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Hindawi Publishing CorporationJournal of CeramicsVolume 2013, Article ID 261914, 6 pageshttp://dx.doi.org/10.1155/2013/261914
Research ArticleEffect of Processing on Synthesis and Dielectric Behavior ofBismuth Sodium Titanate Ceramics
Vijayeta Pal,1 R. K. Dwivedi,1 and O. P. Thakur2
1 Department of Physics and Material Science & Engineering, Jaypee Institute of Information Technology, Noida 201307, India2 Electroceramics Group, Solid State Physics Laboratory, Defence Research and Development Organization (DRDO), Timarpur,Delhi 110054, India
Correspondence should be addressed to Vijayeta Pal; vijayetapal@yahoo.in
Received 29 June 2012; Accepted 20 November 2012
Academic Editor: Baolin Wang
Copyright ยฉ 2013 Vijayeta Pal et al. is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
An effort has been made to synthesize polycrystalline (Bi1โ๐ฅ๐ฅLa๐ฅ๐ฅ)0.5Na0.5TiO3 (abbreviated as BLNT) system with compositionsx = 0, 0.02, and 0.04 by novel semiwet technique. Preparation of A-site oxides of BLNT for composition x = 0 was optimizedusing two precursor solutions such as ethylene glycol and citric acid. e XRD patterns revealed that the sample prepared byethylene glycol precursor solution has single phase perovskite structure with a rhombohedral symmetry at RT as compared tothe sample prepared by citric acid. Ethylene glycol precursor has been found to play a signi๏ฟฝcant role in the crystallization, phasetransitions, and electrical properties. e studies on structure, phase transitions, and dielectric properties for all the samples havebeen carried out over the temperature range fromRT to 450โC at 100 kHz frequency. It has been observed that two phase transitions(i) ferroelectric to antiferroelectric and (ii) antiferroelectric to paraelectric occur in all the samples. All samples exhibit a modi๏ฟฝedCurie-Weiss law above Tc. A linear ๏ฟฝtting of the modi๏ฟฝed Curie-Weiss law to the experimental data shows diffuse-type transition.e dielectric as well as ferroelectric properties of BLNT ceramics have been found to be improved with the substitution of Laelements.
1. Introduction
Lead oxide-based ceramics with perovskite structure havebeen the subject of attraction for high-performance sensors,actuators, transducers, and other applications, owing to theirsuperior dielectric, piezoelectric, and electromechanical cou-pling coefficients. Applications are restricted to temperaturerange โ50 to 150โC [1]. However, in recent years many ๏ฟฝeldshave expressed the need for actuation and sensing which canbe used at higher temperatures (>400โC) such as automotive,aerospace, and related industrial applications. On the otherhand, in most of the cases lead constitutes more than 60% ofthe composition of these piezoelectric devices. Lead, knownto be highly toxic and volatile, is released to the atmosphereduring sintering causing serious environmental and healthproblems. Another cause for concern is the disposal of theseproducts at the end of the life cycle. Considering all thesehealth concerns posed by lead, multinational governments
like the European Union have enacted laws that ban the useof lead in the manufacture of many industrial products [2].is has led to the replacement of lead (Pb) in the ๏ฟฝeld ofpiezoelectric ceramics. A lot of research has been carriedout on lead-free piezoceramic products in the last ๏ฟฝ๏ฟฝy yearsbut in the last few years, the momentum has tremendouslyincreased, accounting for about 75% of all published worksin this ๏ฟฝeld. Bi0.5Na0.5TiO3- (BNT-) and K0.5Na0.5NbO3-(KNN-) based materials are two main material systems withperovskite structure, which have been studied to ๏ฟฝnd thesubstitute of PZT for lead free piezoelectric applications.Pure BNT was discovered by Smolenskii et al. [3] and isa ferroelectric having Bi3+ and Na+ complexes on the A-site of ABO3-type perovskite structure with a rhombohedralsymmetry. Because of a large remanent polarization (๐๐๐๐ =38 ๐๐C/cm2) at room temperature, BNT ceramic is consideredas one of the promising candidates for lead free piezoelectricceramics [4]. However, the poling of pure BNT ceramic is
