Dielectric, Ferroelectric and Piezoelectric study of BNT-BT solid solutions around the MPB region

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1 National Conference on Processing and Characterization of Materials International Conference on Recent Trends in Physics 2016 (ICRTP2016) Journal of Physics: Conference Series 755 (2016) doi: / /755/1/ Dielectric, Ferroelectric and Piezoelectric study of BNT-BT solid solutions around the MPB region T. Badapanda 1*, S. Sahoo 2, P. Nayak 3 1 Department of Physics, C.V. Raman College of Engineering, Bhubaneswar, Odisha, India Department of Physics, Centurion University of Technology & Management, Bhubaneswar, Odisha, India Department of Physics and Astronomy, National Institute of Technology, Rourkela, Odisha , India Corresponding author badapanda.tanmaya@gmail.com Abstract: The (1-x)(Bi 0.5 Na 0.5 )TiO 3 - xbatio 3 (x= ) has been prepared by solid state reaction route. The structural characterizations of the solid solutions were done by X-ray diffraction and Raman spectroscopy, which shows that the morphotropic phase boundary exist between The dielectric, ferroelectric and piezoelectric properties were studied around the MPB region. It is observed from the temperature and frequency dependent dielectric study that the dielectric constant increase with x values up to and then after decreases with x values. The variation of polarization with electric field for all compositions have been done and it is observed that polarization value is high at. The electric displacement of the compositions having MPB with various electric field has been carried out and highest piezoelectric coefficient has been observed at. All the characterizations concluded that the composition will be very suitable for actuator application. 1. Introduction: Keywords: BNT-BT solid solution, MPB, X-Ray diffraction, Dielectric, ferroelectric, Piezoelectric In development of electronic industry, piezoelectric materials play an important role in the applications of actuators, sensors, resonators and ultrasonic transducers [1]. Apart from the piezoelectric properties, ideal ceramic capacitor is associated with some basic requirements such as high dielectric constant, low dielectric loss, and high hysteresis free strain near room temperature over the wide frequency conditions and the performance. These requirements Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1

2 National Conference on Processing and Characterization of Materials were fulfilled by the lead-based piezoelectric materials with superior piezoelectric, ferroelectric, dielectric properties and are most widely used in practical applications [2]. High piezoelectric and ferroelectric properties are found in compositions close to the morphotropicphase boundary (MPB) [2]. As lead is considered as toxic element and cause a serious risk to humanhealth and environmental pollution, there isahunt to replace lead based compositions with lead-free and environment friendly ceramic materials. Recently, extensive research has been done on the development of the lead-free ceramics, which possess high dielectric constant, large field-induced strain, polarization, piezoelectric properties. Among the lead-free piezoelectric materials, sodium bismuth titanate(bi 0.5 Na 0.5 TiO 3, abbreviated as NBT) and Barium titanate (BaTiO 3, abbreviated as BT) are accepted as an important material for the fabrication of lead-free piezoelectric materials. The BNT shows strong ferroelectric property along with high Curie temperature, high remnant polarization and a coercive field at room temperature [3 6]. However, the large coercive field and large conductivity makes pure NBT as hard to pole, which hinders its industrial application. In order to obtain useful lead-free piezoelectric ceramics a lot of work has been done on the NBT-based solid solutions to modify and improve the electrical properties. Recent studies have been focused on improving piezoelectric properties of NBT-based ceramics to use the solid-solution technique to substitute the individuals and seek for the merits of the selected materials. Among these NBT based solid solutions, the (1-x) (Bi 0.5 Na 0.5 )TiO 3 -xbatio 3 is the most attractive one due to their excellent piezoelectric properties at morphotropic phase boundary (MPB) composition as studied by several researchers [7-14]. Morphotropic phaseboundary (MPB) is the boundary between rhombohedralphase of NBT and tetragonal phase of BT. BaTiO 3 is chosen because of its superior dielectric, ferroelectric, piezoelectric, [15 18] that make it an extremely important system for fundamental electro-ceramics research. It was reported that NBT BT 94/6 ceramics shows high piezoelectric constant d 33 =125 pc/n and an electromechanical coupling coefficient of k 33 =0.55 near the rhombohedral/tetragonal MPB region. These properties are suitable for high frequency ultrasonic uses or piezoelectric actuator applications. As for the NBT BT crystals, the highest value of d 33 is 160 pc/n obtained in the NBT BT 94/6 single crystals grown by Bridgman method [18]. The piezoelectric properties of 001 poled NBT BT 95/5 show good piezoelectric properties with d 33 =280 pc/n. However, the d 33 values of the 111 oriented NBT BT 95/5 crystals are 90 pc/n [19]. Ching et al. have reported that NBT BT 94.5/5.5 single crystal on the 001 exhibit d 33 value up to 450 pc/ N, which is close to some PZT ceramics [20].The enhanced piezoelectric and ferroelectric properties are observed around the MPB region in BNT-BT composition. However, there is still a ambiguity in the exact MPB composition and lack of fundamental understanding of the structure-property relations and mechanisms for high piezoelectric properties near the MPB composition..in this paper, we report our work on the preparation and characterization of (1-x) BNT-xBT ceramics with special emphasis on searching the morphotrophic phase boundary (MPB) of this system via structural, dielectric and piezoelectric properties. 2. Experimental Solid-state synthesis method was adopted to powder samples (1-x) BNT-xBT with 0 x 0.1. The raw materials used were Barium Carbonate - BaCO 3 (reagent grade Meark, India), Titanium Oxide -TiO 2 (reagent grade, Merck), Bismuth Oxide - Bi 2 O 3 (reagent grade Meark, India) and Sodium Carbonate - Na 2 CO 3 (reagent grade, Merck). 2

