Grain-Oriented Bi 0.5 Na 0.5 TiO 3 –BaZrO 3 Piezoelectric Ceramics

Piezoelectric ceramics have applications in various electronic devices such as sensors, actuators, energy harvesters, and so on. Most of these devices are manufactured from lead-containing materials because of their excellent electromechanical performance and low cost. However, lead-containing materials are considered as serious threat to the environment and facing restrictions from legislative agencies across the globe. Since last decade, much research efforts have been devoted to produce high-performance lead-free ceramics for industrial applications. Among lead-free candidate materials, bismuth- based perovskite ceramics such as sodium bismuth titanate (Bi 0.5 Na 0.5 TiO 3 , BNT) are considered potential substitute for lead-based materials because of the Bi 3+ and Pb +2 same 6s 2 lone pair electronic configuration. This chapter describes the synthesis of BNT particles by different techniques; conventional mixed oxide (CMO) route along with topochemi‐ cal microcrystal conversion (TMC) methods followed by fabrication of BaZrO 3 -modi‐ fied BNT ceramics with a chemical composition of 0.994Bi 0.5 Na 0.5 TiO 3 –0.006BaZrO 3 (BNT– BZ) by a conventional solid-state reaction method, its texture development by reactive templated grain growth method using BNT templates, and comparison of the structur‐ al and electromechanical properties of the textured and non-textured counterparts.


Introduction
Piezoelectricity is a coupling between a material's mechanical and electrical response. In the simple term, when pressure is applied to a piezoelectric material, an electric charge collects on its surface. On the other hand, when field is applied to a piezoelectric material, it mechanically deforms [1]. This behavior of piezoelectric materials is widely utilized in optics, astronomy, fluid control, and precision machining due to their high generative force, accurate displacement, and rapid response [2,3].
To date, most of the commonly used piezoelectric materials are lead-based, such as lead zirconate titanate (PZT) and its solid solutions. Lead-containing materials display high piezoelectric coefficients, especially near the morphotrophic phase boundary (MPB) and have dominated the market of piezoelectric industry [1,4,5]. However, the hazardous lead content present within these materials raise serious environmental problems. Therefore, environmental issues such as regulations and policies against lead-based materials have been increasingly enacted throughout the world [6][7][8]. In order to circumvent the drawback of lead toxicity, extensive research is focused on the quest for alternate piezoelectric materials. Numerous research efforts have been devoted to the candidate lead-free piezoelectric materials such as BaTiO 3 (BT), K 0.5 Na 0.5 NbO 3 (KNN), and Bi 0.5 Na 0.5 TiO 3 (BNT) because of their interesting electromechanical properties. The BT-based ceramics are interesting from the view point of their good ferroelectricity, chemical, and mechanical stability along with easy processing in polycrystalline form [9,10]; however, they are inadequate for device applications due to their low Curie temperature (T c ≈ 120°C) [11,12]. Alternatively, The KNbO 3 -NaNbO 3 solid solution or the (K,Na)NbO 3 (KNN)-based material exhibits good piezoelectric properties as well as high T c ≈ 420°C and have been studied as substitutes for PZT-based ceramics [13][14][15][16]. Nevertheless, alkali metal elements present in KNN-based ceramics easily evaporate at high temperatures. Moreover, KNN ceramics are not only hygroscopic but also make transition between orthorhombic and tetragonal structure (T O-T ) around 200°C, and this transition temperature shifts toward room temperature (RT) with change in composition through doping or substituting other element, which hinders KNN-based ceramics from practical applications [17,18]. As one of the most feasible lead-free candidates, BNT-based solid solutions have good piezoelectric properties, excellent reproducibility, and high maximum dielectric constant temperature (T m 300°C) [19][20][21][22]. However, poling of pure BNT ceramics is difficult due to their large coercive field (E c ~ 73 kV/cm), which is overcome by certain amount through compositional modification and texture development [22][23][24][25].
