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The Inferences of ZnO Additions
for LKNNT Lead-Free Piezoelectric Ceramics
1
CHENG ,
CHIEN-MIN
2015 International
Conference on
Innovation,
Communication and
Engineering
CHING-HSING
1
PEI ,
MEI-LI
2,
CHEN *,
KAI-HUANG
3,
CHEN *
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
By the conventional solid-state liquid-phase sintering technique, Li0.058(K0.480Na0.535)0.966(Nb0.9Ta0.1)O3 + x wt% ZnO (x = 0, 0.2, 0.4, 0.6, 0.8, 1, 3) lead-free piezoelectric
ceramics were fabricated. The addition of ZnO liquid-phase sintering promoters could improve the grain-growth of LKNNT ceramics significantly and its inferences were
investigated detailed in this paper. The crystal phases and micro-structures were analyzed by means of the X-ray diffraction and scanning electronic microscopy,
respectively. Using the impedance analyzer, the dielectric constant, loss tangent, Curie temperature, phase transition point, and electromechanical coupling factor were
measured. And the piezoelectric constants were measured by the d33 meter. Compared to pure LKNNT ceramic (sintered at 1090 C, d33 = 279 pC/N, and kp = 0.46), for x =
0.6 specimen, even though the optimal d33 and kp values were only 272 pC/N and kp = 0.44, but the optimal sintering temperature have been improved from 1090 C to 1020
C successfully.
Keywords Liquid-phase sintering, Lead-free, Piezoelectric, ZnO.
Results and Discussion
o
1.0wt%
o
o
Intensity (a.u.)
1wt%
*
*
oo
oo
oo
0.8wt%
*
It is shown that for x = 0 and sintering temperature is 1090 C, the
maximum d33 value is 279 pC/N. Whereas increasing ZnO contents, the
maximum d33 value is 265 pC/N for x = 0.4 and 272 pC/N for x = 0.6,
respectively. However, owing to the inferences of ZnO liquid-phase sintering
promoters, their optimal sintering temperature all had been decreased to only
1020 C significantly. For most specimens, the d33 values will first increase,
and then reach to saturated maximum values gradually as sintering
temperature is higher than 1020 C, and finally start to decrease as it higher
than 1090 C. Especially, as x = 0.6, which exhibits the best d33 value (272
pC/N) than other ZnO-doped LKNNT ceramics.
300
0.8wt%
0.6wt%
0.6wt%
0.4wt%
280
0.2wt%
260
0.0wt%
240
5000
x=0.2
x=0.4
x=0.6
x=0.8
x=1.0
4000
Dielectric Constant
o LKNNT
* ZnO
(200)
o
(002)
It is clear that all the ceramics possess a perovskite structure in which the
tetragonal symmetry is dominant at room temperature, though a slight
increase of the orthorhombic symmetry is observed with increasing ZnO
content. From the 44~47 figure presented, we can observe that as 0.6 wt%
ZnO was added, (002) and (200) peaks reveal obvious coexistence of
orthorhombic and tetragonal phases for 0.6 wt% specimen.
3000
2000
1000
0.4wt%
0.2wt%
0
30
40
50
60
44.5
45.0
45.5
46.0
46.5
47.0
2 Theta (deg.)
2 Theta (deg.)
Figure 1. XRD patterns of 1020C-sintered LKNNT + (x)wt% ZnO ceramics.
It can be observed that x = 0.6 specimen reveals the most uniform grains
than others, and the average grain size is about 4 m. As x < 0.6, the surfaces
become dense with increasing of ZnO contents, additionally, for higher ZnO
contents up to 0.6 wt%, the average grain size of matrix-grains increases
significantly and become more non-uniform as x > 0.6.
100
300
400
Temperature (oC)
200
Figure 6. The dielectric constant for the LKNNT + (x)ZnO ceramics as measured
at 1KHz / 25~500 C.
