Ferroelectrics Dielectric Relaxation and Electrical

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Ferroelectrics

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Dielectric Relaxation and Electrical Properties of Lead-Free LiNbO3Substituted Na0.5K0.5NbO3 Ferroelectric Ceramics

Jin Soo Kima; Chang Won Ahna; Ill Won Kima; Sang-Bock Chob; Chang Hee Chungc; Ho Sueb Leec a Department of Physics, University of Ulsan, Ulsan, South Korea b Department of Electrical Eng., University of Ulsan, Ulsan, South Korea c Department of Physics, Changwon National University, Gyeongnam, Changwon, South Korea Online publication date: 01 December 2010

To cite this Article Kim, Jin Soo , Ahn, Chang Won , Kim, Ill Won , Cho, Sang-Bock , Chung, Chang Hee and Lee, Ho

Sueb(2010) 'Dielectric Relaxation and Electrical Properties of Lead-Free LiNbO3-Substituted Na0.5K0.5NbO3 Ferroelectric Ceramics', Ferroelectrics, 404: 1, 180 — 185 To link to this Article: DOI: 10.1080/00150193.2010.482496 URL: http://dx.doi.org/10.1080/00150193.2010.482496

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Ferroelectrics, 404:180–185, 2010 Copyright © Taylor & Francis Group, LLC ISSN: 0015-0193 print / 1563-5112 online DOI: 10.1080/00150193.2010.482496

Dielectric Relaxation and Electrical Properties of Lead-Free LiNbO3 -Substituted Na0.5 K0.5 NbO3 Ferroelectric Ceramics JIN SOO KIM,1,∗ CHANG WON AHN,1 ILL WON KIM,1 SANG-BOCK CHO,2 CHANG HEE CHUNG,3 AND HO SUEB LEE3 Downloaded By: [Cwahn][Kim, Ill Won] At: 01:36 2 December 2010

1

Department of Physics, University of Ulsan, Ulsan 680-749, South Korea Department of Electrical Eng., University of Ulsan, Ulsan 680-749, South Korea 3 Department of Physics, Changwon National University, Changwon 621-749, Gyeongnam, South Korea 2

Na0.5 K0.5 NbO3 ceramics and LiNbO3 -substituted Na0.5 K0.5 NbO3 ceramics were prepared by the solid-state reaction method. The crystallization and grain morphology of the two types of ceramics were confirmed by X-ray diffraction and atomic force microscopy studies, respectively. The 5 mol% LiNbO3 substituted Na0.5 K0.5 NbO3 ceramics shows well-saturated ferroelectric P-E hysteresis loops. The dielectric and electrical properties of the ceramics were investigated in the frequency range from 10 Hz to 1 MHz and the temperature range from 25◦ C to 600◦ C. Analysis of the complex impedance relaxation by the Cole-Cole plot showed one impedance relaxation for LiNbO3 substituted Na0.5 K0.5 NbO3 . The contributions of electrical conduction in the above mentioned temperature and frequency ranges are discussed. Keywords Ferroelectrics; dielectric properties; Na0.5 K0.5 NbO3 ; ABO3 perovskite; P-E hysteresis loop; piezoelectric; lead free

Introduction For environmental safety, it is necessary that ferroelectric and piezoelectric materials be lead-free. One such material, Na0.5 K0.5 NbO3 (NKN), has been studied extensively because it has good piezoelectric and ferroelectric properties [1]. Dielectric and electrical properties of NKN ceramics have been investigated often in the past several years [1–4]. To ensure practical device applications, it is necessary to carry out high quality investigations of the dielectric, ferroelectric, and electrical properties of NKN-based ceramics. Many research groups have conducted studies in an attempt to improve the ferroelectric and piezoelectric properties of ceramics [2–4]. Recently, the effects of ion doping and several solid solutions on ceramic properties were investigated [5–13]. In this study, we prepared NKN- and LiNbO3 -substituted NKN ceramics and investigated the effects of LiNbO3 substitution on the dielectric and electrical properties on the ceramics. We carried

Received August 23, 2009; in final form October 20, 2009. ∗ Corresponding author. E-mail: [email protected]

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Lead-Free LiNbO3 -Substituted Na0.5 K0.5 NbO3 Ferroelectric Ceramics

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out this investigation by means of a P-E hysteresis loop, dielectric measurements, and complex impedance measurements.

