Dielectric and piezoelectric properties of textured and non-textured BNT–BZ ceramics.
Abstract
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 (Bi0.5Na0.5TiO3, BNT) are considered potential substitute for lead-based materials because of the Bi3+ and Pb+2 same 6s2 lone pair electronic configuration. This chapter describes the synthesis of BNT particles by different techniques; conventional mixed oxide (CMO) route along with topochemical microcrystal conversion (TMC) methods followed by fabrication of BaZrO3-modified BNT ceramics with a chemical composition of 0.994Bi0.5Na0.5TiO3–0.006BaZrO3 (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 structural and electromechanical properties of the textured and non-textured counterparts.
Keywords
- Perovskite
- Piezoelectric
- BNT–BZ
- Grain orientation
- Texture
1. 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 [6–8]. 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 BaTiO3 (BT), K0.5Na0.5NbO3 (KNN), and Bi0.5Na0.5TiO3 (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 (
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 BNT-based ceramics, this chapter describes the synthesis of BNT particles by different techniques; CMO route along with TMC methods followed by fabrication of BaZrO3-modified BNT ceramics with a chemical composition of 0.994Bi0.5Na0.5TiO3–0.006BaZrO3 (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.
2. Experimental
Reagent-grade metal oxide powders of Bi2O3, TiO2, and Na2CO3 (purity >99.9%) were used as starting materials to produce plate-like Bi4.5Na0.5Ti4O15 (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 Al2O3 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 as-synthesized product thorough hot de-ionized water. BNT4 platelets, Na2CO3, and TiO2 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 Al2O3 crucible. Finally, NaCl salt was removed through hot de-ionized water from the product and HCl was utilized to remove the bismuth oxide (Bi2O3) by-products. For comparison, BNT particles were also produced by CMO route [39].
Grain-oriented ceramics with a composition of 0.994Bi0.5Na0.5TiO3–0.006BaZrO3 (BNT–BZ) were fabricated through RTGG process utilizing the as-synthesized BNT templates [38]. Commercially accessible carbonate powders such as: Na2CO3 and BaCO3 (99.95%, Sigma Aldrich) along with metal oxide powders such as: Bi2O3, TiO2, and ZrO2 (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
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 CuK
3. 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
A schematic diagram showing the conversion of the layered perovskite into a simple perovskite structure by a topochemical conversion is illustrated in Figure 2. Bismuth layered-structure materials have a chemical formulation Bi2O2(
Figure 3 presents the X-ray diffractogram of the BNT particles produced through TMC (Figure 3a) and CMO (Figure 3b) methods, respectively. BNT particles produced through both methods exhibit a single-phase perovskite structure and both match well with the JCPDS card No. 36–0340 of the Bi0.5Na0.5TiO3. Because of the small rhombohedral distortion, all diffraction peaks were indexed on the basis of the pseudocubic perovskite unit cell. A significant difference in the peak intensities can be observed in BNT particles synthesized by two different routes. BNT particles synthesized by TMC shown in Figure 3a exhibit strong (100) and (200) diffraction peaks while that synthesized by CMO have (110) as major peak (shown in 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 (Bi2.5Na0.5Ti4O13)2− (pseudo-perovskite layers) enclosed in (Bi2O2)2+ fluorite layers where the
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 Bi3+, Na+, Ti4+, and O2− ions in the particles, recommending successful synthesis of BNT particles through both processes. Additionally, the ratio of mBi3+:mNa:mTi4+:mO2− is close to the stoichiometric amount of the receptive BNT composition [38] in both cases.
Figure 6 provides the Raman scattering spectra (in range 100–1000 cm−1) of the polished surface of NBT particles measured at room temperature. The different vibration modes observed in Raman spectra of both sample is consistent with previously reported NBT-based ceramics [40–44]. The Raman-active mode (A1) positioned around 150 cm−1 is associated with the
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 TMC-synthesized 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 single-crystal 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 CMO-synthesized 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 SiO2-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
Where
where
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 (
Figure 11a, b shows the room temperature P–E hysteresis loops of the textured and non-textured 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/cm2 for non-textured sample to 11 and 35 μm/cm2 for it textured counterpart. The coercive field (
Relative density |
|
Strain |
|
|
---|---|---|---|---|
Textured | 93% | 1500 | 0.30% | 428 pm/V |
Non-textured | 96% | 1200 | 0.15% | 214 pm/V |
Increase | - | 25% | 100% | 100% |
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 field-induced strain response of textured specimen (Figure 12a) is higher than that of the non-textured 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 (
4. Summary
For texture development of BaZrO3-modified Bi0.5Na0.5TiO3 (BNT–BZ) ceramics, plate-like Bi0.5Na0.5TiO3 (BNT) template was first synthesized from bismuth layer-structured ferroelectric Bi4.5Na0.5Ti4O15 (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 (
Acknowledgments
This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MOE) (2013R1A1A2058345) and the Basic Research program through the National Research Foundation of Korea (NRF) funded by Ministry of Education, Science and Technology (MEST) (2011–0030058).
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