*3.3.3 BNT based ceramic systems*

*Multifunctional Ferroelectric Materials*

**3.3 Lead-free relaxor ferroelectrics**

*3.3.1 BTO based relaxor ferroelectrics*

Bi(Mg2/3Nb1/3)O3 and so on [47–49].

*3.3.2 KNN based ceramic system*

Although lead-based relaxor ferroelectrics are dominating in the electronic markets, lead-free ceramics have been focused intensively for last few years due to the restriction of hazardous substances such as lead, lead oxide and heavy metals. There is no equivalent alternative as compared to lead-based compounds, particularly PZT based relaxor ferroelectrics till now. However, certain lead-free relaxor ferroelectric groups with a perovskite crystal structure are impressed by the current researchers for their enhanced physical properties in terms of dielectric, ferroelectric, and piezoelectric properties. Those relaxor ferroelectrics are typical classified as follows: (a) barium titanate (BaTiO3/BTO) based-, (b) potassium sodium niobate (K0.5Na0.5NbO3/KNN) based-, (c) bismuth sodium titanate (Bi0.5Na0.5TiO3/BNT)

based- and (d) bismuth layer structured ferroelectrics (BLSFs) [47].

At room temperature, BTO exhibits stable electrical properties (dielectric and ferroelectric), good electrochemical coupling (*k*33 ~ 0.50), high-quality factor, low dielectric loss, but limited by low *T*c (120 °C–135 °C) and *d*33 (~190 pC/N). Also, it follows the subsequent structural phase transitions from cubic (>120 °C–135 °C)-tetragonal (120 °C to 20 °C)-orthorhombic (20 °C to −80 °C)-rhombohedral (<−80 °C). In general, BaTiO3 exhibits normal ferroelectric and follows the Curie–Weiss law at ferroelectric to the paraelectric phase transition. The BaTiO3-BaSnO3 solid solution was the first BTO based compound in which relaxor ferroelectric behavior was observed. After that, the relaxor behavior in BTO has been developed by designing the A and B-sites with incorporation of both heterovalent and isovalent ionic substitutions. Currently, there are several modified BTO ceramics available with diffuse phase transition. The available BTO based relaxor ferroelectric systems are BaTiO3-CaTiO3, BaTiO3-BaZrO3- CaTiO3 [*d*33 ~ 620pC/N for Ba0.85Ca0.15Ti0.90Zr0.10O3], BaTiO3-BiFeO3-Bi(Mg0.5Ti0.5) O3, BaTi0.8Sn0.2O3, Ba(Ti0.94Sn0.03Zr0.03)O3 BaTiO3–La(Mg0.5Ti0.5)O3, BiTiO3-(x)

The KNN system is one of the most promising lead-free alternatives due to its high *T*c (~410 °C), high *P*r (~33μcm−2), and large *K*p (~0.454). Basically, KNN is the solid solution of two perovskite compounds, i.e., KNbO3 (orthorhombic: ferroelectric) and NaNbO3 (orthorhombic: antiferroelectric). In general, the KNN forms the morphotropic phase boundary as similar to PZT [47]. It exhibits moderate dielectric, ferroelectric and piezoelectric properties as compared to PZT. Similar to BTO, the relaxor behavior of KNN has been developed by introducing other elements through interrupting the long rang polar ordering and, forms the PNRs as evidenced by several experimental results.

Some of the KNN based relaxor ferroelectrics with physical properties are (K0.48Na0.535)0.942Li0.058NbO3 [*d*33 ~ 314, *K*33 ~ 41,*T*c ~ 490 °C], (K0.44Na0.52Li0.04) (Na0.86Ta0.10Sb0.04)O3 [*d*33 ~ 416pC/N],0.96(K0.5Na0.5)0.95Li0.05Nb1-xSbxO3–

0.04BaZrO3, 0.5wt%Mn-KNN (*T*c ~ 416 °C, *d*33 ~ 350 pCN−1), (Na0.44K0.515Li0.045) Nb0.915 Sb0.045Ta0.05O3(*d*33 ~ 390pC/N, *T*c ~ 320 °C *K*33 ~ 0.49), 0.96(K0.4Na0.6) (Nb0.96Sb0.04)O3–0.04Bi0.5K0.5Zr0.9Sn0.1O3 (*d*33 ~ 460pC/N, *T*c ~ 250 °C, *K*33 ~ 0.47),(Na0.5K0.5)0.975Li0.025Nb0.76 Sb0.06Ta0.18O3 (*d*33 ~ 352, *T*c ~ 200 °C, *K*33 ~ 0.47)

**60**

[48, 50].

BNT is one of the promising lead-free materials to compete with PZT for actuator applications. It exhibits relaxor ferroelectric properties with relatively large remanent polarizations (*P*r ~ 38 μC/cm<sup>2</sup> ), large coercivity (*E*c ~ 73 kV/cm), and high Curie temperature (~320 °C). It follows the character of an ergodic relaxor with room temperature rhombohedral crystal symmetry. The physical properties of several BNT based binary and solid ternary solutions near MPB composition along with the substitution of various cations have been reported; such as BNT-ATiO3 (A = Ca2+, Sr2+, Ba2+, and Pb2+), BNT-KNbO3, BNT-Bi0.5Li0.5TiO3, BNT-Bi0.5K0.5TiO3(BNT-BKT), BNT-K0.5Na0.5NbO3(BNT-KNN), BNT-BKT-KNN, BNT-BT-KNN, BNT-BKT-BiFeO3, BNT-BKT-BaTiO3-SrTiO3 and so on [50, 51].
