The impact of chemical heterogeneity in lead-free (K, Na)NbO3 piezoelectric perovskite: Ferroelectric phase coexistence

Accepted Manuscript The impact of chemical heterogeneity in lead-free (K, Na)NbO3 piezoelectric perovskite: Ferroelectric phase coexistence Hao-Cheng Thong, Chunlin Zhao, Zhi-Xiang Zhu, Xin Chen, Jing-Feng Li, Ke Wang PII:

S1359-6454(19)30023-0

DOI:

https://doi.org/10.1016/j.actamat.2019.01.012

Reference:

AM 15080

To appear in:

Acta Materialia

Received Date: 3 November 2018 Revised Date:

5 January 2019

Accepted Date: 7 January 2019

Please cite this article as: H.-C. Thong, C. Zhao, Z.-X. Zhu, X. Chen, J.-F. Li, K. Wang, The impact of chemical heterogeneity in lead-free (K, Na)NbO3 piezoelectric perovskite: Ferroelectric phase coexistence, Acta Materialia, https://doi.org/10.1016/j.actamat.2019.01.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The Impact of Chemical Heterogeneity in Lead-Free (K, Na)NbO3 Piezoelectric Perovskite: Ferroelectric Phase Coexistence

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Hao-Cheng Thong1, Chunlin Zhao1, Zhi-Xiang Zhu2,3, Xin Chen2,3, Jing-Feng Li1 and Ke Wang1,* State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing, China 2

State Key Laboratory of Advanced Transmission Technology, Changping District,

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Beijing 102209, P.R. China

Institute of New Electrical Materials, Global Energy Interconnection Research

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Institute, Changping District, Beijing 102209, P.R. China *Corresponding author: [email protected] ABSTRACT

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(K, Na)NbO3 (KNN)-based solid solutions are outstanding lead-free

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piezoelectric materials, yet stable reproduction of these materials has always been an obstacle. In the present study, significant ferroelectric phase coexistence was observed

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in the polycrystalline ceramic sample after sintering. The ferroelectric phase coexistence can be well explained by the chemical heterogeneity in the calcined

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powder, which is a consequence of using monoclinic Nb2O5 as the precursor. It was demonstrated that the ferroelectric phase coexistence, as well as the chemical heterogeneity, could be a barrier of the reproducibility of performances. Therefore, we suggest that extra attention should be paid on the processing of KNN-based materials, from the very beginning, that is, the choice of precursors. Keywords: Piezoelectricity; Ferroelectricity; Ceramics; Heterogeneity, KNN

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ACCEPTED MANUSCRIPT 1. INTRODUCTION In the past, lead-based piezoelectric materials, such as (Pb, Zr)TiO3 and Pb(Mg, Nb)-PbTiO3 systems, have dominated the market of electronic devices that

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worth billions USD [1]. However, owing to the safety concern, many countries have enforced regulations, e.g. Restriction of Hazardous Substances Directive (RoHS) in European Union, to prohibit the use of hazardous lead (Pb) in electronic devices. The regulations were successful on controlling the usage of Pb in various applications but

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not in the case of piezoelectric materials, which is mainly due to the poor reliability of

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substitutes.

After a decade of research, a new era of lead-free piezoelectric materials is finally approaching [2], especially for (K, Na)NbO3 (KNN)-based materials [3, 4]. Since 2004, Saito et. al. [5] demonstrated an extremely high d33 value of 416 pC/N in

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a textured KNN-based ceramic, the d33 record almost doubled very recently [6].

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Except for the continuous breakthrough in the d33 value, KNN-based materials have been proved to possess an outstanding thermal stability and electrical fatigue

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resistance [7-14]. The present situation is described as the second phase of development, that is, the technology transferring state [15]. Researches have become

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application-oriented, focusing on topics like cost reduction, lifetime, and reproducibility because these data will strongly affect the investors’ decision whether it is reasonable to gamble on the future of KNN. Among various problems, the reproducibility of functional properties of KNNbased materials is the most worrying. Many reviews have mentioned the poor reproducibility in KNN-based materials [1, 3, 15, 16]. The poor reproducibility may origin from the hygroscopicity of alkaline carbonates [17], volatilization of alkaline 2

ACCEPTED MANUSCRIPT elements [18], sensitivity to processing conditions [17], complex grain growth behavior and related piezoelectricity dependence [19, 20]. On the other hand, absence of statistical average of performance and incomplete descriptions of experimental condition in papers have also brought distrust to industry [15]. The inconsistent

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reproduction of KNN-based piezoelectric materials is like an impenetrable barrier for further development.

