Processing-Dependent Dielectric and Ferroelectric Properties of BST Ceramics

Abstract

Barium strontium titanate, BaxSr1-xTiO3 (BST) ceramics have been extensively used as in a microelectronics due to its excellent ferroelectric, dielectric, piezoelectric, and pyroelectric properties [1–3]. At room temperature BST is ferroelectric when x is in the range of 0.7–1, and has a maximum dielectric constant around x = 0.8 [4]. Characteristics of BST powders strongly depend on their synthesis. To achieve the desired properties and practical application, BST powder needs to be free of intermediate phases, with a defined stoichiometry and homogenous microstructure. The commonly used techniques to produce BST powders are conventional solid state processing, complex polymerization method, and hydrothermal method. The solid state reaction is most common technique for preparation of BST powders, but it is faced with several disadvantages such as large particle size distribution, non-homogeneity and presence of impurities, which can negatively affect the properties of BST ceramics. Hydrothermal technique is able to produce uniform, nanosized, low agglomerated BST particles at low temperature, but it is hard to control the stoichiometry of the final products. In this study, BST powder Ba0.8Sr0.2TiO3 was obtained by hydrothermal treatment of precursor solution containing titanium citrate, barium and strontium acetates, previously prepared by complex polymerization method. By combining the advantages of both methods a fine, nanosized BST powder was synthesized with homogeneous particle size distribution [5]. The calcined BST powders were pressed into pellets and sintered at 1280°C with different dwelling times, from 1 to 32 h. The phase compositions of the sintered samples were determined by X-ray diffraction (XRD) and EDS analysis. Microstructural properties were investigated by scanning electron microscopy. The phase transitions and dielectric properties were investigated by measuring dielectric permittivity (ε) and dielectric losses (tanδ) as a function of temperature. Ferroelectric properties such as remanent polarization (Pr) and coercitive field (Ec) were determinated by polarization-electric field (P-E) measurements. The main objective of this work was to investigate the changes in the phase composition, structural parameters and microstructure of BST ceramics and their influence on dielectric and ferroelectric properties. XRD analysis of BST ceramics showed that crystallite size and phase composition varied with different sintering times. It was found that the sintering process leads to a formation of secondary phases; Ba6Ti17O40, BaTi2O5 and SrTi21O38 on Ti-rich side, and Ba2TiO4 on Ba-rich side. The sintered samples underwent an abnormal grain growth, whereby some grains grew faster than others due to the presence of multi-phase structure. It was found that dielectric and ferroelectric properties of BST ceramics strongly depended on grain size, density, phase compositions and defects.