Читать книгу Lead-Free Piezoelectric Materials - Jing-Feng Li - Страница 28

Bismuth ferrite (BiFeO₃) possesses a rhombohedrally distorted perovskite structure as shown in Figure 2.6, which can be denoted as a pseudocubic structure with lattice parameter a = 0.396 nm and α = 89.6° [45]. BiFeO₃ is the only room‐temperature multiferroic material with coexisting ferroelectric and magnetic properties in a single compound [46]. Extensive studies have been conducted on this compound in the form of thin films, which are expected to open up new applications [47]. As reported in the epitaxial thin films, BiFeO₃ has a high Curie point (T_c ~ 830 °C) and large remnant polarization (P_r ~ 100 μC/cm²) at the same time [47]. In addition, the cost of resource is low because of the earth abundance of the constituent elements. Most importantly, BiFeO₃ is a lead‐free ferroelectric compound where Bi has a similar electronic structure with Pb. Therefore, recently, increasing attention has been paid to the development of BiFeO₃‐based lead‐free piezoelectrics.

However, BiFeO₃ and BiFeO₃‐based ceramics have high leakage current issues, mainly induced by defects and secondary phases, which hinder the poling process required for piezoelectric ceramics [48]. To resolve this problem, extensive investigations have been conducted based on chemical modifications to BiFeO₃ [49]. In 2008, Takeuchi and coworkers introduced another rhombohedral–orthorhombic (R–O) MPB that was formed by A‐site doping with 14% Sm in BiFeO₃ films grown on SrTiO₃. The piezoelectric coefficient d₃₃ near this MPB could reach 110 pm/V [50]. This report stimulated many studies about Sm‐doping on BiFeO₃, and the addition of other rare earth elements were also investigated to promote the piezoelectricity and ferroelectricity. For example, a large piezoelectricity (d₃₃ ≈ 50 pC/N) was reported in Sm and La co‐doped BiFeO₃ ceramics [51]. Higher properties are obtained in BiFeO₃ ceramics with the addition of ABO₃. In a study by Lee et al., excellent piezoelectric properties with a Curie temperature above 400 °C were reported in BiFeO₃–BaTiO₃ ceramics with the addition of either Bi_1.05GaO₃ or Bi_1.05(Zn_0.5Ti_0.5)O₃ fabricated in a sintering‐and‐quenching process [52]. Such a special process has proved to effectively suppress the formation of point defects and secondary phase and thereby reduce the high current leakage [53].

Figure 2.6 Crystalline structure of BiFeO₃.

The main applications of BiFeO₃‐based ceramics are considered to be high‐temperature piezoelectric sensors, but their lower usage temperatures should be lower as compared with bismuth layer‐structured ferroelectric ceramics (BLSFs). Nevertheless, BiFeO₃‐based ceramics have the potentials to achieve much higher piezoelectric coefficients, so their applications can be extended to broader areas including piezoelectric actuators and transducers at middle and high temperatures.