Recently, our group discovered an irregular grain growth behavior in the

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calcined KNN powder, which might account for the poor reproducibility [21]. In the

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study, the irregularly grown grains were found to possess heterogeneous chemical compositions. Repeated ball-milling and calcination process were inefficient to eliminate or suppress the irregular grain growth. It was suggested that these irregularly grown grains, having heterogeneous compositions, will remain even after sintering and therefore leading to a poor reproducibility of performance. Additionally,

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it was suggested that the irregular grain growth is a consequence of using monoclinic

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Nb2O5 as the precursor for fabrication.

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In the present study, we aimed to determine the influence of chemical heterogeneity to the comprehensive properties of the sintered ceramics. Orthorhombic

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and monoclinic Nb2O5 precursors were used to fabricate pure KNN piezoceramics. Unexpectedly, significant ferroelectric phase coexistence was observed in the sintered ceramic when monoclinic Nb2O5 was used, and its origin was systematically investigated. The ferroelectric phase coexistence, which we suggested to be a consequence of the chemical heterogeneity in calcined powders, can account for the poor reproducibility of KNN. Therefore, homogenization was tried and its efficiency was discussed. 3

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2. EXPERIMENTAL 2.1 Preparation of KNN ceramic samples

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Pure KNN piezoceramics were prepared by the conventional solid-state reaction method. Raw materials, including K2CO3 (99%, Sinopharm, China), Na2CO3 (99.8%, Sinopharm, China), and different kinds of Nb2O5, including (i) Orthorhombic

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Nb2O5 (99.99%, Sinopharm, China), (ii) Monoclinic Nb2O5 (99.95%, Conghua Tantalum and Niobium Smeltery, China), and (iii) Monoclinic + Orthorhombic Nb2O5

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(99.95%, Conghua Tantalum and Niobium Smeltery, China), were weighed accordingly to the stoichiometry composition of (K0.5, Na0.5)NbO3 and subjected to planetary ball milling at 300 rpm for 12 hours with ethanol as dispersant. Then, homogenous powder mixtures were obtained after drying in an oven. Later, these mixtures were calcined in a furnace at 700 oC for 4 hours before subjected to another

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ball milling process for 12 hours. Before sintering, PVA binder was added into these

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powders for granulation. Then, the granulated powders were pressed into a diskshaped compact of 1.5 mm in thickness and 10 mm in diameter. The green compacts

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were subjected to thermal debinding at 600 oC for 6 hours, followed by sintering at

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1080 oC for 4 hours.

2.2 Characterization of calcined KNN powders and Nb2O5 precursors The pure KNN powders prepared by different Nb2O5 precursors were sampled for Scanning Electron Microscopy (SEM) (JSM-6460LV, JEOL) measurement before subjected to the second ball milling. Meanwhile, SEM of Nb2O5 precursors are shown 4

ACCEPTED MANUSCRIPT in Fig. S1. X-ray Diffraction (XRD) (D8 advance, Bruker) measurements of calcined KNN powders were carried out after the second ball milling. Nb2O5 precursors were also measured for reference.

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2.3 Electrical characterization of sintered ceramics Before electrical measurement, all ceramic samples were grounded to 1mm in thickness and subsequently coated with a layer of silver electrode on both surfaces.

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Ferroelectric performances were measured by using a ferroelectric tester (TF-1000, aixACCT Systems GmbH) at a fixed frequency of 1 Hz before poling. For poling

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process, the samples were subjected to an electric field of 3 kV/mm at 120 °C in a silicone oil bath for 30 min. The piezoelectric constant d33 was measured with a quasistatic piezoelectric tester (ZJ-3A, Institute of Acoustics, Chinese Academy of Science) at room temperature. Temperature-dependent permittivity was measured with an

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impedance analyzer (TH2827, Changzhou Tonghui Electronic Co) at 1 kHz.

