Preparation Of Silicon Nitride Nanowires And Their Application In Porous Ceramics By Carbothermal Reduction Nitrification

When the material decreases to a certain scale,it can lead to damage on the surface of material’s cyclical conditions,which makes the dependence between the band structure of the material and its size.The structure’s features of nanomaterials result in better performance than ordinary materials.The nanomaterials of nuclear shell structure not only realize the combination and complementarity of the excellent properties of various nanomaterials,but also have the diversity and controllability of various materials structure and chemical composition.Si3N4 is a kind of engineering ceramics and semiconductor materials that have excellent properties.Because of its high strength,high hardness and excellent corrosion resistance,oxidation resistance,thermal shock and mechanical shock resistance,it is widely used in many fields.In the engineering parts,electrical components,aerospace craft,military field and so on,the Si3N4 materials have a broad prospect.Radome is an important part in missiles and aircraft’s front,the radome have the functions of anti-thermal,impact resistance and carrier.So radome are required to have the performance of high mechanical strength,low dielectric constant and dielectric loss,good corrosion resistance and impact resistance.And then,silicon nitride ceramic materials have the advantages of high temperature resistance,strong thermal stability,low dielectric constant,low dielectric loss and excellent mechanical properties,etc,which have become the research focus of the radome materials in recent years.In this paper,we synthesized nanocarbon spheres with hydrothermal carbonization method,Si O2 coated carbon spheres(C@Si O2)were prepared by TEOS hydrolysis reaction,and Si3N4 fibre were prepared by carbothermal reduction nitriding.On the other hand,direct coagulation casting method via dispersant reaction has been utilized to form green bodies of porous Si3N4 ceramics combining with ceramic hollow sphere method,in view of its advantage on the coagulation mechanism.Finally a new type of porous silicon nitride ceramics is prepared through the silicon source and carbon source in the matrix.The effects of raw material ratio and process parameters on the morphology of nanowires and the comprehensive properties of porous ceramics were studied.The main research contents are as follows:The carbon spheres were prepared by hydrothermal reaction with glucose,and the particle size and morphology of carbon spheres were controlled by changing the concentration,reaction time,and temperature of carbon spheres in the system.The morphology of product was observed by scanning electron microscopy,and the nanoparticles with different particle size distribution were prepared,the sphericity and monodispersity were pretty good.The C@Si O2 nanoparticles were prepared by TEOS hydrolysis and the carbon particles is the core.By adjusting the concentration of the system,reaction temperature and time,and the content of silicon source TEOS,the structure of the core-shell of C@Si O2 was improved.The product morphology was observed by scanning electron microscope,and the product content was determined by transmission electron microscope and EDS energy spectrum.In the end,we obtained C@Si O2 with particle diameter of 200 nm,and spherical shape was good,dispersed uniformly without adhesion.In the high temperature nitrogen atmosphere,the C@Si O2 was heated to prepare silicon carbide nanowires,then change the sintering temperature and raw materials,finally the nitride nanowires with diameters of 2μm,1μm,0.5μm were prepared under the nitrogen atmosphere of 1400℃.The diameter of the nanowires was uniform and the surface was smooth.Direct coagulation casting method via dispersant reaction has been utilized for the first time to form green bodies of porous Si3N4 ceramics combining with ceramic hollow sphere method,in view of its advantage on the coagulation mechanism.It efficiently enhances the interface strength between hollow spheres and ceramic matrix,which contributes to the close contact between hollow spheres and ceramic particles.Owing to its coagulation mechanism,namely the attractive state between particles and spheres.This method turns out to better regulate the performance of porous ceramics especially through the sintering parameters.It is found that porous Si3N4 ceramics prepared with 5 wt.% silica sol addition and treated in argon at 1400℃ possess the optimal performance,and it possesses fracture toughness of 2.45 MPa·m1/2,dielectric constant of 3.49,thermal conductivity of 2.57 W/(m·K).On the other hand,different amounts of silica sol and carbon black for the carbothermal reduction have been added to the suspension,leading to different morphologies of porous Si3N4 ceramics.With the increase of the added amount of silica sol,the porosity of porous ceramics increases from ~48% to 60%.Moreover,sintering atmosphere plays an important role in the performance optimization especially treatment in inert atmosphere,which could be mainly due to the formation of α-Si C phase.High-performance porous Si3N4 ceramics have been achieved,whose porosity is 48.2%,flexural strength is 108.30 MPa,dielectric constant is 3.49 and thermal conductivity is 2.57 W/(m·K).This efficient method provides a promising way to fabricate radome.