Nanoscale ferroelectric/relaxor composites: Origin of large strain in lead–free Bi–based incipient piezoelectric ceramics
Introduction
Piezoelectric ceramics play an important role in electronic device applications, especially lead oxide–based piezoelectric ceramics, such as Pb(Zr,Ti)O3, (Pb,La)(Zr,Ti)O3, and Pb(Mg1/3Nb2/3)O3 [1], [2]. On the other hand, these materials contain more than 60 wt.% lead [3], which is toxic and causes damage to the kidneys, brain, and nervous system. The volatilization of lead oxide (PbO) during high–temperature sintering not only causes environmental pollution but also generates instability in the composition and electromechanical properties of the final products. In recent years, for environmental protection and human health, many countries (European Union, Japan, USA, Korea, etc.) have regulated all of their new electronic products to be lead–free [4], [5], [6]. Therefore, there is a need to develop lead–free piezoelectric ceramics that are environmentally–friendly and possess comparable electromechanical properties to those of existing lead–based materials.
Among the various lead–free piezoelectric candidates, binary (Bi,Na)TiO3–(Bi,K)TiO3 (BNKT) solid solutions are considered to be promising owing to their enhanced electromechanical properties near the morphotropic phase boundary (MPB) [7], [8], [9]. Many studies have reported that the electromechanical properties of BNKT at the MPB composition can be improved further by modification with various dopants or modifiers [10]. For example, BNKT modified with BiAlO3 [11], LiSbO3 [12], or BaTiO3 [13] exhibits excellent ferroelectric and piezoelectric properties. A temperature–insensitive electrostrictive coefficient was also found in (K,Na)NbO3 [14]- or BaZrO3 [15]−modified BNKT. Moreover, Nb [16], Sr(K0.25Nb0.75)O3 [17], Sn [18], Nb and (Ba0.7Sr0.3)TiO3 [19] doped/modified BNKT were found to exhibit giant electric–field–induced strain (EFIS).
Ferroelectricity in piezoelectric crystals is characterized by their non centro–symmetric or polar structures. With the application of an electric field, ions undergo asymmetric displacement that results in a small change in the crystal dimensions, which is defined as the EFIS [20], [21]. The mechanism of EFIS in piezoelectric ceramics was first considered in lead–based materials with the composition of (Pb,La)(Zr,Ti,Sn)O3 [22], [23], which showed a large strain due to an electric–field–induced antiferroelectric to ferroelectric phase transition. In lead–free piezoelectric ceramics, a giant EFIS was reported by Zhang et al. in Bi1/2Na1/2TiO3–BaTiO3–K0.5Na0.5NbO3 ceramics [24], [25]. Initially, the origin of this giant strain was believed to be a significant volume change due to a field–induced phase transformation. Whereas, later studies highlighted that the origin of large EFIS in Bi–based piezoceramics was the electric–field–induced reversible phase transformation from polar (or ferroelectric) to nonpolar (or relaxor) phases [16], [17], [18], [19], [26], [27]. Bi–based ceramics exhibiting large EFIS were recently considered to be incipient piezoceramics [27], [28], in which the materials become macroscopically piezoelectric under an applied electric field. Incipient piezoceramics, however, require high electric fields for inducing a phase transformation to obtain the giant strain. This can be ameliorated by using composites consisting of a relaxor (matrix) and a ferroelectric phase (seed). Through this approach, the critical electric field to initiate a giant EFIS was reported to be lowered in some Bi–based composites [29], [30], [31], [32].
For lead–based piezoceramics, enhanced properties near the MPB are generally attributed not only to polarization rotation and extension, [22], [23] but also to the substantial contributions from nanodomains [33]. Analogous results have also been reported in lead–free systems close to the MPB [34] or polymorphic phase transition (PPT) [35] regions. In situ high–energy X–ray [36], [37], [38], transmission electron microscopy (TEM) [30], [33], [35], [39], and piezoresponse force microscopy (PFM) [34], [40], [41] are powerful tools for providing perspicuous picture of the crystal structure and domain morphology, which is helpful for further uncovering the origin of the improved electromechanical properties. In this study, a new mechanism of giant EFIS in a La–doped (Bi0.5Na0.41K0.09)TiO3 ceramic was proposed on the basis of analyzed data from PFM and TEM.
Section snippets
Experimental procedure
Ceramic specimens were prepared via a conventional solid–state reaction route. The starting powders were weighed according to the compositions of [Bi0.5Na0.41K0.09]1−xLaxTiO3 (abbreviated as La100x; x = 0, 0.03, and 0.05). The powder mixture was ball–milled for 24 h in ethanol with zirconia balls as milling media. The slurries were dried and then calcined at 850 °C for 2 h. The calcined powder was mixed with polyvinyl alcohol as a binder and pressed into green discs of 12 mm diameter at 110 MPa.
Results and discussion
Fig. 1 presents FE–SEM images and the relative density of the La100x ceramic (x = 0; 0.03; and 0.05). All specimens showed similar grain morphologies and a dense microstructure, a result that is consistent with the previous reports on related materials [11], [12], [13], [14], [15], [16], [17], [18], [19]. The mean grain size (estimated from the number of grains in an area from the FE–SEM images) increased from 1.0 μm for La0 (Fig. 1(a)) to 1.3 μm for La3 (Fig. 1(b)) and then decreased to 1.2 μm when
Conclusions
La doping in BNKT induced a ferroelectric–to–relaxor phase transition, resulting in a notable strain enhancement just over the transition point at the composition of 3 mol% La. The PFM and TEM studies confirmed that undoped BNKT (La0) was ferroelectric and 5 mol% La–doped BNKT (La5) was a type of relaxor ferroelectric. On the other hand, a nanoscale ferroelectric/relaxor composite was observed in 3 mol% La–doped BNKT (La3), which induced a giant electric–field–induced strain at a relatively low
Acknowledgments
This work was financially supported by the National Research Foundation (NRF) Grant (2013R1A1A2058917) and the Strategic Core Materials Development Program, contract no. G01201311010111, funded by the Ministry of Trade, Industry, and Energy, Republic of Korea.
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