Synthesis And Tailoring Of One-dimensional Silicon Nitride

Silicon nitride?Si₃N₄?material is widely used in the fields of metallurgy,machinery,chemicals,electronics,defense and energy due to its excellent physical and chemical properties,such as high value of strength,modulus,thermal conductivity and high resistance to heat,oxidation,corrosion as well as low value of thermal expansion,dielectric constant and dielectric loss.Compared with its bulk counterpart,onedimensional Si₃N₄ possesses more uniform microstructure,higher specific area and surface energy.Therefore,it exhibits excellent mechanical,thermal and semiconductor properties,which has great potential applications in composite,micro and nano devices.However,the current studies mainly focus on morphologies and physical properties of one-dimensional Si₃N₄ and those synthesis methods have disadvantages of low yield and high cost.Since its good performance in practice,it is highlighted that synthesis of one-dimensional Si₃N₄ with high purity and well dispersion to satisfy the urgent want from industries.In this study,one-dimensional Si₃N₄ is synthesized by direct nitridation of silicon and characterized by X-ray diffraction,scanning electron microscopy and Raman spectrum.The influences of loading form,particle size and purity of silicon powders,nitriding schedual and catalysts on phase contents,morphologies and microstructures of one-dimensional Si₃N₄ are discussed.The main results are as followings:?1?The conversion rate increases with the decrease of the accumulation density and particle size of silicon powder.When the particle size of silicon powder decreases,the one-dimensional Si₃N₄ gets smaller in diameter and better at dispersion.The purity of silicon powder affects significantly on the content of Si₃N₄.In the case of loose accumulation,the content of Si₃N₄ is as high as 97.9% including ?-Si₃N₄ of 71.6%,in which one-dimensional Si₃N₄ exhibits well dispersed.The content of Si₃N₄ is high up to 99.1% in the case of 1?m silicon powder.In-situ temperature rise due to esothermic reaction increases the content of ?-Si₃N₄.The diameter of one-dimensional Si₃N₄ produced by silicon powder of 1?m and 5?m is about 200 nm and 800 nm.?-Si₃N₄ shows almost one-dimensional morphologies and grows preferentially,while ?-Si₃N₄ exhibits particles.The yield of Si₃N₄ from silicon powder with purity of 98.5% is higher than that of 99.9% due to impurity catalysis.?2?In the atmosphere of pure NH₃,the nitriding rate of silicon powder during slow heating?by 2h?is higher than that of fast heating?by 1h?in the temperature range of rapid nitridation reaction?1300-1380??.The former produces Si₃N₄ as high as 96.9% in content and contains many rodlike Si₃N₄ with diameter of 1.5?m,while the latter includes much molten Si and Si₃N₄ particles.The nitriding efficiency in pure N2 atmosphere is better than that in pure NH₃ atmosphere during fast heating.The former case can produce sample with Si₃N₄ content close to 100% which includes massive,well dispersed and uniform one-dimensional Si₃N₄.?3?The catalyst species have great influences on the morphologies of onedimensional Si₃N₄,especially can tailor the diameter of one-dimensional Si₃N₄ effectively.Using catalysts of Y and Ce can increase the diameter of one-dimensional Si₃N₄ up to 1?m,but the products have residual spheric particles.Li as catalyst can reduce the diameter of one-dimensional Si₃N₄ to 50 nm.The catalysts of Mg and Ca can produce smoother one-dimensional Si₃N₄ with more uniform diameter?500-600nm?,and the content of one-dimensional Si₃N₄ is higher in the case of Mg than that of Ca.