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Dielectric and Polarization Studies of CaSubstituted in Bi4SrTi4O15 CeramicsGagan Anand a , A. R. James b & P. Sarah ca Vignan Istitute of Technology and Science, Vignan Hills, Nalgonda,A.P, Indiab Defence Metallurgical Research Laboratory, Kanchanbagh,Hyderabad, A.P, Indiac Vardhaman College of Engineering, Shamshabad, Hyderabad, A.P,IndiaPublished online: 01 Dec 2010.

To cite this article: Gagan Anand , A. R. James & P. Sarah (2010): Dielectric and Polarization Studiesof Ca Substituted in Bi4SrTi4O15 Ceramics, Integrated Ferroelectrics: An International Journal, 116:1,137-144

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Integrated Ferroelectrics, 116:137–144, 2010Copyright © Taylor & Francis Group, LLCISSN: 1058-4587 print / 1607-8489 onlineDOI: 10.1080/10584587.2010.489411

Dielectric and Polarization Studiesof Ca Substituted in Bi4SrTi4O15 Ceramics

GAGAN ANAND,1 A. R. JAMES,2 AND P. SARAH3

1Vignan Istitute of Technology and Science, Vignan Hills, Nalgonda, A.P, India2Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, A.P,India3Vardhaman College of Engineering, Shamshabad, Hyderabad, A.P, India

The processing conditions, microstructure and dielectric properties of strontium bismuthtitanate (SBT) were studied by means of Ca substitution in the strontium site referred tohereinafter as SCBT. The mechano-chemical activation method permits one to reducethe particle size of the initial products, so the surface area of the powder is increased,improving in most cases its reactivity.

X-ray diffraction analysis showed the formation of a single-phase layered per-ovskite structure of SCBT. Morphological studies were carried out by SEM analysis.Dielectric measurements in the frequency 100 Hz–1 MHz were made using an ImpedanceAnalyzer (HP4192A) interfaced to a computer and the measurements were carried outfrom room temperature to 600◦C. a variation in the dielectric constant was observedby the incorporation of calcium into the layered perovskite structure. The hysteresisloop revealed a variation in remnant polarization with the substitution of calcium instrontium. Resonance studies were made on poled sample using an Impedance Analyzer.Elastic compliance [s33, s11] and coupling factor [k33, k31] were also obtained.

1. Introduction

Bismuth layer-structured ferroelectrics are excellent candidate materials for piezoelectricand pyroelectric sensors requiring a high stability and high operation temperature. Thisis owing to their high Curie temperatures, low ageing rate and strong anisotropic elec-tromechanical coupling factors. Due to their promising fatigue-free nature, bismuth layer-structured ferroelectrics (BLSFs) have attracted considerable attention [1]. The BLSFshave a crystal structure containing interleaved bismuth oxide (Bi2O2)2+ layers and pseudo-perovskite blocks which contain BO6 octahedron and generally formulated as (Bi2O2)2+

(An-1BnO3n+1)2−. In this notation A represents a mono-, bi- or trivalent ion, B denotes atetra, penta or hexavalent ion and n is the number of BO6 octahedron in each pseudo-perovskite block (n = 1, 2, 3 . . . ) [2]. Among the protype BLSFs, SrBi2Ta2O9 is nowstudied extensively for its application in FeRAMs. It has been reported that Sr-deficientand Bi excess SrBi2Ta2O9 shows an increase in Curie temperature TC with increasing com-positional deviation and a marked increase in remnant polarization [3, 4]. Generally, thedisplacive ferroelectrics with a higher TC have a large remnant polarization. Since the TC

of Bi4SrTi4O15 (520◦C) [5] is higher than that of SrBi2Ta2O9 (335◦C) [5], a high remnant

Received December 13, 2009; in final form February 5, 2009.∗Corresponding author. E-mail: gagan4567@rediffmail.com

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Figure 1. Lattice structure of (a) SBT and (b) SCBT ceramics.

polarization can be expected. Substitution of calcium in strontium sites should further in-crease the TC. The compound is tetragonal with slight orthorhombic distortion. The a- andb- axes lie along (110) c where the suffix denotes the cubic perovskite sub cell so that a≈ b≈2ac≈ 0.54 nm. The c- axis is inherently long. The pseudo-structure of BLSF compoundscan be seen in Fig 1. These layered perovskite compounds are ferroelectric with high Curietemperatures [6].

The ionic polarization makes an important contribution to the permanent dipole in theferroelectric state. The structural consequence of this polarization is to distort the octahedralcoordination, and thereby to lower the crystal symmetry. Chen Da Ren and Guo Yan Yi[7] reported piezoelectric properties of a number of compounds by replacing the cationshaving similar structural configurations. According to them the Bi3+ ion has an importanteffect on the bond strength of Ti4+-O2− (along a- axis) in the compound Bi4SrTi4O15.Figure 1 is the lattice structure of SBT and SCBT ceramics. The Bi ion in (Bi2O2) 2+ canonly be substituted by a few metal cations which have a sterochemically active lone pair ofelectrons, such as Pb2+, Sn2+, Sb2+, or Te4+, because Bi has a stable lone electron pair theCa2+, Sr2+, and Bi3+ cat ions disperse in the A sites, and Ti4+ cations occupy the B sites inthe pseudo perovskite layers. The ratio of (Ca, Sr)/Bi in the A site is 1:2 and there are 1

4 ofA sites vacant in the SCBT lattice [8] as shown in Fig. 1.

