Effect of sintering optimization on the electrical properties of bulk BaxSr1−xTiO3 ceramics

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Abstract

BaxSr1−xTiO3 (x=0.6, 0.75, 0.80, 0.85 and 0.9) compositions are prepared by solid-state reaction route using controlled heating and cooling. Density optimization by varying sintering temperature was achieved. X-ray diffraction (XRD) analysis shows the phase pure materials. The lattice constant decreases from 3.9868 Å (x=0.90) to 3.9449 Å (x=0.60) with increasing Sr2+; the tetragonal distortion also decreases. Dielectric constant show sharp peaks for samples having low strontium content (0.10, 0.15) and gets smeared out as the strontium content is increased (0.20, 0.25). For further higher Sr2+ composition (0.40), the dielectric peak could not be observed in the measured temperature range. The peak broadening in Sr2+ rich compositions indicates that diffused transitions and is attributed to the disorder in the arrangement of cations at A-site. A modified Curie–Weiss law using exponent γ and diffuseness parameter δ fits temperature dependence of dielectric constant in paraelectric phase. Both the parameters increase with increasing Sr2+ content showing that with strontium substitution, the material becomes disordered and may be correlated with the compositional fluctuation in solid solution. Frequency dependence of dielectric behavior is analyzed in terms of Debye formalism and nature of dielectric relaxation processes is explored using Cole–Cole formalism. Ferroelectric hysterisis loops reveal spontaneous polarization and coercive field increases with deceasing Sr content.

Introduction

Perovskite materials with high dielectric constant and lead-free compositions attracted much interest as materials for environment-friendly applications (dielectric for capacitors, actuators, insulators in dynamic random access memory (DRAM), etc.). Barium strontium titanate (BaxSr1−xTiO3, BST) is a ferroelectric solid solution which exhibits high dielectric constant, low loss tangent, high dielectric breakdown strength, good tunability, high pyroelectric coefficient and a Ba/Sr composition dependent Curie temperature TC. Due to this, it has been identified as a promising material for ferroelectric DRAM (FE-DRAM), tunable phase shifters, high frequency capacitors and uncooled infrared focal plan arrays [1], [2], [3]. It is one of the most studied ferroelectric materials, exhibiting normal, first-order phase transition behavior [4]. The BST family of normal ferroelectrics is also interesting candidates for field-induced piezoelectric transducers due to the large polarizations, large permittivity and large induced strains achievable. The TC can be modified by using isovalent and aliovalent substitutions [5]. However, the same composition in bulk and in thin films gives different values of TC [6]. Further, it is not clear if TC is a function of sintering temperature. Studies on the dielectric properties of BST solid solutions have shown that the compositions with x⩾0.2 exhibited normal ferroelectric behavior while relaxor behavior is observed in the SrTiO3 rich region (x<0.2) [7]. Many interesting results were obtained on dielectric response in BST; the variation of TC was determined by change in the cell volume; the change of order and the diffuseness of the phase transition in BST could also be attributed to a cell volume effect [8]. There are several reports available on the structure and electrical behavior of these ceramics synthesized by conventional solid state and chemical routes [9], [10], [11], [12], [13], [14]. Most of the studies on ferroelectric materials have been performed for their structural, electrical and positive temperature-coefficient of resistance (PTCR) characteristics only [15], [16]. Comprehensive dielectric studies have been reported on the compound in ferroelectric phase. However, paraelectric dielectric behavior, which is more important for applications [17] is relatively less studied.

The purpose of the present work is to optimize the densification (by varying sintering parameter) and study the temperature and the frequency dependent dielectric responses of the BaxSr1−xTiO3 (for x=0.6, 0.75, 0.80, 0.85 and 0.9). Composition dependence of TC for maximum densification is obtained. Results are analyzed in terms of disorder created at A-site.

Section snippets

Experimental

The BaxSr1−xTiO3, (BST) samples with varying values of x=(0.6, 0.75, 0.85, and 0.90) were prepared through solid solution reaction by standard sintering ceramic technique using AR grade BaCO3 (Merck 99.9%), TiO2 (Merck 99.5%) and SrCO3 (LOBA 99.5%) by the following chemical reaction:xBaCO3+(1−x)SrCO3+TiO2=BaxSr1−xTiO3+CO2.

The Stoichiometric amounts of constituent's powders were thoroughly wet mixed in acetone for 6 h in an agate mortar. The homogeneously mixed powders were calcined at 1200 °C for

XRD studies

Fig. 1 shows XRD patterns of various BST compositions sintered at 1280 °C. The XRD patterns are assigned using POWD software. All major X-ray reflection peaks observed could be fitted satisfactorily in cubic perovskite phase for x⩽0.75, with (1 1 0) as the major peak. No additional peaks were observed indicating the formation of pure phase. For Br concentration ⩾0.75, we get splitting in (2 0 0) peak that indicates the tetragonal phase being stabilized at room temperature. The (c/a) ratio

Ferroelectric properties

Ferroelectric properties of various compositions of BST's were studied using P–E hysterisis loop data. The typical hysterisis loops at room temperature for all compositions are shown in Fig. 14. For x=0.6 composition, no loop is observed, which is obvious as the composition has TC below room temperature. For all other compositions, symmetrical hysterisis loop could be observed. All loop measurements are for as-prepared samples without any poling; therefore the values of spontaneous polarization

Conclusions

The various compositions of BST are prepared using standard solid-state route with controlled cooling and heating rate during sintering. The pure phase materials showing compositional dependent TC are obtained. The sintering was optimized and at 1280 °C, the maximum density of 90% was obtained. XRD data show that the lattice parameter ‘a’ increases with increasing Sr content and decreases with increasing sintering temperature above optimum temperature at 1280 °C. No dielectric dispersion peak is

Acknowledgment

PKB is thankful to the University Grants Commission, New Delhi, for financial support (UGC Major Project Sanction No F-10-16/2004(SR)).

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