Ferroelectric/piezoelectric relaxor ferroelectrics

In such a measurement, the sample is clamped as lightly as possible, and the displacement of the surface in monitored. The amount of sample clamping is important, because the mechanical constraints can impact the ferroelastic response of the sample. That is, in samples where the mechanical coercive stress is low, it is possible to change the domain state of the material by improperly clamping it in the sample fixture. This is especially important in elastically soft piezoelectrics, such as many of the relaxor ferroelectric PbTiC>3 single crystals. 

Park, S.-E. and Shrout, T.R. (1997) Characteristics of relaxor-based piezoelectric single crystals for ultrasonic transducers, IEEE Trans. Ultrasound, Ferroelectrics and Frequency Control, 44, 1140-7. 

Another field of intensive research is the insulating perovskite alloys with exceptional dielectric and piezoelectric properties [74], like the so-called relaxor ferroelectric alloys PZT (PbZrxTii-xOs), PZN-PT (Pb(Zni/3Nb2/3)03-... 

Finally, it is worth mentioning that a phenomenon analogous to the difference between the normal and giant flexoelectricity of calamitic and bent-core nematics, respectively, exists in crystals, ceramics and polymers too. The flexoelectric response (defined in Eq. (3.1)) of perovskite-type ferroelectrics, " of relaxor ferroelectric ceramics and polyvinylidene fluoride (PVDF) films are four orders of magnitude larger than the flexoelectricity of dielectric crystals. In those sohd ferroelectric materials the polarization induced by flexing is evidently of piezoelectric origin. 

Park S, Shrout T (1997) Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals. J Appl Phys 82 1804... 

Relaxor-type electrostrictive materials, such as those from the lead magnesium niobate-lead titanate, Pb(Mgp 3Nb2/3)03-PbTi03 (or PMN-PT), solid solution are highly suitable for actuator applications. This relaxor ferroelectric also exhibits an induced piezoelectric effect. That is, the electromechanical coupling factor kt varies with the applied DC bias field. As the DC bias field increases, the coupling increases and saturates. Since this behavior is reproducible, these materials can be applied as ultrasonic transducers which are tunable by the bias field [12]. 

If all the coefficients of equation (2) are known, one can accurately predict the longitudinal strain under a varying electric field for a given piezoelectric or electrostrictive material, and even for a material exhibiting both piezoelectric and electrostrictive effects, such as irreversible electrostrictive materials. For ideal reversible electrostrictive materials, which possess no remnant polarization at zero electric field, the odd power term of the electric field in equation (2) vanishes. However, we will consider the relaxor PLZT ceramics studied in this chapter as irreversible electrostrictives, to account for any ferroelectric behaviour under dc bias fields, and we will therefore include both terms of the electric field in equation (2). 

Figure 7 shows the ac strain amplitude versus dc bias fields, measured up to the fourth harmonic, with a driving 0.37 MV.m-i peak-to-peak ac field at 120 Hz for PLZT (9.5/65/35). In this case, the first harmonic piezoelectric strain is dominant and seems to increase with the dc bias field until a maximum is reached at 1.2 MV.m-i dc. The theoretical curve seen in this figure is the result of fitting the data collected at 120 Hz to the first harmonic term in equation (4), while fixing the ac field value. This general behaviour of relaxor ferroelectrics has been previously observed for PMN electrostrictive ceramics (Masys et al. 2003). 

The increased strain with increasing dc bias in figure 7 can be explained by previous dielectric measurements of PLZT (9.0/65/35) ceramics as a function of both tempierature and dc bias (Bobnar et al. 1999) They have observed a sharp increase in the dielectric permittivity with increasing dc bias fields at temperatures dose to the ferroelectric-relaxor phase transition, indicating that the dc bias is inducing the creation of electric dipoles at this transition, and hence increasing the overall piezoelectric response. 

Next, the ac strain amplitude was plotted as a function of dc bias fields for various driving ac fields, as seen in figure 8. The first-harmonic piezoelectric strain increased with both the ac and dc fields until a maximum of approximately 0.8 x lO m.m-i occurred at 1.1 MV.m-i dc and 1.09 MV.m-i ac peak-to-peak. These results once again confirm studies of the dielectric behaviour of PLZT (9.0/65/35) (Bobnar et al. 1999), in which above a critical field, called Ec, a phase transition from relaxor to ferroelectric occurs in the PLZT structure. It is perhaps the presence of both phases simultaneously that give rise to the piezoelectric strain further increasing of the fields would just render the samples more and more ferroelectric, therefore decreasing the strain. 

Newnham RE, Sundar V, Yimnirun R, Su J, Zhang QM (1997) Electrostriction nonlinear electromechanical coupling in solid dielectrics. J Phys Chem 101 10141-10150 Ploss B, Ploss B, Shin FG, Chan HLW, Choy CL (2000) Pyroelectric or piezoelectric compensated ferroelectric composites. Appl Phys Lett 76 2776-2778 Poddar S, Ducharme S (2013) Measurement of the flexoelectric response in ferroelectric and relaxor polymer thin films. Appl Phys Lett 103 202901... 

After the discovery of significant piezoelectricity in polymers in the late 60s, they were immediately considered for applications in the early 70s. Interest faded in the early 80s due to problems in producing reproducible devices. After solving these initial problems by developing efficient poling procedures and suitable electrical contacting methods, piezoelectric polymers became a commercial success in many niche applications. Science progresses in waves, and currently, we face a very active research phase where piezoelectric and relaxor ferroelectric polymers are employed... 

---