Pairing High Piezoelectric Coefficients, d33, with High Curie

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Article 33

Pairing high piezoelectric coefficient d with high Curie temperature (T ) in lead-free (K,Na)NbO C

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Muhammad Asif Rafiq, Maria Elisabete V. Costa, and Paula Maria Vilarinho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08199 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 8, 2016

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ACS Applied Materials & Interfaces

Pairing High Piezoelectric Coefficient d33 with High Curie Temperature (TC) in LeadFree (K,Na)NbO3 Muhammad Asif Rafiq#, Maria Elisabete Costa, Paula Maria Vilarinho*

Department of Materials and Ceramic Engineering, CICECO – Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal

*Corresponding author: [email protected] #Current affiliation: Department of Metallurgical and Materials Engineering, University of Engineering and Technology, 54890, Lahore, Punjab, Pakistan

Authors Information: Muhammad Asif Rafiq -


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Current affiliation: Department of Metallurgical and Materials Engineering, University of Engineering and Technology, Lahore, Punjab, Pakistan

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Email: [email protected]

Maria Elisabete Costa -


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Paula Maria Vilarinho -


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Corresponding author

ACS Paragon Plus Environment

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Abstract The largest piezoelectric properties, d33=416pC/N and 490pC/N in KxNa1-xNbO3 ceramics have been reported for compositions close to polymorphic phase transition (PPT), however with Curie Temperature, TC, around 217–304ºC considerably lower than undoped KNN ceramics (420ºC). High d33 along with high TC remains the ideal choice for applications but unfortunately not attained up–to-now. Here we show that using KNN single-crystals as seeds to template grain growth (TGG) of KNN ceramics enables dramatic improvements in the

electromechanical

properties

while

maintaining

a

high

TC.

(001)–oriented

(K0.5Na0.5)0.98Li0.02NbO3 ceramics engineered by TGG using (K0.5Na0.5)NbO3 crystals as templates exhibit a high d33 of 280pC/N while maintaining the high TC of 430ºC. Enhanced piezoelectricity is attributed to long-range ordered ferroelectric domain patterns consisting of 90º and 180º domains, similar to single crystals. It is the first time that pairing high d33 and high TC in KNN, keeping a high PPT temperature, is achieved. This study is an unequivocal proof that it is possible to maximize d33 keeping a high TC in KNN without resorting to heavily doped compositions. This work opens the door to high-performance, rare-earth free, compositionally simple lead-free and low-cost electromechanical compounds, which can largely expand lead-free piezoelectrics applications.

Key words: lead-free, piezoelectrics, ferroelectrics, potassium sodium niobate, template grain growth, single crystals, textured ceramics

Introduction There is a recent article by Wang et al. (2014) [1] that claims a giant piezoelectricity in potassium sodium niobate based lead-free ceramics (KxNa1-xNbO3 (KNN)), i.e. d33 of ~ 490 pC/N measured at room temperature in the highly compositional complex system (1x)(K1−yNay)(Nb1−zSbz)O3−xBi0.5 (Na1−wKw)0.5ZrO3](0 ≤ x ≤ 0.05, 0.40 ≤ y ≤ 0.68, 0 ≤ z ≤ 0.08, and 0 ≤ w ≤ 1) which Curie temperature (TC) is 227 ºC. This is an important result because it confirms that lead free ceramics can reach a piezo-response similar to that of PZT. The relevance of this work is in line with the previous study of Saito et al. (2004) [2] where an improved value of d33 ~ 416 pC/N followed by a TC of 253 ºC is obtained as a result of KNN compositionally complex multiple doping with Li1+, Ta5+ and Sb5. In both works (Saito et al., 2004 and Wang et al., 2014) [1, 2], the polymorphic phase transition (PPT) between


