Structural, optical and electrical properties of the microcrystalline structure of (Ba_1-xY_2x/3)(Zr_0.20Ti_0.80)O₃ ceramics

Highlights

• The effect of Y³⁺ doped BYZT ceramics on structure and electrical properties are reported.

• We explained the relationship between grain sizes and electrical properties with concentration.

• The structure and local structure around the Ti absorbing atoms were investigated by X-ray absorption spectroscopy (XAS).

• The recoverable energy-storage density with excellent energy storage efficiency is reported.

Abstract

Yttrium (Y³⁺) doped barium zirconate titanate, (Ba_1-xY_2x/3)(Zr_0.20Ti_0.80)O₃; BYZT ceramics with varying x (0 = x ≤ 0.10) were prepared by the solid-state reaction method. These samples were analyzed by X-ray diffraction (XRD) and the XRD patterns were fitted using the Rietveld refinement. The local structural changes of the BYZT ceramics were investigated by synchrotron X-ray absorption spectroscopy. The results showed that an increase in the x content in the BYZT lattice structure significantly affected the phase transition behavior and the local structure around the Ti absorbing atoms, which corresponds with the phase transition from a tetragonal to a cubic structure. SEM images showed a uniform and highly dense microstructure with increasing x values. The optical band gap (E_gap) values measured from the UV–visual diffuse reflectance spectra, showed a decrease from ~3.55 eV to ~2.90 eV with increasing values of x. The modified Curie-Weiss law showed that a normal ferroelectric phase transition is observed in the unmodified BZT ceramic and as the concentration of x increased, it induces diffuseness in the phase transition behavior. The largest dielectric constant (ε_r = 13,200), the highest recoverable energy-storage density (W_rec = 1.76 J/cm³) with an excellent energy storage efficiency (η = 91%) under a lower electric field of 50 kV/cm and lowest dielectric loss (tanδ = 0.01) were found in the composition of Ba_0.98Y_0.01337Zr_0.2Ti_0.8O₃ (x = 0.02 mol.%).

Introduction

Many piezoelectric devices are made from Pb(Zr,Ti)O₃ (PZT) based ceramics, which consist of more than 60% of Pb by weight (Xinyou et al., 2006; Cernea et al., 2012). Pb is toxic and there are concerns regarding environmental contamination from the evaporation of Pb during the fabrication process (Yi et al., 2005; Rodel et al., 2009). Therefore, it is imperative to develop non-toxic piezoelectric ceramics with good properties to replace PZT-based ceramics. The BaZrxTi₁₋xO₃ (BZT) ceramics are a potential environmental friendly material for use as dielectrics in commercial multilayer capacitors and in highly tunable applications (Ghosh and Rout, 2016; Chandraiah et al., 2016; Yu et al., 2002; Hennings et al., 1982;Weber et al., 2001; Buscaglia et al., 2014). BZT ceramics possess good dielectric properties due to their low dielectric loss, high electrical resistance and composition-dependent Curie point (Chou et al., 2007; Jiang et al., 2006; Zhang et al., 2006; Tang et al., 2004a, Tang et al., 2004b). Dielectric measurements of BZT ceramics exhibit a normal ferroelectric behavior for 0 < x < 0.1, a diffuse phase transition in the range of 0.1 < x < 0.2 and a relaxor character for 0.2 < x < 0.5 (Hennings et al., 1982; Cai et al., 2010). BZT exhibits a perovskite-type rhombohedral structure at room temperature with zirconium (Zr) content up to 0.05 wt%. When Zr content is increased up to 0.08 wt%, the BZT ceramics form an orthorhombic structure and exhibit a broad permittivity at T_m, which is due to the inhomogeneous distribution of [ZrO₆] clusters into the titanium (Ti) sites and/or the mechanical stresses on the grains (Badapanda et al., 2009). The structure transforms from tetragonal to pseudo-cubic when the Zr content is increased up to x ≈ 0.20 (Hennings et al., 1982). This composition is responsible for the phase transition from a ferroelectric to a relaxor in BZT materials. Recently, the effect of various dopants such as Mn, Ca, Yb, Nd, etc. on the crystal structure, morphotropic phase boundary (MPB) and polymorphic phase transitions (PPT) have been studied and have been shown to improve electrical properties of BZT ceramics (Chandraiah et al., 2016; Cai et al., 2015; Wang et al., 2002; Ghosh et al., 2014). Chandraiah et al. (2016) studied CaO-doped Ba_1-xCa_x(Zr_0.02Ti_0.98)O₃ ceramics produced by the solid-state reaction method. It was found that the piezoelectric charge constant (d₃₃ = 248 pC/N), dielectric constant (ε_r = 2521) and remnant polarization (P_r = 4.22 μC/cm²) improved with the addition of 0.015 M CaO. Wang et al. (2002) reported that Yb-doped BaZr_0.15Ti_0.85O₃ had a lower Curie temperature and enhanced permittivity values compared to the non-doped ceramics. Nd-doping in (Ba_1-xNd_2x/3)(Zr_0.3Ti_0.7)O₃ ceramics induced peak broadening and simultaneously reduced the intensities of the Raman active modes, which suggested an increase in local symmetry caused by undistorted [ZrO₆] and [NdO₁₂] clusters in the lattice (Ghosh et al., 2014). Furthermore, Mn-doped BaZr_0.2Ti_0.8O₃ ceramics exhibited a lower dielectric constant and the Curie point also decreased when compared to unmodified BZT ceramics (Cai et al., 2010). Undoped BZT and doped BZT ceramics have been prepared by different methods, which can be divided into three main categories: solid-phase state, liquid-phase state and gas-phase state (Zheng et al., 2011). Like the traditional solid reaction methods, the solid-phase method has several benefits; for example, a shorter reaction time, less contamination, higher purity, simpler procedures, and it is also currently a popular method of synthesizing ferroelectric powder. Therefore, in this work, the addition of yttrium in [Ba_1-xY_2x/3](Zr_0.20Ti_0.80)O₃; BYZT ceramics, prepared by the solid-state reaction method, was investigated. Moreover, from a survey of the literature, the effect of yttrium content on the microcrystalline, energy density, optical and electrical properties BYZT ceramics prepared by the solid-state reaction method has been rarely studied. In this paper, we studied the influence of yttrium (x) content on the microstructure, phase formation, optical and electrical properties (dielectric and ferroelectric properties) in BYZT ceramics.