2nd International Conference on Ultrafine Grained & Nanostructured Materials (UFGNSM) International Journal of Modern Physics: Conference Series Vol. 5 (2012) 472–479 World Scientific Publishing Company DOI: 10.1142/S201019451200236X

472

EVALUATION OF NANOCRYSTALLINE BA(ZRXTI1-X)O3 PRODUCED BY MECHANICAL ALLOYING

M. Hajfathalian1

Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Azadi Avenue, 14588 Tehran, Iran1 m.hajfathalian@gmail.come

Z. A. Nemati2

Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Azadi Avenue, 14588 Tehran, Iran1 nemati@sharif.edu

H. R. Madaah Hosseini 3 Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box

11365-9466, Azadi Avenue, 14588 Tehran, Iran3 madaah@sharif.edu

BaTiO3-based solid solutions involve high piezoelectric and ferroelectric properties. One of the most favorable systems of this material is Ba(ZrxTi1-x)O3. While nano structured ceramics of which have a great potential for using them in capacitors, actuators, transducers and dielectrics to reach high K values. In this study, nano crystalline Ba(ZrxTi1-x)O3(x=0.1-0.8) which is called BZT composite has been prepared by mechanical alloying. The parameters affecting the process and properties were evaluated. The starting powders that are used here were nano sized high pure ZrO2; submicron sized high pure TiO2 and BaCO3. The prepared powder was calcined, compacted and sintered at different temperatures and in various times. The structural changes and phase developments during mechanical alloying were studied by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) and Transition Electron Microscopy (TEM). The results showed that by using mechanical alloying method, high dense nano crystalline BZT ceramics were obtained successfully at very low sintering temperature. The dielectric constant of BZT was much great, because of using nano materials in this method at the temperatures of 1250-1400˚C. The effect of high energy milling in micro structural development of nano crystalline BZT has been discussed as well as dielectric properties.

Keywords: Ba(ZrxTi1-x)O3; nano structured; mechanical alloying; sintering.

1. Introduction

The ferroelectric-relaxor behavior has been studied in many provskite-type materials, especially in lead-containing complex-compounds Pb(Zn1/3Nb2/3)O3 (PZN), Pb(Mg1/3Nb2/3)O3 (PMN) and (Pb,La)(Zr,Ti)O3, Bi-doped SrTiO3, and doped BaTiO3. The relaxor materials have found wide applications in electronic industry. Very high strain level and piezoelectric activity have been reported in solid solutions, such as, PZN-

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Evaluation of Nano Crystalline Ba(ZrxTi1-x )O3 Produced by Mechanical Alloying 473

PT (PbTiO3) and PMN-PT ferroelectric-relaxor crystals. This leads to extensive interests in the possible correlation of ferroelectric-relaxor behavior and high-piezoelectric performance. Environmental concerns are a contributing factor driving the exploration for lead-free piezoelectric and dielectric materials, account the high toxicity of lead oxide and its high vapor pressure during processing. BaTiO3-based solid solutions represent an alternative for the replacement of Pb-based piezoceramics and recently, there has been continued growth of interest in the use of BaTiO3-based ceramics, because they are promising materials for tunable microwave device application such as electronically tunable mixer, filters, capacitors, resonators and phase shifters. On the other hand, it is well known that BaTiO3 and its solid solutions possess high piezoelectric and ferroelectric properties, and has been extensively studied and widely used. The influences of many isovalant dopants on polymorphism and dielectric properties of BaTiO3 have been studied in detail. A series of impurity-doped BatiO3 systems, such as Sn, Hf, Ce, and Y, have also shown a ferroelectric-relaxor behavior [1-5]. One of the most favorable systems of these materials is Ba(ZrxTi1-x)O3 and their nano structured specimens have a great potential for applications such as capacitors, multi layer ceramic capacitors (MLCCs), actuators, transducers and dielectrics due to their high K values. Ba(ZrxTi1-x)O3 (BZT) ceramics are obtained by substituting ions at the B site of BaTiO3

