Lead-free piezoelectrics

The known lead-free piezoelectric materials can be categorized into three broad families (i) perovskite titanates such as barium titanate and bismuth 

The in situ domain observations and Raman studies on single-crystalline BTO ceramics showed that these structural phase transitions can also be induced by an applied electric field [9]. It has been found that the room-temperature tetragonal structure can transform to a monoclinic phase under an applied electric field amplitude of lOkV/cm and then subsequently to a rhombohedral phase at an amplitude of 30kV/cm [9j. Enhanced piezoelectric coefficient 33 of approximately 295 pC/N is reported for such a monoclinic BTO phase. However, the poor piezoelectric properties ( 33 190 pC/N), the structural phase transformation near room temperature, and relatively low Curie temperature Tq 120°C) of BTO ceramics restricted their wider usage 

Certain ternary solid solutions containing BTO as one of their major phase components show excellent piezoelectric properties in the vicinity of MPB. They have been considered as a potential replacement for existing lead-based piezoelectric perovskite, PbZrOs-PbTiOs, PMT-PT, and so on. In fact, the extremely high piezoelectric response of PZT and PMN-PT has been partially attributed to the presence of a MPB between rhombohedral and tetragonal phases at that concentration [12]. Therefore, the presence of a MPB is considered as a necessary component of a lead-free replacement material due of the enhancement in properties observed for compositions at or near MPB. The basic approach to achieve high piezoelectricity is to choose the composition of the material at the proximity of a MPB of the phase diagram. The strong piezoelectric and dielectric responses at the vicinity of MPB have been ascribed to the low polarization anisotropy (rotation of FE polarization between two equivalent states) and elastic softness at MPB. 

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Kimura T, Yi Y, Sakurai F (2010) Mechanisms of texture development in lead-free piezoelectric ceramics with perovskite stmcture made by the templated grain growth process. Materials 3 4965-4978... 

Tani, T. and Kimura, T. (2006) Reactive-templated grain growth processing for lead free piezoelectric ceramics. Adv. Appl. Ceram., 105 (1), 55-63. 

Takenaka, T., and Nagata, H. 2005. Current status and prospects of lead-free piezoelectric ceramics. Journal of the European Ceramic Society 25, pp 2693-2700. 

T. Takenaka, K. Maruyama andK. Sakata, (Bil/2Nal/2)Ti03-BaTi03 system for lead-free piezoelectric ceramics , Jpn. J. Appl. Phys., 30 [93] Part 1, No. 9B, 2236-2239 (1991). 

KNbOi (LB Number 1A-2). This crystal is ferroelectric below about 418 °C. Further phase transitions take place at about 225 °C and about — 10°C, retaining ferroelectric activity. The crystal has large electromechanical coupling constants and is useful in lead-free piezoelectric elements and SAW (surface acoustic wave) filters in communications technology (Fig. 4.5-13,4.5-14). 

Hosono, Y., Harada, K., and Yamashita, Y. (2001) Crystal growth and electrical properties of lead-free piezoelectric material. Jpn. J. Appl. Phys., 40, 5722-5726. 

Lead-free piezoelectric ceramics of (Bil4Na%)Ti03-/4(Bi203 SC2O3) system. Jpn.J. Appl. Phys., 36, 6055-6057. 

In addition to BZT-xBCT, the other system that has been studied extensively by the researchers for both fundamental understanding and applications is sodiimi bismuth titanate, Nao.5Bio.5Ti03 (NBT). NBT and its solid solutions have been extensively studied because of their exceptionally high electrostrain [20,21]. In addition, NBT coidd be a potential candidate in the search for lead-free piezoelectric materials. From the structural aspects, NBT is... 

Figure 10.5 shows a schematic of a simple experiment that demonstrates the response of NBT ceramics for external mechanical stimuli. A fiber-reinforced composite sheet was fixed at one edge to act like a cantilever sheet An actuator whose frequency and amplitude could be controlled externally was fixed imder-neath the sheet and with no additional power supply the NBT ceramic was fixed on the sheet. The effective response of the NBT ceramic pellets connected to an oscilloscope is seen in Figure 10.5b. The amplitude of the signal was on the order of 3-5 mV, which could be conditioned, amplified, and further utilized later for various apphcations. Thus, NBT ceramics exhibit promising features in the search for lead-free piezoelectrics.

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