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Journal of Alloys and Compounds 462 (2008) 129–134

Dielectric and ferroelectric characteristics of bariumzirconate titanate ceramics prepared from

mixed oxide method

F. Moura a,1, A.Z. Simoes a,∗, B.D. Stojanovic a,b,1,M.A. Zaghete a,1, E. Longo a,1, J.A. Varela a,1

a Chemistry Institute, Department of Chemistry-Physics, UNESP C.P. 355, CEP 14801-970, Araraquara, SP, Brazilb Center for Multidisciplinary Studies University of Belgrade, Department of Materials Science,

Kneza Viseslava 1a, Belgrade, Serbia

Received 27 December 2006; received in revised form 17 July 2007; accepted 20 July 2007Available online 26 July 2007

bstract

Barium zirconium titanate (BZT) ceramics were prepared by mixed oxide method. X-ray diffraction showed the presence of a single phase while

aman scattering confirmed structural transitions as a function of different Zr/Ti ratio. The addition of Zr strongly influenced the crystal structurend electrical properties of the ceramics. A typical hysteresis loops were observed for all investigated compositions. BZT ceramics with 15 mol%r have shown a ferroelectric to paraelectric transition at around 77 ◦C.2007 Published by Elsevier B.V.

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eywords: Ferroelectrics; X-ray diffraction; Dielectric response

. Introduction

Barium zirconium titanate ceramics are attractive can-idates for dynamic random access memories and tun-ble microwave devices. Such, lead-free, environmentalriendly materials are known to exhibit relaxor behaviourn bulk materials with increasing the Zr content. Thenterest in high strain piezoelectric materials is increas-ng for electromechanical transducers and various relatedpplications [1]. Though the large family of lead-basederovskites and relaxors has shown a great potential,ead-free compositions in these families will be of interest dueo obvious environmental concern in the future [2–6]. BaTiO3 isnown to have a large electromechanical coupling factor. Substi-

ution of Ti4+ (atomic weight of 47.9, ionic radius of 0.0745 nm)ith Zr4+ (atomic weight of 91.2, ionic radius of 0.086 nm)

xhibits several interesting features in the dielectric behavior of

∗ Corresponding author. Tel.: +55 16 3301 6600; fax: +55 16 3301 6692.E-mail address: alezipo@yahoo.com (A.Z. Simoes).

1 Tel.: +55 201 6600; fax: +55 16 222 7932.

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925-8388/$ – see front matter © 2007 Published by Elsevier B.V.oi:10.1016/j.jallcom.2007.07.077

aTiO3 ceramics. When the Zr content is less than 10 mol%, theZT ceramics show normal ferroelectric behavior and dielectricnomalies corresponding to cubic to tetragonal (Tc), tetragonalo orthorhombic (T2), and orthorhombic to rhombohedral (T3)hase transitions. At around 27 mol%, Zr-doped BZT ceramicsxhibit typical diffuse paraelectric to ferroelectric phase tran-ition behavior, whereas Zr-richer compositions exhibit typicalelaxor-like behavior in which Tc shifts to higher temperatureith increase of frequency [7]. The modified Ba(Zr,Ti)O3 has

hown the systematic changes in the dielectric, piezoelectric,nd phase transition characteristics in the bulk ceramic and sin-le crystal forms [8]. In the paraelectric state, just above Tc, BZTeramics are attractive candidates for dynamic random accessemories and tunable dielectric devices.It is well known that Zr, like Sr, is an effective substituent in

aTiO3 to decrease and shift the Curie temperature below roomemperature [9]. Moreover, Zr4+ ion is chemically more stablehan Ti4+ and has a larger ionic size to expand the perovskite

attice. Therefore, the substitution of Ti by Zr would depress theonduction by electronic hopping between Ti4+ and Ti3+ and itould also decrease the leakage current of the BaTiO3 system. It

s reported that an increase in the Zr content induces a reduction

130 F. Moura et al. / Journal of Alloys and

Fig. 1. X-ray diffraction pattern of the BZT powders calcinated at 1200 ◦C for2 h in different concentrations: (a) Ba(Ti0.95Zr0.05)O3; (b) Ba(Ti0.9Zr0.10)O3 and(c) Ba(Ti0.85Zr0.15)O3.

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Fig. 2. Linear shrinkage as a function of temperature for BZT powders calcinateBa(Ti0.9Zr0.10)O3 and (c) Ba(Ti0.85Zr0.15)O3.

Compounds 462 (2008) 129–134

n the average grain size, decreases the dielectric permittivityεr), maintaining a low and stable leakage current [10–12]. Thiss possible because Zr4+ ion has larger ionic size (0.087 nm)han Ti4+ (0.068 nm). Thus, BZT ceramics have good propertiesnd can be applied as a storage capacitor for the next DRAMeneration and a dielectric material for MLCC. Zr/Ti ratio ismportant in BZT system and a 0.2/0.8 ratio is known to havexcellent bulk properties [13]. In this work, the effect of Zr/Tiatio on the structural and electrical properties of BZT ceramicsas investigated.

