Microwave dielectric properties and thermally stimulated depolarization of Al-doped Ba4(Sm,Nd)9.33Ti18O54 ceramics

Ba 4 (Sm 0.15 Nd 0.85 ) 9.33 Ti 18- z Al 3 z /4 O 54 (BSNT- z Al, 0.0 ≤ z ≤ 2.5) ceramics were prepared via a solid-state reaction, and the effects of Al doping on the microwave dielectric properties and defect behavior of the title compound were studied. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) photographs suggested that Al ions successfully entered the lattice to form tungsten-bronze-like solid solutions. With a small amount of Al substitution, the relative dielectric constant (ε r ), and the temperature coefficient of resonant frequency ( τ f ) values decreased, whereas the quality factor ( Q × f ) substantially increased by approximately 50%. The defect-related extrinsic dielectric loss was clarified via the thermally stimulated depolarization current (TSDC) technique. With Al doping, the TSDC relaxation of across-grain-boundary oxygen vacancies ( ) vanished, whereas that of defect dipoles ( ) appeared at relatively low temperatures. Therefore, in the BSNT- z Al ceramics, oxygen vacancies were more inclined to interconnect with to form defect dipoles. This could reduce the activity of and account for the notable improvement in the Q × f values. In particular, the excellent characteristics of ε r = 67.33, Q × f = 16 530 GHz, and τ f = +0.87 ppm/°C were achieved in the specimens with z = 1.5 sintered at 1350°C for 4 hours.