Influence of secondary phases and Ni doping on the dielectric properties of Cu1-xNixMoO4 ceramics sintered at ultra-low temperature

Ultralow-temperature sintered ceramics (ULTCCs) are attracting increasing attention due to their excellent dielectric properties, low processing temperatures, and unique microstructures that enable low dielectric loss in high-frequency applications. In this work, the dielectric behavior of Cu _1-x Ni _x MoO ₄ (CNMO, x = 0.02-0.11) ceramics was systematically investigated, with emphasis on the role of secondary phases. X-ray diffraction revealed that samples with x = 0.02 and 0.08 were dominated by the CuMoO ₄ phase, while Cu ₃ (Mo ₂ O ₉ ) and MoO ₃ appeared at x = 0.05 and 0.11, respectively. These secondary phases, together with changes in densification, significantly modified both dielectric permittivity (ε _r ) and the quality factor (Q×f). SEM and EDS analyses confirmed distinct morphological features between the primary and secondary phases. Dielectric permittivity was primarily governed by shrinkage and relative density, whereas Q×f was strongly influenced by the intrinsic loss characteristics of the secondary phases. The temperature coefficient of resonant frequency (τ _f ) was mainly correlated with bond energy, bond valence, and thermal expansion behavior. Raman analysis further revealed that peak shifts and variations in full width at half maximum (FWHM) of Mo-O vibrations were directly linked to ε _r and dielectric loss. Among all compositions, Cu _0.89 Ni _0.11 MoO ₄ sintered at 650 ℃ exhibited optimal microwave dielectric performance (ε _r = 4.61, Q×f = 42,067 GHz, τ _f = -58.96 ppm/℃). These results demonstrate that CNMO ceramics can achieve ultralow-temperature sintering while maintaining excellent frequency stability and low dielectric loss, underscoring their strong potential for ULTCC and microwave communication applications.