Effects of W,Ti Co-Doping on the Microstructure and Microwave Dielectric Properties of Ba5Nb4O15 Ceramics

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W,Ti co-doped Ba5Nb4O15 microwave dielectric ceramics with compositions of Ba5Nb4-x(W1/2Ti1/2)xO15 ($x =0--2$) were prepared by solid-state reaction method. W6+ and Ti4+ substitution for Nb5+ can markedly lower the sintering temperature of Ba5Nb4O15 from 1450 to 1250 °C. X-ray powder diffraction (XRD) analysis reveals that BaWO4 formed as a secondary phase due to the limited solubility of WO3 in perovskite Ba5Nb4O15 when $x>0.4$. Scanning electron microscopy (SEM) observation suggests that a small amount of W and Ti co-doping can suppress the grain growth of Ba5Nb4O15 ceramics, indicating that the grain size decreases from ${\sim}25$ to ${\sim}3$ μm. The microwave dielectric properties of Ba5Nb4O15 ceramics is largely improved by W,Ti co-doping. With increasing $x$, the $Q\times f$ value increases from 25,200 GHz for undoped samples ($x=0$) to 38,600 GHz for $x=0.4$, and then declines to 19,500 GHz for $x=1.2$. In the composition range $x=0--1.2$, the dielectric constant increases slightly from 39.2 to 44, while the temperature coefficient decreases from ${\sim}78$ to ${\sim}50$ ppm/°C. The reasons for the improvement of the dielectric properties were investigated by SEM, XRD, and AC impedance spectrum analysis.
2008-06-25