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Ferroelectric relaxor properties of (1 − x)K0.5Na0.5NbO3–xBa0.5Ca0.5TiO3 ceramics
Current Applied Physics 12 (2012) 1266e1271
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Ferroelectric relaxor properties of (1 x)K0.5Na0.5NbO3exBa0.5Ca0.5TiO3 ceramics C.-W. Cho a, M.R. Cha a, J.Y. Jang a, S.H. Lee a, D.J. Kim a, S. Park a, *, J.S. Bae b, S.D. Bu c, S. Lee d, J. Huh e a
Department of Physics, Pusan National University, Busan 609-735, Republic of Korea Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea c Department of Physics, Chonbuk National University, Jeonju 561-756, Republic of Korea d JPS Micro-Tech, Busan 619-961, Republic of Korea e Chonnam National University, Yeosu 550-749, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2012 Received in revised form 25 February 2012 Accepted 2 March 2012 Available online 16 March 2012
The Ba0.5Ca0.5TiO3 (BCT) composition dependent dielectric and structural properties of (1x) K0.5Na0.5NbO3exBa0.5Ca0.5TiO3 powders were investigated. Room temperature x-ray diffraction revealed the powder structure to transform from orthorhombic to cubic with increasing BCT composition. The frequency dependent dielectric constant measurements revealed a shift in the temperature of the maximum dielectric constant for at frequencies, suggesting that the system exhibits ferroelectric relaxor behavior. The system containing 15% BCT showed the closest calculated CurieeWeiss exponent to 2, which the exponent for a relaxor ferroelectric. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Relaxor ferroelectric Dielectric constant X-ray diffraction
1. Introduction Relaxor ferroelectrics exhibit a high dielectric constant and high strain across a wide range of temperatures and are attractive for various applications, such as capacitors, sensors, actuators, and integrated micro-electromechanical systems [1]. The most commonly used relaxor ferroelectric systems, such as Pb(Mg1/3Nb2/3) O3 (PMN) and Pb(Zn1/3Nb2/3)O3 (PZN) exhibited mainly piezoelectricity and high dielectric constants [2,3]. However, those systems (for example PMN, PZN) contained almost 60 w% Pb [4,5]. The environmental effects of lead have attracted considerable concern, particularly on humans [6]. Therefore, lead-free ferroelectric materials with comparable or even superior properties to the most widely used lead (Pb2þ ion)-based ferroelectric material systems have received a great deal of research interest [7]. Among the many candidates for lead-free relaxor ferroelectrics systems, the BaTiO3 based pseudo-binary system [8] and/or the pseudo-ternary ferroelectric system based on KNN (orthorhombic structure [9]), are promising candidates because they exhibit high piezoelectricity and a high Curie temperature (420 C) [10e14]. As the relaxor behavior in lead-based ceramic systems stems the cationic distribution disorder in the same crystallographic site, lead-free relaxors have also been derived as compositionally
disordered systems. Many studies focused on modifying the atomic arrangements by substituting a similar size of ions while maintaining their structure because the structural symmetry is deformed by substituting atoms with different sized atoms [15]. Recently, Du et al., reported a relaxor ferroelectric system by adding Ba0.5Sr0.5TiO3 (BST) to their host KNN system [16]. The ionic radius of Ba2þ (0.161 nm) is similar to that of Kþ (0.164 nm) with the same coordination number (CN ¼ 12). Similarly, the ionic radii of Naþ and Sr2þ are 0.139 nm (CN ¼ 12) and 0.144 nm (CN ¼ 12), respectively, and the ionic radii of Nb5þ and Ti4þ is 0.064 nm (CN ¼ 6) and 0.074 nm (CN ¼ 6) [17], respectively, suggesting that the addition of BST in the KNN host will reduce the asymmetry of the ionic radius at the individual site. Ba0.5Ca0.5TiO3 (BCT) is another candidate for the rearrangement of ions that can be added to the KNN host because both BCT and BST have the same tetragonal structure [9,18]. Furthermore the ionic radius of Ca2þ is 0.134 nm (CN ¼ 12) which similar to that of Naþ. Therefore, it is expected the addition of BCT to the KNN host system can reduce the crystal asymmetry further due to the ionic size mismatch. This paper reports the structural and dielectric properties of the newly synthesized relaxor ferroelectric ceramics by adjusting the ionic radius in the perovosikte structure. 2. Experiments
* Corresponding author. Tel.: þ82 51 510 2595. E-mail address: [email protected] (S. Park). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.03.006
(1x)K0.5Na0.5NbO3exBa0.5Ca0.5TiO3 ceramics ((1x)KNNexBCT) with a range of BCT contents (x ¼ 0.0, 0.05, 0.10, 0.15, 0.20) were
C.-W. Cho et al. / Current Applied Physics 12 (2012) 1266e1271
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Fig. 1. (a) XRD patterns and (b) SEM images of an expanded region of the diffraction pattern of the (1 x)KNNexBCT powders. For the x ¼ 0.0 sample, the XRD profile (bottom in figure) was plotted with the fitted XRD profile for comparison purposes.
