Microscopic Insight into Electric Fatigue Resistance and Thermally

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Functional Inorganic Materials and Devices

Microscopic Insight into Electric Fatigue Resistance and Thermally Stable Piezoelectric Properties of (K,Na)NbO3-Based Ceramics Peng Li, Xiaoqiu Chen, Feifei Wang, Bo Shen, JiWei Zhai, Shujun Zhang, and Zhiyong Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08445 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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ACS Applied Materials & Interfaces

Microscopic Insight into Electric Fatigue Resistance and Thermally Stable Piezoelectric Properties of (K,Na)NbO3-Based Ceramics Peng Li,† Xiaoqiu Chen,‡ Feifei Wang,‡ Bo Shen,*,† Jiwei Zhai,*,† Shujun Zhang,*,§ and Zhiyong Zhouǁ †

School of Materials Science and Engineering, Tongji University, Shanghai 201804, China

‡

Key Laboratory of Optoelectronic Material and Device, Department of Physics, Shanghai

Normal University, Shanghai 200234, China §

Institute for Superconducting and Electronic Materials, Australian Institute of Innovative

Materials, University of Wollongong, NSW 2500, Australia ǁ

Key Laboratory of Inorganic Functional Material and Device, Shanghai Institute of Ceramics,

Shanghai 201800, China

ABSTRACT: Pb-based piezoelectric materials such as PZTs have been the mainstay for electromechanical devices, however encounter the challenge of sustainable environmental development, which require the Pb-free piezoelectric counterparts. The foremost obstacles in developing Pb-free piezoceramics are their low piezoresponse and inferior temperature stability. In this work, we reported Mn-modified c-textured (K,Na)NbO3 based ceramics to achieve thermally stable piezoelectric properties and enhanced fatigue resistance in conjunction with high piezoelectricity of d33~560 pC/N. The in situ d33 measurement reveals that the temperature stability of the small signal d33 is mainly depended on the temperature induced phase transition. While the local PFM measurements imply that the excellent temperature stability of the large signal d33* (field-induced strain) benefits from the stable domain response to applied electric field at elevated temperatures. Moreover, the good fatigue resistance is proposed to be associated with the decreased defect concentration by Mn doping, 1 ACS Paragon Plus Environment

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based on the analyses of dielectric loss, leakage current and thermally simulated depolarization current. KEYWORDS: textured ceramics, KNN, fatigue behavior, piezoelectricity, thermal stability, microscopic origin █

INTRODUCTION Noncentrosymmetric crystals have the potential to display piezoelectricity, thus

can produce electric charges under applied mechanical stress, or generate strain under the action of electric field. Piezoelectric ceramics have been used in numerous fields, such as actuators, sensors and transducers, etc.1 Among these existing piezoelectric materials, ferroelectrics with perovskite structure exhibit high piezoelectric properties in comparison with other materials such as bismuth-layered and tungsten bronze ferroelectrics. Among the perovskite ferroelectrics, lead based solid solutions such as Pb(Zr,Ti)O3 (PZT)2 and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT)3-5 have dominated the commercial markets due to their outstanding electro-mechanical properties.6 However, the legislation of Restriction of the use of certain Hazardous Substances in electronic equipment (RoHS) requires the replacement of lead based materials due to their harm to environment and human health.7 To circumvent these issues, the exploration of lead-free piezoelectric materials is imperative. Over the last two decades, there have been extensive studies on lead-free piezoceramics, in which, (K,Na)NbO3 (KNN),8,9 (Ba,Ca)(Ti,Zr)O3 (BCZT),10 BiFeO3,11 and (Bi0.5Na0.5)TiO3 (BNT)12,13 based solid solutions have shown promising properties. To improve the piezoelectric performance, two approaches have been generally 2 ACS Paragon Plus Environment

