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; [email protected]
Received 29 June 2012; Accepted 20 November 2012
Academic Editor: Baolin Wang
Copyright Β© 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 (Bi1βπ₯π₯Laπ₯π₯)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 signiοΏ½cant 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 450βC 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 modiοΏ½edCurie-Weiss law above Tc. A linear οΏ½tting of the modiοΏ½ed 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 β50 to 150βC [1]. However, in recent years many οΏ½eldshave expressed the need for actuation and sensing which canbe used at higher temperatures (>400βC) 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 οΏ½eld ofpiezoelectric ceramics. A lot of research has been carriedout on lead-free piezoceramic products in the last οΏ½οΏ½y yearsbut in the last few years, the momentum has tremendouslyincreased, accounting for about 75% of all published worksin this οΏ½eld. Bi0.5Na0.5TiO3- (BNT-) and K0.5Na0.5NbO3-(KNN-) based materials are two main material systems withperovskite structure, which have been studied to οΏ½nd 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 (ππππ =38 ππC/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 diοΏ½cult due to its high coercive οΏ½eld (πΈπΈππ = 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 (750βC) 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 οΏ½rst 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 (Bi1βπ₯π₯Laπ₯π₯)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 π₯π₯ = 0 was prepared by usingtwo different precursor solutionsοΏ½ the οΏ½rst one is citric acid,andοΏ½ the second is ethylene glycol. In the οΏ½rst process, anappropriate amount of citric acid solution was added to thesolution of nitrates of A-site cations in a beaker (C/M βΌ 1 : 1).Aqueous ammonia in the solution form was added drop bydrop to adjust the pH value of the solution in the range of 6β8.e precursor solution was dehydrated by putting on heaterwith continuous stirring at 80βC for 3 hrs to form a viscousgel which was placed in an oven at 150βC 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 βΌ 1 : 1)with continuous stirring for 30min to get homogeneouslymixed solution. e precursor solution was dehydrated bythe same conditions, which is discussed in the οΏ½rst 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 (ππ βΌ 500βC) in the form of ash powder.e ash,highly οΏ½ne, 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 750βC(2 hrs) for BNT-CA and BNT-EG and calcined at 850βC(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
(11
0)
(11
3)
oo
o
(22
0)
o
(21
0)
o
(20
0)
o
(11
1)
o
(10
0)
Perovskite structure
20 30 40 50 60 70
Inte
nsi
ty (
a.u
.)
2ΞΈ (deg)
(a)
20 30 40 50 60 70
Inte
nsi
ty (
a.u
.)
(11
3)
o
o
o
o
o
oo (2
20
)
(21
0)
(20
0)
(11
1)
(11
0)
(10
0)
2ΞΈ (deg)
Bi2Ti2O7β
βββββββββ
β
ββ
(b)
F 1: XRD patterns of (a) BNT-EG and (b) BNT-CA samples.
sintered at temperature 1150βC 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 (π·π· =0.89ππ/ππ πππ πππ΅π΅) where π·π· is the average crystallite size, ππis the wavelength of X-ray radiation, ππ is the full width athalf maximum (FWHM), and πππ΅π΅ 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-KπΌπΌ radiation (DX-1000).e surface morphology of sinteredceramics was observed by scanning electron microscopy(SEM, model JEOL A 800). e dielectric constant ππππ anddielectric loss (tan πΏπΏ) of the ceramic samples at 100 kHz weremeasured as a function of temperature over the temperaturerange from room temperature to 450βC 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 οΏ½eld (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 (π₯π₯) Lattice parameterππ π ππ π ππ (Γ )
Volume(m3)
ππππ at RT Tan πΏπΏ at RT ππππ at ππππ Tan πΏπΏ at ππππππππ ππππ πΎπΎ π·π·(nm)βC βC
BNT (π₯π₯ π π₯) 3.868 5.78 β 10β29 705 0.040 3200 0.08 180 353 1.26 34.98BLNT (π₯π₯ π π₯π₯π₯π₯) 3.931 6.07 β 10β29 1036 0.044 3630 0.03 200 355 1.49 39.33BLNT (π₯π₯ π π₯π₯π₯π₯) 3.922 6.03 β 10β29 3020 0.045 5630 0.05 210 380 1.80 34.81
o
o
oo
oooo
Perovskite structure
20 30 40 50 60 70
Inte
nsi
ty (
a.u
.)
2ΞΈ (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
.)
2ΞΈ (deg)
(Bi0.98La0.02)0.5Na0.5TiO3
(b)
oo
o
o
o
o
(22
0)
(21
0)
(11
3)
(20
0)
(11
1)
(11
0)
(10
0)
o
20 30 40 50 60 70
Inte
nsi
ty (
a.u
.)
2ΞΈ (deg)
Bi0.5Na0.5TiO3
(c)
F 2: XRD patterns of BLNT system with compositions π₯π₯ ππ₯, π₯π₯π₯π₯, and 0.04.
the presence of other phase identiοΏ½ed as Biπ₯Tiπ₯O7 [13].e experimental density observed in BNT-EG sample was5π₯77 gm/cmπ₯ which is 95% of theoretical density and theexperimental density observed in BNT-CA sample was5π₯56 gm/cmπ₯ which is 91% of theoretical density. erefore,a typical (Bi1βπ₯π₯Laπ₯π₯)π₯π₯5Naπ₯π₯5TiO3 (BLNT) systemwith compo-sitions π₯π₯ π π₯π₯π₯π₯ and π₯π₯π₯π₯ was prepared by semiwet techniqueusing ethylene glycol precursor, calcined at 85π₯βC. 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+ (1.03Γ ) whichare close to Bi3+ ionic radius (0.96Γ ) and Na+ (0.99Γ ). evalues in the parentheses refer to the Shannonβs 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 115π₯βC is shown in Figure 3. Ethylene glycol precursorplays a signiοΏ½cant role in the grain growth and densiοΏ½cation.e microstructure of pure BNT-EG ceramics is denser,homogeneous, and uniform grains with grain size of 2.12ππmas compared to BNT-CA with grain size of 1.19ππm.
