Lead-free (1-
x
)(Na
0.5
K
0.5
)NbO
3
-xLiNbO
3
, i.e., NKN-LN
x
(
x
=0.0, 0.1, 0.2, 0.3, 0.4 mol) was prepared using the conventional solid state reaction method. The effects of LN mixing on the ferroelectric properties of NKN-LN
x
ceramics were studied using a dielectric constant and
P-E
(Polarization-electric field) measurements. Ferroelectricity was observed in the composition for x approximately varying between 0.0 and 0.4. Minimum remanent polarization 2
P
r
=5 C/cm
2
was achieved in the composition for
x
= 0.2. The ferroelectric phase transition temperature T
C
increased with increasing LN content. The ferroelectric phase transition of NKN-LN
x
(
x
≥ 0.1) is a second-order phase transition, and that of NKN-LN
x
(
x
≤ 0.2) is a first-order phase transition. These results indicate that the ferroelectric phase transition temperature of NKN-LN
x
change from that of second-order to weak first-order phase transition according to the LN content.
1. INTRODUCTION
In the field of piezoelectric ceramics, sodium potassium niobate ceramics with lead-free piezoelectric material have been investigated as alternative material for PZT-based ceramics
[1
-
13]
. Lead-free ferroelectric materials with perovskite structure have a general formula of ABO
3
. In this structure, cations based on their valence states and coordination numbers occupy the A- or Bsites. Na
1-y
K
y
NbO
3
, NKN is a material with perovskite structure, and it exhibits high piezoelectric properties because its structure permits spontaneous polarization to rotate along three orientations. Sodium potassium niobate (NKN) is a solid solution of potassium niobate (KN) a ferroelectric and sodium niobate (NN), with an Na/K ratio of ~50/50. The piezoelectric applications of Na
0.5
K
0.5
NbO
3
(NKN), ceramics produced by hot-pressing, are better than those produced by sintering in air atmosphere. Hotpressed NKN ceramics have been reported to have a high phase transition temperature (T
c
~ 420℃), good piezoelectric properties (d
33
~ 160 pC/N), and a high planar coupling coefficient (κp~45%)
[1
-
4]
.
However, NKN ceramics are difficult to obtain using the conventional sintering method because their phase stability is limited to 1,140℃ and they are exposed to moisture. Therefore, attempts have been made to improve the sinterability and piezoelectric properties of KNN through the addition and/or substitution of several cationic elements in the A- or B-sites
[10
-
13]
It is known that (1-
x
)(Na
0.5
K
0.5
)NbO
3
-
x
LiNbO
3
, NKN- LN
x
ceramics are good, lead-free piezoelectric and ferroelectric ceramics. A morphotropic phase boundary between the orthorhombic phase and the tetragonal phase of NKN-LN
x
was present when
x
was approximately 0.05 ~ 0.07 mol of LN
[8]
. Guo
et. al.
, observed that the Curie temperatures (T
C
) of NKN- LN
x
ceramics were in the range of 452 ~ 510℃, according to their LN content, which is at least 100℃ higher than that of Pb(Zr, Ti)O
3
. For (Na
0.5
K
0.5
)NbO
3
, Tc values were observed at 420℃ and 200℃, which correspond to the cubic-orthorhombic and orthorhombic-tetragonal phase transitions, respectively. Two phase transitions were present at
x
= 0.04, 0.06 mol, similar to the case for NKN, except that the phase transition temperatures were shifted
[9]
. Many research efforts thus far have been based on the conditions for which a small amount of LN was added to the NKN composition.
In this study, (1-
x
)(Na
0.5
K
0.5
)NbO
3
-
x
LiNbO
3
, i.e., NKN-LN
x
(
x
= 0.0, 0.1, 0.2, 0.3, 0.4mol), was synthesized using the conventional solid state method. The purpose of this study is to investigate the phase transition and electrical properties of (Na
0.5
K
0.5
)NbO
3
in terms of its LiNbO
3
content.