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orthorhombic and tetragonal structures in the first study (Saito et al.) [2] and rhombohedral and tetragonal in the later one (Wang et al.) [1], occurs close to room temperature. In addition, both of these remarkable d33 results imply a dropping of TC to almost 200 ºC below the value of undoped KNN ceramics and single crystals (TC = 420 ºC), due to the complex doping of these compositions [3, 4]. By far the most successful piezoelectric on the market belong to the solid solution PbZrO3-PbTiO3 (commonly designated as PZT). There are however other lead based materials such as Pb(Zn1/3Nb2/3)-PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)-PbTiO3 (PMN-PT) with giant electromechanical response that are commercially important as well [5]. The high electromechanical properties found in PZT compositions, close to the morphotrophic phase boundary (MPB) where x=0.48, are the main reason for the wide use of PZT. Experiments have shown that PZT near MPB composition contains two crystallographic phases, a rhombohedral and a monoclinic one. The piezoelectric response of MPB PZT can be divided in two parts: (i) the extrinsic part, which includes changes in the fractions of the two coexisting phases and domain configuration necessitating domain wall motion, and (ii) the intrinsic part, which is largely determined by the changes within a domain. Recent insitu uniaxial compression neutron diffraction experiments showed the importance of the extrinsic contribution, whereas the hybridization between Pb 6s and O2p states largely explains why the intrinsic piezoelectric response of Pb-based perovskites is much larger than in the case of closed-shell A cations, such as Ba-based perovskites. Near the phase boundaries, the intrinsic part is further enhanced by the phase instability: the increase of certain elastic compliance tensor components is accompanied by the increase of the corresponding piezoelectric tensor ones [6]. Additionally, the high TC results in excellent thermal stability of PZT and allows its use at operational temperatures of about 175º C. In spite of its exceptional electromechanical properties there are currently requirements that PZT cannot meet and are one of the major challenges faced by the electroceramics industry: need of sustainable materials and manufacturing routes, on one hand; lead-free alternatives compatible with the current most restrict environment directives have to be developed, on the other hand; and last, but not the least, elements as rare earth which are normally used to enhance electrical properties of lead based electroceramics need to be avoided or replaced. From the point of view of applications, besides the maximised electromechanical properties, a piezoelectric should have a TC as high as possible. As a rule of thumb, piezoelectric materials must be safely used to approximately 1/2 TC without significant reduction in its piezoelectric activity. Indeed many applications require piezoelectrics having


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stable piezoelectric response over a wide temperature range, as for example in process and plants control, power generation, automotive and aerospace industry. Furthermore, phase boundaries close to room temperature are undesired, as thermal cycling between ferroelectric phases leads to domain instability, depolarization, and eventually to mechanical failure [7]. In lieu of this it is critical to develop KNN-based ceramics that possess high electromechanical properties together with phase transitions as much as possible far from room temperature and high TC. Undoped KNN undergoes a phase transition from orthorhombic or monoclinic [8] at room temperature to tetragonal around 220 ºC, designated the polymorphic phase transition (PPT) and then to cubic near 420 ºC (TC). As referred the giant d33 reported for KNN compositions [Saito et al and Wang et al] [1, 2] besides being both coupled to low values of TC the phase boundaries lye close to the room temperature. Pairing high piezoelectric coefficients and high TC in electroceramics is challenging and not yet attained in KNN or other lead free piezoceramics, mainly due to the fact that up to now optimization of properties has mostly relied on doping strategies. It is known that the piezoelectric coefficient (d33) depends on the relative permittivity and remnant polarization according to [8, 9]

= 2

Equation 1

where stands for the remnant polarization, for the electrostrictive constant of the paraelectric phase which normally varies between 0.05 and 0.1 m4C-2 for different materials, and are the permittivity of free space and of the material, respectively [10]. Permittivity and remanent polarization can be both optimized by doping and controlling the microstructure, which in turn increase d33. Generally in ferroelectrics doping affects directly TC, being even a way of tuning it. The emblematic classical example is the dependence of TC of BaTiO3 on the dopant content and microstructure [11]. Contrarily, texturing or grain orientation is a way of controlling microstructure and electrical properties without doping not affecting TC or phase transition temperatures [12, 13]. Template Grain Growth (TGG), in which a preferential oriented growth of large template grains takes place by consuming the matrix grains during the densification process, is one of the promising routes to control the microstructure and texture of electroceramics. Some of the examples of oriented ceramics with optimised properties are Bi4Ti3O12, Sr2Nb2O7, CaBi4Ti4O15, Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), Sr0.53Ba0.47Nb2O6, SrBi2Ta2O9 and (Na1/2Bi1/2)TiO3 - BaTiO3 [12, 14].


