Study On Rapid Densification Of Barrel Shaped Carbon/Carbon Composites By Chemical Vapor Infiltration

C/C composites have a series of excellent properties,which are widely used in aerospace and photovoltaic fields.Chemical vapor infiltration(CVI)is the best method to prepare C/C composites,but it has the disadvantages of long production cycle,high cost and high energy consumption.Therefore,it is very important to realize fast densification of C/C composites.In this paper,the induction heating multiple coupled physical field CVI method and Cu-based catalyst doping preform CVI method were used to prepare barrel shaped and massive C/C composites respectively,realizing the rapid densification of C/C composites.The effects of temperature,gas pressure,gas flow rate,catalyst content and catalyst time on the densification rate,microstructure,crystal orientation,mechanical and thermal properties of C/C composites were investigated.The main contents and conclusions of this paper are as follows:(1)The induction heating multiple coupling physical field CVI method was invented to realize the rapid densification of the barrel shaped C/C composites.When the total gas flow rate is 3L/min,980℃and 20k Pa are the optimal technological parameters for rapid densification,and the volume density of barrel shaped sample can reach 1.28g/cm~3 within 60h.Under these technological parameters,the volume density of the barrel shaped sample can reach 1.55g/cm~3 within 30h if the total gas flow is increased to 20L/min.(2)The effect of CVI process parameters on the crystal orientation of C/C composites was studied.The results show that the crystal orientation of pyrolytic carbon in the inner wall of barrel sample can be improved only by increasing temperature,gas pressure or total gas flow rate.The crystal orientation of the pyrolytic carbon at the outer wall and core can be improved only by increasing the temperature or the total gas flow rate,while the crystal orientation of the pyrolytic carbons at the outer wall and core can be decreased only by increasing the gas pressure.(3)The effects of carbon nanofiber(CNFs)clusters on CVI densification and crystal orientation of C/C composites were studied by controlling catalyst content and catalyst time.With the increase of catalyst content or the extension of catalyst time,the slope of initial densification curve of block sample increases gradually,and the shape of densification curve shows the changing rule of"S→M→N".When the mass fraction of CNFs inside the carbon felt increased from 2.20%to 167.44%,the volume density of the sample increased from 0.603g/cm~3 to 1.453g/cm~3 for 102h.The crystal orientation of CNFs/Py C region is significantly higher than that of Py C region.For pyrolytic carbon within the same radius of Py C region,the closer it is to CNFs/Py C region,the higher its crystal orientation is.(4)The effect of CNFs clusters on the properties of C/C composites was studied.The compressive strength of CNFS-C/C composites tends to increase gradually only by prolonging the catalytic time or increasing the catalyst content.When the CNFs content increases from 2.20%to 167.44%,the compressive strength increases from 75.5MPa to 140.9MPa and the thermal conductivity increases from 3.177W/(m·K)to 5.373W/(m·K)for204h,with an increase of 87%and 69%,respectively.The influence of CNFs on the compression behavior of C/C composites can be attributed to:the influence of CNFs on the volume density of C/C composites,the"particle enhancement"effect of CNFs/Py C pellets in micron scale,and the"fiber enhancement"effect of CNF in nanometer scale.