**2. 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

### *New Bismuth Sodium Titanate Based Ceramics and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.93921*

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-0.15SrTiO3 ceramics [23]. Later, Zhang et al. introduced (K, Na)NbO3 (KNN) into BNT-BaTiO3 (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<sup>2</sup> 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

### **Figure 1.**

*Properties of 0.89BNT-0.06BT-0.05KNN ceramics: (a) the temperature-dependence of dielectric properties, (b) the polarization-electric field (P-E) loops at 20°C, (c) the P-E loops at 110°C, (d) the energy density as function of temperature [25].*

around 0.59 J/cm3 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-BiAlO3 (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/cm3 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 Sr0.85Bi0.1□0.05TiO3 (□ represents the A site vacancy) and NaNbO3 into the BNT matrix as illustrated in **Figure 3**. The introduced A site

### **Figure 2.**

*(a) The temperature dependence dielectric properties of the 0.93 (0.96BNT-0.04BA)-0.07KNN ceramics. (b) The P-E loops of the (1-x)(0.96BNT-0.04BA)-xKNN ceramics. (c) The P-E loops of the 0.93 (0.96BNT-0.04BA)-0.07KNN ceramics under different frequencies. (d) The energy density of 0.93 (0.96BNT-0.04BA)-0.07KNN ceramics at different temperatures [26].*

*New Bismuth Sodium Titanate Based Ceramics and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.93921*

#### **Figure 3.**

*(a) Schematic image showing energy storage properties under different electric fields. (b) Schematic image showing polar structure in relaxor ferroelectrics under loading and unloading electric fields. (c) The dielectric permittivity and loss as a function of temperature, measured at different frequencies from 0.1 kHz to 1 MHz for the 0.96 (0.65Bi0.5Na0.5TiO3–0.35Sr0.85Bi0.1TiO3)–0.04NaNbO3 ceramics. (d) Energy efficiency versus recoverable energy density value for the 0.96 (0.65Bi0.5Na0.5TiO3–0.35Sr0.85Bi0.1TiO3)–0.04NaNbO3 ceramics compared to other lead-free systems [27].*

vacancy and Sr2+, Nb5+ 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/cm3 ) 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.

### **3. 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.

Bi0.5 Na0.5TiO3 (BNT) is explored as an alternative lead-free candidate for pulse power supply, in view of its high Pr, high breakdown strength Eb, low bulk density, and relatively high Curie temperature (Tc). Gao et al. [31] reported that the BNT

can be fully depolarized by shock compression and generate a giant power output (3.04 × 108 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 O2− 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) Bi0.5Na0.5TiO3-x BiAlO3]-0.03K0.5Na0.5NbO3 (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/cm2 to 8.2 μC/cm<sup>2</sup> , decreasing ~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.

### **Figure 4.**

*Pressure dependence of the phase transition in BNT has been studied by the in situ synchrotron x-ray diffraction [31]. (a) The x-ray diffraction spectra of NBT ferroelectric materials at selected pressures. (b) The XRD peaks of the phase are marked by the red spades. The NBT is rhombohedral (R3c) structure at low pressure, and it changes into orthorhombic structure (Pnma) at high pressure. The normalized P-V curve of NBT according to the Z = 6, and (c)–(e) the schematic diagram of the structure phase transition during the phase transition.*

*New Bismuth Sodium Titanate Based Ceramics and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.93921*

### **Figure 5.**

*First-principles calculations of the R3c and Pnma phases as a function of pressure [31]. (a) The enthalpy (H) calculated by first-principles simulation for R3c and Pnma phases at different pressures, respectively. The enthalpy change of R3c phase could be divided into two regions (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).*

**Figures 6** and **7** show the effect of hydrostatic pressure on the ferroelectric properties of BNT-0.01BA-0.03KNN and BNT-0.04BA-0.03KNN, respectively. It is clear that the Pr and Pm decrease monotonically with increasing pressures, which further confirms the increasing the instability of the long-range FE order and the energy barrier for the formation of FE domains under hydrostatic pressure conditions. In addition, the response of BNT-0.04BA-0.03KNN under pressure is more sensitive than that of BNT-0.01BA-0.03KNN. And the thermally induced depolarization is also stronger for BNT-0.04BA-0.03KNN. These phenomena should be related to their different depolarized temperature values. The ER phase exhibits smaller volume than the FE phase. Therefore, applying compressive pressure favors

#### **Figure 6.**

*(a) P-E loops and (b) I-E curves of BNT-0.01BA-0.03KNN ceramics under different hydrostatic pressures; and (c) the pressure dependence of maximum polarization and remanent polarization of BNT-0.01BA-0.03KNN ceramics [32].*

### **Figure 7.**

*(a) P-E loops and (b) I-E curves of BNT-0.04BA-0.03KNN ceramics under different hydrostatic pressures; (c) the pressure dependence of maximum polarization and remanent polarization of BNT-0.04BA-0.03KNN ceramics [32].*

**Figure 8.**

*Dynamic response behaviors of BNT-BA-0.01NN ceramics in a short-circuit mode under different shock pressures [34]. (a) 2.3 GPa, (b) 5.4 GPa, (c) 6.9 GPa, (d) 8.2 GPa.*

### **Figure 9.** *Pressure-dependent (a) P-E and (b) I-E loops of unpoled BNT-BA-0.01NN ceramics at 70°C [34].*

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 (Bi0.5Na0.5)(Ti0.995Mn0.005) O3–0.02BiAlO3]-0.01NaNbO3 (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<sup>2</sup> . The released J was 96% of thermal-induced discharge density (~40 μC/cm<sup>2</sup> ). 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.

*New Bismuth Sodium Titanate Based Ceramics and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.93921*

**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.