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Thermally stimulated depolarization current (TSDC) measurement was conducted by utilizing an electrometer (KEITHLEY 6517B, Keithley Instruments) and a

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temperature controller (Quatro Cryosystem, Novocontrol).

3. RESULTS AND DISCUSSION 3.1 Chemical heterogeneity in calcined powders After calcination at 700 oC, a pure perovskite KNN structure could be

synthesized because the reaction of alkali carbonate and niobium pentoxide occur in between 400 oC to 700 oC [16]. The XRD patterns of calcined KNN powders are shown in Fig. 1. It was observed that a pure KNN perovskite phase was obtained 5

ACCEPTED MANUSCRIPT when an orthorhombic Nb2O5 was used as a precursor. However, by using a monoclinic Nb2O5 as a precursor, a mixture of KNbO3-like (space group Amm2, a=5.6896 Å, b=3.9692 Å, c= 5.7256 Å [22]) and NaNbO3-like (space group Pbcm, a=5.5071 Å, b=5.5698 Å, c= 15.5245 Å [23]) perovskite could be obtained, as shown

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in Fig. 1C. The measured diffraction peaks of KNbO3 and NaNbO3 are slightly different with the previously reported diffraction peaks, which indicates the variation of lattice structures caused by the diffusion of Na and K into KNbO3 and NaNbO3

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respectively.

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Fig. 1 The XRD patterns of (A) as-received Nb2O5 precursors and (B) calcined KNN powders. The black arrows in (A) indicates the presence of orthorhombic Nb2O5. Cubic KNN lattice structure is used for peak labelling. (C) Magnified 110 peak at 2θ around 32 o. Diffraction peaks of KNbO3-like and NaNbO3-like are coloured with pink and orange, respectively.

Previously, the chemical heterogeneity caused by the monoclinic Nb 2O5

precursor has also been reported [24]. The crystal structure of Nb2O5 can be modified during sol-gel synthesis, which is known as the Pechini route [25]. The synthesized product strongly depends on the annealing temperature, where the crystal structure will go through an orthorhombic to monoclinic phase transformation from 800 oC to 1000 oC. In the present study, three different kinds of niobium pentoxides were used 6

ACCEPTED MANUSCRIPT for the fabrication of KNN powder, which are orthorhombic (denoted as O), monoclinic (denoted as M1), and a mixture orthorhombic and monoclinic (denoted as M2 since the monoclinic phase is the major phase). Therefore, the calcined KNN

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powders are named as KNN-O, KNN-M1 and KNN-M2 accordingly. The morphology of calcined KNN-powders was examined by using SEM, as shown in Fig. 2. A homogeneous particle size distribution was observed in KNN-O powder, having an average particle size around a few hundred nanometers. In contrast,

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a bimodal particle size distribution was observed in both KNN-M1 and KNN-M2.

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These two powders consist of similar small particles having an average particle size around hundreds nanometer and coarse faceted particles having sizes of a few micrometers. The SEM results are in good agreement with the XRD results, where only a set of diffraction pattern was observed in KNN-O while double sets of

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diffraction patterns were observed in KNN-M1 and KNN-M2.

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Fig. 2 SEM of calcined (A) KNN-O, (B) KNN-M1 and (C) KNN-M2 powders. Unimodal particle size distribution was observed in KNN-O while bimodal particle size distribution was observed in KNN-M1 and KNN-M2. The bimodal particle size distribution in calcined powder is somehow

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analogous to that of abnormal grain growth (AGG) in sintered ceramics. However, AGG is a kind of grain growth behavior where certain grains can grow rapidly by consuming the matrix grains [26]. The detailed discussion of AGG in KNN can be referred to our previous work [21]. To avoid any confusion with the well-established term ‘AGG’, the bimodal particle size distribution in calcined powder is termed as irregular grain growth. According to our previous work, by using energy dispersive spectroscopy, the small particle and the large particle were found to have KNbO3-like 8

ACCEPTED MANUSCRIPT and NaNbO3-like composition respectively [21]. The chemical heterogeneity induced by the difference of the Nb2O5 precursors has been discussed by Hreščak et al [24]. They suggested that orthorhombic nanocrystalline Nb2O5 can be obtained during the ball-milling process of submicron monoclinic Nb2O5 precursor. Since it has been

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observed that the rate constant of Na2CO3/Nb2O5 diffusion couple is about one order of magnitude higher than K2CO3/Nb2O5 [27], in the present circumstance, the relatively active element Na will first react with the orthorhombic nanocrystalline

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Nb2O5 while the slower species K can only react with submicron monoclinic Nb2O5. Therefore, hetergeneous mixture of KNbO3 and NaNbO3 will be obtained. Based on

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their explanation, we can reasonably conjecture that the NaNbO3 will grow into a larger crystal size than the KNbO3, due to the difference of reaction rates.