synthesized using a conventional solidestate reaction. As starting materials, K2CO3, Na2CO3, Nb2O5, BaCO3, CaCO3, and TiO2 reagent grade powders were weighed properly and mixed for 30 min before calcination and sintering. The pellets were then calcinated at 900 C for 2 h and sintered at 1100 C for 2 h. The ceramics were ground to a powder to allow x-ray diffraction (XRD, D8 Advance, Bruker) measurements of the compositional dependent crystal structures at ambient temperature. The temperature and frequency dielectric properties of the samples in pellet form were also measured with an Au-wires electrode using a HP4194A. The surface morphology of the pellet was also observed by scanning electron microscopy (SEM, Hitachi).
3. Results and discussion 3.1. Structural properties Fig. 1 shows the room temperature XRD patterns of the (1 x) KNNexBCT powders. Fig. 1(a) presents the XRD data (20 < 2q < 80 ). Fig. 1(b) presents the diffraction profile of the expanded region (40 < 2q < 50 ) along with SEM images. For the x ¼ 0.0 sample (pure KNN), the detailed structure was analyzed from a Reitveld refinement [19] using a PowderCell [20]. The sample was found to have an orthorhombic (Bmm2) with lattice parameters of a ¼ 5.635 Å, b ¼ 3.938 Å, and c ¼ 5.667 Å. Pure KNbO3
Fig. 2. Temperature dependent dielectric permittivity and tangent loss (inset) of (1 x)KNNexBCT ceramics measured at a 1 MHz.
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Table 1 CurieeWeiss temperature (TCW), the temperature above which the dielectric constant follows the CurieeWeiss law (TB), deviation (DTmax), the CurieeWeiss constant (C), and the diffuseness (g) for (1 x)KNNexBCT ceramics at 1 MHz. x Tmax ( C) TCW ( C) C ( C) ( 105) TB ( C) DTmax ¼ TB Tmax ( C)
g
0.0 403 379 1.31 403 0 1.19
0.05 312 286 1.84 357 45 1.45
0.10 226 178 1.44 362 136 1.85
0.15 149 61 1.29 339 190 1.88
0.20 87 67 1.68 171 84 1.75
has the same perovskite structure with lattice parameters of a ¼ 5.697 Å, b ¼ 3.971 Å, and c ¼ 5.721 Å [9]. Therefore, the KNN powder (x ¼ 0.0) has a smaller volume than that of pure KNbO3 due to the substitution of smaller Na ion in the potassium site of the system. Previously, Du et al., examined the (1x) K0.5Na0.5NbO3exBa0.5Sr0.5TiO3 system and observed the structural changes as the composition of Ba0.5Sr0.5TiO3 was varied from x ¼ 0.0 to x ¼ 0.20 [16]. A comparison of the present XRD profile
with Du’s result revealed, the sample to have a tetragonal structure for x ¼ 0.05, a pseudocubic structure for x ¼ 0.10e0.15, and a cubic structure for x ¼ 0.20 at room temperature. The physical origin of the structural changes with different BCT contents is related to the different sizes of the ionic radii because the mean ionic radius of the A site is smaller than that of pure KNN. On the other hand, the mean ionic radius of the B site was larger than that of pure KNN. Adding BCT to the KNN host material resulted in a decrease in the difference in ionic radius between the A and B sites as well as reduced asymmetry due to the difference in ionic radius. The inset in the Fig. 1(b) shows SEM of the (1 x)KNNexBCT ceramics. The grain size decreased with increasing BCT content. Furthermore, ceramics with a smaller grain size distribution exhibited fewer pores and a uniform distribution of grains. 3.2. Dielectric properties Fig. 2 shows the temperature dependent dielectric constants and dielectric loss (inset in Fig. 2) of (1 x)KNNexBCT ceramics
Fig. 3. Temperature dependent dielectric permittivity of (1 x)KNNexBCT ceramics for (a) x ¼ 0.0, (b) x ¼ 0.05, (c) x ¼ 0.10, (d) x ¼ 0.15 and (e) x ¼ 0.20 under various frequencies.
