Fabrication And Properties Of Silicon Carbide Polymer-Derived Ceramics Modified By Heterogeneous Elements

With the rapid development of aviation and aerospace field,higher requirements have been put forward for thermal protection performance.The precursor conversion method,as an ingenious technology that transform materials from polymers to inorganic ceramics,has a simple preparation process and possesses the ability to control ceramic composition,thus as-achieved polymer-derived ceramics(PDCs)are more suitable for complex environment.However,the generation of a large number of bubbles and the existence of thermal stress during pyrolysis process will cause shrinkage,deformation and even cracking,which affect the mechanical properties of products.Consequently,based on the successful preparation of fiber and film modified by heterogeneous elements,AlO6 could improve cross-linking degree of precursors,TiB2 located at the grain boundary could improve the high-temperature resistance.In addition,the precursor conversion method can obtain porous PDCs with excellent properties including lightweight,high strength,low expansion coefficient,high temperature resistance,oxidation resistance and thermal shock resistance.However,with the urgent demand for thermal protection system,porous PDCs are developing in the direction of lower density and higher porosity.Accordingly,in order to solve above problems,Al(acac)3 was used to improve cross-linking degree based on polycarbosilane-vinyltriethoxysilane-graphene oxide(PCS-VTES-GO,PVG)developed by our laboratory,and SiC porous PDCs with lightweight and good mechanical properties were prepared by introducing pore-forming agent and the repyrolysis technology.Subsequently,SiC(Ti,B)PDCs prepared by physical blending possess outstanding mechanical properties and excellent thermal stability.Poreforming agent and re-pyrolysis technology had also been applied to obtain SiC(Ti,B)porous PDCs,which laid a good foundation for thermal protection system.The main research contents and results of this thesis are as follows:(1)SiC porous PDCs modified by Al were prepared by combination of precursor conversion method and pore-forming agent method.The re-pyrolysis technology that can reduce linear shrinkage,improve porosity and ensure good mechanical strength was introduced after analyzing Al-modified SiC(Al,rGO)porous PDCs.It is found that the synergistic effect of outwardly expanding SiC(Al,rGO)P filler and inwardly shrinking PAVG precursor during re-pyrolysis process could form dense skeleton,which has important significance for porous PDCs.Meanwhile,SiC(Al,rGO)porous bulk PDCs composed of SiOxCy,Cfree,rGO,SiO2 and β-SiC have high porosity(53.06%),low linear shrinkage(6.63%),good fracture toughness(1.31 MPa·m1/2)and low thermal conductivity(0.23 W·m-1·K-1).To a certain extent,it possesses the characteristics of lightweight and low thermal conductivity.(2)TiN and B powders were used as active fillers to prepare PVG(Ti,B)precursors.Afterwards,effect of pyrolysis temperature on performance of SiC(Ti,B)PDCs was investigated.It is found that the surface of SiC(Ti,B)PDCs pyrolyzed at 1300℃ is compact and flat,and β-SiC grains are uniformly dispersed in amorphous SiOxCy.Meanwhile,the high temperature phase of TiB2 is also beneficial to release thermal stress and inhibit the growth of β-SiC,so that SiC(Ti,B)PDCs exhibit excellent hardness(23.32 GPa),outstanding fracture toughness(6.97 MPa·m1/2)and high thermal stability.Subsequently,different pore-forming agent contents were added into SiC(Ti,B)PDCs for exploring the application of porous material.After that,introducing repyrolysis technology,SiC(Ti,B)p particles pyrolyzed at 1300℃(acted as fillers)were mixed with PVG(Ti,B)through ball milling to prepare SiC(Ti,B)bulk PDCs,which were then analyzed by morphology,microstructure,chemical composition and mechanical properties.It is worth mentioning that re-pyrolysis technology reduce the process of oxidation cross-linking and could directly achieve poducts with complex shapes.SiC(Ti,B)bulk PDCs pyrolyzed at 1300℃ possess high ceramic yield(96.83%),low linear shrinkage(6.25%),outstanding mechanical properties(hardness 5.44 GPa,fracture toughness 4.70 MPa·m1/2,flexural strength 41.90 MPa,compressive strength 126.33 MPa)and high thermal stability.Finally,under the condition of high porosity,30 wt%pore-forming agent was selected to prepare SiC(Ti,B)porous bulk PDCs through precursor conversion method and re-pyrolysis technology.The characterization analysis further verified the possibility of application in the field of porous materials.