Barium titanate nanoparticles produced by planetary ball milling and piezoelectric properties of corresponding ceramics

Materials Letters 65 (2011) 970–973

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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Barium titanate nanoparticles produced by planetary ball milling and piezoelectric properties of corresponding ceramics K. Chandramani Singh ⁎, A.K. Nath Department of Physics, Sri Venkateswara College, University of Delhi, New Delhi 110021, India

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Article history: Received 2 September 2010 Accepted 20 December 2010 Available online 4 January 2011 Keywords: Perovskite Ceramics Sintering Piezoelectric materials

a b s t r a c t Barium titanate (BaTiO3) nanocrystalline powders were prepared by solid state reaction route starting from BaCO3 and TiO2 powders accompanied by high-energy ball milling. Different samples were prepared by varying the milling time from 1 h to 30 h, keeping the milling speed fixed at 250 rpm. All the milled powders were examined with TEM. The particle size first decreases from 105 nm to a minimum of 28 nm as the milling time increases from 1 h to 20 h and then increases to 38 nm with further increase of milling time to 30 h. Dense ceramics were formed by sintering the nanopowders at 1350 °C for 4 h. With decreasing particle size of the starting nanopowders, the ceramics exhibit gradual increase in density from 5.65 g/cc to 5.84 g/cc, coercive field (Ec) from 2.25 kV/cm to 4.77 kV/cm and piezoelectric charge constant (d33) from 99pC/N to 121pC/N. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Research on lead-free piezoelectric ceramics [1, 2] has been gaining importance due to the environmental concern caused by highly toxic nature of lead-based ceramics. BaTiO3 (BT) is the most widely studied lead-free material due to its potential application as multilayer ceramic capacitors, PTC thermistors, piezoelectric transducers, actuators and dynamic RAM. The characteristics of such electronic ceramic are markedly influenced by the particle size and morphology of the material [3]. A few workers have reported great improvement on the physical, electrical and piezoelectric properties of BT ceramics synthesized from the nanoceramic powders [4, 5]. It is also reported that ferroelectricity of BT decreases with decreasing particle size and disappears below certain critical size [6, 7]. From these standpoints, production of nanoceramic powders having desired particle size has been gaining importance in current time. High-energy ball milling technique is a simple and cost effective method for large scale production of BT powders [4, 8, 9]. In the present work, ferroelectric and piezoelectric properties have been investigated for BT ceramics obtained from nanopowders produced by high-energy ball milling.

homogeneously mixed in isopropanol medium using ball milling. The mixture was dried and then calcined at 1050 °C for 4 h. The calcined powder was then high-energy milled in the isopropanol medium using a Retsch PM 100 planetary ball mill in which the sun wheel and grinding jar rotate in opposite directions with speed ratio 1:−2. Agate balls (each of 10 mm diameter) and vial (125 ml) were used. The milling was performed for different durations of 1, 10, 15, 20 and 30 h, fixing the speed at 250 rpm. During each high-energy milling, a mass ratio of 1:5 for powder and balls was always maintained. The ball mill was set in such a mode that the rotational direction of the vial and the sun wheel changes every 6 min after a rest interval of 2 min.

2. Experimental The starting raw materials BaCO3 and TiO2 (purity ≥ 99%) were weighed according to the stoichiometric formula to yield BaTiO3 and ⁎ Corresponding author. Tel.: +91 11 9868202566; fax: +91 11 24118535. E-mail address: [email protected] (K.C. Singh). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.12.039

Fig. 1. X-ray diffraction pattern of a BaTiO3 ceramic prepared from powders milled at 250 rpm for 20 h.

K.C. Singh, A.K. Nath / Materials Letters 65 (2011) 970–973

The particle sizes of the milled powders were examined by using TEM (Morgagni 268). X-ray diffractometer (Philips Diffractometer PW 3020) with monochromatic CuKa radiation (λ = 1.54178 Å) was used over a 2θ angle from 20° to 70° to characterize the crystalline phase of the powders milled under different conditions. The powders were pressed into pellets of 10 mm diameter and 1 mm thickness under 2 MPa using polyvinyl alcohol (PVA) as a binder. After burning off PVA, the pellets were sintered at 1350 °C for 4 h in closed alumina crucible. The bulk density of the sintered specimens was measured using Archimedes principle. For the electrical measurements, silver paste was coated on both sides of the sintered samples and fired at 300 °C for 1 h to form electrodes. For activating the piezoelectric properties to the ceramics, a poling treatment was conducted at 100 °C in stirred silicone oil at 3 kV/mm for 30 min using a DC power supply, and then the samples were cooled to room temperature

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by maintaining the electric field. The polarization versus electric field (P–E) hysteresis loops of the ceramics were recorded using an automated P–E loop tracer (AR Imagetronics, India) operating at 50 Hz. The piezoelectric charge coefficient (d33) was measured with a piezometer (Take Control, PM 25). 3. Results and Discussion Fig. 1 shows a room temperature diffraction pattern of BT ceramic prepared from powders milled at 250 rpm for 20 h. All the ceramics possess a single-phase perovskite structure. The splitting into (002) and (200) characteristic peaks at 2θ ~ 45.5° indicates that the crystal symmetry is tetragonal. Fig. 2 shows the TEM micrographs of the BT powders milled for different durations of 1, 10, 15, 20 and 30 h, each at the speed of

Fig. 2. TEM micrographs of BaTiO3 nanoparticles milled at 250 rpm for (A) 1 h, (B) 10 h, (C) 15 h, (D) 20 h, and (E) 30 h.

