Study Of Preparation And Properties Of CNT Preform Reinforced SiC Composites

Because of the extraordinary functional and mechanical properties,CNTs have been regarded as the most promising reinforcement material for producing composites.However,applying CNTs in composites by mixing or casting methods makes a uniform dispersion of CNTs with high nanotube concentration very difficult because of their strong tendency to bundle and aggregate.So in this article the new CNT macroscopic preform were used during CVI process.In this way,the disadvantage of difficult to disperse for CNT could be avoided during sintering process and advantage of CNT self-standing,uniform distribution could be fully taken.Thus high-performance,high CNT volume fraction ceramic matrix composites could be prepared.In this article,three CNT macroscopic preform,namely aligned CNT(ACNT),CNT foam(CNT_f),CNT paper(CNT_p)were used as reinforcement,combing with advanced CVI process to preparing CNT preform reinforced SiC matrix composites(CNT/SiC).Mechanical properties,oxidation resistance and electromagnetic properties of CNT/SiC composites were systematically studied and microstructures of the composites were also analyzed.The main results are follows:1.ACNT/SiC composites preparation and oxidation resistance were investigated.The ACNT prepared by CVD with high purity,high crystallinity and high degree of graphitization own the characteristic of self-standing and uniform distribution of CNT.By using TG method ACNT/SiC composite was heated to 1400 oC in static air for oxidation test.Results showed that ACNT/SiC almost had no weight loss.SEM displayed that surface matrix was oxidized to silicon dioxide,and the internal ACNT is intact with no sign of severe oxidation.2.Statically compressive property of ACNT/SiC composites was studied.The results show that,with a porosity of 28% and a density of only 1.54 g/cm~3,the compressive strength of the composite reached 160 MPa.Compressive load displacement curves indicated that the ACNT/SiC composites exhibited non-brittle fracture failure mode,due to the internal ACNT cross-linking the SiC together,playing a good toughening effect.SEM showed a lot of CNT pull-outs were found in the fracture of ACNT/SiC specimen,and this is the strengthening-toughening mechanisms in microscopic level of CNT.3.Preparation and bending performance of CNT_f/SiC composites were studied.The obtained CNT_f/SiC composites own a pore distribution similar to that of C/SiC,macropore concentrated around 100 ?m,while the apertures are concentrated around 1 ?m.With a density of 1.60 g/cm~3,bending strength of CNT_f/SiC reached 150 MPa.By observing the fracture,a large number of CNT pull-outs were found with long pulled length.Due to the presence of CNT,composites performance significantly improved.4.Compression performance CNT_f/SiC composites were studied.The results showed that static compressive strength was 150 MPa,close to its flexural strength.Compression fracture mode is not brittle fracture,but a "false shaping" deformation behavior similar to metal.5.Mechanical properties of the 2D C/SiC composites modified by CNT_p were studied.It was found that the obtained CNT_p-C / SiC composites owned appropriate density and porosity,2.20 g/cm~3 and 8.6%.Highest flexural strength and fracture toughness reached 353 MPa and 15.3 MPa·m~0.5,respectively.CNT_p addtion contributed little to improving the mechanical properties of C/SiC composites.6.Electrical conductivity and electromagnetic shielding performance of CNT_p-C/SiC composites were studied.CNT_p-C/SiC composites conductivity with 8 CVI cycles reached 10.0S/cm,doubling that of C/SiC.when the SiC matrix deposited in the preform,the carbon fibers were separated by the SiC,resulting in decreased conductivity,while CNT maintained as an integral conductive channel.Due to a substantial increase in electrical conductivity,electromagnetic shielding effectiveness CNT_p-C/SiC also increased by 25% than C/SiC,from 29.4 dB to 36.8 dB.