Synthesis of Aluminum Nitride Nanocrystalline

Nanocrystalline aluminum nitride (whisker and particle) has been synthesized by heating the mixed aluminum+aluminum nitride ultrafine powders (UFPs) produced by nitrogen plasmamolten aluminum reaction, under N2 atmosphere at lower temperature. Some bamboo-like nano- whiskers of aluminum nitride were observed in TEM, the possible formation mechanism of the whiskers was also disucssed.     

Preparation and characterization of morph-genetic aluminum nitride/carbon composites from filter paper

Morph-genetic aluminum nitride/carbon composites with cablelike structure were prepared from filter paper template through the surface sol-gel process and carbothermal nitridation reaction. The resulting materials have a hierarchical structure originating from the morphology of cellulose paper. The aluminum nitride/carbon composites have the core-shell microstructure, the core is graphitic carbon, and the shell is aluminum nitride nanocoating formed by carbothermal nitridation reduction of alumina with the interfacial carbon in nitrogen atmosphere. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and transmission electron microscope were employed to characterize the structural morphology and phase compositions of the final products.     

Synthesis of silicon nitride nanowires by the pyrolysis of perhydropolysilazane

Silicon nitride nanowires synthesized by the pyrolysis of perhydropolysilazane without using any catalysts are reported. After pyrolysis at 1073 K in N2/NH3 atmosphere, the synthetic nanowires are discrete and curly with diameters about tens of nanometers and lengths of hundreds of nanometers. While after post-treatment at 1873 K in N2 atmosphere, the nanowires are continuous and randomly distributed with diameters about tens of nanometers and several microns in length. There are no bulbs or droplets on the tips of the nanowires, and two gas-solid mechanisms are proposed to explain their growth.     

Synthesis of aluminum nitride fibres from aluminum silicate fibres by carbothermal reduction method

Aluminum nitride (AlN) fibres were synthesized by carbothermal reduction of aluminum silicate fibres and carbon black in a flowing nitrogen atmosphere. XRD and SEM were employed to study the phase structure, chemical composition and morphologies of the products. It is suggested that aluminum silicate fibres can be converted into AlN fibres by carbothermal reduction. SEM confirmed that the fibres maintained the morphologies of aluminum silicate fibres during reduction, but their details of surface have been altered. A carbothermal nitriding model of the aluminum silicate fibres has been developed.     

微波碳热还原法制备氮化铝粉末的工艺研究

采用微波碳热还原法制备了氮化铝粉末,研究了铝源、碳源和添加剂对制备氮化铝粉末的影响. 通过对所合成的产物进行XRD检测分析表明,氢氧化铝和乙炔黑是最合适的铝源和碳源、单质添加剂的氮化催化效果最明显. 以氢氧化铝和乙炔黑为原料,加入单质添加剂,在氮气气氛下反应温度为1300℃、反应时间为1h时能获得完全氮化的氮化铝粉末,可见微波碳热还原工艺能够大大降低碳热还原法制备氮化铝粉末的反应温度,并缩短反应时间.     

Preparation of 14C by Irradiation of Aluminum Nitride in Nuclear Reactor

Properties of aluminum nitrides prepared by various methods were analyzed with respect to their use as nitrogen-containing materials for preparing ^{1 4}C by irradiation with thermal neutrons in nuclear reactor. It was found that carbon can be completely isolated from irradiated aluminum nitride using potassium dichromate oxidizing agent. Carbon-14 dioxide can be isolated from the gas mixture with nitrogen and nitrogen oxides formed during oxidation of irradiated nitride by passing through potassium permanganate and sodium hydroxide absorbing solutions.     

Non-thermal plasma for oxidation of gaseous products originating from thermal treatment of wastes

A plasma reactor generating non-equilibrium plasma in a gliding discharge was applied as one of the modules of a new laboratory device for hazardous waste destruction. The degradation process of wastes containing an organic part was carried out in two stages. The first one consisted of thermal decomposition of wastes in an inert atmosphere (pyrolysis process in argon flow—the gaseous products are formed from the organic part of wastes). In the second stage products of pyrolysis were oxidized in a gliding discharge. This work was focused on study of the parameters influencing the oxidation process of gases originating from pyrolysis and flowing into the plasma reactor. Oxygen was introduced into the plasma reactor simultaneously with the gases. We investigated two factors significantly affecting the oxidation process: (a) the oxygen concentration in the initial mixture of argon and oxygen and (b) the total flow of argon and oxygen gases. The best oxidation efficiency of the processing gases in the plasma was reached when the oxygen content did not exceed 20% and when the total flow of argon and oxygen was low enough not to cause disturbances of functioning of the plasma reactor.     

