Microwave dielectric properties, vibrational spectrum, and antenna design of a novel melilite-type Ba₂CoSi₂O₇ ceramic

Low-ε_r (the low dielectric constant) microwave dielectric ceramics play an important role in sub-6 GHz band communication by virtue of their small signal delay. In this paper, Ba₂CoSi₂O₇ ceramics were synthesized by the solid-phase reaction method, and X-ray diffraction (XRD) confirms that they belong to a new melilite structure, with C2/c (No.15) space group, and the ceramics have good microwave dielectric properties with an ε_r ∼ 7.37, a Q×f (Q = 1/dielectric loss, f = resonant frequency) ∼ 26,320 GHz and a TCF (the temperature coefficient of resonant frequency) ∼ −55 ppm/°C after being sintered at 1150°C, and the theoretical dielectric constants derived from the Clausius–Mosotti as well as the porosity equation, respectively, are also used to further demonstrate the low-ε_r properties. Based on the group theory, the one-dimensional integrable representations of Γ_acoustic (the phonon modes) and Γ_optical (optical modes) of Ba₂CoSi₂O₇ are obtained, and the internal vibrational modes of Ba₂CoSi₂O₇ are deconstructed and the main modes of influence of the electronic polarization are analyzed in conjunction with the results of the validly fitted peaks of the 17 Raman and 25 IR tests. A microstrip Yagi antenna is also designed based on the ceramic properties, which can achieve a gain of 9.65 dBi at the center tuning frequency of 5.47 GHz, with a S₁₁ (return loss) close to −60 dBi. Based on the above results, the Ba₂CoSi₂O₇ ceramic is a low-ε_r microwave dielectric ceramic with significant potential for communication application.