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K.C. Singh, A.K. Nath / Materials Letters 65 (2011) 970–973

Table 1 Several properties of the BaTiO3 ceramics. Milling time (h)

1

10

15

20

30

Particle size (nm) Bulk density (g/cc) ± 0.01 Pr (μC/cm2) Ec (kV/cm) d33(pC/N)

105 ± 1 5.65 2.7 ± 0.1 2.3 ± 0.1 99 ± 1

61 ± 2 5.69 2.9 ± 0.1 2.6 ± 0.1 103 ± 1

57 ± 2 5.72 5.1 ± 0.2 3.4 ± 0.1 112 ± 1

28 ± 4 5.84 6.1 ± 0.2 4.8 ± 0.2 121 ± 2

38 ± 3 5.79 6.5 ± 0.2 4.1 ± 0.2 116 ± 2

250 rpm. The average particle size calculated from each of these micrographs using ImageJ is given in Table 1. As seen in this Table, the average particle size first decreases from 105 nm to 28 nm with increase in milling time from 1 h to 20 h and then increases to 38 nm as the milling time increases further to 30 h. The energy supplied by the planetary ball mill during high-energy milling of a powder is used in the rupture of interatomic bonds in the crystal and the formation of additional surface as a result of the cleavage of crystalline grains [10]. According to a model proposed by Gusev et al [11], an increase in the milling time should lead to a gradual decrease in the particle size of the milled powder to a saturation value. Such a decreasing trend of crystal size evolution with the milling time has also been predicted to be represented by an exponential function [12, 13]. The observed decrease in average particle size of milled BT powders from 105 nm to 28 nm as the milling time increases from 1 h to 20 h agrees with these predictions. However, in contradiction to these theoretical predictions, the average size of the particles does no further decrease beyond 20 h of milling in the present investigation (Table 1). Instead, the average particle size starts increasing from 28 nm to 38 nm as the milling time increases from 20 h to 30 h. Traiphol et al also, using a laser diffraction particle size analyzer, observed such increase of average particle size from 0.84 μm to 1.85 μm as milling time increases from 40 h to 50 h in lead zirconate titanate aqueous suspension [14]. The increase in particle size was attributed to particle repacking caused by prolonged impact of the grinding media to the wall of the milling bottle. This may be one contributing factor even to the increase in size of nanoscale particles. However, the observed increase in particle size in the present study could be a more complex process involving surface energy and microstrains of the particles. As the particle size goes down to nanometer range the surface to volume ratio of the particle and hence their specific surface energy increases significantly. On the other hand, the microstrains produced in the particles increase as the milling time of the powder increases. Both these processes lead to enhanced

10

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-20

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0

10

20

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instability in the powder particles. Perhaps, a critical stage of instability is reached in the nanoparticles with prolonged grinding when the particles start coalescing to form bigger particles by way of releasing the excess energy on the particles. The bulk densities of the BT ceramics as measured by Archimedes method are listed in Table 1. All samples reached a bulk density of N94% of the theoretical density (6.01 g/cc). The reduction in the particle size to the nanoscale by high-energy milling has promoted the densification of the ceramic system, as the driving force for sintering is inversely proportional to the initial particle size [15]. Fig. 3 shows the P–E hysteresis loops measured at room temperature of BT ceramics. Well saturated hysteresis shape typical of ferroelectric materials is evident for the ceramics. The values of remnant polarization (Pr) and coercive field (Ec), determined from the loops are summarized in Table 1. It is seen that Ec values follow an increasing trend with decreasing particle size of the starting nanopowders. This result agrees with the increase in Ec with grain size reduction reported for sputter-deposited BT thin layers [16]. The piezoelectric properties of the BT ceramic samples are given the Table 1. The value of d33 increases from 99pC/N to 121pC/N as the average particle size of the starting powder decreases from 105 nm to 28 nm. The increase in d33 can be attributed to the denser microstructure and smaller grain size [17]. The piezoelectric effect in perovskite-type ferroelectric ceramics is strongly influenced by the movement of the 90o domain walls [18, 19]. As domain walls with large areas possess heavier inertia masses in motion [18], domain walls with small areas will respond more actively to the external electrical or stress signal. Also the smaller the grain size, the smaller the areas of the domain walls [17]. It is thus expected that the decrease in grain size in the ceramic corresponding to the decrease in particle size of the starting powders will result in an increase in d33 as observed in the present investigation. 4. Conclusions Lead-free BaTiO3 nanocrystalline powders with average particle size varying from 105 nm to 28 nm have been produced by highenergy ball milling. An increase in milling time from 1 h to 20 h leads to a gradually decreasing trend in particle size from 105 nm to a minimum value of 28 nm, in consistent with existing theories. However, the theories fail to explain the increasing particle size from 28 nm to 38 nm as the milling time further increases from 20 h to 30 h. Such a peculiar change in particle size may be related to the microstrains and specific surface energy of the particles. The observed change in particle size with milling time is experimentally corroborated by the associated systematic change in Ec and d33 for the ceramics. Acknowledgements

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Fig. 3. P-E hysteresis loops of the BaTiO3 ceramics milled at 250 rpm for 1 h, 10 h, 15 h, 20 h and 30 h, measured at 25 °C and 50 Hz.

The financial support from UGC of India under the Major Research Project vide No. F. 33-25/2007(SR) is acknowledged. We also acknowledge the Directors, Solid State Physics Laboratory and AIIMS, Delhi, for providing facilities for some measurements.

K.C. Singh, A.K. Nath / Materials Letters 65 (2011) 970–973

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