International Journal of Minerals, Metallurgy and Materials Volume 19, Number 9, Sep 2012, Page 843 DOI: 10.1007/s12613-012-0637-8
Influence of sintering temperature on the structure and piezoelectric properties of ZnO-modified (Li, Na, K)NbO3 lead-free ceramics Hai-tao Li1, 2), Bo-ping Zhang2), Wei-gang Yang2), and Nan Ma2) 1) School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China 2) School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China (Received: 10 November 2011; revised: 14 December 2011; accepted: 27 December 2011)
Abstract: ZnO-modified (Li, Na, K)NbO3 lead-free ceramics with a nominal composition of Li0.06(Na0.535K0.48)0.94NbO3+0.7mol% ZnO (LNKN-Z7) was synthesized normally at 930-1000°C. The Zn ions incorporated into the A-site at a higher sintering temperature, which changed LNKN-Z7 to soft piezoelectric ceramics with the mechanical quality factor decreasing from 228 to 192. A phase transition from tetragonal to orthorhombic symmetry was identified by XRD analysis, and the corresponding calculation of lattice parameters was conducted at 970-980°C. Because of such transitional behavior and fine microstructure, the optimized values of piezoelectric coefficient, planar electromechanical coupling coefficient, and relative dielectric constant were obtained. Keywords: piezoceramics; niobates; zinc oxide; modification; phase transitions; piezoelectricity; sintering
[This work was financially supported by the Beijing Natural Science Foundation (No.2112028), the Research Fund for the Doctoral Program of Higher Education of China (No.20090006110010), and the Doctoral Start-up Fund of Henan University of Science and Technology (No.09001542).]
1. Introduction (K, Na)NbO3 (KNN)-based lead-free ceramics with proper additives were reported to exhibit excellent piezoelectric properties that are comparable to some of hard Pb(ZrxTi1−x)O3 (PZT) [1-6]. However, these systems are difficult to sinter and show high instability when they are exposed to moisture [7-10]. Slightly higher sintering temperature or longer dwell time than the optima can lead to significant volatilization of alkali metal elements, which will result in a compositional segregation and hence deteriorate piezoelectric properties [11-13]. In order to solve the above problems, various processing techniques, such as hot press sintering [14], spark plasma sintering [10], and double crucibles [11] have been applied. Although these methods can yield higher densities and better piezoelectric properties than traditional sintering techniques, they complicate synthesis Corresponding author: Bo-ping Zhang
routes and increase fabricating cost and are unfit for large-scale production. By comparison, traditional sintering at low temperatures, where the volatilization effect could be ignored, should be a practical approach. In fact, some sintering aids, such as CuO and ZnO, have been used to lower the sintering temperature of KNN-based ceramics in virtue of the formation of liquid phase during the sintering process [15-16]. ZnO, as a good sintering aid, is often used to lower the sintering temperature of functional ceramics. For the ZnO-added (Na0.5K0.5)NbO3 ceramics, Park [16] reported the piezoelectric coefficient (d33) value of 123 pC/N and the planar electromechanical coupling coefficient (kp) value of 40% when sintered at 1050°C for 2 h. Zuo [17] reported a d33 of 117 pC/N in (Na0.5K0.5)NbO3+1mol% ZnO ceramics synthesized with the normal sintering method at 1100°C for 2 h. Both d33 are higher than 80 pC/N, which is obtained
E-mail:
[email protected]
© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2012
844
from pure (Na0.5K0.5)NbO3 prepared by the conventional mixed-oxide method. Zhao [5] once reported a high piezoelectric coefficient d33>310 pC/N in Li-doped (Na, K)NbO3 ceramics with the optimum molar ratio of Na:K of 0.535:0.480 by optimizing the sintering temperature. Recently, we obtained a higher d33=250 pC/N in Li0.06(Na0.535K0.48)0.94NbO3+xmol% ZnO ceramics with x=0.7 sintered at 970°C [18]. In this paper, the addition amount of ZnO is fixed at 0.7mol%, samples with the nominal composition of Li0.06(Na0.535K0.48)0.94NbO3+ 0.7mol% ZnO were sintered at 930-1000°C for 2 h, and their microstructures, phase structures, and electrical properties were investigated with a special emphasis on the influence of sintering temperature.
