Preparation,Structure And Properties Of SiC （rGO,MoSi₂） Porous Polymer-derived Ceramics

During the flight of hypersonic vehicles,high temperature on their surface caused by intense friction between vehicles and atmosphere may damage the vehicles.Thermal protection system plays a significant role in protecting flight safety of vehicles and operation of internal equipment.With the development of hypersonic vehicles,the working environment of thermal protection materials becomes increasing worse,and as a result higher demands of them have been put forward to protect vehicles.It is crucial to develop new materials with low density,heat-insulating performance and necessary mechanical properties to meet the demands,among which silicon carbide（SiC）porous polymer-derived ceramics（PDCs）are a promising candidate.However,their advantages have been limited by difficulties of bulk ceramics preparation.The reasons are that the evaporation of gaseous species during pyrolysis could result in serious shrinkage and deformation as well as cracks within bulk PDCs,which has a negative effect on mechanical properties of them（including porous PDCs）.Additionally,presence of SiOxCy amorphous phase and free carbon in SiC PDCs affect their high-temperature resistance and high-temperature oxidation resistance.Therefore,technology of preparing porous PDCS should be improved to make up for abovementioned shortcomings of SiC porous PDCs.MoSi₂ is an ideal filler with excellent high-temperature resistance,mechanical properties and oxidation resistance,which has good physical and chemical compatibility with SiC PDCs.SiC porous PDCs with low density,high porosity,high-temperature resistance and necessary strength could be prepared so that can be applied in insulation tile.Based on the above reasons,main contents and results of this dissertation are as follows:（1）Mo was introduced into precursor by the reaction between MoCl₅ and PCS.And then,Mo-containing precursor（PMCS）was grafted on graphene oxide（GO）by vinyltriethoxysilane（VTES）to obtain PMCS-VTES-GO（PMVG）.SiC(rGO,MoSi_2x)ceramics monolith and SiC(rGO,MoSi_2x)bulk PDCs were successfully prepared from pyrolysis of PMVG and re-pyrolyzing SiC(rGO,MoSi_2x)p/PMVG blends.MoSi₂ phase（formed in pyrolysis process）uniformly distributes in system and is well compatible with β-SiC/SiOxCy/Cfree framework.Furthermore,effect of Mo source content on the microstructure of SiC(rGO,MoSi_2x)bulk PDCs was also investigated.SiC(rGO,MoSi_20.1)bulk PDCs displays higher ceramic yield（92.23%）than others,and show excellent mechanical properties with a fracture toughness of 4.26 MPa·m^1/2 and a compressive strength of 151.45 MPa,and exhibit good oxidation resistance.Additionally,SiC(rGO,MoSi_20.1)porous PDCs with 43%open porosity were practically prepared by adding pore-forming agent（poly styrene microsphere）,of which fracture toughness and compressive strength maintain 1.91 MPa·m^1/2 and 39.20 MPa.（2）The feasibility of preparing bulk porous PDCs was systematically explored.First,MoSi₂/SiC（rGO）p/PCS-VTES-GO（PVG）blends were obtain by ball milling,where MoSi₂ act as inert filler to improve mechanical properties of SiC porous PDCs.And MoSi₂/SiC（rGO）porous PDCs was fabricated by adding pore-forming agent（PS）and re-pyrolyzing MoSi₂/SiC（rGO）p/PVG blends.The effects of microsphere size of pore-forming agent,content of pore-forming agent and pyrolysis temperature on the mechanical properties of porous PDCs were studied by orthogonal method,among which the content of pore-forming agent is the most important factor.Series of MoSi₂/SiC（rGO）porous PDCs with different content of pore-forming agent were prepared by control various method to further investigate properties of MoSi₂/SiC（rGO）porous PDCs.MoSi₂/SiC（rGO）porous PDCs possess uniform pore distribution,and MoSi₂ is well compactible with β-SiC/SiOxCy/Cfree framework.Open porosity of porous PDCs shows upwards tendency,while mechanical properties and thermal conductivity exhibit downward tendency,with rising pore-forming agent content.The shrinkage of MoSi₂/SiC（rGO）porous PDCs,at a low level,varies from 8.00%to 8.38%.When pore-forming agent content increases from 10 wt%to 20 wt%,mechanical properties of porous PDCs decrease dramatically,while thermal conductivity does not drop markedly.Hence,MoSi₂/SiC（rGO）porous PDCs with 10 wt%pore-forming agent content own 40%open porosity and 0.8427 W·m^-1·K^-1,and maintain the highest harness（5.10 GPa）,fracture toughness(2.24 MPa·m^1/2),compressive strength（58.24 MPa）,flexural strength（30.92 MPa）.The mechanical properties and thermal stability of MoSi₂/SiC（rGO）porous PDCs are superior to SiC(rGO,MoSi_20.1)porous PDCs.