Influence of CeO₂ on microstructure and microwave dielectric properties of Na_1/2Sm_1/2TiO₃ ceramics

Crystal structures and microwave dielectric properties of wt% CeO₂-added Na_1/2Sm_1/2TiO₃ (NST) ceramics were investigated systematically (x = 0.0, 0.3, 0.6, 0.9 and 1.2). Samples with CeO₂ content ≤1.2 wt% were crystallized as single phase of orthorhombic perovskite structures. When Ce⁴⁺ initially entered into the A-site of perovskite structure, uniform grain growth was observed and the dielectric constant increased, but then as Ce⁴⁺ began to enter into the B-site of the perovskite structure, abnormal grains appeared and the ε _r value reduced. The Q × f value was firstly enhanced to its maxim value of 9640 GHz due to the improvement of density and elimination of pores as Ce⁴⁺ content was in the range of 0.0–0.6 wt%, then sharply decreased. The temperature coefficient of resonant frequency (τ _f ) slightly decreased from 199.7 to 185.1 ppm/°C. Typically, specimens sintered in air at 1425 °C for 2 h exhibited excellent microwave dielectric properties of ε _r = 103.01, Q × f = 9640 GHz and τ _f  = 192.8 ppm/°C. The relationships between microwave properties and crystal structures were also discussed in this paper.