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 low dielectric constant (ε_r < 15) is the key to improving the signal transmission speed of microwave communication devices. However, the resonant frequency temperature coefficient (τ_f) of most low-ε_r microwave dielectric ceramics is usually negative. Aiming to modify the large negative τ_f of scheelite CaWO₄ and explore the underlying mechanism between the structure and microwave dielectric properties, a series of Ca_1–x(Li_1/2Eu_1/2)_xWO₄ (x = 0.1−1.0) (CLEWO_x) ceramics were prepared at low sintering temperatures (750−875 ℃). The ε_r increased from 10.46 to 18.55, and the Q× f decreased from 39,032 GHz–7425 GHz, mainly due to the enhanced rattling effect of Li⁺. The τ_f rapidly increased from negative (−19.91 × 10⁻⁶ ℃⁻¹) to abnormally positive (+162.15 × 10⁻⁶ ℃⁻¹), influenced by the reduced temperature coefficient of ion polarizability (τ_αm) caused by the rattling Li ⁺ cation. The CLEWO_0.15 sample has good comprehensive performance (ε_r = 12.28, Q×f = 28,027 GHz, and τ_f = −0.5 × 10⁻⁶ ℃⁻¹) and compatibility with the Ag electrode, showing the potential of LTCC applications. Additionally, a dielectric resonator antenna based on CLEWO_0.15 ceramic was designed with a bandwidth of 254 MHz at 4.504−4.758 GHz and a gain of 4.87 dBi at 4.62 GHz, indicating that CLEWO_0.15 may be a potential candidate for dielectric resonator antennas.

Dielectric ceramics with low permittivity (ε_r), high quality factor (Q×f), and near-zero resonant frequency (τ_f) in the microwave bands are key materials used in fifth/sixth-generation (5G/6G) telecommunication, whileτ_f of most low-ε_r microwave dielectric ceramics is relatively negative. In this work, the first low-ε_r Ga-based ceramic SrGa₁₂O₁₉ with an anomalous positive τ_f was reported, and the causes of the positive τ_f, intrinsic polarization, and loss mechanism were systematically studied. X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that the SrGa₁₂O₁₉ ceramic formed a pure hexagonal magnetoplumbite structure with spinel blocks and rock-salt blocks stacked along the crystallographic c-axis. When sintered at 1430 °C, it possessed the optimal microwave dielectric properties of a low ε_r of 14.46, high Q×f of 64,705 GHz, and exceptional positive τ_f of +55.7 ppm/°C, along with a low linear thermal expansion coefficient (α_L) of 11.617 ppm/°C. The large positive deviation between ε_r and ε_r(C–M) of 45.31% resulted from the rattling effect of atoms in the rock-salt block. The unique positive τ_f (+55.7 ppm/°C) was governed by the rattling effect, resulting in a positive τ_αm (the temperature coefficient of ion polarizability) of 8.489 ppm/°C and a large negative temperature coefficient of permittivity (τ_ε) of −132.864 ppm/°C. Phillips–Vechten–Levine (P–V–L) chemical bond theory revealed greater contributions of the spinel block to bond ionicity (f_i, 52.95%), permittivity (ε, 55.15%), bond energy (E, 56.87%), and lattice energy (U, 74.88%) than those of the rock-salt block. The intrinsic dielectric properties were analyzed using infrared (IR) reflectivity spectra. The favorable performance of the SrGa₁₂O₁₉ ceramic indicated that it is a novel τ_f compensator. This selection of compounds with different structural layer combinations provides a new idea for exploring excellent microwave dielectric ceramics.