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accepted: (1) design compositions with morphotropic phase boundary (MPB) or polymorphic phase boundary (PPB); (2) control grain growth along a specific crystallographic orientation. Via constructing MPB or PPB, high piezoelectric properties have been achieved in BCZT and KNN based ceramics. A piezoelectric coefficient d33 exceeding 600 pC/N was reported in BCZT ceramics,10 while d33 higher than 500 pC/N was reported in polycrystalline KNN based ceramics by tailoring the rhombohedral (R)→orthorhombic (O) and orthorhombic→tetragonal (T) transitions to form a R-T polymorphic phase boundary.9,14,15 However, it should be noted that the piezoresponse can’t be fully exploited in randomly oriented polycrystalline ceramics owing to the average effect over all of the possible grain orientations. On the contrary, the best performance can be realized in single crystals and textured ceramics by aligning the crystals in a proper crystallographic direction. In 2004, Saito et al. achieved a high d33 of 416 pC/N in Li, Ta, and Sb modified KNN textured ceramics oriented along [001]c direction.16 More recently, a breakthrough piezoelectric constant, d33, of ~700 pC/N was reported in KNN based textured ceramics based on the [001]c oriented engineered domain configuration.17 Of particular interest is that an ultrahigh electromechanical coupling factor k33 of 95% was reported in (K,Na,Li)(Nb,Ta)O3:Mn single crystal, inherently associated with the engineered domain configuration.18 From practical applications viewpoint, it requires the piezoelectric materials not only possess high piezoelectric properties but also temperature stability over a broad usage range.19 For example, due to the strong curved PPB in KNN-based solid 3 ACS Paragon Plus Environment


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solutions,20 it is difficult to maintain high piezoelectric properties over a broad temperature range. The enhanced temperature stability has been reported by Li et al. in KNN based ceramics by strategies of both widening the phase transition region (constructing diffused PPB)21,22 and Mn doping.23 In addition, introducing the elements with diverse valence states (e.g., Mn, Fe and Ti) into perovskite ferroelectrics can not only tune the “hard” or “soft” behavior but also control the concentration of the defects. Because of the complexity of the cations with variable valence, the impacts of chemical modification on electrical properties in KNN solid solutions have been rarely addressed. In this work, we reported an effective approach to solve the aforementioned challenges by combining the texturing and Mn-doping methods to simultaneously improve the piezoelectric response, thermal stability and fatigue resistance in the KNN-based ceramics. The microscopic origin of both the enhanced thermal stability of piezoresponse and fatigue resistance was deeply uncovered. █

EXPERIMENTAL SECTION

The

x

wt.%

Mn

(x

=

0.1,

0.2,

0.5,

1.0)

modified

0.96(K0.5Na0.5)(Nb0.965Sb0.035)O3-0.01CaZrO3-0.03(Bi0.5K0.5)HfO3 textured ceramics (abbreviated as xMn:KNN-T) were prepared utilizing a template grain growth (TGG) method. The platelet-shaped NaNbO3 microcrystals were selected as templates for texturing process. Mn-modified KNNS-CZ-BKH matrix powders were synthesized by the solid-state reaction method. It should be noted that owing to the Na and Nb elements will diffuse into the matrix lattice during high temperature sintering,24,25 the 4 ACS Paragon Plus Environment

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content of Nb and Na containing in the NaNbO3 templates was eliminated from the precursor KNNS-CZ-BKH during weighing. To prepare tape-casting slurries, Mn-doped KNNS-CZ-BKH matrix powders, 3 mol.% NaNbO3 templates and 20 wt.% binder were mixed in a solvent (50 wt.% alcohol and 50 wt.% toluene). Then, the slurries with good liquidity were casted on glass board to form green tapes. The dried tapes were stacked and then pressed to form dense green bodies. After removing organic binder, the specimens were sintered using a two-step process, where the samples were first heated to a high temperature of 1170 oC, and then they were rapidly cooled to a low temperature of 1070 oC and held for 10 h. The crystal structure was characterized by X-ray diffraction (XRD, D/max 2550V, Rigaku, Japan), and the microstructure was observed using scanning electron microscopy (SEM, HITACHI S-4700, Japan). The ferroelectric domain structures were investigated using transmission electron microscope (TEM, H-800 ELECTRON MICROSCOPE, HITACHI), and piezoresponse force microscopy (PFM, MFP-3D, Asylum Research, USA). After the samples were poled under an electric-field of 30 kV/cm, the longitudinal piezoelectric constant, d33, which is generally defined as the small signal d33, was determined by a quasi-static d33 meter (ZJ-6A, Institute of acoustics, China). The in situ temperature-dependent small signal d33 was measured using a custom-designed high-temperature piezoelectric tester (PEMS-600, Partulab, China). During heating process with a rate of 1 oC/min, a constant force of 0.25 N was applied to the sample, and d33 values were collected every 5 oC. The ex situ temperature-dependent small signal d33 measurement was performed by two steps, i.e., 5 ACS Paragon Plus Environment


