Microwave dielectric ceramics (MWDCs) are pivotal to modern wireless communication systems, with their performance governed by three key parameters: relative dielectric constant (ε_r), Q×f value (product of quality factor Q (reciprocal dielectric loss) and frequency f), and temperature coefficient of resonant frequency (τ_f). This review systematically summarizes the recent research progress of MWDCs from five interrelated aspects. In terms of performance characterization, standardized resonant methods achieve ε_r measurement errors below 1% and a tanδ detection limit as low as 10^-5. Theoretically, frameworks from complex crystal chemistry to the recently elucidated cation rattling effect enable quantitative interpretation of dielectric behavior. In processing, the cold sintering process achieves ceramic densification below 300 °C, reducing energy consumption by over 97% in comparison with conventional sintering. For applications, these materials have been widely deployed in high-performance substrates, resonators, and filters for 5G/6G communications, with device insertion loss maintained below 1 dB. Additionally, data-driven approaches, particularly machine learning, can accurately predict key dielectric properties with a coefficient of determination (R²) higher than 0.9, accelerating the exploration and development of novel MWDCs. By integrating these perspectives, this review offers a systematic insight into the state-of-the-art progress and future development directions of MWDCs research.

The advancement of communication technologies demands dielectric materials with superior performance characteristics, particularly low permittivity and minimal dielectric loss. This study investigates Ge⁴⁺-substituted willemite ceramics, including Zn₂Si_1–xGe_xO₄ (ZS-xGe, x = 0 and 0.1) and Zn_1.8Si_1–yGe_yO_3.8 (ZS-yGe, y = 0 to 0.3), synthesized via the conventional solid-state method. The non-stoichiometric design effectively suppresses the formation of ZnO. The intrinsic and extrinsic losses of the ZS-xGe ceramics are separated by a systematic comparative analysis of the dielectric losses in the terahertz band, and the extrinsic losses are fitted by the Drude term in the Lorentz-Drude dielectric response model. Consequently, ZS-yGe ceramics exhibit lower ε_r and significantly improved Q×f values across microwave to terahertz band. In ZnO-free ceramics, Ge⁴⁺ substitution enhances the ionic polarizability, the unit cell volume and the bond strain, increasing ε_r and Q×f values (decreasing intrinsic losses), and decreasing τ_f. The optimized Zn_1.8Si_0.9Ge_0.1O_3.8 ceramics demonstrate superior dielectric properties with ε_r = 6.66, Q×f = 225,500 GHz and τ_f = −60.0 × 10⁻⁶ °C⁻¹ at 12.45 GHz, and ε_r = 7.02, Q×f = 401,800 GHz at 1 THz. These novel ceramics are positioned as promising candidates for next-generation microwave and terahertz communication devices.

The rapid development of fifth-/sixth-generation telecommunication technologies has increased the demand for silicate ceramic materials with low permittivity and low dielectric loss. However, few silicate ceramics with ultrahigh Q×f values (≥ 200,000 GHz) have been developed to date. In this study, a slight substitution of Ge⁴⁺ ions in MgSi_1−xGe_xO₃ (MSGx, x = 0 to 0.6) ceramics caused a phase transition from clinoenstatite (x = 0) to orthoenstatite (x = 0.2), and the Q×f value increased from 70,600 GHz to 148,800 GHz. Following the phase transition, the cations change from a “compressed” state to a “rattle” state, and the lattice distortion continues to rise with x, resulting in the optimal microwave dielectric properties (ε_r = 7.21, Q×f  = 259,300 GHz) of the MgSi_0.5Ge_0.5O₃ ceramics. Significant discrepancies in the dielectric properties are found in the microwave and terahertz bands. There is an anomalous increase in ε_r and a decrease in the Q×f value in the terahertz band, which is due to the change in polar phonon modes revealed by the terahertz time-domain spectra. Consequently, MgSi_0.7Ge_0.3O₃ ceramics display superior dielectric properties, with ε_r = 7.02, Q×f  = 191,300 GHz in the terahertz band. These novel materials have the potential to serve as promising dielectric materials for future microwave or terahertz mobile communication systems.

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.