Synthesis, structural characterization and dielectric properties of Nb doped BaTiO₃/SiO₂ core–shell heterostructure

Highlights

• Optimal parameters for preparation by sol–gel of core–shell (BT-Nb_0.005)/SiO₂ are presented in this paper.

• Single crystalline BT-Nb_0.005/SiO₂ core–shell composite with ∼34 nm shell thick was prepared.

• The core–shell ceramic exhibits good dielectric properties and ferroelectric characteristics.

Abstract

Perovskite complex ceramic oxides, BaTiO₃ doped with 0.5 mol%Nb₂O₅ and then nanocoated with SiO₂ (abbreviated as BT-Nb_0.005/SiO₂) was successful prepared using conventional sol–gel processing. Phase composition, particle morphology, structure, and electric properties of BT-Nb_0.005 core and BT-Nb_0.005/SiO₂ core–shell were examined and compared, using X-ray diffraction, transmission electron microscopy and, dielectric and ferroelectric measurements. Core–shell composite with well-defined perovskite tetragonal phase of BaTiO₃ was achieved. Furthermore, single crystalline BT-Nb_0.005/SiO₂ core–nanoshell heterostructure with ∼34 nm shell thick was prepared, which is a novelty in ferroelectrics field. The ferroelectric quality of BT-Nb_0.005 has suffered an alteration when the (BT-Nb_0.005)/SiO₂ core–shell heterostructure was realized. One-dimensional BT-Nb_0.005/SiO₂ core–shell heterostructure exhibits an improvement of dielectric losses and a decrease of dielectric constant, compared to uncoated BT-Nb_0.005. The (BT-Nb_0.005)/SiO₂ core–shell material could be interesting for application in the composite capacitors.

Introduction

Barium titanate (BaTiO₃) is extensively investigated because it is chemically stable, exhibits good ferroelectric and dielectric properties in the frequency range 100 Hz–1 MHz. Based on these characteristics, BaTiO₃ has applications in high performance capacitors, multilayer capacitors, energy storage devices and thermistors [1], [2], [3], [4], [5], [6], [7]. Very interesting properties for various applications of barium titanate can be obtained by adding dopants and by an appropriate modification of the grains and grain boundary characteristics due to the recent developments in synthesis of materials [7]. Many rare earth oxides are used as dopants in order to modify the Curie point temperature and to improve the electrical characteristics of the capacitors by controlling the microstructure and the grain size of BaTiO₃ [7], [8], [9], [10], [11], [12], [13]. It is known that BaTiO₃ can be doped with donors like the trivalent ions (e.g. yttrium, europium, lanthanum) on Ba-ion sites or a pentavalent ions (e.g. niobium, tantalum) on Ti-ion sites [11], [12], [13], [14]. There are several reports on the doping BaTiO₃ with niobium [7], [8], [9], [15], [16], [17], [18], [19]. The phase composition, microstructure and dielectric properties of BaTiO₃ show a strong dependence on the amount of added niobium [7]. We have chosen a concentration of Nb of 0.5 at% because it found that the dielectric constant of Nb-doped BaTiO₃ increase with the increase of niobium concentration from 0.4 to 0.6 mol%, in the presence of a small amount of manganese (0.01 mol%Nb⁵⁺) as acceptor dopant [7]. There are substantial discrepancies in reported properties of Nb-doped BaTiO₃ [20], [21], [22], [23]. The differences are due to a slow rate of Nb incorporation into the BaTiO₃ lattice [24] and the temperature range at which the processing is effective [25]. It was found that at temperatures lower than 1225 °C, Nb cannot be incorporated into the BaTiO₃ lattice due to kinetic reason [25]. Niobium has a significant effect on densification and microstructure evolution of BaTiO₃ [7]. It was reported that small concentrations of niobium (<0.06 mol%Nb) enhance grain growth, while above this limit it inhibits the grain growth of barium titanate [26], [27].

The dielectric characteristics (especially dielectric losses) of ferroelectric materials can be improved also by coating the ferroelectric grain with an insulating material with low dielectric loss, i.e. Al₂O₃, SiO₂, MgO, Ta₂O₅ [1], [28], [29], [30], [31], [32]. Coating the ferroelectric particles with amorphous silica is often the first choice for the assembly process since dense-coating of amorphous silica improves the chemical stability and reduces the growth rate of the core particles in the fabricating process [33], [34]. Generally, the grains with core–shell architecture combine diverse functionalities into a single hybrid composite and show improved or new physical properties over their single-component counterparts. Core–shell particles can be used for the fabrication of dense dielectric, ferroelectric and multiferroic composites with 3–0 connectivity or ceramics with controlled composition and property gradients at the level of the single grains [35].

In this work we used a colloidal chemistry method (sol–gel) in order to prepare BaNb_0.005Ti_0.995O₃ coated with SiO₂ (abbreviated as BT-Nb_0.005/SiO₂). The dielectric behaviour of BT-Nb_0.005/SiO₂ core–shell was studied and compared with that of the BT-Nb_0.005 base material. The sintering behavior of SiO₂ shell was investigated also in this paper.