Composition, microstructure and electrical properties of K0.5Na0.5NbO3 ceramics fabricated by cold sintering assisted sintering

Composition, microstructure and electrical properties of K0.5Na0.5NbO3 ceramics fabricated by cold sintering assisted sintering

Accepted Manuscript Title: Composition, microstructure and electrical properties of K0.5 Na0.5 NbO3 ceramics fabricated by cold sintering assisted sin...

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Accepted Manuscript Title: Composition, microstructure and electrical properties of K0.5 Na0.5 NbO3 ceramics fabricated by cold sintering assisted sintering Authors: Ma Jianzhang, Li Hanying, Wang Huajing, Lin Cong, Wu Xiao, Lin Tengfei, Zheng Xinghua, Yu xing PII: DOI: Reference:

S0955-2219(18)30709-X https://doi.org/10.1016/j.jeurceramsoc.2018.11.044 JECS 12196

To appear in:

Journal of the European Ceramic Society

Received date: Revised date: Accepted date:

4 September 2018 23 November 2018 24 November 2018

Please cite this article as: Jianzhang M, Hanying L, Huajing W, Cong L, Xiao W, Tengfei L, Xinghua Z, xing Y, Composition, microstructure and electrical properties of K0.5 Na0.5 NbO3 ceramics fabricated by cold sintering assisted sintering, Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.11.044 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.

Composition, microstructure and electrical properties of K0.5Na0.5NbO3 ceramics fabricated by cold sintering assisted sintering Ma Jianzhang1, Li Hanying1, Wang Huajing1, Lin Cong1,2,3* [email protected] , Wu Xiao1,2*

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[email protected] , Lin Tengfei1,2, Zheng Xinghua1,2, Yu xing3

Department of Material Science and Engineering, Fuzhou University, Fuzhou, China

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Key Laboratory of Eco-materials Advanced Technology, Fujian Province University, Fuzhou,

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China 3 National Analysis

Center for Iron & Steel, Central Iron and Steel Research Institute, Beijing, China

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Corresponding authors:

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Abstract

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In this paper, cold sintering was served as a forming method to assist the conventional

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sintering, which is so-called cold sintering assisted sintering (CSAS) method. Lead-free K0.5Na0.5NbO3 piezoelectric ceramics were prepared by the CSAS method, and the

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effects of the different procedures on the sintering behaviors and electrical properties

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of KNN ceramics were studied. Compared with conventional sintering (CS), cold sintering process can induce potassium-rich phase on the KNN particle surface, and remarkably increase both the green and sintering density of KNN ceramics. Meanwhile,

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the potassium-rich phase would transform to K4Nb6O17 second phase on the grain surface, and subsequently suppress the volatilization of potassium element. The sinterability and electrical properties were greatly improved, and KNN piezoelectric ceramics with high performance can be manufactured in a wide sintering temperature

range (1055°C-1145°C), which proves that CSAS has the potential to be an excellent sintering technique for producing KNN based ceramics.

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Keywords: Cold sintering, KNN, Lead free piezoelectric ceramics, Volatilization

1. Introduction

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High-performance lead free piezoceramics have drawn much attention because of

their excellent piezoelectricity, strong ferroelectricity, and wide application potentials

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in many electronic devices (e.g., sensors, actuators, ultrasound transducers, etc.).[1-5]

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Among all the lead-free piezoelectric ceramics, (K0.5Na0.5)NbO3 (denoted hereafter as

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KNN) has become one of the most extensively studied piezoelectric systems due to its

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large d33 and high Tc.[6-8] However, it is very difficult to obtain high-density KNN

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ceramics by ordinary sinitering method because of their poor sinterability and high volatility.[9] Furthermore, very narrow sintering temperature range is also a big

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challenge for its industrial production.[10, 11]Over the past few decades, numerous

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efforts have been made on KNN-based ceramics to improve the sinterability of KNN based ceramics.[12-15] Very recently, a novel sintering technology, which is called as cold sintering process