180
x=0.0
x=0.4
x=0.6
x=1.0
x=3.0
160
140
410
405
120
400
960
It can be observed that for pure LKNNT (the theoretical density is 4.538
g/cm3), maximum relative density (0.97) can be obtained as the sintering
temperature is 1090 C. Whereas increasing the ZnO contents, the relative
density increases to a saturated and maximum value of 0.98 for x = 0.4, and
also increases to 0.978 for x = 0.6 (the theoretical densities are 4.55 g/cm3
and 4.556 g/cm3 for x = 0.4 and x = 0.6, respectively), however, their optimal
sintering temperature had be decreased from 1090 C (x = 0) to 1020 C (x =
0.4, 0.6) effectively due to these little ZnO additions. But for the ZnO
contents further increase to x = 1 and x = 3, the relative density starts to
decrease (0.962 for x = 1 and only 0.916 for x = 3) due to the inferences of
ZnO second phases, non-uniform grains, and pores.
200
220
980
1000
1020
1040
1060
1080
1100
1120
Sintering Temperature (oC)
Tc (oC)
20
44.0
d33 (pC/N)
0.0wt%
Figure 4. Piezoelectric constant (d33) as a function sintered temperature for the
LKNNT + (x)ZnO ceramics.
0.50
395
390
385
x=0.0
x=0.4
x=0.6
x=1.0
x=3.0
0.45
380
375
0.2
0.4
0.40
0.6
0.8
1.0
kp
ZnO Content x (%)
Figure 7. The Curie temperature vs. ZnO contents of the LKNNT + (x)ZnO ceramics
(measured at 1KHz).
0.35
Table 1. Properties of the LKNNT + (x)ZnO ceramics.
(a) x = 0
(c) x = 0.4
(b) x = 0.2
(d) x = 0.6
0.30
0.25
960
1000
1020
1040
1060
1080
1100
1120
Figure 5. The electromechanical coupling factor (kp) as a function sintered temperature
for the LKNNT + (x)ZnO ceramics.
1.00
It is clear that for x = 0 and sintering temperature is 1090 C, the maximum
kp value is 0.46. Whereas increasing ZnO contents, the maximum kp value is
0.43 for x = 0.4 and 0.44 for x = 0.6, respectively. However, owing to the
inferences of ZnO liquid-phase sintering promoters, their optimal sintering
temperature all had been decreased to only 1020 C significantly. The kp
values will first increase, and then reach to saturated maximum values
gradually as sintering temperature is higher than 1020 C, and finally start to
decrease as it higher than 1090 C. Especially for x = 0.6 specimen, its kp
value (kp = 0.44) is better than others but for x = 0 (kp = 0.46).
0.98
0.96
0.94
0.92
x=0.0
x=0.2
x=0.4
x=0.6
x=1.0
x=3.0
0.90
0.88
The TC values, which are concluded in Table 1 and Fig. 7. The
Curie temperature (TC) in this study is observed to be gradually
increasing with the increase of ZnO content, and maximum relative
dielectric constant (max) is 5081 for x = 0.6.
0.86
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)ZnO ceramics.
X=0.4
X=0.6
X=0.8
X=1
4575
380
100
4660
385
40
5081
395
95
3885
400
40
3682
405
90
Sintering Temperature (oC)
(g) x = 3
(e) x = 0.8
(f) x =1
Figure 2. SEM images of 1020C-sintered LKNNT + (x)ZnO ceramics.
Relative Density
980
εmax
TC (oC)
TO-T (oC)
X=0.2
1120
Conclusions
Lead-free LKNNT + (x)ZnO piezoelectric ceramics were successfully
prepared by adding little amount of ZnO. The structures, dielectric and
piezoelectric properties of LKNNT + (x)ZnO ceramics were investigated. All
samples show pure perovskite phase with typical orthorhombic symmetry.
Compared to pure LKNNT ceramic (sintered at 1090 C, d33 = 279 pC/N, and
kp = 0.46), even though the optimal d33 and kp values were only 272 pC/N
and kp = 0.44 for x = 0.6 specimen, but the optimal sintering temperature
have been improved from 1090 C to 1020 C effectively.