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 Ba₄Nd_9.33Ti₁₈O₅₄ (BNT) ceramics with a chemical formula of Ba₄Nd_9.33+z/3Ti_18-zAl_zO₅₄ (BNT-AN, 0 ≤ z ≤ 2) was proposed to improve the dielectric properties through structural and defect modulation. Together with Al-doped ceramics (Ba₄Nd_9.33Ti_18-zAl_4z/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 (ε_r) and the temperature coefficient of resonant frequency (τ_f) of the ceramics decreased, and the Q×f values of the ceramics obviously increased when z ≤ 1.5. Excellent microwave dielectric properties of ε_r = 72.2, Q×f = 16,480 GHz, and τ_f = +14.3 ppm/℃ 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.

Polymer–ceramic composites were prepared by twin screw melt extrusion with high-density polyethylene (HDPE) as the matrix and polystyrene-coated BaO–Nd₂O₃–TiO₂ (BNT) ceramics as the filling material. Interestingly, the incorporation of polystyrene (PS) by the coating route could significantly improve the thermal behaviors of the composites (HDPE–PS/BNT), besides the temperature stability of dielectric properties and thermal displacement. The microwave dielectric properties of the composites were investigated systematically. The results indicated that, as the volume fraction of BNT ceramic particles increased from 10 to 50 vol% in the composites, the dielectric constant increased from 3.54 (9.23 GHz) to 13.14 (7.20 GHz), which can be beneficial for the miniaturization of microwave devices; the dielectric loss tangent was relatively low (0.0003– 0.0012); more importantly, the ratio of PS to HDPE increased accordingly, making the composite containing 50 vol% BNT ceramics have a low value of temperature coefficient of resonant frequency ( ${\tau }_f$= −11.2 ppm/℃) from −20 to 60 ℃. The GPS microstrip antennas were therefore designed and prepared from the HDPE–PS/BNT composites. They possessed good thermal stability ( ${\tau }_f$= 23.6 ppm/℃) over a temperature range of −20 to 60 ℃, promising to meet the requirements of practical antenna applications.