2 Journal of Ceramics
very di๏ฟฝcult due to its high coercive ๏ฟฝeld (๐ธ๐ธ๐๐ = 73 kV/cm).So, the pure BNT ceramic usually exhibits weak piezoelectricproperties.erefore, a number of BNT-based ceramics wereprepared to improve the electrical properties of this materialby the convectional solid state method [5, 6]. Recently, alot of efforts have been made to prepare the material byvarious chemical methods, such as hydrothermal process[7], citrate method [8, 9], sol-gel, autocombustion [10], andstearic acid gel route [11]. In the present work, A-site oxidesof lead-free Bi0.5Na0.5TiO3 ceramics were optimized usingtwo precursor solutions such as ethylene glycol and citricacid at low calcination temperature (750โC) and furthersome La doped BNTs (BLNTs) have been developed usingethylene glycols precursors by a novel semiwet technique andstructural, dielectric, and ferroelectric properties have beenstudied for both systems. To the best of our knowledge, BLNTsystem has been synthesized for the ๏ฟฝrst time using semiwettechnique. is technique has been applied to make othersystems enhance its properties [12].
2. Experimental Procedures
A novel semiwet technique was used to prepare lead-freeceramic (Bi1โ๐ฅ๐ฅLa๐ฅ๐ฅ)0.5Na0.5TiO3 (BLNT) system. ese com-positions were prepared using analytical-grade metal oxidesor nitrate powders of sigma Aldrich as raw materials suchas Bi2O3 (99%), La2O3 (99%), NaNO3 (99%), TiO2 (99.9%)citric acid, and ethylene glycol. In this method, A-site ofBLNT with composition ๐ฅ๐ฅ = 0 was prepared by usingtwo different precursor solutions๏ฟฝ the ๏ฟฝrst one is citric acid,and๏ฟฝ the second is ethylene glycol. In the ๏ฟฝrst process, anappropriate amount of citric acid solution was added to thesolution of nitrates of A-site cations in a beaker (C/M โผ 1 : 1).Aqueous ammonia in the solution form was added drop bydrop to adjust the pH value of the solution in the range of 6โ8.e precursor solution was dehydrated by putting on heaterwith continuous stirring at 80โC for 3 hrs to form a viscousgel which was placed in an oven at 150โC for overnight tocombust the gel into ash powder. In the second process,stoichiometry amount of the solution of nitrates of A-sitecation was dissolved with the ethylene glycol (E/M โผ 1 : 1)with continuous stirring for 30min to get homogeneouslymixed solution. e precursor solution was dehydrated bythe same conditions, which is discussed in the ๏ฟฝrst methodto form a gel into ash powder. Both precursor solutionsare expected to distribute the cations atomically homoge-neously throughout the polymeric structure forming a stablepolymeric complex, which was combusted at appropriatetemperature (๐๐ โผ 500โC) in the form of ash powder.e ash,highly ๏ฟฝne, homogeneous, and highly reactive powder wasmixed with appropriate amounts of TiO2 powder thoroughlyin ethanol using mortal pestle for 2 hrs followed by solidstate route. ese powders were dried and calcined at 750โC(2 hrs) for BNT-CA and BNT-EG and calcined at 850โC(2 hrs) for BLNT samples. e calcined powder was mixedthoroughly with a polyvinyl alcohol (PVA) binder solutionand then pressed into the form of disk with 10mm diameter.All samples are in pellet form, kept in alumina boat, and
o
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oo
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o
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o
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o
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o
(10
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Perovskite structure
20 30 40 50 60 70
Inte
nsi
ty (
a.u
.)
2ฮธ (deg)
(a)
20 30 40 50 60 70
Inte
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ty (
a.u
.)
