Effect of poling condition on piezoelectric properties of (K_0.5Na_0.5)NbO₃ ceramics

DU Hong-liang(杜红亮)¹, TANG Fu-sheng(唐福生)¹, LI Zhi-min(李智敏)¹,

ZHOU Wan-cheng(周万城)¹, QU Shao-bo(屈绍波)², PEI Zhi-bin(裴志斌)³

1. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China;

2. Key Laboratory of Electronic Materials Research, Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, China;

3. College of Science, Air Force Engineering University, Xi’an 710051, China

Received 10 April 2006; accepted 25 April 2006

Abstract: The lead-free piezoelectric ceramics (K_0.5Na_0.5)NbO₃ (abbreviated as KNN) with  the relative density of 97.6% were synthesized by press-less sintering owing to the careful control of processing conditions. The phase structure of KNN ceramics was analyzed. The results show that the pure perovskite phase with orthorhombic symmetry is in all ceramics specimens. The effect of poling conditions on the piezoelectric properties of KNN ceramics was investigated. The results show that the piezoelectric constant d₃₃ and electromechanical coupling factor k_p increase with poling field, poling temperature and poling time increasing, then decrease because of electric broken. Take into account of poling conditions and piezoelectric properties of pure KNN ceramics, the optimum poling conditions for pure KNN ceramics are poling field of 4 kV/mm, poling temperature of 140 ℃ and poling time of 20-25 min.

Key words: (K_0.5Na_0.5)NbO₃; lead-free piezoelectric ceramics; piezoelectric properties; poling condition; perovskite

1 Introduction

The piezoelectric ceramics have been widely used for sensors, actuators, transducers, buzzers and other electronic devices[1, 2]. However, these piezoelectric ceramics are mostly PZT-based ceramics, which contain more than 60% lead[3]. Lead is a very toxic substances, it can cause damage to the kidney, brain and nervous system, especially the intelligence for the children[4]. Some countries have prohibited electronic devices containing lead[5]. Therefore, it is urgency to develop lead free piezoelectric ceramics to replace PZT-based ceramics. (K_0.5Na_0.5)NbO₃ is considered to be a promising candidate of lead free piezoelectric ceramics because of its high piezoelectric properties, high Curie temperature and compatibility with human tissue[4, 6]. Recently, much attention for lead-free piezoelectric ceramics has been paid to (K_0.5Na_0.5)NbO₃ (abbreviated as KNN)-based piezoelectric ceramics because SAITO et al had developed KNN-based textured ceramics with properties comparable to those of a basic, unmodified PZT ceramics[7]. However, pure KNN ceramics are known to be difficult to densify fully by ordinary sintering method[4, 6]. In order to improve the densification behavior and electric properties, BaTiO₃[8], SrTiO₃[9], LiNbO₃[10], LiTaO₃[11], K₄CuNb₈O₂₃[12], K_5.4CuTa₁₀O₂₉[13], CuO[14] and ZnO[15] were added into KNN ceramics. In addition, the different preparing method were used to obtain the dense KNN ceramics such as the hot pressing and spark plasma sintering[16, 17]. Despite of such difficulties in the sintering of pure KNN ceramics, the authors have succeeded in the synthesis of dense KNN ceramics sintered in air owing to the careful control of processing conditions. The poling is a very crucial step in the processing of piezoelectric ceramice, because piezoelectric ceramic has not piezoelectricity until the random ferroelectric domains are aligned through the poling [18]. Therefore, the poling process significiantly affects the piezoelectric properties. However, until now, there is no report on the effect of poling condition on piezoelectric properties of pure KNN ceramics.

In this paper, the influence of poling condition on piezoelectric properties of pure (K_0.5Na_0.5)NbO₃ ceramics was investigated. The optimized poling condition and piezoelectric properties of KNN ceramics were achieved.

2 Experimental

The (K_0.5Na_0.5)NbO₃ ceramics in this study were fabricated by traditional ceramics process. Reagent-grade oxide and carbonate powders of K₂CO₃, Na₂CO₃ and Nb₂O₅ were used as starting raw materials. Before weighing, these powders were first separately dried in an oven at 120 ℃ for 5 h. They were milled for 24 h using planetary milling with zirconia ball media and alcohol, and then calcined at 900 ℃ for 5 h. The calcined powders were ground, calcined at 900 ℃ for 5 h again. Then these powders were ball milled again for 12 h, dried and pressed into disks of 12 mm in diameter and 1.2 mm in thickness using PVA as a binder. After burning off PVA, the pellets were sintered at 1 100-1 120 ℃ for 2 h soaking period in air. The silver paste was fired on both sides of the samples at 810 ℃ for 20 min as the electrodes for the dielectric and piezoelectric measurements. The samples were poled in silicon oil bath under the different poling conditions.

The bulk densities of sintered samples were measured by Archimedes method. The crystal structures were determined by X-ray powder diffraction analysis using a Co K_α radiation (Philips X-Pert Diffractormeter). The microstructure evolution was observed using scanning electron microscopy (SEM, Model JSM-6360, Japan). The average grain size (diameter) was determined from the number of grains in the fixed area. The piezoelectric constant d₃₃ was measured using a quasi-static d₃₃ meter (Model ZJ-3, Institute of Acoustics Academic Sinica). The electromechanical coupling factor k_p and mechanical quality factor Q_m were calculated by the resonance-antiresonance method on the basis of IEEE standards using an impedance analyzer (Agilent 4294A).

