ULTCC-Compatible LiPO ₃ Ceramic with Ultralow Permittivity for 5G High-Gain Antenna Array Applications

In contemporary radio frequency (RF) systems, organic substrate materials are extensively utilized due to their low permittivity. However, their low thermal conductivity (κ ≈ 0.25 W/(m·K)) can lead to catastrophic thermal management issues in RF systems. This work proposes a LiPO₃ ceramic substrate, which demonstrates ultralow permittivity (ε_r ≈ 4) and exceptional quality factors (tan<i>δ</i> at the 10^-4 level). However, the temperature coefficient of resonant frequency for the LiPO₃ ceramic is a large negative value (TCF ≈ -110 ppm/°C), which requires further adjustment. The ε_r of LiPO₃ is comparable to that of organic laminates, which helps reduce signal delay, and its high quality factor gives it excellent frequency selectivity. The low ionic polarizability of Li⁺ and P⁵⁺ in combination with the unique (PO₃)_<i>n</i> chain structure of the phosphate groups in LiPO₃ is intrinsically responsible for its low permittivity. In addition, the κ of LiPO₃ exceeding 2 W/(m·K) contributes to extending the lifespan of the RF components. Furthermore, LiPO₃ has a sintering temperature of only 630 °C and does not undergo chemical reactions or physical penetration with aluminum. This suggests its potential in the ultralow temperature cofired ceramics (ULTCC) field. The potential of LiPO₃ was validated through an antenna array application in the 5G network. This work effectively resolves the conflict between signal delay and thermal management in high-integration RF systems.