Dielectric and microwave absorption properties of KNN/Al₂O₃ composite ceramics

Potassium sodium niobate (KNN) based ceramics are commonly used in piezoelectric applications. In this work, we paid attention to their dielectric properties in the frequency range of 8.2–12.4 GHz (x-band) to develop a microwave absorbing ceramic. (1-x)KNN-xAl₂O₃ (x = 0.1, 0.2, 0.3) ceramics were prepared by the traditional ceramic processing. With the increase of Al₂O₃ content, the ceramics exhibit smaller particle size and higher porosity. KNN and Al₂O₃ as well as a small amount of Nb₂O₅ can be detected in the ceramics. And the third phase is caused by the volatilization of K and Na elements and the diffusion effect of Al and Nb elements. The complex permittivity of the composite ceramics decreases with the increase of Al₂O₃, and this is attributed to the low permittivity of Al₂O₃ as well as the increased porosity of the ceramics. Both the thickness d and Al₂O₃ content have great influence on the microwave absorption properties of the ceramics. The reflection loss peaks shift to lower frequency with the increase of the thickness d. And when x = 0.3 and d = 0.95 mm, the ceramic has optimum microwave absorption property with the reflection loss peak is − 6.8 dB and the effective frequency range is 8.66–11.27 GHz.