Structural and Electrical Properties of Bi 0.5 Na 0.5 TiO 3 Templates Produced by Topochemical Microcrystal Conversion Method

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1 New Physics: Sae Mulli, Vol. 65, No. 8, August 2015, pp DOI: /NPSM Structural and Electrical Properties of Bi 0.5 Na 0.5 TiO 3 Templates Produced by Topochemical Microcrystal Conversion Method Ali Hussin Adnan Maqbool Rizwan Ahmed Malik Min Su Kim Tae-Kwon Song Myong-Ho Kim School of Advanced Materials Engineering, Changwon National University, Changwon , Korea Arif Zaman Department of Physics, Abdul Wali Khan University Mardan, Khyber Pakhtunkhwa, Pakistan Won-Jeong Kim Department of Physics, Changwon National University, Changwon , Korea (Received 24 April 2015 : revised 5 June 2015 : accepted 5 June 2015) Bi 0.5Na 0.5TiO 3 (BNT) templates with a single phase perovskite structure were produced from a plate-like precursor particles of bismuth-layer-structured ferroelectric Bi 4.5Na 0.5Ti 4O 15 (BNT4) through a topochemical microcrystal conversion process. First, the plate-like BNT4 precursor particles were prepared via molten salt synthesis. The layered structure BNT4 transformed into a single phase perovskite BNT templates after its topochemical reaction with the complementary Na 2CO 3 and TiO 2 reactants at 950 C for 4 h in a NaCl flux. The as synthesized BNT templates exhibited large grains (range from 10 to 15 µm), had plate-like morphology and exhibits a single-phase perovskite structure with a pseudo-cubic symmetry. Furthermore, the temperature dependences of dielectric constant and loss at different frequencies showed a relaxor behavior, and polarization versus electric field curves exhibited a typical ferroelectric response. PACS numbers: cp, Ej Keywords: BNT template, Molten salt synthesis, Topochemical microcrystal conversion, Lead-free ceramics I. INTRODUCTION Environmental problems of lead-based piezoelectric materials stimulate the development of high performance lead-free counter parts [1 3]. Recently, much research has been carried out on Bi-containing perovskite materials because of its similar electronic structure with Pb 2+ ions which has a 6s lone pair configuration, that is considered highly beneficial for superior piezoelectric response [4]. Among, Bi-based perovskite ceramics, Bi 0.5 Na 0.5 TiO 3 (BNT) is considered a promising leadfree candidate material because of its large polarization and high Currie temperature. In the last decades, BNT mhkim@changwon.ac.kr and its solid solutions are intensively studied for the improvement of their electromechanical properties [5 10]. Templated grain growth (TGG) technique has been widely used to produce textured piezoelectric ceramics with enhanced electromechanical properties [11 13]. Anisotropic template particles play a crucial role in TGG processes. For Bi 0.5 Na 0.5 TiO 3 (BNT)-based textured ceramics, bismuth layer-structured ferroelectric (BLSF) Bi 4 Ti 3 O 12 (BIT) templates or plate-like SrTiO 3 (ST) templates were frequently utilized for their grainorientation [14]. Nevertheless, BNT-based ceramics textured by ST templates exhibit a high degree of grain orientation, however, the paraelectric phase of ST ceramics is believed to decrease depolarization temperature and have adverse effects on the electromechanical This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

2 716 New Physics: Sae Mulli, Vol. 65, No. 8, August 2015 properties [11]. Beside this, a large amount of plate-like BIT templates is usually required to produce highly textured ceramics [15], which makes the ceramics difficult to densify. If similar structure, plate-like BNT templates are utilized for fabrication of BNT-based textured ceramics then it is expected that it will improve the degree of grain orientation and will not change the depolarization temperature. BNT has a simple perovskite structure which possesses a high crystal symmetry. It is very hard to produce plate-like large anisotropic BNT templates by conventional procedures, such as mixed oxide route [16], sol-gel [17] or hydrothermal methods [18]. The layered structure ferroelectric materials such as Bi 4.5 Na 0.5 Ti 4 O 15 (BNT4), where the A-site is co-occupied