Study On Thermal Fields Tailoring And Hybrid Synergistic Effect Of SiC By Microwave Heating

Silicon carbide （SiC） can be widely used in many traditional industrial fields such as abrasives,refractories,metallurgy,high-temperature structural ceramics due to its superior features of high strength,high hardness,high elastic modulus,and good chemical stability.It has been received considerable attention in recent years due to its wide band gap,high electronic mobility,strong critical breakdown field,and has been honored as a typical kind of 3^rdgeneration wide band-gap semiconductor.Conventional methods of production SiC show many disadvantages such as long dwell time,high energy consumption,high pollution and cost.Some novel preparation methods as hot pressing sintering（HPS）,spark plasma sintering（SPS）become the focus gradually,but these methods are still in the exploratory stages.It has many advantages to preparate SiC by microwave heating,however,there are some phenomena such as selective absorption,power redistribution and thermal upheaval in the actual heating process.At the same time,the dielectric loss of the materials and the dynamic changes of physics and chemistry at high-temperature directly affect the stability and controllability of microwave heating.Therefore,it is of great significance to study the interaction mechanism between microwave and materials,thermal fields tailoring method,and realize the synergy of different microwave hybrid effects for the popularization and application the technology to preparate SiC by microwave heating.SiC crystals with different morphologies were synthesized by microwave sintering using coal and tetraethoxysilane（TEOS）as raw materials.A sol-gel method was carried out to coat coal mineral particles with silicon dioxide（Si O₂）.The regulation of preforming pressure,carbon content and Ni metal particles on microwave thermal field and the synergistic mechanism of various thermal and non-thermal effects were studied.The synthesis mechanism of SiC with different morphologies was discussed.Phases and morphologies were detected by X-ray diffraction analysis（XRD）,Raman spectrum,Field-emission scanning electron microscopy（SEM）,Tansmission electron microscope（TEM）.The main research contents and results were shown as follows.（1）The pre-forming pressure was selected at 0,1,2,3,4 and 5 MPa respectively.The effects of pre-forming pressure on microwave plasma transient thermal field were studied.The results showed that the changes of the C particles space in the raw material and the amount of gas adsorbed in the precursor with preforming pressure:the field enhancement effect was changed by C particles space,which determined the time of primary plasma excitation;Adsorbed gas in the precursor determined the duration of primary plasma.The plasma transient thermal field was a necessary condition for rapid synthesis SiC.Without pre-forming pressure,the plasma transient thermal field could not be excited,and there was no SiC in the production.The plasma transient thermal field made the original simple C@Si O₂interfacial solid state reaction changing into Si O@CO space gas phase reaction and melting Si O₂@C liquid-solid interface reaction.These effectively improved the synthesis speed of SiC,saved time and reduced energy consumption.At the same time,The reaction of Si O@CO consumed CO gas,prepared SiC crystal,generated O₂and realized"carbon neutralization".～4 MPa might be the optimized pre-forming pressure for both microwave plasma effects and E-field intensification.（2）The regulation of continuous coupling thermal effect by different carbon silicon molar ratio was studied by changing the carbon silicon molar ratio from 1:1 to8:1.The results showed that different carbon silicon molar ratios changed the duration and intensity of the primary coupling thermal effect,and affected the amount of gas adsorbed in the precursor and the field enhancement effect.When the molar ratio of carbon to silicon was 1:1,the primary coupling was weak and the heating time was long,resulting in the escape of adsorbed gas,weak plasma thermal effect and short duration,which was difficult to realize SiC preparation.In other cases,SiC crystals were synthesized,but their morphologies were quite different.The molar ratios of carbon to silicon changed the proportion and total amount of Si O and CO gas,and then affected the structure and morphology of SiC:when the content of C was low or high,the total amount of gas generated was small,and the reaction was mainly interfacial solid-phase and liquid-phase reaction to generate SiC fiber or SiC rod;When the content of C was moderate,the reaction was mainly in gas phase to form SiC particles."Short rod"SiC,"cable type"SiC and"chain beat type"SiC could be synthesized by changing the molar ratio of carbon to silicon and adjusting the content ratio of Si O and CO.Pure SiC particles were prepared when the molar ratio of carbon to silicon was 5:1,which realized the hybrid coordination of various thermal fields,and was the best molar ratio of carbon to silicon.（3）Ni particles with 1,3 and 5 wt%contents were added during the mixing process to synthesize SiC by microwave heating.Heating effect of Ni surface plasmon was studied.The results showed that in the low temperature stage,Ni particles reflecting microwave to repeatedly couple C and microwave,improving the energy conversion efficiency,making the internal temperature field distribution more uniform,and shorten the time of primary coupling thermal effect.In the high temperature stage,the plasmon on Ni surface was excited by microwave,the microwave field distribution was adjusted,the duration of secondary plasma was reduced,and the rapid microwave absorbing melting of Ni particles was realized.The solubility of CO and Si O gas in different nickel silicon Alloys determined the morphology of SiC.