| Thermal protection system (TPS), protecting aerocrafts with high velocity in the aerosphere from invalidation or even burning, is one of the most significant structures of aerocrafts. On the background of thermal protection structure based on thermal radiation of near-space aerocraft, ceramic fiber thermal insulation rigid tiles were studied.Thermal insulation tile preforms were fabricated from short silica fiber and mullite fiber, respectively, which were interspersed in deionized water. Then add boron carbide (B4C) powders, colloidal silica and soluble starch into the slurry. After mixing round, the homogeneous slurry was molded, dried and sintered to form porous skeleton. Porous skeleton was dipped into SiO2-sols and SiO2-Al2O3 binary sols and then the specimen was dried supercritically to obtain aerogel loading porous skeleton composite. The surfaces of thermal insulation rigid tiles were coated with high radiant emissivity coatings, utilizing molybdenum disilicide (MoSi2) as high radiant emissivity agent. Rigid tiles'thermal insulation capabilities were assessed by thermal environment simulation on ground.Technical condition orthogonal test revealed that the dosage of boron carbide powders had most powerful influence on apparent porosity of preforms and molding pressure came next. By restraining dosage of boron carbide less than 15% of fiber weights as well as utilized molding pressure below 79.8 kPa, the porosity of short fiber porous skeleton was higher than 80%. The flexural strength of silica fiber porous skeleton with porosity of 83.95% was 3.582MPa while that with porosity of 92.0% was reduced to 0.585MPa. The flexural strength of silica fiber skeleton with porosity of 82.0% was 1.205MPa while that with porosity of 89.6% was reduced to 0.312MPa. The porosity of porous skeleton had little influence on room temperature thermal conductivity. However, when porosity of short mullite fiber skeleton was reduced from 87.5% to 80.0% and the temperature was higher than 700℃, thermal conductivity was lowered 5%~10%. In the way of adding potassium titanate whishers (PTW) to short mullite fiber porous skeleton, thermal conductivity at 1000℃had decreased about 29%.With equivalent porosity, thermal conductivity of short silica fiber porous skeleton was 25%~30% lower than that of short mullite fiber porous skeleton. According to heat soaking test (1000℃/16hours), the linear shrinkage of short mullite fiber porous skeleton was 0 while that of short silica fiber skeleton was 10.0%.Aeverage pore diameter of aerogel loading porous skeleton composite was 39.2nm. Compared to short fiber porous skeleton, thermal conductivity of aerogel loading porous skeleton composite had decreased 30%~50% (thermal conductivity of SiO2-aerogel loading silica fiber porous skeleton composite was 0.041W·m-1·K-1 at 800℃) as well as mechanical properties of which were improved remarkably: compressive strength was improved 250% at most while flexural strength was improved 200% at most. Further, aerogel loading porous skeleton composite even showed some evidence of tough fracture. SiO2-Al2O3 binary aerogels loading porous skeleton composite had a specific surface area of 470 m2·g-1 at 1000℃.By balanced-state heat meter measurement, the hemispherical radiant emissivity of coatings was 0.92. According to thermal insulation effect comparison experiment, the cold face temperature of mullite fiber porous skeleton (thickness: 17 mm) with 200-micron MoSi2 coatings, compared to that without coatings, was 30 to 100℃lower. In addition, because of boronsilicate glass coating's porous structure, they showed better resistance to crack spreading and could endure 10 cycles of 1100℃-cold water thermal shock. Further, the test of coating's adhesion to rigid tiles showed that the fracture of rigid tiles themselves occupied 55%~90%.Hot face temperature rose from room temperature to 1000℃in 100 seconds and then held for 600 seconds. At the end of the process, the cold face temperature of SiO2-aerogel loading short silica fiber porous skeleton composite (thickness: 16.4 mm) was 320℃, and that of SiO2-Al2O3 binary aerogels loading short mullite fiber porous skeleton (to which PTW was added) composite was 340℃.
|
PDF Full Text Request