C.-W. Cho et al. / Current Applied Physics 12 (2012) 1266e1271
measured at 1 MHz. For pure KNN, two distinct dielectric peaks (w200 C and w400 C) were observed, which were assigned to the orthorhombic-tetragonal and tetragonal-cubic structural phase transitions, respectively [16,21]. As the BCT content was increased, the ceramics showed a dielectric peak within the measured temperature range. The maximum of the dielectric constants was w4500 at x 0.05, whereas the maximum dielectric constants for the x 0.10 samples were w1000. Furthermore, the temperature which was exhibited maximum dielectric constant (Tmax) and dielectric constants decreased with increasing BCT content. The detailed Tmax is listed in Table 1. The reduced dielectric constants and broad nature of the peak around Tmax are typical characteristics of relaxor ferroelectrics. For x 0.10, the dielectric loss of the ceramics reached a minimum (<4%) around the Tmax (100e300 C). Ceramics with smaller dielectric loss and a larger dielectric constant (for example x ¼ 0.05 sample) can be used as high temperature capacitors in the automobile industry. Fig. 3 shows the temperature dependent dielectric permittivity of the (1 x)KNNexBCT ceramics at various frequencies. For the pure KNN (x ¼ 0.0), the dielectric permittivity profile is invariant at various frequencies [21]. With the samples containing x ¼ 0.10, x ¼ 0.15, and x ¼ 0.20, the dielectric permittivity profile became diffusive and the maximum dielectric permittivity shifted to
Fig. 4. Inverse dielectric permittivity at 1 MHz as a function of temperature for (1 x) KNNexBCT ceramics (symbols: experimental data; the solid line: fitting to the Curie eWeiss law).
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a higher temperature with increasing frequency suggesting that the ceramics exhibited relaxor ferroelectric characteristics. The dielectric permittivity of the normal ferroelectric systems can be explained the CurieeWeiss law,
ε ¼
C ; T TCW
(1)
where C is CurieeWeiss constant and TCW is CurieeWeiss temperature [22,23]. The typical magnitude of C which reflects the nature of the ferroelectric transition, can be varied either w105 for displacive transition or w103 for an orderedisorder transition in the high temperature paraelectric phase (T > TCW). The inverse dielectric permittivity was examined as a function of temperature to analyze the behavior of the dielectric perimittivity around TCW is shown in Fig. 4. The linear characteristic of the inverse dielectric permittivity for the pure KNN, at T > TCW suggests that the system obeys the CurieeWeiss law. On the other hand, the dielectric permittivity of the samples with a higher BCT concentration (x 0.05) deviated from the CurieeWeiss laws for TCW < T < TB, where TB (Burns temperature) is the temperature when the linearity begins to deviate (i.e., the temperature that does not follow CurieeWeiss law) [24,25]. The degree of deviation can be defined as
Fig. 5. Log (1/εr 1/εmax) at 1 MHz for (1 x)KNNexBCT ceramics (symbols: experimental data; solid line: fitting to the modified CurieeWeiss law) as a function of the log (T Tmax).
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C.-W. Cho et al. / Current Applied Physics 12 (2012) 1266e1271
a
b
Fig. 6. Inverse temperature of the maximum dielectric constant temperature (Tmax) as a function of ln f for (1 x)KNNexBCT ceramics. The solid line represents the fitting results using the VogeleFulcher law.
DTmax (¼ TB Tmax). The DTmax increased with increasing BCT
content, reached a maximum when x ¼ 0.15, and then decreased at higher BCT contents. This suggests that the diffusive structural phase transition increased with increasing BCT content up to x ¼ 0.15 and then decreased because the DTmax for normal ferroelectrics is approximately zero. Previously, Uchino et al., proposed the modified CurieeWeiss law (so called Uchino and Nourma function),
1 1 ðT Tmax Þg ¼ C εr εmax
(2)
