Fabrication And Properties Of Structural-Functional Integrated 3D-SiC(rGO)PDCs

As an advanced structural material,silicon carbide(SiC)possesses excellent properties,such as high melting point,high strength,good chemical stability and corrosion resistance,etc.It is widely used in many advanced applications including aerospace,semiconductor devices,nuclear energy and microelectromechanical systems(MEMS).Among them,SiC polymer derived ceramics(PDCs)which function as advanced structural materials display desirable combination of exceptional properties containing light weight,high mechanical strength,high chemical stability and corrosion resistance,attracting much attention of researchers.Currently,SiC PDCs realize a significant breakthrough in low-dimensional forms(fibers,films,coatings and membrence).However,their popularity is limited by fabrication techniques of threedimensional(3D)structural-functional integrated ceramics.The main reason is that release of numerous gaseous species during pyrolysis would lead to generation of cracks within the ceramic,and thus destroying its integrity and mechanical properties.Besides,microcracks caused by serious shrinkage of ceramic would decrease fracture toughness and functional properties of SiC PDCs.Therefore,there has been a considerable amount of interest in the development of PDCs route for producing structural-functional integrated SiC PDCs with high ceramic yield,low shrinkage,high hardness,excellent fracture toughness and functional properties.In view of this,graphene and MoSi2 are used as two kinds of structural-functional fillers respectively in this thesis,and a new manufacturing process to synthesis lightweight 3D-SiC PDCs through re-pyrolysis ceramic fillers/precursor blends is applied,which displays great application value in aerospace fields.This research would help to explore the combination of satisfying structural and functional features of PDCs for versatile applications.The main subjects and results of this thesis are summarized as follows:(1)A technology that re-pyrolysis ceramic/precursor/filler blends to prepare 3DSiC PDCs was proposed,and graphene is used as a functional filler.The samples are pyrolyzed at 1300℃ and 1400℃ respectively.By characterizing the characteristics of each sample,it can be demonstrated that the comprehensive performance of samples pyrolyzed at 1300℃ is better than that of samples pyrolyzed at 1400℃.The ceramic yield of all samples is between 89.3%and 91.9%,and linear shrinkage is between 1.2%and 5%,both of which are at a relatively high level.This is attributed to the combination of GO and precursor increases the molecular weight and inhibits the decomposition of small molecules in the process of pyrolyzing.Secondly,the ceramic SiC(rGO)p without mass loss is very stable in the re-pyrolysis process.Under thermal stress,re-pyrolysis ceramic SiC(rGO)p will expand outward while precursor will shrink during the process of re-pyrolyzing,this synergistic effect can effectively reduce the linear shrinkage of 3D-SiC PDCs.The electrical and thermal conductivities of samples undergo a huge increase with graphene content increases.The electrical conductivity of 3D-SiC(rGO,G20%)increases up to three orders of magnitude and thermal conductivity increases up to 12 times of magnitude compared with 3 D-SiC(rGO)without graphene addition.This is attributed to the addition of graphene in the ceramic matrix to form a conductive framework,which makes the number of free-moving electrical-charge carriers in the ceramic matrix increase,forming conductive channels and equivalent phonon-channels to improve the electrical and thermal conductivities of samples.In addition,hardness and fracture toughness slightly decrease with rising graphene addition,but they could still be adequately considered for many potential applications.By characterizing microstructure of samples,it is observed that graphene is uniformly distributed in the SiC/SiOxCy/Cfree system,graphene and SiOxCy phase are well-bonded without gaps and voids at the interface,suggesting graphene might be well compatible with SiC/SiOxCy/Cfree system.Furthermore,bulk 3D-SiC(rGO,Gx)ceramic with a diameter of 38 mm is prepared using a large mold,the feasibility of preparing large-scale structure-function 3D-SiC PDCs by this process is preliminary discussed.(2)MoSi2 is used as a structure-functional filler,and re-pyrolysis ceramic/precursor/filler blends are pyrolyzed at 1300℃ respectively.The electrical and thermal conductivities of samples undergo a huge increase with MoSi2 content increases.The electrical conductivity of 3D-SiC(rGO,MoSi220%)increases up to 0.85 S·cm-1 and thermal conductivity increases up to 8.57 W·m-1·K-1.It is worth noticing that by characterizing mechanical properties of the sample,the hardness of samples increases with increasing MoSi2 content,reaching at a maximum of 10.34 Gpa when the content of MoSi2 is 20 wt%,the fracture toughness first increases with the rise of MoSi2 content,reaching at a maximum value of 5.16 MPa·1/2 when the content of MoSi2 is 10 wt%and then slightly decrease,both of which are significantly improved.Based on the analysis from microstructure of samples,it is observed that MoSi2 has good compatibility with SiC ceramics.MoSi2 is uniformly dispersed in the ceramic matrix after pyrolyzing,filling some holes,thereby forming a three-dimensional percolation network,which makes ceramic matrix more compact.In addition,MoSi2 owns high hardness,and even its properties are complementary to SiC,which makes 3D SiC(rGO,MoSi2x)composite ceramics have excellent comprehensive properties.