by Na + and Bi 3+ has been experimentally verified to form plate-like templates through molten-salt processes. The resemblance of the crystal structures of the layered structure BNT4 and simple BNT facilitates the transformation from the layerstructured BNT4 into a simple perovskite BNT by a topochemical reaction. Here we report, the preparation of plate-like BNT templates from layered structure BNT4 via a topochemical method and investigation of their structural, dielectric and ferroelectric properties. II. EXPERIMENTAL PROCEDURE The starting materials, reagent-grade Bi 2 O 3, TiO 2 and Na 2 CO 3 powders of purity more than (99.9%) were mixed according to the stoichiometric formula of BNT4. Sodium chloride (NaCl) salt (99.95%) was mixed with them in a salt to oxide weight ratio 1.5:1, followed by ball-milling in ethyl alcohol for 24 h. After removal of balls and drying, the dried powder was put in a tightly covered Al 2 O 3 crucible and heat treated at 1100 C for 4 h. After the completion of reaction, NaCl was removed from the as-synthesized product by washing thoroughly with hot de-ionized water. BNT4 platelets, Na 2 CO 3 and TiO 2 were then weighed to give a total composition of BNT. NaCl salt was again mixed to them with same weight ratios followed by mixing for 5 h in ethanol solution with magnetic stirrer. The slurry was dried and heat treated at 950 C for 4 h in a tightly covered alumina crucible. Subsequently, with hot de-ionized water, NaCl was washed from the product and the by-product of bismuth oxide (Bi 2 O 3 ) was eliminated by reacting with HCl solution [19]. Finally, the as synthesized BNT templates were pressed into pallets and sintered at 1150 C for 2 h. Crystalline phase and purity of the as synthesized BNT templates were examined at room temperature by X-ray diffraction machine (XRD, RAD III, Rigaku, Japan) using CuKα radiation (λ = Å). The particle morphology and size were examined with a field emission scanning electron microscope (FE-SEM, JP/JSM 5200, Japan). The bulk density of the sintered BNT was measured through Archimedes method and was found to have 91% of its theoretical value. For electrical measurements, the upper and the lower surfaces of the specimen were polished to become parallel and coated with a silver-palladium paste as electrode by screen printing. The dielectric constant and loss responses were measured through an impedance analyzer (HP4194A, Agilent Technologies, Palo Alto, CA). Polarization versus electric field (P E) hysteresis loops were measured in silicone oil with the aid of ferroelectric test system (Precision LC; Radian Technologies, Albuquerque, NM) at 20 Hz. III. RESULTS AND DISCUSSION Fig. 1 shows the XRD pattern and FE-SEM micrograph of BNT4 templates prepared by molten salt synthesis (MSS). The XRD profile indicates a single phase without any evidence of unwanted phases with all diffraction peaks attributable to a layered perovskite structure. All the diffraction matches well with the JCPDS card no Majority of the peaks such as (006), (008), (0010), (0016), and (0020) shows higher intensities than other peaks (101), (107), (109), (110) etc., suggesting that the surface of BNT4 templates are parallel to (00l) plane and a high degree of grain orientation for BNT4 templates [19]. The FE-SEM micrograph of BNT4 shows that all templates have plate-like morphology with an average grain size (10-15 µm). Some submicron size grains were also observed, which could be the broken pieces of BNT4 crystals. BNT4 belongs to BLSF family which is highly anisotropic with growth along a(b) axis much higher than c-axis. Therefore, it reasonable for them to

