Scholarly article on topic 'Investigation on transition behavior and electrical properties of (K0.5Na0.5)1-xLixNb0.84Ta0.1Sb0.06O3 around polymorphic phase transition region'

Investigation on transition behavior and electrical properties of (K0.5Na0.5)1-xLixNb0.84Ta0.1Sb0.06O3 around polymorphic phase transition region Academic research paper on "Materials engineering"

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Academic research paper on topic "Investigation on transition behavior and electrical properties of (K0.5Na0.5)1-xLixNb0.84Ta0.1Sb0.06O3 around polymorphic phase transition region"

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Investigation on transition behavior and electrical properties of (K0.5Na0.5)1-xLixNb0.84Ta0.1Sb0.06O3 around polymorphic phase transition region

Chen Zhu, Wenchao Wang, Jiquan Huang, Fei Tang, Yang Liu, Honglin Shi, Fangyu Wang, Chong Wang, Yongge Cao, and Xuanyi Yuan

Citation: AIP Advances 4, 017125 (2014); doi: 10.1063/1.4863200 View online: http://dx.doi.org/10.1063/1.4863200

View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/4/1?ver=pdfcov Published by the AIP Publishing

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Investigation on transition behavior and electrical properties of (Ko.5Nao.5)1-xLixNbo.84Tao.1Sbo.o6O3 around polymorphic phase transition region

Chen Zhu,1,2 Wenchao Wang,1,2 Jiquan Huang,2 Fei Tang,1 Yang Liu,3 Honglin Shi,1,2 Fangyu Wang,1,2 Chong Wang,2 Yongge Cao,1,2 and Xuanyi Yuan1,a

1 Department of Physics, Renmin University of China, Beijing 100872, P R China 2Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China

3School of Chemistry and Environment Engineering, Shaoguan University, Shaoguan 512005, PR China

(Received 27 November 2013; accepted 10 January 2014; published online 24 January 2014)

(K0.5Na0.5)1.xLixNb0.84Ta0.1Sb0.06O3 (KNLNTS) lead free ceramics with different Li concentration were fabricated by conventional solid-state reaction method. By increasing Li ions in KNLNTS, the grains grow up and the crystal structure changes from orthorhombic to tetragonal. When 0.03 < x < 0.05, the ceramics structure lays in PPT region. Polarization versus electric field (P-E) hysteresis loops at room temperature show good ferroelectric properties and the remnant polarization decreases by increasing Li content while coercive electric keeps almost unchanged. In PPT region, taking x = 0.04 as an example, the sample shows excellent dielectric properties: the dielectric constant is 1159 and loss tangent is 0.04, while the piezoelectric constant d33 is 245 pC/N and kp is 0.44 at room temperature, it is promising for (K0.5Na0.5)1-xLixNb0.84Ta0.1Sb0.06O3 with 4 at. % Li to substitute PZT. © 2014 Au-thor(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/L4863200]

I. INTRODUCTION

Perovskite lead zirconate titanate (PZT) based ceramics are conventional piezoelectric materials which are widely used as actuators and sensors due to their excellent piezoelectric properties around the morphotropic phase boundary (MPB).1-5 However, lead is toxic and can cause serious environmental pollution.6-8 Therefore, it is necessary to develop lead-free piezoelectric ceramics. Recently, potassium sodium niobate (KNN)-based ceramics are expected to have excellent piezoelectric,9-17 ferroelectric and dielectric properties near orthorhombic-tetragonal polymorphic phase transition (PPT) temperature.18-20 Saito et al. have reported Li, Ta, Sb doped KNN ceramics with high piezoelectric constant of 416 pC/N by templated grain growth (TGG) method.9

When Na/Kratio is near 0.5/0.5, KNN system possesses a PPT similar to that of PZT system.21 It is believed that this PPT separates the orthorhombic state from the tetragonal state, and the existence of these equivalent phases will enhanced piezoelectric and dielectric properties,19 which is reported to be originated from the simultaneous substitutions in the A site and the B site and the co-existence of mixed structures.11 When Li ions are doped in A site and Sb ions or Ta ions are doped in B site, the lattice parameter will be changed. It will lead to the distortion of structure and make it shifts to PPT region at room temperature.18-20 In Li, Sb and Ta co-doped KNN ceramics, the introduction

aTo whom correspondence should be addressed.: +86-10-62517887; Tel: +86-10-82501673. Email: yuanxuanyi@ruc.edu.cn

2158-3226/2014/4(1)/017125/6

4, 017125-1

) Author(s) 2014 i

FIG. 1. SEM patterns of (Ko.5Nao.5)i-xLixNbo.84Tao.iSbo.o6O3 (a) x = 0, (b) x = 0.02, (c) x = 0.03, (d) x = 0.04, (e) x = 0.05, (f) x = 0.06.

of Li can not only promote the sintering and densification of ceramics due to low melt point of Li2O,5 but also has important effect on the phase transition and other behaviors of the structure. However, the mechanism and effects of Li doping on PPT and dielectric property have not been studied thoroughly.