to estimate the degree of the relaxor [26], where C is the Curie constant and g is the diffusion coefficient. g ¼ 1 and 2, represents a normal ferroelectric system and ideal relaxor ferroelectric system, respectively. Fig. 5 shows log(1/εr 1/εmax) as function of log(T Tmax) for (1 x)KNNexBCT ceramics measured at 1 MHz. The solid lines represent the best linear fitted results to estimate the slope (i.e., g). The g values increased (closed to 2) with increasing BCT content until x ¼ 0.15, then decreased with further increases in BCT content (x ¼ 0.20) suggesting that 0.85KNN e0.15BCT ceramics (x ¼ 0.15) most likely exhibited relaxor behavior. Table 1 lists the detailed values of g. Many models have been proposed to explain the origin of relaxor ferroelectrics, such as composition fluctuation theory, superparaelectricity theory, dipole glass model, random field model, and the existence of polar region [2,15,25,27e30]. Among those proposed theories, the existence of polar nanoregions (PNRs) due to the local distortion of the crystal structures is common [15,31]. According to PNR theory, it is plausible that pure KNN could not exhibit relaxor behavior because the ionic radii of Kþ and Naþ is comparable. On the other hand, after adding BCT to the KNN host system, the different number of valence electrons and dissimilar ionic radii of Ba2þ and Ca2þ induced local electric fields due to charge imbalance and a local elastic field due to local structure deformation. Therefore, (1 x)KNNexBCT ceramics are expected to exhibit relaxor behavior when x s 0. An alternative way to examine the relaxor behaviors due to the PNRs is to apply the VogeleFulcher law [31,32],
h i f ¼ fo exp Ea =kB Tmax Tf ;
(3)
where, f is the frequency, fo is a constant, Ea is the activation energy, kB is the Boltzmann constant, Tmax is the temperature at the maximum dielectric constant in Kelvin, and Tf is freezing temperature explained by Du et al. [16]. Fig. 6 shows the inverse temperature dependent logarithm frequency with the fitting results using
VogeleFulcher law for x ¼ 0.15 and x ¼ 0.20. Nevertheless, other compositions could not be to fitted properly. This observation also agreed with the sample except that x ¼ 0.15 and x ¼ 0.20 are no longer relaxor ferroelectrics. The detailed fitted parameters are listed in the figures. For x ¼ 0.15, all the fitted parameters were greater than that of the x ¼ 0.20 sample suggesting that the maximum relaxor behavior can be found from the x ¼ 0.15 sample, which is consistent with other experimental results. 4. Conclusion This study revealed the structural and dielectric characteristics of newly synthesized (1 x)KNNexBCT (x ¼ 0.0, 0.05, 0.10, 0.15, and 0.20) lead-free relaxor ferroelectric ceramics. The structure of (1 x)KNNexBCT ceramics changed from orthorhombic (x ¼ 0.0) to cubic (x ¼ 0.20) as the BCT content was increased. The temperature dependent dielectric constants of the (1 x)KNNexBCT showed that the ceramics exhibited relaxor characteristics when x 0.05. Furthermore, the most probable relaxor behavior appeared for the sample with x ¼ 0.15 from modified CurieeWeiss law which give the potential application in the lead-free relaxor ferroelectrics. Furthermore, the activation energy, which is related to the volume of the PNRs, obtained from the VogeleFulcher law also suggests that the x ¼ 0.15 sample has a higher degree of relaxor behavior. Acknowledgments This study is supported in part by NRF Korea (2011-0002273, 2010-371-B00008, 2011-330-B00044, 2011-0031933) and N000100128. References [1] J.F. Scott, C.A. Paz de Araujo, Science 246 (1989) 1400. [2] I.W. Chen, J. Phys. Chem. Solids 61 (2000) 197 (and references therein). [3] L.E. Cross, Relaxor ferroelectrics in piezoelectricity, in: W. Heywang, K. Lubitz, W. Wersing (Eds.), Springer, vol. 114, Springer, 2008. [4] L.E. Cross, Nature (London) 432 (2004) 24. [5] M. Kosec, V. Bobnar, M. Hrovat, J. Bernard, B. Malic, J. Holc, J. Mater. Res. 19 (2004) 1849. [6] Y. Li, K. Moon, C.P. Wong, Science 308 (2005) 1419. [7] M.D. Maeder, D. Damjanovic, N. Setter, J. Electroceram. 13 (2004) 385. [8] W. Liu, X. Ren, Phys. Rev. Lett. 103 (2009) 257602. [9] http://www.fiz-karlsruhe.de/icsd.html. [10] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Nature (London) 432 (2004) 84. [11] E. Hollenstein, M. Davis, D. Damjanovic, N. Setter, Appl. Phys. Lett. 87 (2005) 182905. [12] S.J. Zhang, R. Xia, T.R. Shout, G.Z. Zang, J.F. Wang, J. Appl. Phys. 100 (2006) 104108.
C.-W. Cho et al. / Current Applied Physics 12 (2012) 1266e1271 [13] H.L. Du, W.C. Zhou, F. Luo, D.M. Zhu, S.B. Qu, Z.B. Pei, Appl. Phys. Lett. 91 (2007) 132914. [14] H.L. Du, W.C. Zhou, F. Luo, D.M. Zhu, S.B. Qu, Y. Li, Z.B. Pei, J. Am. Ceram. Soc. 91 (2008) 2903. [15] L.E. Cross, Ferroelectrics 76 (1987) 241. [16] H. Du, W. Zhou, F. Luo, D. Zhu, S. Qu, Z. Pei, J. Appl. Phys. 105 (2009) 124104. [17] R.D. Shannon, Acta Cryst. A 32 (1976) 751. [18] M.R. Panigrahi, S. Panigrahi, Phys. B 405 (2010) 1787e1797. [19] H.M. Rietveld, Acta Crystallogr. 22 (1967) 151. [20] http://www.ccp14.ac.uk/tutorial/powdcell/index.html. [21] H. Birol, D. Damjanovic, N. Setter, J. Eur. Ceram. Soc. 26 (2006) 861.
