Leadfree (1
x
)(Na
_{0.5}
K
_{0.5}
)NbO
_{3}
xLiNbO
_{3}
, i.e., NKNLN
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 NKNLN
x
ceramics were studied using a dielectric constant and
PE
(Polarizationelectric 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 NKNLN
x
(
x
≥ 0.1) is a secondorder phase transition, and that of NKNLN
x
(
x
≤ 0.2) is a firstorder phase transition. These results indicate that the ferroelectric phase transition temperature of NKNLN
x
change from that of secondorder to weak firstorder phase transition according to the LN content.
1. INTRODUCTION
In the field of piezoelectric ceramics, sodium potassium niobate ceramics with leadfree piezoelectric material have been investigated as alternative material for PZTbased ceramics
[1

13]
. Leadfree 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
_{1y}
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 hotpressing, 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 Bsites
[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, leadfree piezoelectric and ferroelectric ceramics. A morphotropic phase boundary between the orthorhombic phase and the tetragonal phase of NKNLN
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 cubicorthorhombic and orthorhombictetragonal 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., NKNLN
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
Leadfree (1
x
)(Na
_{0.5}
K
_{0.5}
)NbO
_{3}

x
LiNbO
_{3}
, i.e., NKNLN
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 ballmilled 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 Xray 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 PE (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., NKNLN
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 NKNLN
x
ceramics were transformed, again increasing their LiNbO
_{3}
content.
PE hysteresis loops of (1
x
)(Na
_{0.5}
K
_{0.5}
)NbO
_{3}

x
LiNbO
_{3}
, i.e., NKNLN
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 PE hysteresis loops is considered to be evidence that a material is ferroelectric.
The capacitor is characterized by PE hysteresis curves. However, the shapes of the PE 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}
.
Xray diffraction patterns of the (1x)(Na_{0.5}K_{0.5})NbO_{3}xLiNbO_{3}, NKNLNx ceramics.
Ferroelelctric hysteresis loops of the (1x)(Na_{0.5}K_{0.5})NbO_{3} xLiNbO_{3}, NKNLNx 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 NKNLNx 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 NKNLN
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 NKNLN0.06 ceramics are a promising candidate for leadfree hightemperature 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., NKNLN
x
(
x
=0.0, 0.1, 0.2, 0.3, 0.4 mol) ceramics. In the case of NKNLN0.0 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 λtype 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(1x)(Na_{0.5}K_{0.5})NbO_{3}xLiNbO_{3}, NKNLNx 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 midpoint 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 NKNLN0.1, 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 NKNLN0.0. The sharp peaks around T
_{C}
for the NKNLN0.0 and NKNLN0.1 samples show a secondorder phase transition without thermal hysteresis.
In the case of NKNLN
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 firstorder phase transition. A lowtemperature dielectric anomaly was not observed upon heating and cooling. In NKNLN
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 cubicorthorhombic (at T
_{C}
) and orthorhombictetragonal (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 NKNLiNbO
_{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 NKNLNx ceramics on heating and cooling. unit: ℃
Phase transition temperature (T_{OT}, T_{C}) of NKNLNx 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 lowtemperature 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 secondorder phase transition of NKNLN
x
. These results indicate that the phase transition of NKNLN
x
ceramics occurs when T
_{C}
changes from a secondorder to weak firstorder 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 secondorderferroelectric phase transition.
4. CONCLUSIONS
In conclusion, (1
x
)(Na
_{0.5}
K
_{0.5}
)NbO
_{3}

x
LiNbO
_{3}
, i.e., NKNLN
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 PE 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 NKNLN
x
ceramics. The phase transition temperature, T
_{C}
, increased with increasing LN contents. The ferroelectric phase transition of NKNLN
x
(
x
≤ 0.1), is a secondorder transition without thermal hysteresis, and NKNLN
x
(
x
≥ 0.2) is a weak firstorder 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 firsttosecondorderferroelectric phase transition.
Acknowledgements
This work was supported by a research grant from Chinju National University of Education.
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