Improved dielectric properties of Ba_xSr_1−xTiO₃-based composite ceramics derived from core–shell structured nanopowders

Abstract

We performed a study on the dielectric properties of Ba_xSr_1−xTiO₃–Mg_0.9Zn_0.1O (BST–MZO) composite ceramics derived from core–shell structured nanopowders with the shell of zinc doped MgO and core of Ba_xSr_1−xTiO₃. It was found that the ceramics exhibit a significant improvement in dielectric response under a DC electric field. The Curie temperature decreases more significantly in the BST–MZO composite ceramics compared to that of pure BST ceramics. The tunability of Ba_0.75Sr_0.25TiO₃–40%MZO is 23.49% at 20 °C. The dielectric properties at high frequency also show low dielectric constant and great reduction in dielectric loss. These ceramics are expected to be useful in microwave devices.

Introduction

Ba_xSr_1−xTiO₃ (BST) has been considered to be an important material for tunable phase shifters, tunable filters, and high-Q resonators for radar and communication applications because of its large field tunability and variable Curie temperature [1], [2], [3], [4], [5], [6]. The Curie temperature of BST can be controlled by adjusting the ratio of Ba/Sr and/or doping ions to substitute for A or B sites in the ABO₃ perovskite systems. The dielectric constants as well as dielectric loss are usually lower in the paraelectric phases than in the ferroelectric phases, owing to the disappearance of hysteresis. In microwave device applications, the optimum materials should have moderate dielectric constant (ɛ), large tunability, a low loss dissipation factor (tan δ), and low temperature dependence of ɛ. Thus far, composite materials, including ferroelectric BST phases and non-ferroelectric oxides with a low loss (MgO, Al₂O₃, etc.), have attracted considerable attention [7], [8], [9], [10], [11].

The hydrothermal process is one of the commonly used methods for preparing nanoscaled powders because of its low synthesis temperature, low equipment cost and simple synthesis procedures. Qi et al. developed a new and simple method of directly synthesizing nano-BaTiO₃ powders from solution at 60 °C with an average particle size of about 25 nm [12]. A process for fabricating the core–shell MgO-wrapped–BST (BST–MgO) nanopowders, with a grain size close to 17–20 nm, was developed in our previous work [13]. Nanoscaled BST powders were used as the core materials and the core–shell structure was formed by a chemical solution method, by adding magnesium nitrate solution into the BST dispersed base solution by an ultrasonic process.

In the conventional composite, the sintering characteristics usually depend on the easily sintered phase: e.g. in the MgO–BST system, BST can be easily sintered at much lower temperatures than MgO. In MgO-wrapped–BST, however, the sintering behavior was dominated by MgO, making sintering difficult for BST. Some modifications were made in order to solve this difficulty in sintering. In this work, easily sintered Mg_0.9Zn_0.1O₂ (MZO) shell material was used as a substitute for MgO, and a two-step synthesis processing was employed to obtain BST nanopowder uniformly wrapped with Mg_0.9Zn_0.1O (BST–MZO). The core–shell structure of BST nanoparticles wrapped with zinc doped MgO was also analyzed and confirmed elsewhere [14].