[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
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G.A. Samara, Phys. Rev. 151 (1966) 378. C. Kittel, Introduction to Solid State Physics, eighth ed. John Wiley & Sons, Inc, 2005. C.J. Stringer, T.R. Shrout, C.A. Randall, J. Appl. Phys. 101 (2007) 054107. G. Burns, F.H. Dacol, Phys. Rev. B 28 (1983) 2527. K. Uchino, S. Nomura, Ferroelect. Lett. Sect. 44 (1982) 56. M.S. Jang, D.J. Kim, New Phys. 55 (2007) 4. L.E. Cross, Ferroelectrics 151 (1994) 305. Z.-G. Ye, Key. Eng. Mater. 81 (1998) 155e156. A.A. Bokov, Z.-G. Ye, J. Mater. Sci. 41 (2006) 31. D. Viehland, S.G. Jang, L.E. Cross, M. Wuttig, J. Appl. Phys. 68 (1990) 2916. X.F. Long, Z.G. Ye, Appl. Phys. Lett. 90 (2007) 112905.
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Ferroelectric relaxor properties of (1 x)K0.5Na0.5NbO3exBa0.5Ca0.5TiO3 ceramics C.-W. Cho a, M.R. Cha a, J.Y. Jang a, S.H. Lee a, D.J. Kim a, S. Park a, *, J.S. Bae b, S.D. Bu c, S. Lee d, J. Huh e a
Department of Physics, Pusan National University, Busan 609-735, Republic of Korea Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea c Department of Physics, Chonbuk National University, Jeonju 561-756, Republic of Korea d JPS Micro-Tech, Busan 619-961, Republic of Korea e Chonnam National University, Yeosu 550-749, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2012 Received in revised form 25 February 2012 Accepted 2 March 2012 Available online 16 March 2012
The Ba0.5Ca0.5TiO3 (BCT) composition dependent dielectric and structural properties of (1x) K0.5Na0.5NbO3exBa0.5Ca0.5TiO3 powders were investigated. Room temperature x-ray diffraction revealed the powder structure to transform from orthorhombic to cubic with increasing BCT composition. The frequency dependent dielectric constant measurements revealed a shift in the temperature of the maximum dielectric constant for at frequencies, suggesting that the system exhibits ferroelectric relaxor behavior. The system containing 15% BCT showed the closest calculated CurieeWeiss exponent to 2, which the exponent for a relaxor ferroelectric. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Relaxor ferroelectric Dielectric constant X-ray diffraction
1. Introduction Relaxor ferroelectrics exhibit a high dielectric constant and high strain across a wide range of temperatures and are attractive for various applications, such as capacitors, sensors, actuators, and integrated micro-electromechanical systems [1]. The most commonly used relaxor ferroelectric systems, such as Pb(Mg1/3Nb2/3) O3 (PMN) and Pb(Zn1/3Nb2/3)O3 (PZN) exhibited mainly piezoelectricity and high dielectric constants [2,3]. However, those systems (for example PMN, PZN) contained almost 60 w% Pb [4,5]. The environmental effects of lead have attracted considerable concern, particularly on humans [6]. Therefore, lead-free ferroelectric materials with comparable or even superior properties to the most widely used lead (Pb2þ ion)-based ferroelectric material systems have received a great deal of research interest [7]. Among the many candidates for lead-free relaxor ferroelectrics systems, the BaTiO3 based pseudo-binary system [8] and/or the pseudo-ternary ferroelectric system based on KNN (orthorhombic structure [9]), are promising candidates because they exhibit high piezoelectricity and a high Curie temperature (420 C) [10e14]. As the relaxor behavior in lead-based ceramic systems stems the cationic distribution disorder in the same crystallographic site, lead-free relaxors have also been derived as compositionally
disordered systems. Many studies focused on modifying the atomic arrangements by substituting a similar size of ions while maintaining their structure because the structural symmetry is deformed by substituting atoms with different sized atoms [15]. Recently, Du et al., reported a relaxor ferroelectric system by adding Ba0.5Sr0.5TiO3 (BST) to their host KNN system [16]. The ionic radius of Ba2þ (0.161 nm) is similar to that of Kþ (0.164 nm) with the same coordination number (CN ¼ 12). Similarly, the ionic radii of Naþ and Sr2þ are 0.139 nm (CN ¼ 12) and 0.144 nm (CN ¼ 12), respectively, and the ionic radii of Nb5þ and Ti4þ is 0.064 nm (CN ¼ 6) and 0.074 nm (CN ¼ 6) [17], respectively, suggesting that the addition of BST in the KNN host will reduce the asymmetry of the ionic radius at the individual site. Ba0.5Ca0.5TiO3 (BCT) is another candidate for the rearrangement of ions that can be added to the KNN host because both BCT and BST have the same tetragonal structure [9,18]. Furthermore the ionic radius of Ca2þ is 0.134 nm (CN ¼ 12) which similar to that of Naþ. Therefore, it is expected the addition of BCT to the KNN host system can reduce the crystal asymmetry further due to the ionic size mismatch. This paper reports the structural and dielectric properties of the newly synthesized relaxor ferroelectric ceramics by adjusting the ionic radius in the perovosikte structure. 2. Experiments
