Effects of barium substitution on the sintering behavior,dielectric properties of Ca_(2)Nb_(2)O_(7) ferroelectric ceramics

Barium-doped Ca_(2)Nb_(2)O_(7)ceramics were prepared in the form of Ca_(2-x)Ba_(x)Nb_(2)O_(7)(0≤x≤0.6)by solid-state reaction.The solubility limit of barium in Ca_(2)Nb_(2)O_(7)was found to be x=0.4 based on X-ray diffraction and Raman spectroscopy analysis.When x≤0.4,Ca_(2-x)Ba_(x)Nb_(2)O_(7)solid solutions with a monoclinic perovskites-like layered structure(PLS)were formed,whereas beyond x=0.4,a secondary phase CaNb_(2)O_(6) was detected.The dielectric properties obviously depended on the barium substitution.With increasing barium amount,the dielectric constant increased from 33.5 for pure Ca_(2)Nb_(2)O_(7)to 38.6 for x=0.4(f=1 MHz).The Curie temperature(T_(c)) of the x=0.1 sample was 1280±5℃.The phase transition was confirmed to be the second order.

Preparation, crystal structure, and dielectric characterization of Li_(2)W_(2)O_(7) ceramic at RF and microwave frequency range

Single phase Li_(2)W_(2)O_(7) with anorthic structure was prepared by the conventional solid-state reaction method at 550℃ and the anorthic structure was stable up to 660℃.The dielectric properties at radio frequency(RF)and microwave frequency range were characterized.The sample sintered at 640℃ exhibited the optimum microwave dielectric properties with a relative permittivity of 12.2,a quality factor value of 17,700 GHz(at 9.8 GHz),and a temperature coefficient of the resonant frequency of232 ppm/℃ as well as a high relative density~94.1%.Chemical compatibility measurement indicated Li_(2)W_(2)O_(7) did not react with aluminum electrodes when sintered at 640℃ for 4 h.