Ba4(Sm0.15Nd0.85)9.33Ti18‐zAl3z/4O54 (BSNT‐zAl, 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 (εr), and the temperature coefficient of resonant frequency (τf) 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 (VO..) vanished, whereas that of defect dipoles (AlTi′-VO..) appeared at relatively low temperatures. Therefore, in the BSNT‐zAl ceramics, oxygen vacancies were more inclined to interconnect with AlTi′ to form defect dipoles. This could reduce the activity of VO.. and account for the notable improvement in the Q × f values. In particular, the excellent characteristics of εr = 67.33, Q × f = 16 530 GHz, and τf = +0.87 ppm/°C were achieved in the specimens with z = 1.5 sintered at 1350°C for 4 hours.

Microwave communication systems are being developed with the goal to achieve miniaturization and high reliability. There are urgent requirements for microwave dielectric materials to exhibit low sintering temperatures, excellent microwave dielectric properties, and prominent mechanical performance. However, simultaneously achieving these requirements is a significant challenge. These concerns have been addressed in the uniform ultrafine-grained 0.95MgTiO 3 -0.05CaTiO 3 ceramics with an average grain size of ~0.6 µm via a grain size engineering strategy. Interestingly, the sintering temperature in the unique two-step sintering method of 1250℃/1 min and 1130℃/10 h is much lower (200-300℃) than that obtained in the conventional sintering method reported previously (1400-1450℃). Excellent dielectric properties are achieved, with relative permittivity ε r = 20.11, quality factor Q × f = 68 613 GHz, and temperature coefficient of resonant frequency τ f = 1.81 ppm/℃. The bending strength f , Vickers hardness H v , and elastic modulus E reach up to 212.4 MPa, 10.1 GPa, and 200.7 GPa, respectively. These values are significantly enhanced over those of samples obtained via the conventional sintering method with an average grain size of ~2.5 µm. For the first time, uniform ultrafine-grained microwave dielectric materials with excellent microwave dielectric properties and prominent mechanical performance were synthesized. This work provides a guideline for developing other high-performance microwave dielectric materials and makes a significant contribution to the miniaturization and high reliability of microwave dielectric devices.

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Low-loss tungsten—bronze microwave dielectric ceramics are dielectric materials with potential application value for miniaturized dielectric filters and antennas in the fifth-generation (5G) communication technology. In this work, a novel Al/Nd co-doping method of Ba4Nd9.33Ti18O54 (BNT) ceramics with a chemical formula of Ba4Nd9.33+z/3Ti18−zAlzO54 (BNT—AN, 0 ≼ z ≼ 2) was proposed to improve the dielectric properties through structural and defect modulation. Together with Al-doped ceramics (Ba4Nd9.33Ti18−zAl4z/3O54, BNT—A, 0 ≼ z ≼ 2) for comparison, the ceramics were prepared by a solid state method. It is found that Al/Nd co-doping method has a significant effect on improving the dielectric properties compared with Al doping. As the doping amount z increased, the relative dielectric constant (εr) and the temperature coefficient of resonant frequency (τf) of the ceramics decreased, and the Q×f values of the ceramics obviously increased when z ≼ 1.5. Excellent microwave dielectric properties of εr = 72.2, Q×f = 16,480 GHz, and τf = +14.3 ppm/°C were achieved in BNT—AN ceramics with z = 1.25. Raman spectroscopy and thermally stimulated depolarization current (TSDC) technique were firstly combined to analyze the structures and defects in microwave dielectric ceramics. It is shown that the improvement on Q×f values was originated from the decrease in the strength of the A-site cation vibration and the concentration of oxygen vacancies $$\text{V}_\text{O}^{^{ \cdot \cdot }}$$, demonstrating the effect and mechanism underlying for structural and defect modulation on the performance improvement of microwave dielectric ceramics.