High strain response and low hysteresis in BaZrO3-modified KNN-based lead-free relaxor ceramics

The high driving electric field and the large strain hysteresis are subject to a challenge for piezoelectric actuators’ practical applications. In order to obtain the piezoceramics with giant strain and low hysteresis at small electric field, a ternary solid solution (0.97-x)(K0.48Na0.52)Nb0.965Sb0.035–0.03Bi0.5(K0.18Na0.82)0.5ZrO3-xBaZrO3 (x = 0–0.06) was designed and synthesized by the traditional solid-state reaction method. The relationships among phase transition, microstructure, and electrical properties of the ceramics samples were systemically investigated. Under a low electric field of 4 kV/mm, the ceramic with x = 0.02 obtained a high bipolar strain of 0.29% (Smax/Emax = 729 pm/V) and a low hysteresis of 13.8%. The excellent piezoelectric properties are mainly attributed to rhombohedral–orthorhombic–tetragonal (R–O–T) phase boundary and the relaxor-to-ferroelectric phase transition. We believe that our research can not only provide the pathway of achieving KNN-based ceramics with high strain and low hysteresis but also promote the practical application of lead-free piezoelectric actuators.


Introduction
In order to replace the lead-based piezoceramics, lead-free materials have been extensively explored in actuator applications, and many of remarkable achievements have been realized in the aspect of giant strain [1][2][3]. Nevertheless, the giant strain is usually accompanied by a high driving field, for example, a giant strain of 0.45% is obtained at 8 kV/ mm in the BNT-BT-KNN system [4], which may impede their utilization in practical applications. In recent years, because of their excellent piezoelectric properties, high Curie temperature (T C ) and low driving voltage, (K,Na)NbO 3 (KNN)-based ceramics have the focus of research [5,6], which leads to the discovery of many materials with excellent strain properties under an low electric field [7,8].
At present, many studies have proved that the construction of phase boundaries by chemical modification is an effective strategy to improve the strain performance of KNN-based ceramics [6,9,10]. Wu's group [9] has obtained high strain of 0.34% (at 3.7 kV/mm, 901 pm/V) by the doping of Bi 0.5 Na 0.5-Zr 1-z Hf z O 3 and Sb 5? , which commonly construct rhombohedral-tetragonal (R-T) phase boundary by simultaneously moving the rhombohedralorthorhombic (T R-O ) and orthorhombic-tetragonal (T O-T ) phase transition temperature toward room temperature. And high sensitivity and precision is necessary for actuators devices, so low hysteresis is as important as giant strain obtained at small electric field in actuators' practical applications [11]. Relaxors are important materials for high-precision piezoelectric actuators, and they have quite low hysteresis due to the absence of macro-domains. Zuo et al. and Wang et al. [12][13][14] reported that BaZrO 3 and Sb 5? can lead to obvious dispersion and broadening of dielectric peak in KNN-based ceramics, showing the characteristic of relaxor-like ferroelectrics, resulting in lower hysteresis. Therefore, the purpose of this study is to obtain KNN-based ceramics with larger strain and lower hysteresis at small applied electric field through adding appropriate components.
In this work, a new material system comprising (0.97-x)(K 0.48 Na 0.52 )Nb 0.965 Sb 0.035 -0.03Bi 0.5 (K 0.18-Na 0.82 ) 0.5 ZrO 3 -xBaZrO 3 is designed. By constructing the R-O-T phase boundary, a high strain and low hysteresis are achieved in the ceramics with x = 0.02. Effects of BaZrO 3 on their phase structure, microstructure, and electrical properties are comprehensively investigated. The present study provides a guideline for chemical modification to achieve promising strain behavior and a relatively low hysteresis behavior.

Experimental process
Lead-free (0.97-x)(K 0.48 Na 0.52 )Nb 0.965 Sb 0.035 -0.03Bi 0.5 (K 0.18 Na 0.82 ) 0.5 ZrO 3 -xBaZr O 3 (abbreviated as KNNS-BNKZ-xBZ, x = 0, 0.01, 0.02, 0.03, 0.04, 0.06) ceramics were synthesized by the traditional solid-state reaction method, and using K 2 CO 3 (99.0%), Na 2-CO 3 (99.8%), Nb 2 O 5 (99.5%), Sb 2 O 3 (99.5%), BaCO 3 (99.0%), ZrO 2 (99.0%), and Bi 2 O 3 (99.0%) as the raw materials. Firstly, all raw materials were dried and weighed according to the stoichiometry. Then these powders were ball-milled in ethanol medium for 12 h. The powder mixture was calcined at 850°C for 3 h. Under the same conditions of first ball-milling, the calcined powders were another ball-milling. Subsequently, the powders were mixed with a binder of 5 wt% polyvinyl alcohol (PVA) and pressed into the disks with 13 mm diameter and 1.6 mm thickness. Finally, after burning off the PVA at 600°C for 1 h, the disks were sintered in the temperature range of 1175-1220°C for 3 h in air. For measuring their electrical properties, the two sides of all disks are coated with silver paste, and the disks were fired at 750°C for 30 min.
