Influence of sintering additives on dielectric microwave properties of columbite niobates

Additions of 1 wt.% V$_2$O$_5$, 1 wt.% CeO$_2$, 2 wt.% WO$_3$ and 0.5 wt.% CuO had extremely variable and unpredictable effects on the sintering and dielectric loss of columbite niobates. In almost all cases, $\varepsilon_r$ was increased and $\tau_f$ was significantly decreased, and the maximum Q was also generally decreased. However, goodness-of-fit values above 20 000 GHz were often obtained at lower temperatures in poorly sintered niobates. In CaNb$_2$O$_6$ + CuO, V$_2$O$_5$ or CeO$_2$ and CoNb$_2$O$_6$ + CuO or WO$_3$ significantly increased goodness of fit was obtained at lower temperatures, in the case of CaNb$_2$O$_6$ + CuO significantly higher than that of pure columbite. CaNb$_2$O$_6$ + V$_2$O$_5$ or CuO gave 90% sintered or weakly sintered materials with Qf > 10 000 or 25 000 GHz.