Abstract: Phenolic resin-based porous composites are the promising thermal protection materials for aerospace applications. The high-temperature evolution of microstructure due to the decomposition of the resin also presents great challenges to predict the internal heat and mass transport behaviors. This work investigates the effects of microstructural characteristics such as the number of pores, size distribution, pore-throats size and volume fraction on the permeation behaviors of fluid in the needle-punched short-cut fiber reinforced silicon phenolic resin-based porous composites. The specimens are prepared by the sol-gel method and the atmospheric pressure drying process and the pyrolysis experiment are conducted at 400 ℃ and 800 ℃. Then, a scanning electron microscope and a Nano-CT computer tomography are applied to obtain the surface morphologies and the interior slice images of the specimens. The AVIZO software is employed to accurately extract and analyze the pore structural model and simulated calculate the absolute permeability. It is found that the small pores develop gradually during pyrolysis due to the resin decomposition and the quartz fibers rearrangement, resulting in an increase in number of large pores. Nonetheless, the equivalent radii of most pores are less than 1 μm. Very few pores possess a large radius over 5 μm. However, the volume fraction of these large pores exceeds 99%. In addition, with the pore size growing, the connectivity between these pores is enhanced, immediately causing an increase in number and size of the pore-throats. Larger pore and more pore-throats would add the unblocked flow channels for the fluid passing, reducing flow resistance. The seepage simulation also confirms that the absolute permeability gains significant increase after pyrolysis in all directions. For example, the absolute permeability of the pyrolyzed sample is 9.0×10^-13 m² in X direction, which is an order of magnitude greater than that of the unpyrolyzed sample. This study provides important insights for understanding the high-temperature evolution at of microstructure and the permeation behavior of fluid in porous thermal protection materials.

Key words: Permeability, Phenolic resin-based porous composites, Nano-CT scanning, Pyrolysis process, Permeability simulations

摘要： Phenolic resin-based porous composites are the promising thermal protection materials for aerospace applications. The high-temperature evolution of microstructure due to the decomposition of the resin also presents great challenges to predict the internal heat and mass transport behaviors. This work investigates the effects of microstructural characteristics such as the number of pores, size distribution, pore-throats size and volume fraction on the permeation behaviors of fluid in the needle-punched short-cut fiber reinforced silicon phenolic resin-based porous composites. The specimens are prepared by the sol-gel method and the atmospheric pressure drying process and the pyrolysis experiment are conducted at 400 ℃ and 800 ℃. Then, a scanning electron microscope and a Nano-CT computer tomography are applied to obtain the surface morphologies and the interior slice images of the specimens. The AVIZO software is employed to accurately extract and analyze the pore structural model and simulated calculate the absolute permeability. It is found that the small pores develop gradually during pyrolysis due to the resin decomposition and the quartz fibers rearrangement, resulting in an increase in number of large pores. Nonetheless, the equivalent radii of most pores are less than 1 μm. Very few pores possess a large radius over 5 μm. However, the volume fraction of these large pores exceeds 99%. In addition, with the pore size growing, the connectivity between these pores is enhanced, immediately causing an increase in number and size of the pore-throats. Larger pore and more pore-throats would add the unblocked flow channels for the fluid passing, reducing flow resistance. The seepage simulation also confirms that the absolute permeability gains significant increase after pyrolysis in all directions. For example, the absolute permeability of the pyrolyzed sample is 9.0×10^-13 m² in X direction, which is an order of magnitude greater than that of the unpyrolyzed sample. This study provides important insights for understanding the high-temperature evolution at of microstructure and the permeation behavior of fluid in porous thermal protection materials.

关键词: Permeability, Phenolic resin-based porous composites, Nano-CT scanning, Pyrolysis process, Permeability simulations