合粉体和纯Al5O6N粉体进行了分析.用差热一热重分析确定了最佳氧化除碳温度.结果表明:碳/Al5O6N混合粉体的最佳氧化除碳温度为600℃,煅烧1 h后可得到球形立方相Al5O6N粉体,平均粒径为30～40nm,无明显团聚.冲击波等离子体制备纳米粉体时,由于水介质的强制快冷,可减少粉体生长的时间,控制粉体的团聚.     

搅拌法制备C/Al复合材料的界面问题

在碳纤维表面镀上一层金属铜镀层,用硼酸对其进行防氧化处理,采用机械搅拌法制取C/Al复合材料。利用扫描电镜、能谱分析和X射线衍射等手段对液态机械搅拌法制备的碳纤维增强铝基复合材料的界面微观结构进行了分析,结果表明,碳纤维表面镀铜,既可有效解决碳纤维与铝的润湿性问题,又可有效地阻挡碳纤维与铝的过度化学反应。硼酸作为保护剂,有效地解决了高温复合时镀铜层的氧化难题。Cu/Al界面生成大量的CuAl₂金属间化合物,C-Cu/Al界面为混合界面：C/Cu界面和C/Al界面。