2. Experimental procedures Sodium carbonate (Na2CO3, 99.8%), potassium carbonate (K2CO3, 99%), niobium oxide (Nb2O5, 99.95%), lithium carbonate (Li2CO3, 99.5%), and zinc oxide (ZnO, 99.5%) were used as raw materials. These materials were mixed with the nominal compositions of Li0.06(Na0.535K0.48)0.94NbO3+ 0.7mol% ZnO (abbreviated as LNKN-Z7) by ball milling with a little of ethanol solution in a planetary ball mill. After ball milling for 4 h, the slurry was dried and calcinated at 750°C for 4 h. The synthesized powders were then compacted into a disk of 10 mm in diameter and 1.5 mm in thickness, followed by normal sintering in air at 930-1000°C for 2 h. The sintered sample was poled under a dc field of 4-5 kV/mm at 120°C in a silicone oil bath for 30 min. Density of the samples was determined by the Archimedes method. The phase structure was determined using X-ray powder diffraction with a Cu Kα radiation (λ=0.15416 nm) filtered through a Ni foil (Rigaku; RAD-B system, Tokyo, Japan). The microstructure was observed by scanning electron microscopy (SEM, S-450, HITACHI, Japan). The temperature dependence of dielectric properties was examined using a programmable furnace with an LCR analyzer (TH2828S, TongHui Electronics, China) in the temperature range of 20-530°C. The piezoelectric constant d33 was measured using a quasistatic piezoelectric coefficient testing meter (ZJ-3A, Institute of Acoustics, Chinese Academy of Sciences). The planar electromechanical coupling coefficient kp and the mechanical quality factor Qm were calculated by the resonance-antiresonance method using an Agilent 4294A precision impedance analyzer (Hewlett-Packard, Palo Alto, CA) on the basis of IEEE standards. Ferroelectric hysteresis loop was measured at room temperature using a ferroelectric tester (RT6000HVA, Radiant
Int. J. Miner. Metall. Mater., Vol.19, No.9, Sep 2012
Technologies Inc., Albuquerque, NM).
3. Results and discussion 3.1. Microstructure Fig. 1 shows the SEM images of the thermally etched surface for LNKN-Z7 ceramics sintered at 930-1000°C for 2 h. As shown in Fig. 1(a), the grain size of the ceramics sintered at 930°C shows a bimodal distribution with the small ones of 1-2 µm and the large ones of 5-6 µm, and some pores lay in the grain boundaries. With the sintering temperature increasing, a dense and homogeneous microstructure without pores is formed accompanying with the grain growth as shown in Figs. 1(b)-1(e). However, when the sintering temperature is raised to 1000°C, a few pores are found, and the grain distribution becomes inhomogeneous again (Fig. 1(f)). The ZnO-added NKN-based ceramics was reported to form a liquid phase during the sintering process due to a mixture of Na2O, Li2O, and ZnO or Na2O, Li2O, Nb2O5, and ZnO, which decrease the optimum densification sintering temperature [16]. Here, for the sample sintered at 930°C, most of the grains are small, possibly due to the small amount of liquid phase. The amount of the liquid phase increases with the sintering temperature increasing, which leads to a lower sintering temperature and produces a dense microstructure with large grains (Figs. 1(d)-1(e)). Further increasing the sintering temperature results in the appearance of some pores in grain boundaries, which may be due to the formation of large amounts of liquid phase [15]. Therefore, an optimum sintering temperature is required to produce a proper amount of liquid phase in order to obtain a dense microstructure for LNKN-Z7 ceramics. 3.2. Crystalline phase Fig. 2 shows the X-ray diffraction (XRD) patterns of LNKN-Z7 ceramics sintered at different temperatures for 2 h. All the samples possess a single perovskite structure, indicating that Zn2+ has diffused into LNKN lattices to form a homogeneous solid solution without any traces of second phase. From the characteristic diffraction peaks of 22º and 45º, it is concluded that the samples sintered below 970°C show a typical tetragonal phase and become orthorhombic symmetry when the sintering temperature is raised to 1000°C. Therefore, orthorhombic and tetragonal phases co-exist in the samples synthesized at 970-980°C. This can also be verified from the change in 2θ of the mean diffraction peak of (101) as highlighted in a detailed XRD pattern in Fig. 2(b). The 2θ of the mean diffraction peak of (101) is almost constant when the sintering temperature is below
H.T. Li et al., Influence of sintering temperature on the structure and piezoelectric properties of ZnO-modified ...
845
Fig. 1. SEM images of the thermally etched surface of LNKN-Z7 ceramics sintered at different temperatures: (a) 930°C; (b) 950°C; (c) 960°C; (d) 970°C; (e) 980°C; (f) 1000°C.
Fig. 2. X-ray diffraction patterns for LNKN-Z7 ceramics sintered at 930-1000°C: (a) 20°-60°; (b) magnification of 30°-33°.
970°C. The constant crystal lattice may attribute to that the concentration of A-site cations is almost invariable, owing
to the negligible volatilization effect of alkali metals. However, when the sintering temperature is increased to 970°C, the diffraction peak of (101) shifts to a low angle abruptly, suggesting that a transition phase occurs initially at 970°C. With the sintering temperature further raising, the diffraction peak of (101) shifts to a high angle, and the corresponding space distance decreases, which could be deduced by the Bragg’s equation, 2dsinθ=λ, herein λ=0.15416 nm. The reason for the decreased space distance may be that more Zn2+ would incorporate into the A-site because of the formation of more A-vacancies at a higher sintering temperature. The ionic radius of Zn2+ (0.074 nm) is less than that of A-site ions (K: 0.133 nm, Na: 0.097 nm), which leads to the shrinkage of the crystal lattice. Fig. 3 shows the varying trend of lattice parameters as a function of sintering temperature, which further evidences the phase transitional behavior from tetragonal to ortho-
846
Int. J. Miner. Metall. Mater., Vol.19, No.9, Sep 2012
Fig. 3. Lattice parameters and angle β as a function of sintering temperature for LNKN-Z7 ceramics.
rhombic structure. Here, the subcell of orthorhombic structure is treated as the pseudomonoclinic symmetry with lattice parameter am=cm>bm, while for the tetragonal symmetry, the lattice parameter at=bt