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3.2 Ferroelectric performance

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The calcined powders were formed into a green compact and sintered at 1100 C. Dense ceramics were fabricated, having relative densities around 92-94%

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(theoretical density of pure KNN is 4.51 g/cm3), as shown in Table 1. Electrical measurement, including polarization, strain, and current-density loops as a function of

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applied electric field were measured on these samples, as shown in Fig. 3.

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Fig. 3 Polarization, strain and current-density loops of unpoled KNN-O, KNN-M1 and KNN-M2 samples measured under 1 Hz. The black arrows represent the domain switching of (K0.5, Na0.5)NbO3, while the grey arrows represent the domain switching of a secondary phase (K1-x, Nax)NbO3.

First, a leakage current was clearly observed in all the samples by judging from the shape of the polarization loop, i.e. the remnant polarization (Pr) is higher than the maximum polarization (Pmax). A Pr that is higher than a Pmax is actually an artifact occurs during the measurement, where the switched charge (Q) is measured, instead of the Pr [28]. Therefore, for a slightly conductive ferroelectric material, the Q value will be determined by contributions from both the ferroelectric part and the 10

ACCEPTED MANUSCRIPT conductive part, as described as follows [29]: , where A is the electrode area, σ is the conductivity, Ea is the applied electric field, and

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t is the measuring time. The leakage current in KNN solid solution probably originates from the mobile charged defects, e.g. oxygen vacancies, induced by nonstoichiometry [30], which is a possible consequence of the volatilization of alkali

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( )

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elements. The defect chemistry of K and Na volatilization are shown as follows:

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To maintain the electrical neutrality, cation vacancies will always be compensated by doubly ionized oxygen vacancies. Compared to cation vacancies, oxygen vacancies

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are rather mobile in perovskite because there are always nearest-neighbour oxygen

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sites for exchange their position [31]. Under applied electric field, the oxygen vacancies can freely diffuse through a long distance and thus increase the conductivity.

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Potential presence of oxygen vacancies was also observed in the TSDC result, as shown in Fig. S2.

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Domain switching behavior can be observed from a current-density loop

because the switching of domains will cause polarization/depolarization current. Under applied electric field, a sharp current peak can be observed in the current density loop when domains suddenly switch to a similar direction accordingly. Besides, the electric-field-induced strain can also reflect the domain switching behavior. In the present study, multiple domain switching behaviors were observed in all sintered ceramic samples. In the current density loop, except for the current peak 11

ACCEPTED MANUSCRIPT of KNN domains at around 0.6 kV/mm (marked with black arrows), it is worth noting that there is an extra current peak (marked with grey arrows). These current peaks can also be observed in the strain loop relatively. Compared to the sharp peak of KNN, the extra current peak is relatively diffused. This diffused current peak might

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correspond to the rotation switching of a series of secondary ferroelectric domains having heterogeneous compositions of (K1-x, Nax)NbO3, which are formed by the inter-diffusion of KNbO3 and NaNbO3 during sintering. To check the possibilities of

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secondary phases, the phase structures of sintered ceramics were examined by using XRD. As shown in Fig. S3, KNN of orthorhombic perovskite phase, as well as a

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small portion of non-perovskite secondary phase, is observed in all samples. Note that there is no non-perovskite secondary phase observed in calcined powders. The nonperovskite secondary phase found in sintered ceramics might originate from the formation of liquid phase upon sintering close to solidus temperature [32]. Since the

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formation of liquid phase will result in an exacerbated volatilization of alkalis, a large

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deficiency of A-site element will favourably result in the formation of the secondary phase, having a tungsten-bronze structure [33], e.g. (K, Na)4Nb6O17. Anyway, such

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little portion of secondary phase might not be able to explain the extra current peak observed in the current-density loop. Therefore, it is conjectured that there are other

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secondary phases which possess very similar orthorhombic perovskite structures, i.e. (K1-x, Nax)NbO3, yet it is difficult to be identified based on a lab-scale XRD of low resolution. The analysis of the secondary phase will be further discussed in subsequent sections.