3 Results and discussion

Fig.1 shows the XRD patterns of KNN powder calcined at 900 ℃ and the ceramics sintered at 1 120 ℃. The phase structure in all samples is pure perovskite phase with typical orthorhombic symmetry and no any secondary impurity can be certified. Fig.2 shows the SEM micrograph of fracture surface of KNN ceramics. The microstructure of the sample sintered at 1 120 ℃ is uniform and fine, the grain boundary is clear, their microstructure is dense, the average grain size is about 5 μm, the fracture surface is mainly intergranular fracture as shown in Fig.2.

Fig.1 XRD patterns of KNN powder calcined at 900℃(a) and ceramics sintered at 1 120 ℃(b)

Fig.2 SEM micrograph of fracture surface of KNN ceramics sintered at 1 120 ℃

Fig.3 shows d₃₃ and k_p values of KNN ceramics sintered at 1 120 ℃ as a function of poling field with poling time (20 min) and temperature(100 ℃) being fixed as constants. It can be seen that the poling field significantly affects the piezoelectric properties of KNN ceramics, d₃₃ and k_p increase with poling field increasing in the range of 2-4 kV/mm. However, when the poling field exceeds 4 kV/mm, samples are electrically broken. The poling field can cause domain switching and rotation, make the ceramics exhibit piezoelectric properties. Because the phase structure of KNN ceramics at room is perovskite phase with orthorhombic symmetry, there are 60°, 90°, 120° and 180° domains in the orthorhombic phase of KNN. During the course of the poling, the 180° domain switching has no spontaneous strain, thus the 90°, 120° and 180° domains can cause spontaneous strain, consequently, the 180° domain can easily switch than 90°, 120° and 180° domains. It can be observed from Fig.3 that the increase of d₃₃ and k_p can result from the 180° domain switching during lower poling field(＜2 kV/mm), then 90°, 120° and 180° domains switching is the dominant factor affecting d₃₃ and k_p during higher poling field( 2-4 kV/mm). However, when the poling field exceeds 4 kV/mm, samples are electrically broken. This is due to pores and other physics flaws, which result in the increase of conductivity. Therefore, it is expected that the optimum poling field should be 4 kV/mm.

Fig.3 d₃₃ and k_p of KNN ceramics sintered at 1 120 ℃ as function of poling field

In order to investigate the influence of the poling temperature on the d₃₃ and k_p values of KNN ceramics sintered at 1 120 ℃, all samples are polarized under the condition of poling field 4 kV/mm and poling time 20 min. Fig.4 shows the d₃₃ and k_p as a function of poling temperature with poling time and temperature. In general, the increase of poling temperature can make easily domains switching, and result in the increase of piezoelectric properties. As seen in Fig.4, the d₃₃ and k_p values increases remarkably as the poling temperature increases under 140 ℃, however, when the poling temperature exceeds 140 ℃, the leakage current significantly increases, KNN ceramics samples are electrically broken. Therefore, d₃₃ and k_p decrease remarkably when the poling temperature is higher than 140 ℃.

Fig.4 d₃₃ and k_p of KNN ceramics sintered at 1 120 ℃ as function of poling temperature

Fig.5 shows the d₃₃ and k_p values of KNN ceramics sintered at 1 120 ℃ as a function of poling time under the condition of poling field 4 kV/mm and poling temperature 140 ℃. The variations of d₃₃ and k_p with poling time are very similar to those of the poling field and poling temperature. The d₃₃ and k_p increase remarkably with increasing the poling time in the initial period of poling. This is because that 180° domains switching is the dominant factor affecting d₃₃ and k_p during the initial period of poling, which can easily switch than 60°, 90° and 120° domains. However, KNN ceramics samples are very sensitive to poling time. When the poling time exceeds 20 min, the leakage current significantly increases, KNN ceramics samples are electrically broken. This is because that the conductivity increases rapidly when the poling time exceeds 20 min.

Fig.5 d₃₃ and k_p of KNN ceramics sintered at 1 120 ℃ as function of poling time

Taking into account of the above results, it can be concluded that the optimum poling conditions for pure KNN ceramics are poling field of 4 kV/mm, the poling temperature of 140 ℃ and poling time of 20 min.

4 Conclusions

The lead-free piezoelectric ceramics (K_0.5Na_0.5)- NbO₃ with the relative density of 97.6% have been synthesized by press-less sintering owing to the careful control of processing conditions. The optimum poling conditions for pure KNN ceramics are poling field of 4 kV/mm, poling temperature of 140 ℃ and poling time of 20 min.

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(Edited by YUAN Sai-qian)

Foundation item: Project(10474077) supported by the National Natural Science Foundation of China; Project(2002CB613304) supported by the National Basic Research Program of China

Corresponding author: DU Hong-liang; Tel: +86-29-88488007; E-mail: duhongliang@126.com