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.     

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.     

Formation of SiC whiskers from compacts of raw rice husks

The formation of SiC whiskers from compacts of raw rice husks without coking and catalyst has been studied. A pyrolysis temperature of 1600°C has yielded a considerable quantity of SiC whiskers. The formation of spherical particles of silica is observed. Whisker formation occurs by reaction between SiO_(g) and CO_(g).     

氮化硅晶须对反应烧结氮化硅多孔陶瓷介电性能的影响

以硅粉和氮化硅晶须为原料,通过添加30%(质量分数)成孔剂球形颗粒,以聚乙烯醇作粘结剂,采用干压成型工艺,反应烧结制备了多孔氮化硅陶瓷,分析对比了氮化硅晶须对反应烧结氮化硅多孔陶瓷介电性能的影响.实验结果表明,随着氮化硅晶须加入量的升高,氮化硅多孔陶瓷的介电常数和介电损耗都升高,介电性能恶化.     

Formation of silicon carbide whiskers and their microstructure

Thermodynamic and kinetic conditions for the formation of SiC whiskers are established. The mechanism of their nucleation and growth are studied and, on this basis, the magnitude of the thermally activated barrier is determined from the rate of reduction data. The microstructures of whiskers are analysed and the role of interfacial tension between the nuclei and impurities, and the metallic iron catalyst is studied in relation to the formation of SiC whiskers. A possible reason for polytypism in SiC whiskers is also proposed.     

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.     

The synthesis of silicon nitride whiskers from SiO2-N2-Na3AlF6 system

Silicon nitride (Si₃N₄) whiskers were synthesized from SiO₂-N₂-Na₃AlF₆ system. Whiskers, which were synthesized when the molar ratio of SiO₂ to Na₃AlF₆ (SiO₂/Na₃AlF₆) ranged from 2 to 8, were prismatic with a stable diameter ranging from 0.1 to 0.5 m. Therefore, the whiskers were considered to have grown by a VS mechanism. The effect of the addition of iron oxide (Fe₂O₃ : SiO₂ = 1.5–7.5 : 100) was examined when SiO₂/Na₃AlF₆ was 3, at which the maximum amount of whiskers was obtained. Since some of the whiskers, synthesized when the weight of Fe₂O₃ to that of SiO₂ ranged from 3.0 to 6.25 (Fe₂O₃ : SiO₂ = 3.0–6.25 : 100), have droplets on their tips, they were assumed to have grown by VS and VLS mechanisms. The composition of the droplets were found to be Al-Si by elemental analysis by EDAX. Since droplets composed of Al-Si have never been reported, we performed a detailed analysis of the droplets in this study.     

Size-strength effects in sapphire and silicon nitride whiskers at 20° C

Tensile tests at 20° C have been carried out on seventy-three sapphire whiskers and on seventeen silicon nitride whiskers. The sapphire whiskers were of 0001, 1¯120, 10¯10, and 10¯11 orientations, while the silicon nitride whiskers were 0001, 11¯20, and 10¯13. Tensile strengths were in the range 45 to 1500 kg/mm², and deformation was found to be purely elastic. The tensile strength data have been analysed and fitted to empirical equations describing the effect of size on strength for different orientations. These empirical equations have been used to deduce possible fracture nucleation mechanisms. It is concluded that, in the case of 0001 sapphire whiskers, fracture nucleation may be due to dislocation pile-ups or interactions, while in the other cases a Griffith flaw mechanism is probably applicable.     

Characterization of SiC whiskers

The composition and microstructure of SiC whiskers from three different suppliers were studied to understand their physical and chemical properties. The following analytical methods were utilized: complete chemical analysis, morphological examination by scanning electron microscopy and by high-resolution transmission electron microscopy, selected-area electron diffraction, X-ray diffraction, and thermogravimetric analysis of the oxidation rate of the three SiC whiskers at 1050C in oxygen and at 1150C in air.     

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.     

Anisotropic etching of SiC whiskers

We have demonstrated a method of producing nanoplatelets or complex well-ordered nanostructures from silicon carbide (SiC) whiskers. Preferential etching of SiC whiskers in a mixture of hydrofluoric and nitric acids (3:1 ratio) at 100 degrees C results in the selective removal of cubic SiC and the formation of complex structures resembling a pagoda architecture. Possible mechanisms governing selective etching are discussed. Reproducible results on SiC whiskers manufactured in different laboratories suggest that the self-patterning phenomena are common in SiC whiskers, and the same electroless etching procedure can be used to synthesize various complex nanostructures from more conventional nano- and microscale objects for use as building blocks in the fabrication of sensors, cellular probes, and electronic, optoelectronic, electromechanical, and other devices.