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(CSP), is established to achieve dense ceramic solids at dramatically reduced sintering temperatures.[16-22] Basically, CSP needs a temporary aqueous environment to accelerate densification by a mediated dissolution–precipitation process under some external pressure ranged from 50MPa to 500MPa.[17] With the assistance of pressure,

a proper liquid medium can wet the particle interfaces to dissolve the particle edge, and meanwhile can lead to a facilitated particle rearrangement. Both of these effects will help to increase the density of ceramics compacts. By far, the feasibility of CSP to densify more than 50 chemistries with 80%-99% of theoretical density below the

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temperature of 300°C have been demonstrated.[16] For some specific ingredients, although they cannot be directly densified through CSP method, the samples proceeded

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by CSP can be easily fabricated by the following conventional sintering at much lower

temperature than usual. For example, benefited from the CSP, Guo et al. successfully

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reduced the sintering temperature from 1400°C to 1200°C for zirconia ceramics during

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the following conventional sintering.[20] Congruent dissolution can be observed in this

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case, and no segregation appeared during the precipitation.[23]For some substances

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such as BaTiO3, previous studies have revealed an incongruent dissolution phenomenon

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when BaTiO3 particles are cold sintered with the assist of Ba(OH)2/TiO2 suspension.[24] Dense BaTiO3 ceramics with ~95% relative density containing the carbonate-rich glass

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phase was first obtained at the low temperature of 180oC via a hydrothermal synthesis

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assisted cold sintering process, and then was crystallized into BaTiO3 after heat treatment. [17] Whatever congruent or incongruent dissolution is involved, these two methods are derived from cold sintering technique and then followed by conventional

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sintering or a heat treatment, and can be called as cold sintering assisted sintering (CSAS). It has been proved that CSAS method can play a positive role in increasing the green density of ceramics, and then lowering down the sintering temperatures. Similar to BaTiO3, incongruent dissolution would be expected when the KNN

particles were cold sintered because of their similar perovskite structure of ABO3, where A-site cations have higher leaching rate than B-site. Meanwhile, deduced from the previous chemical stability study of KNN and relevant binary compounds in aqueous environment, potassium cations in A-site of perovskite are preferentially

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leached out, and the faster leaching rate of K+ ions might precipitate into K-Nb-O compound and recrystallize on the surface of KNN particles.[25, 26] One can predict

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that microstructures and properties KNN ceramics would be greatly affected after cold

sintering due to the segregation of composition and the uneven leaching rate of A-site

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cations. In this study, pure KNN ceramics were prepared by cold sintering assisted

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sintering process, and the effects of CSAS on the composition, microstructure and

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2. Materials and Experiments

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electrical properties of KNN ceramics were carefully studied.

K0.5N0.5NbO3 powders were synthesized through a solid-state reaction route. For

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powder synthesis, the raw materials K2CO3 (99.0%), Na2CO3 (99.8%), and Nb2O5

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(99.99%) (Sinopharm Chemical Reagent Co., Ltd, CN) were dried under vacuum at 120°C for 8h to remove the absorbed moisture, and weighed according to the stoichiometric ratio. The carbonates and oxides were milled at 380 rpm for 10h in a

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planetary ball-mill using ZrO2 as milling balls and ethanol as milling medium. The slurry was dried and calcined at 850°C for 4h to form KNN powders, and then the powders were ball-milled and sieved to remove the aggregates. The fully dried KNN powders were uniformly mixed with different ratio of deionized water (from 5 wt.% to

15 wt.%) so that liquid phase can be intentionally introduced between the particleparticle interfaces. Then the moist powders were uniaxially pressed under 350 MPa at 120°C for a cold sintering process for 30 min. The as-prepared ceramic pellets were baked at 120°C for 6h to remove the residual water, and then were sintered in a

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conventional electric furnace at different temperatures for 4h. These samples are referred to CSAS ceramics hereafter. To make a comparison, the same powders, mixed

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with 2.5 wt.% polyvinyl alcohol (PVA), were also pressed under 350 MPa at room temperature (named as CS samples), and subsequently were sintered in the same batch

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with CSAS samples.

