Large electric field-induced strain in AgNbO3-modified 0.76Bi0.5Na0.5TiO3-0.24SrTiO3 lead-free piezoceramics
Introduction
Piezoelectric actuator is an important device that can output precisely controlled displacement during the application of electric field, it is widely used to drive motors in lots of advanced engineering systems, such as fuel injection, ink-jet printers, micromachining metals, and so on [1]. Take fuel injectors as an example, its most crucial properties include strain output, blocking stress, operating temperature, operating frequency, weight and reliability [2], [3], [4], [5], [6], [7]. Up to now, the commercially available materials used for piezoelectric actuator are dominated by lead-based ceramic due to their excellent piezoelectric properties, such as perovskite lead zirconate titanate (Pb(ZrxTi1-x)O3, abbreviated as PZT). However, It is well known that PZT contains high content toxic element Pb, which does harm to the environment as well as human health. Moreover, the Restriction of Hazardous Substances Directive (RoHS) has been widely applied in many countries to restrict the use of hazardous substances including lead in electrical and electronic equipment. Consequently, it is of great importance to develop environmentally friendly lead-free piezoelectric materials [8], [9], [10], [11].
Tremendous efforts have been made to find out the substitution of lead-based piezoceramics over the decades [12], [13], [14], [15], [16]. BNT-based lead-free piezoceramics are among the most promising candidates for the substitution of lead-based piezoceramics. An ultrahigh strain response of 0.45% at 80 kV/cm was observed in incipient piezoceramics 0.92Bi0.5Na0.5TiO3-0.06BaTiO30.02K0.5Na0.5NbO3 (92BNT-6BT-2KNN), which is even higher than the strain obtained with established ferroelectric PZT ceramics [17]. This significant improvement has attracted much attention to BNT-based lead-free piezoceramics. It's evident that the KNN addition into BNT-BT appears to interrupt the long-range ferroelectric order of BNT-BT [18], [19], [20]. Along this line, many similar works have been reported, such as (1-x)[0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3]-xLiNbO3 (BNT-6BT-100xLN), 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3-0.03AgNbO3 (BNT-6BT-3AN), ((Bi1/2(Na0.84K0.16)1/2)0.96Sr0.04)(Ti1−xNbx)O3 (BNT-100xNb), Bi1/2(Na0.82K0.18)1/2Ti1-x(Fe0.5Nb0.5)xO3 (BNKT-100xFN) and so on [21], [22], [23], [24]. It has been demonstrated that this giant strain is attributed to a reversible electric-field-induced phase transition from a nonpolar relaxor state to a polar state with a long-range ferroelectric order [13]. However, this giant strain due to reversible phase transition can only be induced by a quite high electric field, which limits the practical application of these lead-free piezoceramics. Different strategies have been carried out to modify the actuator performance of these lead-free piezoelectrics, which can be classified as composition engineering and structure engineering [25]. It is well known that electromechanical properties can be enhanced around phase instabilities [26]. Two kinds of phase instabilities have been reported in literature i.e. morphotropic phase boundary (MPB) and polymorphic phase boundary (PPB). In former, rhombohedral and tetragonal phases coexist, such as BNT-xBT, BNT-xBKT [27], [28], [29], [30]. Additionally, in later, shifting relaxor-to-ferroelectric phase transition temperature is also able to achieve the same goal [24], [31]. As for structure engineering, textured lead-free piezoceramics present great potential to obtain excellent properties which can be close to a single crystal theoretically [22]. Some other methods to form heterogeneous materials, including relaxor/ferroelectric composites and core-shell structure, were also proved to be effective [32], [33]. However, compared with those complicated structure engineering methods, developing novel lead-free piezoceramics by composition modification is an easier and cost-effective method.