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In KNN based ceramics, texturing has been mainly done by combining tape casting techniques with various particle morphologies of NaNbO3 templates although plate-like BaTiO3 templates have also been reported. In general, properties improved and piezoelectric constant increased to more than 300 pC/N, but in all cases either TC is low or PPT temperature becomes close to room temperature turning. Table 1 shows the properties of textured KNN (undoped and doped) ceramics reported so far. In this study we exploit the development of KNN-based ceramics with high electromechanical properties together with phase transitions as much as possible far from room temperature and high TC via texturing KNN ceramics with KNN single crystals seeds. We do not rely on heavily doped KNN compositions to pair high piezoelectric coefficient d33 with high Curie temperature (TC) in lead-free KNN. (001) oriented (K0.5Na0.5)0.98Li0.02NbO3 (KNNL) ceramics were prepared by using single crystal templates. (001)-oriented KNN single crystals having monoclinic phase at room temperature with improved properties as compared to those of bulk counterparts were prepared by a high temperature flux method. The detailed production method and properties have already been reported in some of our previous work [15-17]. The use of single crystals as templates for TGG process has been reported for other materials like SrBi2Ta2O9 [14], but up to now, to the best of our knowledge, not reported in KNN ceramics. The dielectric, ferroelectric and piezoelectric properties were measured and the relationship between domain patterns and electric properties established.

Experimental High purity starting reagents of K2CO3 (Merck, ≥99%), Na2CO3 (Chempur, ≥ 99.5%), Nb2O5 (Chempur, 99.9%), Li2CO2 (Merck, 99%) and B2O3 (Merck, 95%) were used to prepare (K0.5Na0.5)NbO3 single crystals and (K0.5Na0.5)0.98Li0.02NbO3 (KNNL) non-templated and templated ceramics. The details of KNN single crystals growth were presented somewhere else [15]. KNNL powders were first synthesized by a conventional solid-state reaction. After being dried at 230 °C for 5 h, the starting reagents were weighed according to the proper stoichiometric ratio and then mixed by ball-milling in teflon pots with ethanol for 5 h using zirconia balls. Then the mixture was calcined at 800 °C for 2 h to obtain the perovskite phase. After calcination the powders were milled again for 5 h and dried. At this stage10 wt % of single crystal templates (crushed single crystals with sizes < 45 µm) were mixed with the powder in ethanol by ultrasonic mixing for 2 h to obtain templated ceramics. Then pellets both non-templated and templated were pressed by uniaxial pressure at 170 MPa followed by a cold isostatic pressing at 200 MPa. Uniaxial pressing is able to induce


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preferential orientation of the platelet templates inside the matrix in such a way that the template major faces become perpendicularly positioned to the uniaxial pressing direction. This orientation effect of uniaxial pressing is well known and reported in the literature [18]. Uniaxial pressing ensures a reproducible condition of initial orientation state of the template single crystal. Sintering was carried out at 1100 ºC regardless their template condition. The sintering time was varied between 2 to 24 h to optimise the texturization of the microstructure. The crystalline phases were evaluated by X-ray diffraction (XRD, Rigaku, D/Max-B, Cu-Kα radiation) at room temperature in the 20° to 60° 2θ range with a step length of 0.02°. The texture fraction of (001) planes in the obtained ceramics was estimated from the XRD patterns in terms of the Lotgering factor. The Lotgering factor (f) is defined as the fraction of the area oriented with the crystallographic plane of interest as:

=

− 1 −

Equation 2

/ ∑ , being and the intensities of (hkl) where, = ∑ / ∑ and = ∑

peaks for the oriented and randomly oriented samples, respectively. Rocking curves and pole figures were used to assess the quality of the orientation of the textured ceramics. Rocking curves are often used to reveal the orientation of planes with an orientation close to the parallel surface of the sample. For a fixed - 2 position, plane positioned at different tilt position with are brought to diffraction by varying tilt. In pole figures, the intensity of a given XRD peak is measured as the sample rotates about two orthogonal axes, thus collecting the sum of the lattice plane reflection signals from a large number of crystallites in a polycrystalline material. The principle, to determine the orientation of a given lattice plane, (hkl), is as follows: the detector is first set to the proper Bragg angle, 2θ, of the diffraction peak of interest and then the sample is rotated in a goniometer until the lattice plane hkl is in the reflection condition, i.e. the normal to the lattice plane is the bisectrix between incident and diffracted beam. The microstructure was assessed by Scanning electron microscopy (SEM / EDS, Hitachi S-4100). Using ImageJ software, the grain size distribution of the sintered samples was established. The area of the section of the grains was measured and then its equivalent diameter calculated. The average grain size, G, was determined from the average equivalent diameter, by using a multiplying factor of 1.22 [3].


