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Materials Chemistry and Physics 130 (2011) 191– 195

Contents lists available at ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

tructural, dielectric and ferroelectric study of microwave sintered lanthanumubstituted BaTiO3 ceramics

onia Sharmaa, R.K. Patela, C. Prakashb, P. Kumarc,∗

Department of Chemistry, National Institute of Technology, Rourkela 769008, IndiaDRDO Bhawan, Rajaji Marg, New Delhi 110011, IndiaDepartment of Physics, National Institute of Technology, Rourkela 769008, India

r t i c l e i n f o

rticle history:eceived 22 August 2010eceived in revised form 23 May 2011ccepted 17 June 2011

eywords:

a b s t r a c t

Lanthanum substituted barium titanate (BT), Ba(1−x)LaxTi(1−x/4)O3 (where x = 0.02,0.04,0.06 and0.08)/BLT1, BLT2, BLT3 and BLT4 ferroelectric ceramic samples were synthesized in single perovskitephase by microwave (MW) processing technique. Dense packing of grains with higher density at lowersintering temperature (1100 ◦C) signifies the importance of MW process over conventional process. Thestructure changes from tetragonal to cubic and grain size gradually decreases with the increase in La3+

xidesinteringEMielectric propertieserroelectricity

ions substitution concentration in BT ceramic samples. Presence of pore free uniform grains suggested theadvantage of using microwave sintering process. Transition temperature (Tc) decreases with the increasein La3+ ions substitution concentration in BT system and for BLT3 and BLT4 systems Tc is below roomtemperature (RT). Temperature coefficient of capacitance is negligible from RT temperature to 75 ◦C and50 ◦C for BLT1 and BLT2 ceramic samples, respectively. Ferroelectric study confirms the transformation ofstructure from tetragonal to cubic with the increase of La3+ ion substitution concentration in BT system.

. Introduction

Barium titanate (BT) is the most common ferroelectric oxide inhe perovskite ABO3 phase and often used in multilayer ceramicapacitors due to high dielectric constant (εr) and low dielectricoss (tanı) characteristics [1]. Many efforts have been tried to mod-fy this compound by different means in order to achieve stableapacitors with satisfactory operational capacity. Earlier reportshow that substitution of host lattice cation significantly changeshe dielectric properties. An excellent example is La3+ ions substi-uted BT system; it has both an exceptionally high (εr) for selectedomposition and temperature [2,3]. However, the performance ofT ceramic samples significantly depends on the microstructurend sintered body. The microwave (MW) processing of ceramicsan be utilised as an alternative approach for conventional sinteringf ceramics because of potential advantages such as rapid heat-ng, penetrating radiation, more uniform microstructure and henceigher density [4–10]. The literature data mainly reports the prepa-ation of La3+ ions substituted BT system by conventional route

3,11–13]. Microwave sintering of ceramics offers several poten-ial advantages over conventional sintering. Microwave heatinglso has the potential for energy and cost savings when compared

∗ Corresponding author. Fax: +91 0661 2462999.E-mail address: pvn77@rediffmail.com (P. Kumar).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.06.028

© 2011 Elsevier B.V. All rights reserved.

with conventional heating. Besides that nearly theoretical densityand uniform grains, which in turn improves the ceramics physi-cal properties, can be achieved in microwave processed ceramics.Many researchers have reported the enhancement in the solid statereaction rate or the solid state diffusion rate in a microwave dielec-tric field [14,15]. Ceramic in conventional furnace require generallyrather long time owing to inertia of furnaces and much energy islost by radiation.

These disadvantages could be avoided by using microwave pro-cessing of ceramics. It is also worth pointing out that the use ofmicrowave allows heating to be started at the core of ceramics con-trary to conventional heating which starts at the surface; this couldchange the microstructure and dielectric properties [16]. Therefore,substitution of La3+ ions in BT system and its microwave processingcan modify the structure and dielectric properties.

In the present work, Ba2+ and Ti4+ ions in BT system are sub-stituted by La3+ ions at A site (off-valent substitution) and B site(to maintain charge neutrality). Here, we report the dielectric,microstructure and ferroelectric studies of La3+ ions substituted BTsystem sintered by microwave technique.