3 Structural and Electrical Properties of Bi 0.5Na 0.5TiO 3 Templates Produced by Ali Hussin et al. 717 Fig. 1. (a) XRD patterns and (b) FE-SEM micrograph of Na 0.5 Bi 4.5 Ti 4 O 15 precursor particles produced by molten salt synthesis at 1100 C for 4 h. Fig. 2. (a) XRD patterns and (b) FE-SEM micrograph of Bi 0.5 Na 0.5 TiO 3 templates produced from Bi 4.5 Na 0.5 Ti 4 O 15 precursor. form plate-like morphology during appropriate processing. These as synthesized BNT4 templates were used as precursor materials for topochemical microcrystal conversion (TMC) process in this work. The XRD pattern along with the FE-SEM micrograph of BNT templates produced from the BNT4 using TMC method was recorded and is shown in Fig. 2. BNT templates produced by TMC method shows a singlephase perovskite structure, which matches well with the JCPDS card No of the Na 0.5 Bi 0.5 TiO 3 ceramics. Because of a small rhombohedral distortion, all the intensity peaks were indexed on the basis of pseudocubic perovskite unit cell. The XRD profile provides clear information that after the TMC process, the layerstructured BNT4 templates have been completely transformed into a simple perovskite BNT templates, which reserved the parent plate-like morphology. Most of the peaks in plate-like BNT templates laid down with the c- axis aligning along the vertical direction during the sample synthesis. So, they exhibit strong (100) and (200) diffraction peaks [19]. Fig. 2(b) shows the FE-SEM micrographs of the BNT templates synthesized from the BNT4 templates through TMC method. Analogous to the BNT4 templates, most of the BNT templates have plate-like morphology and large grain size. Such types of large and plate-like particles are quite suitable for producing textured ceramics by the tape-casting process. BNT4 belongs to the family BLSFs, which contain (Bi 2.5 Na 0.5 Ti 4 O 13 ) 2 (pseudo-) perovskite layers enclosed in (Bi 2 O 2 ) 2+ fluorite layers, where the A site is co-occupied by Bi 3+ and Na + in a Na/Bi ratio of 0.2. This conversion from the layered structure to the simple perovskite is comprised of two processes: first, the diffusion of Na + and Bi 3+ in the perovskite layers, while second is the change of the (Bi 2 O 2 ) 2+ fluorite layers to the perovskite structure. It has been also reported that this transformation is from a lamellar phase to a perovskite phase [20], the process involving the (Bi 2 O 2 ) 2+ layers changing to the perovskite structure still need further verifications. Fig. 3(a), and Fig. 3(b) shows XRD profile and FE- SEM micrograph of the sintered BNT templates produced through TMC process. Within the detection

4 718 New Physics: Sae Mulli, Vol. 65, No. 8, August 2015 Fig. 4. (Color online) Temperature dependent dielectric response of Bi 0.5 Na 0.5 TiO 3 ceramics produced by TMC method. Fig. 3. (a) XRD patterns and (b) FE-SEM micrograph of Bi 0.5 Na 0.5 TiO 3 ceramics sintered at 1150 C for 2 h in air atmosphere. limit of XRD, the pattern shows a single phase perovskite structure without any traces of unwanted parasite phases. This suggest that BNT produced by TMC form a complete solid solution after sintering. Close inspection of the XRD pattern disclosed no splitting of the characteristic peaks at 2θ angle 40 and 46 and all peaks matches well with the standard data of pseudocubic perovskite unit cell. This study also shows that TMC process does not bring an obvious change in the crystal structure of BNT templates. The FE-SEM picture of BNT indicates a dense and compact microstructure. Small and big grains are homogeneously distributed with some pores. Sintering has a significant effect on the grain growth and grain morphology of BNT templates. Overall, the grain morphology of the sample is changed from plate like to spherical shape having a polycrystalline nature. Before sintering, polyvinyl alcohol (PVA) binder was mixed with the plate-like BNT templates, crushed and then pressed into pellet at a pressure of 50 MPa. The crushing and pressing are expected to break the platelike morphology, where the subsequent heat treatment results in equaxial grain morphology. The temperature dependence of the dielectric constant and loss of sintered BNT templates with different frequencies (1-100 khz) are shown in Fig. 4. Two dielectric anomalies, commonly known as the depolarization temperature (T d ) and permittivity maximum temperature (T m ) are appeared at temperature 195 C and 340 C, respectively, These two anomalies are consistent with the dielectric behavior of other BNT and BNT-based ceramics [16, 21]. Both T d and T m anomalies are broadened with increasing frequency. Furthermore, the value of dielectric constant at T m decrease with increasing frequency, however, no significant change in the T m temperature can