(11
3)
o
o
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o
o
oo (2
20
)
(21
0)
(20
0)
(11
1)
(11
0)
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2ฮธ (deg)
Bi2Ti2O7โ
โโโโโโโโโ
โ
โโ
(b)
F 1: XRD patterns of (a) BNT-EG and (b) BNT-CA samples.
sintered at temperature 1150โC for 2 hours. Two pellets ofeach composition were electroded with silver paint on boththe surfaces of the samples for the subsequent electricalmeasurements.e crystallite size of all the samples in BLNTsystem was calculated using Debye-Scherrer formula (๐ท๐ท =0.89๐๐/๐๐ ๐๐๐ ๐๐๐ต๐ต) where ๐ท๐ท is the average crystallite size, ๐๐is the wavelength of X-ray radiation, ๐๐ is the full width athalf maximum (FWHM), and ๐๐๐ต๐ต the diffraction angle. ecorresponding values are reported in Table 1.
e crystalline structure of the sintered samples wasexamined using X-ray diffraction (XRD) analysis with Cu-K๐ผ๐ผ radiation (DX-1000).e surface morphology of sinteredceramics was observed by scanning electron microscopy(SEM, model JEOL A 800). e dielectric constant ๐๐๐๐ anddielectric loss (tan ๐ฟ๐ฟ) of the ceramic samples at 100 kHz weremeasured as a function of temperature over the temperaturerange from room temperature to 450โC using an LCRmeter (Hioki 3522). A conventional P-E loop tracer (MarineIndia), which is based on Sawyer-Tower circuit, was used tomeasure the polarization-electrical ๏ฟฝeld (P-E) hysteresis loopat 50Hz.
3. Results and Discussion
In the present work, preparation of pure Bi0.5Na0.5TiO3 wasoptimized using two chemical precursors, citric acid andethylene glycol. ese samples are abbreviated as BNT-CAand BNT-EG (Figures 1(a) and 1(b)). It is observed fromthe XRD patterns that the sample prepared by ethyleneglycol precursor solution has shown better phase forma-tion, whereas BNT-CA has formed partially along with
Journal of Ceramics 3
T 1: Dielectric properties of all BLNT system, prepared using ethylene glycol precursor.
Composition (๐ฅ๐ฅ) Lattice parameter๐๐ ๐ ๐๐ ๐ ๐๐ (ร )
Volume(m3)
๐๐๐๐ at RT Tan ๐ฟ๐ฟ at RT ๐๐๐๐ at ๐๐๐๐ Tan ๐ฟ๐ฟ at ๐๐๐๐๐๐๐๐ ๐๐๐๐ ๐พ๐พ ๐ท๐ท(nm)โC โC
BNT (๐ฅ๐ฅ ๐ ๐ฅ) 3.868 5.78 โ 10โ29 705 0.040 3200 0.08 180 353 1.26 34.98BLNT (๐ฅ๐ฅ ๐ ๐ฅ๐ฅ๐ฅ๐ฅ) 3.931 6.07 โ 10โ29 1036 0.044 3630 0.03 200 355 1.49 39.33BLNT (๐ฅ๐ฅ ๐ ๐ฅ๐ฅ๐ฅ๐ฅ) 3.922 6.03 โ 10โ29 3020 0.045 5630 0.05 210 380 1.80 34.81
o
o
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Perovskite structure
20 30 40 50 60 70
Inte
nsi
ty (
a.u
.)
2ฮธ (deg)
(Bi0.96La0.04)0.5Na0.5TiO3
(a)
o
o
oo
o
o o
20 30 40 50 60 70
Inte
nsi
ty (
a.u
.)
