A-site substitution effect of strontium on bismuth layered CaBi4Ti4O15 ceramics on electrical and piezoelectric properties

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Abstract

Strontium substituted CaBi4Ti4O15 ceramics with the chemical formula Ca1−xSrxBi4Ti4O15 (CSBT) (x = 0.0–1.0) have been prepared through conventional solid state route. The formation of single phase material with orthorhombic structure was verified from X-ray diffraction with incorporation of Sr substitution. Decrease in a-axis displacement of Bi ion in the perovskite structure in the CSBT ceramics were observed from the relative changes in soft mode (20 cm−1) in the Raman spectra, and increase in Sr incorporation shows the shift in ferroelectric to paraelectric phase transition temperature. The dielectric properties for all the CSBT ceramic compositions are studied as a function of temperature over the frequency range of 100 Hz–1 MHz. Curie's temperature was found to be function of Sr substitution and with increase in the Sr concentration the phase transition becomes sharper and phase transition temperature gets shifted towards lower temperature (790–545 °C). The behavior of ac conductivity as a function of frequency (100 Hz–1 MHz) at low temperature (<500 °C) follows the power law and attributed to hopping conduction mechanism. Sr substitution results in the increase in piezoelectric coefficients (d33) whereas piezoelectric charge coefficient values were found comparable to that of PZT at room temperature. Relative changes in soft modes due to Sr incorporation results in high piezoelectricity in the CSBT ceramics.

Introduction

Bismuth layered structured material are of great technological interest because of their applications as thermally stable capacitors, positive temperature coefficient devices, microwave applications, non-volatile ferroelectric memories, high temperature piezoelectric transducers, sensors, actuators and photonics devices [1], [2], [3]. High temperature sensing devices are required for detecting vibration in aerospace, automotive, power generating industry and oil well drilling under harsh thermal condition [4], [5]. Bismuth layered structured ferroelectric (BLSF) ceramics with high ferroelectric phase transitions are the potential candidates for high temperature sensing applications [4], [5].

The general formula of BLSF is (Bi2O2)2+(Am−1BmO3m+1)2− where A is mono, di, or trivalent ion or a mixture of them to form a dodecahedron, and B is a combination of cations well suited to form an octahedron and m is an integer usually lying in the range 1–4. The CaBi4Ti4O15 with layer m = 4 are subgroup of BLSF family and is potential candidate for high temperature piezoelectric applications due to its high Curie temperature (Tc = 790 °C) and piezoelectric voltage coefficient g33 (8 × 10−3 V m N−1), which is comparable to lead based piezoelectric ceramics, i.e. for PZT g33 = 26 × 10−3 V m N−1 [6].

It has been shown that the properties of the BLSF compounds are strongly dependent on the number of perovskite unit and the chemical elements at the A or B site and properties of a material can be tailored by doping with a suitable dopant at either of these sites. There are many reports available in the literature for A and B site substitution in the perovskite layers of CBT ceramics and its influence on the structural, dielectric, piezoelectric and ferroelectric properties has been studied [7], [8], [9]. Wu and coworkers studied the influence of various A-site dopants (Ca2+, Ba2+) and B-site dopants (W6+, V5+) on dielectric and piezoelectric properties of BLSF material and correlated with the difference in ionic radii [7]. Substitution of host cations by dopant having smaller ionic radius will result in an increased rattling space leading to a higher transition temperature and large dielectric constant at transition point, while dopant having larger radius will result in lower transition temperature with lower dielectric constant at transition temperature. Enhanced piezoelectricity due to the changes observed in soft modes in Raman spectra have been reported by various authors [10], [11]. However no report are available in the literature regarding the changes observed in soft modes due to Sr substitution corresponds to a-axis displacement of Bi ion in perovskite of CSBT ceramics which yield enhanced piezoelectric and ferroelectric properties. Furthermore the detailed studies on ac conductivity and dielectric properties of Sr substituted CBT ceramics have not been reported as a function of temperature. In the present study single phase Sr substituted CBT ceramics were prepared from solid state route and effect of Sr substitution were analyzed on structural and electrical properties of Calcium bismuth titanate ceramics and attempts have been made to correlate the softening of Raman modes with the ferroelectric and piezoelectric properties.

Section snippets

Experimental

Ceramic samples of Ca1−xSrxBi4Ti4O15 (CSBT) were prepared by conventional solid-state reaction technique. Reagent-grade oxide or carbonate powders of Bi2O3, SrCO3, TiO2, and CaCO3 with 99% purity were weighed and mixed in stoichiometric ratio. The materials were thoroughly mixed and ball milled for 24 h with ZrO2 ball using distilled water as mixing media. After calcining at 1000 °C the ground and ball milled powders were mixed with binder (5%PVA). The dry powder was pressed into circular disks

Structural properties

Fig. 1 shows the X-ray diffraction pattern for Ca1−xSrxBi4Ti4O15 (CSBT) ceramics samples where x varying from 0.0 to 1.0 in step of 0.25, sintered at 1200 °C for 1 h. All the peaks obtained in the XRD pattern were well indexed with the reported data on CBT ceramic indicating the formation of single phase of the bismuth oxide layer type structure with m = 4 [12], [13], [14], [15]. The lattice parameters were estimated and results indicates that the crystal symmetry of both CBT (a = 5.44 Ǻ, b = 5.47 Ǻ and c

Conclusions

To summarize, strontium substituted CBT ceramics Ca1−xSrxBi4Ti4O15 (x = 0.0–1.0 in step of 0.25) were synthesized by solid state reaction method. Structural distortion with the Sr substitution has been observed by XRD and Raman analysis. Substitution of Sr concentration at Ca site increase the a-axis displacement of the Bi ion in the perovskite structure and thus soft mode at 20 cm−1 was found to increase with Sr incorporation. Temperature dependent dielectric studies indicate phase transition

Acknowledgements

Authors are thankful to Prof. R.S. Katiyar, University of Puerto Rico, San Juan, USA for Raman studies and fruitful discussion. The support from the DRDO and DST, Government of India are highly acknowledged. One of the authors (A.T.) would like to thank the Council of Scientific & Industrial Research (CSIR), India for the research fellowship.

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