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Ferroelectric Phase Transition of Lead Free (1-<italic>x</italic>)(Na<sub>0.5</sub>K<sub>0.5</sub>)NbO<sub>3</sub>- xLiNbO<sub>3</sub> Ceramics
Ferroelectric Phase Transition of Lead Free (1-x)(Na0.5K0.5)NbO3- xLiNbO3 Ceramics
Transactions on Electrical and Electronic Materials. 2012. Dec, 13(6): 297-300
Copyright ©2012, The Korean Institute of Electrical and Electronic Material Engineers
This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
  • Received : April 04, 2012
  • Accepted : November 11, 2012
  • Published : December 25, 2012
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About the Authors
Jong-Ho Park
Department of Science Education, Chinju National University of Education, Jinju 660-756, Korea
parkkdp@cue.ac.kr
Hui-Jin Park
Department of Physics, Pukyong National University, Busan 608-737, Korea
Byung-Chun Choi
Department of Physics, Pukyong National University, Busan 608-737, Korea
Abstract
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.
Keywords
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 .
Lager Image
X-ray diffraction patterns of the (1-x)(Na0.5K0.5)NbO3-xLiNbO3, NKN-LNx ceramics.
Lager Image
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
Lager Image
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: ℃
Lager Image
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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