炭纤维增强氮化物复合材料的制备及其烧蚀性能

合成了硼吖嗪和全氢聚硅氮烷的混杂先驱体并对其结构进行了表征;以混杂先驱体和3D炭纤维编制体为原料,采用先驱体浸渍-裂解(PIP)工艺制得了炭纤维增强氮化物基体的复合材料,并对复合材料基体的抗氧化性以及抗烧蚀性能进行了研究。结果表明:混杂先驱体中含有B-N、B-H、Si-N、Si-H、N-H等结构,无其他杂质;基体材料在空气中具有优良的抗氧化性能,温度升至1000℃时仍未发生明显的质量变化,明显优于C/C复合材料;四个PIP工艺循环所制得的复合材料烧蚀表面平整,氮化物基体比增强炭纤维具有更好的耐烧蚀性能。     

The effect of aluminum additive on structure evolution of silicon oxycarbide derived from polysiloxane

Silicon oxycarbide (SiOC) and aluminum-containing silicon oxycarbide (SiAlOC) glasses were obtained through pyrolysis in argon atmosphere at 1000 °C of a polymethyl(phenyl)siloxane resin and aluminum tri-sec-butoxide-derived siloxane networks. These glasses were further annealed at 1200, 1300, and 1400 °C in vacuum atmosphere to investigate their high-temperature behavior. The two types of glasses were characterized by X-ray diffraction, ²⁹SiMASNMR, ²⁷AlMASNMR, and chemical element analysis. The aluminum incorporated into structure plays a major role on the thermal stability of SiOC by hindering carbothermal reductions. It can be found that introducing aluminum into structure should be an effective way to enhance the thermal stability of SiOC glasses.     

Direct nitridation of aluminum compacts at low temperature

Direct nitridation of aluminum compacts (relative density 65%)consisting of commercial atomized powder was examined at temperaturesfrom 500 to 700°C, near or below the melting point ofaluminum, and under a pressured nitrogen atmosphere between0.5–7 MPa. Complete nitridation was achieved at a temperature as lowas 540°C by controlling the nitrogen pressure. Thenitridation process and the structures of generated aluminum nitride(AIN) were drastically influenced by nitrogen pressure. Consideringthe relations between the reaction conditions and the reactionprocesses, it is suggested that low nitridation temperature andpre-heating in vacuum have a good effect on the nitridation ratio.     

THE INFLUENCE OF SYNTHESIS PARAMETERS ON THE PRODUCTION OF MULTIWALLED CARBON NANOTUBES BY THE FERROCENE CATALYZED PYROLYSIS OF TOLUENE

A high yield (∼32 wt.%) of multiwalled carbon nanotubes was obtained in an iron catalyzed reaction. This was achieved in the temperature range 800-1000°C under an atmosphere of H₂/Ar by an improved solution injection method in a horizontal reactor using toluene as carbon source and ferrocene as catalyst precursor. The pyrolysis temperature, ferrocene concentration, solution feeding rate and carrier gas flow rate all influenced the yield of carbon nanotubes and the thickness of the aligned carbon nanotube films. The carbon nanotubes was prepared in high purity using optimized pyrolysis conditions.     

A molecular-level analysis of gas-phase reactions in chemical vapor deposition of carbon from methane using a detailed kinetic model

This work describes a modeling study of methane pyrolysis in chemical vapor deposition (CVD). The model consists of a detailed chemical kinetic model, which includes 241 species and 909 gas-phase reactions for methane pyrolysis mechanism, and a plug-flow model, which describes the transport conditions in CVD. Reasonably good agreements were obtained between the simulation results and the experimental results of methane pyrolysis in CVD of pyrocarbon in a vertical hot-wall deposition reactor without any artificial adjustments. The mole fractions of hydrogen, acetylene, ethylene, and benzene increased with a decreasing growth rate as the residence time and the initial methane pressure increased. Sensitivity analysis and reaction paths were conducted to identify the crucial reaction steps and explain how they impact in this pyrolysis process. Results showed that methane pyrolysis had an incubation stage to form a necessary gas atmosphere for the pyrolysis to move forward and C₃ species were the main direct source for benzene formation. These results should be useful to understand methane pyrolysis at a molecular level in CVD, as well as the relationship between the gas species and the pyrocarbon.     

Embrittlement of electrical steel laminations by nitrogen pick-up during heat treatment

Heat treated electrical steel laminations have shown evidence of low ductility behavior, characterized by a small number of bends till fracture, on repeated bending tests. The laminations were produced using a new grade of electrical steel with much lower aluminum content than usual. The problem happens when the oxygen potential (measured by the dew point of the atmosphere) of the heat treatment atmosphere is abnormally high. Furthermore, ductility can be restored by a low-oxygen potential heat treatment. Although the heat treatment resulted in a loss of ductility, the magnetic properties were not deteriorated. The low ductility samples always show intergranular fracture, whereas the un-treated laminations fracture by cleavage. The low ductility is associated with the formation of silicon–manganese nitride precipitates formed at grain boundaries, although they are not the cause of the low ductility. Ductility could be restored by a low dew point heat treatment but the inclusions remained in the grain boundaries. The low ductility and its recovery must be ascribed to the presence of nitrogen atoms segregated to the grain boundaries when the heat treatment atmosphere has a high oxygen potential. The lack of aluminum in the composition of the steel hinders the scavenging effect of this element on nitrogen atoms in solution in the steel.     