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For the electrical characterization, the faces of the ceramics, perpendicular (⊥) and parallel (∥) with reference to the pressing direction (a schematic is shown in Figure 1) were polished and then coated with silver paint to be used as top and bottom electrodes. We probed their dielectric and ferroelectric properties using metal – oxide - metal capacitor structures. The relative permittivity (εr) was measured with a Precision LCR Meter (HP 4284A) as a function of the frequency and temperature from 100 Hz to 1 MHz and from room temperature to 550 ºC, respectively. For the determination of the polarization versus the electric field hysteresis loop (P-E), a ferroelectric tester (aixACCT TF analyzer 1000) was used. The piezoelectric constant d33 was assessed at room temperature by a Berlincourt piezoelectric meter, Sinocera YE 2730A, that operates at 110 Hz. Specimens to be tested were placed between the two probes (top and bottom probe) and the direct piezoelectric coefficient was measured by applying a fixed force of 250 x 10-3 N. KNN ceramics were poled prior to d33 measurements. In an oil bath a voltage amplifier (Glassman high voltage, EQ020R060 - 22) was used to apply a dc bias and an electric field of 2 kV/cm was applied for half an hour at 100 °C. The ceramics were kept in the oil bath at 100 ºC for 2 h before the electric field application to assure a uniform temperature distribution along the specimens and yielding the maximum poling efficiency. The ferroelectric domain structure was mapped by piezoresponse force microscopy (PFM) using a Multimode, NanoScope IIIA (Veeco Instruments), which is a modification of the contact atomic force microscopy (AFM), specially adapted for local piezoelectric measurements. The top faces of the samples were polished before the measurements and silver paint was used as bottom electrode. The PFM was equipped with an external lock-in amplifier (SR-830, Stanford Research), function generator (FG120, Yokogawa) and voltage amplifier (7602, Krohn-Hite). A conductive SPM probe tip by PPP-NCHR Nanosensors, Switzerland (length: 125 µm, thickness: 4.0 µm, width: 30 µm, resonance frequency: 355 kHz, spring constant: 50 N/m) was used. An external ac voltage signal with amplitude of 10 – 30 V, frequency of 5 kHz and 50 kHz was applied to induce piezoelectric deformation, for out-of phase and in-phase imaging, respectively.

Results and discussion Figure 2a presents the XRD pattern of KNN single crystals grown by a high temperature self-flux method [15]. More details on the growth of these single crystals can be found as supplementary information (S1). Under the resolution limits of the equipment, no


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impurity phases were found, and the XRD pattern corresponds to (001)-oriented crystals having a monoclinic crystal structure [15, 16]. The calculated unit cell parameters of these KNN crystals are a =3.9997 Å, b =3.9478 Å, c =3.9981 Å with β =90°22′, being close to the values previously reported in the literature [19]. Figure 2b illustrates the SEM microstructure of KNN template particles showing a plate like particle morphology with a thickness of 8 - 10 µm and a particle width ~ 45 µm. These template particles were obtained by crushing bigger KNN crystals. XRD spectra recorded on both non-templated and templated KNNL ceramics sintered for different sintering times are presented in Figure 3a. Both ceramics (non-templated and templated) show a mono-phasic perovskite structure and no additional second phases are detected within the resolution limits of the equipment, as well. The lattice parameters a = 4.002 Å, b = 3.946 Å, c = 3.982 Å, and β = 90.44º were calculated considering a monoclinic system (Pm space group) [20, 21]. To note is the fact that the (00l) family peaks intensity increases with the sintering dwell time for templated ceramics (Figure 3a), which translates an increase in the crystallographic texture degree, in accordance with (00l) diffraction peaks of KNN single crystals (Figure 2a). To further analyse the crystal orientation and to quantify the crystallographic texture we calculated the Logtering factor (f) of non template and templated KNN ceramics based on these XRD patterns. Templated ceramics sintered at 1100 º C for 2 h exhibit (f) of 12 %, whereas for 24 h of sintering (f) reaches 40 %. Figure 2b allows the comparison of the dependence of the Lotgering factor (f) on the sintering time (for the sintering temperature of 1100 ºC) for templated and non-templated ceramics. Nontemplated ceramics maintain a low Lotgering factor, lower than 9 % even after 24 h of sintering dwell time, whereas templated ceramics exhibit higher values that increase with the increase of the sintering temperature, reaching a maximum of f = 40 % for 24 h of sintering dwell time. For a better and more complete understanding of the crystallographic orientation of these ceramics rocking curves (Figure 3c and d) and pole figures of the template and non template ceramics (001) orientation were acquired (Figure 3e and f). Analysis of the rocking curves reveal broad peaks for the non-templated ceramics as compared to the templated ones (Figure 3c and d) For the templated ceramics a full width at half maximum (FWHM) was calculated as 0.42º and 7.5º from the rocking curve and pole figure, (Figure 3f), respectively. These values indicate a very good texture quality as smaller FWHM values indicates better orientation development, which is even better than those reported for textured KNN based compositions, i.e. 7 - 8.5º calculated from the rocking curves for Li-doped, Ta-doped and Sb-


