Preparation and defect control of sixth-order Mg2SiO4 microwave dielectric ceramic filters based on the hydantoin epoxy resin gel casting

Abstract

Microwave dielectric ceramics have extensive applications in communication. Filters are critical components in microwave communication. The fabrication of high-order complex-shaped filters still faces considerable difficulties. This study utilized an innovative and environmentally friendly gel system (hydantoin epoxy resin, HER system) to prepare a sixth-order waveguide filter. Mg₂SiO₄–MgTiO₃ powder was used as the dielectric material. This powder had a dielectric constant ε_r of 9.8, a Q × f value of 59 000 GHz, and a τ_f of ±11 ppm °C⁻¹. A ceramic slurry with a solid content of 78 wt% was prepared, which contained 8 wt% HER and 1.5 wt% polyacrylic acid dispersant. Rheological property analysis confirmed the shear thinning behavior with a low viscosity (less than 1 Pa s at 100 s⁻¹), indicating excellent fluidity and suitability for casting. The gelation kinetics revealed the formation of a stable three-dimensional network structure. Thermogravimetric analysis was used to determine the optimal debinding schedule. A sixth-order waveguide filter was designed, with a topological structure featuring cross-coupling, achieving simulated performance. The center frequency was 4.0 GHz, the bandwidth was 200 MHz, the insertion loss was less than 0.5 dB, and the return loss exceeded 13 dB. To address cracking and deformation defects in large-size green bodies, the curing conditions were optimized to 40 °C and 80% relative humidity. This optimized condition effectively reduced the internal temperature gradient of the green body. It accelerated the formation of the network structure and prevented particle precipitation. Moreover, after sintering at 1360 °C, a complete component structure was formed. Compared with the traditional dry pressing process, this HER-based gel casting simplified the processing flow. It has the advantages of low cost and excellent near-net-shape preparation capability for large components with complex shapes. This method showed strong application potential in next-generation 5G/6G microwave equipment.