Among BNT-based lead-free piezoceramics, (1-x)(Bi0.5Na0.5)TiO3-xSrTiO3 (BNT-100xST) can obtain high strain response within a low electric field, which was firstly reported by K. Sakata and Y. Masuda [34]. Y. Hiruma et al. investigated BNT-100xST ferroelectric ceramics systematically, it revealed that BNT-100xST forms an MPB of rhombohedral ferroelectric and pseudocubic paraelectric at x = 0.26–0.28, whereas a very large bipolar strain and normalized strain of 0.29% and 488 pm/V were obtained at x = 0.28, respectively [35]. M. Acosta et al. studied temperature- and frequency- dependent properties of BNT-25ST, a high normalized strain of 600 pm/V at 40 kV/cm for frequencies ranging from 0.1 to 100 Hz was observed [36]. Considering the favourable properties of BNT-100xST binary solid solution, some efforts had been made to further improve the actuating performance by forming a ternary solid solution. With the modification of 4 mol.% BT, a high strain of 0.424% at 60 kV/cm in NBT-ST-100xBT was observed by S. Praharaj et al. [37]. A similar work showed BNT-BT-22ST achieved a large strain response of ~ 0.2% (under a moderate electric field of 40 kV/cm) with a normalized strain of 490 pm/V [38]. For BNT-BKT-5ST, a large unipolar strain of 0.36% (Smax/Emax = 600 pm/V) at a driving field of 60 kV/cm was also obtained at room temperature [39].
AgNbO3 (AN) presents orthorhombic phases in rhombic orientation at room temperature and have an extremely large polarization of 52 μC/cm2, which is likely to optimize the BNT-ST phase composition [40]. Moreover, it has been demonstrated that AN doping can improve the actuating performance of BNT-6BT according to our previous work [22]. In this work, AN was chosen to modify BNT-24ST, and the correlation between structures, dielectric, ferroelectric, and piezoelectric properties of AN-modified BNT-24ST was systematically studied.
Section snippets
Experimental procedure
BNT-24ST-based lead-free piezoceramics with nominal compositions of (1-x)(0.76Bi0.5Na0.5TiO3-0.24SrTiO3)-xAgNbO3 (BNT-24ST-100xAN, x = 0, 0.005, 0.01, 0.015, and 0.02) were synthesized by conventional solid-state reaction method. The raw materials Bi2O3 (99.0%), Na2CO3 (99.8%), Ag2CO3 (98.0%), SrCO3 (99.0%), Nb2O5 (99.5%), and TiO2 (99.5%) were weighted according to the nominal compositions after drying at 110 °C for 24 h. The raw powders were ball milled with stabilized zirconia balls in
Phase and microstructure analysis
The XRD patterns of BNT-24ST-100xAN system are plotted in Fig. 1. It can be observed that all samples show typical perovskite structure without any trace of a secondary phase, which demonstrates silver ions (Ag+) and niobium ions (Nb5+) completely diffuse into the BNT-24ST lattice. In order to display the effects of AN addition on BNT-24ST clearly, the diffraction patterns in the range of 45°–48° which refers to (200) peaks are enlarged, as shown in Fig. 1(b). The peak splitting of (200)
Conclusion
In summary, lead-free BNT-24ST-100xAN were prepared by solid-state reaction method, all samples showed pure perovskite phase and dense structures. The AN substitution into BNT-24ST further interrupted the long-range ferroelectric order, resulting in a ferroelectric-to-relaxor phase transition in BNT-24ST-100xAN, as well as the significant decrease of remanent polarization, coercive field, negative strain and piezoelectric coefficient. The origin of this process is suggested to be the decreased
Acknowledgments
The authors acknowledge also the generous support by the National Natural Science Foundation of China under grant no. 51672092 and U1732117. H.Z. thanks the generous support by Basic Research Program of Shenzhen City (Grant No. JCYJ20160414101859817), Natural Science Foundation of Hubei Province of China (Grant No. 2016CFB533) and Wuhan Morning Light Plan of Youth Science and Technology (No. 2017050304010299). The authors also wish to thank the Analytical and Testing Center of Huazhong
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