. Experimental procedure

The powders were homogenized in a ball mill using isopropyl alco-ol. All oxides were analytical grade: BaCO3 (Vetec), ZrO2 (Inlab), TiO2

Vetec). The investigated systems were Ba(Ti0.95Zr0.05)O3, Ba(Ti0.9Zr0.10)O3

nd Ba(Ti0.85Zr0.15)O3. After drying, the powders were compacted in the formf pellets with 1 mm of thickness and 12 mm of diameter, isostatically pressedt 210 MPa. The pellets were sintered at 1550 ◦C for 4 h in static air and cooledo room temperature (5 ◦C/min).

X-ray diffraction data were collected with a Rigaku Rint 2000 diffractome-

er under the following experimental condition: 50 kV, 150 mA, 20 ◦≤ 2θ ≤ 80◦,

2θ = 0.02◦, λCu k� monocromatized by a graphite crystal, divergencelit = 2mm, reception slit = 0.6 mm, step time = 10 s. Dilatometric analy-es were performed in a Nezstch dilatometer 402E, up to 1550 ◦C,t a heating rate of 5 ◦C/min in static atmosphere and the results of

d at 1200 ◦C for 2 h in different concentrations: (a) Ba(Ti0.95Zr0.05)O3; (b)

F. Moura et al. / Journal of Alloys and Compounds 462 (2008) 129–134 131

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ig. 3. Linear shrinkage rate as a function of temperature for BZT powders ca(Ti0.9Zr0.10)O3 and (c) Ba(Ti0.85Zr0.15)O3.

inear shrinkage rate (d(�l/l0)/dT) and linear shrinkage �l/l0 were deter-ined.

Raman measurements were performed using an ISAT 64000 tripleonochromator. An optical microscope was employed to focus the 514.5-nm

adiation from a Coherent Innova 99 Ar + laser on the sample and to collect theack-scattered radiation. The scattered light dispersed by the spectrometer wasetected by a charge-coupled device (CCD) detection system. Specific surfacerea was determined based on nitrogen adsorption isotherms by using the BETethod (Sorptomatic 1990).

Gold electrodes for electrical measurements were applied by evapo-ation through a sputtering system on a polished surfaces of sinterediscs. Ferroelectric properties were measured on a Radiant TechnologyT6600A tester system equipped with a micrometer probe station in a vir-

ual ground mode. The dielectric characterization was accomplished with HP192 impedance analyser, and measurements of the capacitance as a func-ion of temperature at a frequency of 10 kHz were performed. From theapacitance dependence temperature curves, the Curie temperature was deter-ined.

. Results and discussion

Fig. 1 shows the room temperature XRD plots ofa(Ti0.95Zr0.05)O3, Ba(Ti0.9Zr0.10)O3 and Ba(Ti0.85Zr0.15)O3

owders. All the ceramics composition were crystallized intosingle-phase perovskite structure. This is a clear indica-

ion that the addition of Zr is forming a stable solid solutionith the BaTiO3 lattice. As reported in other studies [2–6],

ownc

ted at 1200 ◦C for 2 h in different concentrations: (a) Ba(Ti0.95Zr0.05)O3; (b)

epending upon the Zr content, BZT may have orthorhombic,hombohedral and tetragonal structures at room tempera-ure.

The linear shrinkage �l/l0 and the linear shrinkage rated(�l/l0)/dT) as a function of temperature for different Zr/Tiatio are shown in Figs. 2 and 3. The sintering process is stronglynfluenced by Zr/Ti ratio leading to a reduction in the densifica-ion temperature. The maximum shrinkage rate occurred around410 ◦C for the composition Ba(Ti0.95Zr0.05)O3. Meanwhile, aeduction in the maximum shrinkage rate was observed for thea(Ti0.85Zr0.15)O3 composition, probably due to the segregationf ZrO2 at the grain boundaries. The peak close to 980 ◦C indi-ates powder agglomeration during the sintering process (intra-nd inter-agglomerates) as a consequence of different surfacerea of the raw powders [ZrO2 (5.67), BaCO3 (1.35) and TiO253.71)].

Room temperature Raman spectra are displayed in Fig. 4.ecause of the random grain orientations in the powders, theirections of the phonon wave vectors are randomly distributedrom one grain to another with respect to the crystallographicxes. The evolution of Raman spectra with Zr substituting

n Ti sites shows some interesting changes. As observed,ith increasing Zr content the Raman line at 123 cm−1 hasot been observed. Taking the mass ratio Zr/Ti = 1.9 intoonsideration this mode frequency is expected to be about

132 F. Moura et al. / Journal of Alloys and Compounds 462 (2008) 129–134

Fig. 4. Room temperature depolarized Raman spectra for BZT powders calcinated at 1200 ◦C for 2 h in different concentrations: (a) Ba(Ti0.95Zr0.05)O3; (b)B

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Measurements on dielectric permittivity as a function of

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S

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a(Ti0.9Zr0.10)O3 and (c) Ba(Ti0.85Zr0.15)O3.