The phase structure of unpoled sintered ceramics was detected by using X-ray diffraction (XRD, Smartlab 9 kW, Rigaku, Japan) with Cu Ka radiation. The surface microstructure morphology was measured by scanning electron microscopy (SEM, JSM-6510, JEOL, Japan). Dielectric constant curves were obtained by an LCR meter (TZDM-RT-600, Heilongjiang, China), and the ceramic was unpoled. The ferroelectric hysteresis (P-E) loops and strain-electric field (S-E) curves were carried out by utilizing ferroelectric instrument (Precision Premier II, Radiant Technology, USA).
The fitting curve matches the original curve, and it can know that the presence of R, O and T phases. Therefore, the coexistence of R, O and T phase appears in the ceramics with the composition range of 0 B x B 0.02. The temperature-dependent permittivity (e r -T) is studied to further identify the phase transition of ceramics, measured from -100 to 200°C at 10 kHz, as shown in Fig. 2c. As is shown in the figure, both T R-O and T O-T shift simultaneously toward room temperature with the increasing x. Considering the results of both XRD patterns and e r -T curves, the R-O-T multiphase coexistence at x = 0-0.02, the T R-O and T O-T peak disappear for x = 0.04-0.06 and are replaced by T m (temperature corresponding to the maximum dielectric constant (e m )) peak, the ceramics possess a pseudo-cubic phase structure.
The e r -T (30-450°C) of KNNS-BNKZ-xBZ ceramics are shown in Fig. 3, measured at 0.1-100 kHz. It can be seen that T m gradually reduced with the increasing BZ content. And the ceramic of x = 0 appeared a relatively sharp dielectric peak (T m ) around 312.1°C, and T m represent the ferroelectric-paraelectric phase transition. There is no frequency dependence between T m . As the BZ content increased, the dielectric peak becomes progressively broadness. Notably, the frequency dispersion becomes more apparent, e m decreases significantly and T m moves to a higher temperature as the frequency increases. This phenomenon can also be found in another systems modified by other additives, such as (Bi 0.5 Na 0.5 )ZrO 3 [16], BaZrO 3 [17] and Sb 5? [12]. The diffuseness of ferroelectric materials is usually quantified by utilizing the modified Curie-Weiss law [18]: where C is the Curie-Weiss constant, and c is the diffuseness degree ranging from 1 to 2, c = 1 represent classical ferroelectric, and c = 2 represent ideal relaxor. The plots of ln(1/e r -1/e m ) versus ln(T -T m ) curves at 10 kHz are also shown in Fig. 3g [19,20]: At x = 0, 0.01, DT relax = 0°C. With increasing x, relaxation showed a monotonously increasing trend, the relaxation degree of ceramics at x = 0.02, 0.03, 0.04, 0.06 is 5.9°C, 10°C, 13°C, 13.9°C, respectively. Above results suggest that KNNS-BNKZ-xBZ ceramics exhibit relaxation-like characteristic, the dielectric properties not only show diffuse phase transition behavior but also a slight frequency dispersion at x C 0.02, and c values is closer to 2 at x = 0.06, as the BZ content increased, the relaxation characteristic of ceramics is more obvious, and the ceramics is closer to the ideal relaxor. Therefore, the addition of BZ can induce an obvious ferroelectricrelaxor transformation.
To analyze the evolution of microstructure, the SEM surface morphologies patterns of KNNS-BNKZ- xBZ ceramics are shown in Fig. 4. As is shown in the picture, all of the ceramics exhibit an inhomogeneous and dense microstructure (density [ 96%) with the rectangular grain. The grain size distribution, average grain size and relative density of the KNNS-BNKZ-xBZ ceramics are plotted in Fig. 5. For the ceramics with x = 0, the grain size of is mainly concentrated in 0.7-1.3 lm, and the grain size of ceramics with x = 0.01-0.03 is concentrated in 0.75-1.65 lm, and the grain size of ceramics with x = 0.04 is concentrated in 0.7-1.3 lm, and the grain size of ceramics with x = 0.06 is concentrated in 0.45-0.95 lm. Overall, their average grain sizes first rise and then drop with increasing BZ contents. At x = 0.06, the average grain size of the ceramics decreases to approximately 0.72 lm, indicating that excessive BZ can inhibit the growth of grains. At present, it has been reported that excessive Ba 2? and Zr 4? are not uniformly aggregated in the grain boundary, which will hinder the growth of grains [21][22][23].