2ฮธ (deg)
(Bi0.98La0.02)0.5Na0.5TiO3
(b)
oo
o
o
o
o
(22
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(21
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(11
3)
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20 30 40 50 60 70
Inte
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2ฮธ (deg)
Bi0.5Na0.5TiO3
(c)
F 2: XRD patterns of BLNT system with compositions ๐ฅ๐ฅ ๐๐ฅ, ๐ฅ๐ฅ๐ฅ๐ฅ, and 0.04.
the presence of other phase identi๏ฟฝed as Bi๐ฅTi๐ฅO7 [13].e experimental density observed in BNT-EG sample was5๐ฅ77 gm/cm๐ฅ which is 95% of theoretical density and theexperimental density observed in BNT-CA sample was5๐ฅ56 gm/cm๐ฅ which is 91% of theoretical density. erefore,a typical (Bi1โ๐ฅ๐ฅLa๐ฅ๐ฅ)๐ฅ๐ฅ5Na๐ฅ๐ฅ5TiO3 (BLNT) systemwith compo-sitions ๐ฅ๐ฅ ๐ ๐ฅ๐ฅ๐ฅ๐ฅ and ๐ฅ๐ฅ๐ฅ๐ฅ was prepared by semiwet techniqueusing ethylene glycol precursor, calcined at 85๐ฅโC. e XRDpatterns of all the samples in BLNT system have shown singlephase formation with a rhombohedral symmetry in Figure 2.
Pure BNT and BLNT powders have rhombohedral sym-metry at room temperature. However, rhombohedral struc-ture is hard to distinguish due to the overlapping of peaks that
could be due to nearly cubic lattice parameter. Owing to smalldegree of rhombohedral distortion, diffraction lines wereindexed on the basis of pseudocubic unit cell [14] and latticeparameters of all the BLNT samples, prepared by ethyleneglycol, are calculated by using the Unit Cell program package[15]. ere is a variation in the lattice parameters becauseof the different sizes of ionic radius of La3+ (1.03ร ) whichare close to Bi3+ ionic radius (0.96ร ) and Na+ (0.99ร ). evalues in the parentheses refer to the Shannonโs effective ionicradius with the coordination number of six taken from [16].
e microstructure of the pure BNT ceramics, preparedby semiwet using ethylene glycol and citric acid, sinteredat 115๐ฅโC is shown in Figure 3. Ethylene glycol precursorplays a signi๏ฟฝcant role in the grain growth and densi๏ฟฝcation.e microstructure of pure BNT-EG ceramics is denser,homogeneous, and uniform grains with grain size of 2.12๐๐mas compared to BNT-CA with grain size of 1.19๐๐m.
e dielectric measurements of electroded samples ofBNT-EG, BNT-CA, and BLNT samples were carried at1๐ฅ๐ฅ kHz frequency over temperature range from room tem-perature to ๐ฅ5๐ฅโC and are shown in Figures 4(a), 4(b), 4(c),and 4(d), respectively.
Two dielectric anomalies in all the samples are shown attemperatures ๐๐1 and ๐๐๐ฅ, termed as โ๐๐๐๐โ and โ๐๐๐๐โ respec-tively, which corresponds to dielectric transitions from ferro-electric (FE) to anti-ferroelectric (AFE) and anti-ferroelectric(AFE) to paraelectric (PE), respectively. e correspondingpeaks also appear in tan ๐ฟ๐ฟ versus ๐๐ plots. However, thelow value of dielectric constant of the sample, preparedusing citric acid (BNT-CA), may be due to formation ofother phases of Bi๐ฅTi๐ฅO7, which hampers the one dielectricanomaly and relative value of dielectric constant in thissample. It has been observed that the ๐๐๐๐ and ๐๐๐๐ shi tohigher temperature with increasing the concentration ofLa3+. e partial replacement of A-site cation, with largerionic radius cations and larger amount, decreases the relativedisplacement of B-site cation with respect to the oxygen octa-hedral cage and hence the increase in transition temperatureis observed with substitution of La at A-site. It is obvious thatLa3+ (1.03ร ) can occupy the A-site of Bi3+ (0.96ร ) or Na+
(0.99ร ). When La3+ occupies A-site of Na+, it will lead tocharge imbalance, which creates defects on A-site. In general,A-site (Bi3+ and Na+) substitution by La3+ in BNT ceramicscan be formulated in the following way:
La๐ฅO3 + BNTโถ๐ฅLaBi + 3Oo (1)
La๐ฅO3 + BNTโถ๐ฅLaโขโขNa + ๐ฅ๐๐
๏ฟฝNa + 3Oo (2)
4 Journal of Ceramics
(a) (b)
F 3: Microphotographs patterns of (a) BNT-EG and (b) BNT-CA samples.
