Analysis of the pyrolytic behavior of benzoxazine-derived carbon/carbon composites
Date
2023
Authors
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Publisher
University of Delaware
Abstract
Carbon-carbon composites (CCCs) are a form of carbon-fiber reinforced materials that exhibit excellent thermomechanical properties under extreme environmental conditions. To expand the applicability of CCCs, the fabrication process must be modified such that there is a reduction in cost or processing time. In this study, benzoxazine resin derived carbon precursors were utilized as they offer greater flexibility in design and thermal processing as they do not require high pressures to form the carbon matrix (as in the case of thermoplastic pitch), nor do they require lengthy gas-phase infiltrations (as is the case of chemical vapor infiltration). In using thermoset resins, increased fabrication efficiencies have been previously achieved via modifications to the material properties. However, this approach is empirical in nature as any gains in efficiency are unique to a given system. Therefore, the goal of this research was to understand how the microstructure evolves during the heat treatment process as a function of time, temperature, and material conversion. ☐ To track the evolution of the porous microstructure, a heat treatment protocol was developed using a multi-stage nth – order kinetic model to separate each reaction distinctly. For each reaction, the microstructure was evaluated using 3D X-ray micro-Computed Tomography to analyze the formation of cracks and voids volumetrically. This provided a stepwise progression where it was shown that void growth was restricted to the first reaction, the onset of small microcracks the second reaction, and the formation of large interconnected macrocracks to the third reaction. This knowledge of the pore structure formation was then used to formulate an optimized carbonization protocol. This involved using reaction-rate constraints to limit the pressure evolved in the system until the condition where it was expected that there would be a high permeability due to crack formation. This process resulted in a reduction of 41 hours (from 48 to 7 hours) while producing a comparable microstructure to the original cycle. ☐ Further optimizations were proposed using a predictive DiBenedetto expression to estimate when the glass transition temperature exceeded the process temperature (TG/TP > 1), as this condition was believed to be responsible for the crack formation observed. This behavior was substantiated through dynamic scanning calorimetry where it was found that the TG varied linearly with decomposition conversion. The model was then validated through thermomechanical analysis where the elongation behavior of the composite provided insight into the microstructural progression in real-time.
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Keywords
Carbon-carbon composites, Pyrolytic behavior, Heat treatment, Thermomechanical properties, Material properties