Silicon Nitride Fiber Materials Prepared By Direct Nitridation Process Of Silicon Porous Green Body

Silicon nitride fiber materials have excellent properties,such as good mechanical,high elastic modulus,low thermal expansion coefficient,low dielectric constant and dielectric loss,and low thermal conductivity.It has great application prospect in some special heat insulation fields,such as energy-efficient aluminum electrolyzer and microwave-sintering furnace.However,the preparation of the traditional precursor spinning method is high cost.A novel method was proposed to prepare Si3N4 fiber materials by our research.In this paper,Si₃N₄ fiber materials were successfully fabricated via nitridation of porous Si green bodies which were firstly prepared by foaming method combined with gel-casting using commercial silicon（Si）powders as the main starting materials.The effects of nitrogen flow rate,nitriding temperature,heating rate,foaming volume times of green body,the addition amount and morphology of Si₃N₄ on the structure of Si₃N₄ fiber materials were investigated.The mechanisms of the morphology control and the growth of Si₃N₄ nanofibers were discussed.The main results are as follow:（1）The formation region of Si₃N₄ nanofibers which is controlled by temperature and oxygen partial pressure is obtained by thermodynamic analysis of Si-O-N system.The formation region of Si3N4 nanofibers is enlarged with the increase of temperature.The range of reasonable oxygen partial pressure can promote the growth of Si₃N₄ nanofibers.（2）When the temperature and nitrogen partial pressure are determined,the growth area of Si3N4 nanofiber is controlled by oxygen partial pressure.With increasing temperature,the growth region of Si₃N₄ nanofibers is not only enlarged,but also the stable region of Si₂N₂O and SiO₂ is reduced and the maximum partial pressure of SiO in Si-O-N system is increased.（3）The growth of Si3N4 nanofibers is controlled by nitrogen flow rate,nitriding temperature and heating rate.Larger nitrogen flow rate,higher nitriding temperature and lower heating rate can increase the growth of Si₃N₄ nanaofibers and promote the collapse of pore wall of porous green body.Furthermore,the diameter of Si3N4 nanofibers is coarsened with the increase of nitriding temperature and low heating rate.When the nitrogen flow rate is 250 mL/min,and the nitriding temperature is 1400℃and the heating rate is 0.5℃/min,the fiber grow well.（4）The collapse of pore wall of porous green body,the growth of Si₃N₄ nanaofibers,low volume density and high apparent porosity are controlled by increasing foaming volume times of green body.When the foaming volume times of green body is two times,the growth of Si₃N₄ nanofibers is in good condation.（5）With changing the addition amount of Si₃N₄ particles and whiskers from 2wt%to20wt%,the length of Si₃N₄ nanofibers increases and from 20wt%to 30wt%,the length of Si3N4 nanofibers become slightly shorter.The diameters of Si3N4 nanofibers can become fine by addingα-Si₃N₄ particles and whiskers.The growth of Si₃N₄ nanofibers mainly follows vapour-solid（VS）mechanism,accompanied by vapour-liquid-solid（VLS）mechanism.Si₃N₄ nanofibers are formed by the reaction of nitrogen and silicon monoxide（SiO）which originated from the active oxidation of Si and the decomposition of silicon dioxide（SiO₂）in the VS mechanism.The collapse of pore wall of porous green body is also due to the generation of SiO.（6）The growth ofα-Si₃N₄ nanofibers occurs（100）plane along the[210]direction when Si₃N₄ is not added to the green body.And whenα-Si₃N₄ particles and whiskers is added to the green body,the growth ofα-Si₃N₄ nanofibers occurs（100）plane along the[210]direction and（101）plane along the[213]direction.