Scholarly article on topic 'Microstructure and piezoelectric properties of K5.70Li4.07Nb10.23O30-added K0.5Na0.5NbO3 ceramics'

Microstructure and piezoelectric properties of K5.70Li4.07Nb10.23O30-added K0.5Na0.5NbO3 ceramics Academic research paper on "Materials engineering"

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Academic research paper on topic "Microstructure and piezoelectric properties of K5.70Li4.07Nb10.23O30-added K0.5Na0.5NbO3 ceramics"

Journal of Advanced Ceramics

2014, 3(2): 147-154

DOI: 10.1007/s40145-014-0105-1

Research Article

ISSN 2226-4108 CN 10-1154/TQ

Microstructure and piezoelectric properties of K5.7oLi4.o7Nbio.23O3o-added Ko.5Nao.5NbO3 ceramics

Xuming PANG6, Jinhao QIU°*, Kongjun ZHUa

aState Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, China Department of Mechanical Engineering, Nanjing Tech University, Nanjing 210009, China

Received: January 19, 2014; Revised: March 26, 2014; Accepted: April 12, 2014 ©The Author(s) 2014. This article is published with open access at Springerlink.com

Abstract: Lead-free piezoelectric ceramics K0.5Na0.5NbO3-xmol%K5 70Li4.07Nb10.23O30 (x = 0-2.5, KNN-xmol%KLN) were prepared by conventional sintering technique. The phase structure and electrical properties of KNN ceramics were investigated as a function of KLN concentration. The results showed that small amount of KLN introduced into the lattice formed a single phase perovskite structure. The KLN modification lowered the phase transition temperature of orthorhombic-tetragonal (TO-T) and increased the Curie temperature (TC). Some abnormal coarse grains were formed in a matrix when the content of KLN was relatively low (1 mol%). However, normally grown grains were only observed when the sintering aid content was increased to 2 mol%. Proper content of KLN decreased the amount of defects, thus the remanent polarization increased and the coercive field decreased markedly, and the sinterability of the KNN ceramics was simultaneously improved with significant increase of piezoelectric properties.

Keywords: ceramics; sintering aid; phase transformation; electrical properties

1 Introduction

Lead-based ferroelectric materials, such as Pb(Zr,Ti)O3 (PZT), Pb(Mgi/3Nb2/3)O3-PbTiO3 (PMN-PT) and Pb(Zni/3Nb2/3)O3-PbTiO3 (PZN-PT), show excellent piezoelectric properties [1-3] and have been adopted for many applications. However, the development of harmless lead-free piezoceramics has gained a great deal of attention because of the near future restriction on the lead-based materials due to the environmental issues.

In the past several years, much attention has been paid to the alkaline niobate-based materials, and especially to the potassium sodium niobate

* Corresponding author. E-mail: qiu@nuaa.edu.cn

K0.5Na0.5NbO3 (KNN) family. KNN is one of the most promising candidates for lead-free piezoelectric ceramics because of its high Curie temperature (about 420 °C) and large electromechanical coupling factors [4,5]. However, the difficulty in sintering KNN under the atmospheric conditions is a serious drawback, and various techniques such as hot pressing and spark plasma sintering have been utilized in order to improve the sinterability of KNN ceramics [6,7]. Since these techniques are found unsuitable for use in industrial production, many sintering aids are researched by several researchers in order to sinter KNN under atmospheric conditions, such as K54Cu13Ta10O29, CuO and MnO2 [8-15]. Nevertheless, the piezoelectric properties of the KNN system are degraded in these cases although these sintering aids can improve the sinterability of the KNN ceramics. Therefore, the novel

sintering aids, which improve both the sintering behavior and piezoelectric properties, are key research.

Because K5.70Li4.07Nb10.23O30 (KLN) as aid has not been studied, the sintering behaviors and piezoelectric properties of KNN ceramics with KLN added are investigated by conventional sintering technique.

2 Experimental

A conventional ceramic fabrication technique was used to prepare K05Na0.5NbO3-xmol%K5.70Li4.07Nb1023O30 (0 ^ x ^ 2.5, KNN-xmol°/oKLN) ceramics using analytical-grade metal oxides or carbonate powders: K2CO3 (99%), Na2CO3 (99.8%), Li2CO3 (98%) and Nb2O5 (99.5%). The KNN and KLN powders were first synthesized at 900 °C for 5 h by a solid-state reaction method. After the calcination, KNN and KLN powders were weighted according to the formula of KNN-xmol%KLN and ball milled for 12 h. The resulting mixture was further mixed with polyvinyl alcohol binder solution thoroughly and then pressed into disk samples. The disk samples were sintered at 1100 C for 2 h in air.