be observed. In addition, the T d temperature increases with increasing frequency suggesting relaxor like behavior of the sample. The sample exhibits low dielectric loss at room temperature, however, this loss increase at high temperatures 400 C and above. The high value of the dielectric loss at higher temperature may be due to transport of ions with higher thermal energy. The rapid increase in the dielectric loss curves beyond the 400 C may be due to the scattering of the thermally activated charged carriers and some defects in the samples. At elevated temperatures the conductivity begins to dominate which in turn is responsible for the rise in dielectric loss (since σ tan δ) [22]. It is well known that the hysteresis loops is a useful probe to investigate the ferroelectric behavior of the sample. The measurement of P E was conducted to investigate the ferroelectric properties of the sintered BNT templates. Fig. 5 shows the room temperature P E

5 Structural and Electrical Properties of Bi 0.5Na 0.5TiO 3 Templates Produced by Ali Hussin et al. 719 and frequency dispersion response of the sample. P E hysteresis loops show well saturated curve at an applied electric field of 90 kv/cm with a large remanent polarization 32 µc/cm 2 and a coercive field of 56 kv/cm. Our results reveal that plate-like BNT templates prepared by TMC method are suitable for the development of high performance BNT-based textured ceramics. ACKNOWLEDGEMENTS Fig. 5. (Color online) P E hysteresis loops of Bi 0.5 Na 0.5 TiO 3 ceramics produced by TMC method measured at different fields. hysteresis loops of the BNT ceramics at a measuring frequency of 50 Hz under different fields (70-90 kv/cm). It is interesting to note that the loops show a typical ferroelectric behavior at all fields, which is characterized by definite squareness in the P E hysteresis loop with a certain remanent polarization (P r ) and a coercive field (E c ). However, at 70 kv/cm the loop is not well saturated. When the field is increased from 70 to 90 kv/cm, polarization as well as coercive field increase and the loops become well saturated. At 90 kv/cm, high remanent polarization 32 µc/cm 2 and a coercive field (E c ) of 56 kv/cm can be observed, which show successful synthesis of BNT templates by TMC method. The well-defined dielectric and ferroelectric response of BNT along with its plate-like morphology and uniform distribution are expected to act as a substrate for epitaxy and as a seed in the matrix powder of BNT-based ceramics to develop high performance textured ceramics by reactive template grain growth method. IV. CONCLUSION Large platelet single crystal type BNT templates were prepared from BNT4 precursor through a topochemical microcrystal conversion process. XRD analyses of the as synthesized BNT templates and sintered BNT ceramics show the formation of single phase pseudocubic perovskite structure. The surface morphology of BNT templates preserves the platelet morphology of the parent BNT4 precursor. The dielectric curves show diffuse This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government, Ministry of Education (MOE) (2013R1A1A ) and Basic Research program through the National Research Foundation of Korea (NRF) funded by Ministry, Science and Technology (MEST) ( ). REFERENCES [1] Y. Saito, H. Takao, T. Tani, T. Nonoyama and K. Takatori et al., Nature 432, 84 (2004). [2] J. Rödel, W. Jo, K. T. P. Seifert, E. M. Anton and T. Granzow et al., J. Am. Ceram. Soc. 92, 1153 (2009). [3] S. Zhang, R. Xia and T. R. Shrout, J. Electroceram. 19, 251 (2007). [4] M. R. Suchomel, A. M. Fogg, M. Allix, H. J. Niu and J. B. Claridge et al., Chem. Mater. 18, 4987 (2006). [5] G. A. Smolenskii, V. A. Isupov, A. I. Agranovskaya and N. N. Krainik, Sov. Phys. Solid State 2, 2651 (1961). [6] J. Suchanicz, Ferroelectrics 209, 561 (1998). [7] G. O. Jones and P. A. Thomas, Acta Crystallogr. Sect. B: Struct. Sci. 56, 426 (2000). [8] A. Maqbool, J. U. Rahman, A. Hussain, J. K. Park and T. G. Park et al., Trans. Nonferrous Met. Soc. China 24, s146 (2014). [9] R. A. Malik, J. K. Kang, A. Hussain, C. W. Ahn and H. S. Han et al., Appl. Phys. Express 7, (2014).

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