2. Experimental

The polycrystalline ceramic samples of Ba(1−x)LaxTi(1−x/4)O3 (wherex = 0.02,0.04,0.06,0.08))/BLT1, BLT2, BLT3 and BLT4 compositions were syn-thesized using grade reagents BaCO3, TiO2, and La2O3 powders (all Aldrich with99.8% purity). These starting precursors were taken in stoichiometric ratio asthe initial raw material. Stoichiometric weights of all the powders were mixed,

192 S. Sharma et al. / Materials Chemistry and Physics 130 (2011) 191– 195

rzostausspTsdpJs(dcp

2 0 3 0 4 0 5 0 6 0

(d )

(c )

(b )

(a) BLT(98/2)(b) BLT(96/4)(c) BLT(94/6)(d) BLT(92/8)

(112

)(1

12)

(002

)

(200

)

(110

)

(211

)(2

11)

(200

)(0

02)

(200

)

(111

)

(101

)(1

10)

(101

)

(110

)

Inte

sity

in (a

.u)

2 θ (degrees)

(a ) (201

)(1

02)

(201

)(2

10)

(100

)

Fig. 1. Schematic diagram of microwave sintering system.

espectively as per different compositions and ball milled with acetone for 8 h, usingirconia balls as the grinding media. After drying the slurry in oven, the calcinationsf the powders was carried out at 1100 ◦C for 4 h in conventional furnace andingle perovskite phase formation was confirmed by the X-ray diffraction (XRD)echnique. The calcined powders were mixed thoroughly with 2 wt% polyvinyllcohol (PVA) and pressed into disks of diameter ∼10 mm and a thickness ∼1.5 mmnder ∼60 MPa pressure. Schematic diagram of microwave sintering system ishown in Fig. 1. The microwave sintering of the La3+ ions substituted BT ceramicamples was carried out at 1100 ◦C for 1 h with a heating rate of 40 ◦C min−1 bylacing the pellets in the centre of a 4.4 kW, 2.45 GHz multi mode microwave cavity.he microwave furnace temperature was recorded by using a Raytek non-contactensor (XRTG5). XRD analysis of the pellets were performed on PW 3020 Philipsiffractometer using Cu K� (� = 0.15405 nm) radiation in order to examine thehases present in the material. The sintered microstructures were observed using

EOL T-330 scanning electron microscope (SEM). Silver paste was applied on bothides of the ceramic samples for the electrical measurements. Dielectric constant

εr) and dielectric loss (tanı) were measured as a function of temperature atifferent frequencies using computer interfaced HIOKI 3532-50 LCR-HITESTER. Aonventional computer interfaced Sawyer–Tower circuit was used to measure theolarization vs. electric field (P–E) hysteresis loops at 20 Hz frequency.

Fig. 3. SEM images of MW sintered (a) BLT1 (

Fig. 2. XRD peaks of MW sintered (a) BLT1 (b) BLT2 (c) BLT3 and (d) BLT4 samples.

3. Results and discussions

Fig. 2 shows the XRD pattern of La3+ ions substituted BT ceramicsamples. The diffraction patterns show the development of intenselines of single perovskite phase for different La3+ ions substitutionconcentrations in BT ceramic samples XRD diffraction peaks arefound to be sharp and distinct, indicating good homogeneity andcrystallization of La3+ ions modified BT ceramic samples [17]. Fig. 2also shows the changes of splitted 2� ∼45◦ peak with the variation

of La3+ ions substitution concentration in BT ceramic samples. Asthe La3+ ions substitution concentration in BT ceramic samples isincreased, the well resolved doublet till to x = 0.04 merges, which

b) BLT2 (c) BLT3 and (d) BLT4 samples.

S. Sharma et al. / Materials Chemistry and Physics 130 (2011) 191– 195 193

(b) (a)

(d) (c)

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Temperature (oC)Temperature (oC)

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es of M

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microwave sintered samples is due to the rapidity of microwaveheating which avoids the undesirable grain growth [23]. This is thecharacteristic of microwave processing of ceramics as the heating