* Corresponding author. Tel.: þ82 51 510 2595. E-mail address: [email protected] (S. Park). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.03.006
(1x)K0.5Na0.5NbO3exBa0.5Ca0.5TiO3 ceramics ((1x)KNNexBCT) with a range of BCT contents (x ¼ 0.0, 0.05, 0.10, 0.15, 0.20) were
C.-W. Cho et al. / Current Applied Physics 12 (2012) 1266e1271
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Fig. 1. (a) XRD patterns and (b) SEM images of an expanded region of the diffraction pattern of the (1 x)KNNexBCT powders. For the x ¼ 0.0 sample, the XRD profile (bottom in figure) was plotted with the fitted XRD profile for comparison purposes.
synthesized using a conventional solidestate reaction. As starting materials, K2CO3, Na2CO3, Nb2O5, BaCO3, CaCO3, and TiO2 reagent grade powders were weighed properly and mixed for 30 min before calcination and sintering. The pellets were then calcinated at 900 C for 2 h and sintered at 1100 C for 2 h. The ceramics were ground to a powder to allow x-ray diffraction (XRD, D8 Advance, Bruker) measurements of the compositional dependent crystal structures at ambient temperature. The temperature and frequency dielectric properties of the samples in pellet form were also measured with an Au-wires electrode using a HP4194A. The surface morphology of the pellet was also observed by scanning electron microscopy (SEM, Hitachi).
3. Results and discussion 3.1. Structural properties Fig. 1 shows the room temperature XRD patterns of the (1 x) KNNexBCT powders. Fig. 1(a) presents the XRD data (20 < 2q < 80 ). Fig. 1(b) presents the diffraction profile of the expanded region (40 < 2q < 50 ) along with SEM images. For the x ¼ 0.0 sample (pure KNN), the detailed structure was analyzed from a Reitveld refinement [19] using a PowderCell [20]. The sample was found to have an orthorhombic (Bmm2) with lattice parameters of a ¼ 5.635 Å, b ¼ 3.938 Å, and c ¼ 5.667 Å. Pure KNbO3
Fig. 2. Temperature dependent dielectric permittivity and tangent loss (inset) of (1 x)KNNexBCT ceramics measured at a 1 MHz.
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Table 1 CurieeWeiss temperature (TCW), the temperature above which the dielectric constant follows the CurieeWeiss law (TB), deviation (DTmax), the CurieeWeiss constant (C), and the diffuseness (g) for (1 x)KNNexBCT ceramics at 1 MHz. x Tmax ( C) TCW ( C) C ( C) ( 105) TB ( C) DTmax ¼ TB Tmax ( C)
g
0.0 403 379 1.31 403 0 1.19
0.05 312 286 1.84 357 45 1.45
0.10 226 178 1.44 362 136 1.85
0.15 149 61 1.29 339 190 1.88
0.20 87 67 1.68 171 84 1.75
has the same perovskite structure with lattice parameters of a ¼ 5.697 Å, b ¼ 3.971 Å, and c ¼ 5.721 Å [9]. Therefore, the KNN powder (x ¼ 0.0) has a smaller volume than that of pure KNbO3 due to the substitution of smaller Na ion in the potassium site of the system. Previously, Du et al., examined the (1x) K0.5Na0.5NbO3exBa0.5Sr0.5TiO3 system and observed the structural changes as the composition of Ba0.5Sr0.5TiO3 was varied from x ¼ 0.0 to x ¼ 0.20 [16]. A comparison of the present XRD profile
with Du’s result revealed, the sample to have a tetragonal structure for x ¼ 0.05, a pseudocubic structure for x ¼ 0.10e0.15, and a cubic structure for x ¼ 0.20 at room temperature. The physical origin of the structural changes with different BCT contents is related to the different sizes of the ionic radii because the mean ionic radius of the A site is smaller than that of pure KNN. On the other hand, the mean ionic radius of the B site was larger than that of pure KNN. Adding BCT to the KNN host material resulted in a decrease in the difference in ionic radius between the A and B sites as well as reduced asymmetry due to the difference in ionic radius. The inset in the Fig. 1(b) shows SEM of the (1 x)KNNexBCT ceramics. The grain size decreased with increasing BCT content. Furthermore, ceramics with a smaller grain size distribution exhibited fewer pores and a uniform distribution of grains. 3.2. Dielectric properties Fig. 2 shows the temperature dependent dielectric constants and dielectric loss (inset in Fig. 2) of (1 x)KNNexBCT ceramics
Fig. 3. Temperature dependent dielectric permittivity of (1 x)KNNexBCT ceramics for (a) x ¼ 0.0, (b) x ¼ 0.05, (c) x ¼ 0.10, (d) x ¼ 0.15 and (e) x ¼ 0.20 under various frequencies.
