Effects of Cu²⁺ Substitution and LiF Additive on Low-Temperature Sintering and Microwave Performance of CoTi_0.2Zr_0.8Nb₂O₈ Ceramics

A²⁺B⁴⁺Nb₂O₈-type ceramics (where A = Co, Ni, Zn and B = Ti, Zr) are critical materials for satellite communication and mobile base station applications due to their moderate relative permittivity (ε_r) and high-quality factor (Q × f). Among these, CoTi_0.2Zr_0.8Nb₂O₈ ceramic demonstrates superior microwave dielectric properties (i.e., ε_r of 26.5, Q × f value of 76921 GHz, and τ_f of –15×10^–6/ ℃). However, its high sintering temperature (1350 ℃) has a major challenge for practical implementation in low-temperature co-fired ceramic (LTCC) systems, where lower processing temperatures are essential for compatibility with silver electrodes and multilayer integration. This study was to prepare Cu_xCo_1–xTi_0.2Zr_0.8Nb₂O₈ (0.0≤x≤0.3) ceramic system via the partial substitution of Co²⁺ ions with Cu²⁺ ions and LiF flux doping. The optimal Cu²⁺ substitution level was determined via the systematic analysis of the quality factor (Q × f) of ceramics with varying compositions. LiF was introduced as a sintering aid, effectively reducing the sintering temperature of Cu_0.2Co_0.8Ti_0.2Zr_0.8Nb₂O₈ ceramic. The results demonstrated that Cu_0.2Co_0.8Ti_0.2Zr_0.8Nb₂O₈ ceramic with 30% LiF additive, sintered at 950 ℃ exhibited a superior compatibility with silver electrodes and achieved optimal microwave dielectric properties (i.e., a relative permittivity (ε_r) of 30, a high Q × f value of 44,571 GHz, and a temperature coefficient of resonant frequency (τ_f) of –42.0×10^–6/℃).

The raw materials were precisely weighed according to stoichiometric ratios and subjected to ball milling in a horizontal ball mill with zirconia balls with ethanol as a dispersant for 24 h. The resulting slurry was dried at 85 ℃ for 4 h and calcined at 900 ℃ for 3 h. Afterwards, the powders were further ground in the mill under identical conditions for 24 h to ensure the material homogeneity. After subsequent drying at 85 ℃ for 4 h and sieving through a 40-mesh screen, the finer powders were mixed with 5% PVA as a binder in an agate mortar, passed through a 200-mesh sieve, and uniaxially pressed into green pellets with 10 mm diameter × 5 mm height. These pellets were first debindered at 600 ℃ for 2 h before being sintered between 1200–1300 ℃ for 4 h. For low-temperature sintering optimization, LiF at different concentrations (i.e., 0–40.0%, 99.9% purity, Macklin Biochemical Co.) was introduced during secondary milling, and the modified samples were prepared under the same processing condictions but sintered at a reduced temperature of 950 ℃ for 4 h.

This study obtains a low-temperature sintering and performance optimization of Cu_xCo_1–xTi_0.2Zr_0.8Nb₂O₈ (0.0≤x≤0.3) ceramics via a strategy of partial Cu²⁺ substitution for Co²⁺ and LiF flux doping. The XRD patterns confirm the effective solid solution of Cu²⁺ without secondary phase formation, though the lattice contraction appears due to the smaller ionic radius of Cu²⁺ (i.e., 0.73 Å), compared to Co²⁺ (i.e., 0.74 Å). The sample with x of 0.2 demonstrates an optimal comprehensive performance.

For the introduction of LiF as a flux agent, the sintering temperature of Cu_0.2Co_0.8Ti_0.2Zr_0.8Nb₂O₈ can be reduced from 1200 ℃ to 950 ℃. The Rietveld refinement indicates the formation of a (Li, Cu)NbO₃ secondary phase, and the content increases at a higher LiF doping concentration (i.e., 10%–40%). The SEM images show that the 30% LiF-doped sample has the most homogeneous microstructure (grain size: 0.52 ± 0.12 μm) and a high relative density, leading to an optimal microwave property (i.e., ε_r = 30, Q × f = 44,571 GHz, and τ_f = –42.0×10^–6/℃). The results of Co-firing experiments confirm an excellent chemical compatibility between the 30% LiF-doped ceramic and silver electrodes at 950 ℃ without any interfacial reactions.

This study prepared Cu_xCo_1–xTi_0.2Zr_0.8Nb₂O₈ (0.0≤x≤0.3) ceramic system via the partial substitution of Co²⁺ ions with Cu²⁺ ions and LiF flux doping, enabling dense sintering at 950 ℃. The optimal 30% LiF-doped Cu_0.2Co_0.8Ti_0.2Zr_0.8Nb₂O₈ ceramic sintered at 950 ℃ for 4 h achieved an exceptional performance (i.e., ε_r = 30, Q × f = 44,571 GHz, and τ_f = –42.0×10^–6/℃) with an excellent Ag electrode compatibility, demonstrating a promising potential for microwave dielectric applications.