Density of the samples was determined by the Archimedes method. The crystalline phase was analyzed using an X-ray diffractometer (D8 Advance). The microstructure was observed by a scanning electron microscope (JSM-5610LV/Noran-Vantage). Dielectric properties as functions of temperature and frequency were measured by an impedance analyzer (HP4294A). Polarization vs. electric field hysteresis loops were measured using a ferroelectric test system (TF Analyzer 2000). Silver electrodes were fired on the top and bottom surfaces of the sintered samples. The ceramics were poled under a DC field of 2 kV/mm at 110 C in a silicon oil bath for 30 min. The piezoelectric constant d33 was measured using a quasistatic piezoelectric constant testing meter (ZJ-3A, Institute of Acoustics, Chinese Academy of Sciences, Beijing, China).

3 Results and discussion

Figure 1 shows the X-ray diffraction (XRD) patterns of KNN-xmol%KLN ceramics. All of the ceramics exhibit single phase perovskite structure which indicates that excess Li+ and K+ may incorporate into the lattice. There are two peaks at about 45° which change obviously with different KLN contents. The

x=2.5 s ll M H

x=1.5 1 M rA A

x=1.0 ji m A i\

x=0 1 if M a /\

10 20 30 40 50 60

Fig. 1 XRD patterns of KNN-xmol%KLN ceramics with different KLN contents.

crystal structure can be distinguished from the relative intensity of these two peaks. For orthorhombic structure, the left peak at about 45° has higher intensity than that of the right peak, and it was indexed as (202) and (020), respectively. For tetragonal structure, the left peak at about 45° has lower intensity than that of the right peak, and it was indexed as (002) and (200), respectively. The phase structure of KNN-1mol%KLN is the orthorhombic structure while KNN-1.5mol%KLN exhibits tetragonal structure. Orthorhombic and tetragonal phases coexist in the ceramics when x is in the range of 1 < x < 1.5.

The lattice parameters of KNN-xmol%KLN ceramics are calculated by fitting the diffraction peak profile, as shown in Fig. 2. Clearly, there is a transition zone between the orthorhombic and tetragonal phases in the range of 1 < x < 1.5. When x is larger than 1.5, the materials become pure tetragonal phase. The tetragonality c/a is ~1.011 for KNN-1.5mol%KLN and increases to ~1.013 for the composition with x = 2.5.

J___□-- —□

- a=b

-------

- b —-0

0.0 0.5 1.0 1.5 2.0 2.5 KLN content (mol%)

Fig. 2 Lattice parameters of KNN-xmol%KLN ceramics as a function of the KLN contents.

An increase in tetragonality usually corresponds to a rise in the Curie temperature for a couple of perovskite solid solution ceramics, such as Pb(Zr,Ti)O3 [16].

Figure 3 shows the microstructures of KNN-xmol%KLN ceramics sintered at 1100 °C. For the pure KNN ceramic (i.e., x = 0), the grains have a diameter in the range of 10 ^m, and small amount of pores are observed (Figs. 3(a) and 3(b)). By the increasing x to 1, the grains become larger and more nonuniform (Figs. 3(c) and 3(d)). The ceramics are denser and almost no pore is observed. These results clearly show that the addition of KLN can improve the sintering performance of the ceramics. The grain growth is inhibited and average grain size is decreased with increasing x to 1, while the amount of pores decreases. This result explains that the grains of the KNN-1mol%KLN sample grow sufficiently.

For the KNN-1mol%KLN ceramic sintered at 1100 C, the average grain size of matrix grains in Figs. 3(c) and 3(d) is 3 ^m. However, some coarse grains (or areas), which are indexed by grains 1 and grains 2, are also observed. In particular, the extremely large grains with diameter up to 20-30 ^m as indexed by grains 1, can be clearly seen in Fig. 3(d). Obviously, this is a kind of abnormal grain growth (AGG) behavior, whose characteristic is the formation of some exceptionally large grains in a matrix of fine grains [17-19]. However, it seems that the AGG in the present material is different from the classical AGG behavior reported by other authors [20,21], because the abnormal grains in this study are much more like finer matrix grains aggregated together probably due to the formation of a liquid phase [22,23]. Because K+ incorporates into the lattice which gives rise to the relatively higher content of K in KNN-xmol%KLN samples, the solidus temperature of KNN decreases with increasing the content of KLN from phase diagram of the KNbO3-NaNbO3 system [24]. On the other hand, the amount and fluidity of liquid phase increase with increasing KLN at 1100 °C. When the content of KLN increases to 1 mol%, the numbers of large grains apparently increase. Meanwhile, two kinds of large grains can be classified. The one is abnormal large grains indexed by grains 1, while the other one is normal large grains indexed by grains 2 in Figs. 3(c) and 3(d). Furthermore, by increasing x to 2, only normal grain growth (NGG) behavior takes place and no abnormal large grain is observed as shown in Figs. 3(g)-3(j).

