Lei Li

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 B ‐site ordered Ba‐based complex perovskites represent a significant class of medium permittivity and low‐loss microwave dielectric ceramics, demonstrating extensive applications in microwave communication fields. Single‐phase Ba _{1− x} Sr _x (Mg _1/3 Nb _2/3 )O ₃ ( x  = 0 ∼ 1) ceramics were designed and synthesized using the standard solid‐state sintering method to evaluate the co‐effects of ordered domain structure and octahedral tilting on microwave dielectric properties in complex perovskites. A comprehensive investigation was conducted to elucidate the composition‐dependent evolution of crystal structures, ordered domain structures, and octahedra tilting. B ‐site 1:2 ordering was observed in all the compositions, while the highest ordering degree S = 0.98 and maximum Qf value of 47 400 GHz were achieved in Ba _0.75 Sr _0.25 (Mg _1/3 Nb _2/3 )O ₃ . With increasing Sr, the crystal structure transforms from a triangle P 3 ̅ m 1 phase to a space group of P2 ₁ /c with both antiphase and in‐phase tilting, while a near‐zero τ _f of −7.8 ppm/°C was obtained at x  = 0.7. The variation of τ _f followed the empirical law well with the tolerance factor. Moreover, by annealing at 1350°C for 12 h, τ _f shifted closer to zero ( τ _f = −3 ppm/°C) with improved Qf value, stable ε _r , and increased S value in ceramics with x  = 0.7. This systematic study revealed the underlying mechanisms governing the influence of these structural parameters on the microwave dielectric performance, and provided useful guidance for collaborative optimization of microwave dielectric properties.

Introducing pores is an effective approach for fabricating ultralow-ε r ceramics; however, it is still unclear how the microstructures affect the microwave dielectric properties. In the present work, Al2O3-A and Al2O3-B porous ceramics were prepared by incomplete sintering at 1,000-1,650 °C and sintering at 1,650 °C with a porogen, respectively, so that the effects of porosity and ceramic connectivity can be clarified. The introduction of pores led to a significant decrease in ε r, Qf , and |τf| , and Al2O3-B ceramic exhibits a slightly higher ε r, a larger |τf| , and a much higher Qf value compared to the Al2O3-A counterpart with a similar relative density. For Al2O3-A, increasing the sintering temperature and relative density notably enhanced ceramic connectivity, while Al2O3-B's ceramic connectivity remained relatively insensitive to density variations. The improved ceramic connectivity in Al2O3-B enhanced the contribution of the ceramic phase to ε r and τf , while also increased the Qf value by reducing ceramic-pore interfaces. Notably, Al2O3-B, with a low relative density of 48.31%, demonstrated a good combination of microwave dielectric properties, with ε r = 4.16, Qf = 38,400 GHz, and τf = -44.3 ppm/°C. These findings reveal the strong dependence of microwave dielectric properties on ceramic connectivity in porous ceramics, and also inspire the development of ultralow-ε r materials by regulating both the porosity and ceramic connectivity.