the samples were firstly annealed at specific temperatures for 20 min at a heating oven and then tested using a quasi-static d33 meter at room temperature. Polarization hysteresis (P-E), field-induced strain (S-E) and temperature dependence of d33(E) loops were obtained by a ferroelectric test system (aix-ACCT TF Analyzer 2000, Germany). For d33(E) measurement, a trigonometric waveform excitation signal (frequency of 1 Hz, amplitude of 40 kV/cm) with a superimposed small signal AC voltage (frequency of 100 Hz, amplitude of 200 V/mm) was applied to the unpoled sample. The planar electromechanical coupling factor kp was measured using a impedance analyzer (HP 4294A, Agilent, USA). The temperature dependence of dielectric properties was determined by using an LCR meter (E4980A, Agilent, USA) connected to a high temperature furnace. Thermally stimulated depolarization current (TSDC) was measured using a PA meter (Keithley 6517A, Cleveland, OH). █

RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of the polished xMn:KNN-T ceramics. It can

be observed that all samples show single perovskite phase without visible second phase. Obviously, the (001)/(100) and (002)/(200) diffraction peaks become much stronger over 2θ range of 20-60o, indicating that all ceramics have preferred orientation along (00l)c direction. The degree of c texture is determined by Lotgering factor method,26 and the highest texture degree f~97% was achieved in 0.5Mn:KNN-T sample. The high degree of texture benefits from large aspect ratio of NaNbO3 templates (see Supporting Information Figure S1), and their high alignment under shear force during tape casing process. According to the relative ratio of peak 6 ACS Paragon Plus Environment

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intensity of (200) and (002) reflections and temperature dependence of dielectric constant (see Supporting Information Figure S2), the phase structure evolution can be determined to be from major orthorhombic phase with minor rhombohedral for x = 0.1, 0.2 and 0.5 samples to major orthorhombic with minor tetragonal phase for x = 1.0 samples. A slight shift of the diffraction peaks toward higher angle with increasing of Mn concentration was visible indicating lattice contraction. This result can be attributed to that the Mn4+ ions with small radius (RMn4+ = 0.67 Å) were substituted into Nb5+ (RNb5+ = 0.78 Å)27 sites during high temperature sintering. Figure 1b gives the cross-sectional SEM image of

Figure 1. (a) XRD patterns of xMn:KNN-T (x = 0.1, 0.2, 0.5, 1.0) ceramics. The inset shows the texture degree dependence of Mn concentration. (b) Cross-sectional SEM image of 0.5Mn:KNN-T sample. Note that the normal vector of cross section was parallel to the casting direction. (c) Bright field TEM image and (d) out-of-plane (OP) PFM phase image of domain structure for 0.5Mn:KNN-T ceramic. 7 ACS Paragon Plus Environment


0.5Mn:KNN-T ceramic as a representative, where the homogeneous microstructure and c-oriented grain growth are observed. Note that clear trace of liquid phase was present during sintering in the Mn-modified ceramics, which was beneficial for the densification and texture process. The elements distribution and content in the sintered 0.5Mn:KNN-T ceramic are displayed in Figure S3. It can be seen that the component elements were homogeneously distributed in the ceramics. The slightly deviation of the final component from the initial stoichiometry primarily originated from elements evaporation during high temperature sintering process. The representative domain structures of 0.5Mn:KNN-T ceramic are given in Figure 1c and 1d. Both bright field TEM image (Figure 1c) and PFM phase image (Figure 1d) clearly reveal that the laminar 180o domains with width over the range of 50-100 nm presented in the textured ceramics.

