M A T E R I A L S C H A R A C T E R I Z A T I O N 6 5 ( 2 0 1 2 ) 1 2 6 – 1 3 2

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Nanocrystalline non-stoichiometric SBT: Effect of millingduration on structural and electrical characteristics

Sugandhaa, A.K. Jhaa, b,⁎aThin Film andMaterial Science Laboratory, Department of Applied Physics, Delhi Technological University (Formerly Delhi College of Engineering),Delhi-110042, IndiabDepartment of Applied Sciences, Ambedkar Institute of Technology, GGSIPU, Geeta Colony, Delhi-110092, India

A R T I C L E D A T A

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E-mail addresses: miglani.sugandha@gm

1044-5803/$ – see front matter © 2012 Elseviedoi:10.1016/j.matchar.2011.12.011

A B S T R A C T

Article history:Received 28 August 2011Received in revised form14 December 2011Accepted 23 December 2011

In the present work, nanocrystalline specimens of non-stoichiometric Strontium BismuthTantalate (SBT) ferroelectric ceramics were synthesized by mechanical activation processusing a high energy planetary ball mill. The powders were milled for different milling dura-tions (5, 10, 20 h) keeping the milling speed fixed at 300 rpm. Microstructural characteriza-tions have been performed using X-ray diffraction, scanning electron microscopy,electron diffraction and transmission electron microscopy. Nanocrystallites with averagegrain size in the range of 30–50 nm are observed to be formed. Grain size is observed to de-crease with increasing milling duration. Detailed dielectric study as a function of tempera-ture has been carried out. It is observed that dielectric constant increases and dielectric lossdecreases with increasing milling duration. The observed characteristics have beenexplained in terms of increased number of grain boundaries due to the reduction of granu-lar size.

© 2012 Elsevier Inc. All rights reserved.

Keywords:Electro-ceramicsNanocrystalline SBTDielectric property

1. Introduction

Nanocrystalline ferroelectric ceramics are important electron-ic materials having a wide range of scientific and industrialapplications such as high dielectric constant capacitors, pie-zoelectric transducers, pyroelectric sensors, non volatile fer-roelectric random access memories, etc. It is well knownthat performance of any material is closely related to its syn-thesis process. The methods of synthesis of ferroelectric pow-ders play a significant role in determining themicrostructural,electrical and optical properties of these materials [1–3]. Con-ventionally these ceramics are synthesized by solid state reac-tion process, using constituent oxides/carbonates as thestarting materials. In the last decades, various wet chemistrymethods such as chemical co-precipitation, sol–gel, hydro-thermal synthesis [4–6], etc. have also been explored to syn-thesize nanocrystalline ferroelectric ceramics. Although

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significant progress has been achieved, there are certain prob-lems associated with above techniques. For example, sol–gelprocess uses metal alkoxides as the starting material whichare very expensive and extremely sensitive to the environ-mental conditions such as moisture, light and heat. Co-precipitation technique involves repeated washing in orderto eliminate anions coming from the precursor salts used,making the process complicated and time consuming. Fur-thermore, it is difficult to produce large batches by usingmost of the chemical solution processing routes. Therefore,exploring alternative methods for the preparation of ferro-electric ceramics is still of scientific and technologicalsignificance.

Mechanical activation (high energy ball milling) is a tech-nique which has been recently used to synthesize various fer-roelectric ceramics such as PbZr1−xTixO3 (PZT), BaTiO3,Ba5SmTi3Nb7O30 and various bismuth containing aurivillius

nstitute of Technology, GGSIPU, Geeta Colony, Delhi-110092, India.

kjha@gmail.com (A.K. Jha).

127M A T E R I A L S C H A R A C T E R I Z A T I O N 6 5 ( 2 0 1 2 ) 1 2 6 – 1 3 2

compounds such as Bi4Ti3O12 (BiT), SrBi2Ta2O9 (SBT), SrBi4Ti4-O15, etc. [7–10]. The most significant characteristic of thistechnique is the formation of the desired compound is dueto the reaction of the oxide precursors which are activatedby mechanical energy, instead of the heat energy required inconventional solid state reaction technique. Secondly, ittakes place at room temperature in well sealed containers,thus effectively minimizing the loss of the volatile compo-nents such as lead and bismuth.