2. EXPERIMENTS
Lead-free (1-
x
)(Na
0.5
K
0.5
)NbO
3
-
x
LiNbO
3
, i.e., NKN-LN
x
(
x
= 0.0, 0.1, 0.2, 0.3, 0.4 mol), was prepared by mixing the oxides, K
2
CO
3
(99% purity), Na
2
CO
3
(99% purity), LiNbO
3
(99% purity) and Nb
2
O
5
(99% purity) in a molar ratio used in the conventional solid state reaction method. Before being weighed, the K
2
CO
3
and Na
2
CO
3
powders were first dried in an oven at 200℃ for 10 h to minimize the effect of moisture. These powders were then milled with ZrO
2
balls for 20 h using ethyl alcohol as a medium and dried. The dried powders were calcined at 850℃ for 2 h. After calcination, the powders were ball-milled again for 20 h and, dried, after which PVA(4 wt%) was added as a binder. They were then pressed into disks with diameter of under 13 mm. After burning off the PVA, the pellets were sintered at 1,070℃ for 2 h. The crystal structures were determined by X-ray power diffraction analysis using CuКα radiation (Philips X’ Pert - MPD system). The remnant polarization
P
r
and coercive field
E
c
were determined from the P-E (Polarization - Electric field) hysteresis loops, as measured by a Radiant Precision Workstation. To examine their dielectric properties, the ceramics were polished and painted with silver paste on both surfaces, and fired at 800℃ for 30 min. The real and imaginary dielectric constants were measured using an SI1260 impedance analyzer at temperature ranging from room temperature to ~ 600℃ with heating and cooling rates of 0.2℃/min in the frequency range of 1 Hz to 1 MHz.
3. RESULTS AND DISCUSSION
Figure 1
shows the XRD patterns of the (1-
x
)(Na
0.5
K
0.5
)NbO
3
-
x
LiNbO
3
, i.e., NKN-LN
x
(
x
= 0.0, 0.1, 0.2, 0.3, 0.4 mol) ceramics. Studies have reported that a phase of K
3
Li
2
Nb
5
O
15
(KLN) with a tetragonal tungsten bronze structure starts to appear at x ≥ 0.08
[9]
. In this study, it appeared at x ≤ 0.2 but for x ≥ 0.3, the KLN phase and LiNbO
3
phase coexisted. This implies that the structures of the NKN-LN
x
ceramics were transformed, again increasing their LiNbO
3
content.
P-E hysteresis loops of (1-
x
)(Na
0.5
K
0.5
)NbO
3
-
x
LiNbO
3
, i.e., NKN-LN
x
(
x
= 0.0, 0.1, 0.2, 0.3, 0.4 mol) ceramics measured at room temperature under a driven electric field are plotted in
Figs. 2
(a)- (f). Generally, the presence of P-E hysteresis loops is considered to be evidence that a material is ferroelectric.
The capacitor is characterized by P-E hysteresis curves. However, the shapes of the P-E loops changed slightly with increasing LN contents. As shown in
Fig. 2
(f), the value of 2
P
r
decreases with an increasing LN content below a certain critical level. 2
P
r
has a minimum value of 5 C/cm
2
near
x
= 0.2, and it first increases and then decreases after reaching this value. The coercive field 2
E
c
increases for an increase in the amount of LN in the range between
x
= 0.0 and
x
= 0.1 mol., and a further increase in the amount of LN above
x
= 0.2 mol causes an increase in 2
E
c
.
X-ray diffraction patterns of the (1-x)(Na0.5K0.5)NbO3-xLiNbO3, NKN-LNx ceramics.
Ferroelelctric hysteresis loops of the (1-x)(Na0.5K0.5)NbO3- xLiNbO3, NKN-LNx ceramics for (a) x =0.0, x =0.1, (c) x =0.2, (d) x =0.3, and (e) x =0.4 mol, (f) remanent polarization and coercive field of NKN-LNx ceramics as a function of the LN contents x.
The tendency of varying 2
P
r
is similar to that of 2
E
c
when the range of
x
is approximately above
x
= 0.2 mol.
Du
et al.
[8]
reported the dielectric properties of NKN-LN
x
ceramics for the case that the amount of LN is below
x
= 0.2 mol; when the amount LN is
x
= 0.06 mol,
E
c
achieves its minimum value of 13.4 kV/cm and
P
r
reaches its minimum value of 20 C/ cm
2
. They proposed that NKN-LN0.06 ceramics are a promising candidate for lead-free high-temperature piezoelectric ceramics.
Figures 3
(a) and (e) show the real (
ε
') dielectric constant at 1 MHz as a function of temperature for of (1-
x
)(Na
0.5
K
0.5
)NbO
3
-
x
LiNbO
3
, i.e., NKN-LN
x
(
x
=0.0, 0.1, 0.2, 0.3, 0.4 mol) ceramics. In the case of NKN-LN0.0 ceramics, the values of
ε
' increase with decreasing temperature. At T
C
(the temperature at which
ε
' is maximized) = 409℃,
ε
' beings to decrease, forming a large λ-type peak in the dielectric constant
vs
. temperature curve upon heating and cooling.