In the recent years, extraordinarily large strain has been reported in compositionally designed BNT-based ceramics [20,21,26,27], which seems to be alternative for PZT in specific actuator applications. Beside, compositional design, texture engineering of polycrystalline ceramics is inimitable and vibrant approach to enhance the piezoelectric properties of ceramics without any major change in the base compositions. Several texture development techniques, such as templated grain growth (TGG) [28], reactive TGG (RTGG) [18], oriented consolidation of anisometric particles [29], screen printing [30], multilayer grain growth [31], and directional solidification technology [32], have been employed to improve the electromechanical properties of piezoelectric ceramics. Among all these techniques, RTGG is more suitable for the texture development of perovskite-type materials [33,34]. In this process, the plate-like template particles that have specific microstructure and crystallographic characteristics are oriented in a matrix ceramic powder through a tape-casting process, and then the consequent heat treatment results in the nucleation and growth of desired crystals on aligned template particles to bring into being textured ceramics [35]. Because of the grain orientation effect induced by templates particles through RTGG process, textured ceramic samples can be more easily poled and thus deliver much higher dielectric and piezoelectric response in comparison with non-textured counterparts prepared by a conventional mixed oxide (CMO) routes [18,33,34].
For texture development of BNT-based ceramics, simple BNT templates are considered the most promising because of their large size and plate-like nature. Nevertheless, BNT templates prepared by CMO route have equiaxial morphology which cannot satisfy the requirement as seed in the RTGG process. An alternative convenient approach is to synthesize plate-like perovskite templates by a topochemical microcrystal conversion (TMC) process. This process involves substitution or modification of the interlayer cations, however, retaining the structural and morphological features of plate-like layered-perovskite precursors by ion exchange and intercalation reactions at low temperatures [36,37]. Considering the importance of BNTbased ceramics, this chapter describes the synthesis of BNT particles by different techniques; CMO route along with TMC methods followed by fabrication of BaZrO 3 -modified BNT ceramics with a chemical composition of 0.994Bi 0.5 Na 0.5 TiO 3 -0.006BaZrO 3 (BNT-BZ) by a conventional solid-state reaction method, its texture development by RTGG method using BNT templates, and comparison of the structural and electromechanical properties of the textured and non-textured BNT-BZ ceramics.

Experimental
Reagent-grade metal oxide powders of Bi 2 O 3 , TiO 2 , and Na 2 CO 3 (purity >99.9%) were used as starting materials to produce plate-like Bi 4.5 Na 0.5 Ti 4 O 15 (BNT4) precursors by molten salt synthesis (MSS) [38]. The stoichiometric amount of the raw BNT4 powder mixture was first mixed with NaCl (99.95%) in a weight ratio of 1:1.5 and then ball milled in polyethylene jar for 24 h. Consequently, the balls were removed and the slurry was dried and then brought to a firmly covered Al 2 O 3 crucible for heat treatment at 1100°C for 4 h. The reaction was assumed complete in accordance with the chemical Eq. (1); the NaCl salt was washed away from the assynthesized product thorough hot de-ionized water. BNT4 platelets, Na 2 CO 3 , and TiO 2 were then further weighed to provide a total BNT composition in accordance with the chemical Eq.
(2). NaCl salt was again added to the powder mixture with 1:1.5 weight ratios, and then milling was carried out in the presence of ethanol through a magnetic stirrer for 5 h. Subsequently, the slurry was dried and a heat treatment at 950°C for 4 h was performed in a firmly covered Al 2 O 3 crucible. Finally, NaCl salt was removed through hot de-ionized water from the product and HCl was utilized to remove the bismuth oxide (Bi 2 O 3 ) by-products. For comparison, BNT particles were also produced by CMO route [39].  Bi Na Ti O + 2Na CO + 5TiO 9Bi Na TiO + 2CO ® Grain-oriented ceramics with a composition of 0.994Bi 0.5 Na 0.5 TiO 3 -0.006BaZrO 3 (BNT-BZ) were fabricated through RTGG process utilizing the as-synthesized BNT templates [38].