800
1600
2400
3200 BNT-EG
0
0.3
0.6
0.9
100 kHz
100 200 300 400
T (โฆC)
ฮตr
Tan
ฮด
Tm
Td
(a)
400
800
1200
1600 BNT-CA
0
0.3
0.6
0.9
100 200 300 400
T (โฆC)
ฮตr
Tan
ฮด
(b)
100 200 300 400
1400
2100
2800
3500
0
0.2
0.4
0.6
0.8
T (โฆC)
ฮตr
Tan
ฮด
(Bi0.98La0.02)0.5Na0.5TiO3
(c)
3200
4000
4800
5600Tm
Td
100 200 300 4000
0.2
0.4
0.6
0.8
T (โฆC)
ฮตr
Tan
ฮด
(Bi0.96La0.04)0.5Na0.5TiO3
(d)
F 4: Variations of ๐๐๐๐ and tan ๐ฟ๐ฟ with temperature for samples (a) BNT-EG, (b) BNT-CA, (c) BLNT with ๐ฅ๐ฅ ๐ฅ ๐ฅ๐ฅ๐ฅ๐ฅ, and (d) ๐ฅ๐ฅ ๐ฅ ๐ฅ๐ฅ๐ฅ๐ฅ.
โ0.2 โ0.1 0 0.1 0.2โ8
โ4
0
4
8
Applied field E (kV/cm)
Po
lari
zati
onP
(ยตC
/Cm
2)
Pr = 5.3
Ec = 0.12
(a)
โ80 โ40 0 40 80โ20
โ10
0
10
20
Applied field E (kV/cm)
Po
lari
zati
onP
(ยตC
/Cm
2)
Pr = 13.16
Ec = 25.02
(b)
F 5: e P-E hysteresis loops at RT (50Hz) (a) for BNT-CA and (b) BNT-EG.
Journal of Ceramics 5
T 2: Ferroelectric properties of all saturated BLNT system,prepared using ethylene glycol precursor.
Composition (๐ฅ๐ฅ) ๐๐๐๐ (๐๐C/cm2) ๐๐max (๐๐C/cm
2) ๐ธ๐ธ๐๐ (kV/cm)BNT-EG (๐ฅ๐ฅ ๐ฅ ๐ฅ) 13.16 16.90 25.02BLNT (๐ฅ๐ฅ ๐ฅ ๐ฅ๐ฅ๐ฅ๐ฅ) 13.33 17.86 24.96BLNT (๐ฅ๐ฅ ๐ฅ ๐ฅ๐ฅ๐ฅ๐ฅ) 17.70 20.60 36.73
When La3+ occupies Bi-site, as shown in (1), the substitu-tion of Bi3+ by La3+ may cause the slack of BLNT lattice.e lattice deformation can make the ferroelectric domainsreorientation more easily. It leads to the enhancement ofdielectric as well as ferroelectric properties. Additionally,La3+ can also occupy the A-site of Na+, as shown in (2).In this case, the valence of La3+ ion is higher than that ofNa+ ion. To maintain overall electrical neutrality, La3+ actsas a donor leading to some Na-site vacancies [๐๐๏ฟฝNa], whichcan relax the strain caused by reorientation of domains.erefore, the movement of the domains becomes easierand thus the electrical properties of the BLNT ceramics areimproved signi๏ฟฝcantly. us, the substitution of La3+ in BNTsystem has signi๏ฟฝcantly in๏ฟฝuenced the phase transition anddielectric behavior (Figures 4(c) and 4(d)). e highest valueof ๐๐๐๐ (โผ3020) and lowest value of Tan ๐ฟ๐ฟ (โผ0.045) are obtainedwith the composition ๐ฅ๐ฅ ๐ฅ ๐ฅ๐ฅ๐ฅ๐ฅ in the BLNT system at roomtemperature. e physical and dielectric properties of all theBLNT samples are tabulated at RT (1๐ฅ๐ฅ kHz) in Table 1.
