New Bismuth Sodium Titanate Based Ceramics and Their Applications

Ferroelectric materials are widely investigated due to their excellent properties and versatile applications. At present, the dominant materials are lead-containing materials, such as Pb (Zr,Ti)O 3 solid solutions. However, the use of lead gives rise to environmental concerns, which is the driving force for the development of alternative lead-free ferroelectric materials. (Bi 0.5 Na 0.5 )TiO 3 -based ceramics are considered to be one of the most promising lead-free materials to replace lead-containing ferroelectric ceramics due to their excellent ferroelectric properties, relaxation characteristics, and high Curie point. After decades of efforts, great progress has been made in the phase structure characterization and properties improvement of BNT based ceramics. However, most of the studies on BNT system mainly focuses on its piezoelectric properties and application of piezoelectric sensors and strain actua-tors, little attention is paid to its ferroelectric properties and related applications. In this chapter, new BNT-based ceramics via composition modification and special focuses on the ferroelectric properties, phase transition behaviors under external fields and related applications, such as application in energy storage, pulsed power supply and pyroelectric detection were proposed.


Introduction
.5 Na 0.5 )TiO 3 (BNT) was first reported by Smolenskii et al. in 1960 [1]. BNT ceramic is a kind of ABO3 type ferroelectrics which is replaced by Na + and Bi 3 + complex ions at A-site. The A-site ions of BNT ceramics are located at the eight corner positions of octahedron, and the B-site ions are at the body center of octahedral structure [2]. Well sintered BNT ceramics have been obtained by hot pressing sintering method with d 33 of 94-98 pC/N [3]. BNT ceramics exhibit a high Curie temperature (~320°C) and high polarization of 38 μC/cm 2 , which is considered to be one of the most promising environment-friendly ceramic system to replace lead-based ceramics [4].
Pure BNT ceramics exhibits some problems such as high conductivity, and large coercive field, consequently giving problems in the poling process [4], which seriously hinder its practical application. Studies show that the comprehensive properties of BNT system can be significantly improved by doping or by incorporation

New BNT-based ceramics for energy storage applications
BNT-based materials possess a superior potential for energy storage due to their high saturation polarization which originates from hybridization between the Bi 6p and O 2p orbitals. However, the pure BNT materials at room temperature own a ferroelectric perovskite structure with the polar R3c space group, usually exhibiting a saturated polarization loop with high remnant polarization, which is very unfavorable to obtain good energy storage performance [19]. Fortunately, the BNT materials can show an antiferroelectric-like behavior at around 200-320°C, which opens a door to the energy storage application of BNTbased materials, and the 200°C is identified as the depolarization temperature (T d ) of the BNT materials, which correspond with a peak in the temperaturedependent dielectric loss curve. The structure at this temperature range is still under debate. Zvirgzds et al. [20] proposed a rhombohedral (R3c)-tetragonal (non-polar P4bm) phase transition over the broad temperature range (255-400°C). Moreover, Schmitt et al. [21] suggested the phase transformation from non-polar P4bm phase to polar R3c phase under applied electric field BNT-based piezoelectric ceramics d 33 (pC/N) Ref.