3 Intensity (a.u) Intensity (a.u) National Conference on Processing and Characterization of Materials All the powders are weighed according to the stochiometry ratio and powders were ball milled with Zirconium media in ethyl alcohol for 12 hrs. The mixed dry powders were calcined respectively at C for 3hrs in a programmable furnace. The phase purity of the samples was investigated using X-ray diffraction (XRD), (Xport MPD, Philips, UK). The granules of all the compositions were made by adding 5% polyvinyl alcohol as a binder and pressed to obtain discs. The discs were sintered at 1150 o C for 3 hrs. Silver paste was applied on both faces followed by heat treatment at C (20mins) of the samples for electrical measurements. The relative permittivity against the temperature was measured at various frequencies using an LCR meter. The polarization versus electric field (P E) loops of the specimens, were studied using modified (Radiant Technologies) radiant P E loop tracer circuit by applying AC field. 3. Result and Discussion: 3.1 X-ray diffraction Study Fig 1 (a) shows the XRD patterns of (1-x) (Bi 0.5 Na 0.5 )TiO 3 - x BaTiO 3 ceramics with 0 x 0.1 sintered at 1150 o C for 3 hrs. All the compositions exhibit a pure perovskite structure and no second phases are observed, which implies that BaTiO 3 (BT) ceramic has diffused into the Bi 0.5 Na 0.5 TiO 3 lattices to form a solid solution. All the peaks in the XRD pattern of (1-x) BNT-xBT are correspond to the BNT phase with rhombohedral structure as reported by different researchers [21]. With the increasing of BT content the peak positions shift towards lower angle and the peaks are also get broaden, it implies that crystallite size was decreasing with addition of barium titanate. Fig. 1(b) shows the XRD patterns of the ceramics in the 2θ range of The rhombohedral symmetry of BNT ceramic at room temperature can be characterized by a (0 0 3)/(0 2 1) peak splitting between 39 and 41 0 and a single peak of (2 0 2) between 42 and It shows that the (2 0 2) peak became broad and asymmetric up to x = 0.04 and a distinct (0 0 2)/(2 0 0) peak splitting appears after x =0.05, referring to a tetragonal symmetry.this characterizes a coexistence of rhombohedral (R) and tetragonal (T) phases, which is consistent with a MPB composition [22]. The lower angle peak, splits into (100),(001) and it may be due to the deformation and distortion created by BT. (a) x=0.0 x=0.01 x=0.02 x=0.03 x=0.04 x=0.10 x=0.04 (b) Fig. 1(a) X-Ray diffraction pattern of (1-x) (Bi 0.5 Na 0.5 )TiO 3 -xbatio 3 ceramics, (b) reduced X-Ray diffraction pattern of (1-x) (Bi 0.5 Na 0.5 )TiO 3 -xbatio 3 ceramics 3