It shows the typical character of a ferroelectric behavioraround transition temperature because of diffused phasetransition. Dielectric constant exhibits strong frequencydependence above๐๐๐๐ and themaximumvalue of ๐๐๐๐ decreasesas frequency increases in the BNT-EG samples suggestingthat the ceramic is relaxor ferroelectric. e diffuseness inthe phase transition can be described by 1/๐๐๐๐ โ 1/๐๐๐๐max ๐ฅ๐ถ๐ถโ๐พ๐พ(๐๐ โ ๐๐๐๐)
๐พ๐พ in relaxor ferroelectrics [17], where ๐๐๐๐max isthe maximum value of dielectric constant at ๐๐๐๐, ๐พ๐พ is thedegree of diffuseness, and ๐ถ๐ถ is the curie-like coefficient. ๐พ๐พcan have a value ranging from 1 for normal ferroelectric to 2for an ideal relaxor ferroelectric.is sample exhibits a linearrelationship. e value of the exponent โ๐พ๐พโ was determinedby least-squared ๏ฟฝtting experimental data to the equationwhich is in the range of 1.50 to 1.80.is con๏ฟฝrms the diffusephase transition in BLNT-EG system. e polarization-electrical ๏ฟฝeld (P-E) hysteresis loop is shown in Figures 5(a)and 5(b). It has been found that the P-E loop of BNT usingcitric acid is round shaped, not well developed (saturated),Figure 4(a). BNT-CA sample gets breakdown with increasingelectric ๏ฟฝeld.is may be due to (I) large leakage current and(II) due to insufficient annealing process. P-E loop for BNTsample prepared using ethylene glycol is well developed orsaturated, Figure 5(b).
is may be attributed to smaller grain size and relativelybetter density of the samples and con๏ฟฝrm that all the samplesare ferroelectric in nature.
e ferroelectric properties of all the BLNT system aretabulated at RT (50Hz) in Table 2.
4. Conclusion
In summary, ethylene glycol prepared samples have revealedbetter crystallization of pure phase for BNT-EG sample. XRDpatterns have revealed that the sample BNT-EG has singlephase perovskite structure with a rhombohedral symmetry atRT.e structural, phase transition, and electrical propertiesof all the samples were investigated. e Bismuth sodiumtitanate, prepared by ethylene glycol precursor, has not onlyshown excellent dielectric but also ferroelectric behavior.eBNT sample has high value of dielectric constant (๐๐๐๐ ๐ฅ 7๐ฅ5),dielectric loss (Tan ๐ฟ๐ฟ ๐ฅ ๐ฅ๐ฅ๐ฅ๐ฅ), remnant polarization (๐๐๐๐ ๐ฅ13๐ฅ16 ๐๐C/Cm๐ฅ), and Coercive ๏ฟฝeld (๐ธ๐ธ๐๐ ๐ฅ ๐ฅ5๐ฅ๐ฅ6 kV/Cm) atroom temperature. e relatively highest value of dielectricconstant for BLNT with composition ๐ฅ๐ฅ ๐ฅ ๐ฅ๐ฅ๐ฅ๐ฅ may beattributed to the La doping. Composition with La substitu-tion of ๐ฅ๐ฅ ๐ฅ ๐ฅ๐ฅ๐ฅ๐ฅ has shown the highest value of dielectric(3๐ฅ๐ฅ๐ฅ) constant and low loss (๐ฅ๐ฅ๐ฅ๐ฅ5) as well as the highestvalue of remanent polarization (๐๐๐๐ ๐ฅ 17๐ฅ7๐ฅ ๐๐C/Cm
๐ฅ). etransition temperature Tm is also maximum (385โC) for thiscomposition which reveals that the material can be useful forhigh-temperature device applications.
Acknowledgment
One of the authorsMs. V. Pal is thankful to JIIT for providingteaching assistance ship and other research facilities to carryout her research work at JIIT, Noida (India).
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