Low sintering temperature, interface characteristic and microwave/terahertz dielectric properties of ternary-phase Nd2O3-P2O5 based ceramics for patch antenna application

In this work, LiF was chosen as a sintering aid to lower the sintering temperature required for monoclinic NdPO₄ (NPO) ceramic. A comprehensive investigation was conducted to analyze the impact of LiF on the sintering characteristics, microstructures, phase compositions, crystalline structures, interface characteristics, and microwave dielectric properties of NPO ceramic. Structure-property relationships were discussed based on the density, crystallite size, and lattice strain. The complex impedance spectroscopy revealed the mechanism underlying the interface characteristics of LiF on NPO ceramic. Additionally, far-infrared reflectance spectra and THz time-domain spectra revealed the intrinsic dielectric properties of NdPO₄–1 wt.%LiF (NPO1). Notably, 1 wt.% LiF effectively reduced the sintering temperature from 1300 °C to 850 °C, significantly improving densification and dielectric properties. Typical properties such as Q × f = 55,840 GHz (at 9.58 GHz), ε_r = 10.4, and τ_f = −43.1 ppm °C⁻¹ were obtained for NPO1 ceramic after sintering at 850 °C. Furthermore, its compatibility with Ag was explored as a prerequisite for LTCC applications. Finally, the NPO1 ceramic was designed as a microstrip antenna with high gain (6.83 dB) and low return loss (−18.85 dB) at a centre frequency of 2.56 GHz, demonstrating its potential for Wi-Fi and Bluetooth applications. This work provides a theoretical foundation and practical guidance for developing orthophosphate ceramics.