4 Normalized Intensity (a.u) Normailzed Intensity (a.u) National Conference on Processing and Characterization of Materials 3.2 Raman Study: The Raman spectroscopy is an effective technique to investigate the presence of functional groups in solid solutions, which provides better insight into the structural study. Fig 2(a) represents the Raman spectroscopy study of (1-x) (Bi 0.5 Na 0.5 )Ti0 3 -xbatio 3 (x= ) to verify the existence of MPB region. The Raman peaks are relatively broad, which can be caused by the distorted octahedral [BiO 6 ] and [NaO 6 ] clusters or disorder in A-site of rhombohedral structureand the overlapping of Raman modes due to the lattice anharmonicity. The observed peaks are categorized into three wave number ranges. There are (1) low wavenumber range of cm -1 which is believed to be related to Na O vibrations; the peak splitting in this range indicates that there are two kinds of ABO 6 interactions in the NBT system. (2) A mid wave number range of cm -1 which is believed to be related to Ti O vibrations; and (3) a high wave number range of cm -1 which is believed to be related to oxygen octahedral vibrations and rotations [23].The first Raman-active A 1 (TO 1 ) mode at around (146 cm -1 ) is related to network modifiers or distorted octahedral [BiO 6 ] and [NaO 6 ] clusters. The Raman-active E(TO 2 ) mode in the regions of 279 cm -1 is assigned at stretching arising from the bonds due tothe presence of octahedral [TiO 6 ] clusters at shortrange. The (TO 3 ) modes situated at around 542 cm -1 is described to the ( O Ti O ) stretching symmetric vibrations of the octahedral [TiO 6 ] clusters. Finally, the (LO 3 ) mode found at 812 cm -1 is due to presence of the sites within the rhombohedral lattice pre containing octahedral distorted [TiO 6 ] clusters. Fig. 2(b) shows the Raman spectroscopy plot for x= of (1-x) (Bi 0.5 Na 0.5 )Ti0 3 -xbatio 3 solid solution. The mode around the region 542 cm -1 splitfor x 0.05, shows a MPB region which also confirms the X-ray result. To find the most prominent composition among the MPB region which will be suitable for industrial application, we study the dielectric, ferroelectric and piezoelectric study. (a) x=0.0 x=0.01 x=0.02 x=0.03 x=0.04 x=0.1 (b) x= wave number (cm-1) wave number (cm-1) Fig. 2. Raman Spectroscopy study of (1-x) (Ba 0.5 Na 0.5 )TiO 3 -xbatio 3 ceramic (a)x= (b) x=0.0,0.05,0.06,0.07,0.08, Dielectric behaviour: The temperature dependence of dielectric constant ε' of (1-x)BNT- (x)bt system at various frequencies near the MPB region are shown in Fig.3. Two dielectric peaks have been observed in each composition as shown in Fig. 3. The observed two dielectric peaks can be attributed to the factors caused by the phase transitions from ferroelectric to anti-ferroelectric, 4