Gefügemodifizierung durch Bornitrid

Grain refinement by boron nitride Grain refinement of magnesium alloys aims at better deformation behaviour, higher strength and improved corrosion resistance. Besides mechanical treatment like pressing, it is possible to refine the grainsize by using nucleation materials. Whereas calcium and rare earth elements are already widely used, the use of boron nitride offers a cheap alternative to refine grains of magnesium aluminum alloys. The effect is achieved by the reaction of boron nitride with aluminum which cracks the chemical compound to form aluminum nitride with the nitrogen while boron is forming different magnesium borides. These two compounds both exhibit very high melting points and are stable in this environment so that they can act as seed crystals. Because boron nitride shows a bad wettability to metal molds, it would float on top of the mold. Therefore, it is necessary to produce pellets out of boron nitride and aluminum powder to improve contact to the mold and enhance reaction velocity.     

等离子体法制备超细粉体氮化铝的研究

以微米级铝粉为原料 ,用N2 热等离子体法制备了超细氮化铝粉体。在等离子体功率12kW ,运行N2 流量 2m3/h ,急冷NH3流量 0 6m3/h ,送粉N2 流量 0 8m3/h条件下 ,铝粉全部转化为纳米氮化铝。采用SEM技术和粒度分析仪对产品进行了分析 ,制得的氮化铝粉末平均粒径为10 0nm ,粒度分布为 4 0～ 14 0nm     

Synthesis of aluminum nitride nanolayers

The interaction between the surface of polycrystalline aluminum and nitrogen at a slow heating to a temperature of 623°C and a pressure of 1.25 atm is studied by ellipsometry. It is shown that the formation of a continuous nanolayer of aluminum nitride occurs in a temperature range of 520–580°C and above. The maximum thickness of the synthesized aluminum nitride nanolayer is 32 nm. The formation of nitride on the surface of aluminum is confirmed by IR spectroscopy of powders with a specific surface area of 6.4 m²/g.     

Preparation of aluminum nitride granules by a two-step heat treatment method

The sintered aluminum nitride granules were prepared in a continuous process through a two-step heat treatment at 1600?°C for 5?h and 1850?°C for 5?h using the spherical γ-alumina/phenol-resin precursor obtained by spray granulation as the starting material. In the first step of heat treatment, the alumina powders were nitrided to form porous aluminum nitride granules via a carbothermal reduction reaction and then were continuously heated in the second step of heat treatment to produce dense spherical aluminum nitride granules without using sintering aids. The obtained porous and sintered aluminum nitride granules were characterized by XRD, SEM and BET measurements and exhibited an average grain size of about 1 and 3?µm, respectively. Furthermore, the specific surface area of the obtained porous and sintered aluminum nitride granules dramatically decreased from 2.49 to 0.16?m²/g showing that the low-porosity, dense, polycrystalline aluminum nitride granules were successfully prepared.     

Effects of Cation Impurities on Thermal Conductivity of Yttria-Doped Aluminum Nitride

Effects of cation impurities such as Fe and Si on thermal conductivity of yttria-doped aluminum nitride were investigated. Two types of aluminum nitride powders, Tokuyama F-grade and Denka AP-10, were compared. The powders have similar contents of oxygen but different levels of cation impurities. Since the thermal conductivity of sintered aluminum nitride is virtually affected by the residual oxygen contents in the grains, effects of yttria additions and prolonged firing time are also investigated. The resultant thermal conductivity is thought to be influenced by the sample size of sintering bodies because extracting oxygen from the aluminum nitride lattice in larger samples requires longer time resulting from a longer diffusion path. In comparing the AlN powders, impurity contents of Fe and Si are somewhat higher in Denka powder than Tokuyama powder. These impurity elements degrade the thermal conductivity of sintered aluminum nitride. Degradation effects in thermal conductivity due to presence of Fe and Si are evaluated by artificially introducing these elements to Tokuyama AlN powder. The difference in the thermal conductivity resulting from the two types of powders is discussed.     

THE INFLUENCE OF SYNTHESIS PARAMETERS ON THE PRODUCTION OF MULTIWALLED CARBON NANOTUBES BY THE FERROCENE CATALYZED PYROLYSIS OF TOLUENE

ABSTRACT

A high yield (～32?wt.%) of multiwalled carbon nanotubes was obtained in an iron catalyzed reaction. This was achieved in the temperature range 800–1000°C under an atmosphere of H₂/Ar by an improved solution injection method in a horizontal reactor using toluene as carbon source and ferrocene as catalyst precursor. The pyrolysis temperature, ferrocene concentration, solution feeding rate and carrier gas flow rate all influenced the yield of carbon nanotubes and the thickness of the aligned carbon nanotube films. The carbon nanotubes was prepared in high purity using optimized pyrolysis conditions.     

Preparation of silicon carbide whiskers from silicon nitride

Silicon carbide whiskers have been prepared by sintering silicon nitride powder in a graphite reactor at 1800°C under a nitrogen atmosphere. The whiskers differ in morphology: tubular needles, hollow faceted fibers with a square cross section, and solid fibers with a triangular cross section. The average diameter of the needles is 0.5?5 μm, and that of the faceted fibers is up to 20 μm. The fibers range in length up to several millimeters. Such silicon carbide whiskers can be used as reinforcing agents for structural ceramics based on nonoxide materials.