Commercially accessible carbonate powders such as: Na 2 CO 3 and BaCO 3 (99.95%, Sigma Aldrich) along with metal oxide powders such as: Bi 2 O 3 , TiO 2 , and ZrO 2 (99.9% Junsei Co., Limited) were first weighed according to the stoichiometric formula of BNT-BZ and then mixed by ball milling for 24 h at 250 rpm. The slurry was dried and then calcined at 850°C for 2 h to form a perovskite phase. The as-prepared calcined powders of BNT-BZ were mixed thoroughly with a solvent (60 vol.% ethanol and 40 vol.% methyl-ethyl-ketone, MEK) and triethyl phosphate (dispersant) in a ball mill for 24 h. Subsequently, polyvinyl butyral (binder) and polyethylene glycol/diethyl-ophthalate (plasticizer) were added to the mixtures and the milling was continued again for another 24 h. BNT templates of 15 wt% were then added to the mixture and ball milled with a slow rotation for another 12 h to form a slurry for tape casting. The viscous slurry was tape cast to form a green sheet with a thickness of ~100 μm on a SiO 2 -coated polyethylene film by a doctor blade apparatus. Afterward drying, a single layer sheet was cut, laminated, and hot-pressed at a temperature of 45°C and a pressure of 50 MPa for 2 min to form a 2-mm-thick green compact. The compacts were further cut into small samples of about 1 × 1 cm 2 and then heated at 600°C for 12 h with intermediated steps of 250 and 350°C for 6 and 8 h and to remove organic substances prior to sintering. The samples were sintered at 1150°C for 15 h in air atmosphere and were then brought to room temperature at cooling rate of 5°C/min. For comparison, non-textured BNT-BZ ceramics were also prepared through conventional solid-state reaction [38].
Crystalline phase and purity information of the as-synthesized BNT particles and BNT-BZ ceramics were checked by X-ray diffraction machine (XRD, RAD III, Rigaku, Japan) using CuKα radiation (λ = 1.541 Å) at room temperature. The XRD patterns were collected in the Braggs-Brentano configuration operated at 10 mA and 20 kV with a step size of 0.02°. The particle size and shape was observed through field emission scanning electron microscope (FE-SEM, JP/JSM 5200, Japan). At room temperature, Raman scattering investigation was performed using a dispersive Raman spectrometer (ALMEGA, Nicelot, USA). Selected area electron diffraction (SAED) pattern and high-resolution transmission electron microscopy (HRTEM) images were obtained by transmission electron microscope (TEM) using a FE-TEM (JEOL/JEM-2100F version) operated at 200 kV. Both surfaces of the samples were polished and coated with a silver-palladium paste to form electrodes for electrical properties measurements. The dielectric constant and loss response were measured through an impedance analyzer (HP4194A, Agilent Technologies, Palo Alto, CA). The ferroelectric hysteresis loops were measured using a Precision Premier II device (Radiant Technology, Inc.) at 50 Hz. Fieldinduced strain response was measured using a contact-type displacement sensor (Model 1240; Mahr GmbH, Göttingen, Germany) at 50 mHz.

