Preparation and properties of (K_0.5Na_0.5)NbO₃-LiNbO₃ ceramics

TANG Fu-sheng(唐福生)¹, DU Hong-liang (杜红亮)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 (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃(abbreviated as KNLN) were synthesized by a traditional solid state reaction. The effects of Li⁺ on the sintering characteristic, the phase structure and piezoelectric properties of KNLN ceramics were investigated. The sintering temperature of KNN-based ceramics is decreased by doping Li⁺ and the range of the sintering temperature is narrow. The KNLN ceramics exhibit an enhanced piezoelectric properties with the piezoelectric constant d₃₃ value of 180-200 pC/N, The electromechanical coupling coefficients k_p is 35%-40%. The results show that (1-x)(K_0.5Na_0.5)- NbO₃-xLiNbO₃ (x=0.05, 0.06) is a promising high-temperature lead free piezoelectric ceramic.

Key words: (K_0.5Na_0.5)NbO₃-LiNbO₃; lead-free piezoelectric ceramics; piezoelectric properties; perovskite

1 Introduction

The lead oxide based piezoelectric ceramics (mainly PZT-based ceramics) have been widely used for sensor, actuators, transducers, buzzers and other electronic devices[1]. However, The PZT-based ceramics which contain up to 60% PbO have been an environmentalist’s nightmare because PbO is a very toxic substances[2]. At present, some countries have prohibited using of some kinds of electronic devices containing lead[3], therefore it is very urgent to find some candidates for lead oxide based piezoelectric ceramics. Several kinds of lead free materials have been extensively investigated world widely in order to develop a suitable candidate for PZT-based ceramics[4-7]. Among these materials, alkaline niobate-based ceramics attract considerable attentions, especially (K_0.5Na_0.5)NbO₃ (abbreviated as KNN) 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[8].

It is difficult to prepare high dense KNN ceramics by ordinary sintering process [9]. In order to prepare highly dense KNN ceramics some new techniques have been applied, such as hot-press and spark plasma sintering(SPS)[10,11]. Recently a textured KNN-based ceramics was prepared using reactive templated train growth technique (RTGG), its properties can be comparable with the modified PZT based ceramics[12]. Unfortunately these techniques are unsuitable for using in industrial production, therefore it is necessary to investigate KNN-based ceramics fabricated by ordinary processing. Doping to modified KNN ceramics is an effective way to improve the sintering characteristic as well as the electrical properties[13, 14] and  Li⁺ modified  KNN ceramics is considered to be one of the most promising ways to obtain high properties KNN based ceramics[16]. Its properties have been investigated by several researchers [15, 16], However, until now, the influences of the processing parameters such as the amount of Li⁺ and sintering temperature on the properties of KNLN ceramics have not been reported. In this paper, the effects of Li⁺ on the sintering characteristic, the phase structure and piezoelectric properties of KNLN ceramics were investigated. The results show that the enhanced electric properties can be achieved at optimal processing condition.

2 Experimental

The (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ (x=0.04, 0.05, 0.06, 0.07) ceramics were synthesized by traditional ceramics process. The reagent-grade oxide and carbonate powders of K₂CO₃, Na₂CO₃, Li₂CO₃ and Nb₂O₅ were used as starting raw materials. They were weighed and mixed by ball milling in alcohol for 24 h using ZrO₂. After calcination at 850 ℃ for 5 h, the calcined powders were ball milled again for 12 h and pressed into disks using PVA as a binder. After burning off PVA, the green compact were fired in air at temperatures ranging from 1 000 ℃ to 1 100 ℃ for 2 h.

The crystal structures were determined by X-ray powder diffraction analysis obtained using a CoK_α radiation (Philips X-Pert Diffractometer). The micro- structure evolution was examined using a scanning electron microscopy (SEM; Model JSM-6360, Japan). For electrical characterization, silver electrodes were prepared on both side of a disk and samples were immersed in silicon oil and poled in 30 kV/cm field at 120 ℃ for 30 min. Piezoelectric constant d₃₃ was measured by a quasi-static meter (Model ZJ-3, Institute of Acoustics Academic Sinica). The electromechanical coupling coefficients k_p were determined from resonance-antiresonance methods on basis of IEEE standards by using an impedance analyzer (Agilent 4294A).

3 Results and discussion

It has been reported that for ceramics with composition of (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃, a complete solid solution with perovskite structure can be formed when x≤0.07[16]. Fig.1 shows the XRD profiles of the (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃(x=0.06) powders calcined at 850 ℃ for 5 h and the KNLN ceramics sintered at 1 060 ℃ for 2 h. It can be seen that the phase structure in all samples is pure perovskite phase. Since LiNbO₃ has an illumenties crystal structure, however NaNbO₃ and KNbO₃ have a perovskite crystal structure[15], the effects of doping Li⁺ on the structure of  KNLN can be detected in this figure, which also indicates a morphotropic phase boundary (MPB) occured at 0.05≤x≤0.07.