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Figure 2. (a) Polarization-electric field (P-E) and bipolar strain-electric field (S-E) curves of the representative 0.5Mn:KNN-T ceramic. (b) The variation of d33, kp and θ with respect to Mn concentration.

The polarization hysteresis (P-E) and bipolar strain (S-E) loops measured at 40 kV/cm and 10 Hz for 0.5Mn:KNN-T ceramic are presented in Figure 2a. The saturated P-E loop with high remnant polarization (Pr~25 µC/cm2) and low coercive field (Ec~10 kV/cm) were achieved. A symmetric butterfly-shaped strain curve with maximum strain Smax~0.23% was observed. Figure 2b depicts the dependence of d33, kp and phase angle θ on the level of Mn doping. The 0.5Mn:KNN-T ceramic displays a d33 of ~560 pC/N (with a fluctuation range of ±2%) and Tc ~253 oC, which are comparable to the state-of-the-art PZT-5H ceramic (d33~590 pC/N, Tc~200 oC). Furthermore, the value of kp, calculated by the resonance (fr) and antiresonance (fa) frequencies (see Supporting Information Figure S4) is up to 73% for the 0.5Mn:KNN-T ceramic. It is believed three primary factors contributed to the 9 ACS Paragon Plus Environment


excellent piezoelectric performance. First, textured grains enable much more efficient alignment of the polar vectors under external field, which will improve the poling efficiency and enhance the piezoelectricity.28 Second, the enhanced piezoelectric response benefit from the inherent piezoelectric anisotropy due to the favored engineered domain morphology in the c textured ceramics with coexisted R and O phases.17,29-31 Lastly, from a microstructural perspective, the nanoscale ferroelectric domains with reduced domain wall energy can facilitate polarization rotation during electrical poling, contributing to the enhanced piezoelectric performance.32,33 The high mobility of nanodomains is more readily to achieve saturated polarization state, which can be confirmed by the high phase angle of ~83o in 0.5Mn:KNN-T ceramic, as shown in Figure 2b. From practical application viewpoint, the reliability and stability of the material properties are essential. Therefore, the fatigue resistance determining service life/reliability and temperature stability determining the operating temperature range were evaluated systematically for xMn:KNN-T (x = 0.1, 0.2, 0.5, 1.0) ceramics in this work. Polarization hysteresis (P-E) loops, unipolar strain curves and related parameter variations with respect to electric field cycling are given in Figure 3a-c. Compared to KNN-3T ceramics without Mn dopant (see Supporting Information Figure S5), 0.5Mn:KNN-T ceramic exhibited almost no degradation behavior after 105 cycles, indicating fatigue free characteristic. It is well known that due to the inevitable volatilization of alkali metal oxides at high-temperature sintering, the alkali metal vacancies (denoted as ܸ஺ᇱ ) and corresponding oxygen vacancies (ܸை∙∙ ) would readily 10 ACS Paragon Plus Environment

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form in KNN based ceramics. The defects (i.e. ܸை∙∙ and ܸ஺ᇱ ) can negatively impact the domain activity thus the main factor affecting the fatigue behavior. The evolution of the defects can be revealed by dielectric loss, leakage current and thermally stimulated depolarization current (TSDC) analyses. Figure 3d gives the temperature dependent dielectric loss for xMn:KNN-T ceramics. The decreased dielectric loss can be observed over the temperature range of -80~150 oC by adding moderate amount (x = 0.2, and 0.5) of Mn. The dielectric loss anomalies, as shown in Fig. 3d, were caused by the R-O phase transition due to the lattice softening and polarization rotation when the ceramics approaching the phase transition temperature. Additionally, the leakage current density vs. electric field curves for x = 0 and x = 0.5 samples are shown in Figure 3e. It is evident that Mn modification reduced the leakage current density. Both decreased dielectric loss and leakage current can be interpreted by two aspects, i.e., reduced defects (ܸை∙∙ and ܸ஺ᇱ ) concentration and inhibited defects migration by Mn doping.34