Results and discussion
Plate-like particles play a crucial role in the texture engineering. For texture development of BNT-BZ ceramics, plate-like BNT templates were first produced by a topochemical reaction from bismuth layered-structure ferroelectric (BLSF) BNT4 precursor through molten salt process. Figure 1 shows the crystalline phase and FE-SEM micrograph of BNT4 precursor particles synthesized by molten salt process. The X-ray diffraction pattern of the BNT4 precursor (Figure 1a) reveals the development of a single phase with no traces of secondary or parasite phases. All diffraction peaks inherit the characteristics features of the typical layered-perovskite structure. All diffraction peaks matches with the JCPDS card no. 74-1316 of the BLSF. Maximum number of peaks, for instance (006), (008), (0010), (0018), and (0020), were observed to possess higher intensities than the other peaks, signifying that the surface of BNT4 particles is parallel to the (00l) plane and suggesting that the BNT4 particles have high degree of preferred grain orientation. The FE-SEM micrograph of the BNT4 particles (Figure 1b) shows a plate-like morphology with size ranging from 15 to 20 μm. Some small grains of size less than one micron can be also observed that might be the broken pieces of BNT4 crystals. BNT4 has the characteristics anisotropic BLSF materials, in which the growth along the a and b axis is much higher than that of the c-axis. Thus, it is reasonable for them to adopt a plate-like morphology. The as-synthesized BNT4 particles produce in this work were used as a precursor materials for the TMC process [38]. A schematic diagram showing the conversion of the layered perovskite into a simple perovskite structure by a topochemical conversion is illustrated in Figure 2.    Figure 3b). This diffraction profile clearly indicate that the layered structure of BNT4 particles completely transformed into perovskite BNT templates after the TMC process (Figure 3a). Furthermore, the perovskite BNT preserves the plate-like morphology BNT4. Analogous to BNT4 templates, most of the large and plate-like BNT particles laid down with the c-axis aligned along the vertical direction during the sample preparation for the XRD analysis. Accordingly, they show strong diffraction peaks of (100) and (200). BNT4 belongs to the BLSFs family that possess (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 Na + as well as Bi 3+ in a Na/Bi ratio of 0.2. This conversion from the layered structure into a simple perovskite is composed of two stages: firstly, the transmission of Na + and Bi 3+ through the perovskite layers; secondly, the transfiguration of (Bi 2 O 2 ) 2+ fluorite layers into the perovskite phase. It has been also reported that this transformation implies a change from a lamellar phase to a perovskite phase [36]; nevertheless, the process involving the change of the (Bi 2 O 2 ) 2+ layers to perovskite structure still needs further detailed investigations [38].  Figure 4 provides the FE-SEM micrographs of the BNT templates produced from the BNT4 precursor by TMC process along with the BNT particles prepared via a CMO route. Similar to BNT4 particles, most of the BNT templates have large grains of plate-like morphology. Such kind of large and plate-like particles are reasonably more suitable for the preparation textured ceramics by tape-casting process. Beside this, the BNT particles prepared by CMO possess small-and spherical-type grains. This kind of spherical grains is not appropriate to use as templates in development textured ceramics by RTGG technique. The EDS spectra of the BNT particles produced by TMC and CMO processes are displayed in Figure 5. This spectra clearly show the presence of Bi 3+ , Na + , Ti 4+ , and O 2− ions in the particles, recommending successful synthesis of BNT particles through both processes. Additionally, the ratio of m Bi3+ :m Na :m Ti4+ :m O2 − is close to the stoichiometric amount of the receptive BNT composition [38] in both cases.   