Effects of cation distribution on microwave dielectric properties of Mg1−xZnxAl2O4 ceramics

The effects of duration of ball-milling on cation distribution of calcined-MgAl2O4 and -ZnAl2O4 powders were investigated by 27Al solid-state nuclear magnetic resonance (NMR) measurement; the degree of inversion (λC), which corresponds to the fraction of Al3+ in tetrahedral site, of the MgAl2O4 powder increased from 0.33 to 0.42 when the duration of ball-milling varied from 0 to 24 h, though that of the ZnAl2O4 powders ranged from 0.04 to 0.07. Thus, the preferential occupation of Al3+ cation in tetrahedral site was obtained for the calcined-MgAl2O4 powders prepared by ball-milling for 24 h. As a result, the Q·f value of the MgAl2O4 ceramics extremely increased from 85,100 to 187,200 GHz with increasing the duration of ball-milling of the calcined powders, while that of the ZnAl2O4 ceramics was almost constant with a Q·f value of approximately 100,000 GHz. Moreover, the effects of Zn substitution for Mg on the microwave dielectric properties and cation distribution of the Mg1−xZnxAl2O4 ceramics were also investigated. 27Al NMR measurement revealed that the high λ0 values, implying the degree of the redistribution of the Al3+ cation in the octahedral site during firing, were observed in the composition range of 0-0.75 and the highest Q·f value of 222,600 GHz was obtained for the Mg1−xZnxAl2O4 ceramics at x = 0.75. Thus, it is considered that the preferential occupation of Al3+ cation at tetrahedral sites of the calcined-powders caused by the ball-milling is effective to enhance the Q·f value of the Mg1−xZnxAl2O4 ceramics.