| The radome is an important component of the front of missile or aircraft, the functions of which integrate heat resistance, wave permeability and bearing capacity. Therefore, radome materials must have the characteristics of low dielectric constant and dielectric loss, high mechanical strength, good thermal shock resistance, good heat performance, as well as rain erosion resistance. Ceramic materials have high temperature resistance, good heat stability, low hygroscopic, low expansion coefficient, enough strength and suitable dielectric properties, all of which make ceramic materials become the hot issue in the study of high temperature radome materials at home and abroad. Porous ceramic is a novel kind of functional material with a lot of excellent properties, including low density, high specific surface area, low thermal expansion coefficient, low dielectric constant and dielectric loss, etc. Silica ceramics process the advantages of low dielectric constant and dielectric loss. When the silica material is made into porous structure, the dielectric constant, dielectric loss and density of which will be reduced greatly. It is proved that porous silica matrix ceramic is a kind of promising radome material. Since 1950 s and 1960 s, hollow microsphere, which has developed to be a new particle material with wide applications in the field of astronautics, mechanical manufacturing and building materials due to their characteristics of large specific surface area, light weight, high liquidity and packing density. In this paper, we took advantages of hollow microspheres to prepare ceramics radome material through different methods. We also studied the factors on microstructures, mechanical strength and dielectric properties. The main research contents include as follows:Si O2 hollow microspheres were directly sintered to prepare porous silica ceramics in this paper, and effects of sintering atmosphere and temperature on the preparation and properties of samples were analyzed. The results showed that the optimal temperature for uniform microstructure was 1650 ℃ for 1 h under N2 atmosphere. Accordingly, the porous silica ceramics had the porosity of 71.46%, the bulk density of 0.66 g/cm3, dielectric constant of 1.78, dielectric loss of 1.85×10-3 at test frequency of 10.66 GHz. However, their mechanical performance was low, the compressive strength was only 0.75 MPa.Experiments were performed to prepare porous mullite matrix ceramic radome materials by directly sintering Si O2-Al2O3 ceramic microspheres, the purpose of which was to improve the poor mechanical properties of porous silica radome material. The effects of sintering temperatures and chemical compositions of green microspheres on the microstructure of samples and the comprehensive performances were studied. Results showed that samples sintered at 1550 ℃ for 1 h, samples with good dielectric properties were obtained, with dielectric constant of 3.37, dielectric loss of 4.16×10-3 at test frequency of 10.88 GHz, porosity of 81.37%, and compressive strength of 6.25 MPa, which indicated that the mechanical performance of samples was greatly improved.Using Si O2 hollow microspheres as pore-forming agent, gel-casting method was adopted to prepare porous Si2N2O-Si3N4 composite ceramic radome materials, to further improve mechanical properties. The influences of Si O2 hollow microspheres contents on the phase composition, microstructure, mechanical and dielectric properties were studied. Results showed that when Si O2 hollow microspheres content was 15 wt.%（based on Si3N4 slurry）, the samples with the optimal microstructure, uniform pore size distribution, good dielectric properties and comprehensive performance were obtained, with the porosity of 51.85%, bulk density of 1.43 g/cm3, dielectric constant of 4.52, loss tangent of 6.01×10-3, flexure strength of 43.92±4.72 MPa, which proved that the obtained samples had better performance than both porous silica ceramics and porous mullite matrix composite ceramic. |
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