Morphologies and growth mechanisms of aluminium nitride whiskers

Morphologies of AlN whiskers grown by the vapour–liquid–solid mechanism (VLS) were investigated. Several types of whisker structures, such as growth hill, wavy, crossed and stack structures, were found due to the variation of growth conditions. Growth mechanisms and orientations of AlN whiskers were also studied. Besides preferential crystallographic planes, several other planes were found to be growth layers due to the perturbation of the AlN lattice change caused by dissolution of oxygen. A screw dislocation growth mechanism was clearly confirmed. An oblique growth mechanism was found in this work, which may be the result of two processes: the vapour–liquid–solid process and dissolution of oxygen. This revised version was published online in November 2006 with corrections to the Cover Date.     

Growth of oriented aluminium nitride films on silicon by chemical vapour deposition

Aluminium nitride films were grown on silicon substrates using the chemical vapour deposition (CVD) method. The properties of the films were studied by scanning electron microscopy (SEM), atomic force microscope (AFM) measurements, X-ray diffraction and Raman scattering. The resulting films were strongly textured and had a preferential orientation with the c-axis normal to the surface, the Raman spectra showed two peaks at 607 and 653 cm^–1 and two large bands at 750 and 900 cm^–1 of smaller intensity. Both the macro- and micro-Raman spectra showed the same peaks.     

Microstructure of alpha-silicon nitride whiskers

Alpha-Si₃N₄ whiskers grown by the vapour-liquid-solid process have been investigated in a transmission electron microscope. The growth directions of the whiskers are determined to be {1 0 ¯1 1}^*, {1 0 ¯1 0}^* and {0 0 0 1}^*. Defects in the three kinds of whisker have been characterized and branched phenomena are explained on the basis of the TEM observations.     

直接氮化法制备氮化铝粉末的结构特性

用金属镁（Mg）作催化剂,氮气和铝块为反应物,采用直接氮化法制备出氮化铝（AlN）粉末样品。运用X射线衍射仪（XRD）、扫描电子显微镜（SEM）和拉曼光谱仪（Raman）对样品进行结构特性分析发现,AlN样品为纯六方相结构,呈现纳米线堆积形貌,纳米线直径约60nm,且尺寸均匀。拉曼散射光谱峰值较单晶AlN向低波数方向移动,表明此方法制备的AlN纳米线存在表面拉应力。     

Wettability of aluminium nitride by tin–aluminium melts

The wettability of aluminium nitride by Sn–Al melts was studied by the sessile drop method in a vaccum of 2 × 10^–3 Pa at 1100 °C over the whole concentration region. The minimum interval on the contact-angle concentration dependence curve was observed at intermediate composition. For comparison, experiments were also performed on porous AlN. Wetting of porous nitride is worse than the dense nitride. The results have been analysed on the basis of the relation between wettability and the chemical interface reactivity in solid–liquid metal systems.     

Formation of SiC whiskers from silicon nitride

Attempts have been made to produce SiC whiskers through vacuum pyrolysis of Si₃N₄ without any addition of extraneous carbon. Vacuum pyrolysis of Si₃N₄ granules and powder compacts, has been carried out at 1550 and 1700°C using a graphite resistance furnace. The products of pyrolysis have been identified through XRD and SEM as SiC whiskers and particles. Small amounts of elemental silicon at 1550°C and free carbon at 1700°C have been detected through X-ray diffraction. Detection of elemental silicon through X-ray diffraction and solidified silicon droplets at the whisker tips in the SEM provide important clues regarding the mechanism of SiC_w formation, as the one involving the reaction 2Si(l) + CO(g) SiC(s) + SiO(g) Silicon carbide whiskers, 3–4 mm long, have been grown from Si₃N₄ compacts at 1550°C over a short period of 0.5 h. It has been shown in the present study that Si₃N₄ can be completely converted to SiC_w, when a loose bed of Si₃N₄ in the form of granules is pyrolysed in the presence of CO at about 1550°C.     