Downloaded By: [Cwahn][Kim, Ill Won] At: 01:36 2 December 2010

Experimental Work Powders of pure Na0.5 K0.5 NbO3 and 0.95Na0.5 K0.5 NbO3 :0.5LiNbO3 ceramics were prepared by the solid-state reaction method using K2 CO3 , Na2 CO3 , Nb2 O5 , and Li2 CO3 (99.0–99.9%) powders as starting materials. The two powders were mixed separately with ethanol by ball milling for 24 h and dried at 80◦ C for 5 h. They were then sintered at 800◦ C for 2 h and cooled to room temperature. Thereafter, they were again milled with ethanol and pressed into pellets (13 mm in diameter and 1 mm in thickness). Finally, the pellets of Na0.5 K0.5 NbO3 and 0.95 Na0.5 K0.5 NbO3 :0.5 LiNbO3 (abbreviated as NKN and NKN:LN05, respectively) ceramics were sintered at 1100◦ C∼1200◦ C for 2 h in air. The NKN phase formation and crystal structure were investigated by X-ray diffraction (XRD). A silver electrode was coated on the ceramic samples to investigate their electrical properties. The capacitance and loss tangent (tanδ) were measured at a rate of 1◦ C/min over the frequency range of 10 Hz∼1 MHz by a dielectric spectrometer (SI1260). The remanent polarization (Pr ) and coercive electric field (Ec ) were obtained from the ferroelectric P-E hysteresis loop by using a Sawyer-Tower circuit.

Results and Discussion NKN and NKN:LN05 ceramics were prepared by the solid-state reaction method. The XRD peaks of NKN and NKN:LN05 ceramics were in agreement with that of the previous NKN phase with the ABO3 perovskite structure. This indicates that the NKN:LN05 single phase was formed after LiNbO3 substitution [2, 4]. Figure 1 shows the ferroelectric P-E hysteresis loops of NKN and NKN:LN05 ceramics at room temperature. The ferroelectric P-E hysteresis loop of NKN ceramic showed a round shape, which is caused by high leakage conductivity of the NKN ceramic. However, the ferroelectric P-E hysteresis loops of NKN:LN05 ceramic changed a round shape of NKN to a typical P-E hysteresis shape of NKN:LN05. The value of the remanent polarization of NKN:LN05 ceramic is Pr = 10 µC/cm2. The value of the coercive field of NKN:LN05 ceramic is Ec = 12 kV/Cm. Thus, the remanent polarization increased slightly with substitution of LiNbO3 , but the coercive field decreased with this substitution.

Figure 1. Ferroelectric P-E hysteresis loops of Na0.5 K0.5 NbO3 and NKN:LN05 ceramics.


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Figure 2. Temperature dependence of dielectric constants of NKN and NKN:LN05 ceramics at a frequency of 100 kHz. (See Color Plate XXXII)

Figure 2 shows the temperature dependence of the dielectric constant at 100 Hz, 1 kHz, 10 kHz, 100 kHz, and 1 MHz. Two dielectric peaks, i.e., phase transitions, of the NKN ceramic were observed at 200◦ C and 420◦ C. The peak at 200◦ C is attributed to an orthorhombic-tetragonal (TO-T ) transition. In contrast, the peak at 420◦ C is attributed to the phase transition temperature from the cubic-orthorhombic phase (Curie temperature, Tc ). The dielectric behaviors of the NKN ceramic with two dielectric peaks were in agreement with results of previous studies [7]. The phase transitions of the NKN:LN05 ceramic are observed at 140◦ C (TO-T ) and 460◦ C (Tc ). The Curie temperature Tc of the dielectric peak increased slightly and the magnitude of the peak decreased. In particular, the dielectric peaks at 140◦ C became broader. The LiNbO3 substitution resulted in an inhomogeneous composition and a more disordered crystal structure of the NKN ceramic, resulting in the broad dielectric maximum. The dielectric peak with the broad maximum can be explained by the coexistence of A-site ions [1, 8]. Figure 3 shows the plot of the ac conductivity, σ ac , versus reciprocal temperature, 1/T, which was measured at a frequency of 1 kHz. The conductivity of the NKN:LN05 ceramic decreased by as much as one order of magnitude in the high-temperature range, which indicates that LiNbO3 substitution decreased the electrical conductivity caused by the space charge or ionic charge carriers. The ac conductivity can be divided into different regions. The ac conductivity versus 1/T is described by an Arrhenius relation, σ (T) = σ o exp(-Ea/kT), where Ea is the activation energy and k is the Boltzmann constant. The slope of the curve of ln(σ ) versus 1000/T gives the value of Ea for the conduction process [8, 9]. Above Tc, the electrical conductivity is dominated by intrinsic defects. The activation energies of the NKN and NKN:LN05 ceramics obtained at 1 kHz were 0.98 eV. For ionic conduction, most of the mobile charges in the Bi-layered oxides are generally oxygen vacancies [6]. The activation energy for oxygen vacancy migration is approximately 1 eV. A similar value of activation energy (0.9∼1.1 eV) for oxygen-ion diffusion has been reported for titanate- and niobate-based crystals, which is related to the oxygen vacancy transport within the system. Experimentally measured parallel values of resistance and capacitance were converted to their equivalent qualities; these values are shown in the form of complex impedance (Z ∗ =



