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Characteristics Improvement of
Li0.058(K0.480Na0.535)0.966(Nb0.9Ta0.1)O3 Lead-Free Piezoelectric
Ceramics by LiF Additions
2015 International
Conference on
Innovation,
Communication and
Engineering
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. E-mail: [email protected]
October 23 - 28, 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
oo
o
o
Intensity (a.u.)
0.5wt%
oo
*
oo
* LiF
oo
*
0.5wt%
0.4wt%
0.4wt%
0.3wt%
0.3wt%
0.2wt%
0.2wt%
0.1wt%
0.1wt%
0.0wt%
0.0wt%
20
30
40
50
60
44.0 44.5 45.0 45.5 46.0 46.5 47.0
2 Theta (deg.)
(a) 2 = 20 ~ 60
2 Theta (deg.)
(b) 2 = 44 ~ 47
For pure LKNNT (the theoretical density value is 4.538 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 0.978 for x = 0.1 (the theoretical densities is
4.522 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 0.942 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.
300
Figure 1. XRD patterns of 990C-sintered LKNNT + (x)LiF ceramics.
280
5000
x=0.0
x=0.2
x=0.3
x=0.4
x=0.5
4000
3000
2000
1000
260
d33 (pC/N)
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.
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.
Dielectric Constant
o LKNNT
(200)
(002)
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.
0
240
150
220
180
250
300
350
400
450
Temperature (oC)
x=0.0
x=0.1
x=0.2
x=0.3
x=0.5
200
200
Figure 6. The dielectric constant for the LKNNT + (x)LiF ceramics as measured at
1KHz / 25~500 C.
430
425
160
960
980
1000
1020
1040
1060
1080
1100
1120
Sintering Temperature (oC)
Tc (oC)
420
Figure 4. The d33 values vs. sintering temperature of the LKNNT + (x)LiF ceramics.
0.48
x=0.0
x=0.1
x=0.2
0.46
415
410
x=0.3
x=0.5
0.44
Figure 2. SEM images of 990C-sintering LKNNT + (x)LiF ceramics.
405
0.42
kp
400
1.00
0.40
0.1
0.2
0.3
0.4
0.5
LiF Content x (wt%)
0.38
0.95
Relative Density
0.0
Figure 7. The Curie temperature vs. LiF contents of the LKNNT + (x)LiF ceramics
(measured at 1KHz).
0.36
Table 1. Properties of the LKNNT + (x)LiF ceramics.
0.90
X=0
X=0.2
X=0.3
X=0.4
X=0.5
εmax
4452
4952
4539
4287
3853
TC (C)
425
425
415
410
405
TO-T (C)
210
210
210
210
210
0.34
960
0.85
980
1000
1020
1040
1060
1080
1100
1120
Sintering Temperature (oC)
Figure 5. The electromechanical coupling factor vs. sintering temperature for the
LKNNT + (x)LiF ceramics.
x=0.0
x=0.1
x=0.2
x=0.3
x=0.5
0.80
0.75
960
980
1000
1020
1040
1060
1080
1100
Sintering Temperature (oC)
Figure 3. Relative Density variation as a function sintered temperature for the
LKNNT + (x) LiF ceramics.
1120
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
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.