overlapping of A 1 (longitudinal optical) and E (longitudinal optical) bands. The NBT particles produced by TMC method show broadened and diffuse peaks around 270 cm −1 (Ti-O) and 450-700 cm −1 (TiO 6 ) vibration modes. It can be noted that the two individual TiO 6 octahedra vibration modes tend to diffuse, where the later mode centered around 550 cm −1 of the TMCsynthesized BNT sample exhibits weakening in intensity, which indicates the softening behavior in modes. Such phonon behavior suggests higher asymmetry in structure. Figure 7 presents the TEM pictures of the BNT particles synthesized by TMC and CMO methods, respectively. The SEAD pattern of the BNT particles synthesized by TMC method shows dot pattern (Figure 7a), suggesting its single-crystal-type behavior, while that synthesized by CMO exhibits a ring-type configuration (Figure 7b) indicating a polycrystalline nature. The lattice spacing calculated from the HRTEM image (Figure 7c) of the TMCsynthesized BNT particles is 0.389 nm, which is consistent with the lattice spacing of the cubic (100) plane. This calculation suggests that TMC-synthesized BNT particles have a singlecrystal nature and preferentially grow along the [100] direction which is further confirmed by the SAED pattern provided in (Figure 7a). Beside this, the lattice spacing of the CMOsynthesized BNT particles calculated from the HRTEM image (Figure 7b) is 0.275 nm; this value is consistent with the (110) plane lattice spacing of the BNT composition signifying a grain orientation of particles along [110] direction. These findings verify the XRD analysis of BNT particles synthesized by different routes. Therefore, the variation in microstructure along with the difference in lattice spacing of the CMO-and TMC-synthesized BNT particles indicates their successful fabrication [38]. The BNT particles synthesized by TMC method were further utilized as templates for texture development of BNT-BZ ceramics. Figure 8 shows a schematic diagram for the development of textured BNT-BZ ceramics by tape-casting method. A green sheet of a thickness about 100 μm was formed on SiO 2 -coated polyethylene film through a doctor blade technique, and 25 different green sheets were laminated and hot-pressed to develop a thick green compact of about 2 mm. The green compacts were then sintered to measure their structural and electromechanical properties.  Figure 9a, b provides a comparison of the XRD pattern of the textured and non-textured BNT-BZ ceramics. For texture development of BNT-BZ ceramics, 15 wt% BNT templates were used as seed particles. The diffraction pattern of both samples display a single-phase perovskite structure. Textured sample (Figure 9a) produced by RTGG process shows (100) and (200) peaks much higher than that of the corresponding non-texture sample, suggesting a preferred grain orientation. The non-textured ceramic (Figure 9b) produced by conventional method shows a strong (110) diffraction peak indicating a random orientation. The overall comparison shows that the textured ceramics exhibit [100] diffraction peak dominant, indicating that a large fraction of grains are aligned with their a-axis normal to the sample surface. The degree of grain orientation was estimated by the Lotgering factor (F), which is given by where I and I o are the relative intensity of the diffraction peaks of the textured and non-textured ceramics, respectively, and (h00) and (hkl) are their miller indices. The degree of grain orientation obtained by the method is not very accurate due to the intensity of the diffraction peaks from which the slightly misaligned grains are not counted in the measurement of I (h00) and I (hkl) . However, the F values are used in this work because of their convenience in the measurements. The F values vary from zero for a randomly oriented sample, to one, for a completely oriented sample. The F values were calculated from the diffraction peaks lies in the 2θ range from 20° to 70°. A high Lotgering factor F about 0.83 was obtained for the textured BNT-BZ sample, signifying a high degree of preferred grain orientation. This result also indicates that the plate-like BNT templates are proficient in prompting grain orientation in the BNT-BZ ceramics. Previously, Tam et al. [45] utilized (20 wt%) BNT templates for Bi 0.5 Na 0.35 K 0.1 Li 0.05 TiO 3 (BNKLT) ceramics and reported a Lotgering factor of 0.6. The results obtained in this work indicate that the present high-quality BNT template by TMC process has the potential to develop oriented ceramics with enhanced grain orientation factor in regular perovskite-type ceramics. Moreover, high grain orientation factor of the textured BNT-BZ ceramics usually improves the piezoelectric properties in comparison with the non-textured sample of the same material [38].