It is worth mentioning here that in the mechanical activa-tion technique, milling duration has significant effect on thephase formation and final particle/grain size in the synthe-sized specimens. Excessive milling is known to introduce var-ious defects such as contamination, lattice distortion, stress,vacancies, etc. Elimination of these defects takes place at theexpense of grain growth [11]. Therefore, in this procedure it ispossible to control the final particle/grain size of synthesizedspecimen by optimizing the milling duration. As discussedearlier, mechanical activation technique is a promisingtechnique for the synthesis of bismuth layer structured ferro-electric ceramics. This is due to the fact that this techniquedoes not require high temperature calcination and lowers thesintering temperature, which is themain drawback in the con-ventional solid state reaction technique.

Among bismuth layer structured ferroelectrics, stoichio-metric SBT has generated great interest due to its promisingproperties such as high dielectric constant, high remnant po-larization, low dielectric loss, low leakage current, etc [12].However, due to bismuth volatilization, stoichiometric SBTsuffers from many intrinsic defects such as oxygen vacancies.This results in degraded ferroelectric properties and fatigueendurance [13,14]. It has been reported that by varying Sr/Biratio in starting composition comparatively enhanced electri-cal properties are obtained at lower processing temperature[15,16]. The lack of reports in mechanically activated non-stoichiometric SBT has prompted the authors to synthesizeSr0.8Bi2.2Ta2O9 compound by varying the milling durationand investigate its effects on the structural and electricalproperties.

Fig. 1 – X-ray diffraction patterns of Sr0.8Bi2.2Ta2O9 powderssubjected to different milling durations of mechanicalactivation process.

2. Experimental Procedure

Highly pure powders of SrCO3, Bi2O3, Ta2O5 (all from M/sAldrich) were mixed to yield Sr0.8Bi2.2Ta2O9. This mixture ofpowders was milled in a high-energy planetary ball mill(Retsch, PM 100) for 5, 10, and 20 h at a milling speed of300 rpm. Milling was carried out in toluene medium with ahigh wear-resistant 10 mm zirconium oxide balls in a zirconi-um oxide vial with a ball-to-powder weight ratio of 10:1. Themixture was then admixed with 2 wt.% polyvinyl alcohol asa binder and then pressed at 200 MPa into a disk shapedpellet. These pellets were then sintered at 1100 °C for 2 h inair on alumina crucibles.

Phase development in the synthesized compound wasmonitored using an X-ray diffractometer (Bruker, D8 Advance)with CuKα radiation (λ=1.5405 Å) at a scanning rate of 1°/min.Transmission electron microscopy (Jeol, 2100-F) and scanningelectron microscopy (Hitachi, S-3700N) were used to studytheir structural morphology. The sintered pellets were

polished and silver pasted on both sides and cured at 500 °Cfor 1 h. The dielectric measurements were carried out usinga precision LCR meter (Agilent 4284A) at an oscillation ampli-tude of 1 V. The dc electrical resistivities of the samples weremeasured using a programmable electrometer (Kiethley,6517A).

3. Results and Discussions

3.1. Structural and Microstructural Studies

Fig. 1 compares the X-ray diffraction (XRD) patterns of theSr0.8Bi2.2Ta2O9 powders milled for different durations i.e. 5,10, and 20 h. The XRD peaks were indexed using the observedd-values and software package, Powder X [17]. It can be seenthat phase development starts from 5 h and improves withthe duration of milling. A shift of the peaks toward higher an-gles is seen on increasing the milling duration. The specimenmilled for 20 h clearly exhibit layered perovskite phase forma-tion with distinct (115), (008), (200) and (220) peaks. Further,the broadening of the characteristic peaks such as (115),(006), (220), etc. indicates the reduction of particle size andthe formation of nanocrystallites in the specimen milled for20 h. When the specimen is milled for longer duration like20 h using zirconium balls and vial, it is likely that some zirco-nium atoms enter the perovskite structure [18]. However, no

Fig. 3 – Transmission electron micrograph and electrondiffraction pattern of 20 h milled Sr0.8Bi2.2Ta2O9 powder.

128 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 5 ( 2 0 1 2 ) 1 2 6 – 1 3 2

such extra peaks corresponding to ZrO2 or the compound in-volving zirconium are seen in the XRD diffractograms. Fur-ther, had zirconium entered the structure during the millingprocess the peak should have shifted to lower angles insteadof the observed shift towards higher angles as the ionic radiiof Zr4+ (0.79 Å) is higher that that of Ta5+ (0.64 Å). This is pos-sibly due to high wear resistant balls and vials used in the pre-sent work [19,20].

