“…Compared with the results by the solid-state reaction method, Khalam et al [6] reported a lower Qf result of 116,000 GHz, Fu et al [8] reported a similar Qf value of 175,000 by the high-energy ball-milling method, while Kan et al [5] reported a relatively higher Qf value of 193,000. As for the Qf values of the dielectric ceramics, it is known that porosity, secondary phase, structure defect and grain boundary of the ceramics produce a deterioration in the Qf values [28].…”