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doped textured KNN ceramics [22, 23] and also when compared with values reported for other perovskite systems [12, 24]. These results confirm that templated KNN ceramics developed a preferred crystallographic orientation along 00l-orientation whereas the nontemplated ceramics maintained a random orientation. The microstructures of randomly oriented and oriented KNNL ceramics with f =40 % are shown in Figure 4a 4b respectively. Regardless their sintering and template conditions, all the ceramics have a relative density of ~96%. The microstructure of the non-templated ceramics sintered at 1100 ºC for 24 h is homogeneously composed of small sized grains having an average size of ~ 2.5 µm and a narrow size distribution. On the other hand the microstructure of templated ceramics sintered for 24 h with f = 40 % reveals a striking different appearance: the microstructure consists of a matrix of small grains in which very large grains are embedded. The small matrix grains have an average size of ~ 2.5 µm (as in the case of non templated ceramics) and the large grains are one order of magnitude higher, with sizes varying between 15 to 65 µm. The coexistence of large and small grains observed in the templated ceramics indicates that abnormal grain growth took place, as the crystallographic texture develops. Indeed, the increase in the degree of orientation induced by the development of large grains coexisting with small matrix grains has also been observed in (K, Na, Li)(Nb,Ta)O3 ceramics oriented by platelike NaNbO3 templates [25] and BaTiO3 ceramics oriented by platelike Ba6Ti17O40 hetero-template grains [26]. Having established the difference in the microstructures and crystallographic orientations of the different KNN ceramics we measured their dielectric and ferroelectric properties at a macro and nanoscale. The variation of the relative permittivity and dielectric losses as a function of the temperature at a frequency of 1 kHz is illustrated in Figure 5. For comparative purposes, the dielectric response of randomly oriented KNNL ceramics is also included. Two peaks in the permittivity are observed in these curves, which can be correlated to the phase transitions from cubic to tetragonal (TC) and tetragonal to monoclinic ( → ) upon cooling. For randomly oriented ceramics TC and → / were found to be 439 ºC and 155 ºC which are close to the values reported in the literature [27], whereas for oriented ceramics and → / were found to be ~ 430 ºC and 167 ºC, respectively. Permittivity of ~ 550 at room temperature was almost the same for both randomly oriented and textured ceramics (f = 40 %). However textured ceramics showed a maximum of 13017 at TC when compared to 7013 observed for the randomly oriented ceramics. This relative permittivity for randomly oriented ceramics (7013) measured at TC is higher as compared to values reported


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in the literature for Li and Sb co-doped ceramics

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(~ 6500) [28] and 0.058LiNbO3–