with Zr ions. It is reported that BZT system has high dielectric constant and low loss tangent at room temperature. The ferroelectric properties of BZT are largely dependent on the amount of Zr substitution [6, 7]. The dielectric data of Ba(ZrxTi1-x)O3 ceramics suggest a normal ferroelectric behavior for compositions 0≤x≤0.1 and relaxor character for 0.1≤x≤0.42 when degree of diffuseness of the ferro-para phase transition is increasing with x [8] and antiferroelectric properties for Zr-rich compositions. The relaxor state is characterized by a strong frequency dependence of the permittivity below the maximum permittivity temperature Tm, a shift of Tm towards higher values with increasing frequency and deviations from the Curie-Weiss law in the paraelectric phase [9, 10]. Investigation has shown that the phase transition behavior of Ba(ZrxTi1-x)O3 materials can be changed from a normal phase transition to diffusive and further become a relaxor phase transition. The modified BZT has shown systematic variations in the dielectric, piezoelectric and phase transition characteristics in the ceramics [11]. To achieve fine microstructure and high performance, it is necessary to start with

preparing fine, stoichiometric and narrow size distribution powder. In the present study nano crystalline Ba(ZrxTi1-x)O3 (x=0.1, 0.8) which is called BZT( BZ0.1T0.9, BZ0.8T0.2) composite has been prepared by high energy milling and/or solid state reaction method. The parameters affecting the process and properties were evaluated. For the composition x = 0.1, 0.8 that prepared by high energy milling, samples with nanostructure sized were obtained. The influence of Zr content on the dielectric and ferroelectric properties of the BZT ceramics with x = 0.1, 0.8 were examined and the results were discussed.

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474 M. Hajfathalian, Z. A. Nemati & H. R. Madaah Hosseini

2. Materials and Methods

Ba(ZrxTi1-x)O3 (BZT) ceramics were prepared by solid-state reaction using submicron precursors of BaCO3, TiO2 and nanometric precursors ZrO2 reagent. They were mixed in stochiometric proportions and milled by vibration milling in a Tungsten carbide jar and Tungsten carbide disc for 4 h and calcined at 10000C/6h. The calcined powders were milled and compacted at 200MPa in pellets of 12mm in diameter and a bout 2mm in thickness and then sintered at 1150-14000C/2-4h. The phase purity was evaluated by X-ray diffraction (XRD) with Cu Ka radiation (Philips PW3710). The microstructures and nanostructures were examined by Scanning Electronic Microscopy (SEM) and Transition Electronic Microscopy (TEM) analysis. The apparent densities were measured by the immersion method in water. For the electric measurements, Ag paste was deposited on the plane parallel polished clean surfaces of the ceramics followed by annealing in air at 600 0C/1h.The capacitance was measured with using Auto transformer and Static converter Based Circuit for applying high voltage and frequency to samples and the dielectric constant of each sample was calculated from the measured capacitance and the dimensions of the specimen.

3. Results

Fig. 1 shows XRD patterns of Ba(ZrxTi1-x)O3 (x = 01, 0.8) ceramics. All the samples show the typical XRD pattern of the perovskite structure. The XRD patterns that prepared by high energy milling, show a decrease in grain size. The grain size, D, is estimated from the full-width at half-maximum (FWHM) of the perovskite (011) peak in XRD patterns using Scherrers formula “Eq. (1)”:

(1)

Where B is FWHM, cosθ is the diffraction angle, and λ is the wavelength of the X-ray. By comparing with the widths of the diffraction peaks of standard materials, the width of finite crystallite size can be obtained. It is well known that grain size in ABO3 perovskites ferroelectric materials are investigated, showing important influence on lattice vibration, phase transitions and optical properties [12]. It was observed that the diffraction peaks shifted to a lower angle with an increase in Zr content. Due to fact that the ionic radius of Zr4+ (0.72 A0) is larger than that of Ti4+ (0.605 A0), thus the substitution of Ti4+ by Zr4+ could increase the lattice parameter of ceramic[13].

