Materials Chemistry and Physics 112 (2008) 858–862

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Materials Chemistry and Physics

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A comparative study of Ba0.95Ca0.05Zr0.25Ti0.75O3 relaxor ceramics prepared byconventional and microwave sintering techniques

Sandeep Mahajana, O.P. Thakura,∗, D.K. Bhattacharyaa, K. Sreenivasb

a Electroceramics Group, Solid State Physics Laboratory, Timarpur, Delhi 110054, Indiab Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India

a r t i c l e i n f o

Article history:Received 7 February 2008Received in revised form 3 June 2008Accepted 21 June 2008

Keywords:

a b s t r a c t

A comparative study on the dielectric properties of (Ba0.95Ca0.05Zr0.25Ti0.75O3 (BCZT) ) bulk ceramics pre-pared by conventional and microwave sintering (MWS) has been done. High-density ceramic materialswere obtained in 4 h of cycle time for microwave sintered sample whereas it took 22 h for conventionalmethod. Samples prepared by both sintering techniques showed single-phase formation in XRD withcubic structure. The dielectric properties of the samples were measured as a function of temperature inthe frequency range of 100 Hz–100 kHz. Relaxor behaviour was observed in both types of samples. It was

BZTCeramicsMicrowave sinteringDR

observed that the peak value of dielectric constant was higher for microwave sintered sample comparedto conventional one and also the dielectric loss was lower. The value of activation energy was found to be

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. Introduction

Barium titanate is widely used as dielectrics because of itsigh dielectric constant and low loss [1,2]. It is well known thatemi-conducting BaTiO3 ceramics are also used for grain boundaryarrier layer (GBBL) capacitors [3,4]. Substitution of both isovalentnd aliovalent cation for the host ones in perovskite lattice playsvery important role. Ba(ZrxTi1−x)O3 ceramics are generally used

or making capacitors, and thus most studies are focused on theemperature dependence of the dielectric constant, the nature ofhase transitions and the relaxor behaviour of this material [5–9].etailed ferroelectric and piezoelectric properties were less studied

or this system [8].On the other hand, the high piezoelectric and electrostrictive

roperties were found only in lead based ferroelectric relaxors, suchs Pb(Mg1/3Nb2/3)O3 (PMN), Pb(Zn1/3Nb2/3)O3 (PZN) and (Pb, La)Zr, Ti)O3 (PLZT) ceramics [10–13]. The relaxor materials have foundide applications in electronics industries. Very high strain level

nd piezoelectric activity have been reported in these solid solu-ions. This leads to extensive interest in the possible correlation of

erroelectric relaxor behaviour and high piezoelectric performance.owever, lead is toxic element, and with increasing demand ofnvironmental protection, lead-free materials are highly desirable.aTiO3 is a well-known lead-free material. The strain is as high as

∗ Corresponding author. Tel.: +91 11 23903433; fax: +91 11 23913609.E-mail address: omprakasht@hotmail.com (O.P. Thakur).

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254-0584/$ – see front matter © 2008 Published by Elsevier B.V.oi:10.1016/j.matchemphys.2008.06.054

wave sintered and conventional sintered samples, respectively.© 2008 Published by Elsevier B.V.

1% in the form of single crystals, but the hysteresis is quite large,hich blocks practical application of this material [14–17].

It is well known that the doping is an effective way to improvehe materials performance for electro-ceramics. Zirconium-dopedaTiO3 single crystal were recently grown and showed promisingiezoelectric/electrostrictive properties [18–21]. At the same time,alcium substituted BZT ceramics is reported to exhibit a broadielectric constant-temperature curve near Tc with high value ofielectric constant [22]. It was reported that microwave technique

s superior to conventional sintering (CS) due to its unique char-cteristics, such as rapid heating, enhanced densification rate andmproved microstructure [23–28]. Microwave heating is funda-

entally different from conventional heating. In the microwaverocess, the heat is generated internally within the material insteadf originating from external sources, and hence there is an inverseeating profile. The heating is very rapid as the material is heatedy energy conversion rather than by energy transfer, which occursn conventional techniques. Surprisingly, not much work has beenerformed on the piezoelectric and dielectric properties of BCZTeramic prepared by microwave sintering (MWS) despite of theromising characteristics of the material and the preparation tech-ique. This has been our motivation for the study, which is reportedere.