As the temperature decreases, if we assume that the phase
The temperature dependence of the real dielectric constant ε' in(1-x)(Na0.5K0.5)NbO3-xLiNbO3, NKN-LNx ceramics at 1 MHz on heating (symbol) and cooling (solid line), (a) x=0.0, (b) x =0.1, (c) x =0.2, (d) x =0.3, and (e) x =0.4 mol.
transition temperature is the mid-point of the steepest curve of
ε
', then the lower transition occurs at T
OT,C
(low temperature phase transition point) = 176℃ upon cooling and at T
OT,H
=195℃ upon heating with a thermal hysteresis of 19℃ This result is similar to that reported by Guo
et al.
[9]
.
In the case of NKN-LN0.1, a low temperature anomaly was not observed at T
OT
upon heating or cooling.
At high temperatures, the complex dielectric response of NKNLN0.1 was found to be similar to that of NKN-LN0.0. The sharp peaks around T
C
for the NKN-LN0.0 and NKN-LN0.1 samples show a second-order phase transition without thermal hysteresis.
In the case of NKN-LN
x
(
x
≥ 0.2), the ferroelectric phase transition temperature T
C
shifted to a higher value with an increase in the LN content, whereas the dielectric peak broadened. The temperature anomaly of the real dielectric constant appeared at T
C
in all the samples upon heating and cooling with a small thermal hysteresis, which corresponded to at weak first-order phase transition. A low-temperature dielectric anomaly was not observed upon heating and cooling. In NKN-LN
x
samples with 0 ≤
x
≤ 0.07, Guo et al.
[9]
reported that the phase transition of NKNLN0.0 was observed at 420℃ and 200℃, which corresponds to the cubic-orthorhombic (at T
C
) and orthorhombic-tetragonal (at T
OT
) phase transitions. Also, LiNbO
3
has lithium niobate structure, which can be described as a heavily distorted perovskite or an ordered phase derived from the corundum structure with space group R
3C
(C
3V
6
). So, it is evident that two effects on the structure of NKN ceramics have been observed in NKN-LiNbO
3
ceramics. At lower LiNbO
3
concentrations, Li mainly replaces Na and K in the
A
sites of ABO
3
perovskite structure (i.e. form a solid solution), leading to a linear shift of the Curie point (
T
C
) to higher temperature
[9]
. However, the structure of solid solution transforms from orthorhombic to tetragonal symmetry due to the large distortion caused by Li
+
[9]
.
The phase transition temperatures also shifted increasing the
Phase transition temperature (TOT, TC) of NKN-LNx ceramics on heating and cooling. unit: ℃
Phase transition temperature (TOT, TC) of NKN-LNx ceramics on heating and cooling. unit: ℃
LN content. T
C
shifted to a higher value, and T
OT
, to a lower value
[11]
. Thus, we expect that a low-temperature phase transition of this sample should appear at room temperature because these phase transition temperatures decrease with an increase in LN contents.
The values of T
OT
, T
C
, and ΔT obtained for all the samples are presented in
Table 1
. Here, ΔT indicates the degree of the firstand second-order phase transition of NKN-LN
x
. These results indicate that the phase transition of NKN-LN
x
ceramics occurs when T
C
changes from a second-order to weak first-order phase transition with increasing LN contents. Our results also show the possibility that the concentration of
x
= 0.2 may be the critical concentration for a first- to second-order-ferroelectric phase transition.
4. CONCLUSIONS
In conclusion, (1-
x
)(Na
0.5
K
0.5
)NbO
3
-
x
LiNbO
3
, i.e., NKN-LN
x
(
x
=0.0, 0.1, 0.2, 0.3, 0.4mol) ceramics, were synthesized using the solid state reaction method. The effects of LN mixing on the ferroelectric properties of these two ceramics were studied through dielectric and P-E measurements. The value of
P
r
increased with increasing Nb content. (1-
x
)(Na
0.5
K
0.5
)NbO
3
-
x
LiNbO
3
ceramics exhibited a minimum remanent polarization of 2
P
r
=5 μC/cm
2
at an LN content of
x
~ 0.2. These results indicate that LN doping can change the ferroelectric properties of NKN-LN
x
ceramics. The phase transition temperature, T
C
, increased with increasing LN contents. The ferroelectric phase transition of NKN-LN
x
(
x
≤ 0.1), is a second-order transition without thermal hysteresis, and NKN-LN
x
(
x
≥ 0.2) is a weak first-order transition with small thermal hysteresis. Thus, our results demonstrate the possibility that the concentration of
x
~ 0.2 may be the critical concentration for a first-to-second-order-ferroelectric phase transition.
Acknowledgements
This work was supported by a research grant from Chinju National University of Education.
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