Fig. 2 shows the scanning electron microscope images ofthe specimen milled for different durations. The size of theparticles is seen to be decreasing with increasing millingtime. It is observed that the sample milled for 5 h exhibitlarge particles with many small size grains are seen to stickto the surface of large grains. In the specimen milled for10 h, the number of large particles gets reduced formingloose agglomerate. The sample milled for 20 h shows adense microstructure indicating agglomeration of particlesdue to further reduction in the grain size. The imagesobtained from transmission electron microscopy and electrondiffraction of the studied specimen milled for 20 h are shownin Fig. 3. The average particle size is seen to be in the range of30–50 nm. Also, the formation of particles of nano dimensionsis further confirmed by the appearance of diffraction ringsconsisting of discrete diffraction spots in the electron diffrac-tion pattern. As shown in the diffraction pattern, five distinctrings have been indexed [21,22]. The phase formation of ferro-electric nanocrystallite specimen using high energy ball mill-ing is a complex process. Mechanical activation of startingoxides/carbonates, to form desired ferroelectric phase, takesplace in two stages. In the first stage, the milling process re-duces the particle size of the starting oxides. This results inthe formation of high defect densities, shorter diffusion dis-tances, more intimate contacts of precursors, etc. At the

Fig. 2 – Scanning electron micrographs of Sr0.8Bi2.2Ta2O9 powders prepared at different milling durations.

Fig. 4 – X-ray diffraction pattern of Sr0.8Bi2.2Ta2O9 sintered samples milled at different durations.

129M A T E R I A L S C H A R A C T E R I Z A T I O N 6 5 ( 2 0 1 2 ) 1 2 6 – 1 3 2

same time fresh/cleansed surface interfaces are also created.All these factors contribute to enhance the reactivity and sin-terability of the starting oxide mixture. In the later stage, after

Table 1 – Comparison of different parameters in Sr0.8Bi2.2Ta2Odifferent milling durations.

Milling duration (h) a (Å) b (Å) c (Å)

5 5.4772 5.5089 24.845110 5.4719 5.5058 24.702120 5.4573 5.5027 24.6451

Fig. 5 – Scanning electron micrographs of Sr0.8Bi2.2Ta2

sufficient activation, nucleation and growth of the nanocrys-tallite structure takes place resulting into the formation of de-sired perovskite ferroelectric phase. This is attributed to the

9 samples prepared by mechanical activation process for

V (Å3) Crystallite size (nm) εmax Tc (K)

749.659 60.8 578.532 653742.168 39.0 759.786 643740.089 27.6 859.325 628

O9 sintered samples milled at different durations.

130 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 5 ( 2 0 1 2 ) 1 2 6 – 1 3 2

local heating and high localized pressure during collision ofzirconium balls with the vial. It is worth mentioning herethat although the overall temperature of a milling system islower than 100 °C, the “in situ” impact temperature is suffi-ciently high to activate the solid state reaction [23,24].

Fig. 4 shows the X-ray diffractogram patterns of the sin-tered specimen milled for different durations. It is observedthat single phase layered perovskite structure is formed in allthe samples. In these diffractograms the shift of peaks toslightly higher angles with increasing milling duration is ob-served. It was observed that the d-values of the characteristicpeaks are matching with that obtained in the X-ray diffracto-gram patterns (Fig. 1) of the as-milled powders suggestingthe initiation of the development of the layered perovskitephase in the milled powders. The lattice parameters for thesynthesized sampleswere calculated using the observed inter-planar spacing, d-values, obtained from the diffractogramsand refined using the least square refinement method by the

Fig. 6 – Temperature variation of (a) dielectric constant (ε′r) and (samples at different milling durations.

computer program package Powder X are given in Table 1. Itis observed that the lattice parameters and unit cell volumedecrease with increase in milling duration. The crystallitesize calculated from the broadening of X-ray diffractogrampeaks using the Scherrer's formula [25] ofmilled Sr0.8Bi2.2Ta2O9

decreases with increasing milling time (Table 1). The corre-sponding scanning electron micrographs of the sample pre-pared at different milling times are shown in Fig. 5. Theaverage grain size of the sintered specimen decreases from 1to 2 μm in the sample milled for 5 and 10 h to<1 μm in thesample milled for 20 h. Also, the calculated surface area in-creases from 22 m2/g in the sample milled for 5 h to 28 m2/gand 43 m2/g respectively in the samplesmilled for 10 and 20 h.