Oxidation of aluminium nitride

The oxidation kinetics of polycrystalline aluminium nitride substrates in air at temperatures in the range 1150 to 1750°C have been studied by measuring the weight increase in the oxidized samples. At the lowest temperature, the oxide layer was not continuous on the AlN surface and the oxidation kinetics followed a linear rate law with an activation energy of 175 kJ mol^–1. At all the higher temperatures, the growth kinetics followed a parabolic rate law with an activation energy of 395 kJ mol^–1. Samples oxidized at these higher temperatures were covered with a dense oxide layer having a fine-grained microstructure.     

Synthesis and characterization of silicon nitride whiskers

Silicon nitride whiskers were synthesized by the carbothermal reduction of silica under nitrogen gas flow. The formation of silicon nitride whiskers occurs through a gas-phase reaction, 3SiO(g)+3CO(g)+2N₂(g)=Si₃N₄()+3CO₂(g), and the VS mechanism. The generation of SiO gas was enhanced by the application of a halide bath. Various nitrogen flow rates resulted in different whisker yields and morphologies. A suitable gas composition range of N₂, SiO and O₂ is necessary to make silicon nitride stable and grow in a whisker form. The oxygen partial pressure of the gas phase was measured by an oxygen sensor and the gas phase was analysed for CO/CO₂ by gas chromatography. Silicon nitride was first formed as a granule, typically a polycrystalline, and then grown as a single crystal whisker from the {1 0 0} plane of the granule along the 210 direction. The whiskers were identified as-sialon with Z value ranging from 0.8 to 1.1, determined by lattice parameter measurements.     

A1直接氮化法制备纳米A1N晶须的形成过程及形貌观察

通过同步热分析（simultaneous thermal analysis，STA）分析了Al在非高压氮气气氛下（压力0．2MPa，流速30ml/min）生成纳米AlN晶须的反应过程。分析了氮化的动力学过程。用环境扫描电子显微镜（environmental scanning electron microscopy,ESEM）观察了AlN及Al-AlN界面的显微结构。结果表明：在300-450℃之间，氮气即可在固-气界面处与气相铝发生反应生成AIN；从450℃开始，反应生成的AlN在未反应的Al表面沉积，以后一直持续至铝熔融；从熔点开始，熔融后的铝粉体在芯层中迅速团聚，阻碍氮化的进一步进行，但是在壳层中，氮化在液-气界面进一步进行，形成最终的芯-壳结构。ESEM的分析结果表明，在持续升温过程中，壳层中主要生成毛纤维状的纳米AIN晶须。芯层中主要通过VS机制生成大的AlN晶体。     

Hydrophobing of aluminium nitride powders

The hydrophobing of AIN powders through adsorption of capric acid, stearic acid and cetyl alcohol on the particle surface was investigated by statistical analysis. Stearic acid as surface adsorbent and cyclohexane as solvent were identified as the best combination for achieving highly effective hydrophobicity of AIN. The adsorption data obtained for this combination indicated a Langmuir chemisorption isotherm. Even after 96 h leaching in water, no crystalline phase other than AIN could be detected by X-ray diffraction (XRD).     

Oxidation of aluminium nitride substrates

The growth of oxide films on two types of aluminium nitride substrates of different origin has been studied as a function of temperature. At a given set of oxidation reaction parameters, the oxide layers grown on substrates with a relatively large grain size and high concentrations of Y-Al-O-based liquid sintering aid phases (type I substrates) were observed to be thicker and more diffuse than those obtained on substrates with an average particle size of approximately 3 m and low liquid sintering aid concentrations (type II substrates). The effects of the oxygen partial pressure variation on the oxide film growth have been investigated for the oxidation of type II AIN substrates. The kinetics of the growth of oxide films on such substrates were analysed and determined to fit best to a linear rate law. This type of rate law indicates that the rate-limiting step in the growth of oxide films on high-quality type II aluminium nitride substrates is an interface reaction-controlled process.     

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.     

