Frequency-dependent Qf value of low-loss Ba2Ti9O20 ceramics at microwave frequencies

The microwave dielectric properties of low-loss Ba2Ti9O20 ceramics were evaluated by the resonant cavity method, and the frequency dependence of Qf value was investigated. TE011 resonant mode was used for measurement, and the measurement frequency was changed by adjusting the sample size. The measurement frequency could also be tuned over a wide range by using higher-order TE0np modes for a sample with a fixed size. The measured Qf value of Ba2Ti9O20 ceramics increased significantly with increasing the frequency from 4.55 to 10.74 GHz. The result conflicted with the common recognition that the Qf value of microwave dielectric ceramics was a frequency-independent constant at microwave frequencies. The frequency dependence of Qf value was attributed to the extrinsic dielectric loss, which originated from extrinsic factors such as grain boundary, oxygen vacancy and pore.