Effect of Co2+ substitution on phase composition, microstructure and microwave dielectric properties of Zn1.8-xCoxSiO3.8 ceramics

Low-permittivity microwave dielectric ceramics with nominal compositions Zn _{1.8- x} Co _x SiO _3.8 ( x = 0-0.12) were synthesized by a solid-state reaction route. The influence of Co ²⁺ substitution on phase evolution, lattice structure, densification behavior, and microwave dielectric performance was systematically evaluated. X-ray diffraction coupled with Rietveld refinement verifies that all specimens in the investigated range preserve a single-phase Zn ₂ SiO ₄ (willemite) solid-solution structure, with no detectable secondary phases. Microstructural observations indicate that a small Co ²⁺ addition facilitates mass transport along grain boundaries and assists pore removal, thereby enhancing densification at relatively low sintering temperatures. The composition with x = 0.04 exhibited the best overall properties, delivering a bulk density of 4.0873 g/cm³ and a relative density of 96.15%, together with ε _r = 6.55, Q × f = 62,559.8 GHz, and τ _f = −45.2 ppm/℃; meanwhile, the optimum sintering temperature was reduced from 1300 ℃ to 1275 ℃. In contrast, further increasing Co content ( x ≥ 0.06) introduces more grain-boundary imperfections and residual porosity, which compromises densification, decreases Q × f , and and weakens the improvement in τ _f . Benefiting from the combination of low permittivity and high-quality factor, the developed Zn ₂ SiO ₄ -based ceramics show potential for high-frequency communication components. These results demonstrate that Co ²⁺ doping offers an effective pathway to achieve low-temperature densification while coordinating dielectric performance in willemite-type low-loss microwave ceramics.