(Bi 1/2 Na 1/2 )TiO 3 94-98 [3] (Bi 1/2 K 1/2 )TiO 3 69 [10] (1-x) (Bi 1/2 Na 1/2 )TiO 3 -x(Bi 1/2 K 1/2 )TiO 3 140-192 [11] (Na 1-xK x ) 0 accounted for the antiferroelectric-like characteristic, but this could not reasonably explain a large temperature hysteresis of different physical properties about the phase transition between 200 and 320°C. Dorcet et al. [22] revealed a modulated phase at 200-300°C through in-situ Transmission electron microscope (TEM) characterization, it was formed of Pnma orthorhombic sheets which are locally analogous to an antiferroelectric phase, and these sheets are twin boundaries between R3c ferroelectric domains. The phase structure evolution disclosed by Zvirgzds et al. [21] well matches the macroscopic physical properties of BNT materials during the heating process. In 1947, Sakata et al. reported an antiferroelectric-like behavior in the 0.85BNT-0.15SrTiO 3 ceramics [23]. Later, Zhang et al. introduced (K, Na)NbO 3 (KNN) into BNT-BaTiO 3 (BT) ceramics to low the phase transition temperature and achieved the antiferroelectric-like behavior in BNT-BT-KNN ceramics with slanted polarization hysteresis loops at room temperature [24]. In 2011, Gao et al. [25] first investigated the energy storage properties of the BNT-BT-KNN system, the 0.89BNT-0.06BT-0.05KNN ceramics was chosen as the object, Figure 1(a) is the temperature-dependent dielectric properties of the 0.89BNT-0.06BT-0.05KNN ceramics, it can be seen that these ceramics showed much lower T d compared with pure BNT materials, indicating the antiferroelectric-like behavior at a lower temperature. Figure 1(b, c)show the temperature dependence of polarization hysteresis loops of the 0.89BNT-0.06BT-0.05KNN ceramics under different electric fields. At 20°C, the polarization hysteresis loop was more of ferroelectric featured with coercive field E c = 0.9 kV/mm and remnant polarization P r = 6.2 μC/cm 2 under 6 kV/mm. At 110°C, the polarization hysteresis loop was more of an antiferroelectric-like feature with a pronounced shrinkage in both E c and P r compared with those at 20°C. The energy density as a function of the temperature of the 0.89BNT-0.06BT-0.05KNN ceramics are displayed in Figure 1(d). An energy density of  [25]. around 0.59 J/cm 3 under 5.6 kV/mm at 10 Hz was obtained in 0.89BNT-0.06BT-0.05KNN ceramics from 100 °C to 150 °C, indicating high stability of temperature in the antiferroelectric-like region. Although the obtained energy density was very small and only existed above 100°C, this work is still meaningful because it inspires the further way for studying energy-storage in BNT-based materials. After, researches about the energy storage properties in BNT-based ceramics have been extensively reported.
Ren et al. [26] reported that the introduction of KNN would decrease the T d of BNT-BiAlO 3 (BA) ceramics and the KNN content exerts a significant influence on the polarization hysteresis loops of BNT-BA-KNN materials as shown in Figure 2b. For 0.93 (0.96BNT-0.04BA)-0.07KNN ceramics, the T d was below the room temperature as depicted in Figure 2a and these ceramics were more of antiferroelectriclike behavior. Ren et al. [26] also investigated the energy storage properties of 0.93 (0.96BNT-0.04BA)-0.07KNN ceramics, an energy storage density of 0.65 J/cm 3 was obtained under 8 kV/mm at room temperature, and these ceramics exhibited good stability of energy density as a function of temperature and frequency at 7 kV/mm, which can be seen from Figure 2c,d.
Due to the high energy loss of the antiferroelectric-like BNT-based materials, the BNT-based relaxor ferroelectrics have attracted more and more attention for energy storage and usually can show superior energy storage performance. In fact, by modifying composition and temperature in BNT-based systems, a normal or square P-E loop can transform into a slim P-E loop due to the occurring of an ergodic relaxor phase, which can be contributed to the energy storage properties. Wu et al. [27] focused on the energy storage characteristics of BNT-based relaxor ferroelectric ceramics and introduced Sr 0.85 Bi 0.1 □ 0.05 TiO 3 (□ represents the A site vacancy) and NaNbO 3 into the BNT matrix as illustrated in Figure 3. The introduced A site  vacancy and Sr 2+ , Nb 5+ ions replaced the A-and B-sites ions respectively, which led to the stress mismatch and charge imbalance. These effects acted together to effectively form a local random field, which broke the long-range ordered structure of the dipole in the matrix and formed a weakly coupled polar nanodomain. Under the applied electric field, the modified ceramics exhibited a small hysteresis and a small remnant polarization, achieving high energy storage density (3.08/cm 3 ) and high energy storage efficiency (81.4%). To evaluate the practicability of the modified ceramic, energy storage performance test in a wide range of temperature and frequency found that the variations of its energy storage performance at RT ~ 100°C and 1 Hz ~ 100 Hz was less than 10%. The modified ceramics with excellent application prospects are excellent candidate materials for dielectric energy storage capacitors.

New BNT-based ceramics for pulse power supply application
Ferroelectric materials have an important application in pulse power supply due to their shock compression induced depolarization behavior [28]. At present, the main material systems studied are PZT52/48 piezoelectric ceramics [28], PZT95/5 ceramics [28,29] and PIN-PMN-PT single crystals [30]. However, due to the toxicity of Lead, it is urgent to develop lead-free materials for high ferroelectric pulse power supply.