e dielectric measurements of electroded samples ofBNT-EG, BNT-CA, and BLNT samples were carried at1π₯π₯ kHz frequency over temperature range from room tem-perature to π₯5π₯βC 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 ππ1 and πππ₯, termed as βππππβ and βππππβ 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 πΏπΏ versus ππ plots. However, thelow value of dielectric constant of the sample, preparedusing citric acid (BNT-CA), may be due to formation ofother phases of Biπ₯Tiπ₯O7, which hampers the one dielectricanomaly and relative value of dielectric constant in thissample. It has been observed that the ππππ and ππππ 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+ (1.03Γ ) can occupy the A-site of Bi3+ (0.96Γ ) or Na+
(0.99Γ ). 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:
Laπ₯O3 + BNTβΆπ₯LaBi + 3Oo (1)
Laπ₯O3 + BNTβΆπ₯Laβ’β’Na + π₯ππ
οΏ½Na + 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 (β¦C)
Ξ΅r
Tan
Ξ΄
Tm
Td
(a)
400
800
1200
1600 BNT-CA
0
0.3
0.6
0.9
100 200 300 400
T (β¦C)
Ξ΅r
Tan
Ξ΄
(b)
100 200 300 400
1400
2100
2800
3500
0
0.2
0.4
0.6
0.8
T (β¦C)
Ξ΅r
Tan
Ξ΄
(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 (β¦C)
Ξ΅r
Tan
Ξ΄
(Bi0.96La0.04)0.5Na0.5TiO3
(d)
F 4: Variations of ππππ and tan πΏπΏ with temperature for samples (a) BNT-EG, (b) BNT-CA, (c) BLNT with π₯π₯ π₯ π₯π₯π₯π₯, and (d) π₯π₯ π₯ π₯π₯π₯π₯.
β0.2 β0.1 0 0.1 0.2β8
β4
0
4
8
Applied field E (kV/cm)
Po
lari
zati
onP
(Β΅C
/Cm
2)
Pr = 5.3
Ec = 0.12
(a)
β80 β40 0 40 80β20
β10
0
10
20
Applied field E (kV/cm)
Po
lari
zati
onP
(Β΅C
/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 (π₯π₯) ππππ (ππC/cm2) ππmax (ππC/cm
2) πΈπΈππ (kV/cm)BNT-EG (π₯π₯ π₯ π₯) 13.16 16.90 25.02BLNT (π₯π₯ π₯ π₯π₯π₯π₯) 13.33 17.86 24.96BLNT (π₯π₯ π₯ π₯π₯π₯π₯) 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 [πποΏ½Na], 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 signiοΏ½cantly. us, the substitution of La3+ in BNTsystem has signiοΏ½cantly inοΏ½uenced the phase transition anddielectric behavior (Figures 4(c) and 4(d)). e highest valueof ππππ (βΌ3020) and lowest value of Tan πΏπΏ (βΌ0.045) are obtainedwith the composition π₯π₯ π₯ π₯π₯π₯π₯ in the BLNT system at roomtemperature. e physical and dielectric properties of all theBLNT samples are tabulated at RT (1π₯π₯ kHz) in Table 1.
It shows the typical character of a ferroelectric behavioraround transition temperature because of diffused phasetransition. Dielectric constant exhibits strong frequencydependence aboveππππ and themaximumvalue of ππππ decreasesas frequency increases in the BNT-EG samples suggestingthat the ceramic is relaxor ferroelectric. e diffuseness inthe phase transition can be described by 1/ππππ β 1/ππππmax π₯πΆπΆβπΎπΎ(ππ β ππππ)
πΎπΎ in relaxor ferroelectrics [17], where ππππmax isthe maximum value of dielectric constant at ππππ, πΎπΎ is thedegree of diffuseness, and πΆπΆ is the curie-like coefficient. πΎπΎcan 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 βπΎπΎβ was determinedby least-squared οΏ½tting experimental data to the equationwhich is in the range of 1.50 to 1.80.is conοΏ½rms the diffusephase transition in BLNT-EG system. e polarization-electrical οΏ½eld (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 οΏ½eld.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 conοΏ½rm 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 (ππππ π₯ 7π₯5),dielectric loss (Tan πΏπΏ π₯ π₯π₯π₯π₯), remnant polarization (ππππ π₯13π₯16 ππC/Cmπ₯), and Coercive οΏ½eld (πΈπΈππ π₯ π₯5π₯π₯6 kV/Cm) atroom temperature. e relatively highest value of dielectricconstant for BLNT with composition π₯π₯ π₯ π₯π₯π₯π₯ may beattributed to the La doping. Composition with La substitu-tion of π₯π₯ π₯ π₯π₯π₯π₯ has shown the highest value of dielectric(3π₯π₯π₯) constant and low loss (π₯π₯π₯π₯5) as well as the highestvalue of remanent polarization (ππππ π₯ 17π₯7π₯ ππC/Cm
π₯). etransition temperature Tm is also maximum (385βC) 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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Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2013
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Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2013
Journal ofNanomaterials