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The densities of all green pellets were determined by measuring the mass and the

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volume. Archimedes method was used to test the densities of the sintered specimens.

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The KNN powders were pressed into pellets by both conventional method and cold

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sintering method, and then grinded carefully into particles. The surface morphology of KNN particles were observed by transmission electron microscopy (TEM: TECNAI

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G2 F20, FEI, USA). Energy dispersive spectrometer (EDS: X-MAX 50, OXFORD

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U.K.) was also applied to perform the composition distribution of the particles. Sintered pellets were crushed into powders for analyzing the phase structure of KNN ceramics sintered in different temperatures by X-ray diffraction (XRD: ULTIMA III, Tokyo,

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Japan) with Cu-K radiation. The morphology and microstructures of sintered samples were polished and thermal etched before being observed by scanning electron microscopy (SEM: SUPRA 55 SAPPHIRE, Carl Zeiss, Germany). Crushed ceramics were dissolved into hydrofluoric acid, and the relative contents of potassium and

sodium elements were determined using inductively coupled plasma mass spectrumetry (ICP: ICAP-MS-QC, Thermo, USA). For electrical properties characterization, KNN ceramics were polished and pained with silver paste on the sample surfaces. The specimens were immersed in silicon oil

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and poled at 150°C by applying a DC field of 2.5kV/mm for 20 min, and aged for 24 h before the piezoelectric and dielectric properties measurements. The piezoelectric

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properties were determined by a quasistatic piezoelectric coefficient testing meter

(YE2730A, SINOCERA, China). To investigate the temperature stability of CSAS and

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CS produced ceramics, the d33 of KNN ceramics sintered at 1115°C were measured

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after being annealed at different temperatures from room temperature to 325°C.

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Dielectric properties were tested as a function of temperature by a LCR meter

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(HP 4294 A, Agilent Technologies, Inc. Palo Alto, CA). The ferroelectric hysteresis

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loops were observed at room temperature using a ferroelectric tester (TF2000,

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AixACCT, Aachen, Germany) at room temperature.

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3. Results and Discussion The relative densities of green and sintered KNN samples prepared by CS and CSAS

methods at 1130oC are plotted in Fig.1, as well as the piezoelectric constants d33. The

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CSAS samples were prepared from x wt.% water contents (named as CSASx samples). Compared with the green density of 53% for CS samples, the density of green pellets after cold sintering procedure greatly increased to more than 65% of relative density. The improvement of particle mobility by the water-layer on the particle surface will be

beneficial to rearrangement of particles, which results in the increase of packing density of ceramics.[27] And also, the water may dissolve the particle edges and wet the particle interfaces, and the precipitates will fill the interstitials of particles, which would also increase the green density.[28] [29]For sintered ceramics, the maximum relative density

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can reach to 98% when the added water content is 10 wt.%, which is comparable to those sintered by pressure assisted sintering methods such as spark plasma sintering or

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hot press sintering.[30, 31] A further increase of water content to 15 wt.% will lead to a slight decline of both green and sintered densities. The expansion of pore volumes

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from evaporation of superfluous water may be the reason for the decline of green

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density, and would ultimately reduce the sintering density.