Bi 0.5 Na 0.5 TiO 3 (BNT) is explored as an alternative lead-free candidate for pulse power supply, in view of its high P r , high breakdown strength E b , low bulk density, and relatively high Curie temperature (T c ). Gao et al. [31] reported that the BNT can be fully depolarized by shock compression and generate a giant power output (3.04 × 10 8 W/kg). This power output is mainly attributed to a two-step polar-nonpolar phase transition from rhombohedral to orthorhombic under shock pressure. Figure 4 shows that BNT is polar phase and rhombohedral (space group R3c) at low pressure, and transforms via a first-order phase transition to a nonpolar phase (space group Pnma), which is orthorhombic and centrosymmetric. The electrical output of BNT from depoling under shock compression can be attributed to the ferroelectric-to-paraelectric (R3c − Pnma) phase transition. The energy output under shock compression in BNT is larger than that reported in other ferroelectric materials, mainly due to a first-order R-O phase transition under high dynamic pressure. This phase transition undergoes two steps, which correspond to the unitcell shrinkage and O 2− ions chain rearrangement, respectively, as shown in Figure 5. These results will extend the potential application of the pressure induced depolarization effects and guide the application and development of BNT ferroelectric materials.
Liu et al. [32] report the pressure driven depolarization behavior in 0.97[(1-x) Bi 0.5 Na 0.5 TiO 3 -x BiAlO 3 ]-0.03K 0.5 Na 0.5 NbO 3 (BNT-xBA-0.03KNN) ceramics. Particularly, with increasing hydrostatic pressure from 0 MPa to 495 MPa, the polarization of BNT-0.04 decreases from 30.7 μC/cm 2 to 8.2 μC/cm 2 , decreas-ing~73%. The observed depolarization effect is associated with the pressure induced polar ferroelectric -nonpolar relaxor phase transition. The results revealed that BNT-xBA-0.03KNN ceramics as promising lead-free candidates for energy conversion applications based on the pressure driven depolarization effect.   A and B). When the pressure is below 1.9 GPa (region A), the enthalpy of R3c increases sharply due to the volume decreasing as shown in (b). When the pressure is above 1.9 GPa (region B), the enthalpy of R3c phase increases gently, which is mainly due to the O2−ions displacing following the red arrows in (c).  the FE-ER phase transition. This is quite similar to the case of Nb doped PZT95/5, in which pressure can drive the larger volume FE phase to transform into the smaller volume AFE phase.
Peng et al. [33,34] report the depolarization behavior of lead-free ternary 0.99[0.98 (Bi 0.5 Na 0.5 )(Ti 0.995 Mn 0.005 ) O 3 -0.02BiAlO 3 ]-0.01NaNbO 3 (BNT-BA-0.01NN) ferroelectric ceramics under shock wave compression. Particularly, approximately complete depolarization under shock compression was observed in the poled BNT-BA-0.01NN ceramics, releasing a high discharge density J of 38 μC/cm 2 . The released J was 96% of thermal-induced discharge density (~40 μC/cm 2 ). This discharge density J was 18% higher than that of PZT95/5 ceramics [29]. The shock-induced depolarization mechanism can be attributed to the ferroelectric-ergodic relaxor phase transition. These results reveal the BNT-based ceramics as promising candidates for pulsed power applications.    Figure 8 shows the BNT-based ceramics were almost completely depolarized, similar to PZT95/5 ceramics [29] and PIN-PMN-PT crystals [30], which indicate a similar depolarization mechanism, that is, a stress-induced phase transition. Although the released J in BNT-based ceramics is 26% lower than that obtained in PIN-PMN-PT crystals, the simple preparation methods together with environmental friendliness will be a benefit to their applications in the future. Figure 9 unveils the possible shock-induced depolarization mechanism of BNT-BA-0.01NN ceramics. The pinched P-E loops gradually emerge and the sharp current peak splits into four peaks, indicating a pressure-induced FE-ER phase transition. It is suggested that applying compressive pressure favors the formation of the ER phase for its smaller volume.  [41][42][43][44]. Among them, BNT-based ceramics have been regarded as one of most promising alternative lead-free ceramics due to its high pyroelectric coefficient (p), high remnant polarization P r (around 38μC/cm 2 ), high Curie temperature T c (around 320°C), low-cost, and simple synthesis process. In recent decades, pyroelectric properties of BNT-based materials, including pyroelectric coefficient and detection rate, have been greatly improved. The pyroelectric coefficient of BNT-based lead-free pyroelectric materials has been comparable to commercial PZT [45][46][47]. However, the enhanced pyroelectric property is usually at the cost of degraded depolarization temperature (<150°C) and thermal stability, which are the hurdles to application. The BNT-based pyroelectric ceramics with low Td will depolarize partially or completely during the heat treatment (typically >100°C) processes, causing degradation of pyroelectric performance. Therefore, from the viewpoint of practical application, it is urgent for BNT-based materials to optimize their depolarization temperature, thermal stability and pyroelectric performance, thus further to promote their applications in infrared detection [48,49].