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Figure 3. AC conductivity of NKN and NKN:LN05 ceramics measured at 1 kHz. (See Color Plate XXXIII)

Z − iZ ) plots [8–11]. The complex impedance of many condensed matter systems can be expressed by introducing a temperature-dependent factor n into the Debye expression, i.e., the Cole-Cole expression [9, 11], Z ∗ (ω) =

Z0 , τ0 = 1/ω0 = 1/2πfr 1 + (iωτ )n

(1)

Here, τ0 is the mean relaxation time and the factor n take values in the range 0 ≤ n ≤ 1. If n is unity, the relaxation is explained by a Debye-type response [9]. The real and imaginary parts of the complex impedance can be written from Eq. (2) in the following forms: Z = Z∞ + (Zs − Z∞ ) Z = (Zs − Z∞ )

1 + (ωτ )n cos(nπ/2) 1 + 2(ωτ )n cos(nπ/2) + (ωτ )2n

(ωτ )n sin(nπ/2) 1 + 2(ωτ )n cos(nπ/2) + (ωτ )2n

(2) (3)

where Zs and Z∞ are the static impedance and impedance at very high frequencies, respectively. Figure 4 shows the complex impedance Cole-Cole plots of NKN and NKN:LN05 ceramics at various temperatures. The ceramics showed an impedance semicircular arc in the wide frequency and temperature ranges. In particular, the impedance semicircular arc in the measured frequency region is consistent with that in the temperature range in which the real part of the dielectric constant has a dielectric dispersion (Fig. 3). These facts suggest that the dielectric anomaly and impedance plot are representative of the bulk grain effect. The NKN:LN05 ceramic exhibited impedance relaxation explained by the Cole-Cole type response. Defects such as Na and K and oxygen vacancies were formed at high sintering temperature during NKN preparation [5, 7, 11–13], which caused strong low-frequency dielectric dispersion and high electrical conductivity. The defects are trapped at grain boundaries and grain-electrode interfaces, which act as a space charge. However, LiNbO3 substitution reduces the contribution of space charge or ionic charge carriers. Therefore, the electrical conductivity of the ceramics decreased and their ferroelectric properties improved by LiNbO3 substitution.


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Figure 4. Complex impedance plots of NKN and NKN:LN05 ceramics. (See Color Plate XXXIV)

Conclusions Lead-free ferroelectric and piezoelectric ceramics, 5 mol% LiNbO3 substituted Na0.5 K0.5 NbO3 (NKN:LN05) ceramics, were successfully synthesized by the solid-state reaction method. By LiNbO3 substitution, the dielectric constant peak of the NKN ceramic became broader, owing to the increase in disorder at the A-site. The ferroelectric P-E hysteresis loop of the NKN:LN05 ceramic changed from the round shape of NKN to a typical P-E hysteresis shape of NKN:LN05, which implies a enhanced ferroelectricity of the materials. The NKN:LN05 ceramic exhibited a well-saturated P-E hysteresis loop with remanent polarization Pr = 10 µC/cm2. The NKN:LN05 ceramic exhibited one impedance semicircular arc, which is explained by the Cole-Cole type response. Therefore, the substitution of a small amount of LiNbO3 decreased the electrical conductivity of the ceramics, which in turn improved their ferroelectric properties.

Acknowledgments This work was supported by National Research Foundation of Korea Grant funded by the Korean Government (2009-0072488). Also supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2009-0093818).

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