0.942[(Na0.535K0.480)NbO3 [29] (~ 3000 - 5000). Also, the dielectric permittivity of 13017 for the

textured

ceramics

at

TC

exceeds

the

values

reported

for

textured

(Na0.50K0.47Li0.03)(Nb0.8Ta0.2)O3 (3000-7000 at 100 kHz) with sintering aids [30] and is comparable to the values for Li content of 0.058 to 0.85 mole % and other doped KNN ceramics (10000 to 15000) [31, 32]. Other than the higher dielectric constant, the → / transition temperature and TC are quite high as compared to the corresponding values in ceramics with higher amount of Li, and lower dielectric constant values [33]. From Figure 5b, it can be seen that randomly oriented and oriented ceramics both showed low dielectric loss values, which were lower than 0.1 upto 250 ºC, which is considered low loss in this temperature range. Figure 5c compares the temperature dependence of the dielectric constant of oriented ceramics (f = 40%) measured under an electric field applied perpendicular (⊥) and parallel (∥∥) with reference to the pressing direction. As observed, at room temperature, a dielectric constant of ~ 550 is found along (∥∥) direction (for f = 40%) and of ~ 400 in the (⊥) direction. This trend remains at high temperatures but the difference between (∥∥) and (⊥) directions becomes higher. This permittivity variation of ~28 % reflects a property dependence on the ceramic crystallographic orientation. The higher values of permittivity were observed in the parallel direction as compared to both perpendicular direction and to randomly oriented polycrystalline ceramics hence indicating that the (001) crystallographic orientation is a favourable polarization direction. Polarization hysteresis (P-E) loops measured at room temperature and at a frequency of 50 Hz are presented in Figure 6, for randomly oriented and oriented KNNL ceramics (f = 40 %). Well saturated P-E loops with significant improvement in remanent polarization (Pr) from 13.1 µC/cm2 to 21.4 µC/cm2 and increased coercive field (Ec) from 9.6 kV/cm to 11.7 kV/cm are observed for template ceramics as compared to non-templated ones. The values obtained for the templated ceramics are very similar to the ferroelectric characteristics of undoped KNN single crystals previously prepared by us, characterised by a remnant polarization (Pr) and coercive field (Ec) of 19.4 µC/cm2 and 10.6 kV/cm, respectively at room temperature as previously reported [15]. The enhanced polarization for template KNN ceramics is likely to be due to the crystallographic orientation direction (easy direction of spontaneous polarization) and higher permittivity as explained in Figure 4.


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To better understand the reasons of the observed higher permittivity and polarization in textured ceramics, we examined the domain structure of these ceramics by piezoforce response microscopy (PFM). Figure 7a-c shows the topography and piezo-response images of randomly oriented KNNL ceramics. It was also observed that there is no cross talk in topography and piezo – response image and PFM contrast is independent of topography effects, indicating that the acquired PFM signal did not suffer any influence from cross talk with topography signal in both cases, i.e. oriented and randomly oriented ceramics. Bright and dark contrast in the PFM images represent the direction of orientation of the polarization vectors in their respective planes: dark regions orientation into the bulk of the textured ceramics and bright regions polarization orientation towards the surface. For the bulk nontemplated ceramics, randomly oriented domains were observed both in-plane (IPP) and out of plane (OPP) PFM images. Corroborating literature reports regular domain patterns are absent in small grain size microstructures, as here observed for non templated KNN ceramics. Furthermore, it is also clear from PZT and BT studies that complex self-assembled domains like those observed in the oriented KNNL ceramics, can only be formed in ceramics with large grains of several square micrometers [34]. Figure 7d-f depicts the topography and piezo-response images of templated KNNL ceramics, respectively. A complex domain pattern is detected in the oriented ceramics, which consists of herringbone type (V shaped) domains, assembled in a regular, repetitive fashion as shown in the OPP image of Figure 7f, which mimics closely the domain pattern observed in KNN single crystals [15].Complex self assembled domains of similar type have been reported in BaTiO3 [34] with large grains of several square micrometers or in PZT single crystals [35, 36] and more recently in our grown KNN single crystals [15]. For clarity purposes a zoomed image of the herringbone area is shown in Figure 8a, b (topography and piezo-response image, respectively) along with a schematic representation. The domain length varies between 1.8 – 3.4 µm and domain width varies between 0.3-0.9 µm as shown in the schematics. It has been reported that internal stresses plays a very important role in the formation of different types of domains, i.e. small stress favours 180º domains while high stresses privilege 90º domains formation [15, 37]. We have recently observed in our previous study on KNN single crystals that the dominant domains are of 180º type which is an indication that internal stresses in the single crystals are low, may be due to the very slow cooling rate used during their production [21]. Stresses are unavoidable in ceramics and in the present case not only due to the heating / cooling cycles, but also because of microstructure differences between oriented / bulk ceramics and single crystals templates: a ceramic grain is clamped by its neighbouring grains in all the three