In this paper, KNN ceramics doped with Li, Ta and Sb, (K0.5Na0.5)1-xLixNb0.84Ta0.1Sb0.06O3 (KNLNTS) were prepared by conventional solid-state reaction method, and their electrical properties and phase transition behavior near phase transition temperature were investigated through varying the content of Li.

II. EXPERIMENTAL DETAILS

(Ko.5Nao.5)i-xLixNbo.84Tao.i Sb0.06O3 (KNLNTS with x = 0,0.03,0.04,0.05,0.06) were prepared by conventional method. K2CO3 (99.8%), Na2CO3 (99.5%), Li2CO3 (99.99%), Nb2O5 (99.5%), Ta2O5 (99.99%) and Sb2O5 (99.95%) were used as raw materials, and they were mixed by ball milling in ethanol for 24 h. The obtained slurry was dried and then calcined at 850 °C for 4 h. The obtained powers were mixed with 0.5 wt% PVB by ball milling in ethanol for 24 h and then pressed into pellets with a diameter of 10 mm. After removing binder under 600 °C for 10 h, they were cold isostaitc pressed, and the disks were sintered at 1090 °C-1140 °C for 2 h in the air.

The structure characterization was performed by X-ray diffraction (XRD, MiniFlex II, Rigaku) by using Cu Ka (X = 1.54178A) at room temperature, and high temperature structure characterization was performed by X-ray diffraction (X' Pert PRO, PANalytical). Microstructure of surface was observed by a scanning electron microscopy (SEM, Phenom G2, FEI). Density of ceramics was measured by Archimedes method with ethanol. Sliver electrodes were printed on both surfaces of each disk. And samples were poled in silicon oil at 120 °C (below Curie temperature) for 30 min by applying a DC electric field of 3 kV/mm. The piezoelectric constant d33 was measured by using piezoelectric constant testing meter (ZJ-3A, Institute of Acoustics, Chinese Academy of Science, Beijing, China). Dielectric properties and planar electromechanical coefficient kp were measured by an impedance analyzer (TH2828s) from room temperature to 450 °C. Polarization versus electric field (P-E) hysteresis loops were tested by Novocontrol Alpha-A High Performance Frequency Analyzer.

III. RESULTS AND DISCUSSION

Figure 1 shows the SEM images of the KNLNTS ceramics. It is clearly that all samples have quadrate grains, but the sizes were affected by the amount of Li. When x = 0, the small quadrate particles with homogeneous size of about 1 ^m were observed, as shown in Fig. 1(a). With the increase of x, the average size of grains increases and the size distribution tends to bimodal: one

FIG. 2. (a) XRD patterns of (K0.5Na0.5)1-xLixNb0.84Ta0.1Sb0.06O3 while x varies from 0 to 0.06. (b) expanded XRD patterns of KNLNTS range from 40 ° to 50 °.

FIG. 3. (a) High temperature XRD of (K0.5Na0.5)0.96Li0.04Nb0.84Ta0.1Sb0.06O3 measured at 120°C, 260 °C, 325 °C and 450 °C, (b) expanded patterns range from 43 ° to 48 °.

increases from 1 /m (for x = 0.02) to 2-3 /m (for x = 0.06), while the other increases from 2 /m (for x = 0.02) to 6-8 /m (for x = 0.06), as shown in Figs. 1(b)-1(f). The increase of particle size with increasing Li content is probably due to low melting temperature of Li2CO3 (about 720 °C). When calcined at 850 °C, the liquid phase appears in the pellets,6 which can promote the crystal growth.

Figure 2 shows the XRD patterns of KNLNTS ceramics with Li content varying from 0 to 0.06 while Fig. 2(b) is the enlarged patterns with 20 ranging from 40 ° to 50°. It is found that all the samples are pure KNLNTS. When x = 0 (K0.5Nao.5Nbo.84Tao.1Sbo.o6O3), the diffraction pattern is similar to that of pure KNN (orthorhombic phase), suggesting that it has the structure of orthorhombic.22 As x increases to 0.03, tetragonal phase appears.23,24 Then, the structure changes to the tetragonal phase when x = 0.06. As 0.03 < x < 0.05, KNLNTS ceramics show coexistence of orthorhombic and tetragonal phase, indicating the presence of PPT region.22-26 Therefore, the electrical properties of KNLNTS with varying Li content from 3 at. % (x = 0.03) to 6 at. % (x = 0.06) will be discussed to reveal the phase transition behaviors.22,27

Figure 3 gives high temperature XRD analysis for the sample with x = 0.04 measured at 120 °C, 260 °C, 325 °C and 450 °C respectively. With temperature increases, the crystal structure of ceramics changes from tetragonal to cubic phase. At 260 °C there is coexistence of tetragonal and cubic phase. But above 300 °C, the phase are all cubic and there is almost no change of the diffraction patterns between 325 °C and 450 °C.