C.-W. Cho et al. / Current Applied Physics 12 (2012) 1266e1271
measured at 1 MHz. For pure KNN, two distinct dielectric peaks (w200 C and w400 C) were observed, which were assigned to the orthorhombic-tetragonal and tetragonal-cubic structural phase transitions, respectively [16,21]. As the BCT content was increased, the ceramics showed a dielectric peak within the measured temperature range. The maximum of the dielectric constants was w4500 at x 0.05, whereas the maximum dielectric constants for the x 0.10 samples were w1000. Furthermore, the temperature which was exhibited maximum dielectric constant (Tmax) and dielectric constants decreased with increasing BCT content. The detailed Tmax is listed in Table 1. The reduced dielectric constants and broad nature of the peak around Tmax are typical characteristics of relaxor ferroelectrics. For x 0.10, the dielectric loss of the ceramics reached a minimum (<4%) around the Tmax (100e300 C). Ceramics with smaller dielectric loss and a larger dielectric constant (for example x ¼ 0.05 sample) can be used as high temperature capacitors in the automobile industry. Fig. 3 shows the temperature dependent dielectric permittivity of the (1 x)KNNexBCT ceramics at various frequencies. For the pure KNN (x ¼ 0.0), the dielectric permittivity profile is invariant at various frequencies [21]. With the samples containing x ¼ 0.10, x ¼ 0.15, and x ¼ 0.20, the dielectric permittivity profile became diffusive and the maximum dielectric permittivity shifted to
Fig. 4. Inverse dielectric permittivity at 1 MHz as a function of temperature for (1 x) KNNexBCT ceramics (symbols: experimental data; the solid line: fitting to the Curie eWeiss law).
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a higher temperature with increasing frequency suggesting that the ceramics exhibited relaxor ferroelectric characteristics. The dielectric permittivity of the normal ferroelectric systems can be explained the CurieeWeiss law,
ε ¼
C ; T TCW
(1)
where C is CurieeWeiss constant and TCW is CurieeWeiss temperature [22,23]. The typical magnitude of C which reflects the nature of the ferroelectric transition, can be varied either w105 for displacive transition or w103 for an orderedisorder transition in the high temperature paraelectric phase (T > TCW). The inverse dielectric permittivity was examined as a function of temperature to analyze the behavior of the dielectric perimittivity around TCW is shown in Fig. 4. The linear characteristic of the inverse dielectric permittivity for the pure KNN, at T > TCW suggests that the system obeys the CurieeWeiss law. On the other hand, the dielectric permittivity of the samples with a higher BCT concentration (x 0.05) deviated from the CurieeWeiss laws for TCW < T < TB, where TB (Burns temperature) is the temperature when the linearity begins to deviate (i.e., the temperature that does not follow CurieeWeiss law) [24,25]. The degree of deviation can be defined as
Fig. 5. Log (1/εr 1/εmax) at 1 MHz for (1 x)KNNexBCT ceramics (symbols: experimental data; solid line: fitting to the modified CurieeWeiss law) as a function of the log (T Tmax).
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a
b
Fig. 6. Inverse temperature of the maximum dielectric constant temperature (Tmax) as a function of ln f for (1 x)KNNexBCT ceramics. The solid line represents the fitting results using the VogeleFulcher law.
