The effects of sintering aids upon dielectric microwave properties of columbite niobates, M²⁺Nb₂O₆

The columbite niobate ceramics ZnNb₂O₆, MgNb₂O₆, CaNb₂O₆ and CoNb₂O₆ have low dielectric losses at microwave (1–10 GHz) frequencies, resulting in Qf values between 40 000 and 90 000 GHz, making them suitable materials for use in dielectric resonator applications. However, their temperature coefficient of resonant frequency (τ_f) values were high, at between −50 and −90 ppm, and the optimum sintering temperatures were found to be between 1150°C and 1350°C. This paper details the doping of these ceramics with 1 wt%V₂O₅, 1 wt% CeO₂, 2 wt% WO₃ and 0.5 wt% CuO, in an attempt to reduce the sintering temperature. It was found that in many cases the dopants also had an extremely beneficial effect upon microwave properties, increasing ε_r, and decreasing τ_f considerably. Although the dopants often had a deleterious effect upon the quality factor (Q), in some cases they caused an increase in Q. Qf values of over 20 000 GHz were often obtained at lower temperatures, even in poorly sintered niobates, and CuO-doped CaNb₂O₆ yielded Qf values in excess of 65 000 GHz. CoNb₂O₆+V₂O₅ or CuO gave 90% sintered and poorly sintered materials with Qf over 10 000 GHz and 25 000 GHz, respectively, at temperatures within low temperature Co-fired ceramic limits.