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ACCEPTED MANUSCRIPT 3.3 Temperature-dependent dielectric performance Apart from the ferroelectric performance, a significant difference was again

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observed in the dielectric performance, shown in Fig. 4.

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Fig. 4 (A) Dielectric permittivity and (B) loss tangent of different KNN ceramic samples measured at 1 kHz during cooling from 500 oC to room temperature.

The transition temperatures, i.e. orthorhombic to tetragonal phase transition temperature (TO-T) and Curie temperature (TC), of KNN-M1 and KNN-M2 are slightly higher than that of KNN-O. Shifting of the transition temperatures can be explained by the K1-xNbO3-NaxNbO3 phase diagram [3]. According to the phase diagram, if KNN consists of a lower volume fraction of Na (e.g. K1-xNbO3-NaxNbO3, x < 0.5), the transition temperatures should be higher, and vice versa. Therefore, the KNN-M 13

ACCEPTED MANUSCRIPT (including KNN-M1 and KNN-M2) might possess a lower volume fraction of Na, compared to KNN-O. However, the phase diagram was constructed based on a rather small dataset. A direct composition identification based on the phase diagram can be misleading. Nevertheless, shifting of transition temperatures suggests a possible

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variation of KNN composition.

According to the modified Curie-Weiss Law, the temperature-dependent

(

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permittivity curve of a ferroelectric can be described as follows [34]:

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Where ε is dielectric permittivity, εm is the maximum dielectric permittivity, T is temperature, Tm is the temperature where permittivity is maximized, C is a Curie-like constant and γ is the critical exponent of nonlinearity. The diffuseness of the phase transition at Tm, i.e. TC in ferroelectrics, can be represented by the γ factor. The γ

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Fig. 5.

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factors of all samples were derived from the permittivity curve (Fig. 4A) and fitted in

Fig. 5 Plot of ln (1/ε − 1/εm) as a function of ln (T−Tm) for (A) KNN-O, (B) KNNM1 and (C) KNN-M2 ceramic samples.

The fitted γ factors of KNN-O, KNN-M1 and KNN-M2 samples were also 14

ACCEPTED MANUSCRIPT summarized in Table 1. Higher values in KNN-M1 (γ = 1.06) and KNN-M2 (γ = 1.11) imply that relatively heterogeneous compositions exist in both KNN-M ceramics, compared to KNN-O ceramic (γ = 1.03). The high γ values in KNN-M1 and KNN-M2 further confirm the presence of the ferroelectric secondary phases, having varying

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compositions of (K1-x, Nax)NbO3.

As shown in Fig. 4B, the temperature-dependent dielectric loss tangents of various KNN piezoelectrics are quite different. Ferroelectric loss is the major factor

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for the dielectric loss in ferroelectric materials, largely contributed by the extrinsic

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domain part [30]. The result indicates a possible variation of domain structures among three types of KNN ceramics. However, it should be noted that conductivity plays a critical role as well. A further experiment is required to classify the contributions from the domain and the conductivity.

Properties o

KNN-O

KNN-M1

KNN-M2

KNN-M2

KNN-M2

KNN-M2

KNN-M2

700 4.21(6) 108(3) 0.36(1) 176(24) 407 183 285 0.018 1.031(3)

700 4.27(10) 105(6) 0.36(1) 166(31) 416 186 359 0.035 1.060(1)

700 4.24(5) 102(4) 0.36(2) 153(8) 411 186 392 0.032 1.110(4)

850 4.14(2) 114(4) 0.38(2) 172(9) 410 189 414 0.027 1.117(4)

900 4.15(5) 114(5) 0.38(2) 180(17) 409 187 396 0.027 1.099(4)

950 4.11(3) 115(4) 0.36(1) 176(22) 413 189 367 0.027 1.076(5)

1000 4.13(4) 116(4) 0.40(1) 184(7) 414 190 400 0.028 1.122(5)

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Tcal ( C) ρ (g/cm3) d33 (pC/N) kp Qm TC (oC) TO-T (oC) εr tan δ γ

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Table 1 Properties of KNN-O, KNN-M1 and KNN-M2 sintered ceramics. Tcal: calcination temperature; ρ: density; d33: piezoelectric coefficient; kp: electromechanical coupling factor; Qm: mechanical quality factor; TC: Curie temperature; TO-T: orthorhombic to tetragonal phase transition temperature; εr: dielectric permittivity; tan δ: dielectric loss tangent; γ: gamma factor.

