Yuhan Ren,Biao Zhang,Zhaoxin Zhong,Jian Ye,Jiawei Zhang,Zhiyong Fang,Feng Ye
In this study, monolithic Al 2 O 3 –SiO 2 aerogels with excellent thermal stability and high specific surface area were synthesized using boehmite nanorods (L=20–30 nm, D=3–5 nm). The high specific surface area of aerogels was maintained after calcination at 800 °C (663.54 m 2 /g), 1000 °C (486.91 m 2 /g), and 1100 °C (230.76 m 2 /g). The anti-sintering mechanism of the Al 2 O 3 -SiO 2 aerogels was discussed. Thanks to the composite of Al 2 O 3 -SiO 2 aerogel and mullite fibers, the disadvantages of brittleness and low strength of aerogel are overcome. The thermal conductivity of the composite is extremely low at 0.034 W/(m·K)-0.089 W/(m·K) (25 °C-1000 °C). Five muffle cycle back-temperature tests at 1000 °C and a butane blowtorch back-temperature test at 1350 °C also proved that the composites have excellent thermal insulation properties. Composites with excellent thermal insulation properties combined with lightweight, thinness, machinability, and good dielectric properties meet the need for high-temperature thermal insulating-wave transmitting integrated materials for hypersonic missile radomes.
Traditional layered broadband radomes are challenging to meet the needs of the high-temperature application, due to thermal mismatch between their layers. In this study, porous Si 3 N 4 ceramics with a novel continuous porosity gradient from 54.7% to 82.4% were achieved by lamellar gel-casting, freeze-drying, and subsequent sintering process. The porosity-graded structure in Si 3 N 4 ceramics shows no failure or cracks after sintering, benefiting from the low shrinkage after sintering at 1900 °C with the rapid sintering rate. The continuous porosity-graded structure without obvious interfaces was architected by ice crystal growth during solidification and the growth of rodlike β-Si 3 N 4 grains across the interfaces during sintering. This approach can be beneficial to designing high wave transmission performance at specific broadband for radome.
Yi-Ning He,Ren-Zhong Zhang,Jia-Min Wu,Ze-Yu Deng,Ze-Yuan Li,Jia-Xin Zheng,Lin Guo,Zhi-Xiang Cai,Yu-sheng Shi
SiO 2 -based ceramics are an ideal material for manufacturing radomes due to their thermal stability, high dielectric properties, and simple processing techniques. However, traditional manufacturing methods heavily rely on molds, making it difficult to form complex SiO 2 -based ceramic structures. Digital Light Processing (DLP) technology was applied to fabricate SiO 2 -based ceramics doped with Al 2 O 3 -MgO in this work. The influence of the Al 2 O 3 -MgO content on the SiO 2 -based ceramics was systematically investigated. Increasing Al 2 O 3 -MgO content causes a shrinkage to first decrease then increase, contrasting with the flexural strength's initial increase and subsequent decrease. The main reason is magnesium-aluminum spinel (MgAl 2 O 4 ) generated during the sintering process and thus retards crack propagation, promoting a transition in fracture mode. During the fracture stage, the fracture mode of samples with 10.0 wt% Al 2 O 3 -MgO is mainly transgranular fracture, instead of intergranular. This transition in fracture mode enhanced the flexural strength. Meanwhile, as the Al 2 O 3 -MgO content continues to increase, excessive spinel causes agglomeration within the ceramic microstructure. When the Al 2 O 3 -MgO content reached 10.0 wt%, the shrinkage decreases to a minimum of 3.32 %, and the flexural strength increases to a maximum of 15.94 MPa, making it suitable for manufacturing ceramic radomes. The research results provide a new solution for manufacturing SiO 2 -based ceramic radomes by DLP.
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