Isothermal grain growth and effect of grain size on ...

SMM-10804; No of Pages 4 Scripta Materialia xxx (2015) xxx–xxx

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Isothermal grain growth and effect of grain size on piezoelectric constant of Na0.5Bi0.5TiO3 ceramics M. Muthuramalingam a, D.E. Jain Ruth a, M. Veera Gajendra Babu a, N. Ponpandian b, D. Mangalaraj b, B. Sundarakannan a,⁎ a b

Department of Physics, Manonmaniam Sundaranar University, Tirunelveli 627012, Tamil Nadu, India Department of Nanoscience and Technology, Bharathiar University, Coimbatore 641046, Tamil Nadu, India

a r t i c l e

i n f o

Article history: Received 3 September 2015 Accepted 7 September 2015 Available online xxxx Keywords: Sintering SEM Piezoelectric ceramics Grain growth Critical grain size

a b s t r a c t Grain growth behavior as a function of sintering time and effect of grain size on piezoelectric constant of Na0.5Bi0.5TiO3 are investigated. Grain sizes of NBT ceramics are 0.38, 1.23, 1.73, 1.99, 2.31 and 3.65 μm. Interface controlled grain growth occurs during isothermal sintering with a grain growth exponent of 2.1. Piezoelectric constant is 17pC/N for a sample with grain size of 0.38 μm and around 73pC/N for the remaining samples. Drastic reduction of piezoelectric constant for the sample with sub-micron size grains is due to decrease of non-180o domain wall movement as evidenced by the intensity ratio of (111)/(111) of poled samples. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Sodium bismuth titanate (NBT) is one of the potential lead-free candidate materials with perovskite structure [1]. NBT belongs to monoclinic crystal system with a space group Cc [2,3]. Monoclinic structure is susceptible to thermal, mechanical, electrical and chemical perturbations and eventually changes to rhombohedral system with a space group of R3c [4,5]. It is ferroelectric with a Curie temperature of 320 °C, remnant polarization is 38μC/cm2 and piezoelectric constant is 73pC/N [6]. It exhibits morphotrophic phase boundary (MPB) with tetragonal perovskites, K0.5Bi0.5TiO3 and BaTiO3, and orthorhombic KNbO3 where piezoelectric constant (d33) is enhanced [7–9]. Variation of Curie temperature, dielectric constant, coercive field, remnant polarization, piezoelectric constant and electromechanical coupling coefficients with grain size has been clearly reported for PZT ceramics [10]. Specifically, piezoelectric constant decreases in submicron grain sizes due to a reduction in domain variants with increased domain density which limits the extrinsic contribution [10]. Maximum d33 occurs at 1 μm grain size due to high mobility of domain walls while it rapidly decreases with further reduction of grain size in BaTiO3 ceramics [11,12]. Dielectric constant at room temperature has a maximum at 1 μm grain size and ferroelectricity disappears at around 100 nm in Na0.5Bi0.5TiO3 ceramics [13]. However, dependence of

⁎ Corresponding author. Tel.: +91 9487410736. E-mail address: [email protected] (B. Sundarakannan).

piezoelectric constant of Na0.5Bi0.5TiO3 with grain size, has not been reported in the literature, is essential for choosing the processing condition since it is an important constituent of many lead-free MPB systems. The objective of the present work is to investigate the effect of grain size on piezoelectric constant. To obtain NBT ceramics with different grain sizes, isothermal sintering was performed for six different durations at the temperature of 1180 °C. Grain sizes are estimated from SEM micrographs. Critical grain size for drastic reduction of piezoelectric constant is identified for NBT ceramics. 2. Experimental procedures Stoichiometric amount of oxides and carbonates, Bi2O3 (99.9%), Na2CO3 (99.5%), TiO2 (99.8%) from Sigma Aldrich, ground using mortar and pestle. Powders calcined at 850 °C for 3 h and subsequently compacted into pellet using PVA binder under a pressure of 7 t. Pellets were sintered at 1180 °C for 10, 20, 30, 60,120 and 240 min. Relative densities of the ceramics are above 97%. Powder X-ray diffractions were recorded using Cu-Kα radiation in a diffractometer (Panalytical Xpert PRO, Netherland). Bragg profiles of Cu-Kα2 were stripped by Highscore Plus software of Panalytical. Electron micrographs were recorded in field emission scanning electron microscope (FESEM, Quanta 250FEG, USA). Poling field of 4 kV/mm was applied on well-polished pellets, with Ag electrode on both surfaces, at room temperature using high voltage D.C. power supply (H5KO2N, Aplab Ltd., India). Piezoelectric constants were measured using a d 33 meter (YE2730A, APC International, USA).

http://dx.doi.org/10.1016/j.scriptamat.2015.09.011 1359-6462/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article as: M. Muthuramalingam, et al., Isothermal grain growth and effect of grain size on piezoelectric constant of Na0.5Bi0.5TiO3 ceramics, Scripta Materialia (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.09.011

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M. Muthuramalingam et al. / Scripta Materialia xxx (2015) xxx–xxx

Fig. 1. (a) Relative density of Na0.5Bi0.5TiO3 ceramics sintered at different temperatures for 2 h and different durations at 1180 °C. (b) Powder XRD pattern of NBT sintered at 1180 °C for 20 min.

Fig. 2. SEM micrographs NBT ceramics sintered for (a) 10, (b) 20, (c) 30, (d) 60, (e) 120, and (f) 240 min at 1180 °C.



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estimated from the micrographs by linear intercept method. Grain growth is expressed by m

m

dt −d0 ¼ kt

Fig. 3. (a) Grain size to the power of 2.1 is plotted against isothermal sintering time. Solid line is the best fit to grain growth equation. Subplot shows the variation of correlation coefficient for different grain growth exponents. (b) Piezoelectric constant is shown for different grain sizes of NBT ceramics.

