Investigation On Mechanism And Application Of Ferrous Silicon Nitride Synthesized By Flashing Combustion

Ferro-silicon nitride (Fe-Si3N4) is a metal-ceramic composite materials, prepared by flash combustion synthesis process with the cheap FeSi75 as raw material, and it has been widely used in waterless taphole clay of large blast furnace, iron runner castable of blast furnace, ladle slide plate, and other fields, reducing the costs of blast furnace smelting and ladle refining. As a silicon nitride-based materials with excellent performance, ferro-silicon nitride products have potential applications in many occasions. For the purpose of extending the application scope of ferro-silicon nitride, the issue is focused on the mechanism of the flash combustion synthesis process, the mechanism of nitride reaction, the theory of crystal growth mechanism and the possible application of ferro-silicon nitride in some occasions, in order to control the structure, phases and properties of ferro-silicon nitride, promoting the development of flash combustion synthesis technology and the application of ferro-silicon nitride material.Ferro-silicon nitride is prepared by Industrial flash furnace with the FeSi75 powder as raw materials. Various performances of ferro-silicon nitride are tested and the phase composition, microstructure and element distribution are ananlysed by using XRD、SEM、EDS and TEM. The main phases and content of Fe-Si3N4 are β-Si3N4 (wt59%),α-Si3N4 (wt23%), SiO2 (wt3%), Fe3Si (wtl5%). There are three kinds of microstructure in Fe-Si3N4 matrix:a.β-Si3N4 single crystals with a random radial microstructure and regular hexagonal prism morphology; b. Fe3Si that are naked or partially covered by dense areas. c. dense area in the center of radial columnar crystal, a-Si3N4 and SiO2 are located in a dense area and SiO2 exist in the form of crystalline and amorphous state. The pore structure of Fe-Si3N4 is mainly in the scope of sub-millimeter with the diameters distributing between 30 μm～990 μm and almost all of the pores are interconnected.The flash combustion nitride reaction of FeSi75 is the nitride reaction of Si particles and ζ phase. The Si particles were melts and the ζ phase were Fe-Si melts at high temperature. When the surface nitridation of Si melt was proceeding, the volume of Si melt continuously shrinked. When the surface nitridation of Fe-Si melt was proceeding, a mass of Fe ions in the Fe-Si melt surface migrated to the interior, while a lot of Si ion internal migrated to the surface. At the same time, Fe-Si melt also continuously shrinked. The melt of Si and Fe-Si occured surface nitridation- silicon nitride shell rupture-surface nitridation repeatedly, then the stable mixture of Fe3Si, a-Si3N4 and β-Si3N4 was obtained. In the falling process, the silicon nitride obtained just now reacted with O2 in N2, forming a layer of amorphous SiO2 film on the surface. Fe3Si and silicon nitride coated with silica film fell into the product pool, and stacked loosely. In product pool, α-Si3N4 transformed into β-Si3N4 by dissolution-precipitation mechanism, and the α-Si3N4 crystals which did not have enough time to dissolve remained in the dense area, and the SiO2 film developed into the mix of crystalline and amorphous state under high temperature.The β-Si3N4 crystals developed from dense area with a mode of spiral growth and layer growth, and (100) crystal face of Pβ-Si3N4 crystals can obtain a large number of grow straight steps with different angles. The matrix of β-Si3N4 crystals was Si-N-O molten phase. At the later stage of β-Si3N4 crystal growth, the driving force of crystallization decreased, the long range diffusion was disappeared, the short range diffusion leaded to the decreasing of N content near the crystallization interface in the Si-N-0 melt phase, resulting in the gradient distribution of N element by different distances between the Si-N-O melt phase and crystalline interface.At high temperature in vacuum, a lot of Fe volatilized from the Fe3Si melt and reacted with Si3N4 surface, forming new Fe-Si melt, then Fe continued to volatilize from the new Fe-Si melt. The above-described process repeated, causing Fe-Si alloy particles more uniform and finer in ferro-silicon nitride. At high temperature in vacuum, the hexagonal columnar Si3N4 crystals began to decompose partially, each edge of the top hexagon and each edge of the six square prism gradually disappeared.Ferro-silicon nitride refractory material was prepared by reaction sintering with ferro-silicon nitride as aggregate and Si as fine powder. The main phases and content are β-Si3N4 (72wt%),α-Si3N4(9wt%), Si2N2O(6wt%), SiO2(lwt%), Fe3Si(12wt%). The new generated combined phase, such as Si3N4 and Si2N2O, which were characterized by cylindrical, fibrous and granular, were generated due to the reactions between Si vapor, SiO and N2, and O2.