TiO 2 -added Ba 4 (Sm 0.65 Nd 0.35 ) 9.33 Ti 17.75 Al 0.33 O 54 (BSNTA) ceramics were fabricated with a solid-state reaction method. Slight TiO 2 addition increased the dielectric constant ( ε r ) of BSNTA ceramics and brought about a near-zero temperature coefficient of resonant frequency ( τ f ). Sintering in oxygen increased the density of TiO 2 -added BSNTA ceramics and surprisingly concurrently improved the ε r and Q × f value. Thermally stimulated depolarization current (TSDC) measurements indicated that TiO 2 addition introduced oxygen vacancies ( V O .. ) and Ti Ti ' − V O .. defect dipoles and that an oxygen-sufficient sintering atmosphere decreased the concentration of these defects and thus increased Q × f values. From the viewpoint of defect behaviors, this observation offered original insight into the mechanism by which the sintering atmosphere had impacts on the Q × f value of titanate ceramics. Satisfying dielectric properties ( ε r = 79.5, Q × f = 14,840 GHz, τ f = 0 ppm/°C) were attained in oxygen-sintered BSNTA-0.6%TiO 2 ceramics that might be suitable for the manufacture of miniature microwave devices such as filters and antennas.
Weijia Guo,Jie Zhang,Yu Luo,Zhenxing Yue,Longtu Li
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.
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