BNT-BNN pyroelectric ceramics
(1-x)(Bi 0.5 Na 0.5 )TiO 3 -xBa(Ni 0.5 Nb 0.5 )O 3 lead-free pyroelectric ceramics (abbreviated as (1-x)BNT-xBNN) were synthesized by a conventional solid-state reaction method [50], and the thermal stability and depolarization temperature is enhanced at the same time as the excellent pyroelectric performance is maintained. BNN is a compound with a mixed valence state at the b position, which can be solidsolved with BNT and expand a wide range of composition adjustment. The (1-x) BNT-xBNN take into account the advantages of b-position acceptor substitution and donor substitution. The effect of BNN content on phase structure, electrical properties and thermal stability was systematically studied. After the solid-state reaction of BNN, (1-x)BNT-xBNN exhibits enhanced pyroelectric performance with a high depolarization temperature. In addition, it can be exposed to temperature up to ~145°C with negligible deterioration of pyroelectric properties, showing excellent thermal stability.
The temperature-dependent properties of poled (1 − x)BNT-xBNN ceramics are displayed in Figure 10a-d. With the increasing BNN content, the Curie temperature Tc indicated by the maximum dielectric constant decreases and dielectric constant and dielectric loss of BNN decrease first and then increase. The minimum value of dielectric constant and dielectric loss occurs when the BNN content is 2%, which further improve the pyroelectric detection rate figure of merit. The depolarization temperature T d can be characterized by the first anomal point of temperature dependent dielectric properties, and the content of 2% has the highest depolarization temperature. As shown in Figure 10e, after the increase of BNN, the room temperature p values rise from 3.01 × 10 −8 C/cm 2 K of pure BNT to 5.94 × 10 −8 C/cm 2 K of 0.96BNT-0.04BNN with the increasing addition of BNN, which gains advantage compared with many other lead-free ceramics. The p value of (1-x)BNT-xBNN ceramics increases sharply, which indicates that the (1-x)BNT-xBNN sample is sensitive to ambient temperature. In addition, it can be seen that the p value increases with the increasing temperature, which indicates that the (1 − x)BNT-xBNN samples are sensitive to the surrounding temperature. Besides, 0.98BNT-0.02BNN ceramics have the best thermal stability and it can withstand heat treatment at 145°C without depolarization (Figure 10f), which is attributed to the domain switching and phase transition.

BNT-BT pyroelectric ceramics
BNT-BT possesses a rich phase structure, which can be easily adjusted by varying the BT content. Because of the low tripartite-tetragonal transition barrier, the  morphotropic phase boundary (MPB) of BNT-BT, located at where the BT content is approximately 6%, exhibits the best pyroelectric properties and has received much attention. But it is not advisable to blindly pursue a high pyroelectric coefficient. The improvement of pyroelectric performance is often at the cost of low depolarization temperature, which is not helpful to practical applications. However, it is found that the sample with high BT content is in the tetragonal phase, which brings a higher T d than that of the tripartite, but there is no relevant report on the pyroelectric performance of high BT content.
Based on the above ideas, the tetragonal phase 0.8BNT-0.2BT lead-free pyroelectric material with high BT content was successfully prepared, and the microstructure, dielectric properties, pyroelectric properties, and thermal stability were studied [51]. Owing to its high T d , this composition can endure high-temperature environment (180°C) for half hour with the value of p at room temperature remains ~90% of its initial value, demonstrating that the 0.8BNT-0.2BT samples show excellent thermal stability. Moreover, the T d of the samples is up to ~209°C, which is far higher than that of the reported BNT-based, pyroelectric materials, and it is also comparable to the commercial PZT materials.