0.2 0.4 0.6 0.8 1. 0 1.2 1. 4 1.6

-6.0

-5.5

-5.0

-4.5

-4.0

Log

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r-1/ε

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.) γ =1.88

Fig. 4. Variation of εr with temperature at different frequenci

xhibits tetragonal to cubic structural transition. These diffractionines of La3+ ions modified BT ceramic samples are indexed in differ-nt crystal structures and unit cell configurations using a computerrogram package ‘Powdmult’ [18]. Out of different structures auitable structure, given in Table 1, is selected for different La3+

ons substitution concentration in BT ceramic samples for whichhe standard deviation (SD), (��d = dobs − dcal), where ‘d’ is inter-lane spacing, is found to be minimum. The lattice parametersf the unit cell were refined using least square fit method. Thisonfirms the transition of structure from tetragonal to cubic withhe increase of La3+ ions substitution concentration in BT ceramicamples [19]. A change of structure from non centro-symmetrico centro-symmetric with La3+ ions substitution in BT ceramicamples suggest the transformation from double minimum in thenergy vs. displacement of ions to a single minimum in the energyid-way between sites leading to paraelectric state at RT in BLT3

nd BLT4 ceramic samples [20]. The ion size of La3+ ions is lessompare to Ba2+ ions and with the increase in La3+ ions substi-ution concentration and rate with which the small La3+ions hops so high that on average there is no net polarization and hencearaelectric state for BLT3 and BLT4 ceramic samples [21].

Fig. 3 illustrates the SEM micrographs of microwave sintereda3+ ions modified BT ceramic samples. Presences of pore free uni-orm grains suggest the advantage of using microwave sinteringoute. Density, measured by Archimedes method and average grainize, estimated by linear intercept method, are given in Table 1. The

btained densities of MW sintered BLT samples sintered at signif-cantly lower temperatures are higher than the earlier reports on

odified and unmodified BT ceramic samples processed throughther processes including MW technique [6–8,22]. This signifies

W sintered (a) BLT1 (b) BLT2 (c) BLT3 and (d) BLT4 samples.

the importance of processing La3+ ions modified BT ceramic sam-ples through MW technique. Grain size of microwave sintered La3+

ions modified BT ceramic samples is lower than the same systemsprocessed by conventional solid state route [22]. Better densifica-tion with the formation of finer and uniform grains in the case of

Log(T-Tmax.)

Fig. 5. Variation of log(1/εr − εrmax) vs. log(T − Tmax) of BLT1 sample.

194 S. Sharma et al. / Materials Chemistry and Physics 130 (2011) 191– 195

Table 1Effect of variation of La substitution concentration in BT system on different parameters at RT.

System Structure Density g cm−3 Grain size (�m) εr at 1 kHz Pr (�C cm−2)

BLT1 Tetragonal 5.584 0.95 �m 940 5.45

BLT2 Tetragonal 5.658 0.7 �m 2400 0.95

BLT3 Cubic 5.903 0.52 �m 1400 0.28

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BLT4 Cubic 5.842

ate is fast and the mechanism of heating rate is different than con-entional sintering process [4,5]. Grain size is decreasing with thencrease in La3+ ions substitution concentration in BT ceramic sam-les. This indicates that the increase in La3+ ions concentration in BTeramic samples inhibits grain growth. It can also be thought that

change in crystal structure may make the mass transportationore difficult, as BLT ceramic samples change from a tetragonal

o a cubic structure with the increasing of La3+ ions substitutiononcentration in BT ceramic samples [24].

Fig. 4 shows the temperature variation of εr at different fre-uencies of La3+ ions modified BT ceramic samples sinteredy microwave process. Observed transition temperature (Tc)ecreases drastically with the increase in La3+ ions content in BTeramic samples. This indicates that the substitution of smaller La3+

ons at Ba2+ ions position causes decrease of relative displacementf A site ions with respect to oxygen octahedral cage in ABO3 per-

vskite type of systems. Since, Tc is directly related to the square ofhis relative displacement [25], therefore with the increase in La3+

ons substitution concentration in BT ceramic samples, the Tc is

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Ec=001 .409k V/cmPr=00 0.28 4μC/cm2Emax=006 .44 7kV/cmPs=001.133μC/cm2

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Ec=002.601kV/cm

Pr=005.443μC/cm2

Emax=006.554kV /cm

Ps=008.366μC/cm2

Fig. 6. P–E hysteresis loops of (a) BLT1 (b)

