Ebru Mensur-Alkoy a, Ayse Berksoy-Yavuz b c & Sedat Alkoy b c a Maltepe University, Faculty of Engineering, Maltepe, Istanbul,

Transcription

1 This article was downloaded by: [Maltepe Universitesi] On: 24 September 2013, At: 02:21 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Ferroelectrics Publication details, including instructions for authors and subscription information: Electrical Properties and Impedance Spectroscopy of Lithium Modified Potassium Sodium Niobate Ceramics Ebru Mensur-Alkoy a, Ayse Berksoy-Yavuz b c & Sedat Alkoy b c a Maltepe University, Faculty of Engineering, Maltepe, Istanbul, 34857, Turkey b Department of Materials Science and Engineering, Gebze Institute of Technology, Kocaeli, Turkey c ENS Piezodevices Limited, Gebze, Kocaeli, Turkey Published online: 20 Sep To cite this article: Ebru Mensur-Alkoy, Ayse Berksoy-Yavuz & Sedat Alkoy (2013) Electrical Properties and Impedance Spectroscopy of Lithium Modified Potassium Sodium Niobate Ceramics, Ferroelectrics, 447:1, , DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at

2 Ferroelectrics, 447:95 107, 2013 Copyright Taylor & Francis Group, LLC ISSN: print / online DOI: / Electrical Properties and Impedance Spectroscopy of Lithium Modified Potassium Sodium Niobate Ceramics EBRU MENSUR-ALKOY, 1, AYSE BERKSOY-YAVUZ, 2,3 AND SEDAT ALKOY 2,3 1 Maltepe University, Faculty of Engineering, Maltepe, Istanbul, 34857, Turkey 2 Department of Materials Science and Engineering, Gebze Institute of Technology, Kocaeli, Turkey 3 ENS Piezodevices Limited, Gebze, Kocaeli, Turkey I. Introduction Lead-free potassium sodium niobate (KNN) ceramics were prepared with lithium and tantalum modification. Structural properties were investigated. Impedance spectroscopy was used as a tool to analyze electrical behavior of ceramics as a function of frequency from 100 Hz to 10 MHz at a temperature range from 573 K to 798 K. Neither Li nor Li + Ta modification caused any secondary grain boundary phase, however both of them caused a densification of the ceramic. Increasing the lithium addition was found to decrease the impedance, while the Li + Ta addition drastically increased it. Finally, a temperature induced relaxation process was observed in the samples with the relaxation frequency shifting with temperature. Keywords Lead-free; dielectric properties; Li-modified KNN; Curie temperature; impedance spectroscopy Lead-based piezoelectric materials are widely used in sensors, resonators, buzzers and transducers. They exhibit excellent piezoelectric and ferroelectric properties for the mentioned applications. Lead is toxic material and it can evaporate easily even at low sintering temperatures, so lead-based ceramics have caused environmental and health problems. Thus, lead-free piezoelectric ceramics have received increasing attention for these reasons. Among various lead free piezoelectric materials, ceramics based on potassium sodium niobate (KNN) are one of the best candidates because of high piezoelectric properties comparable to PZT, high Curie temperature, and large electromechanical coupling coefficient [1 3]. When pure KNN was produced using conventional solid state calcination method, the densification of KNN ceramics are insufficient. It was explained in the early studies that the major challenges are the hygroscopic behavior of alkaline carbonate raw materials used in KNN and the volatility of alkaline during sintering process. Choosing low temperatures may lead to an incomplete removal process of humidity, whereas temperatures too high may cause additional mass losses because carbonate decomposition starts. Besides these, Received September 28, 2012; in final form March 18, Corresponding author. ebrualkoy@maltepe.edu.tr [223]/95

3 96/[224] E. M. Mensur-Alkoy et al. the phase stability of pure KNN is 1140 C but melting of KNN occurs at over 1100 C. These two points are very effective on deviation of stoichiometry and low densification of KNN ceramics fabricated by ordinary solid state method. These are all very important for electrical properties of KNN. In the literature, in order to improve its densification and electrical properties, there are some approaches such as addition of sintering aid (CuO, ZnO i.e.), modification of stoichiometry and/or structure (addition of LiNbO 3,LiTaO 3,SrTiO 3, BaTiO 3 ), using alternative processing methods such as spark plasma sintering. In this study, lithium substitution is the main approach for the structure in (K 0.50-x Na 0.50-x Li x )NbO 3 composition. Li ratio was chosen as x = 0.04 and 0.07 using literature [1, 3] for two reasons. KNN has ABO 3 perovskite structure. There are many elements such as Li, Ta, Sb etc. that can replace the cations in the A-site (Na, K) or B-site (Nb). Firstly, Li + has a small ionic radius compared to Na and K. This will affect the unit cell of the KNN. Lithium niobate (LiNbO 3 ) is a typical ferroelectric material with high Curie temperature (1142 C). Thus, secondly, it was reported that phase transition temperatures will shift with Li doping [3]. This high Curie temperature is advantage for practical applications and this temperature can be changed by adjusting the amount of LiNbO 3 in the structure. The effect of the LiTaO 3 on structure was also investigated. Impedance spectroscopy is a convenient method to characterize the electrical properties of ceramic materials. This experimental technique also allows a correlation of the dielectric properties of a ceramic material with its microstructure. From the measured capacitance and tangent loss complex dielectric parameters can be computed such as impedance, electric modulus, admittance and permittivity. The complex impedance (Z ) has two parts: Real ( Z ) and imaginary ( Z ) parts. The real and imaginary parts are interpreted as resistance (R) and capacitance(c), respectively. Thus, impedance can be written as Z = [ R 1 + jωc ] 1 where ω is the angular frequency of ac field and j = 1. The equivalent circuit for a polycrystalline bulk material can ideally be represented by a resistive and a capacitive component connected in parallel. In the case of a bulk polycrystalline material containing grain boundary interfacial layers, the equivalent circuit would include an additional RC unit which is connected in series to the first one. These two RC elements represent the bulk of the grains and the grain boundary [4 6]. Impedance data gives a semicircle when Z versus Z is plotted. Grain and grain boundary resistance could be determined from this representation which is called the Nyquist diagram [4 9]. The main goal and original part of this study is to investigate effect of Li and Ta substitutions on the structural and electrical properties of KNN through the impedance spectroscopy. (1) II. Experimental All ceramic samples were prepared using conventional solid state calcination method. Potassium carbonate, sodium carbonate, niobium oxide, lithium oxide and tantalum oxide were used to prepare the powders. The compositions were chosen as (K 0.50-x/2 Na 0.50-x/2 Li x )NbO 3 with x = 0.04 and 0.07 in this study. These samples were named as KL4 and KL7 according to Li content, respectively. Additionally, (K 0.50-x/2 Na 0.50-x/2 Li x )(Nb 1-y,Ta y )O 3 compositions were prepared as x = 0.04 and y = 0.10). The powders were mixed by ball-milling in ethanol

