Effect of sintering aids upon dielectric microwave properties of ZnNb 2O6, CoNb2O6, MgNb2O 6 and CaNb2O6 columbite niobates

Of the columbite niobate ceramics with the formula M2+Nb 2O6, several compounds (M2+ = Zn, Mg, Ca and Co) display 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 (τf) values were high, at between -50 to -90 ppm, and the optimum sintering temperatures were found to be 1150°C, 1200°C, 1300°C and 1350°C for M = Co, Zn, Mg and Ca respectively. Doping with relatively large amounts of TiO2 and CaTiO3 was found to reduce τf in most cases, but with a large increase in dielectric losses (decrease in Q). This paper details the doping of these ceramics with sintering aids (<2 wt%) such as V2O5 and 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, decreasing τf considerably and substantially increasing the quality factor (Q).