Trans. Nonferrous Met. Soc. China 22(2012) s133-s137

Structure and piezoelectric properties of (1-x)K_0.5Na_0.5NbO₃-xLiBiO₃ lead-free piezoelectric ceramics

JIANG Min-hong^{1, 2, 3}, DENG Man-jiao¹, YANG Zhu-pei², FU Zhen-xiao³

1. Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China;

2. Key Laboratory for Macromolecular Science of Shaanxi Province, Shanxi Normal University, Xi’an 710062, China;

3. Research Institution of Fenghua Advanced Technology Holding Co., LTD., Zhaoqing 526020, China

Received 9 July 2012; accepted 20 August 2012

Abstract: Lead-free piezoelectric ceramics (1-x)(K_0.5Na_0.5)NbO₃-xLiBiO₃ [(1–x)KNN-xLB] (x=0, 0.0005, 0.002, 0.004, 0.006, 0.008, 0.010) were prepared by an traditional solid-state reaction. The microstructure and electrical properties of the ceramics were investigated. The results show that all (1-x)KNN-xLB ceramics possess pure perovskite structure when x≤0.01, no trace of any secondary phase is detected, and the phase structure of the ceramics transits abnormally from orthorhombic to cubic. With the increase of the LB content, the size of grain gradually becomes small, the piezoelectric constant d₃₃ and the planar electromechanical coupling coefficient k_p first increases and then decreases. The d₃₃ and k_p of the ceramics reach their maximum values 115 pC/N at x=0.002 and 0.2701 at x=0.001, respectively. The dielectric constant ε_r of the ceramics firstly increases evidently and then decreases with the increase of x, the maximum value 871.8 is obtained at x=0.006.

Key words: lithium bismuthate; sodium potassium niobate; lead-free piezoelectric ceramics; piezoelectric properties; dielectric properties

1 Introduction

Piezoelectric ceramics are widely applied in the devices such as sensors and actuators [1,2]. However, at present most of piezoelectric ceramics used are PbTiO₃-PbZrO₃ (PZT) system. It is recently desired to use lead-free materials for environmental protection during the waste disposal of products. So, it is very urgent and important for people to develop new lead-free piezoelectric materials. To today, people have already developed many lead-free ceramic systems [3-5]. And among these systems, KNbO₃-NaNbO₃(KNN)-based lead-free piezoelectric ceramics is one of the leading candidates because of its strong piezoelectric properties, low density and high Curie temperature, etc [2].

According to the previous reports [6,7], there are several studies on the critical ratio of Na, K and Li elements in KNN systems and the effect of Bi additions on its properties. LI et al [8] reported that Bi or Li elements single-doping could evidently improve piezoelectric properties of (Bi_0.5Na_0.5)TiO₃ (BNT)-based ceramics. However, there is a contrary result for Bi and Li co-doping 0.94(Na_0.8K_0.16Li_0.04)Bi_0.5TiO₃-0.06Ba- (Zr_0.055Ti_0.945)O₃ system. According to Ref. [9], the compound LiBiO₃ is pervoskite structure which is similar to LiSbO₃. Furthermore, it is reported that the addition of LiSbO₃ can well improve the piezoelectric properties of KNN. For example, the piezoelectric constant d₃₃ of the LiSbO₃-doping KNN ceramic is increased from 80 pC/N of the pure KNN to over 200 pC/N [10].

To today, the structure and piezoelectric properties of LiBiO₃ doped K_0.5Na_0.5NbO₃ lead-free piezoelectric ceramics are not reported. Single LiBiO₃ isn’t very stable at high temperature and changes easily to the compound LiBiO₂ as one dopant. Furthermore, the physical and chemical reactions in the ceramics during the sintering are very complex with the additions of many elements, which will probably affect the structure and property and get some new phenomenon. In this paper, LiBiO₃ is selected as the dopant, and the effect of its content on the microstructure, phase transition and piezoelectric properties is investigated.

2 Experimental

Nb₂O₅ (99.5%), K₂CO₃ (99%), Na₂CO₃ (99.8%), Li₂CO₃ (99%) and Bi₂O₃ (99.97%) were used to prepare (1-x)K_0.5Na_0.5NbO₃-xLiBiO₃ [(1-x)KNN-xLB] (x=0, 0.0005, 0.001, 0.002, 0.004, 0.006, 0.008, 0.010) ceramics by the conventional mixed-oxide method. All powders were separately dried in an oven for 4-6 h prior to mix. The stoichiometric powders were mixed by ball-milling in ethanol for 12 h, then dried and calcined at 880 °C for 6 h. And ball milled for 6 h once again. The calcined powders were mixed with 5% (mass fraction) poly vinyl alcohol (PVA) solution, and then pressed into pellets with a diameter of 1.8 cm under 100 MPa pressure. The green disks were sintered at 1 100 °C for 3 h. Silver electrodes were formed on both surfaces of each sintered disk by firing at 600 °C for 30 min. The samples were polarized in silicon oil for 15 min at 60-80 °C. The applied poling fields were 3.5 kV/mm for 15 min. After 24 h, the properties were measured.