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dimensions and the deformation to some extent is possible only by cooperative motion of the adjacent grains which may result in either high internal stress or mechanical twinning, which is not the case for single crystals where the grain structure is absent [34]. In this work we found an obvious correlation between the local piezoresponse / domain patterns and the macroscopic dielectric and ferroelectric enhanced response, evidencing the benefits from the presence of SC templates. Piezoelectric coefficient d33 of 280 pC/N was measured for KNNL ceramics with f = 40%, which is more than twice as compared to that of randomly oriented ceramics. This increase in d33 values can be attributed to both higher relative permittivity and polarization and also to a very well defined domain pattern. Figure 9 represents d33 as a function of TC for several lead free compositions reported in the literature and also for the textured KNNL ceramics of this work. Several observations can be taken from this graph. In a similar way to PZT compounds, d33 decreases as TC increases. BT based compositions exhibit some of the highest d33 values but accompanied with the lowest TC. BNT compounds with a large range of TC exhibit some of the lowest d33. Of relevance is that textured KNNL ceramics of the present work present the highest TC values (> 400 ºC) while keeping a reasonably high d33. For the first time pairing of high d33 and high TC in KNN and keeping PPT temperature is achieved. Simultaneously superior dielectric (permittivity of 13017), and ferroelectric (remanent polarization, Pr = 21.4 µC/cm2 at room temperature) performance is described. The results here reported are of high significance as they couple a non heavily doped KNN composition with enhanced dielectric, ferroelectric and piezoelectric properties which are as good or even better than those reported for KNN with higher Li content (4-8 mole % of Li) or for other KNN ceramics differently doped [20, 21, 38-41], while the phase transition temperatures, i.e. → of 167 ºC and TC of 430 ºC are kept high, even exceeding the reported values for → close to room temperature and TC lying between 200 to 300 ºC [1, 2]. Here we prove that it is possible to maximize d33 keeping a high TC in KNN without resorting to highly doped compositions. The long range ordered ferroelectric domain patterns of the ceramics consisting of 90º and 180º domains, similar to the ones observed in single crystals give rise to an enhanced electromechanical response. These results have implications for the possible discovery of high-performance, rare-earth free and not compositionally complex lead-free electromechanical alternatives where sustainable materials or formulations are not available, tackling one of the major challenges currently faced by the electroceramics industry, that is its reliance on rare-earths and toxic elements.


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Conclusions This works pairs for the first time a high piezoelectric coefficient d33 with high Curie temperature (TC) and high PPT temperature in lead-free (K,Na)NbO3 ceramics. In this work (K0.5Na0.5)0.98Li0.02NbO3 (KNNL) textured ceramics with monoclinic phase at room temperature were fabricated by Template Grain Growth (TGG) using (K0.5Na0.5)NbO3 (KNN) single crystal particles as templates. Textured KNNL ceramics with a preferential (001) orientation and a lotgering factor (f) of 40% exhibit a dramatic improvement in the electromechanical properties while maintaining a high TC. (001) – oriented KNNL ceramics exhibit a d33 of 280 pC/N with a high (TM) phase transition temperature and a high TC of 430 ºC. Simultaneously superior dielectric (permittivity of 13017 at TC)), and ferroelectric (remanent polarization, Pr = 21.4 µC/cm2 at room temperature) performance is attained as well. The presence of a long range ordered ferroelectric domain structure detected by Piezoforce Response Microscopy (PFM) and consisting of 90º and 180º domains, similar to the one observed in single crystals, which was absent in the randomly oriented ceramics, is advocated to account for the reported improvement in the electrical properties. Besides being the unequivocal proof that it is possible to maximize d33 keeping a high (TM) phase transition temperature and a high TC in KNN these results are also relevant since they open the door to the possible discovery of high-performance, rare-earth free and not compositionally complex lead-free electromechanical compounds, which can greatly decrease costs and largely expand the application of lead-free piezoelectric materials.

Acknowledgments This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID /CTM /50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. Muhammad Asif Rafiq acknowledges FCT for financial support (SFRH/BD/66942/2009). We are thankful to Dr. Sebastian Zlotnik for editing support with the figures.


