The advancement of millimeter-wave communication desires the ceramic substrate with low permittivity ( ԑ r ) to meet the requirements of high transmission rates, low latency and wide bandwidth. However, the thermal conductivity of most low- ԑ r ceramics is too low to deal with heat dissipation in millimeter-wave applications. In this paper, we reported novel dielectric ceramics Li M SiO 4 ( M = Ga, Sc and Y) with excellent performances of low ԑ r (< 10) and high thermal conductivity (> 6 W mK −1 ). Their dielectric properties in both microwave and THz were investigated, where the LiGaSiO 4 ceramic achieved the lowest ԑ r of ∼5.2, the LiScSiO 4 ceramic presented extremely low loss ( Q × f ∼ 96,700 GHz, Q = 1/dielectric loss, f is resonant frequency), and the LiYSiO 4 ceramic showed a positive temperature coefficient of f (TCF ∼ +32 ppm °C −1 ). The distinct dielectric behavior was subsequently studied by structure-performance relationship in terms of M -site cations and bond parameters using bond valence theory, Phillips–Van Vechten–Levine chemical bond theory and so on. Moreover, a 36 GHz microstrip array antenna was designed and simulated using the LiGaSiO 4 ceramic substrate, obtaining high realized gain, high radiation efficiency and low sidelobe. The result demonstrated the great potential of LiMSiO 4 -type dielectric ceramics in millimeter-wave communications.
Orthorhombic olivine-structured LiMSiO 4 (M = Ho, In) ceramics were synthesised using the solid-state reaction. Promising microwave dielectric properties of ε r = 10.04, τ f = 23.3 ppm/°C and a high-quality factor ( Q × f ) of 11,415 GHz for LiHoSiO 4 were obtained at 1050 °C, and ε r = 7.78, τ f = −46.8 ppm/°C and Q × f = 23,390 GHz for LiInSiO 4 were achieved at 1140 °C. According to the bond valence result, Ho 3+ presented a slightly compressed state but Li + and Si 4+ exhibited a strong rattling state in LiHoSiO 4 , which led to higher ε r than the ε theo . The integrated effect of rattling cations (Li + and In 3+ ) and a compressed cation (Si 4+ ) in LiInSiO 4 resulted in a small positive deviation Δ ε r . The larger packing fraction of LiInSiO 4 contributed to a higher Q × f than LiHoSiO 4 . The Si–O bond in both ceramics played a leading role in influencing the Q × f according to the Phillips−Van Vechten−Levine chemical bond theory.
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