29 cm−1 for Zr replacing Ti sites, which will be reduced fur-her by an increase in the ionic radius [R(Ti4+) = 0.0745 nm,(Zr4+) = 0.086 nm]. The additional mode could thereforee associated with a normal mode involving Zr atoms.ince this mode disappears in the Ba(Ti0.9Zr0.10)O3 anda(Ti0.85Zr0.15)O3 compositions, it may be considered an indi-ation of the orthorhombic to rhombohedral phase transition.uch observations could not be observed in the X-ray studiesue to the different coherence length and time scale involved in

he process.

SEM analysis of BZT powders calcinated at 1200 ◦C for 2 hre shown in Fig. 5. We observe an increase in the agglomer-

tpc

able 1haracteristics of BZT powders and pellets for different Ti/Zr rat

amples Surface area (nm) Pr (�C/cm2)

a(Ti0.95Zr0.05)O3 0.83 5.4a(Ti0.9Zr0.10)O3 0.27 5.3a(Ti0.85Zr0.15)O3 0.16 3.6

tes size for different Zr/Ti ratio leading to a reduction in thepecific surface area. The agglomerates are rounded in formhich is typical for the mixed oxide method. Values of the spe-

ific surface area of BZT powders were measured from BETnalyses (Table 1). There is a tendency to decrease the specificurface area with the increase in the zirconium content due to thetrong presence of agglomerates and coarse-grained structure,s observed in the SEM analysis.

emperature reveal anisotropic behavior (Fig. 6). The highestermittivity (εr = 14,500) is observed for the Ba(Ti0.95Zr0.05)O3omposition at 100 kHz. Three distinct phase transitions were

Ec (kV/cm) Dielectric permittivity at (100 kHz)

186 14,400134 11,660157 5300

F. Moura et al. / Journal of Alloys and Compounds 462 (2008) 129–134 133

F in different concentrations: (a) Ba(Ti0.95Zr0.05)O3; (b) Ba(Ti0.9Zr0.10)O3 and (c)B

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ig. 5. SEM analysis for the BZT powders calcinated at 1200 ◦C for 2 ha(Ti0.85Zr0.15)O3.

bserved. The Curie temperature reduces with the increase ofirconium content due the changes in crystal structure, as shownn Raman spectra. A strong reduction in the dielectric permit-ivity for the Ba(Ti0.85Zr0.15)O3 system is a consequence ofifferent crystal structure. A sharp phase transition is indicativef a ferroelectric-relaxor behavior, as observed in the literature14–16].

Well saturated hystereses loops with regular shape, typical oferroelectric materials were evident for all investigated systemsFig. 7a–c). There is no evidence of “imprint” phenomena indi-ating that our ceramics present low defects as oxygen vacancieshich pinning the domain walls difficulting the saturation ofystereses loops. Consequently, there is no increase in the coer-

ive voltage in one direction. The reduction in the remnantolarization for the Ba(Ti0.85Zr0.15)O3 system can be relatedo changes in crystal structure which can explain the relaxorehaviour observed in Fig. 6.

Fig. 6. Temperature dependence of dielectric permittivity (ε) for BZT pelletssintered at 1550 ◦C for 4 h in different concentrations: (a) Ba(Ti0.95Zr0.05)O3;(b) Ba(Ti0.9Zr0.10)O3 and (c) Ba(Ti0.85Zr0.15)O3.

134 F. Moura et al. / Journal of Alloys and Compounds 462 (2008) 129–134

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Shrout, Jpn. J. Appl. Phys., Part 1 38 (1999) 5505.[15] S.E. Park, S. Wada, L.E. Cross, T.R. Shrout, J. Appl. Phys. 86 (1999)

2746.[16] J. Ravez, A. Simon, Eur. J. Solid State Inorg. Chem. 34 (1997) 1199.

ig. 7. P–E hysteresis loops for BZT pellets sintered at 1550 ◦C for 4 h ia(Ti0.85Zr0.15)O3.

. Conclusions

Single phase BZT powders were obtained from mixed oxideethod at 1200 ◦C for 2 h. Raman spectroscopy indicated a

hange in the crystal structure with the increase of zirconiumontent. The transition temperature (ferroelectric to paraelectrichase) was found to be systematically reduced for Zr contentqual to 15.0 mol%. In this case, a typical relaxor behavior wasbserved. A decrease in the dielectric permittivity and remnantolarization with the increase of zirconium content is indica-ive of changes in crystal structure and predominance of relaxorehaviour.

cknowledgments

The authors gratefully acknowledge to the financial supportf the Brazilian agencies FAPESP, CNPq, and CAPES and Min-stry of Science and Environmental Protection of Serbia.

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