Effect of Bi2O3 and B2O3 additives on the sintering temperature, microstructure, and microwave dielectric properties for Sm(Mg0.5Ti0.5)O3 ceramics

The microwave dielectric properties of Sm(Mg0.5Ti0.5)O3 incorporated with various amount of Bi2O3 and B2O3 additives have been investigated systematically. In this study, both Bi2O3 and B2O3 additives acting as a sintering aid can effectively lower the sintering temperature from 1550 °C to 1300 °C. The ionic radius of Bi3+ for a coordination number of 6 is 0.103 nm, whereas the ionic radius of B3+ is 0.027 nm. Clearly, the ionic radius of Bi3+ is greatly larger than one of B3+, which resulted in the specimens incorporated with Bi2O3 having larger lattice parameters and cell volume than those incorporated with B2O3. The experimental results show that no second phase was observed throughout the entire experiments. Depending on the interfacial tension, the liquid phase may penetrate the grain boundaries completely, in which case the grains will be separated from one another by a thin layer as shown in Sm(Mg0.5Ti0.5)O3 ceramics incorporated with Bi2O3. Whereas, in Sm(Mg0.5Ti0.5)O3 ceramics incorporated with B2O3, the volume fraction of liquid is high, the grains may dissolve into the liquid phase, and rapidly rearrange, in which case contact points between agglomerates will be dissolved due to their higher solubility in the liquid, leading plate-like shape microstructure. A dielectric constant (ɛr) of 29.3, a high Q × f value of 26,335 GHz (at 8.84 GHz), and a τf of −32.5 ppm/°C can be obtained for Sm(Mg0.5Ti0.5)O3 ceramics incorporated with 10 mol% Bi2O3 sintered at 1300 °C. While Sm(Mg0.5Ti0.5)O3 ceramics incorporated with 5 mol% B2O3 can effectively lower temperature coefficient of resonant frequency, which value is −21.6 ppm/°C. The Sm(Mg0.5Ti0.5)O3 ceramic incorporated with heavily Bi2O3 and B2O3 additives exhibits a substantial reduction in temperature (∼250 °C) and compatible dielectric properties in comparison with that of an un-doped one. This implied that this ceramic is suitable for miniaturization in the application of dielectric resonators and filters by being appropriately incorporated with a sintering aid.