3.4 Chemical homogenization 15

ACCEPTED MANUSCRIPT Aforementioned results, including the multiple domain switching behavior, high γ factors, and variation of dielectric loss, are strong evidence that a secondary ferroelectric phase which possesses a composition of (K1-x, Nax)NbO3 might exist in the sintered KNN-M samples. The presence of the ferroelectric secondary phases is

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believed to be a result of the chemical heterogeneity in calcined powders. Therefore, we designed an experiment to examine the hypothesis whether the secondary phase can be homogenized.

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To alleviate the chemical heterogeneity in calcined KNN-M powders, repeated

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calcinations at higher temperatures, ranging from 850 oC to 1000 oC, were applied. Similarly, the double-calcined powders were sintered at 1100 oC, as shown in Fig. 6. Fig. 6A shows the current density loop of KNN-M2 ceramic. It can be observed that the current peak of (K0.5, Na0.5)NbO3 domains at around E = 0.6 kV/mm sharpens with increasing calcination temperature and reach a maximum at 950 oC, indicating

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the increasing amount of (K0.5, Na0.5)NbO3 domains. Furthermore, the current peak of

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the secondary phase moves towards the (K0.5, Na0.5)NbO3 current peak. The shifting of the current peak position, together with the variation of γ factor, are plotted against

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the increasing calcination temperature, as shown in Fig. 6B. The peak position of the

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secondary phase reaches at a minimum of 1.17 kV/mm. In addition, the γ factor decrease to 1.076 for the second calcination temperature of 950 oC. On the other hand, it is worth noting that the dielectric performance shares a similar trend, as shown in Table 1. There are two possible explanations: (1) the new ferroelectric phases (Kx, Na1-x)NbO3 have a lower dielectric constant compared to the composite of KNbO3like and NaNbO3-like ferroelectric phases [35], and (2) the influence of space charges at low frequencies. However, the influence of space charges might be excluded since 16

ACCEPTED MANUSCRIPT the dielectric loss does not change significantly. From the above results, it can be concluded that the chemical heterogeneity of KNN can be homogenized by the repeated calcination, and the efficiency of homogenization is optimized at 950 oC. Additionally, the homogenization can be observed from the XRD results, shown in

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Fig. S4 and S5.

Fig. 6 (A) Current density loop of KNN-M2 ceramics prepared by powders calcined at different temperatures. (B) γ factor of the sintered ceramics and the current density peak position of the secondary phase plotted as a function of the second calcination temperature. Optimal homogenization temperature was found at 950 oC.

Homogenization of a mixture of separately prepared KNbO3 and NaNbO3 at 17

ACCEPTED MANUSCRIPT 950 oC has been previously performed by Malic et al. [36]. In contrast to the second calcination at 950 oC, a further increase of calcination temperature was found to be less efficient to homogenization. The reason can be possibly explained by the grain

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growth behavior shown in Fig. 7.

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Fig. 7 SEM of KNN-M2 powder calcined at (A) 850 oC, (B) 900 oC, (C) 950 oC and (D) 1000 oC.

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For the KNN-M2 powder calcined at 850 oC, the unique bimodal particle size distribution can still be observed. As the calcination temperature increase to 900 oC, a

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significant grain growth was observed: large rounded grains gradually appeared in the calcined powder, while half of the powder remain the morphology observed at 850 oC. As the temperature kept rising until 950 oC, a unimodal-like particle size distribution was observed. However, two different kinds of particle morphologies were observed, one with cuboidal shape and one with rounded shape. Owing to the variation of surface free energy on different crystallographic planes, each kind of materials has their own preferred crystal shape. Judging from the crystal shape, the cuboidal and 18

ACCEPTED MANUSCRIPT rounded particles are conjectured to be KNbO3-like [37] and NaNbO3-like [38] grains respectively. Finally, when the calcination is implemented at 1000 oC, very large rounded grains were found coexisted with small faceted grains.