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Table and Figure Captions Table 1 KNN textured ceramics: composition, preparation method and properties. Figure 1 Schematic representation of the faces of the ceramics, perpendicular (⊥) and parallel (∥) with reference to the pressing direction.

Figure 2 Some characteristics of KNN single crystals: (a) XRD pattern of (001) - oriented crystal with a monoclinic crystal structure and (b) SEM micrograph revealing the morphology of KNN single crystal templates.

Figure 3 (a) XRD patterns of KNNL ceramics sintered at 1100 ºC and showing the variation of Lotgering factor with the sintering time (from the bottom to the top: randomly oriented ceramics, sintered for 2, 4, 8, 16 and 24 h respectively); (b) the variation of lotgering factor versus sintering time for non-templated and templated ceramics. Ceramics sintered for 2 h is taken as reference to calculate the lotgering factor. The intensity of (001)-peaks increase with increasing the sintering dwell time, (c, d) rocking curves of templated and non-templated ceramics and (e, f) pole figures of templated and non-templated ceramics.

Figure 4 KNNL ceramics sintered at 1100 ºC for 24 h: (a) non-templated ceramics; (b) template ceramics with f = 40%. Large grains are clearly observed in template ceramics. Grain size distribution of KNNL sintered ceramics: (c) non-templated ceramics and (d) template ceramics with f = 40%.

Figure 5 Temperature dependence of the (a) relative permittivity and (b) dielectric loss of KNNL randomly oriented and oriented ceramics. Oriented ceramics with f = 40 % exhibit higher dielectric permittivity and lower dielectric loss values. (c) Relative permittivity for textured ceramics (f = 40%) measured along parallel (∎) and perpendicular (•) directions with reference to the pressing direction.


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Figure 6 P-E loops for KNNL randomly oriented and oriented ceramics. Oriented ceramics show higher polarization as compared to randomly oriented ones.

Figure 7 Topography (a, d), in plane, IPP (b, e) and out of plane, OPP images (c, f) of randomly oriented and oriented KNNL with f = 40 % respectively. There is no influence of the topography on the piezo-response in all the cases and a long range ordered domain pattern can be clearly observed in the oriented ceramics specifically in (f).

Figure 8 Closer look of a PFM image of oriented KNNL ceramics (a) topography and (b) out of plane OPP image and (c) schematic of the domain pattern of KNNL with f = 40%. A long range ordered domain pattern was observed. Figure 9 d33 as a function of TC for various lead based and lead free compositions. The d33 values obtained in this work along with high TC are better than other lead free materials. It is worth mentioning that the compositions in this work are far away from phase coexistence and enhancement in properties is attributed to the long range ordered domain pattern.


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Table 1. KNN textured ceramics: composition, preparation method and properties. Composition

(K0.5Na0.5)0.98Li0.02NbO3

Method

Tape Cast

Lotgering

r

tan δ

%

1 kHz

98

263

NaNbO3,

TC

Pr

Ec

pC/N

°C

(µC/cm2)

(kV/cm)

439

22.1

12.7

99

413

192, 208

Plate-like (K0.5Na0.5)(Nb0.85Ta0.15)O3

d33

Refs.

factor

184

[23]

322

[23]

Screen K0.45Na0.55NbO3

432

printing,

(∥)

0.06

(∥)

18.4

(∥)

10.1

(∥)

401

plate-like

331 (⊥)

0.04 (⊥)

[42] 18.6 (⊥)

10.4 (⊥)

11

14

NaNbO3. Tape (Na0.50K0.47Li0.03)(Nb0.8Ta0.2)

casting,

O3

plate-like

310

320

[30]

NaNbO3 Tape cast Plate K0.5Na0.5NbO3

like

810

(∥)

29.5

BaTiO3

(∥)

163

[43]

331 (⊥)

(BT)

14.4(⊥)

templates Tape cast (KxNa1-x)0.946Li0.054NbO3

plate-like NaNbO3

80

646

0.05

231– 254

450

24.9

[44]

particles * the symbols (∥) and (⊥) indicate that the application of the electric field was perpendicular (⊥) and parallel (∥) with reference to the pressing direction.


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Table of Contents graphic


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Pairing High Piezoelectric Coefficients, d33, with High Curie

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