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

The columbite mobate 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 (tau(f)) values were high, at between -50 and -90 ppm, and the optimum sintering temperatures were found to be between 1150degreesC and 1350degreesC. 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 epsilon(r), and decreasing tau(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.