0.45 �m 1760 0.028

down shifted. This can also be explained by the increase in internalcompressive stress with the increase of La3+ ions substitution con-centration in BT ceramic samples. As, given in Table 1, the averagegrain size decreases with the increase of La3+ ions substitution con-centration in BT ceramic samples. It is a well established conceptthat the internal compressive stress increases with the decrease ofgrain size [26]. Since, Tc is inversely proportional to internal com-pressive stress; therefore there is decrease of Tc with the increaseof La3+ ions substitution concentration in BT ceramic samples. Withthe substitution of La3+ ions in BT ceramic samples, stability ofcapacitance increases for BLT1 and BLT2 ceramic samples. Tem-perature coefficient of capacitance is negligible from RT to 75 ◦Cand 50 ◦C for BLT1 and BLT2 ceramic samples, respectively. Valueof εr at 1 kHz and at RT (given in Table 1) is found to be higherthan the earlier reports on La3+ ions modified BT ceramic samplesprocessed through other techniques and BT ceramic samples pro-

cessed through MW technique [6–8,22]. Value of εr at 1 kHz and atRT is maximum for BLT2 ceramic samples. This is because the Tc ofBLT2 system is just below RT and value of εr is maximum near Tc

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BLT2 (c) BLT3 and (d) BLT4 samples.

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S. Sharma et al. / Materials Chem

n ferroelectric systems [20]. Value of εr of BLT4 ceramic sampless more than BLT3 ceramic samples. This suggests that the effectf fine grain nature on the dielectric properties of BLT4 ceramicamples is dominating. It is well established that in BT ceramic sam-les, below a submicrom size grain arrangement the value εr starts

ncreasing [27]. Grain size of BLT4 ceramic samples is lower thanLT3 ceramic samples, which explains the higher εr of BLT4 sam-les. This again hints towards the importance of processing of La3+

ons modified BT ceramic samples through MW technique at sig-ificantly lower sintering temperature in the present study. Fig. 5hows the variation of log(1/εr − 1/εrmax) vs. log(T − Tmax) for BLT1eramic sample in the paraelectric region. From the slope of theraph, value of ‘� ’ is calculated. Value of � for normal ferroelectricss 1 and it increases with the increase in diffusivity of the samples20]. Value of ‘� ’ is found to be 1.88 in BLT1 samples. Tc of othera3+ ions modified BT ceramic samples is lower than RT thereforeiffusivity is not calculated. But, the variation of εr vs. tempera-ure hints towards the diffusive nature for BLT2, BLT3 and BLT4amples. This implies that the diffusivity increases with the sub-titution of La3+ ions in BT ceramic samples. This further suggestshe introduction of structural disorder and compositional fluctu-tions by La3+ ions substitution in BT ceramic samples [28]. Theiffuse transition behaviour might also be due to the larger num-er of the La3+ ions occupying the Ti4+ site, eventually introducing

arger compositional and structural disorder, which gives rise toiffuse phase transition. Variation of εr with temperature for BLT3nd BLT4 ceramic samples suggest that the Tc is below RT and BLT3nd BLT4 are in paraelectric state at RT. This is confirming the XRDtudy of BLT3 and BLT4 ceramic samples having centro symmetricubic structure.

Fig. 6 shows the P–E hysteresis loops of La3+ ions modified BTeramic samples sintered by microwave process. Development of–E hysteresis loops confirms the ferroelectric nature of La3+ ionsodified BT ceramic samples for BLT1and BLT2 ceramic samples.hereas, non occurrence of well saturated P–E hysteresis loops

uggest the paraelectric nature of the BLT3 and BLT4 ceramic sam-les at RT. This is as per the XRD and dielectric study. Higheroercive field (Ec) for BLT2 than BLT1 ceramic samples can bexplained on the basis of grain size. Grain size of BLT2 is lower thanLT1 ceramic samples and Ec is inversely proportional to grain size.ower the grain size, lower will be the domain size and more resis-ance in rotating the domains and domains walls [29]. Therefore,ower grain size of BLT2 ceramic samples is giving rise to higherc. The low value of remnant polarization (Pr) of BLT2 than BLT1eramic samples can again be explained on the basis of domainize. Lower grain size in a ferroelectric material will lead to loweromain size and lower domain wall motion, which further will leado lower polarization [29].