FIG. 4. Temperature dependence of dielectric constant and loss tangent at 100 kHz for samples of x from 0 to 0.06.

FIG. 5. Piezoelectric constant d-33 and planar electromechanical coefficient kp changes as a function of x.

Figure 4 shows the temperature dependence of dielectric constant for KNLNTS samples from room temperature to 450 °C at 100 kHz. It can be observed that, with increasing Li doping concentration, the Curie temperature (Tc) shifts to higher temperature gradually. Besides, the transition temperature from orthorhombic to tetragonal phase (To-t) is about 130 °C when x = 0 while it decreases to about 70 °C when x = 0.03. However, after x > 0.03, we did not detect To-t peak, which suggests that To-t of these samples are below room temperature, and tetragonal phase occurs when 0.04 < x < 0.06 at room temperature. As shown in Fig. 4, for all the samples, the dielectric loss is about 0.04 when the temperature ranging from room temperature to about 200 °C, which suggests good temperature stability. There is a loss peak around 300 °C, which is due to the transition from tetragonal phase to cubic phase.18 It is also found that the dielectric loss increases with increasing temperature dramatically when T > 400 °C, which is caused by ionic conduction dominating at high temperature.28

Figure 5 shows that piezoelectric constant d33 and the planar electromechanical coefficient kp as a function of x (0 < x < 0.06). Piezoelectric constant is about 160 when x = 0. With increasing x, d33 increases and reaches the maximum of 245 pC/N when x = 0.04, which lay in the PPT region. With the further increase of x from 0.04 to 0.06, d33 decreases. At the same time, the samples changes from PPT (coexistence of orthorhombic and tetragonal phase) to pure tetragonal phase. In other words, the piezoelectric properties are greatly enhanced when x = 0.04 and 0.05, whose compositions are in PPT region. Obviously, PPT plays a very important role in enhancing the piezoelectric of KNLNTS

FIG. 6. (a) Polarization versus electric field hysteresis loops of (Ko.5Nao.5)i-xLixNbo.84Tao.iSbo.o6O3 at room temperature. (b) Remnant polarization (Pr) and coercive field (Ec) of (Ko.5Nao.5)i-xLixNbo.84Tao.i Sbo.o6O3 changes at room temperature.

ceramics.29 Meanwhile, with increasing Li content, kp has the similar change tendency with d33 and reaches the maximum of o.44 as x = o.o4.

Figure 6(a) display polarization versus electric field (P-E) hysteresis loops at room temperature. It can be seen that most of these ceramics show well-saturated polarization versus electric field (P-E) hysteresis loop under the electric field. It is found that by increasing Li content the ferroelectricity of ceramics decreases progressively, that is adding Li in KNLNTS ceramics have negative influence on the ferroelectricity, which is similar to the previous reports.22 As shown in Figure 6(b), with the increase of Li content after x = o.o3, the remnant polarization (Pr) decreases while the coercive field (Ec) change very little. The decrease of remnant polarization is probably due to phase transition from orthorhombic to tetragonal caused by the addition of Li. The phase transition can lead to the symmetry increase and eliminates spontaneous polarization of ceramics,21 thus affect the remnant polarization of KNLNTS.

IV. CONCLUSION

In summary, Li-doping KNNTS ceramic were prepared by conventional solid-state reaction method. By doping Li in KNNTS, the grains grow up and the crystal structure changes from orthorhombic to tetragonal. When o.o3 < x < o.o5, the ceramics structure lays in PPT region. The KNLNTS ceramic with 4at. % Li exhibits excellent dielectric property with dielectric constant of 1159, loss tangent of o.o4 at room temperature, d33 of 245pC/N and kp of o.44. These results show that (Ko.5Nao.5)o.96Lio.o4Nbo.84Tao.1Sbo.o6O3 piezoelectric ceramics is a promising substitution of PZT for different applications and devices.

ACKNOWLEDGMENTS

This work was financially supported by National Natural Science Foundation of China (51272282) and (51302311), the Beijing Committee of Science and Technology, China (008004-3581300100(01-15)), the Education Commission of Beijing, China (2011010329), the Foundation of Renmin University of China (12XNLF09).

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