. Experimental

The starting materials BaCO3 (Aldrich, 99+%), CaCO3 (Aldrich 99.9%), TiO2Aldrich 99.9+%) and ZrO2 (Aldrich, 99.9%) were weighed according to the stoichio-

etric composition Ba0.95Ca0.05Zr0.25Ti0.75O3 (BCZT). The weighed powders were

S. Mahajan et al. / Materials Chemistry and Physics 112 (2008) 858–862 859

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wsCdtocECmicrowave route, respectively. It is seen that the dielectric constantof Ba0.95Ca0.05Zr0.25Ti0.75O3 ceramic follows the Curie–Weiss lawat temperature much higher than the Tεm. An empirical parameter�Tm, defined as �Tm = Tdev − Tm, is often used as a measure of the

Table 1Physical and Electrical Parameters for Conventional & Microwave ProcessedSamples.

Conventional Microwave

Sintering temperature 1400 ◦C 6 h−1 1400 ◦C 2 h−1

Total cycle time (h) 22 h 4 hLattice parameter (Å) 4.04 4.06Expt. density (dexp) (gm cc−1) 6.00 6.10X-ray density (dx) (gm cc−1) 7.04 6.98Average grain size ((m) 9 4Peak dielectric constant (εm

′) 12043 13571Loss (tan ı) at Tm 0.030 0.025Curie constant (C) 3.87 × 105 9.50 × 105

T0 (K) 308 316� 1.88 1.98�Tdif (K) 29 38

Fig. 1. Time-temperature sintering profile for (a) CS and (b) MWS BZT sample.

et ball milled for 24 h using high-purity zirconia balls. After drying, calcinationas done in high-purity alumina crucible at 1200 ◦C for 6 h in a conventional fur-ace. The calcined powder was again ball milled for about 24 h and then sieved. Theynthesized powders were pressed in a rod form with applied pressure of 200 MPasing Cold Isostatic Press (M/S Autoclave Engineers, USA). The cold isostatic processives homogeneous and better compaction of green rods. For CS, one set of theseods was sintered at 1400 ◦C for 6 h in an air with 3 ◦C min−1 heating and coolingate, the total cycle time was about 22 h.

Another set of compacted rods was sintered using a microwave oven (1 kW,.45 GHz) at 1400 ◦C for 2 h. The heating and cooling rate was kept at 30 ◦C min−1

by controlling the input power to microwave oven) for the microwave sinteringxperiment. Time-temperature profile of microwave and conventional sintering isllustrated in Fig. 1. Density and microstructural information were obtained by therchimedes principle and SEM (Leo 1430, Japan), respectively. The phase purity of

he compound was confirmed by X-ray diffraction technique. XRD analysis of theample was carried out using Philips Diffractometer model PW-3020 with Cu-K�� = 1.5418 Å) radiation in a wide range of 2� (20◦ < 2� < 80◦) at a scanning rate of◦ min−1. For electrical measurements, slices (cut from rods) of dimension 0.5 mmhick and 8 mm diameter were used. Both the sides of sintered ceramics were pol-shed, properly cleaned and sputtered by gold serving as the electrode using Sputteroater (Desk II TSC Cold Sputter/Etch Unit). Measurements of capacitance (C), dissi-ation factor (tan ı) was carried out by Agilent 4284 Impedance analyzer interfacedith a PC.

. Results and discussion

The phase formation and lattice parameters of the samples pre-ared by CS and MWS techniques were deduced from the XRDattern. The XRD pattern was(Fig. 2) comprised of sharp diffrac-ion peaks and these peaks were indexed and identified with cubictructure. The lattice parameter was found to be 4.0461 Å and.0561 Å for CS and MWS samples, respectively. From the latticearameters, X-ray density, dx, was calculated. Experimental den-ity, dexp, was determined by a standard water immersion method.he value of dexp was found to be 5.98 gm cc−1 and 6.10 gm cc−1

or CS and MWS samples, respectively. The calculated values of-ray density, porosity, and percentage densification are listed inable 1. The microstructure of the sintered pallets is shown in Fig. 3.he average grain size of the samples was found to be 9 �m and�m for samples sintered by conventional and microwave sinter-

ng, respectively. Rapidity of microwave method avoids undesirablerain growth and provides a finer and uniform microstructure,hich is an attractive feature for the processing of electroceramics

23–28].The temperature dependence of dielectric constant (ε) and dis-

ipation factor (tan ı) for the Ba0.95Ca0.05Zr0.25Ti0.75O3 samples at.1 kHz, 1 kHz, 10 kHz, 100 kHz is shown in Fig. 4. It is seen that thealue of dielectric constant is higher for microwave sintered sample

��FAF

Fig. 2. XRD patterns for (a) CS and (b) MWS BCZT samples.

