| High performance thermal insulators with ultra-high temperature stability, lowthermal conductivity, and moderate mechanical properties are required for the thermalprotection system in advanced high speed aerospace vehicles. Carbon aerogels arenanoporous materials consisted of three-dimension necklace chains of nanosize carbonparticles. They have low density, low radiative thermal conductivity, and ultra-hightemperature stability, but they are weak, brittle, and shrink inherently and largely duringpreparation process, these drawbacks limit their use as thermal insulators in aerospaceapplication. Based on thorough research on the structural evolution and shrinkingmechanisms of resorcinol-formaldehyde (RF) aerogels, for the first time, thisdissertation proposed the use of carbon precursor fibers (preoxidized PAN, preoxidizedcellulose, and phenolic fibers) as reinforcements, impregnated by RF sol, then viagelation, aging, solvent exchange, and supercritical drying processes to prepare carbonprecursor fiber-reinforced RF aerogel composites firstly, then co-carbonizing to formcarbon fiber-reinforced carbon aerogel composites (CF/CAs). The carbon precursorfibers shrink with the RF aerogel matrix during carbonization, thus improving theshrinking compatibility between the reinforcements and the matrix, and results incrack-free co-pyrolysis CF/CAs for thermal insulators. Conclusions are listed as follow:(1) In order to obtain carbon aerogels with low thermal conductivity and smallshrinkage, the effects of water (W), sodium carbonate (C) molar ratios, and dryingmethods on the properties and microstructural evolution of the carbon aerogels wereresearched. The densities of carbon aerogels reduce with increasing the W/R ratios. Theaverage pore sizes decrease with decreasing the W/R ratios. The drying and carbonizingshrinkage of the gels both decrease with increasing the R/C ratios. The average poresizes decrease with decreasing the R/C ratios. When W/R ratios are60~100and R/Cratios are300~600, combining with petroleum ether supercritical drying, carbonaerogels with drying shrinkage less than8%, carbonizing shrinkage less than28%,densities lower than0.15g/cm3, and pore sizes less than150nm can be obtained forpreparing CF/CAs with low thermal conductivity.(2) The physically and chemically structural evolutions of the RF aerogels duringcarbonization were researched, and the shrinking mechanism of RF aerogels wasanalyzed. The carbonizing shrinkage is caused by the mass loss during carbonization,and exhibits two stages. At below460℃, the main reactions are dehydration andcondensation between hydroxyls, and the aerogel exhibits large mass loss and shrinkagerates, there are not any micropores (pore size less than2nm) forming in the aerogel atthis stage. At above460℃, the main reactions are the break of methylene andmethylene ether linkages, and the rearrangements of carbon atoms, a lot of micropores form because of the volatilization of low molecular weight gases. The specific surfacearea is still higher than300m2/g when carbonized at2000℃, testifying the ultra-hightemperature stability of carbon aerogels.(3) To improve the shrinking compatibility between the reinforcements and the RFaerogel matrix, for the first time, carbon precursor fibers were used to reinforce RFaerogels, then co-carbonized to form a new crack-free CF/CA. The shrinkingcompatibilities between three kinds of carbon precursor fibers (preoxidized PAN,preoxidized cellulose, and phenolic fibers) and RF aerogels were researched. The linearshrinkage of these three kinds of composites are small (4.0~5.4%) during supercriticaldrying, the shrinkage in the through-plane and in the in-plane directions are similar.During carbonization, the through-plane shrinkage of them are18.4~21.2%, the in-planeshrinkage are11.4,17.8, and18.7%, respectively. Phenolic fiber is the most suitablereinforcement for RF aerogel.(4) The effects of aerogel matrix densities, kinds of carbon precursor fibers, andfiber fractions on the mechanical properties of the CF/CAs were researched. Thecompressive and bend strengths of the CF/CAs both increase exponentially withincreasing the aerogel matrix densities. But when the aerogel matrix densities are higherthan0.128g/cm3, the increasing rates of bend strengths slow down. The bend strength ofthe CF/CA with fiber fraction of8.25vol%is the highest, larger fiber fraction than8.25vol%will not result in higher strength. The compressive strengths of the CF/CAswith PAN-, phenolic-, and cellulose-based carbon fiber reinforcements are1.72,0.79,0.59MPa, the bend strengths are6.50,2.03,1.38MPa, respectively. The improvedmechanical properties of the CF/CAs are due to fiber pull-out mechanism.(5) The thermal insulation properties and the thermal-insulating mechanism at hightemperature of the carbon aerogels were researched. At100~300℃in air, and in thedensity range of0.054~0.182g/cm3, the thermal conductivity of the carbon aerogel withdensity of0.066g/cm3is the lowest (0.026W/m·K at200℃). Under2000℃,0.15MPaargon atmosphere, the thermal conductivities of carbon aerogels with densities of0.128and0.052g/cm3are0.223and0.601W/m·K, respectively. Carbon aerogel with higherdensity (0.128g/cm3) is more effective in reducing radiative conductivity then reducingtotal conductivity at higher temperature. Due to the mechanisms of phonon scattering,photon absorbing and scattering, and inhibition of gas molecule collision, resulted fromthe nanoparticle and nanoporous structure of carbon aerogel, the solid, radiative, andgaseous conductivities of the carbon aerogel (0.052g/cm3) are only1/2,1/2,1/3of thoseof micrometer pore size carbon foam with similar density (0.054g/cm3).(6) The effects of kinds of carbon precursor fibers, fiber fractions, andcarbonization temperatures on the thermal insulation properties of the CF/CAs wereresearched. The thermal conductivities of the CF/CAs with different fiber reinforcements from large to small are: phenolic-based＞PAN-based＞cellulose-based. The thermal conductivities of the CF/CAs increase with increasing thefiber fraction, and the experimental data fit the geometrical model well. The thermalconductivities increase with increasing the carbonization temperature, and the thermalconductivity of the CF/CA carbonized at1800℃is2times of the one carbonized at1000℃.(7) The high temperature thermal insulation properties of the CF/CAs wereresearched. Under2000℃,0.15MPa argon atmosphere, the thermal conductivity of theCF/CA is0.325W/m·K, much lower than that of the micrometer pore size carbon foam(1.745W/m·K). The results tested by arc jet facility clearly demonstrate the ultrahightemperature thermal insulation property of CF/CAs, the CF/CAs can be used asultra-high temperature thermal insulators under vacuum, inert, or tightly closedatmospheres for thermal protection system in aerospace vehicles or industrial furnaces. |
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