Figure 3. (a) P-E loops as a function of electric-field cycles for 0.5Mn:KNN-T sample. (b) Pr, Ps and Ec variation dependent electric-field cycles of 0.5Mn:KNN-T ceramic. (c) Unipolar strain curves measured at virgin state and after 105 cycles of 0.5Mn:KNN-T sample. (d) Temperature dependence of the dielectric loss measured at 100 kHz during heating. (e) Leakage current density of xMn:KNN-T (x = 0, 0.5) samples. (f) Temperature dependent thermally stimulated depolarization current of poled samples with x = 0 and 0.5.

These effects can be modulated by Mn through the following two approaches. First, the Mn dopant can compensate the alkali metal vacancies, and the oxygen vacancies can be suppressed by the multivalence nature of Mn cation.35,36 Furthermore, the substitution of Mn4+ (Mn3+, and Mn2+ are also possible) into the Nb5+ would induce ᇱ ᇱ the formation of defect complex (‫݊ܯ‬ே௕ − ܸை∙∙ − ‫݊ܯ‬ே௕ ), which effectively restrain ܸை∙∙

migration and decrease the electrical conductivity.37 For dielectric materials, thermally simulated depolarization current (TSDC) measurement is a powerful tool to gain the defects-associated information.23,38 Therefore, temperature dependence of depolarization current measurement was performed on the DC poled samples, as shown in Figure 3f. Three diffused peaks were evident over the temperature range from RT to 500 oC. The current peaks marked by “Peak A” and “Peak B” are induced by the O-T and T-C phase transitions, respectively, which is well consistent with TO-T and Tc determined in εr-T curves (Supporting Information Figure S2). The relaxation peak denoted by “Peak C” at the high-temperature region is not related to phase 12 ACS Paragon Plus Environment

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transition but only connected to the migration of oxygen vacancies.39,40 It is evident that the current density released in 0.5Mn:KNN-T is much lower than that of 0Mn:KNN-T ceramics. According to the Langevin function, lower values of TSDC density indicate lower concentration of mobile defects.38 The TSDC analysis further confirms that Mn doping reduces the concentration of oxygen vacancies. Considering the domain switching dynamics are closely associated with defects (ܸை∙∙ and ܸ஺ᇱ ), the enhanced fatigue resistance via Mn doping can be interpreted by the domain pinning models, as schematically depicted in Figure 4a. It is evident that ܸை∙∙ have a tendency to cluster at the domain boundaries under repeated electric cycling. These accumulated ܸை∙∙ are supposed to restrict the domain walls motion, and lead to serious fatigue. For the ceramics doped with appropriate content of Mn, the low concentration of ܸை∙∙, as confirmed in Figure 3d-f, significantly weakens the pinning effect and increases flexible mobility of domain walls, leading to the superior fatigue resistance in 0.5Mn:KNN-T ceramics.



Figure 4. (a) The model of defects (dominant ܸை∙∙ ) evolution under cyclic electric field loading in the ceramic. (b) Out-of-plane PFM amplitude image, and (c) PFM phase image and corresponding line scanning patterns across the domains. Note that the scanning areas A (6 × 6 µm2) and C (2 × 2 µm2) were poled at +20 V, while areas B (4 × 4 µm2) and D (0.5 × 0.5 µm2) were poled at -20 V.