. Conclusions

La3+ ions modified dense BT ceramic samples were synthesizedy microwave processing technique. MW sintering was carried

[[[[

nd Physics 130 (2011) 191– 195 195

out at 1100 ◦C for 1 h, which is significantly lower than conven-tional sintering process. With the increase in La3+ ions substitutionconcentration in BT ceramic samples, the structure changes fromtetragonal to cubic. Presence of pore free uniform grains sug-gested the advantage of using microwave sintering process overconventional process. Diffuse phase transition hints towards theintroduction of compositional and structural disorder with La3+

ions substitution in BT ceramic samples. Capacitance stabilityincreases for BLT1 and BLT2 ceramic samples. With the increase inLa3+ ions substitution concentration, BLT systems transforms fromferroelectric to paraelectric state at RT. Higher density, lower pro-cessing time and temperature with good dielectric properties ofMW sintered La3+ ions modified BT ceramics than conventionalprocessing technique signifies the importance of MW processingtechnique.

References

[1] J.Y. Sakabe, K. Minai, C. Gallardo, J.M. Criado, F.J. Gotor, A. Bhalla, Ferroelectrics127 (1992) 59.

[2] F.D. Morrison, D.C. Sinclair, J.M. Skakle, A.R. West, J. Am. Ceram. Soc. 81 (1998)1957.

[3] F.D. Morrison, D.C. Sinclair, A.R. West, J. Appl. Phys. 86 (1999) 6355.[4] C.T. Hu, et al., Jpn. J. Appl. Phys. 37 (1) (1998) 186.[5] E.T. Thostenson, T.W. Chen, Compos. Part A 30 (1999) 1055.[6] O.P. Thakur, C. Prakash, D. Agrawal, Int. J. Ceram. Process. Res. 3 (2) (2002)

75.[7] O.P. Thakur, C. Prakash, D. Agrawal, J. Mater. Sci. Eng. B 96 (2002) 221.[8] O.P. Thakur, C. Prakash, D. Agrawal, Mater. Lett. 56 (2002) 970.[9] M. Fu, D. Agrawal, Y. Fang, J. Microwave Power Electromagn. Energy 40 (3)

(2007) 133.10] C.Y. Fang, C. Wang, A.V. Polotai, D.K. Agrawal, M.T. Lanagan, Mater. Lett. 62

(2008) 2551.11] F.D. Morrison, D.C. Sinclair, A.R. West, Int. J. Inorg. Mater. 3 (2001)

1205.12] M. Aparna, T. Bhimasankaram, S.V. Suryanarayana, G. Prasad, G.S. Kumar, Bull.

Mater. Sci. 24 (5) (2001) 497.13] P. Yongding, L. Yunhe, Y. Wenhu, J. Rare Earths 25 (2007) 167.14] C. Masingboon, P. Thongbai, S. Maensiri, T. Yamwong, S. Seraphin, Mater. Chem.

Phys. 109 (262) (2008).15] W.P. Chen, W. Xiang, M.S. Guo, W.C. You, X.Z. Zhao, H.L.W. Chan, J Alloys Compd.

422 (2006) L99.16] S. Gopalan, A.V. Virkir, J. Am. Ceram. Soc. 82 (1999) 2887.17] H.R. Rukmini, R.N.P. Choudhary, D.L. Prabhakara, Mater. Chem. Phys. 64 (171)

(2000).18] E. Wu, POWD, An Interactive Powder Diffraction Data Interpretation And Index-

ing Program, Ver 2.1, School of Physical Science, Finder’s University of SouthAustralia, Bedford Park.

19] R. Zhang, J.F. Li, D. Viehland, J. Am. Ceram. Soc. 87 (2004) 864.20] M.E. Lines, A.M. Glass, Principles and Applications of Ferroelectrics and Related

Materials, Clarendon Press, Oxford, 1977.21] C.D. Hu, Phys. Rev. B 77 (2008) 174418.22] M.M. Vijatovic, B.D. Stojanovic, J.D. Bobic, T. Ramoska, P. Bowen, Ceram. Int. 36

(2010) 1817.23] B. Vaidhyanathan, A.P. Singh, D.K. Agrawal, T.R. Shrout, R. Roy, J. Am. Ceram.

Soc. 84 (2001) 1197–1202.24] R. Zuo, M. Wang, B. Maa, J. Fu, T. Li, J. Phys. Chem. Solids 70 (2009)

750.

26] F.J. Chen, Y.N. Pan, C.Y. Lee, C.S. Lin, J. Electrochem. Soc. 157 (13) (2010) D154.27] G. Arlt, D. Hennings, G. de With, J. Appl. Phys. 58 (1985) 4.28] N. Setter, L.E. Cross, J. Mater. Sci. 15 (1980) 1573.29] G.H. Heartling, J. Am. Ceram. Soc. 82 (1999) 797.