DTmax (¼ TB Tmax). The DTmax increased with increasing BCT
content, reached a maximum when x ¼ 0.15, and then decreased at higher BCT contents. This suggests that the diffusive structural phase transition increased with increasing BCT content up to x ¼ 0.15 and then decreased because the DTmax for normal ferroelectrics is approximately zero. Previously, Uchino et al., proposed the modified CurieeWeiss law (so called Uchino and Nourma function),
1 1 ðT Tmax Þg ¼ C εr εmax
(2)
to estimate the degree of the relaxor [26], where C is the Curie constant and g is the diffusion coefficient. g ¼ 1 and 2, represents a normal ferroelectric system and ideal relaxor ferroelectric system, respectively. Fig. 5 shows log(1/εr 1/εmax) as function of log(T Tmax) for (1 x)KNNexBCT ceramics measured at 1 MHz. The solid lines represent the best linear fitted results to estimate the slope (i.e., g). The g values increased (closed to 2) with increasing BCT content until x ¼ 0.15, then decreased with further increases in BCT content (x ¼ 0.20) suggesting that 0.85KNN e0.15BCT ceramics (x ¼ 0.15) most likely exhibited relaxor behavior. Table 1 lists the detailed values of g. Many models have been proposed to explain the origin of relaxor ferroelectrics, such as composition fluctuation theory, superparaelectricity theory, dipole glass model, random field model, and the existence of polar region [2,15,25,27e30]. Among those proposed theories, the existence of polar nanoregions (PNRs) due to the local distortion of the crystal structures is common [15,31]. According to PNR theory, it is plausible that pure KNN could not exhibit relaxor behavior because the ionic radii of Kþ and Naþ is comparable. On the other hand, after adding BCT to the KNN host system, the different number of valence electrons and dissimilar ionic radii of Ba2þ and Ca2þ induced local electric fields due to charge imbalance and a local elastic field due to local structure deformation. Therefore, (1 x)KNNexBCT ceramics are expected to exhibit relaxor behavior when x s 0. An alternative way to examine the relaxor behaviors due to the PNRs is to apply the VogeleFulcher law [31,32],
h i f ¼ fo exp Ea =kB Tmax Tf ;
(3)
where, f is the frequency, fo is a constant, Ea is the activation energy, kB is the Boltzmann constant, Tmax is the temperature at the maximum dielectric constant in Kelvin, and Tf is freezing temperature explained by Du et al. [16]. Fig. 6 shows the inverse temperature dependent logarithm frequency with the fitting results using
VogeleFulcher law for x ¼ 0.15 and x ¼ 0.20. Nevertheless, other compositions could not be to fitted properly. This observation also agreed with the sample except that x ¼ 0.15 and x ¼ 0.20 are no longer relaxor ferroelectrics. The detailed fitted parameters are listed in the figures. For x ¼ 0.15, all the fitted parameters were greater than that of the x ¼ 0.20 sample suggesting that the maximum relaxor behavior can be found from the x ¼ 0.15 sample, which is consistent with other experimental results. 4. Conclusion This study revealed the structural and dielectric characteristics of newly synthesized (1 x)KNNexBCT (x ¼ 0.0, 0.05, 0.10, 0.15, and 0.20) lead-free relaxor ferroelectric ceramics. The structure of (1 x)KNNexBCT ceramics changed from orthorhombic (x ¼ 0.0) to cubic (x ¼ 0.20) as the BCT content was increased. The temperature dependent dielectric constants of the (1 x)KNNexBCT showed that the ceramics exhibited relaxor characteristics when x 0.05. Furthermore, the most probable relaxor behavior appeared for the sample with x ¼ 0.15 from modified CurieeWeiss law which give the potential application in the lead-free relaxor ferroelectrics. Furthermore, the activation energy, which is related to the volume of the PNRs, obtained from the VogeleFulcher law also suggests that the x ¼ 0.15 sample has a higher degree of relaxor behavior. Acknowledgments This study is supported in part by NRF Korea (2011-0002273, 2010-371-B00008, 2011-330-B00044, 2011-0031933) and N000100128. References [1] J.F. Scott, C.A. Paz de Araujo, Science 246 (1989) 1400. [2] I.W. Chen, J. Phys. Chem. Solids 61 (2000) 197 (and references therein). [3] L.E. Cross, Relaxor ferroelectrics in piezoelectricity, in: W. Heywang, K. Lubitz, W. Wersing (Eds.), Springer, vol. 114, Springer, 2008. [4] L.E. Cross, Nature (London) 432 (2004) 24. [5] M. Kosec, V. Bobnar, M. Hrovat, J. Bernard, B. Malic, J. Holc, J. Mater. Res. 19 (2004) 1849. [6] Y. Li, K. Moon, C.P. Wong, Science 308 (2005) 1419. [7] M.D. Maeder, D. Damjanovic, N. Setter, J. Electroceram. 13 (2004) 385. [8] W. Liu, X. Ren, Phys. Rev. Lett. 103 (2009) 257602. [9] http://www.fiz-karlsruhe.de/icsd.html. [10] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Nature (London) 432 (2004) 84. [11] E. Hollenstein, M. Davis, D. Damjanovic, N. Setter, Appl. Phys. Lett. 87 (2005) 182905. [12] S.J. Zhang, R. Xia, T.R. Shout, G.Z. Zang, J.F. Wang, J. Appl. Phys. 100 (2006) 104108.
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