In order to further elucidate the mechanism responsible for the fatigue resistance, the local switching characteristic of domains were studied by switching spectroscopy piezoresponse force microscopy (SS-PFM) via cyclic positive and negative electric field loading. To experimentally observe the domain switching behavior under electric field, vertical PFM amplitude and phase images were measured for 0.5Mn:KNN-T ceramic, as shown in Figure 4b and 4c. By applying a +20 V tip bias on the rectangle region A, most domains were switched to the +Ps state, while most domains in the region B were easily switched to the –Ps state when loading a -20 V voltage, as shown in the PFM images and corresponding profiles collected from line scanning across domains. Note that the slight degradation of poled regions (partial back-switching) in phase image can be observed due to asynchronous operation between domain writing and reading. The same operation was performed in the regions C and D, respectively. A clear contrast between the upward and downward polarization after cycling positive and negative bias operations, as shown in the amplitude and phase images, suggests high mobility of the domain walls, which should be responsible for the superior fatigue-resistant behavior observed in the 0.5Mn:KNN-T sample.41 Both the heat generated during service and the change of ambient temperature


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may lead to unstable piezoelectric properties. Notwithstanding extensive experiments have been carried out to study the thermal stability of piezoelectrics, there are still some deficiencies in evaluating the temperature-dependent in situ piezoelectric stability and understanding of the affecting factors. Figure 5a provides the temperature-dependent in situ d33 measurements for the xMn:KNN-T (x = 0.1, 0.2, 0.5, 1.0) samples. For comparison, the ex situ measurements of d33 vs. temperature were given in Figure 5b. Additionally, the d33(E)42 hysteresis loops used to study the temperature dependence of in situ small signal d33 were also shown in Figure S6. These data not only enable one to monitor the real d33 variation over the operation temperatures, but also give the difference among the three characterization methods. Intriguingly, it was observed in Figure 5a that with increasing temperatures, the d33 values first slightly increase and then decrease gradually for all the studied compositions, exhibiting a broad peak value region around TO-T. This feature is consistent with temperature-dependent dielectric constants with a diffused O→T phase transition region. When the temperature deviated from the TO-T, d33 values monotonously decline with temperature. The overall changing trend of d33 can be attributed to the interplay between permittivity εr, which shows peak values at TO-T, and remnant polarization Pr, which gradually decreases with temperature (see Supporting Information Figure S7), based on the relationship d33∝εrPr.43 Note that the small signal d33 variation with temperature determined by in situ d33(E) loops (Figure S6) also follows the similar tendency, but this feature is absent in ex situ d33 vs. temperature tests. It is worth noting that a high d33 over 300 pC/N is remained up 15 ACS Paragon Plus Environment


to 150 oC for 0.5Mn:KNN-T ceramic, which is considered to be enough for most applications. Furthermore, a dramatic change of kp with temperature was also observed around the temperatures of O-T phase transition in the temperature dependent in situ kp measurements (see Supporting Information Figure S8). Hence, the variations of small signal d33 and kp with temperature are closely related to the PPB effect in KNN based ceramics.44 The results of temperature dependence of in situ d33 test provide two potential strategies to boost the temperature stability of piezoelectricity for the KNN-based ceramics characterized by non-vertical PPB: i.e., (1) adjusting the PPB temperature away from room temperature, and (2) broadening the temperature range of polymorphic phase transition. The temperature-dependent unipolar field-induced strain (S-E) curves and normalized large signal piezoelectric coefficient d33* (Smax/Emax) of 0.5Mn:KNN-T ceramic were depicted in Figure 5c and 5d, respectively. The temperature-dependent strain behaviors (in terms of normalized d33*) for several representative ceramic systems are also provided in Figure 5d for comparison. It is noted that the rate of variation of


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Figure 5. (a) The temperature-dependent in situ d33 and (b) temperature-dependent ex situ d33 measurements for xMn:KNN-T (x = 0.1, 0.2, 0.5, 1.0) ceramics. (c) Temperature dependence of unipolar strain of 0.5Mn:KNN-T ceramic at an electric field of 40 kV/cm. (d) Temperature dependence of normalized d33* for 0.5Mn:KNN-T and several typical piezoelectric ceramic systems. Note that the d33* parameters of KNN-LF4, CZ5, BNT-BT-KNN, KNNS-BNKH ceramics are obtained from previous reports,12,14,16,44 and soft PZT-5H ceramics are provided by the supplier.