The phase structure of sintered ceramics was measured by X-ray diffraction (XRD, D8-2-Advance) with Cu K_α radiation (step was 0.02°). Surface microstructure and composition analysis were carried out using a scanning electron microscope (SEM, JSM- 5610LV) with the Energy Disperse Spectroscopy (EDS, Noran). The piezoelectric coefficient d₃₃ was recorded from 1 d aged sample using a quasi-static piezoelectric d₃₃ meter (ZJ-3AN, China). Dielectric properties were obtained using an impedance analyzer (Agilent 4294A) by measuring the capacitance and loss. The planar coupling coefficient (K_p) and the mechanical quality factor (Q_m) were determined by the resonance and antiresonance technique using an impedance analyzer (Agilent 4294A).

3 Results and discussion

Figure 1 shows that XRD patterns of (1-x)KNN- xLB ceramic samples sintered at 1150 °C for 3 h. Seen from Fig. 1, for (1-x)KNN-xLB ceramic, the XRD patterns show single pervoskite structure, and no secondary phase is detected. At x=0.0005, the ceramics have orthorhombic perovskite phases, which agree with the pure KNN ceramic (at x=0). However, with the increase of the LB content, the crystalline system of the ceramics changes abnormally. When x<0.002, the ceramic has orthorhombic phase, and mixed phases between orthorhombic and cubic phase at x=0.002-0.006, and then single cubic phase at x≥0.008.

Fig. 1 XRD patterns of (1-x)KNN-xLB ceramic samples sintered at 1150 °C for 3 h

According to the previous reports [11,12], usually for LiNbO₃, LiTaO₃ or LiSbO₃ single doping KNN ceramics, a transition between orthorhombic and tetragonal phase will appear with the increase of the dopant content, and excellent piezoelectric properties are gained near the morphotropic phase boundary (MPB) between orthorhombic and tetragonal phase. However, in this paper a transition between orthorhombic and cubic phase appears with increase of the BL content, and excellent piezoelectric properties are gained near the morphotropic phase boundary (MPB) between orthorhombic and cubic phase. Especially at x= 0.002- 0.005, some big single crystals (the average size of 3-5 mm) appear in the ceramics, which is probably related with the above abnormal phase transition. The reasons will be studied in-depth in the future.

Figure 2 shows the surface SEM pictures of (1-x)KNN-xLB ceramics. Seen from Fig. 2, with the increase of the LB content, the grain size of the ceramics refines and becomes more homogeneous. This indicates that the addition of LB promotes the nucleation but inhibits the growth of grains in the ceramics. This is because after adding LB into KNN, Li⁺ and Bi⁵⁺ ions distribute evenly in the KNN ceramics and form nucleation points.

Fig. 2 Surface SEM photographs of (1-x)KNN-xLB ceramics sintered at 1150 °C for 3 h

Figure 3 shows the dependence of d₃₃，k_p and Q_m of the (1-x)KNN-xLB ceramics on x. Seen from Fig. 3, d₃₃ of the ceramics first increases when x≤0.002 and then decreases when x>0.002 with the increase of the LB content. When the LB content is small, the ceramics have orthorhombic phase structure with ferroelectric property which is the origin of good piezoelectric properties for the ceramics. What’s more, the uniform and compact morphology of the ceramic at x=0.002 is also one of the reasons. The ceramic has almost no piezoelectric properties when the LB content x is over 0.006. It can be supported by the above XRD results. Seen from Fig. 1, when the LB content x is over 0.006, the crystal structure of the ceramic already changes from orthorhombic and cubic phase. Of course, the decrease of the ceramic density also affects its property. Table 1 shows the property parameters of LB doped KNN ceramics. Seen from Table 1, the profile of k_p–x relationship agrees with that of d₃₃–x. Although the structure of the ceramic at x=0.006 is among the MPB, it is close to the cubic phase side. So, its piezoelectric properties are low. Also seen from Fig. 3, with the increase of the LB content, the Q_m of the ceramics increases accordingly. This is because that some vacancies appear in the KNN ceramics companied with the addition of LB. These vacancies reduce the resistance force of the ferroelectric domain rotation in the ceramics, which makes the domain rotation easy.