The temperature dependence of the dielectric constant (ε) and loss (tanδ) of the textured and non-textured BNT-BZ ceramics measured at different frequencies (1, 10, and 100 kHz) is shown in Figure 10. Both samples show increase in dielectric constant with increase in temperature up to certain value and then decrease with further increase in temperature. At all measured frequencies, textured sample exhibits two visible diffuse dielectric anomalies, termed as a depolarization temperature (T d ) appeared at 140°C along with a permittivity maximum temperature (T m ) around 280°C. Nevertheless, the T d anomaly is not visible in the dielectric curves of the non-textured specimen and its T m values are lower than that of the textured sample. The fading of T d anomaly in the non-textured specimen signifies the weakening of the ferroelectric phase and formation of the relaxor phase as reported previously for BNT-BZ system [44]. The dielectric constant measured at room temperature increased from 1200 for the non-texture sample to 1500 for the textured sample. Furthermore, the dielectric loss of the textured sample is slightly higher than that of non-textured sample, probably due to its lower density and defects/impurities introduced into the green laminates during the tapecasting process.  Figure 11a, b shows the room temperature P-E hysteresis loops of the textured and nontextured BNT-BZ ceramics measured at 50 Hz. The textured sample (Figure 11a) exhibits a pinch-type P-E loop, while the non-textured sample (Figure 11b) exhibits a slim at an applied field of 70 kV/cm. Moreover, texture development improved the ferroelectric response of the BNT-BZ ceramics. The remnant and maximum polarization at an electric field of 70 kV/cm, respectively, increased from 5 and 26 μm/cm 2 for non-textured sample to 11 and 35 μm/cm 2 Figure 11. Ferroelectric response of BNT-BZ ceramics (a) textured sample, (b) non-textured sample.
for it textured counterpart. The coercive field (E c ) increased from 13 kV/cm for the non-textured sample to 18 kV/cm for the textured sample. This enhancement in the polarization can be associated with the crystallographic grain orientation of the textured sample.  The field-induced unipolar strain of the textured and non-textured BNT-BZ ceramics was examined at an applied electric field of 70 kV/cm and is shown in Figure 12. The overall fieldinduced strain response of textured specimen (Figure 12a) is higher than that of the nontextured sample (Figure 12b). The unipolar field-induced strain level raised from 0.15% for the non-textured sample to 0.30% for the textured sample. The corresponding normalized strain (d * 33 = S max /E max ) obtained for these specimens are 214 and 428 pm/V, respectively. Table 1 provides the density, dielectric, and piezoelectric response of the textured and non-textured BNT-BZ samples at room temperature. The comparison of both specimens clearly shows that textured ceramics have a better electromechanical properties than non-textured counterparts. The dielectric constant measured at room temperature increased from 1200 to 1500, while the unipolar field-induced strain level enhanced from 0.15 to 0.30% for the non-textured and textured samples, respectively. This is almost 25 and 100% enhancement after texturing process. This increase in dielectric and field-induced strain response can be attributed to the (100) preferred orientation developed by plate-like BNT templates. The results obtained from the BNT-BZ ceramics evidently show that texture development significantly improve the electromechanical properties of the BNT-BZ ceramics and enable them as promising lead-free candidates in piezoelectric industry.

Summary
For texture development of BaZrO 3 -modified Bi 0.5 Na 0.5 TiO 3 (BNT-BZ) ceramics, plate-like Bi 0.5 Na 0.5 TiO 3 (BNT) template was first synthesized from bismuth layer-structured ferroelectric Bi 4.5 Na 0.5 Ti 4 O 15 (BNT4) precursor through a TMC method. The BNT template produced by TMC method presented high aspect ratio plate-like grains that inherit the morphology of the BNT4 precursor particles and are different from the CMO-synthesized BNT particles that exhibit round-type grains of submicron size. The TMC-synthesized BNT particles show strong (h00) intensity peaks signifying (100) preferred grain orientation; however, the CMO-synthesized particles exhibit (110) intense peak, revealing a random orientation. The structural characteristics observed in XRD analysis further inveterate the TEM analysis, verifying the grain growth along (100) plane for TMC process and along (110) plane for the same BNT composition produced through CMO route. The BNT templates produced by TMC process are very effective in the texture engineering of BNT-BZ ceramics. A high Lotgering factor (F = 83%) was observed in BNT-BZ ceramics textured with 15 wt% of BNT templates. Furthermore, room temperature dielectric constant increased by 25%, maximum polarization 34%, and fieldinduced strain response by 100%. The results obtained in this work shows that BNT templates play an important role in the texture engineering of BNT-BZ ceramics and improving the dielectric, ferroelectric, and field-induced strain properties.