3.2. Dielectric Studies

The temperature dependence of dielectric constant (ε′r) anddielectric loss (tanδ) measured at 100 kHz are shown in

b) dielectric loss (tanδ) for mechanical activation processed

131M A T E R I A L S C H A R A C T E R I Z A T I O N 6 5 ( 2 0 1 2 ) 1 2 6 – 1 3 2

Fig. 6(a) and (b) for all the studied samples. All the samplesexhibit sharp ferroelectric to paraelectric phase transition attheir respective Curie temperature (Tc). It is observed inFig. 6(a) that dielectric constant value increases with increasein milling time and is maximum in the sample milled for20 h. Further, Curie temperature is observed to decreasewith increase in milling duration (Table 1). It is known thatin the fine grained specimen, the dielectric constant in-creases due to an increase in residual internal stress[26–28]. Thus, the increase in ε′r value can be attributed tothe decrease in the average grain size with an increase inmilling time (Fig. 5). The observed decrease in transition tem-perature with increasing milling time is attributed to the re-duction in grain size [29,30]. It is known that specimenswith smaller grains posses more interfacial or grain bound-ary energy as it contains larger number of grain boundaries.This results in the requirement of smaller amount of thermalenergy to undergo the phase transition resulting in the re-duction of Tc [31].

Fig. 6(b) shows the variation of dielectric loss (tanδ) as afunction of temperature for all the studied samples. It is ob-served that dielectric loss decreases with increasing millingduration and is minimum in the sample milled for 20 h. Thisis possibly due to improved and dense mirostructure havingless porosity (as observed in Fig. 5) [32,33]. In all the samplesit is observed that tanδ remains nearly constant in the lowertemperature region and thereafter increases rapidly at highertemperature. However, the loss in the sample milled for 20 hdoes not considerably increase in higher temperature region.The sharp increase of dielectric loss at high temperature re-gion can be attributed to the increased mobility of spacecharges arising from defects or vacancies (oxygen vacancies)in the sample [34].

3.3. D.C. Conductivity

Fig. 7 shows the variation of dc conductivity (σdc=1/ρ) with in-verse of temperature (103/T) of the studied specimens. For all

Fig. 7 – Variation of dc conductivity (lnσdc) with inverse of tempedifferent milling durations.

the studied samples, conductivity is constant in the lowertemperature region and thereafter increases considerablywith temperature. This indicates negative temperature coeffi-cient of resistance (NTCR) type behavior. As seen in the figure,there are two predominant types of conduction mechanism,one is the lower temperature region in which dc conductivityremains invariant with temperature and the other is highertemperature region in which dc conductivity increases sharp-ly with temperature. In lower temperature region from roomtemperature to approximately ~573 K the electrical conduc-tion is dominated by extrinsic defects such as unintentionallyadded impurities. Whereas, in higher temperature region~573 K to ~973 K the conduction is dominated by intrinsic de-fects [35,36]. It is observed that dc conductivity decreases withmilling duration and is lowest for 20 hmilled sample. This canbe attributed to the increase in scattering of charge carriers. Itis known that various types of defects such as grain bound-aries, pores, vacant spaces, etc. affect the motion of chargecarriers [37]. In nanocrystalline specimens, scattering bygrain boundaries is more pronounced as compared to scatter-ing by defects or impurities. This is due to the fact that nano-crystalline samples have high grain boundary area to volumeratio resulting into an increase in electron scattering. There-fore, the sample milled for 20 h exhibit lower electrical con-ductivity as compared to the samples milled for 5 and 10 h[38].

4. Conclusion

Nanocrystalline Sr0.8Bi2.2Ta2O9 has been successfully synthe-sized by the mechanical activation process. The synthesizedsamples exhibit single phase layered perovskite structure.Transmission electron microscope image and electron diffrac-tion patterns reveal that the particle size in the specimenmilledfor 20 h reduces to nano range. As the milling duration has in-creased dielectric constant increases considerably whereas theCurie temperature decreases slightly. Sr0.8Bi2.2Ta2O9 compound

rature (103/T) for mechanical activation processed samples at

132 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 5 ( 2 0 1 2 ) 1 2 6 – 1 3 2

prepared by above technique for 20 hmilling exhibit highest di-electric constant, dc resistivity and lowest dielectric loss.

Acknowledgment

Authors expressed their thanks to University Grants Commis-sion (UGC), New Delhi for the research grant (Grant No-F. No.39-469/2010 (SR)).

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