**4. New BNT-based ceramics for pyroelectric applications**

At present, the most widely used intrinsic pyroelectric materials are perovskite-type lead-containing materials, such as Pb(Zr, Ti)O3 (PZT), PbTiO3 doped with Ca (PCT), xPb(Mg1/3Nb2/3)O3-(1-x)PbTiO3 (PMN-PT) [35–40]. Over the last few decades, continuous efforts have been devoted to the development of lead-free pyroelectric ceramics. Many lead-free ceramics such as Sr0.3Ba0.7Nb2O6 based, (Ba, Ca)TiO3-based, CaBi4Ti4O15-based bismuth layer-structured and Bi0.5Na0.5TiO3-based ceramics have been investigated [41–44]. 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 Pr (around 38μC/cm<sup>2</sup> ), high Curie temperature Tc (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 [45–47]. 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].

### **4.1 BNT- BNN pyroelectric ceramics**

(1-x)(Bi0.5Na0.5)TiO3-xBa(Ni0.5Nb0.5)O3 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.

**Figure 10.**

*Temperature dependence of dielectric constant (*ε*r) and dielectric loss (tan* δ*) for poled (1-x)BNT-xBNN (a) x = 0, (b) x = 0.02, (c) x = 0.03, (d) x = 0.04 in frequency range between 10 kHz and 1 MHz; (e) Pyroelectric coefficient (p) of poled (1-x)BNT-xBNN ceramics as a function of temperature; (f) d33RT/d33T at room temperature after annealing at Ta [50].*

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 Td 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 × 10−8C/cm2 K of pure BNT to 5.94 × 10−8C/cm2 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.

### **4.2 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

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

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 Td 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 Td, 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 Td 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

### **Figure 11.**

*(a) Pyroelectric coefficient (p) and figures of merit (Fi, Fv, Fd) as a function of temperature on heating during the range of RT to 70°C. The figures of merit are determined based on the values of p, CV,* ε*r, and tan*δ*; (b) temperature-dependent dielectric constant (*ε*r) and dielectric loss (tan*δ*) of poled 0.8BNT–0.2BT ceramics; (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 Td. 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.

### **4.3 BNT-BA***-***NN pyroelectric ceramics**

A new ternary system 0.98BNT-0.02BA-xNN ceramic was obtained by solid solution of NaNbO3 (NN) in the BNT-BA system and Mn element substitution

### **Figure 12.**

*(a) Temperature dependent dielectric constant (*ε*r) and tangent loss (tan*δ*) of 0.98BNT-0.02BA-xNN ceramics; (b) temperature-dependent pyroelectric coefficient of 0.98BNT-0.02BA-xNN ceramics; (c) the temperature-dependent pyroelectric coefficient p of 0.98BNT-0.02BA-xNN ceramics in the temperature range from 10–80°C; merit (d) Fi, (e) Fv and (f) Fd of 0.98BNT-0.02BA-xNN ceramics measured at 1 kHz over the range of 20–80°C [52].*

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

modification [52]. The NN solution significantly affect the microstructure, phase transition and pyroelectric properties of 0.98BNT-0.02BA-*x*NN 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<sup>2</sup> , of which the Pr of the x = 0.03 component is the largest, reaching 45 μC/cm2 .

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 Td, corresponding to the sudden drop in the polarization intensity Pr. the largest peak appears at the composition x = 0.03, reaching 441.0 × 10−8 C/cm2 K, which is much larger than other BNT-based ceramics reported. As the NN content increases, the Td continuously decreases. Notably, t the Td 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/cm2 K), which improved about 54% compared to the matrix (x = 0, *p* = 3.87 × 10−8 C/cm2 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 m2 /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-*x*NN ceramics for infrared detector applications.

### **5. 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.

### **Acknowledgements**

The authors would like to thank the financial support by Youth Innovation Promotion Association, CAS (Grant No. 2017296), National Natural Science Foundation of China (NSFC) (Grant No. 51872312), and Natural Science Foundation of Shanghai (Grant NO.18ZR1444900).

### **Conflict of interest**

The authors declare no conflict of interest.

*Advanced Ceramic Materials*
