Microwave Dielectric Ceramics for 5G/6G Wireless Communication: Characterization, Theoretical Modeling, Cold Sintering, Device Integration, and Machine Learning Prediction
Microwave dielectric ceramics (MWDCs) are indispensable for modern wireless communication systems, with performance governed by relative dielectric constant (εr), quality factor (Q×f), and temperature coefficient of resonant frequency (τf). This review systematically examines recent progress across five interrelated domains. Standardized resonant methods achieve εr measurement errors below 1% and a tanδ detection limit of 10^-5. Theoretical frameworks, from complex crystal chemistry to the cation rattling effect, enable quantitative interpretation of dielectric behavior. Cold sintering achieves ceramic densification below 300 °C, reducing energy consumption by over 97% compared to conventional sintering. These materials are deployed in high-performance substrates, resonators, and filters for 5G/6G communications, with device insertion loss maintained below 1 dB. Data-driven approaches, particularly machine learning, predict key dielectric properties with a coefficient of determination (R²) higher than 0.9, accelerating discovery of novel MWDCs. Despite these advances, critical gaps persist: accurate measurement of ultra-low dielectric loss (tanδ < 10^-5) remains challenging due to conductor loss and spurious mode interference; the temperature dependence of τf is often neglected; the role of multiscale defects in anharmonic lattice vibrations is not fully quantified; and the cation rattling effect has not been integrated into classical dielectric theory. Conventional high-temperature sintering (>1000 °C) causes severe energy consumption, volatilization of active elements, and incompatibility with low-melting-point electrodes, hindering co-firing integration for miniaturized devices. This review integrates these perspectives to provide a systematic insight into the state-of-the-art and future directions of MWDC research.