Preparation And Properties Of CNT Buckypaper/SiC Composites

Silicon carbide（SiC）has the characteristics of high strength,high hardness,high modulus,and low density.SiC also has good oxidation resistance,corrosion resistance,wear resistance,and low coefficient of thermal expansion.SiC can also be used as an electromagnetic shielding material.However,due to brittleness,the application of SiC in actual production is limited.Carbon nanotubes（CNTs）have excellent electromagnetic properties and mechanical properties,and they show good potential application in the field of composite.However,the dispersibility of CNTs is poor,and CNTs are not easy to form a good interface with the ceramic matrix.These shortages will limit the development and application of CNT/SiC composites.In this project,CNTs and Silicon（Si）powder were used as raw materials,boron carbide（B₄C）was used as a modified filler,and CNT buckypaper was prepared by directional pressure filtration technology.The prepared CNT buckypaper was smooth and flat,without damage and cracks.The density of CNT buckypaper was about 0.6-0.9 g/cm³,and the porosity was about 60%-75%.The pores were mainly nanoscale pores and submicron pores,and the pores were interconnected and distributed evenly.The CNT buckypaper was impregnated with a phenolic resin solution using vacuum-directional pressure infiltration technology.The infiltrated CNT buckypaper was cured on a hot press,then pyrolyzed and sintered in situ.Phenolic resin pyrolyzed into carbon.The resin-derived carbon and Si in CNT buckypaper reacted in situ to form Si C ceramic matrix,and then the single-layered CNT buckypaper/SiC composite was prepared.This process solved the problems of CNTs dispersion and improved CNT/SiC interface bonding.The thickness of the as-prepared single-layered CNT buckypaper/SiC nanocomposite was 300-600μm,with a bulk density of 1.75-1.86 g/cm³ and a porosity of 4%-6%.The single-layered CNT buckypaper/SiC nanocomposite was composed of SiC,B₄C and C（CNTs and residual resin-derived carbon）.Si reacted with the resin-derived carbon to produce SiC,and the remaining resin-derived carbon served as the interface between CNTs and SiC.CNTs and B₄C were closely bonded with the matrix.The hardness of single-layered CNT buckypaper/SiC nanocomposite reached 8-10 GPa,and the indentation fracture toughness was 9-11 MPa·m^1/2.Both B₄C and CNTs played the role of dispersion strengthening and toughening,and B₄C also chemically modified resin-derived C intermediate interface.The surface resistivity and the volume resistivity of the single-layered CNT buckypaper/SiC nanocomposite were respectively 146 and 155μΩ·m,which means a good conductivity.The electromagnetic shielding effectiveness of single-layered CNT buckypaper/SiC nanocomposite reached 30 dB,the main shielding mechanism was surface reflection,and the ratio of transmitted power to total incident power was less than 0.0012.Based on the preparation of single-layered CNT buckypaper/SiC nanocomposites,the single-layered CNT buckypaper/SiC material was welded with resin-derived carbon to prepare a multi-layered CNT buckypaper/SiC graded composite,which achieved the uniform dispersion of high-content CNTs at the macro scale to realized the transition of CNTs composite from single-layered structure to multi-layered structure.The as-prepared multi-layered CNT buckypaper/SiC graded composite consisted of some CNT buckypaper/SiC nanocomposite structure layers and some interface layers made of resin-derived carbon,expanded graphite and silicon carbide whiskers（SiC_w）.The thickness of the structure layer was 150-200μm,and the thickness of the interface layer was 10-50μm,and the number of structure layers was 11-21 layers.The bulk density was 1.62-1.65 g/cm³,and the porosity was 6%-8%.The flexural strength reached 55.63MPa,and the fracture toughness reached 10.56 MPa·m^1/2,and the strain ratio was as high as 11.27%,indicating that the multi-layered CNT buckypaper/SiC graded composite was a ductile material.The surface resistivity was 185.2μΩ·m,and the volume resistivity was 1112μΩ·m.The electromagnetic shielding effectiveness was 35 dB,and the ratio of the transmission power to the total incident powder was less than0.0003.The shielding efficiency was improved compared with the single-layered CNT buckypaper/SiC nanocomposite,and the internal absorbtion loss was improved,but the main shielding mechanism was still surface reflection.