Elsevier

Materials Science and Engineering: B

Volume 211, September 2016, Pages 101-109
Materials Science and Engineering: B

Nano/micro Sr2Bi4Ti5O18 crystallites: Size dependent structural, second harmonic and piezoelectric properties

https://doi.org/10.1016/j.mseb.2016.06.003Get rights and content

Highlights

  • Nano/microcrystallites of Sr2Bi4Ti5O18 were synthesized via sol-gel route.

  • Crystallite size dependent structural and physical properties were studied.

  • SHG intensity (1.4 times that of KDP powder) from these crystallites was recorded.

  • PFM studies on isolated crystallite of 480 nm exhibited d33 as high as 27 pm/V.

  • Single domain nature of the crystallites below 160 nm was observed.

Abstract

Strontium bismuth titanate (Sr2Bi4Ti5O18) powders comprising crystallites of average sizes in the range of 94–1400 nm were prepared via citrate-assisted sol-gel route. With an increase in the average crystallite size there was a change in the lattice parameters and shift in the Raman vibration modes. Second harmonic signal (532 nm) intensity of the Sr2Bi4Ti5O18 powders increased with the increase in the average crystallite size and the maximum intensity obtained in the reflection mode was 1.4 times as high as that of the powdered KH2PO4. Piezo Force Microscopic analyses carried out on isolated crystallite of size 74 nm, established a single domain nature with the coercive field as high as 347 kV/cm. There was a systematic increase in the d33 value with an increase in the size of the isolated crystallites and a high piezoelectric coefficient of ∼27 pm/V was obtained from an isolated crystallite of size 480 nm.

Graphical abstract

Synthesis of Sr2Bi4Ti5O18 nano/micro crystallites and their size dependent non-linear optical and piezoelectric responses.

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Introduction

Ferroelectric materials belonging to AURIVILLIUS family of oxides crystallize in non-centrosymmetric structures formed by bismuth oxide ([Bi2O2]2+) layers interleaved with ‘n’ number of pseudo-perovskite type [An+1BnO3n+1]2− units. Here, ‘A’ stands for a mono-, di- or trivalent ions or a combination of them, and B represents a small ion with high valency such as Ti4+, Nb5+, Ta5+, W6+ etc, or a combination of them [1], [2], [3]. Smolenskii et al. [4] reported ferroelectricity in the n = 2 member (PbBi2NbO9) of this family, whereas chemical and structural criteria for the formation of these layered compounds were elucidated by Subbarao [5]. These bismuth layer-structured ferroelectrics (BLSF) are well known owing to their superior polarization fatigue endurance coupled with high Curie temperatures. These materials, including SrBi2M2O9 (M = Ta, Nb) are popular for modern microelectronic applications, particularly for ferroelectric random access memories (FeRAM) and dynamic random access memories (DRAM) [6], [7], [8], [9], [10], [11]. One of the major requirements in the aerospace and automotive industries is the piezoelectric device that can function in the wider temperature range and under extreme conditions. For these applications Aurivillius-type compounds are promising candidates, as these are lead-free and high temperature piezoelectric [12], [13], [14], [15], [16], [17]. Sr2Bi4Ti5O18 (SBT) belongs to n = 5 member of the BLSF family, where Bi2O2 layers alternate with (Sr2Bi2Ti5O16) perovskite blocks built by five TiO6 octahedral layers and Sr/Bi occupy A sites. SBT has a Curie temperature of 280 °C, piezoelectric coefficient (d33) of 25 pC/N, electromechanical coupling factor k33 of ≈15% (and kp  7%) and mechanical quality factor (Qm) as high as 1400. Because of these interesting characteristics it finds applications in piezoelectric resonators, filters and pyroelectric sensors with large figures of merit [18], [19], [20], [21], [22], [23].

Several reports on the grain size dependent ferroelectric properties of various materials exist in the literature. These studies mainly constitute simple perovskites e.g. BaTiO3, PbTiO3 and Bi4Ti3O12 which is a BLSF [24], [25], [26]. The grain size dependent dielectric, piezoelectric and ferroelectric properties of the SBT ceramics were studied [27]. However, for fabricating miniaturized piezoelectric devices such as nano generators, nano-actuators, it is very much essential to have prior knowledge about the piezoelectric responses from the isolated crystallites, particularly a few nanometers sized [28]. Piezo Force Microscopy (PFM) is a unique and well-established technique as the other conventional methods fail to probe the piezo-response of individual nano/micro crystallites [29], [30], [31], [32], [33]. We thought that it was worth investigating into the piezoelectric properties of individual nano/micro crystallites of SBT using PFM. The nanocrystallites of SBT for this study were synthesized via citrate assisted sol-gel technique and the heat treatment temperatures were varied in order to obtain different crystallite sizes. In this report we describe the details with regard to the effect of average crystallite size on the Raman vibration modes, optical properties and piezoelectric properties of the individual nano/micro crystallites of SBT.

Section snippets

Synthesis of nano/micro crystallites

Nanocrystalline powders of Sr2Bi4Ti5O18 were synthesized via citrate assisted sol-gel route using titanium (IV) isopropoxide [TIP] (Ti[OCH(CH3)2]4, Spectrochem, 98+%), bismuth (III) nitrate pentahydrate [BN] (Bi(NO3)35H2O, AR, Alfa Aesar 98+%), strontium nitrate [SN] (Sr(NO3)2, Merck, 99%), citric acid (SDFCL, 99.5%), acetyl acetone (Spectrochem, 99%) and ammonia solution (SDFCL) as starting materials. To begin with TIP (0.05 mol) was mixed with acetyl acetone (0.05 mol) and stirred for 10 min

Structural and micro structural studies

The XRD patterns obtained as shown in Fig. 1(a–f) are for the SBT powders heat treated at 700 °C, 800 °C, 900 °C, 1000 °C, 1100 °C and 1150 °C respectively. All the Bragg peaks encountered for these samples could be indexed to the space group B2cb of SBT [34].

The refined lattice parameters obtained by profile refinement (Le Bail method) using Jana 2006 software [35] are summarized in Table 1. Peak profile was refined using Pseudo-Voigt function and Legendre polynomials (15 terms) were used to fit the

Conclusions

Fine powders of Sr2Bi4Ti5O18 (SBT) synthesized by citrate assisted sol-gel route on subsequent heat treatment at appropriate temperatures resulted in powder batches comprising average crystallite sizes lying in the range of 94–1400 nm. The crystallite size effects on the lattice parameters, Raman vibration modes and optical band gap were established. Optical second harmonic intensity was found to increase with the increase in crystallite size of SBT. The single domain nature of SBT crystallites

Acknowledgements

Authors are thankful to Prof. K.K. Nanda, Materials Research Centre, Indian Institute of Science, Bangalore, for providing the Raman facilities. One of the authors Tukaram Shet acknowledges AICTE, Govt. of India for granting QIP fellowship and Rajasekhar Bhimireddi acknowledges the financial support provided by University Grants Commission, Govt. of India through the Dr. D. S. Kothari Postdoctoral Fellowship.

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