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Fig. 2 shows the XRD patterns of CSAS and CS ceramics after sintering at 1130°C

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for 4 h, and the synthesized KNN powders were also examined for comparison. For CS

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ceramics and CSAS ceramics with low water content (CSAS5), a pure perovskite phase with typical orthorhombic symmetry can be observed in the XRD patterns. Compared

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with that of the calcined powders, no noticeable secondary phase was formed which

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suggests that the addition of minor amount of water would not induce any noticeable phase transformation during CSAS procedure. However, very few amount of second phases in the XRD pattern of CSAS10 samples were found, identified as K4Nb6O17

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(JCPDS 14-0287), and a further increase of water content to 15 wt.% will increase the content of K4Nb6O17 phase. Therefore, for the fact that CSAS15 samples possess higher green density but lower sintered density if compared with CSAS5 sample, we realized that the formation of K4Nb6O17 second phase might be a more important reason for the

decrease of ceramic density for the CSAS15 samples, and consequently be detrimental for the performance of KNN ceramics. As shown in Fig. 1(c), the piezoelectric coefficient (d33) first increases with added water amounts, and then decreases greatly due to the formation of secondary phase. The optimized d33 value (d33 ~ 130 pC/N) was

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achieved in the ceramics with water content of 10 wt.%. Because of the high density, low second phase content and excellent electrical properties, the CSAS samples for the

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following research, if not specified, were all prepared by adding 10 wt.% water for cold sintering. R.D. of green pellets (%)

70

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60

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(b)

90 130

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95

(c)

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120

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R.D. of ceramics (%)

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55

50 100

d33 (pC/N)

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(a) 65

110

100

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CS

CSAS5

CSAS10

CSAS15

Fig. 1 Column chart of relative densities and d33 for the CS and CSAS ceramics sintered at 1130oC.

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Fig. 2 (a) XRD patterns of the KNN powders, CS ceramics and CSASx ceramics with x wt.% deionized water as additive and sintered at 1130oC; (b) a magnified view of the patterns in the 2θ range from 25° to 30°.

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In order to clarify the formation mechanism of K4Nb6O17 second phase during cold

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sintering process, TEM tests were performed for the grinded KNN powders with and without cold sintering, and their corresponding EDS results are also displayed in Fig.

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3. As shown in Fig. 3(a), in the case of cold sintered powders with 10 wt.% water, some

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amorphous precipitates with several nanometers in size can be observed at the surface of KNN particles, which is pointed out in the enlarged photograph by a black arrow in

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Fig. 3(c). For comparison, the powders without cold sintering (Fig. 3(b), (d) and (e)) shows up a clean and smooth particle surfaces. EDS results are shown in Fig. 3(f) to

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distinguish the chemical constitution. The results reveal that for the precipitates on the cold-sintered grain surface, potassium content was several times higher (K/Na molar ratio of 12.7) than both the grain interior (K/Na ratio close to 1.2) and the grain surface (K/Na ratio of 2.1) of the samples without cold sintering process. Ozmen has studied the chemical stability of K and Na element in the aqueous medium, and found out that

due to the lower ionic strength of potassium ions, they can dissolve in water more than three times in content than the sodium ions.[25, 26] Therefore, it suggests that these amorphous K-rich precipitates was formed at grain surface through a “dissolutionprecipitation” process during the cold sintering process, where water will promote the

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dissolution process and heat can accelerate the precipitation process. Without the assistant of heating or water during cold sintering, the samples were extremely brittle

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and hard to be set. No matter how much water was added, all the samples pressed at

room temperature were broken into pieces after drying, which prove that the amorphous

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precipitates can bind the particles together and strengthen the connection among KNN

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particles, and ultimately increase the strength of green pellets. According to the results

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of XRD and TEM, it is reasonable that the amorphous K-rich phase covered on the

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following sintering procedure.

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KNN particles (in Fig.3) will transform into K4Nb6O17 second phase during the

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Fig. 3 TEM images of the crushed and grinded powders from (a) CSAS and (b) CS green pellets; (c, d, e) The enlarged images of the yellow square regions and (f) the corresponding EDS results by point analysis.

