Study On The Synthesis Of Si₃N₄-SiC Composite From Crystalline Silicon Cutting Waste

With the development of photovoltaic and electronic information industry,the demand for polysilicon is increasing rapidly.In the process of water-based cutting high-purity silicon,up to 50-52%of crystal silicon with the form of silicon powder enters the SiC abrasive contained cutting fluid,which becomes waste slurry because it cannot meet the cutting requirements.Silicon powder and SiC in slurry have small particle size and similar physicochemical properties,it’s difficult to separate them that the recovery is extremely low.Due to the excellent thermal shock resistance,high-temperature creep resistance,acid-base resistance,oxidation resistance and corrosion resistance,SiC-Si3N4 products have broad applications in metallurgical,chemical,energy and other industries.The purpose of this study is to find a routine to prepare of Si3N4-SiC composite materials from the crystalline silicon cutting waste.In this thesis,the silicon nitride bonded silicon carbide refractories have been investigated by the method of the high-temperature nitriding with crystalline silicon cutting waste.We determine the physical properties of the crystalline silicon cutting waste,explore the nitridation process of Si3N4-SiC composite powder prepared by electromagnetic induction heating and Si3N4/SiC composite ceramics prepared by reaction sintering,and get the sample of performance under optimal conditions.The main results are as follows:(1)The physical properties of the cutting waste material were investigated using XRD,SEM,XRF,chemical quantitative analysis,and granularity.It has been found that the cutting waste consist of 53.45wt%silicon,40.60wt%silicon carbide and 4.02wt%iron oxide.Cutting waste exits in the form of fine particles,and the particles have the size distributing in the range of 0.3-19.8μm.Among them,the particles having size between 0.3-4.8μm occupy 50%of the total volume,and the particles with size between 4.8-19.8μm occupy 40%of the total volume.(2)The composite powder preparation research results show that SiC-Si3N4 composite powders were synthesized by electromagnetic induction heating in nitrogen atmosphere with polysilicon waste as main raw material and ammonium chloride(NH4CI)as additive and the main components are α-Si3N4,β-Si3N4 and SiC.The reaction mechanism of NH4CI is not simply nitridation and carbonization,but also the reaction of silica fume with the decomposition products of ammonium salt.With the increase of NH4CI,the fibrous α-Si3N4 in the synthetic products increased.And the Si3N4 in the composite powders also increased with the rise of heating time.(3)In phase ceramic preparation results show that we have studied the effect of the pressure and dwell time on green density.The optimized technology parameters for preparing green body are as follows:the pressure of making billet is 300MPa and dwell time is 3min.The density of sample prepared under such condition is 1.953g/cm3.We have studied the influence of the final nitriding temperature and the holding time at the final nitriding temperature on the density,porosity and compressive strength of the samples.The obtain optimal nitriding conditions are as follows:the final nitriding temperature is 1400℃,the holding time at the final nitriding temperature is 3h.The samples prepared under such conditions have the biggest density and compressive strength,and the lowest porosity.Main toughening mechanism for SiC particle toughening and Si3N4 toughening.With the increasing pressure of the green body,the density and compressive strength of the products after nitriding gradually increase,and the porosity gradually decreases.Effect of thickness on the compressive strength is small,but when the thickness exceeds a certain value,the density of products decreases and porosity increases.The obtained optimal nitriding conditions are as follow:the final nitriding temperature is 1400℃,the holding time at the final nitriding temperature is 3h,the green body thickness is 8mm.The samples prepared under such conditions have the density of 2.341 g/cm3,the porosity of 18.7%,the compressive strength of 164.9MPa.These results show it is feasible to prepare refractory using the crystalline silicon cutting waste.In the subsequent experiments,we can consider new electromagnetic induction heating equipment with temperature control to explore the influence of temperature when preparing the Si3N4-SiC composite materials.And then we can also consider Y2O3、Al2O3 and other additives on the properties of Si3N4-SiC composite products.The performance of the sample could be improved by adding binder when making green body,and making the green body under vacuum and so on.