θλ

BCosD

9.0=

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Evaluation of Nano Crystalline Ba(ZrxTi1-x )O3 Produced by Mechanical Alloying 475

Fig 1. XRD patterns of a) BZ0.1T0.9, b) BZ0.8T0.2, powders after high energy milling

The TEM analysis shows nano particles for samples with using high energy milling and confirms the results that obtained from XRD patterns (Fig2), the average particle diameter was found to be approximately 20 nm. This value confirms the nano sized nature of powders after 4h high energy milling.

Fig 2. TEM image of nano crystalline BZT powders obtained in a) 50 nm and b) 500nm magnification.

The SEM analysis shows good ultra fine grains and nano structures and a small degree of porosity for the samples (Fig3). The agglomerates are rounded in form which is typical for the mixed oxide method[14]. The increase of Zr amount acts as an inhibitor of the grain growth during sintering and milling in the process. The decreasing grain size with

a b

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476 M. Hajfathalian, Z. A. Nemati & H. R. Madaah Hosseini

increasing x is associated with the lower grain-growth rate caused by slow diffusion of Zr4+ion, which has a larger ionic radius than Ti4+ [15] and thus the diffusion of Zr4+ requires higher energy and this increases the sintering temperature. As a result a reduction of the average grain size of the BZT ceramics sintered at 12500C-13000C for samples is obtained for higher x. The samples are sintered to a density of ~97% of their theoretical values in the temperature range 1250-13000C with the Zr-containing compositions requiring temperatures close to 13000C. The suitable sintering condition for the BZT ceramics with x=0.1 was designed as 12500C for 2.5 h whereas that for the BZT ceramics with x=0.8 was designed as 13000C for 2.5 h, The optimum conditions of sintering temperature is lower than reported by Nanokorn and et.[13] because of using high energy milling for providing nano particles.

Fig3. SEM of a) BZ 0.1T0.9 samples that sintered at 12500C (10µm), b, c) BZ 0.8T0.2 samples that sintered at

13000C (5&1µm)

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Evaluation of Nano Crystalline Ba(ZrxTi1-x )O3 Produced by Mechanical Alloying 477

The densification is higher in BZ0.1T0.9 (~97%) as compared to BZ0.8T0.2 (~94%). The increase in Zr content increased the sintering temperature of BZT solid solution [16] and hence resulted in less densification of BZ0.8T0.2 (fig4).

Fig4. Relative density of sintered Ba(Ti1-xZrx)O3 ceramics as a function of x

Dielectric constant was increased with using nano structured BZT in high energy milling in comparison with conventional milling and was decreased with increased of Zr amount because of relaxor and antiferroelectric behavior (Table1).

Table1. Dielectric constant of BZT samples

BZT Dielectric constant

BZ0.1T0.9 15000

BZ0.8T0.2 600

It indicates that dielectric constant is strongly dependent on the grain size. As the grain size increased, the dielectric constants decreased at most of temperatures studied in ferroelectric materials same as BZ0.1T0.9 [3, 17]. These parameters are reasonably high for lead-free materials and show potentials for practical applications. 4. Conclusion

1. Fine BZT nano particles, with an average particle size of 20 nm, were successfully earned with high energy milling.

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478 M. Hajfathalian, Z. A. Nemati & H. R. Madaah Hosseini

2. The process has strong effect on grain size. High density (97%) Ba(ZrxTi1-x)O3 nano structured ceramics with x=0.1-0.8 were prepared via high energy milling. The nano powders are more reactive and are found to yield dense samples at lower temperatures. 3. It has been shown that well-densed BZT will be obtained in lower temperature than previous works with heating for 2.5h in 1250°C. Nano structure BZ0.8T0.2 ceramic with high density can be obtained by sintering in 1300°C for 2.5 h. 4. The Zr ions substitution of Ti ions affects the grain size and electrical properties of final samples. The grain sizes decrease with increasing of Zr amount. Dielectric constant was increased with using nanostructure BZT in high energy milling and was decreased with increasing of Zr amount.

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