A Series of Ultra-low Permittivity ALaP₄O₁₂ (A = Li, Na, K) Metaphosphate Microwave Dielectric Ceramics for Ultra-wideband Dielectric Resonant Antenna Application

The employment of ultra-low permittivity materials in the configuration of antennas has been demonstrated to augment the antenna bandwidth and diminish signal delay effectively. This study presents three ultra-low permittivity metaphosphate microwave dielectric ceramics (MWDCs). The ALaP₄O₁₂ (A = Li, Na, K) metaphosphate ceramics, which all belong to the monoclinic crystal system, exhibit extremely low permittivity (ϵ_r ≈ 5) and excellent quality factor (Q·f > 10,000 GHz) at a low sintering temperature (T < 950 °C). Terahertz time-domain spectroscopy indicates that the ϵ_r of ALaP₄O₁₂ at terahertz frequencies is comparable to that observed in the microwave band and its value remains stable over an extensive frequency range. Furthermore, the relationship between the crystal structure and the dielectric properties of ALaP₄O₁₂ has been analyzed through the lens of chemical bond theory. The highest Q·f value observed for LiLaP₄O₁₂ can be attributed to the high chemical bond strength and stability of its crystal structure. The lowest ionic polarizability per unit volume is exhibited by NaLaP₄O₁₂, which results in the lowest pore-corrected permittivity. The thermal expansion of the chemical bonds within KLaP₄O₁₂ is considerable, resulting in the highest coefficient of thermal expansion. Finally, the performance of a LiLaP₄O₁₂-based dielectric resonator antenna (DRA) excited by slot-coupled microstrip lines was designed and optimized by using different ceramic radius-to-height (RH) ratios. It was found that when the RH ratio of DRA reached 1.85, both the fundamental mode (HEM_11δ) and the higher-order mode (HEM_12δ) of DRA were simultaneously excited. The two modes overlap significantly, resulting in an ultra-wideband (UWB) of 46.8% (bandwidth = 7.93 GHz). Concurrently, the maximum radiation efficiency and gain of the DRA, obtained from the simulation, are 97.9% and 7.92 dBi, respectively. The findings of this study may inform the investigation of ultra-low permittivity phosphorus-based MWDCs and UWB DRAs.