εm ∼13,570 at Tm ∼270 K for 1 kHz) as compared to conventionalεm ∼12,043 at Tm ∼265 K for 1 kHz) one. Strong frequency disper-ion was observed around the dielectric peak and typical relaxorehaviour was observed for both the samples.

It is well known that the classical ferroelectrics follow theurie–Weiss law above Curie temperature and is expressed by the

ollowing relationship,

1ε

= (T − T0)C

(at T > TC) (1)

here T0 is the Curie temperature and C is the Curie–Weiss con-tant. In substituted ferroelectric like PMN, deviation from theurie–Weiss behaviour has been observed [29]. The occurrence ofeviation has been attributed to short-range correlation betweenhe nanopolar domains [29]. Fig. 5(a) and (b) shows the inversef dielectric constant as a function of temperature at 1 kHz foronventional and microwave sintered samples and fitted withq. (1). The fitting parameters are C = 3.87 × 105, T0 = 308 K and= 9.50 × 105, T0 = 316 K for samples sintered by conventional and

Tm (K) 87 95Trelax (K) 19 24

requency (f0) (Hz) 4.32 × 1018 1.82 × 1022

ctivation energy Ea (eV) 0.29 0.53reezing temperature (K) 136 84

860 S. Mahajan et al. / Materials Chemistry and Physics 112 (2008) 858–862

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egree of deviation from the Curie–Weiss law. Here, Tdev is devi-ted Curie–Weiss temperature. The value of Tdev for conventionalnd microwave sintered samples are given in Table 1.

Uchino and Nomura [30] modified the Curie–Weiss law for theiffuseness of the phase transition and given as;

1ε

− 1εm

= (T − Tεm )�

C1(at T > Tm) (2)

here � and C1 are modified constants, with 1 < � < 2. The value ofhe parameter � gives information about the character of the phaseransition. Its limiting values are � = 1 and � = 2. The value of � is 1or the case of a normal ferroelectric and � = 2 (quadratic) is validor an ideal ferroelectric relaxor [31–33]. Thus, the value of � alsoharacterizes the relaxor behaviour. A plot of log (1/ε − 1/εm) as aunction of log(T − Tεm ) is shown in Fig. 6(a) and (b) for CS and MWS

amples. By fitting with Eq. (2), the exponent � , determining theegree of the diffuseness of the phase transition, is obtained fromhe slope of the log (1/ε − 1/εm) vs. log(T − Tεm ) plot. We obtainedhe value of � , which is ∼1.98 and ∼1.88 for MWS and CS sam-les, respectively. The diffuseness of the phase transition can be

raaba

Fig. 4. Temperature dependence of ε and tan ( for BCZT ceramic

(a) CS and (b) MWS BCZT sample.

escribed by an empirical parameter �Tdif, defined as

Tdif = T0.9εm(100 Hz) − Tεm (100 Hz), (3)

.e. the difference between T0.9εm (100 Hz) (the temperature corre-ponding to 90% of the maximum of the dielectric constant (εm) atigher temperature side) and T0.9εm (100 Hz)

(T0.9εm at 100 Hz)). Thealue of �Tdif was found to be 29 K and 38 K for conventional andicrowave sintered samples and shows the more diffusive nature

f microwave sintered BCZT ceramics as compared to conventionalne.

On the other hand, the degree of relaxation behaviour (fre-uency range 100 Hz–100 kHz) could be described by a parameterTrelax, which is defined as

Trelax = Tεm(100 kHz) − Tεm(100 Hz) (4)

he value of �Trelax is shown in Table 1 and indicates degree of

elaxation behaviour is more in microwave sintered sample. Thebove empirical characterization with the Curie–Weiss law (�Tm)nd the parameter (�Trelax, �Tdif, and �) show that (i) dielectricehaviour of the BCZT ceramics follows the Curie–Weiss law onlyt the temperature higher than Tεm , (ii) the parameters, �Tdif and

at different frequencies for (a) CS and (b) MWS samples.

