1. Characteristics Improvement of Li0. 058(K0. 480Na0. 535)0. 966(Nb0
Characteristics Improvement of Li0.058(K0.480Na0.535)0.966(Nb0.9Ta0.1)O3 Lead-Free Piezoelectric Ceramics by LiF Additions
Chien-Min Cheng1, Ching-Hsing Pei1, Mei-Li Chen2,*, KAI-HUANG CHEN3,*
1Department of Electronic Engineering, Southern Taiwan University of Science and Technology, Tainan, Taiwan, R.O.C.
2Department of Electro-Optical Engineering, Southern Taiwan University of Science and Technology, Tainan, Taiwan, R.O.C.
3Department of Electronics Engineering and Computer Science, Tung-Fang Design Institute, Kaohsiung, Taiwan, R.O.C.
*Corresponding author.
2015 International Conference on Innovation, Communication and Engineering
October , 2015, Xiangtan, Hunan, P.R. China
Abstract
In this study, a series of Li0.058(K0.480Na0.535)0.966(Nb0.90Ta0.10)O3 + (x)LiF (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5 wt%) lead-free piezoelectric ceramics were fabricated by a conventional solid-state reaction method. The incorporation of LiF could significantly improve the sintering ability of LKNNT ceramics by reducing the optimal sintering temperature from 1090 C to 1020 C. The crystal phases and micro-structures were analyzed by means of the X-ray diffraction and scanning electronic microscopy, respectively. The impedance analyzer was used to measure the Curie temperature, phase transition point, and electro-mechanical coupling factor. And the d33 meter was used to measure the piezoelectric constants.
From the results, due to the addition of 0.2 wt% LiF, uniform and condensed grains can be obtained and hence the sintering temperature can be lowered down. As the contents of LiF increased, the orthorhombic to tetragonal phase transition points TO-T were almost no changed, but the Curie temperature TC decreased from 425 C (x = 0) to 405 C (x = 0.5). And furthermore, the electro-mechanical coupling factor kp and piezoelectric constant d33 were all decreased with increases of LiF contents. Hence, even though the reducing of little amount of piezoelectric characteristics, the LiF addition can improve the sintering ability of the LKNNT ceramics effectively.
Keywords Curie temperature, Lead-free, LiF, Piezoelectric.
Results and Discussion
An orthorhombic to tetragonal phase transition happened between 0.5 wt% and 0.4 wt%. The crystal structure of pure LKNNT suffered a polymorphic phase transition (PPT) between tetragonal and orthorhombic. Besides, corresponding to the compositional variation, the diffraction peaks shift slightly to lower angles with increasing of addition of LiF contents.
For pure LKNNT (the theoretical density value is g/cm3), maximum relative density (0.97) can be obtained as the sintering temperature is 1090C. Whereas increasing the LiF contents, the relative density increases to a saturated and maximum value of for x = 0.1 (the theoretical densities is g/cm3), and the optimal sintering temperature had be decreased from 1020 C (x = 0) to 990 C (x = 0.1) effectively due to these little LiF additions. But for the LiF contents further increase to x = 0.4 and x = 0.5, the relative density starts to decrease (0.947 for x = 0.4 and only for x = 0.5) due to the inferences of LiF second phases, non-uniform grains, and pores.
It is clear that for x = 0, as the sintering temperature increased from 950 to 1090 C, the d33 values first increased, and reach to maximum of 279 pC/N for 1090 C, and then decreased for higher than 1090 C. Furthermore, as x = 0.2, the d33 values exhibit the same tendency of x = 0, but reach to a maximum of 285 pC/N for 1020 C, and then decreased for higher than 1020 C. Hence, the optimal sintering temperature can be reduced significantly about 70 C. But for x = 0.5, according to Fig. 2(e) and 2(f), due to melted grains and pores, it is difficult to obtain better d33 values, and the optimal d33 value was only 256 pC/N as sintered at 990 C.
x = 0 (kp = 0.46), and gradually decreased for higher than 1020 C. However, for too much LiF content (x = 0.5), due to melted grains and pores as shown in Fig. 2(e) and Fig. 2(f), it is difficult to obtain better kp values and was only 0.37 as sintered at 1090 C. Finally, as the sintering temperature is too high, the kp value starts to decrease.
As Fig. 6, Fig. 7, and Table 1 shown, whereas the measured condition is 1 kHz / 25 ~ 500 C, the Curie temperature (TC) in the present study is observed to be decreasing with the increase of LiF contents. However, the phase transition point TO-T is almost no changed for all composition of LKNNT ceramics, and the range of maximum relative dielectric constants max was 3853 ~ 4952.
(a) 2 = 20 ~ 60 (b) 2 = 44 ~ 47
Figure 1. XRD patterns of 990C-sintered LKNNT + (x)LiF ceramics.
Figure 6. The dielectric constant for the LKNNT + (x)LiF ceramics as measured at
1KHz / 25~500 C.
Figure 4. The d33 values vs. sintering temperature of the LKNNT + (x)LiF ceramics.
It can be seen from Fig. 2(a) and 2(b) that as x = 0 and 0.1, non-uniform grains and more pores can be found. And then the grains become dense as x increased to 0.2 and 0.3, as shown in Fig. 2(c) and 2(d), in which almost no visible pores can be found. However, melted grains and pores can be seen easily from Fig. 2(e) and 2(f), and these demonstrated that too much LiF will cause melted grains generated, probably suggesting that liquid phase related to LiF promotes the growth of local grains only for x < 0.3.
(a) x = 0
(b) x = 0.1
(c) x = 0.2
(d) x = 0.3
(e) x = 0.4
(f) x =0.5
Figure 2. SEM images of 990C-sintering LKNNT + (x)LiF ceramics.
Figure 7. The Curie temperature vs. LiF contents of the LKNNT + (x)LiF ceramics
(measured at 1KHz).
Figure 5. The electromechanical coupling factor vs. sintering temperature for the
LKNNT + (x)LiF ceramics.
Figure 3. Relative Density variation as a function sintered temperature for the
LKNNT + (x) LiF ceramics.
Table 1. Properties of the LKNNT + (x)LiF ceramics.
X=0
X=0.2
X=0.3
X=0.4
X=0.5
εmax
4452
4952
4539
4287
3853
TC (C)
425
415
410
405
TO-T (C)
210
Conclusions
The optimal sintering temperature was improved from 1090 C (0 wt% LiF) to 1020 C (0.2 wt% LiF), and hence the doping of 0.2 wt% LiF can lower down the sintering temperature of pure LKNNT ceramics about 70 C. Furthermore, the TC were also varied significantly from 425 C (0 wt% LiF) to 405 C (0.5 wt% LiF), but the TO-T is also no changed for all compositions of LKNNT ceramics. All samples show pure perovskite phase with typical orthorhombic symmetry.
It is clear that for x = 0, as the sintering temperature increased from 950 to 1090 C, the kp values first increased, and gradually reach to a maximum value of 0.46 for 1090 C, and finally decreased for higher than 1090 C. Furthermore, as x = 0.2, the kp values exhibited the same tendency of x = 0, reach to a maximum of 0.45 for 1020 C, and that is better than others but for