Preparation And Dielectric Properties Of NaNbO₃-Li_0.5Sm_0.5TiO₃Microwave Dielectric Ceramics

The rapid development of mobile communication technology made an increasing demand for microwave dielectric ceramics which have been used in the fields of filters and resonators. The miniaturization of devices require a combination of high relative permittivity (εr), low dielectric loss in the high frequency region,(i.e. high quality factor, Qf) and near zero temperature coefficient of resonant frequency (τf). NaNbO3is an orthorhombic perovskite with εr=290and a positive temperature coefficient, while Li0.5Sm0.5TiO3is a perovskite with εr=52and a negative temperature coefficient of resonant frequency. Combining NaNbO3with Li0.5Sm0.5TiO3could lead to a near zero temperature coefficient of resonant frequency and relatively high relative permittivity.Based on the relationship among the composition, microstructure and dielectric properties, the microwave dielectric ceramics were fabricated via traditional solid state method and analyzed using XRD, SEM and network analyzer. The main contents were exhibited as following:Firstly, xNaNb03-(1-x)Li0.5Sm0.5TiO3(NN-LST) microwave dielectric ceramics were prepared, and the effects of calcination and sintering process on the crystallinity and the microwave dielectric properties were investigated. The results showed that orthorhombic perovskite as a main phase were synthesized after calcining at1050℃for2h, and the most dense ceramics with x=0.2were obtained after sintering at1200℃for2h in the air. With the optimized processing, xNaNbO3-(1-x)Li0.5Sm0.5Ti03ceramics were prepared. It was clear that as the content of NaNbO3increased relative permittivity and temperature coefficient of resonant frequency grew up while the quality factor decreased. When x=0.2, the microwave dielectric properties of ceramics were:εr=110, Qf=3220GHz, τf=4.3ppm/℃.Secondly, the effects of doping neodymium oxide on the microstructure and the dielectric properties of0.2NaNb03-0.8Lio.5(Sm1-xNdx)0.5TiO3(x=0,0.1,0.2,0.3,0.4) were analyzed. Compared with the properties of the ceramics without doping neodymium oxide, the relative permittivity increased from112to121, the quality factor decreased from3010to2100, and the temperature coefficient of resonant frequency changed from-22to4ppm/℃(measured at200KHz). When x=0.3, the microwave dielectric properties of ceramics sintered at1200℃for2h were:εr=118, Qf=2640GHz, τf=-2ppm/℃(measured at200KHz)Finally, as a kind of sintering aid, different content of Bi2O3was added in0.2NN-LST. It could be seen that a small amount of Bi2O3addition had not changed the crystalline structure, but when the content of Bi2O3reached3wt%Sm2Ti2O7pyrochlore were detected as a second phase in the XRD patterns. With the appropriate content of Bi2O3(2wt%), the sintering temperature was decreased from1200℃to1180℃, and the microwave properties of the ceramics were:εr=108, Qf=2140GHz, τf=-43ppm/℃(200KHz).