2. Theory and Literature Reviews
2.3 Relaxor ferroelectrics
2.3.4 Lead-free relaxor ferroelectrics
Since Takenaka at el.28) reported the solid solution of BNT and BT system. Lead-free BNT-BT-based system has been researched a lot because of two interesting issues. The first issue is the crystallographic identity and characters of morphotropic phase boundary (MPB) which is shown in 6~7 mol% BT added BNT-BT system, the second issue is the fact that antiferroelectric phase exists as high temperature phase in compositionally broad range from pure BNT to 85BNT-15BT. Presently, the dispute of the issues on BNT-BT is getting settled since BNT-BT system has been turned out as relaxor ferroelectrics29). However, it is still dispute on phase transition behavior near MPB compositions. Therefore, related researches are in active progress through various measurement technologies30, 31).
Fig. 33 Strain measured with an initial unpoled state of BNT-BT and BNT-6BT-2KNN system (b) compared strain curves of 94BNT-6BT below and above TF-R.
The large strain piezoelectric is firstly reported by Zhang et al.32-34) with (94-x)BNT-6BT-xKNN system.
It is observed that a strain value of 0.45% at 2 mol% KNN substitution level, when they studied on strain hysteresis curves with an increase in KNN substitution level for 94BNT-6BT gradually. It is proposed that giant strain is caused by reversible phase transition between antiferroelectrics and ferroelectrics since there was lack of understanding on the mechanism of large strain at that times35). After that, it was found that the large strain is caused by reversible changes between ER and NR36), and it was also revealed NR is the induced from high temperature ER at room temperature by KNN substitution for BNT-BT system 29).
The material state is ER in the temperature range, but it is observed that the shape of strain curves is exactly matched as 92BNT-6BT-2KNN’s curve. It is implied that the large strain induced by substituting BNT for KNN is closely related to the decrease of Td by chemical substitution. It is confirmed that Td
of large strain composition is slightly under room temperature in BNT-BKT-ST system37), which is due to a transition from a relaxor to ferroelectric state. The microstructural deformation of relaxor ferroelectrics are roughly shown in Fig. 33. At high temperature, the BNT-BT-KNN system is a cubic structure and paraelectric state. The properties of ER depend on the correlation between electrical
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dipoles and thermal energy. As the temperature decreases, thermal energy continuously decreases but the density of electrical dipoles is increased because the correlation among electrical dipoles is increased, then polar nano regions (PNRs) arise, which are groups of electrical dipoles aligned in a few nano scale2). As the creation of PNRs begins, the frequency dependence of a dielectric constant also begins to appear, related to the size and distribution of PNRs. Generally, the broader distribution and bigger size of PNRs lead to the larger frequency dependency. As the total size of polar nano regions get over any certain size, the numbers of PNRs which cannot response to specific frequency increase, and consequently the maximum dielectric constant depending on frequency appears. The study of Viehland at el.5, 10) shows that the activation energy of PNRs diverges as it reaches Vogel-Fulcher temperature (TVF) estimated from Vogel-Fulcher equation while they researched on kinetics of PNRs and they named the phase existing under the certain temperature to NR.
Fig. 34 Compared strain of incipient piezoelectricity with commercial PZT (PIC151) and textured KNN38), CaZrO3 modified KNNLT (KNNLT-CZ and KNNLT-5CZ)39), SrTiO3
modifiend BNT (BNT-28ST)40), textured lead-free ternary system of BNT-BKT-BA (BNKT- 22BA).
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Fig. 35 Comparison of Tf and TF-R of PLZT (left) and BNT-6BT (right).
With the increase in temperature, electric-field-induced ferroelectric domain textures become randomized at the depolarization temperature Td, determined from a peak of thermally-induced- depolarization current (TSDC)41). In canonical relaxors such as Pb(Mn1/3Nb2/3)O3 (PMN) 42, 43) and PLZT 29, 44), Td and TF-R have been considered to be the same. Recently, in BNT-based relaxors, it was demonstrated that TF-R does not have to be identical with Td9, 41, 45). It implies that the depolarization and the transition to ergodic relaxor state of electrically-induced ferroelectric state are separate processes.
which seems that different consecutive domain transitions take place at each the Td and TF-R46). It was proposed that this consecutive transition across Td and TF-R be a detexturization of macroscopic poled state and a miniaturization of ferroelectric domains with a long-range order, respectively9).
The Td is related to freezing temperature, but not have to be exactly matched to lead-free relaxors29). However, NR can be changed to a ferroelectric state over certain electric field due to dynamic characteristics of PNRs; however, it returns to initial ER when an application of electric field is removed.
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