Dilute Phase Entranment Characteristics Of Ultrafine Silicon Powder And Silicon Nitridation & Crystallization Mechanism

Mainstream high-quality ultrafine Si3N4 powder is generally produced by expensive chemical vapor deposition and thermal decomposition of silicon diimide.Atmospheric pressure suspension bed using direct nitridation method can produce ultrafine Si3N4 powder as a low-cost alternative,which uses much cheaper ultrafine Si powder and nitrogen.The process will greatly reduce the production cost of high-quality ultrafine Si3N4 powder and promote the large-scale application of high-performance Si3N4 ceramic products if applied on a large scale.This paper addresses two major problems of the process,i.e.,dilute phase entrainment of ultrafine Si powder(Geldart C),and the mechanism of Si powder nitridation and the relation with product crystal phase.Benefits from the huge specific surface area ultrafine Si powder is very favorable for chemical reactions.However,the strong surface force at the same time makes the powder easily aggregate,and the fluidization and dilute phase entrainment is a worldwide challenge.In this paper,the effects of vibration,stirring,powder layer height-to-diameter ratio and fluidization number on the fluidized entrainment concentration at ambient temperature were investigated using the orthogonal experimental method.The key factors affecting the fluidized dilute phase entrainment concentration at ambient temperature were obtained,and the role of these factors on fluidized dilute phase entrainment was investigated.In suspension bed nitridation process there will be high temperature tail nitrogen remaining after the direct nitridation reaction,which can be sent back for recycling.However,the mixing of high-temperature tail nitrogen will lead to an increase in the temperature of the nitrogen gas flow,the effect on ultrafine powder fluidization and dilute phase entrainment is unclear.From the perspective of powder aggregation,increasing the nitrogen temperature will increase the surface which is beneficial to powder aggregation and promoting the adsorption of ultrafine powder with the inner surface of the flow way,which is detrimental to the dilute phase entrainment of ultrafine Si powder.While,from the perspective of nitrogen flow,the increase of temperature will improve the gas flow rate and viscosity,increase the fluidization number,and facilitate the entrainment of ultrafine Si powder.In this paper,a theoretical model is developed focusing the temperature effect on entrainment,where other parameters are the optimal data obtained from the orthogonal experiment.The powder to gas ratio at room temperature reached 0.228,which is three times more than the reported results.When the nitrogen temperature raised to 443K,the maximum powder to gas ratio further increased to 0.341.There have been many research results on silicon-nitrogen reactions,such as kinetic investigations using Arrhenius method,reaction process investigations using the sharp interface model,and crystallization studies using Vapor-Crystal theory and Vapor-Liquid-Solid theory,etc.While studying the relationship between the Si powder size and the reaction completion time,it was found that the α/β ratio in the reaction products increased significantly with increasing powder diameter even under the same heating regime,holding temperature,and atmospheric environment.The existing theories cannot explain the phenomenon.The detailed correlation between reaction process and crystallization theoretically lacking,which restricts the large-scale production of ultrafine Si3N4 powder.The relationship between the reaction process and crystalline formation was investigated experimentally using a thermogravimetric-differential thermal analyzer(TGA).The repeated melting-solidification phenomenon was observed and investigated by conducting the experiment just below the Si melting point of 1414℃ and using Si powders with different diameters.The morphology of the products was observed by scanning-electron microscopy,and the morphological characteristics of β—Si3N4 gradually increased with increasing powder diameter,which was further corroborated by X-ray diffraction(XRD)analysis.The 4-stage silicon nitridation reaction process was summarized by further analysis the TGA curves,especially the heat flow curves and their time derivatives.Through layered observation of the products,the vapor products were found in dense plate form on the surface of the crucible,and the liquid-gas products in dendrite form appeared in the middle of the crucible,which confirmed the repeated endothermic and mass stagnation phenomena in the TGA curve during the theoretical exothermic and mass increase reaction process.As Si diameter increases,the powder will experience longer liquid-gas reactions as well as gas-liquid-solid reactions,resulting in a rapid increase in β-Si3N4 content.A theoretical model was built to further study the silicon nitridation reaction process.Generalized diffusion model using physical parameter was introduced instead of the existing common methods such as macroscopic fitting kinetic parameters from Arrhenius theory and sharp interface model simulation.The influence of parameters on the reaction interface and completion time was analyzed,including diffusion coefficient and reaction coefficient which are interrelated in chemical reactions.Combined with the TGA experimental results,physical parameters were obtained,the problem of cross-order magnitude differences in the reaction coefficients fitted by Arrhenius method from different researchers was solved,and a reasonable reaction coefficient range was recommended.The model can completely replace the sharp interface model,and by using physical parameters it breaks through difficulties while using Arrhenius fitting parameters in other models.Therefore,the introduced generalized diffusion model can be applied to other numerical simulation model and analysis the reaction details with different atmospheric concentrations and conditions.The relationship between nitridation process and Si core melting-solidification was built by heat transfer and phase change model based on the diffusion model.Through simulation,together with TGA curves,the correlation between the particle temperature,the solid-liquid state of Si core,and the crystal phases of Si3N4 was obtained.The model achieves the prediction of α/β-Si3N4 ratio with a given temperature and concentration(partial pressure),as well as the products and crystal phases timely.Through this model,it is clear for the first time that only α-Si3N4 generated at the solid-gas interface,corroborates the generation of β-Si3N4 in the liquid-gas reaction once again,and β-Si3N4 is obtained at the liquid-solidgas interface,which updates and enriches the understanding of the existing nitridation reaction mechanism.The average reaction rate is higher when the reaction region is in the liquid phase,while the rate is lower in the solid phase.During the condensation(solicitation)of the liquid Si core,the liquid Si near the core side will converse α-Si3N4,which was produced by solid-gas reaction on outer side,to β-Si3N4.