Porous Characteristics And Their Influences On Mass And Heat Transfer Of C_f/SiC Composites Fabricated Via The PIP Process

Besides fiber,matrix and their interface,pore is another important component of carbon fiber reinforced silicon carbide(C_f/SiC) composites fabricatd via precursor infiltration and pyrolysis(PIP).Typical porosity of PIP-C_f/SiC composites amounts to 10～20%.These unignorable pores have considerable influence on the mechanical properties,oxidation behaviour;mass and heat transfer of the composites,and provide them some potential functions.In this dissertation,the PIP-C_f/SiC's porous microsturcture characterization techniques were investigated.The pore structure's evolution during fabrication was tracked,its relationship with processing factors was discussed,and the control means were concluded consequently.Aiming at the application of active-cooled aerospace thermal components,the PIP-C_f/SiC's porous structure's effects on its thermal properties and permeability were also researched.Through evaluating and improvig the general porous media's measuring techniques,an integrated characterization procedure was established,including the MIP, BET,SEM and bubble point method etc.,then applied to research the PIP-C_f/SiC's porous characters successfully,e.g.porosity,morphology,pore size distribution, topology and so on.Based on the results,a detailed physical model was constructed.Three dimensional braided(3D) fabrics,two and a half dimensional braided(2.5D) fabrics,and carbon cloth laminates(2D) reinforced C_f/SiC composites' pore structures were investigated respectively.It is found that there are some common features of pores in C_f/SiC despite the reinforcements architectures' differences,e.g.the "chamber-throat/channel" binary pattern of the most pores,the existence of nanopores, and the small fraction of the fully-open pores etc..Nevertheless,the reinforcements have decisive effects on porous structures,e.g.from the aspect of porosity,2.5D-C_f/SiC＞3 dimesional,4 directional(3D4d)-C_f/SiC＞2D-C_f/SiC＞3 dimensional,5 directional (3D5d)-C_f-/SiC.As for the configuration,3D-C_f/SiC's pores are three-dimensional network style,composed of some hundreds of microns inter-bundle pore chambers, dozens of microns inter-bundle pore throats/channels,and nano-micropores.The channels communicate such chambers and micropores,integrating all pores inside. While the pores in 2.5D- and 2D-C_f/SiC have a laminar structure,whose topology is obviously anisotropic.Surface characterization indicates the specific surface areas of the aforementioned C_f/SiC are much lower than traditional porous media's,the isotherm sorption curve isâ…¡style,which are all determinated by the nano-micropores' contents and size distributions.By tracing the structural variations of the semi-articles before finally produced,the porous structure's evolution is recognized during the PIP fabrication.It is found that the SiC matrix keeps filling the inter-bundle channels of the 3D-C_f/SiC firstly,and changes to fill the pore chambers as the densification reachs some level.On the contrary,the channels at the warp-weft crossply of 2.5D and 2D-C_f/SiC have come into being at the fabrication beginning,the laminar pore chambers are the matrix filling targets all along, and the pores' connectivity evolutes consequently.Owing to the fractions of nano-micropores,the aforementioned C_f/SiC's specific areas develop differently; however,their surfacial toughness is similar,which is due to the covering SiC matrix on the surfaces of pores.The influences of infiltration conditions on the pore structure were researched systematically concerning infiltrate types,pressures,and holding time etc.The results indicate low-viscosity,good wettability,and heat-cure capacities are preferable for infilates to fill the reinforcements' inner spaces efficiently.Thus,polysiloxane(PSO) has higher densification efficiency than polycarbosilane(PCS)/Xylene/SiC_p(slurry), PCS/Xylene,and PCS/DVB.The composites infiltrated by PSO have a much lower porosity(less than 5%),and contain 0.1～20μm micro-cracks mainly.To overcome the resistance and accelerate the infiltrates penetrating,various pressure levels are required for different sized pores,e.g.1atm is sufficient for the pores above 1μm,while at least 4MPa additional pressure are necessary for micropores below 0.1μm.Attributing to the good wettability of aforementioned infiltrates on the carbon fibers,present infiltration time esures all inliltrate permeate into the fabrics deeply.The mass transfer features were evaluated by measuring the gas permeability of PIP-C_f/SiC.The flowrate-pressrue curves indicate that gas(N₂) flows through the C_f/SiC in three different modes,i.e.laminar,non-laminar,and slipping flow.The flowmode conversion pressures are affected by the densification level,reinforcements' architectures,and infiltrates types et al.It is found that the full-open pores' characters are the crucial factor determining the gas permeability of C_f/SiC,because they provide the flowing channels for the fluids. Through the bubble point method,their size distribution functionα(D) is obtained. Insertingα(D) into the parrell or series capillary model is able to predict the composites' permeabilities accurately.There are no direct relationship between the permeabilities and porosities of C_f/SiC reinforced by different fabrics,and the relative ranking of their permeabilities is 3D4d-C_f/SiC＞2D-C_f/SiC＞3D5d-C_f/SiC＞2.5D-C_f/SiC,in which the level of 3D4d-C_f/SiC is 10^-14m².However,the increase of densification can deduce the composite's permeability effectively,for each C_f/SiC above.Moreover,infiltrating PSO, PCS/DVB,PCS/Xylene/SiC_p,and PCS/Xylene produced 3D composites are able to be ranked as:C_f/SiC/Si-O-C＜C_f/SiC/C＜C_f/SiC/SiC_p＜C_f/SiC,according to their permeabilities.And the lowest one of 3D-C_f/SiC/Si-O-C is only 10¹⁸m².The equivalent concept was adopted in studying the thermal properties of PIP-3D-C_f/SiC. At room temperature,the specific heat(C_p) is 0.74 J/g/K,thermal diffusivity (TD) is 0.0183 cm²/s,effective thermal conductivity(K_eff) is 2.45 W/m/K,and total emssivity is 0.75(80～90℃).With the temperature inceasing,the C_p and K_eff both present positive temperature dependence,while the TD shows negative one over the full temperature range.The K_eff values reflect the exact mechanisms of the PIP-C_f/SiC's heat transfer,which involves the solide skeleton(fiber,matrix) thermal conduction via phonons and the radiation contribution of pores inside.Moreover,it is supposed that the latter mechanism is capable of compensating the decrease of thermal conduction occurred at high temperatures,which results in the positive temperature dependence of K_eff.The influencing factors of PIP-C_f/SiC's thermal properties are also discussed.Due to the inherent micro-cracks and low crystalline,the SiC presents low K_eff(1.56～1.47 W/m/K,from RT to 700℃) derived from PCS,which is the main reason of relatively low heat transfer capacity of PIP-C_f/SiC.Besides the matrix,the reinforcements' architectures determine the configuration of solide skeleton and pore,thus have inevitably great effects on the thermal conduction and radiation,e.g.the K_eff is direct proportion to the fiber fraction of fabrics along some direction.There is no fixed correlationship between the densifcation levels and thermal properties of C_f/SiC.For instance,as the porosity of 3D-C_f/SiC decease below 15%,following densification process can not make K_eff increase continuously,but cause it deduce a little,while for 2.5D-C_f/SiC,this inceasing trend is lasting in spite of the densification level.The diversity of radiation capacity accounts for this phenomenon,deriving from the different pores' configurations.The interfacial contact thermal resistance is coincident with the literature reported value of PIP-C_f/SiC calculated by H-J model.The interfacial thermal resistance is insensitive to the reinforecements' architectures and fiber types,but is determined by fabrication process,e.g.the PIP-C_f/SiC value is obviously higher than the CVI one.Due to some interfacial cracks' closure,temperature increasing make the thermal resistance fall off a bit.