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From the kinetics perspective, assuming that diffusion mechanism dominates the solid-state grain growth behavior, the diffusion coefficient can be expressed as follows [39]: )

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(

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where D0 is the pre-exponential diffusion coefficient, QA is the activation energy, kB is the Boltzmann constant, and T is the temperature. Note that D0 and Qa are both material-dependent parameter. As these two parameters can differ significantly between KNbO3-like and NaNbO3-like particles, at certain temperature T, a significant difference of grain growth behavior should be observed. For better

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8.

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description, the diffusion coefficient as a function of temperature is illustrated in Fig.

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Fig. 8 Illustrated diffusion coefficients of various particles, including NaNbO3-like and KNbO3-like grains, are plotted as a function of temperature. The grain growth behaviour of stoichiometric KNN, as shown in Figure 10, is added for comparison.

For example, if the calcination temperature is 900 oC, NaNbO3-like grains can grow

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significantly as the diffusion coefficient is greatly enhanced at such temperature,

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while KNbO3 can barely grow. When the calcination temperature is over 950oC, both types of grains can grow rapidly since the diffusion mechanisms were activated, and

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thus a unimodal-like particle size distribution was observed. However, when the

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temperature is too high, e.g. over 1000 oC, the NaNbO3 grains will grow rapidly into an enormous size and consume most of the driving force, i.e. surface free energy. Consequently, grain growth of KNbO3 will be strongly inhibited. Therefore, mitigated homogenization efficiency was observed at 1000

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C. Nevertheless, further

experiments are required to verify this explanation. On the other hand, the grain growth behavior of KNN-O is also very interesting, as shown in Fig. 9. Small aggregated submicron grains grow into 20

ACCEPTED MANUSCRIPT micrometer-sized cuboidal grains at 900 oC and maintain such grain morphology until 1050 oC. Typical abnormal grain growth behavior was observed at 1050oC, which is a high temperature that is very close to the mostly reported sintering temperature (around 1080 oC). The reason for the abnormal grain growth is likely to be originated

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from the presence of liquid phase at solidus temperature [20, 40]. Step and kink structures was observed at 1050 oC, as shown in Fig. S6, which is probably due to the activation of 2-dimensional nucleation when liquid phase is involved [41].

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Additionally, liquid phase has been continuously reported in different KNN systems

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[42-44].

Fig. 9 SEM of KNN-O powder calcined at (A) 800 oC, (B) 900 oC, (C) 950 oC and (D) 1050 oC.

3.5 Ferroelectric phase coexistence The coexistence of ferroelectric phases in the pure KNN system is an unexpected discovery for the development of KNN. Based on the experimental results, a qualitative model of ferroelectric phase coexistence is proposed, as shown in Fig. 10. 21

ACCEPTED MANUSCRIPT There are two major effects that determine the final phase structure of a sintered ceramic: (i) the chemical heterogeneity of calcined powder, and (ii) the formation of

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liquid phase during sintering.

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Fig. 10 The origin of ferroelectric phase coexistence. (A) The heterogeneous calcined powder which consists of KNbO3-like and NaNbO3-like particles. (B) Polycrystalline ceramic sintered by using heterogenous calcined powder. (C) The homogeneous calcined powder which consists of (K0.5, Na0.5)NbO3 powder. (D) Polycrystalline ceramic sintered by using homogeneous calcined powder. Liquid phase formed during sintering at high temperature will be volatile easily and crystallize upon cooling, leading to the formation of alkaline-deficient phase, (K0.5x, Na0.5-y)NbO3.