Destruction of silicon nitride whiskers by reaction with metals at high temperatures

A preliminary study has been made of the reaction, at elevated temperatures, between whiskers of silicon nitride and pure metals (aluminium and nickel). The subject is important in the context of composite materials using refractory whiskers as a reinforcing component. The method involves reacting the whiskers with a thin, evaporated layer of the metal concerned and observing the results under the electron microscope. The effects of reaction are described and may be explained in terms of either crystallographic etching or recrystallisation of the whisker, though the former appears the more likely. The results do not necessarily reflect the behaviour of bulk composites made from the same constituents.     

Chemical preparation of ultra-fine aluminium nitride by electric-arc plasma

Ultra-fine aluminium nitride has been obtained by interaction of aluminium and nitrogen in electric-arc plasmA. It has been proved that the rate determining step of the process is the evaporation of the aluminium powder. The most appropriate thermodynamic and kinetic conditions for obtaining pure, finely dispersed product has been determined. The prepared aluminium nitride has dimensions of about 50 nm and specific surface of up to 100 m²g^?1. The items made by the baking of this product at 1600 K have zero porosity. The product has a greater chemical reactivity than the one producted by conventional methods.     

Evaporated aluminium nitride encapsulating films

AIN films were deposited onto GaAs and vitreous carbon substrates held at room temperature, by reactive evaporation of aluminium in the presence of nitrogen and/or NH₃ gas mixture. These films and their combination with very thin layers of Si₃N₄ were successfully used as encapsulants for GaAs and were found to withstand annealing temperatures of up to 1100°C. Films grown by this novel method were analysed by Rutherford backscattering spectrometry and reflection high energy electron diffraction. Oxygen, nitrogen and aluminium were the only elements detected in the encapsulants. However, the best encapsulants were found to have the lowest oxygen content. The deposition conditions were found to be very important in preventing the reaction of the films with the surface of GaAs during heat treatment.     

Novel growth of aluminium nitride nanowires

This work describes novel growth of aluminium nitride (AIN) nanowires by nitridation of a mixture consists of aluminium and ammonium chloride powders (Al:NH4Cl = 1.5:1 weight ratio) at 1000 degrees C for 1 h in flowing nitrogen gas (1 l/min). XRD analysis of the product showed the formation of pure hexagonal AIN. SEM micrographs of as-synthesized product revealed the growth of homogeneous AIN nanowires (phi 40-150 nm). No droplets were observed at the tips of obtained nanowires which suggests that they were grown mainly by a vapor-phase reactions mechanism. Thermodynamic analysis of possible intermediate reactions in the operating temperatures range illustrates that these nanowires could be grown via spontaneous vapor-phase chlorination-nitridation sequences.     

Synthesis of oxygen-free aluminium nitride ceramics

The aluminum nitride raw material in the form of powder was synthesized using the Self propagating High temperature Synthesis (SHS) method which provides no oxygen impurities. Then AIN powder was sintered to the full density without sintering additives and under a high pressure in a belt apparatus. For the AIN ceramics obtained the temperature dependences of the thermal diffusivity were measured with the laser-flash method. Finally we produced oxygen-free aluminium nitride ceramics with parameters comparable with theoretical data.     

High temperature oxidation of hot-pressed aluminium nitride by water vapour

Hot pressed AIN without additives was oxidized et 1100 to 1400°C in dry air, wet air and wet nitrogen gas atmospheres with 1.5 to 20 kPa of water vapour pressure. AIN was oxidized by both air and water vapour, and formed -Al₂O₃ film on the surface above 1150°C. The oxidation kinetics in air were parabolic end were promoted by water vapour. On the other hand, the oxidation kinetics in wet nitrogen were linear below 1250°C and parabolic above 1350°C. The oxidation rate in wet nitrogen was much greater than that in wet air. The rate of oxidation increased with increasing temperature until 1350°C, and then decreased. The parabolic rate constant decreased with increasing temperature and increased linearly with increasing water vapour pressure. The linear rate constant at 1150 to 1250° C increased with increasing the temperature with the apparent activation energy of 250 kJ mol^–1. The relation between the linear rate constant and water vapour pressure was of the Langmuir type.