The low crystal symmetry and high coercive fields observed in these compounds resultin the high stability of the piezoelectric properties for ceramic materials of Bi4SrTi4O15

either under high temperature or under one-dimensional stress. Such compounds are impor-tant for many applications involving piezoelectric properties of ceramics. Different methods

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of preparation of the compounds have been suggested viz. hot forging, hot rolling, hot ex-trusion and super plastic deformation. These methods give ceramics with grain orientationsand different densities [9]. Ferroelectric and dielectric properties of SCBT are investigated,prepared via high-energy mechano-chemical process.

2. Experimental Details

2.1 Sample Preparation

The initial compounds SrCO3, Bi2O3, CaCO3, TiO2 (Sigma Aldrich, 99.9% pure, ARgrade) were mixed in appropriate ratios for the synthesis of the desired compound. Thepolycrystalline samples of Bi4SrTi4O15 [SBT] and Bi4Sr0.9Ca0.1Ti4O15 [SCBT] were pre-pared by the method of reactive sintering. The standard ceramic fabrication procedure hasbeen followed for the preparation. The oxide mixture, weighing 50 g, was subjected to highenergy milling in a planetary ball mill (Fritsch Pulverisette 6) at a speed of 150 rpm for 5 hr.A cylindrical tungsten carbide (WC) milling jar, with a usable volume of 250 ml, filled with50 WC balls, each of 10 mm diameter, was used for the purpose of milling. Both vial andballs were from Fritsch GmbH. A total of 50 g of powder was placed in the vial using wateras the milling medium. The milling was stopped for 5 min after every 30 min of milling tocool down the system. Figure 2 illustrates that during each collision the powder particles gettrapped between the colliding balls, between the ball and the inner surface of the vial andlead to smaller particle size. The particle size of the initial materials ranged from 1–2 µm.The mixture was stacked in a crucible and calcined in air at 800◦C for 4 hr and then cooled.The calcined powders were milled for a few minutes to crush any lumps that were formedduring the calcinations. The calcined powders were cold isostatically compressed into theform of a cylindrical rod at a pressure of 3000 Mpa and sintered at 1200◦C for 2 hr and thenallowed to furnace cool. Pellets having a diameter of 1 cm and thickness of 1 mm were cutfrom the sintered samples using a low speed isomet saw. The entire sintering was done in amicroprocessor-controlled furnace. The densified pellets were used for all measurements.

Figure 2. Milling by crushing of particle between the vial surface and balls.

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2.2 Characterization and Measurements

The density of the pellets was determined by Archimedes principle. For electrical measure-ments, samples were polished and coated with Au electrodes on the larger faces. Typicalsample dimensions were 10 mm diameter and 1 mm thickness. In order to make electricalmeasurements, it is necessary to make the samples ‘piezoelectrically active’. Therefore,electrical poling was done at a field of 5 kV cm−1 for 30 min at 100◦C immersing thesamples in a silicon oil bath (Dow Corning 704 R©). The field was retained while coolingas well. The dielectric measurements were made on electrically poled samples with anImpedance Analyzer (HP4192A) at different frequencies from room temperature to 600◦C.These measurements were carried out on silver coated pellets by placing them in betweentwo electrodes, which in turn were connected to the leads of the impedance analyzer.The resonance measurements were carried out in the frequency range 100 Hz–1 MHz atroom temperature using HP-4294A interfaced to computer. The ferroelectric hysteresisloop measurements were done placing the silver coated samples in a silicon oil bath inbetween two electrodes using a ferroelectric hysteresis loop analyzer based on a modifiedSwayer-Tower circuit interfaced to computer. Samples used for P−E measurements werenot poled. Resonance data was used to calculate elastic compliance [s33, s11] and couplingfactor [k33, k31] for SBT and SCBT [10] respectively.

3. Results and Discussions

3.1 Dielectric Measurements

The SBT, SCBT phase evolution was studied by X-ray analysis. Figure 3 contains the XRDpatterns of SBT and SCBT ceramics. It can be observed that the substitution of Ca ions forSr ions didn’t have much impact on the change of the phase. The density of the sampleswas found to be > 90% of the XRD density.

Figures 4(a) and 4(b) show the SEM scan for SBT and SCBT ceramics. Figures 5(a)and 5(b) show the relative dielectric constants and dielectric losses at different frequencies

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Figure 3. XRD trace of (a) SBT and (b) SCBT ceramics. (See Color Plate XI)

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Figure 4. SEM trace of (a) SBT and (b) SCBT ceramics.

as a function of temperature for SBT. An increase in the dielectric constant can be seenwith the substitution of calcium ions in strontium site. An increase in the Curie temperaturecan be observed from the Figs. 5(a) and 5(c) as a result of calcium substitution. When thetemperature rises, the thermal vibration leads to increase in Ca mobility. Therefore, thecontribution from the ionic polarization and the dielectric permittivity are enhanced [11].