In addition, the presence of amorphous K-rich phase would increase the surface

energy of the particles, and be able to enhance the surface diffusion, therefore lower down the sintering temperature. The influence of the sintering temperature on the sintered density is shown in Fig. 4. It can be easily observed that the relative densities

of CSAS produced ceramics are much higher than those of CS produced ceramics within all the sintering temperature range. It can reach to 92% R.D. at 1055°C, while only 85% relative density can be achieved for the CS samples. The sintering density experienced a rising trend until the temperature exceeded 1130°C. The drastic

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volatilization might be the reason for the reduction when the sintering temperature reached to 1145°C[32]. As a result, high relative density over 94% can be achieved for

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the CSAS samples in a very wide sintering range from 1070°C to 1145°C, while 1130°C is the only sintering temperature point for conventional sintering to obtain the similar

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value of density. It can conclude that the cold sintering process can effectively lower

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down the sintering temperature of KNN ceramics and extend the range of feasible

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sintering temperature.

Fig. 4 Relative densities of KNN ceramics sintered at various sintering temperatures.

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Fig. 5 Microstructure of the polished and thermal etched (a) CS and (b) CSAS produced KNN ceramics at different sintering temperatures

Fig. 5(a) and (b) gives the SEM images of the microstructure of polished surface for

CS and CSAS produced KNN ceramics sintered at various temperatures. Compared with the CS samples, less pore volume and smaller pore size can be found in the ceramics prepared by CSAS method, which is consistent with the density results.

Meanwhile, larger grains with more uniform grain size distribution can be observed in CSAS ceramics when sintered below 1130oC, while some abnormal grains can be found in the CS samples. A further increase of sintering temperature to above the solidus temperature for K0.5Na0.5NbO3 (such as 1145oC) would cause many coarse grains with

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abnormal growth for both CS and CSAS samples. The appearance of liquid phase leads to abnormal grain growth,[11, 33] and meanwhile accelerates the evaporation of alkali

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metal elements, which may result in severe density decline and obvious phase segregation.

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As known, sintering temperature is a very sensitive factor for producing KNN based

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ceramics because of the uncontrollable evaporation of alkali elements, which may lead

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to the formation of unexpected secondary phase.[34] The XRD profiles of the ceramics

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sintered at various temperatures are shown in Fig. 6(a). The results suggest that the

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structures of KNN ceramics were kept in the orthorhombic perovskite structure within a wide sintering temperature range from 1055°C to 1130°C, except the appearance of

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K4Nb6O17 phase for CSAS samples. Beyond that, there are no obvious phase

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transformation in this temperature range for both CS and CSAS samples. With higher sintering temperature of 1145°C (above the solidus temperature for KNN ceramics), a secondary phase out of the perovskite background can be clearly identified for CS and

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CSAS samples. The secondary phase match well with a partially filled tetragonal tungsten bronze (TTB) structure (K6Nb10.88O30, JCPDS 87-1856) as shown in Fig. 6(b). The compositional precipitation may be attributed to the volatilization of alkaline elements during sintering.[34, 35] The appearance of the TTB phase would play a

negative role in densification, and can be detrimental for the electrical properties of KNN ceramics. The enlarged XRD patterns in the range of 2 from 45° to 45.8° are shown in Fig. 6(c). The XRD peaks seem to shift slightly forward to large angle with the increasing

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temperature, indicating a decrease of lattice parameters of KNN crystals. Since no dopants were added in this study, the reduction of lattice parameters may result from

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the volatilization of A-site elements such as K and Na. It can be suggested from Fig.

6(c) that the evaporation of alkali ions was much heavier for CS samples, whose XRD

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peaks exhibited a more distinct shift. Meanwhile, the decrease of cell parameters may

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atom radius of K is larger than Na.[34]

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also be ascribed to the different molar ratio of sodium and potassium ions since the

Fig. 6 (a) XRD patterns of KNN ceramics prepared by CSAS and CS methods at different sintering temperatures; (b) and (c) a detail of the XRD diffraction pattern in the 2θ range from 25° to 30° and 45.0° to 45.8°, respectively.