5 dielectric constant dielectric constant dielctric constant dielctric constant National Conference on Processing and Characterization of Materials which is called depolarization temperature (T d ) and from anti-ferroelectric to paraelectric phase, at which the maximum value of dielectric constant corresponding temperature is Curie temperature (T m ). The value of T d and T m are found to decrease with increasing the concentration of BTupto and then increases with further addition. The T d plays an important role in the practical applications of the materials. The ferroelectric domains with lower T d are less stable, hence T d can be seen as the indication of the stability in ferroelectric domains. The MPB compositions exhibit a lower depolarization temperature, which implies a reduction of the stability of ferroelectric domains. The coexistence of a mixed rhombohedral tetragonal phase could lead to more powerful stress in the MPB compositions, due to the incompatibility of their crystal lattices, resulting in a decrease of thermal stability in the longrange ferroelectric domains[24,25].this can be regarded as the origin of the lower depolarization temperature at the MPB compositions.it was also observed that T m decreases with increasing BT content and reached a minimum value at 8 mol% BT and with further additions of BT increase in T m value was observed. The enlargement of the unit cell and achange in the relative fractions of rhombohedral and tetragonal phases was thought to play an important role as the larger ionic radii of Ba 2+ increase in lattice constant and consequently reduced the T m value.it is again observed that the dielectric constant increases upto and then decreases with further addition Hz 5000 Hz 0 Hz Temperature ( 0 C) Temperature ( 0 C) Temprature ( 0 C) Temperature ( 0 C) 5

6 Polarization ( C/cm 2 ) Polariation ( C/cm 2 ) dielectric constant dielectric constant National Conference on Processing and Characterization of Materials Temprature ( 0 C) khz 1 MHz 500 Temperature ( 0 C) x=0.10 Fig 3. Temperature dependent dielectric study of (1-x) (Ba 0.5 Na 0.5 )TiO 3 -xbatio 3 ceramic 3.4 Ferroelectric and piezoelectric study: At Room-temperature both unipolar and bipolar polarization electric field (P E) hysteresis data and strain electric field (S E) data for (1-x)BNT xbt ceramics measured an electric field of 20 kv/cm at frequency 1Hz are shown in Figs. 4(a,b) and 5(a,b), respectively. Furthermore, the remnant polarization (P r ), the maximum polarization (P max ), the coercive field (Ec), the maximum strain (S max ), and the normalized strain coefficient (d 33 = S max /E max ) values are also listed in Table 1. It could be noted that the observed large strain seems to exist only over a narrow compositional region. However, the addition of a stable ferroelectric endmember BT in the solidsolution should result an increase in domain stability. Itcan be clearly seen from the table that the P r ande c increased considerably as the amount of BT increased upto 8 mol% and decreases with further addition of BT. The bipolar strain electric field (S E) loops also showed a normal butterflyshape with increased negative strain. The S max and d 33 values increases up to and then decreases. A slight asymmetry isobserved in the (S-E) loops at, which may be due to the internal fieldinduced by the presence of defects [26]. It can be seen that low-field piezoelectricproperties exhibited a strong compositional and phase dependence within the MPB region of BNT BT.Development of typical butterflyshaped loops and the existence of remnant strain in the NBT-xBTceramics near MPB indicates their piezoelectric nature [27] (a) Electric Field (kv/cm) x= x= Electric Field (kv/cm) Fig. 4. P-E hysteresis loops of BNT-xBT (, 0.07, 0.08,0.09,0.1) ceramics (a) Unipolar (b) Bipolar (b) 6

7 National Conference on Processing and Characterization of Materials Fig. 5 Variation of induced strain% vs. electric field in the BNT-xBT ceramics with BNTxBT (, 0.07, 0.08,0.09, 0.1) ceramics (a) Unipolar (b) Bipolar Parameters obtained from Dielectric, ferroelectric and piezoelectric study of (1-x) (Bi 0.5 Na 0.5 )TiO 3 -xbatio 3 ceramic Composition Dielectric constant T m P r (µc/cm 2 ) S max (%) d 33 (pm/v) x= Conclusion: (1-x)BNT-xBTsolid solutions were successfully prepared by the solid state reactiontechnique. The X-ray diffraction and Raman spectroscopy study shows that there exist a MPB within the region The dielectric data shows that the dielectric constant increases up to and then decreases with further addition of BT. The ferroelectric and piezoelectric study was performed for both unipolar and bipolar field and enhanced behaviour was observed at. All the above study concludes that the highest MPB observed at. High dielectric, large strain and highest remanent polarization was observed for x-0.08 which makes the composition suitable for memory, transducer and actuator application. 7

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