Sintering behavior and morphology control of porous Al ₂ O ₃ ‐SiO ₂ ceramics for radome applications

Abstract

Ceramics from porous Si 3 N 4 and its derivatives SiAlON and Si 2 N 2 O were once considered the most promising high-temperature wave-transmitting materials. However, their large-scale application in the field of radomes is greatly restricted due to their poor oxidation resistance, high preparation costs, and expensive raw materials. Therefore, the development of low-cost porous oxide ceramics remains of significant interest to the field of high-temperature wave transmission. Surprisingly, mullite ceramics, which are representative of the Al 2 O 3 -SiO 2 -system of ceramics, are ultra-low-cost materials with the potential to replace ceramics from Si 3 N 4 and its derivatives. In this paper, integrated porous Al 2 O 3 -SiO 2 -system ceramics were successfully prepared for load-bearing/wavetransmitting applications, using inexpensive calcined kaolin and alumina powder as the main raw materials. Calcined kaolin can provide seeds for the growth and development of mullite crystals in the ceramic system. High-strength and high-porosity ceramics were obtained with the mullite morphology controlled through the molar ratio of Al 2 O 3 to SiO 2 and the resulting content of mullite seeds. With increasing of mullite seed content, the length and radial width of mullite whiskers with "interlocking structure" gradually change from rodshaped "long and thick" to needle-like "short and thin." The prepared porous Al 2 O 3 -SiO 2 ceramics have high flexural strength, fracture toughness, and good dielectric properties.

A simple and efficient hydratable alumina gel‐casting method for the fabrication of high‐porosity mullite ceramics

“…The dielectric constants at 30 GHz for porous mullite ceramics with the solid loadings of 7.5, 10, 15, and 20 Vol.% are 1.34, 1.42, 1.60, and 1.79, respectively. Lichtenecker's logarithmic mixture formula 29,30 can be used to calculate the dielectric constant of composite material. Porous mullite ceramics can be seen as composites of mullite and air, with dense mullite dielectric constant is 6.7 19 and air is defined as 1.…”