Investigation of the effect of Sr-substitution on the structural, morphological, dielectric, and energy storage properties of BaTiO₃-based perovskite ceramics

Highlights

• Synthesis of pure BT and BST ceramic perovskite phase.

• The effect of the substitution of Ba by Sr in site A of the BT has proven an improvement in the dielectric properties of the material due to the reduction in dielectric losses with the appearance of a relaxor behavior for BST ceramics in the vicinity of room temperature, very promising for industrial and technological applications.

• The BT compound proved a considerable energy storage properties.

Abstract

In this work, Ba_1-xSr_xTiO₃ (x = 0 and x = 0.3) samples, abbreviated respectively BT and BST, have been prepared by conventional solid state reaction. The pure phase formation and crystal structure were approved by X-ray powder diffraction analysis. The morphology of these samples were inspected by the transmission electron microscopy. The structural and morphological findings were confirmed by Raman measurements. The effect of the substitution of barium by strontium on the dielectric properties and on the energy storage of these compounds which has been discussed. In this issue, the results indicate the improvement of the dielectric properties of BT-based ceramics by substitution of Ba by Sr due to the decrease in dielectric losses and especially to the appearance of a relaxor behavior for the BST compound in the vicinity of room temperature. However, the P–E loops have proven a degradation of energy storage properties by Sr-substitution in BT ceramics.

Introduction

Perovskite oxides, including titanates, are complex oxides having an ABO₃ structure with a random distribution of the different charges between the two species of cations occupying the A and B sites. These cations are metallic, mono-, di- or tri-valent for the A-site, tetra-, Penta- or hexa-valent for the B-site. They have been widely studied in recent years owing to their excellent dielectric and ferro/pyro/piezoelectric properties and low cost. Such properties support them to occupy a leading place in various industries such as infrared identifiers, microelectronics, memory cells, multilayer capacitors, multilayer ceramics capacitors (MLCC), electro-optic components, PC recollections, pyroelectric indicators, thermistors and transducers, high-temperature piezoelectric sensors and ferroelectric random-access memories (FRAM) [1], [2], [3], [4], [5], [6], [7], [8].

Among this large family of perovskite, dielectric materials, having low dielectric losses (tan δ) and excellent temperature stabilization, have been used efficaciously in energy storage and microelectronic devices, such as capacitors [9], [10], [11], [12].

Furthermore, recent research has paid special attention to this category of materials, in particular lead-free ceramics which take into account the requirements of environmental protection. Of particular relevance to this issue, BaTiO₃-based (BT) ceramics have often been reported to exhibit several promising dielectrics and piezoelectric properties that make them potential strong candidates for several industrial applications [13], [14], [15], [16], [17].

From an electrical and dielectric point of view, a significant number of works have indicated that the dielectric stability near the temperature of the ferroelectric phase transition is of major importance in the performance of any material. The improvement of this stability can be achieved by implementing special compositions resulting from the diversity of introduction at A or B-site of BT-system [13], [14], [15], [16]. In fact, the literature presents ample evidence for the ferroelectric relaxor behavior acquired through the introduction of an additional cation to the A or B-site. Given its capacity to host ions of different sizes, while keeping its perovskite structure, various dopants can be localized in barium titanate [13], [14], [15], [16], [17].

The various studies carried out on relaxor ferroelectric oxides with a perovskite structure have highlighted relevant industrial applications as well as hopeful technological perspectives [18], [19], [20], [21]. Currently, the tendency is to gather such lead-free pottery having a certainly relaxing behavior at room temperature. Nevertheless, BT has been confirmed as a non-relaxor ferroelectric or an anti-ferroelectric [22], [23]. Fortunately, a complex structure of doped-BT has typically been migrated to the desired relaxor behavior.

In this regard, Raman spectroscopy is an experimental technique suitable for detecting and monitoring the progress of the ferroelectric phase. Based on variable-temperature spectroscopy, Bruce et al. [24] have proved a close relationship between room temperature tetragonality and the Curie purpose of BT. More advanced than the above, different Raman spectra have been performed at different temperatures and high-pressure fields on various types of BT solids such as single crystals, polycrystalline powders, masses, and thin films [25], [26], [27]. The common main objective of these studies was always to accurately determine the crystal structure and phase transition temperature of BT and to look for an eventual correlation between crystallographic properties and dielectric behavior.

More pertinent to the aims of the present study, the increased energy requirements for civil, commercial, and military applications, make improving sustainable energy storage technologies a global emergency [28], [29], [30], [31]. Dielectric ceramics with good energy storage efficiency have competed and even surpassed electrochemical batteries for many industrial uses. Therefore, it has been reported that the fulfillment of a high storage density can be achieved through a high electrical breakdown strength (BDS), a small remanent-polarization $(P_r)$ and a high maximum-polarization $(P_{\mathit{max}})$ [32], [33], [34]. In addition, the adjustment of the energy storage density can be accomplished by lead-free ferroelectric ceramics with relaxor behavior. More recently, in their study, Ma et al. [35] have stated that slim polarization electric field (P-E) loops and excellent energy storage properties can be reached by optimizing the synthesis parameters of BT-based ceramics.

Accordingly, the present paper aims to present an experimental study to better highlight the effect of Sr-substitution on the physical properties of the BT ceramics. This study is carried out on BT and BST samples synthesized by conventional solid-state chemistry reaction. In order to achieve a better understanding of the key issues involved in the process, a structural study was carried out by X-ray diffraction followed by a morphological study by transmission electron microscopy. The structural and chemical results were confirmed by Raman spectroscopy analysis. These studies were reinforced by a dielectric investigation allowing to reveal the impact of the substitution of barium (Ba) by strontium (Sr) in the A-site of the lattice on the dielectric behavior and on the energy storage of the BT-based ceramics.