Mechanical And Functional Properties Of Orderly Assembled 3D CNTs Structure Reinforced Ceramic Matrix Composites

Advanced silicon carbide(SiC)structural ceramic possesses many advantages such as high temperature resistance,oxidation resistance and low density,etc.They are extremely important strategic thermal structural materials in aerospace and other fields,however,the inherent brittleness greatly limits its application.The reinforcement can be added to the ceramic for improving the brittleness and the reliability.The scale refinement of the reinforcement from millimeter to micrometer to nanometer is the key to further improve its toughness.Carbon nanotubes(CNTs)are typical one-dimensional nanomaterials that are widely added to SiC ceramic due to their excellent mechanical and functional properties for improving their mechanical and functional properties.At present,the preparation methods of CNT-reinforced SiC ceramic composites(CNT/SiC)are mostly blending/sintering methods,which involve the dispersion and high-temperature sintering damage of easily agglomerated CNTs.The reverse matrix infiltration of CNT perform frameworks can avoid the dispersion,agglomeration,high temperature sintering,mechanical damage and bad interface of CNTs.The subject of this study is mainly to assemble a self-assembled macroscopic assemblies of CNTs with large specific surface area and easy agglomeration obtaining CNT-enhanced skeleton and functional network,and then use thess as preforms combine with chemical vapor infiltration(CVI)SiC ceramic matrix for obtaining CNT/SiC composites.According to the different structures of each assembly,the preparation process,microstructure,toughening mechanism,electromagnetic,and thermal properties of CNT/SiC composites were systematically studied.The main research contents and results are as follows:(1)The basic structural units and network structures of CNT aerogel,sponge and film were studied,the results show that the basic structural unit of CNT aerogel produced by freeze-drying is irregular CNT lamellac,and the basic structure units of 3D CNT sponge and2D film prepared by floating catalytic chemical vapor deposition are independent CNTs interlaced network.Different structures of CNT aerogel can be prepared by controlling the freeze-drying method,Multiple sets of parallel lamellac aerogel can be obtained by transverse spreading,and longitudinal spreading can obtain directional pore aerogel composed of lamellac.The pores between CNT lamellac are mainly micron-sized and CNTs in CNT lamellac are connected by physical bonding.The independent CNTs in CNT sponge are self-assembled into a network structure,and the pores are mainly nano-scale.CNT film is obtained by densifying the CNT sponge along the thickness,and has a lower void than the sponge.(2)The CVI process adaptability of CNT/SiC composites from CNT assemblies was studied,the results show that three CNT assemblies show good structural stability during CVI deposition,and CNT aerogel has the best CVI adaptability,the film has the worst.The size of the CNT/SiC composites is consistent with the original assembly size,there are no macroscopic defects such as collapse and shrinkage,and the internal structure of the assemblies is also well preserved.The CVI process relies on the gas diffusion principle,so the micron-sized aerogel has the best CVI process adaptability,the density of CNT aerogel/SiC can reach 2.6 g/cm³.Followed by sponge,and the density of CNT sponge/SiC is 1.5 g/cm³.The film is the worst,only the SiC coating is deposited on the surface,and the amount of internal ceramic deposition is extremely small.(3)The two-stage micro-nano interface structure of CNT/SiC composites was studied,the results show that CNT aerogel,sponge and film combined with CVI to prepare CNT/SiC can form a nano-and micro-scale interface with effective transfer load.The ceramic matrix is deposited inside and outside the CNT aerogel/SiC lamellac,the large surface effect of CNTs is transformed into strong nano-interface effect,the micron interface is formed between lamellac and the matrix on both sides,and the CNT pull-out is shorter and exhibit needle tip.CNT sponge/SiC contain three parts,from inside to outside,namely porous CNT/SiC network,dense CNT/SiC layer and SiC coating,SiC matrix