Preparation Of Lightweight Thermal Insulation Carbon Foam Composites By Gel-Casting And Performance Study

Carbon foam materials bring excellent performance of low density,tunable thermal conductivity,and high temperature stability due to their tunable 3D porous structure.The new high-speed spacecraft thermal protection system has an urgent demand for high-performance thermal insulation materials with both low density,low thermal conductivity and high strength,and the adjustable properties of carbon foam materials make them an ideal alternative for thermal insulation materials.In this paper,we have considered the preparation cost,molding method,carbon source selection,and performance matching,and conducted a study on the preparation of sucrose-based carbon foam materials with sucrose as the carbon source,acrylamide hydrogel system as the template,and microcapsules as the pore-forming agent.The monomer initiation process in precursor forming,the pyrolysis process of precursors were studied,and the optimization conditions of the structure and properties of the carbon foam materials were explored.The specific studies are as follows:（1）The monomer initiation process in precursor preparation was analyzed,and the effects of monomer,reaction temperature and initiator dosage on the molding time were investigated.The results showed that with 15g of monomer,4g of cross-linker,50℃ reaction temperature and 1g of initiator,the molding time was controlled between 60～82s,which satisfied the requirements of the gel-casting process for the operation time of the injection mold.（2）The pyrolysis process of the precursor was analyzed and kinetic calculations were performed.The results showed that the precursor pyrolysis was mainly divided into four stages,the first stage was from room temperature to 140℃,and the sample weight loss was about 24%,mainly caused by water evaporation;in 140～220℃,the sample weight loss was about 15%,mainly caused by microcapsule decomposition and evaporation of unevaporated water;in 230～330℃,the sample weight loss was about 16%,mainly caused by the decomposition of sucrose;above 340℃,the sample weight loss was 14～28%at different heating rates,mainly caused by the decomposition of polymer materials in the carbonization process.The kinetic calculations of the pyrolysis process were carried out with the help of CR method,and the results showed that the whole process of pyrolysis was a 3-stage reaction at a heating rate of 5℃/min.The increase of the heating rate makes the reaction level in the third stage gradually decrease from 3 to 2,and the reaction level in the rest of the stage is still 3.The reaction level is proportional to the activation energy,while the heating rate is not directly related to the decrease of activation energy.（3）The optimized conditions for the structure and properties of carbon foam materials were studied.The results showed that:fixing the remaining variables and changing the content of microcapsules（88.23～92.59vol.%）,the bulk density,compression strength and thermal conductivity of the foam carbon materials were 0.44～0.51g/cm³,5.02～8.80 MPa and 0.041～0.054W·m^-1·K^-1,respectively;fixing the remaining variables and changing the sucrose content（45～65wt.%）,the bulk density,compression strength,and thermal conductivity of the carbon foam material were0.37～0.50g/cm³,1.84～18.58MPa,and 0.054～0.088 W·m^-1·K^-1,respectively;fixing the remaining variables and changing the carbonization temperature（600～1200℃）,the closed pore structure of the carbon foam material collapses with the increase of temperature,1000℃ is the carbonization temperature to maintain a better pore structure,and the thermal conductivity is 0.038～0.049 W·m^-1·K^-1（600～1000℃）,which increases with the increase of carbonization temperature.