The ferroelectric hysteresis (P-E) loops of KNNS-BNKZ-xBZ ceramics are plotted in Fig. 6a, measured at 1 Hz and room temperature. For the ceramics with x = 0, a typical and saturated ferroelectric hysteresis loop is observed. As the BZ content increased, the loops become much slender, especially for the sample of x = 0.06. In order to further analyze the influence of BZ content (x) on the ferroelectric properties of ceramics, the maximum polarization (P max ), remanent polarization (P r ), and coercive field (E c ) as functions of x are shown in Fig. 6b. With the increase of x, P max , P r and E c showed a downward trend on   [24]. It is believed that the properties of relaxors are closely related to the existence of polar nanoregions (PNRs) [25]. At x = 0.06, the ceramics are relaxors, when the applied electric field is removed, the back-switch behavior of nano-domain is easier than that of micron-domain, which result in a decrease in the amount of domains with similar or same direction relative to the applied electric field. Hence, the value of P r decreased markedly for x = 0.06. Similarly, the easier domains switching leads to smaller E c value, which will also be discussed later. Meanwhile, the corresponding current-electric field (I-E) hysteresis loops can also be obtained, as shown in Fig. 6c. The current peaks around E c suggest typical ferroelectric characteristic, which caused by ferroelectric domain switching [26]. The current peak value decreases with the increase of BZ content, which is consistent with the changing trend of the polarization electric field curve of ceramics, which indicates that the ferroelectric property of ceramics is declining. This is mainly originated from the introduction of BZ destroys the long-range ferroelectric order, and the ceramics are gradually transformed into relaxor [14]. It is of note that the current peak moved to the left, which is caused by easier domain switching and domain wall motion [27]. Figure 7 presents the bipolar strain curve (S-E) of KNNS-BNKZ-xBZ ceramics, measured at 1 Hz and room temperature. We can see from Fig. 7 that S-E curve is not a typical symmetrical curve, which results from the large internal bias field in ceramics [28,29]. The variation of S max with x is shown in Fig. 7b. As x increased, S max first increased and then decreased. The value of S max are maximized at x = 0.02, and is 0.29%, and its corresponding normalized strain value (d 33 * ) is 729 pm/V. The free energy difference between ferroelectric states in the multiphase coexistence region is small, which result in the low degree of polarization anisotropy energy, and the polarization vector is more easily switch to the electric-field-favored polar direction under an applied electric field, which enhances the piezoelectric property of ceramics and enlarges the strain [24,30,31]. The ceramic with x = 0.02 exist between ferroelectric and relaxor states, and the long-range ordered ferroelectric domains are destroyed, leading to the formation of micro-domains or PNRs, which can further promote the polarization rotation and improve the piezoelectric properties [32][33][34][35]. Hence, the giant strain is attributed to the R-O-T multiphase region (which mainly originated from the converse piezoelectric response) and the electric-field-induced phase transition from a ferroelectric to relaxor. The S max is the minimum at x = 0.06, and S neg is zero, indicating that the electrostriction is the predominant contribution in the strain [16,36,37]. Figure 8a shows the unipolar S-E curves of KNNS-BNKZ-xBZ ceramics, and its variation trend is consistent with bipolar strain. The maximum value of unipolar strain (S max ) is maximized at x = 0.02, and is 0.239%, and its corresponding d 33 * value is 598 pm/ V. The remanent strain (S rem ) of ceramics with x = 0, 0.02, and 0.06 is shown in the insets of Fig. 8a, and S rem is caused by irreversible non-180°domain switching. At x = 0, 0.02, S rem can be clearly observed. However, S rem can almost not found in the composition with x = 0.06, indicating that its strain is original from the electrostriction effect [36][37][38]. This confirmed the previous results. In order to evaluate the electrostriction effect, we plot strain (S) as a function of polarization (P) of KNNS-BNKZ-0.06BZ according to the following equation [39]: where Q 33 is the electrostriction coefficient, and P is the polarization. As shown in Fig. 8b, the Q 33 value can be determined by the quadratic coefficient of the fitting line, and it is 0.019 m 4 /C 2 . To meet the low hysteresis demand of practical actuator application, the hysteresis curve of all ceramic samples is shown in Fig. 9. The insets of Fig. 9 present the calculation method of hysteresis. And the hysteresis is 13.8% at x = 0.02. The S max , its corresponding d 33 * , and H ys. of KNNS-BNKZ-0.02BZ and other reported BNT-based and KNN-based ceramics were summarized in Table 1 [28,[40][41][42][43][44][45][46]. Although BNT-based ceramics have usually a large strain, their hysteresis is high ([ 20%), e.g., BNKT-SBTZ6 and BNT-2.5Nb. Instead, a low hysteresis and small strain (\ 0.2%) can be obtained in KNN-based ceramics. In this study, a high strain response of 0.29% and low hysteresis of 13.8% have simultaneously obtained. Generally, hysteresis is the contributions from non-180°domain switching. Due to microdomains or PNRs instead of the macroscopic domains, which respond to the external field much faster than that of macroscopic domains [25]. So the low hysteresis with KNNS-BNKZ-0.02BZ is the existence of micro-domains.

Conclusions
The KNNS-BNKZ-xBZ (x = 0-0.06) ceramics were synthesized by the traditional solid-state reaction method. The influence of BZ content on the phase structure and resulting electrical properties were investigated in detail. Through the design and  optimization of components, a high strain of 0.29% is obtained at R-O-T multiphase coexistence boundary. Simultaneously, the KNNS-BNKZ-0.02BZ ceramics that exhibit a relatively low hysteresis of 13.8% were achieved, which mainly benefit from relaxor behavior. We believe that it will undoubtedly be potential application in actuators.