High dielectric tunability in composites prepared using SiO₂ coated Ba_0.5Sr_0.5TiO₃ nanoparticles

Ba_0.5Sr_0.5TiO₃ (BST) nanoparticles were coated with SiO₂ using a chemical process. SiO₂-coated BST was used as the starting material to prepare BST-xSiO₂ (x = 0, 1.0, 2.0, 3.0, 4.0, and 5.0 wt%) composites via a conventional ceramic process. The influence of SiO₂ on the structure and properties of these composites was systematically studied. Scanning electron microscopy (SEM) results showed that the porosity of the composites was obviously reduced by the addition of SiO₂. X-ray diffraction (XRD) results indicated that, besides the perovskite phase of BST, a traceable secondary phase of BaTiSiO₅ also existed. The formation of BaTiSiO₅ was due to the reaction between SiO₂ and barium from BST. This resulted in a lower phase transition temperature of BST phase in the composites. It was found that the dielectric constant and loss decreased with increasing SiO₂ content. All the composites exhibited a low dielectric loss (< 0.01) and an almost frequency independent dielectric constant in a frequency range from 100 Hz to 100 kHz. All the composites exhibited a high tunability (> 47%). Although the tunability decreased with increasing SiO₂ content, a higher tunability was achieved in the composites because the composites exhibited a much higher breakdown strength than the pure BST ceramics. Additionally, it was found that the tunability of the composites increased with the decrease of the operating temperature.