S. Mahajan et al. / Materials Chemistry and Physics 112 (2008) 858–862 861

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tBvc

l

wttaeiVeB

Fa

aramerBsttbswsicaBm

ig. 5. The inverse ε as a function of temperature at 1 kHz for (a) CS and (b) MWSCZT samples.

Trelax, showed significant diffuseness of the phase transition andome frequency dispersion. (iii) The value of ‘� ’ was very close to, implying typical ferroelectric relaxor behaviour of the sampleintered by conventional and microwave sintering routes.

The frequency dispersion near dielectric maxima in relaxor fer-oelectrics has been attributed to the distribution of relaxationimes. A large number of theoretical models have been proposedo understand the diffusiveness of the dispersion. Among these theogel–Fulcher model has been considered to be the most successfulathematical representation for the divergent nature of relaxation

ime below a certain temperature [34,35].The frequency dependence of the maximum tempera-

ure Tm of the imaginary part of the permittivity for thea0.95Ca0.05Zr0.25Ti0.75O3 sample is shown in Fig. 7 as an ln(f)s. 1000/Tm plot [36]. The observed frequency dependence of Tm

ould be described by Vogel–Fulcher’s empirical relation as

og f = log fo − Ea/kB(Tm − Tf) (5)

here Ea is the activation energy, Tf the freezing temperature ofhe polarization fluctuation, and the pre-exponential factor fo ishe Debye frequency. As the temperature was lowered, the relax-tion time increased and at the critical temperature Tf, it became

xtremely large, and consequently the fluctuation in the polar-zation got frozen into a glassy state [37] Excellent fitting of theogel–Fulcher relation with the experimental data successfullyxplained the relaxor behaviour. The value of Ea, Tf and fo fora0.95Ca0.05Zr0.25Ti0.75O3 samples are shown in Table 1.

witrt

ig. 6. log (1/ε − 1/εm) vs. (log(T − Tεm )) plot for BCZT ceramic at 1 kHz for (a) MWSnd (b) CS samples.

The relaxor behaviour can be induced by several reasons suchs microscopic composition fluctuation, the merging of micro-polaregions into macro-polar regions, or a coupling of order parameternd local disorder mode through the local strain [10,38,39]. Vug-eister and Glinichuk [40] reported that the randomly distributed

lectric field/or strain field in a mixed oxide system was the maineason leading to the relaxor behaviour. In the solid solution ofaZrxTi1−xO3, Ba2+ ions occupied the A-site of the ABO3 perovskitetructure. Zr4+ and Ti4+ ions occupied the B sites. Due to the fact thathe ionic radius of Zr4+ (0.98 Å) is larger than that of Ti4+(0.72 Å),herefore at higher Zr contents (y > 0.08), Ba0.95Ca0.05Zr0.25Ti0.75O3ulk ceramic showed a broad dielectric peak in dielectric con-tant (ε) vs. temperature plot near transition temperature, whichas caused by the inhomogeneous distribution of Zr ions on Ti

ites and the mechanical stress in the grain. Stress was introducednto the lattice during cooling after sintering, and was due to theubic to rhombohedral phase transition below transition temper-ture [5]. Another reason for higher value of diffusiveness of thea0.95Ca0.05(ZrxTi1−x)O3 sample by microwave sintering techniqueight be the fine grain size and large grain boundary volume;hich exerted internal stress on the grains. Therefore, the decrease

n grain size is reasonably considered to be an important factoro change the delicate balance between the long-range and short-ange forces, which decides relaxor state [41]. On the other hand,he non-ferroelectric behaviour of BaZrO3 at room temperature

862 S. Mahajan et al. / Materials Chemistry

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R

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[[

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ig. 7. Frequency dependence of Tm for BCZT ceramic for (a) CS and (b) MWS sam-les.

nd above is well known and the substitution of Zr in BaTiO3 lat-ice would divide the crystal structure into micro-domains, henceesulting in a relaxor behaviour [41].

. Conclusions

Dense and single-phase perovskite BCZT ceramic was obtainedsing microwave sintering and conventional routes. Decrease in the

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and Physics 112 (2008) 858–862

rain size is attributed to fast heating and cooling rate in the casef microwave sintering. The results shown here revealed the sig-ificant improvement in relaxor behaviour by microwave sinteringpart from time and energy saving.

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