3. Results and discussion Fig. 1(a) shows relative density of NBT ceramics for three different sintering temperatures and relative density is greater for the sample sintered at 1180 °C. At 1200 °C, relative density decreases due to the increase of pore volume which is attributed to the increase of grain size. In order to study grain growth and its effect on the piezoelectric constant, sintering temperature of 1180 °C is chosen. Relative densities as a function isothermal sintering time are also plotted in Fig. 1(a). Relative density is greater for 10 min sintered pellet than that of the others and it decreases with the increase of sintering time that indicates an increase in pore volume driven by isothermal grain growth. A typical powder XRD pattern of NBT ceramics sintered at 1180 °C for 20 min is shown in Fig. 1(b). All the peaks are indexed to a rhombohedral crystal system with a space group R3c. Absence of un-indexed peaks suggests that the ceramics are phase pure. Fig. 2 shows SEM micrographs of NBT ceramics sintered at 10, 20, 30, 60, 120 and 240 min. It is seen from the micrographs that grain size increases with isothermal sintering time. Average grain sizes were

where dt and d0 are grain sizes at time t and t = 0, respectively, m is grain growth exponent and k is material constant [14,15]. Value of the exponent, m, provides mechanism of the grain growth; m = 2 indicates interface controlled grain growth and m = 3 indicates diffusion controlled grain growth. Grain growth exponent has been reported to vary from 2 to 10 [15]. To find the empirical exponent, a least square fitting was employed when m varied from 1.5 to 2.5. Correlation coefficient was used to find a best fit [16]. In Fig. 3(a), the subplot shows the correlation coefficient is maximum for m = 2.1. Fig. 3(a) shows the variation of grain size, raised to the power of 2.1, as a function of sintering time with the best fit (solid line). Therefore, isothermal grain growth of NBT ceramics occur through interface controlled mechanism governed by parabolic grain growth law. Small deviation of the exponent from 2 may probably be due to non-spherical shapes of the grains. Grain growth exponent of 3 has been reported for lanthanum modified PZT ceramics [17] and PZN– PZT ceramics [18]. Piezoelectric constant is plotted versus grain size in Fig. 3(b). It is seen from the figure that piezoelectric constant is almost the same value for the NBT sintered for 20 to 240 min but it is low for ceramics sintered for 10 min. Therefore, it can be concluded that the critical grain size of NBT ceramics is ~ 1 μm. Below this size, the piezoelectric constant decreases drastically and above this size it is almost independent. Piezoelectric constant of ferroelectric ceramics arise from intrinsic and extrinsic contributions. Intrinsic contribution originates from atomic displacements within the unit cell while extrinsic contribution comes from domain wall motions when materials are stimulated by a poling field. Poled NBT ceramics exhibit rhombohedral symmetry and spontaneous polarization direction is [111]. That means, it has 8 domain variants which results into 71o, 109o and 180o domain walls. The non180o domains align with a poling field through domain wall motion, nucleation of domains and domain switching. It has been reported that extrinsic contributions can be responsible of up to ~ 75% of dielectric and piezoelectric constants [19]. Luchaninov et al. have reported that 60– 70% of piezoelectric constant comes from extrinsic contribution in BT and PZT [20]. In the present work, the piezoelectric constant decreases from 73pC/N to 17pC/N (Fig. 3(b)) which amounts to a reduction of 75%. Comparison of intensity of Bragg peaks, {111} in the case of rhombohedral, of poled samples of different grain size helps to infer the degree of domain alignment [21,22]. Fig. 4 provides powder XRD patterns of poled samples with different grain sizes. It is clearly seen from the figure that (111) peak is stronger in the samples with grain sizes of 1.23 μm and 1.99 μm while (111) is stronger in the sample with grain size of 0.38 μm. This suggests that domain alignment through domain wall motion is copious in samples with grain sizes 1.23 μm and 1.99 μm. Whereas domain alignment with poling field through domain wall motion is not facilitated in the sample with grain size of 0.38 μm. Therefore it can be inferred that extrinsic contribution to piezoelectric constant in submicron grain sized sample is reduced. To find the impediments of domain alignment, microstructure of domains has to be quantified. It has been reported that domain size ∝ (grain size)n, where n = 1/2 for grain sizes in the range of 1 to 10 μm and large value of n for submicron grain size [23]. Further work is required to quantify the domain size as a function of grain size to reveal the reason for the impediments of domain wall movements. Parabolic grain growth mechanism and drastic reduction of piezoelectric constant of NBT ceramics in submicron grain size range are desirable to the sintering of the lead-free piezoelectrics.


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reduction in the domain alignment with the poling field which was inferred from reduced intensity of (111) peak in comparison to that of (1 11) peak of poled pellet. NBT is a major constituent of potential lead-free piezoelectric MPB systems and sintering of the ceramics for 2 h is sufficient to obtain the maximum piezoelectric constant though there is an increase in grain size if sintering duration is prolonged. Acknowledgments Authors acknowledge the Department of Science and Technology (Dy. No.100/IFD/3149/2009-2010), New Delhi for funding through DST-FIST program to create a powder XRD facility. References

Fig. 4. (111) and (111) reflections of poled NBT pellets of different grain sizes.

4. Conclusion NBT ceramics with grain sizes in the range from 0.38 μm to 3.65 μm were formed when the sintering duration varied from 10 to 240 min at an optimized sintering temperature of 1180 °C. Isothermal grain growth follows the parabolic grain growth law with an exponent of 2.1 indicating an interface controlled grain growth mechanism. Piezoelectric constant drastically decreased in a submicron grain size sample due to the

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