The pyroelectric properties of 0.8BNT-0.2BT pyroelectric ceramics between 25 and 70°C are investigated. With the increase of temperature, the pyroelectric performance shows an increasing trend, indicating that the material has good pyroelectric performance in a wide temperature range. Meanwhile, because the 0.8BNT-0.2BT sample has a low dielectric constant and dielectric loss, it will show a larger detection merit (Figure 11a). In order to study the depolarization temperature of the material, the dielectric thermo diagram of the sample was shown in Figure 11b. When the temperature rises to about 209°C, the dielectric constant of the sample suddenly increases with a dielectric loss peak appearing, indicating that this temperature is the (c) pyroelectric coefficient at room temperature after annealing at Ta. The inset shows the temperaturedependent pyroelectric coefficient on heating after annealing at Ta [51]. depolarization temperature T d . Notably, the depolarization temperature of reported BNT-based pyroelectric materials is generally lower than 180°C. The materials with high Td (209°C) and high pyroelectric coefficient discovered lay the foundation for the further development of lead-free pyroelectric materials. Moreover, it can be observed from Figure 11c that the room temperature pyroelectric coefficient of 0.8BNT-0.2BT maintains about 90% of the original data after being treated at 180°C, indicating that the material has good temperature stability and can withstand high temperature treatment up to180°C without pyroelectric performance loss.

BNT-BA-NN pyroelectric ceramics
A new ternary system 0.98BNT-0.02BA-xNN ceramic was obtained by solid solution of NaNbO 3 (NN) in the BNT-BA system and Mn element substitution  modification [52]. The NN solution significantly affect the microstructure, phase transition and pyroelectric properties of 0.98BNT-0.02BA-xNN ceramics. It was found that NN addition tends to reduce the rhombohedral phase while favoring the formation of the tetragonal phase. The compositions exhibit excellent pyroelectric performance. All components exhibit excellent ferroelectric properties at room temperature, and the Pr values are all higher than 35 μC/cm 2 , of which the Pr of the x = 0.03 component is the largest, reaching 45 μC/cm 2 .
Furthermore, the influence of NN solid solution on the relaxation characteristics and phase transition of BNT-BA-based ceramics was analyzed by testing the temperature-changing dielectric properties in Figure 12a. Figure  12b shows the change curve of the pyroelectric coefficient of 0.98BNT-0.02BA-xNN after polarization with temperature changing. The FE-RE phase transition occurs at T d , corresponding to the sudden drop in the polarization intensity P r . the largest peak appears at the composition x = 0.03, reaching 441.0 × 10 −8 C/ cm 2 K, which is much larger than other BNT-based ceramics reported. As the NN content increases, the T d continuously decreases. Notably, t the T d of the x = 0.02 component is still as high as 155°C. It can be observed from Figure 12c that the introduction of NN significantly improves the room temperature pyroelectric coefficient. With the increase of NN content, the p under room temperature (25°C) first increases and then decreases, and the maximum value is obtained at x = 0.03 (p = 8.45 × 10 −8 C/cm 2 K), which improved about 54% compared to the matrix (x = 0, p = 3.87 × 10 −8 C/cm 2 K). Moreover, the optimal figure of merit (FOMs) at room temperature were obtained at x = 0.02 with F i = 2.66 × 10 −10 m/V, F v = 8.07 × 10 −2 m 2 /C, and F d = 4.22 × 10 −5 Pa −1/2 (Figure 12d-f). Furthermore, the compositions with x ≤ 0.02 possess relatively high depolarization temperature (≥155°C). Those results unveil the potential of 0.98BNT-0.02BA-xNN ceramics for infrared detector applications.

Conclusion
Due to its strong ferroelectric properties, BNT-based ceramics exhibit great potential in the fields of energy storage, pulsed power supply and pyroelectric applications. In this chapter, new bismuth sodium titanate ceramics were synthesized and characterized via composition modifications, the ferroelectric properties, phase transition behaviors under external fields and related applications were proposed in this chapter. To detail, BNT-BT-KNN, BNT-BA-KNN, and BNT-SBT-NN ceramics for energy storage application, BNT, BNT-BA-KNN, and BNT-BA-NN ceramics for pulsed power supply, as well as BNT-BNN, BNT-BT, and BNT-BA-NN for pyroelectric detection application were presented.