Effects of silver doping on the sol-gel-derived Ba4(Nd0.7Sm0.3)9.33Ti18O54 microwave dielectric ceramics

Tungstenbronze-type Ba-4(Nd0.7Sm0.3)(9.33)Ti18O54 (BNST) microwave dielectric ceramics doped with 0-10 wt% silver (Ag) particles were successfully fabricated by a citrate sol-gel method. The influence of Ag doping on the sinterability, microstructure, bulk conductivity, and dielectric properties of BNST was investigated. The desired tungstenbronze-type phase was obtained at 900 degrees-950 degrees C. The sintering temperature of BNST decreased to 1100 degrees C with the aid of a small amount of Ag addition (1 wt%). No chemical reaction between the tungsenbronze phase and Ag was detected. The particle size of the powders decreased with increasing Ag content up to 1 wt% and it then increased with a further increase in the Ag content. The dense fine-grained ceramics with submicrometer grains (similar to 300 nm) were obtained with 1 wt% Ag addition. The submicrometer-grained ceramics had excellent dielectric properties of epsilon(r) similar to 81 and Q x f similar to 11 000 GHz. Both the dielectric constant and dielectric loss significantly increased with large additions (> 3 wt%) of Ag due to the percolation effect.