Fig. 3 Dependence of d₃₃, k_p and Q_m of (1-x)KNN-xLB ceramics on x

Table 1 Piezoelectric properties of (1-x)KNN-xLB ceramics sintered at 1150 °C for 3 h at 1 kHz

Figure 4 shows the dependence of ε_r and tan δ of the (1-x)KNN-xLB ceramics on x. Seen from Fig. 4, with the increase of x, the dielectric constant ε_r of the ceramics first increases evidently and then decreases slightly, and at x=0.006, ε_r reaches its maximum value (871.8). In a whole, the dielectric loss tan δ of the ceramic decreases gradually. These agree with the above microstructure analysis results.

Figure 5 shows the dependence of d₃₃ of the (1-x)KNN-xLB ceramics on the sintering temperature. Seen from Fig. 5, with the increase of the sintering temperature from 1125 to 1160 °C, d₃₃ of 0.2%LB (atom fraction) doped KNN ceramics first increases and then reduces, and when the sintering temperature is 1150 °C, d₃₃ reaches its maximum value (115 pC/N). At the same time, k_p and Q_m of the ceramics first increases and then decreases. Table 2 lists the piezoelectric properties of 0.998KNN-0.002LB ceramics. Also seen from Table 2, ε_r of the ceramics increases in a whole with the addition of the sintered temperature, and reaches its best value (557). The dielectric loss tan δ of the ceramics first decreases and then increases quickly. When sintering at 1150 °C, tan δ of the ceramics reduces 4.93%.

Fig. 4 Dependence of ε_r and tanδ of (1-x)KNN-xLB ceramics on x

Fig. 5 Dependence of d₃₃ of the (1-x)KNN-xLB ceramics on different sintering temperatures

Table 2 Piezoelectric properties of 0.998KNN-0.002LB ceramics sintered at different temperatures

4 Conclusions

1) Lead-free piezoelectric ceramics (1-x)(K_0.5Na_0.5)- NbO₃-xLiBiO₃ are prepared by the traditional solid-state reaction. The ceramics at x≤0.01 possess pure perovskite structure, and no trace of any secondary phase is detected. The phase structure of the ceramics changes abnormally from orthorhombic to cubic phase.

2) With the increase of LiBiO₃ content, the piezoelectric constant d₃₃ and planar electromechanical coupling coefficient k_p at first increases and then decreases, and achieved their maximum value, i.e., 115 pC/N and 0.2701 at x=0.001 and 0.002, respectively. The dielectric constant ε_r of the ceramics firstly increases evidently and then decreases slightly with the increase of x.

3) With the increase of the sintering temperature from 1125 °C to 1160 °C, d₃₃ of 0.2% (atom fraction) LiBiO₃ doped K_0.5Na_0.5NbO₃ ceramics first increases and then reduces, and at 1150 °C d₃₃ reaches its maximum value (115 pC/N). At the same time, k_p and Q_m of the ceramics first increase and then decrease.

References

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(1-x)K_0.5Na_0.5NbO₃-xLiBiO₃无铅压电陶瓷的结构与压电性能

江民红^{1, 2, 3}，邓满姣¹，杨祖培²，付振晓³

1. 桂林电子科技大学 广西信息材料重点实验室，桂林 541004；

2. 陕西师范大学 陕西省大分子科学重点实验室，西安 710062；

3. 广东风华高新科技股份有限公司研究院，肇庆 526020

摘  要：采用传统固相反应制备了(1-x)(K_0.5Na_0.5)NbO₃–xLiBiO₃ [(1–x)KNN–xLB] (x =0，0.0005，0.001, 0.002，0.004，0.006，0.008，0.010)压电陶瓷，并分析研究了其微结构及电性能。结果表明，LB掺杂的KNN陶瓷主要形成了钙钛矿结构，没有检测到第二相的存在，并且陶瓷的相结构出现直接由正交相过渡到立方相的“反常”转变；随着LB掺杂量的增加，晶粒尺寸逐渐细化，陶瓷的压电常数d₃₃、平面机电耦合系数k_p先略有增加后显著下降，且分别在x=0.002和x=0.001时达到最大值，分别为115 pC/N和0.2701；陶瓷的介电常数ε_r随x增大先增加后略有降低，当x=0.006时获得最大值，为871.8。

关键词：铋酸锂；铌酸钾钠；无铅压电陶瓷；压电性能；介电性能

(Edited by CHEN Can-hua)

Foundation item: Project (51102056) supported by National Natural Science Foundation of China; Projects (2011GXNSFB018008, 2012GXNSFGA060002) supported by Guangxi Natural Science Foundation; Project (2011no.40) supported by Program for Excellent Talents in Guangxi Higher Education Institutions; Project (201012MS085) supported by Research Program of Guangxi Education Department; Project (2012M511975) supported by China Postdoctoral Science Foundation Funded

Corresponding author: JIANG min-hong; Tel: +86-13978341435; E-mail: jmhsir@tom.com