Fig. 3 SEM micrographs of KNN-xmol%KLN ceramics with various KLN contents: (a) and (b) x = 0; (c) and (d) x = 1; (e) and (f) x = 1.5; (g) and (h) x= 2; (i) and (j) x = 2.5.

On the basis of the microstructure evolution and our previous study on sintering aid, it is thought that AGG and NGG are related to the presence of a liquid phase. The formation mechanisms of a liquid phase in KNN and KNN-based ceramics have been just discussed. A small amount of liquid phases may form at first in some local areas probably when the content of KLN is 1 mol%, and the liquid phase amount in different local areas may be nonuniform owing to low fluidity and volatilization of alkali metal ions [25]. Based on the powder sintering theory [26], the grain growth is controlled by dissolution and precipitation mechanisms for the more amount of liquid phase, while it is controlled by diffusion for the less one. Therefore, classical large grains are observed in the local areas where the amount of liquid phase is more. Because of less liquid phase amount in the other areas, the small grains can not dissolve and become self-organized to be aligned into clusters, as can be seen in Fig. 3(d) where the small grains obey a discipline of controlled alignment by diffusion. Generally, a liquid phase

contributes to sintering by accelerating particle redistribution because of the enhanced high atom mobility. Besides, similar to the organic additives that accelerate transformation as the surfactants, liquid phase is supposed to act as a kind of surfactant during sintering at high temperature [22]. Also, the volatilization of alkali metal oxides in KNN-based ceramics might play a specific role in the microstructure formation [25]. Thus, the self-assembly of aggregation can be aided through a combined action of small grain surfactant interactions. Another positive effect of a liquid phase on the sintering is enhancement of the final sample density via a high capillary force. Figure 4 is a schematic showing how the AGG structure of a coarse grain is formed. As shown in Fig. 4, when the less liquid phase appears, the small grains distribute randomly and some small holes are even not eliminated. As the sintering time increases, the surfactant-mediated interactions accelerate the formation of self-assembled clusters by small grains. However, when the sintering time further increases, several groups of clusters with small disorientations are supposed to be bounded together to form coarse clusters. According to the classical grain growth [26], the grain growth rate is strongly dependent on the differences in grain radius and disorientation angles with the surrounding grains. For the small grains in the inner cluster structure, the grain boundary driving force is too small to move the boundary migration due to their small disorientations. Therefore, the small grains in the inner cluster structure have almost no growth, as shown in Fig. 3(d). Because of the more amount and higher fluidity of liquid phase, the matrix is easily filled and the proportion of NGG is improved

with increasing KLN as shown in Figs. 3(e) and 3(f). When the liquid phase amount exceeds a certain value, the grain growth is controlled by dissolution and precipitation mechanisms, so AGG completely disappears.

Figure 5 shows the density and property variations of KNN-xmol%KLN ceramics sintered at 1100 °C for 2 h. As shown in Fig. 5, it has been observed that the KNN ceramic without KLN has a lower bulk density. The density of the KNN-xmol%KLN samples increases when the content of KLN increases from 0 to 1 mol%, and then slightly decreases when the content of KLN is above 1 mol%. Proper amount of KLN modification can increase the piezoelectric properties markedly. The J33 and kp for KNN-1 mol%KLN are 121 pC/N and 0.39, respectively. The significant enhancement in the piezoelectric properties results from the increase of bulk density and the phase structure of KNN-1 mol%KLN which is around the polymorphic phase transition of the orthorhombic and tetragonal phases, as shown in Fig. 1. The polymorphic phase transition causes the higher piezoelectric activity owing to the more possible polarization states resulting from the coexistence of the orthorhombic and tetragonal phases. For x > 1, d33 and kp decrease probably because lower density with the increasing content of KLN. The Qm value of KNN-xmol%KLN ceramics is enhanced with increasing KLN within the compositional range of KLN from 0 to 1 mol%. The highest Qm value of 68 is achieved in the KNN-1 mol%KLN ceramic. Qm might be related to domain motion difficulty. The decrease in Qm value is caused by the low density and the excess addition of KLN. As shown in Fig. 5, er increases with increasing

Fig. 4 Schematic diagram showing the formation procedure of AGG.