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Figure 5. Temperature dependence of (a) dielectric constant and (b) loss tangent of SBT measuredat different frequencies. Temperature dependence of (c) dielectric constant and (d) loss tangent ofSCBT at different frequencies. (See Color Plate XII)

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Table 1The values of TC and Dielectric constant for SBT and SCBT samples

Sample Dielectric Constant TC (◦C)

SBT 1765 527SCBT 1949 556

The transition is considered to be the Curie point and is of second-order [12]. The dielectricconstant and transition temperature for SBT and SCBT samples are shown in Table 1.

The dielectric constant vs. temperature curve obeyed a Curie-Weiss law.

ε = C/(T − Tc),

Where ε is the dielectric constant, C is the Curie constant and Tc is the transition temperature.Figures 5(b) and 5(d) depict the variation of tanδ as a function of both temperature and

frequency for SBT and SCBT respectively. The value of tanδ increases appreciably beforethe dielectric transition. It decreases with increasing frequency at higher temperatures andthe peak in tanδ becomes broader. From Figs. 5(b) and 5(d) it is observed that the tanδ

value decreases considerably by the incorporation of Ca in SBT.Figure 6 shows the polarization Vs electric field (P Vs E) Hysteresis loop for the

sintered SBT and SCBT samples. The values of Pr and Ec are shown in Table 2. Previousstudies on similar compounds have shown relatively lower values of Pr. In the current casewe have found significant enhancement of Pr which is noteworthy. The enhancement ofPr could be due to any or all of the following reasons: absence of a significant numberof domain wall pinning centers, uniform distribution of grain size in the milled samples,uniform distribution of composition, low defect density etc. In the present case samplesof SCBT were prepared so as to obtain a grain size distribution, which was in a narrowwindow. Thereby clamping of domain wall grain boundary from neighbours due to verysmall size could be avoided.

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Figure 6. Polarization vs. Electric field Hysteresis loop for SBT & SCBT. (See Color Plate XIII)

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Table 2The values of Pr and Ec for SBT and SCBT samples

Coercive field Remnant PolarizationSample (kV/cm) (µC/cm2)

SBT 11.011 0.773SCBT 34.451 3.220

It is possible that conditions used during synthesis of SCBT viz high-energy millingprovided an environment conducive for formation of a significant number of 180◦ domainwalls thereby, resulting in significant enhancement in remnant polarization. The 2Pr reachesa maximum value of 6.23 [µC/cm2]. On the other hand the tolerance factor, t, for theperovskite structure is given by

t = (rA + r0)√2(rB + r0)

Where rA, rB and ro are ionic sites of an A-site cation, a B-site cation and oxygen ionrespectively. In SCBT, rA is 0.117 nm (Bi3+) or 0.132 nm (Sr2+) or 0.099 nm (Ca), rB is0.0559 nm (Ti4+) and ro is 0.14 [13].

Obviously the tolerance factor of SCBT is less than 1.0 and the substitution of Ca forSr should lead to the increase in ‘t’. The maximum field applicable was limited on accountof the power supply limitations.

3.2 Piezo-Electric Electromechanical Coupling

Ceramic samples as per IEEE standards were used to determine piezoelectric and elec-tromechanical coefficients. The resonance and anti-resonance frequencies were determinedfrom the first minimum and maximum impedance peaks respectively, from the impedanceVs frequency scan. Table 3 shows the Elastic compliance [s33, s11] and electromechanicalcoupling factor [k33, k31] for SBT and SCBT.

Table 3The Measured Electromechanical Coefficients

Constant SBT SCBT

k33 0.38 0.23k31 0.39 0.23kp 0.41 0.23s33

D 1.62 × 10−12m2/N 1.34 × 10−12m2/N

s33E 1.90 × 10−12m2/N 1.42 × 10−12m2/N

s11E 1.84 × 10−12m2/N 1.41 × 10−12m2/N

s11D 1.56 × 10−12m2/N 1.33 × 10−12m2/N

c33D 1.139 × 1011N/m2 1.055 × 1011N/m2

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4. Conclusions

The electric properties of SBT are notably improved by Ca substitution. The TC of SBTshows a slight shift to higher temperature upon Ca substitution. This implies that Ca-substitution enhances the TC of the un-doped material SBT thereby increasing the operatingrange of the material. Further Ca substitution also results in a significant enhancement ofPr of SCBT. The sample SCBT has a large remnant polarization and a high TC near 556◦C.It is believed that SCBT is potentially an attractive candidate for piezoelectric sensorapplications.

Acknowledgments

The authors thank the Director DMRL for providing experimental facilities, Dr KC JamesRaju, School of Physics, HCU, for resonance data, Department of Physics, MRL, OsmaniaUniversity, for dielectric measurements. One of the authors (GA) thanks Principal andManagement of Vignan Isntitute of Technology and Science, Nalgonda for their constantencouragement.

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