is uniformly deposited around each CNT in CNT sponge,forming a nano-interface,a micron interface is formed between internal porous CNT/SiC network and external SiC coating,and CNT pull-out is longer.Nano-interface is also formed between independent CNT and deposited matrix in CNT film/SiC,however the deposition amount is relatively rare compared with sponge.The micro-interfaces such as dense CNT/SiC and PyC layer are formed between the inner composite layer and the outer SiC coating,the CNT pull-out is the longest.On the whole,the amount of ceramic deposition in CNT aerogel is the largest,the CNT pull-out is the shortest,the deposition in the film is the least,and the CNT pull-out is the longest.(4)The mechanical properties of CNT/SiC were studied,the results show that the mechanical properties of CNT aerogel/SiC are anisotropic,and the CNT sponge/SiC is isotropic,pre-deposited PyC interface layer can improve the fracture toughness of CNT film/SiC.The bending strength of transverse spreading CNT aerogel/SiC is relatively discrete and low.The mechanical properties of longitudinal spreading CNT aerogel/SiC increased with the increase of ceramic deposition.The flexural strength and fracture work were as high as240±5 MPa and 4188±118 k J/m²,respectively,their in plane compressive strength were much higher than out plane compressive strength.The mechanical properties of CNT sponge/SiC increased with the increase of SiC coating thickness.The flexural strength and fracture work were 150±7 MPa and 1320±80 k J/m².Compared with CNT aerogel/SiC,the sponge was mainly nano-porous,the amount of internal ceramic deposition is small and the mechanical properties are poor.The tensile strength,fracture strain and fracture work of CNT film/SiC are153±11 MPa,0.67±0.07%,and 516±108 k J/m²,respectively,pre-deposited PyC interface layer can increase tensile strain and fracture work to 0.80±0.02%,609±70 k J/m².(5)The electromagnetic properties of CNT/SiC were studied,the results show that the CNT sponge/SiC with nano-scale network structure possesses lower density and higher shielding effectiveness(SE_T)than CNT aerogel/SiC with micron-sized lamellar pore structure.As the amount of ceramic deposition increases,the absorption of electromagnetic waves by SiC matrix and CNTs increases,so SE_T of CNT/SiC increases.As the thickness is same,the SE_T of longitudinal spreading CNT aerogel/SiC with a density of 1.76 g/cm³ is 19.4?20.3 d B,the SE_T of transverse spreading CNT aerogel/SiC with a density of 2.60 g/cm³ is 30.0?34.5d B,while the SE_T of CNT sponge/SiC with a density of only 1.55±0.07 g/cm³ is as high as37.2?38.6 d B.Due to the nano-scale network of CNT sponge/SiC,electromagnetic waves are reflected multiple times in the composites until they are absorbed and dissipated,electromagnetic waves are reflected less and absorption dissipation in CNT aerogel/SiC with micron-sized lamellar pore structure,so the CNT sponge/SiC has a lower density and a higher SE_T,which can shield more than 99.9%of the incident electromagnetic waves.(6)The thermal properties of CNT/SiC were studied,the results show that CNT sponge/SiC with nano-scale network structure possesses good thermal insulation performance,and the thermal conductivity of CNT aerogel/SiC with micron-sized lamellar pore structure is better.The thermal diffusion and thermal conductivity of CNT/SiC decrease with increasing temperature.The nano-scale network structure in CNT sponge/SiC make a more tortuous heat conduction path,however,the micron-sized lamellar structure in CNT aerogel/SiC is beneficial to heat conduction,so the thermal diffusion and thermal conductivity of CNT sponge/SiC at 25?1200°C are lower.With the increase of deposition amount,the density of CNT aerogel/SiC increased,thermal diffusion and thermal conductivity increased to15.27?4.71 mm²/s?27.43?20.28 W·m^-1·K^-1.However,for CNT sponge/SiC,as the amount of ceramic deposition increases,the coating is mainly deposited on the surface.When the heat flow is radiated to the SiC coating,the heat flow will rapidly spread to the periphery,and the heat conducted through the nano-scale network will be greatly reduced in both speed and quantity,the thermal diffusion and thermal conductivity was reduced to 2.80?1.40 mm²/s and3.50?2.70 W·m^-1·K^-1,respectively.