Fig. 7 shows the ICP results of the variation of K and Na element contents after

sintering, as well as the K/Na ratio. The measured contents are normalized to Nb element, which is considered as a non-volatized element during sintering. K and Na contents both decreased with the increasing of sintering temperature. It can be specifically manifested from the ICP results that the volatilization of potassium is much

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more severe than sodium, especially when the sintering temperature exceeded to 1115°C. This should be ascribed to the low melting point of KNbO3 (~1090oC) if

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compared with NaNbO3 (nearly 1420oC)[33].

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Fig. 7 ICP test results of KNN ceramics prepared by CSAS and CS procedures at various sintering temperatures: (a) molar content and (b) molar ratio of K to Na elements. The nominal composition of the specimen is (K0.5Na0.5)NbO3. All the alkaline elements ratio have been normalized to Nb content.

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In addition, it should be noted that the volatilization of K and Na elements was much

more moderate for the CSAS samples. If comparing the K/Na ratio of CSAS and CS

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samples, the difference of K/Na ratio values between CSAS and CS samples were enlarged with increase of sintering temperature. At the final stage of sintering, the pore volume and the amount of open pores will greatly influence the level of evaporation. In this sense, the higher packing density and the resulting higher sintering density of CSAS samples should be one of the most important reasons for suppressing the strong

volatilization of the alkali metal elements, especially for KNbO3, which has lower melting point than NaNbO3. Besides the higher sintering density, another probable reason for the reduction of evaporation is the formation of K4Nb6O17 phase for CSAS samples. As mentioned

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before, the K-rich amorphous precipitates will transform into K4Nb6O17 second phase during sintering procedure, because the formation of K4Nb6O17 phase is greatly related

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to the added water in the cold sintering process. The coverage of K4Nb6O17 on the grain surface of KNN would help to restrict the evaporation of alkali elements due to the

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higher melting point of K4Nb6O17 (1177oC) than KNN (~1110oC).[36] In this sense, the

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increase of sintering density and the formation of K4Nb6O17 coverage can greatly

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suppress the evaporation of alkali elements during the final stage of sintering, and result

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in compositional uniformity and accurate stoichiometry, which would have positive

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effects on the electrical properties of KNN ceramics. The dielectric and piezoelectric properties of KNN ceramics prepared by CSAS and

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CS methods are shown in Fig. 8. As shown in Fig. 8(a), benefiting from high density

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and stable sintering behavior, KNN ceramics produced by CSAS possess high d33 values in the sintering temperature range. The sintering temperature for CSAS samples can lower down to 1055°C to maintain a practicable d33 value of 105pC/N, whereas

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only 65pC/N for CS samples sintered at the same temperature. The trend of piezoelectric coefficient with the rising sintering temperature is very similar to the density profile in Fig. 4, except the fact that the maximum value of d33 was obtained at 1115°C, a little bit lower the temperature where the highest density was achieved. This

phenomenon must be relative to the different volatilization degree of K and Na, which results from the severe evaporation of KNbO3 when the heating temperature exceeds its melting point[37]. Meanwhile, the value of d33 for CSAS produced ceramics decreased only 3% (from 131 to 127pC/N) when the environmental temperature

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increased from 25°C to 230°C (show in Fig. 8(b)), which means they are very stable in a large scale of operation temperature range.

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The temperature dependence of relative dielectric constants and dielectric loss of the KNN ceramics are shown in Fig. 8(c) and (d), and the results indicate that both of the

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CSAS and CS samples exhibited relatively high curie temperatures (Tc) and

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orthorhombic-tetragonal transition temperature (To-t). The (To-t, Tc) of the CSAS and

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CS produced ceramics sintered at 1115°C are (213°C, 411°C) and (205°C, 409°C),

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respectively. The slightly shift of To-t maybe caused by the fact that the loss of A-site

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elements in CSAS is less than CS samples. It should be noted that the CSAS produced ceramics maintains higher dielectric constant and lower loss from room temperature to

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500°C, and the highest value of dielectric constant is close to 11000 (over 70% higher

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than the maximal value for CS samples), indicating that CSAS produced KNN ceramics

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possess excellent dielectric properties.