-n— Density " -o- Relative density --- ___ □— —""d -o

□ — ' ................. .

'-0-1/33 -o-(ip LI -'............... ________________u. --°x - o- --

O—--- —o— -o

-0-Er _□ -□

- V Qm o—--

---- -□

—--— —o--

0.5 0.4 0 3 0.2 150

0.0 0.5 1.0 1.5 2.0 2.5

KLN content (mol%)

Fig. 5 Density, d33, kp, er and Qm values of KNN-xmol%KLN ceramics with 0 x 2.5.

KLN. These results confirm that the properties of KNN-1mol%KLN ceramic become optimum. The sample with x = 1.0 exhibits good properties: d33, kp, er and Qm show their peak values of 121 pC/N, 39%, 404 and 64, respectively.

Figure 6 shows the temperature dependence of dielectric constant er (at 1 kHz) for KNN-xmol%KLN (x = 0, 1.5, 2.5) ceramics, and the inset shows the low temperature ferroelectric-ferroelectric transition (TO-T). With the increase of KLN, the TC increases and the maximum dielectric constant at TC decreases. The ferroelectric-ferroelectric phase transition shifts to lower temperature, and the peak is slightly broadened which shows a diffusive nature with the increase of KLN. TO-T is near room temperature at x = 1.5. When the content of KLN increases, Li+ and K+ could be incorporated into the lattice and compensate the volatilization of K+ [25], so TC increases. According to previous results [27,28], the incorporation of Li+ will also cause the increase of TC and decrease of TO-T.

Figure 7 shows the P-E hysteresis loops of KNN-xmol%KLN ceramics. The Pr and EC values for pure KNN ceramic are 6.36 (j,C/cm2 and 1.26 kV/mm, respectively. Proper amount of KLN modification increases the remnant polarization and decreases the coercive field. It should be noted that the remnant polarization increases by 140% but the coercive field decreases by 24% when x = 1 (Pr = 15.21 ^C/cm2, EC = 0.957 kV/mm). There are some mechanisms which are related to the increase of polarization and decrease of coercive field: the incorporation of Li+ and the

50 100 150 200 250 300 350 400 450 500 550

Temperature (°C) Fig. 6 Temperature dependence of dielectric constant er for KNN-xmol%KLN ceramics.

ro o Q-

-2-10 1 2 Electric field (kV/mm) Fig. 7 P-E hysteresis loops of KNN-xmol%KLN ceramics.

decrease of defects. Li+ has smaller ionic radius than K+ and will cause the tilting of the Nb-O octahedron which increases the remnant polarization. The decrease of coercive field is mainly attributed to the decrease of the amount of defects. It is well-known that K2O is volatile and K+ vacancies will be left after sintering. The K+ in KLN can compensate these vacancies and the amount of defects decreases. After x ^ 1, Pr decreases and EC increases due to low density and poor microstructure as shown in Figs. 3(e)-3(j).

4 Conclusions

KNN-xmol%KLN ceramics were prepared by the solid-state reaction method. The results show that the small amount of KLN (0 ^ x ^ 2.5) incorporates into the lattice and forms the single phase perovskite

structure. It exhibits the polymorphic phase transition from orthorhombic structure to tetragonal structure when the content of KLN increases from 1 mol% to 1.5 mol%. The KNN-1 mol%KLN ceramic shows AGG behavior. However, when the amount of KLN is increased above 1.5 mol%, abnormally grown grains disappear and relatively uniform microstructure with normally grown grains is formed. The AGG and NGG behaviors should be related to the volatilization of alkali metal oxides and the presence of liquid phase amount under different amount of KLN. KLN modification lowers TO-T and increases TC with increase of x. KNN-1mol%KLN ceramic shows a very high remnant polarization and low coercive field, and its piezoelectric properties are also the best among them.

Acknowledgements

This work was supported by the Natural Science Foundation of Jiangsu Province, China (BK20130791), Projects of International Cooperation and Exchanges NSFC (51161120326), Aeronautical Science Fund (20131552025), the NUAA Fundamental Research Funds (NS2013008), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Open Access: This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.

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