Temperature stability, low loss and defect relaxation of MgO-TiO2 microwave dielectric ceramics modified by Ca0.8Sr0.2TiO3

Temperature-stable and low-loss microwave dielectrics based on the MgO-TiO2 system with nominal formation Mgn+1TinO3n+1 (n = 5, MT) were prepared via the conventional solid-state reaction method. Ca0.8Sr0.2TiO3 (CST) was chosen as a τf compensator for matrix MT to form the composite ceramics (1-x)Mg6Ti5O16-xCa0.8Sr0.2TiO3 (0.10 ≤ x ≤ 0.26, MT-CST). The effects of CST additions on the phase composition, defect relaxation behavior, and microwave dielectric properties of MT were investigated. It revealed that undoped MT was basically consisted of MgTiO3 as a major phase and Mg2TiO4 as a minor phase, and such two phases coexisted well with CST additions. Interestingly, τf could be tuned close to zero (−1.28 ppm/°C) for the MT-CST ceramics at x = 0.22, accompanied with a high Q×f value ~ 74,200 GHz and a proper εr ~ 20.25 (9.90 GHz). These materials possessed a good potential for applications in microwave components and devices. Meanwhile, significant relaxation phenomena were observed in all the MT-CST samples using dielectric spectroscopy and thermally stimulated depolarization current (TSDC) techniques. The oxygen-vacancy-related defects, shown as (TiTi′)-(VO
-
- ) dipoles and VO
-
- , were the main types of defects in MT-CST, which was responsible for the relaxation behavior; meanwhile, the defect concentrations increased with the increase of CST content, thus resulting in the increase of dielectric loss at low and high frequencies.