3.5.1 Chemical Heterogeneity Chemical heterogeneity is a long-existing problem in KNN-based materials. Wang et al. [45] demonstrated a serious chemical heterogeneity problem in Li- and Ta-modified KNN piezoelectric material. They discovered that a thermodynamically 22

ACCEPTED MANUSCRIPT stable pairing of elements, i.e. K-Nb and Na-Ta, exist in the solid solution, which is invulnerable to prolonged annealing time. However, if Nb2O5 and Ta2O5 are precalcined before subjected to the calcination with other raw materials, e.g. K2CO3, Na2CO3 and Li2CO3, the chemical heterogeneity can be effectively circumvented. We

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believe that this is analogous to the A-site chemical heterogeneity discovered in the present study, namely, the coexistence of KNbO3 and NaNbO3 might be thermodynamically stable as well. Meanwhile, from the kinetics perspective, the

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diffusion-dependent grain growth behavior is also a determinant factor for heterogenous calcined powders. If an effective homogenization is skipped during the

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calcination, the ferroelectric phase coexistence will be mostly expected in sintered ceramics, resulting the multiple domain switching behavior shown in Fig. 3. Recently, Hinterstein et al. [46] have also revealed the coexistence of Na-rich and K-rich phases in pure KNN polycrystalline material by using high-resolution diffraction method and

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3.5.2 Liquid phase

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electron probe microanalysis.

For KNN system, perfect chemical homogeneity is always hard to achieve,

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due to the volatilization of alkalis during fabrication. Although a relatively homogeneous KNN calcined powder has already been obtained by using orthorhombic Nb2O5 as precursor, the ferroelectric phase coexistence was still observed in the sintered ceramic. Volatilization of alkalis can occur during the whole fabrication process, including calcination and sintering. It is worth noting that the volatilization of alkalis in the reagent form is more serious than in the synthesized product form [47]. For instance, at 900 oC, the vapor pressure of K over K2O is around 23

ACCEPTED MANUSCRIPT five orders of magnitude higher than the vapor pressure of K over KNbO3. Therefore, it is suggested that a serious volatilization might have already been experienced during calcination. The volatilization of alkalis during calcination is one of the major barriers of obtaining stoichiometry compounds. Furthermore, an adequate attention

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should be paid toward the volatilization during sintering. Upon the solidus temperature, formation of liquid phase will drastically accelerate the volatilization. Formation of liquid phase at grain boundaries will not only lead to abnormal grain

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growth, but also a core-shell structure [18, 20, 43]. In liquid phase sintering, when slowly cooled down from the eutectic point, the liquid phase will crystallize into

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secondary phases along grain boundaries, having segregated compositions [33, 43]. The multiple domain switching behavior observed in the strain loop (Fig. 3D) and the current-density loop (Fig. 3G) of KNN-O could be a sign of the coexistence of a secondary phase at the shell and a stoichiometry KNN phase at the core. Also, a little

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amount of secondary phase has been observed in the Fig. S3, might correspond to the

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outer shell layer. The secondary phase will possibly have an A-site deficient KNN

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is extreme.

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composition or even a B-site rich tungsten bronze composition when the volatilization

4. CONCLUSION

The fabrication of KNN piezoelectric material by the conventional solid-state

reaction method was revisited. Accompanied by a significant chemical heterogeneity, irregular grain growth was discovered in the calcined KNN powder when monoclinic Nb2O5 was used as a precursor. Chemical heterogeneity in calcined KNN-M powders led to the formation of multiple ferroelectric phases in sintered ceramics. We suggest 24

ACCEPTED MANUSCRIPT that the grain growth kinetic of the heterogenous calcined powder plays a decisive role. On the other hand, although the calcined KNN-O powder was relatively homogeneous, a secondary ferroelectric phase was still observed in the KNN-O ceramic sample. It is proposed that the liquid phase formed during sintering will

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crystallize into a new ferroelectric phase along grain boundaries by the end of firing.

Although the d33 performance in KNN-O, KNN-M1 and KNN-M2 are quite similar, a significant difference can still be observed in dielectric and ferroelectric

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performance. Apart from the “primary” d33, all of the “secondary” properties (e.g. εr,

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kp, and Qm) are equally important when it comes to a practical application scenario. As aforementioned, the development of KNN-based materials has been entering the technology transferring period, the reproducibility of functional properties should be highly stabilized. Chemical homogeneity in calcined powders, which is the key-point of the fabrication of KNN-based solid solutions, should be paid much attention

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because it will become much troublesome in complex KNN-based compositions.

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5. ACKNOWLEDGEMENTS

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This work was supported by National Nature Science Foundation of China (Grants

No.

51572143,

51332002),

and

Science

Challenge

Project

(No

JCKY2016212A503). The authors wish to thank Jürgen Rödel and Xiaowen Zhang for many useful discussions.

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