Research On BST Ferroelectric Composites For Tunable Microwave Application

Barium strontium titanate (BST) is a perovskite-structured ferroelectric material. For itsfavorable dielectric and ferroelectric properties such as high dielectric constant, lowdielectric loss and high tunability of dielectric behavior, it has been widely used in variousmicrowave tunable devices, such as microwave phase shifters, tunable filters,voltage-controlled oscillators. However, high dielectric constant of pure barium strontiumtitanate has precluded their use in most applications. Therefore, the dielectric constant anddielectric loss of barium strontium titanate must be reduced while maintaining a hightunability. And that the dielectric constant could be suppressed effectively to formingferroelectric-dielectric composite with linear dielectrics with low dielectric constant and loss.Ba0.5Sr0.5TiO3/MgO/Mg3B2O6composite ceramics and Ba0.45Sr0.55TiO3/MgO/Mg2TiO4composite ceramics were prepared by a conventional solid-state reaction method. Bothtunability and the microwave dielectric properties were investigated. A cubic paraelectricBa0.5Sr0.5TiO3, a cubic MgO and an orthorhombic Mg3B2O6or a spinel structure Mg2TiO4are clearly observed without other phases being detected for the composite ceramics. Fixedthe Ba0.5Sr0.5TiO3content in Ba0.5Sr0.5TiO3/MgO/Mg3B2O6composite ceramics, the tunabilityand the Q f value of Ba0.5Sr0.5TiO3/MgO/Mg3B2O6composite ceramics increase along withthe increase of MgO content. The tunability of Ba0.5Sr0.5TiO3/MgO/Mg3B2O6compositesceramics increase from5.9%to8%with the increase of MgO content at10kHz under2.4kV/mm, and the Q f value increase from399GHz to554GHz. This is beneficial fortunable microwave applications. In Ba0.45Sr0.55TiO3/MgO/Mg2TiO4composite ceramics, thedielectric constant and the tunability of Ba0.45Sr0.55TiO3/MgO/Mg2TiO4composites ceramicsincrease with the increase of Mg2TiO4content. However, the dielectric loss of thiscomposites ceramics range from0.00035to0.0007.The effect of WO3and La2O3doping on the dielectric properties of BST/MgO/Mg2TiO4composite ceramics were also studied. Both WO3and La2O3doping can decrease thedielectric constant of BST/MgO/Mg2TiO4composite ceramics, but the dielectric lossincrease and the tunability decrease.The relation of ε versus E can be well described by the “multi-polarization mechanism” model, which is based on modified Landau–Ginzburg–Devonshire (LGD) theory combiningwith a reorientational polar-cluster term. The results indicate that the analysis of the relationof ε versus E might be an alternative to distinguish the intrinsic part and the extrinsicdielectric responses in BST and other related materials.