| Carbon/carbon (C/C) composites are pyrocarbon-based materials reinforced bycontinuous carbon fibers. They have become potential materials in thermalprotection components of aerospace crafts for their excellent properties, such as highspecific strength, high specific modulus, low density, low thermal expansioncoefficients, good thermal shock resistance and ablation resistance. However, thehigh thermal conductivity of carbon/carbon composites induces high temperatureinside the crafts, which cripples the security and reliability of the crafts. In view ofthe fact, inner thermal barrier should be developed. Contrastly, few work on thissubject has been reported.Carbon foams are ideal thermal-insulation materials for advanced hightemperature thermal protection system because of its low thermal conductivity, lightweight and good high temperature resistance. The paper aims at the achievement ofC/C-carbon foams system. For this purpose, insulated carbon foams were preparedby phenolic resin or mesophase pitch as the precursor firstly; then the elementsilicon, boron or zirconium were added into carbon foams separately for much betterproperties. On this basis, carbon foams were connected with C/C composites viaadhesive joint to form C/C-carbon foams combinational materials. FT-IR, SEM,EDS, XRD, Raman, TEM, HRTEM, compression and shear tests were applied tocharacterize the microstructure, mechanical properties, heat insulation, thermalshock resistance and high temperature ablation performance of the as-preparedcarbon foams and combinational materials respectively. Effects of modifiers’ contenton the performances of carbon foams were investigated. The main content andconclusions were summarized as follows:Resin-based carbon foams Ⅰ and carbon foams Ⅱ were prepared by usingwater-soluble phenolic resin as matrix, incorporating self-made hollow microspheresand commercially available BJO-0930hollow microspheres respectively.Pitch-based carbon foams Ⅲ were prepared by high pressure foaming methodusing mesophase pitch as precursor. The microstructure, mechanical properties andhigh-temperature thermal-insulation properties were studied. The results showed thatall of them mainly consisted of hollow carbon microspheres, resin carbon matricesand pores; incorporation of hollow microspheres into carbon foam decreased density, and improved thermal-insulation properties. Carbon foams Ⅱ contained smallestclosed-pores, whose diameter were substantially less than100μm, and densitieswere between0.35and0.43g·cm-3. Its compressive fracture showed the gradientbrittle mode. The compressive stress-strain curves mainly contained two regions,linear elasitcs and plateau. As the content of microspheres increasing, compressionstrength of carbon foam Ⅱ increased at first and then decreased; while the fracturetoughness remained increasing. Its compression fracture mode was gradient brittlefracture. Carbon foam CF-70showed good mechanical and thermal insulationproperties. Its compressive strength and specific compressive strength were16.0MPa,43.3MPa·cm3·g-1respectively, and the thermal conductivity at800℃was0.76W·m-1K-1. Compared with carbon foams Ⅱ, carbon foams Ⅰand carbon foams Ⅲowned much larger hollow pores, which induced their higher thermal conductivity,and worse mechanical properties.CF-70, which showed the better heat-insulation and mechanical strength, wasselected as the matrix for further modification. Three types of modified carbonfoams (graphite foams) were prepared by adding the element Si, B and Zr intomatrix separately. The results showed that proper amount of additional elementscould improve the mechanical strength of carbon foams. Compressive strength andshear strength of carbon foam were increased firstly and then decrease, with theincreasing dopant’s content. The compressive stress-strain curves of Zr-modifiedcarbon foams exhibited gradient brittle fracture, while the shear load-displacement ofwhich showed certain plastic behavior. Zr-7presented excellent mechanicalproperties: the compressive strength, specific compressive strength and shearstrength were27.8MPa、67.8MPa·cm3·g-1and9.87MPa respectively, increasing by73.7%,56.6%and47.3%correspondingly. Proper amount of element Zr coulddecrease thermal conductivity, thus improve high-temperature thermal insulation.Zr-10showed the lowest thermal conductivity,0.597W·m-1·K-1at800℃, which was21.4%lower than that of unmodified one. Boron element showed better effect onimproving anti-oxidation of carbon foams. BO-7presented excellent anti-oxidationperformance: the5%weight-loss temperature was500.3℃, which increased by29.4%than that of unmodified one.The microstructure of carbon foam Ⅱ changed from amorphous carbon intographitic carbon after high temperature graphitization, which decreased mechanicalstrength and thermal insulation properties. Incorporation of the element Si, Zr into graphite foams reduced ablation rate, thus improved their high-temperatureanti-ablation performance, because of the oxidation products, SiO2or ZrO2, whichretarded the graphite foams substrate from oxidation or thermo-chemistry ablation.Via adhension bonding method, CF-70, BO-4and Zr-7were connected to C/Ccomposites separately, to form different kinds of C/C-carbon foam joints. Thebonding strengths were4.31MPa,4.49MPa and4.67MPa, respectively. Shearfailure positon of joints all located near the interface between resin-carbonintermediate layer and the carbon foam substrate, because the interface shearstrength were approximately equal to that of the carbon foams. Their thermalconductivities were1.55W·m-1K-1,1.39W·m-1K-1and1.35W·m-1K-1at800℃respectively, much lower than that of C/C composites by95%, because of theincreased thermal-resistance of combined material obviously after adhereing carbonfoams to its interior. |
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