Abstract: Mullite ceramic is prepared by hot-pressing method from a mixture of SiO₂ sol and Al₂O₃ sol and from a mixture of SiO₂ and Al₂O₃ powders, respectively. The relationships between permittivities and sintering conditions of mullite ceramic are discussed. The samples’ permittivities are measured in the frequency range of 8.2~12.4GHz. The results show that the real and imaginary parts of permittivities of the samples increase with the sintering densities. The real and imaginary parts of permittivity of the samples increase when additive MgO is used. Mullite ceramics with different permittivities can be obtained through varying their preparation condition. When the as-formed mullite ceramic is used as an impedance transfer layer of the microwave absorbing material, it greatly enhances the microwave absorbing efficiency of absorbing materials. The mechanism of the enhancement of absorbing efficiency is that the addition of mullite ceramic to and absorbing material lowers the interface reflection between air and the absorbing material, more microwave enters the absorbing material and is dissipated. There exist different effects from ceramics which are prepared under different conditions. So an exact preparation condition of the mullite ceramic is required for expected characteristics of the microwave absorbing material.

Key words: mullite ceramic, microwave absorbing material, permittivity, reflectivity, impedance