d33* values for 0.5Mn:KNN-T ceramic is less than ±10% over the temperature range of RT to 150 oC, which is better than other lead-free piezoelectric ceramics and far outperforming the commercial PZT-5H ceramics. The different characteristics of temperature-dependent in situ d33 and d33* in the same composition mean different mechanisms should be responsible for the temperature stability of small signal d33 and electric-field-induced strain. The microscopic domain response to external field is an important attribution of piezoelectricity, namely the macroscopic strain is strongly



affected by both domain switching and the amount of switching domains. Therefore, the microscopic domain response to external field with respect to elevated temperatures may provide clues for understanding the temperature stability of macroscopic strain. To obtain a better insight into the superior strain stability, we performed the in situ PFM measurement to monitor the variations of microscopic domain structure and local piezoelectric response. The microscopic effective piezoelectric coefficient d33,eff, obtained from the amplitude vs. voltage loops at zero electric field, can be used to reveal the variation of macroscopic piezoresponse. To attain d33,eff, a DC bias of amplitude of 10 V with a superimposed AC signal of 4 V was applied to different areas by probe scanning. The typical rectangular phase hysteresis loops and butterfly-shaped amplitude curves captured at three representative temperatures for 0.5Mn:KNN-T sample were shown in the inset of Figure 6. The PFM amplitude images display clear mixed domain pattern independent of temperature, indicating the domain structure is robust at elevated temperatures. The minimal change of domain morphology may be related to the phase transition-induced reorganization of domain structures with increasing temperatures. The d33,eff and phase contrast values relative to their RT values obtained by averaging multiple points were presented in Figure 6. Distinctly, the variations of local effective d33,eff and phase contrast in the 0.5Mn:KNN-T ceramic are within 10% over the studied temperature range, revealing a stable microscopic piezoelectric response with respect to temperature. The microscopic piezoresponse as a function of temperature obtained by PFM measurements was in good agreement with the temperature dependence of d33* 18 ACS Paragon Plus Environment

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indicating qualitative similarity in temperature

Figure 6. Temperature dependence of local effective piezoelectric coefficient d33,eff and phase contrast relative to their RT values measured by in situ PFM. The insets show the PFM amplitude images as well as phase and amplitude vs. DC voltage curves measured at RT, 80 oC and 150 oC, respectively.

stability

of

macroscopic

and

microscopic

piezoelectric

response.

The

temperature-dependent in situ PFM measurements confirm that the stability of piezoelectric strain is associated with the stable microscopic domain response to the external electric field at elevated temperatures. █

CONCLUSIONS

In summary, Mn-modified KNN-based textured ceramics were fabricated by template grain growth method. The structural characterizations by XRD, TEM and PFM reveal that the crystallographic orientation and laminar nanodomains contribute to high 19 ACS Paragon Plus Environment


piezoelectric response. The superior fatigue resistant was interpreted by the decreased concentration of defects (dominated oxygen vacancies) and flexible domain mobility by Mn doping. The small signal d33 and large signal d33* constants show distinct temperature dependent behavior. The temperature stability of d33 is closely associated with the PPB effect, while d33* is dominated by the temperature stability of the microscopic domain response. The microscopic insight into the fatigue behavior and thermally stable piezoelectric properties in KNN-based ceramics can provide guidance for subsequent design of high performance piezoelectric ceramics. █

ASSOCIATED CONTENT

Supporting Information Crystal structure and microstructure of NN templates; temperature dependent dielectric permittivity; impedance spectroscopy; P-E loops as a function of electric-field cycles; temperature-dependent d33(E) and P-E hysteresis loops. █

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] (B.S.) *E-mail: [email protected] (J.W.Z.) *E-mail: [email protected] (S.J.Z.) Notes The authors declare no competing financial interest. █

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support from National Nature Science Foundation of China (Grants No. 51332003, 51372171). S.Z. acknowledges the support of ONRG (N62909-16-1-2126) and ARC (FT140100698). █

REFERENCES

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Microscopic Insight into Electric Fatigue Resistance and Thermally

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