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Fig. 8 Piezoelectric coefficient (a) at different sintering temperatures and (b) at different operating temperature for ceramics sintered at 1115°C; Temperature dependence of relative dielectric constant and dielectric loss for the (c) CSAS and (d) CS produced ceramics at 1115 °C; (e) P-E hysteresis loops of the KNN ceramics sintered by different sintering methods at 1115 °C measured at room temperature at 10 Hz.

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Fig. 8(e) shows the P-E hysteresis loops of the KNN ceramics sintered at 1115°C by

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CSAS and CS methods. Both of the P-E hysteresis loops are well saturated, which confirms excellent ferroelectric properties. The remnant polarization Pr and coercive

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field Ec for CSAS and CS produced KNN ceramics are about 17.3C/cm2, 8.3kV/cm2 and 12.2C/cm2, 13.5kV/cm, respectively. As known, the evaporation of A-site cations

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would increase the oxygen vacancy concentration, which may enhance the pinning effects and decrease domain wall mobility, and correspondingly increase the coercive field. In this sense, higher remnant polarization Pr and lower coercive field Ec for CSAS samples might relate to fewer volatilization contents of A-site cations such as Na+ and K+.

4. Conclusions High quality KNN ceramics were prepared by cold sintering assisted sintering method, and the effects of the sintering process on the composition, microstructure, and

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electrical properties were carefully investigated. Compared to the green pellets that were pressed with PVA as binder (53% of theoretical density), the cold-sintered green

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pellets demonstrated an improved density (above 65% R.D.), and the final density can

reach to nearly 98% after following sintering procedure. The TEM and EDS results

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demonstrated that incongruent dissolution was dominant during cold sintering resulting

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in a faster leaching for A-site cations than B-site. And meanwhile potassium ions had a

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much higher leaching rate than sodium ions, and formed some K-rich amorphous

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precipitates on the KNN particle surface after cold sintering, which would transform to

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K4Nb6O17 second phase during the subsequent sintering process. Meanwhile, the increased green density can effectively lower down the sintering temperature, and

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greatly increase the sintering density of final products. The formation of K4Nb6O17

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phase and the increase of sintering density can effectively reduce the evaporation of alkali elements (especially for potassium), and consequently improve the properties of KNN ceramics. The CSAS produced ceramics sintered at 1115°C exhibited excellent

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ferroelectric, piezoelectric, and dielectric characteristics. Its piezoelectric coefficient, dielectric constant, dielectric loss at 10 kHz, remnant polarization and coercive field were 131pC/N, 10922, 4.86%, 17.3C/cm2 and 8.3kV/cm, respectively. These values are reasonably good for pure KNN system without any special dopants. Moreover, another

inspiring result for CSAS technique is that one could produce KNN ceramics in a very wide range of sintering temperature (1055-1145oC) without sacrificing their properties. This may be attributed to the enhancement of densification during CSAS procedure at relatively low sintering temperature, and the reduction of alkali volatilization at high

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sintering temperatures. To sum up, the method of CSAS could be an effective solution

fabrication of high-performance KNN-based ceramics.

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Acknowledgement

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for the problem of poor sinterability of KNN ceramics, and a great help to the

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The authors would like to thank for the support of the National Key Research and

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Development Program of China (No. 2016YFB0700203) and the Natural Science

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Foundation of Fujian province (No. 2018J01753). And this work was also sponsored

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by the National Natural Science Foundation of China (No. 51602055), and the “Qishan Scholar”Scientific Research Startup Project of Fuzhou University (No. 650338). The

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authors would also like to appreciate Dr. Xinqi Zhang from testing center of Fuzhou

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University for the TEM tests and discussion.

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