Fig.2 shows the SEM photographs of thefracture surface of the (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃(x=0.06) ceramics sintered at different temperatures. As shown in

Fig.1 XRD patterns of KLNN ceramics sintered at 1 060℃ and powder specimens calcined at 850 ℃

Fig.2, little grains form when the sample is sintered at  1 040 ℃, simultaneously many pores and little grains are observed. The grains grew up and the size was nearly well-proportioned at 1 060 ℃ and the pores reduced accordingly. This results is attributed to the sintering temperature of KNLN ceramics decreasing with the content of Li⁺ increasing [15]. Therefore, the sintering temperature of Li⁺ modified KNN ceramics is lower than that of pure KNN ceramics. However, insufficient growth grains can still be observed, which may be caused by a very short sintering time. An abnormal growth was observed when samples sintered at 1 080 ℃ and the amounts of pores increased again. This problem became more serious and led to a sharp decrease on the density when samples sintered at 1 100 ℃. The results indicate that the suitable sintering temperature of the (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃(x=0.06) ceramics is about 1 060 ℃. In addition, we found that the range of sintering temperature is very narrow, besides the sintering time also has an great influence on the density which will be investigated in the future.

Fig.3 shows the sintering temperature dependence of the piezoelectric constant d₃₃ of KNLN ceramics with different amounts of Li⁺. As shown in this figure, the piezoelectric properties are enhanced greatly and the d₃₃ values of all KNLN ceramics exceed 100 pC/N, however, the pure KNN ceramics which is prepared via traditional solid state reaction is only 80 pC/N[9]. The d₃₃ value of (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ (x=0.04, 0.07) ceramics reach their maximum at 1 080 ℃ and 1 060 ℃ respectively. This is likely caused by the effects of different amounts of Li⁺ on the sintering temperature of KNN ceramics. It is well known that the presence of morphotropic phase boundary (MPB) can enhance the electric properties largely. The Li⁺ modified KNN ceramics with a composition near the MPB exhibit an excellent piezoelectricity, as we can seen that the d₃₃

Fig.2 SEM photographs of fracture surface of (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃(x=0.06) ceramics sintered at 1 040 ℃(a), 1 060 ℃(b), 1 080 ℃(c) and 1 100 ℃(d) for 2 h

Fig.3 Temperature dependence of piezoelectric constant d₃₃ of KNLN ceramics

values of  (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ (x=0.05, 0.06, 0.07) reach 180-200 pC/N. But then an interesting phenomenon appears that for (1-x)(K_0.5Na_0.5)NbO₃- xLiNbO₃ (x=0.05, 0.06) ceramics which is sintered at their sintering temperature (1 060 ℃), when the maxi- mum density is obtained, the piezoelectric constant does not reach their maximum, inversely is much lower than that sintered at other temperatures.

Fig.4 shows the piezoelectric constant d₃₃ and electromechanical coupling coefficients k_p of KNLN as a function of the content of Li⁺. It is indicated that the presence of MPB can strengthen the electrical properties. The electromechanical coupling coefficients k_p and piezoelectric constant d₃₃ of (1-x)(K_0.5Na_0.5) NbO₃- xLiNbO₃ (x=0.05) ceramics reach their maximum 37.4% and 200 pC/N respectively.

Fig.4 Electrical properties of KLNN ceramics as function of content of Li⁺

4 Conclusions

The lead free piezoelectric ceramics (1-x)(K_0.5- Na_0.5)NbO₃-xLiNbO₃ with a pure perovskite phase structure were prepared through traditional solid reaction. The sintering temperature of KNN based ceramics decreases with the increase of the amount of Li⁺ and the range of sintering temperature is narrow. The electrical properties of KNLN ceramics are greatly strengthened compared with that of pure KNN ceramics, and (1-x)(K_0.5Na_0.5) NbO₃-xLiNbO₃ (x=0.05, 0.06) ceramics exhibit excellent piezoelectric properties due to the presence of a MPB. The d₃₃ and k_p values of (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO (x=0.05) ceramics are 200 pC/N and 37.4%, respectively. The results show that (K_0.5Na_0.5)NbO₃-LiNbO₃ ceramics possess an excellent electric properties and good sintering characteristics, which indicates that it is a promising lead free piezoelectric ceramics.

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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: TANG Fu-sheng; Tel: +86-29-88488007; E-mail: ts220@163.com