4 Lithium Modified Potassium Sodium Niobate Ceramics [225]/97 for 24 h with K: Na ratio of 1:1 followed by calcination conducted at 900 C for 1 hour. Discshaped samples were dry pressed and then sintered at 1070 C for 4 hours. Silver palladium electrode was applied to the parallel surfaces of the discs for electrical measurements. Scanning electron microscope SEM (XL30, FEI, USA) was used to examine the microstructural features of the samples and X-ray diffraction XRD (DMAX 2200, Rigaku, Japan) was used for structural analysis. Impedance analyzer (4194A, Agilent, USA) was used for impedance-temperature measurements. Measurements were carried out from 300 K to 798 K in 100 Hz-10 MHz frequency range because of the limitation of the system used in this study. The samples were placed in a sample holder with two parallel conductors. The sizes of the samples were identical. The diameter and thickness of the samples were fixed and measured approximately 10,8 mm and 1,12 mm, respectively. III. Results and Discussion Figure 1 shows XRD patters of modified KNN ceramics with 7% Li, 4% Li and 4% Li +10% Ta. The peaks were named based on a primitive cubic structure model. All samples have a perovskite structure. Un-modified pure KNN has orthorhombic symmetry at room temperature [10]. Similar to KNN, the 4% Li modified KNN sample, KL4, has orthorhombic symmetry as shown in Fig. 1. With increasing Li content in the structure phase change occurred. So it can be seen in Fig. 1 that the 7% Li modified KL7 sample has shown tetragonal symmetry (see Fig. 1) [10]. Small peaks were also seen in between for KL4 and KL7 samples. These peaks belong to a secondary phase of K 3 Li 2 Nb 5 O 15 (ICDD: ) [11]. Figure 2 shows the temperature dependence of dielectric constants of the samples. The curves were taken at 100 khz frequency between room temperature-525 C. It was clearly seen from Fig. 2 that KL4 and KLT samples showed two peaks; one above 200 C and another above 400 C which corresponds to the orthorhombic tetragonal phase transition (T ot ) and tetragonal cubic phase transition (Curie temperature-t c ), respectively. KL7 sample only had T c peak at 502 C. This peak has shifted to 468 C for KL4 sample. The peaks of all three ceramics at the Curie temperature are sharp. The reason of high T c observed with increasing Li content is due to the large crystal anisotropy and high Curie temperature of LiNbO 3 [10]. So, T c was increased from 468 Cto 502 C for KL4 and KL7 samples, respectively. Beside addition of lithium, tantalum was also Figure 1. XRD patterns of modified KNN samples (Color figure available online).

5 98/[226] E. M. Mensur-Alkoy et al. Figure 2. Variation of dielectric constant with temperature measured at 100 khz (Color figure available online). added to the structure. The Curie temperature of KLT, (K 0.48 Na 0.48 Li 0.04 )Nb 0.90 Ta 0.10 O 3, sample was measured as 394 C at 100 khz. Singh et al. [12] also measured T c temperature between C for the composition (K 0.5-x Na 0.5 Li x )Nb 0.9 Ta 0.1 O 3 where x = 0, 0.015, 0.045, and These compositions are similar but as expected due to the variation in the K/Na ratio, the Curie peak has shifted further. If there is only lithium in the structure, T c peak shifts up with increasing Li content, but T ot temperature decreased below the room temperature. But opposite of this tendency, in the case of increasing of Ta substitution, both the T c and T ot decreased. For KLT composition of this study T ot was observed around 110 C. When the results of Fig. 1 and Fig. 2 were evaluated together, the temperature KL4 and KLT samples showed orthorhombic symmetry at the room temperature. The dielectric constants values are 345 and 525 for KL4 and KLT samples at 26 C, respectively. KL7 sample exhibited tetragonal symmetry and dielectric constant was measured as 615 for KL7 sample at 26 C. These results are in good agreement with literature [12, 13] SEM micrographs of 7 mole% Li (KL7), 4 mole% Li (KL4) and 4 mole% Li and 10 mole% Ta (KL4T) modified samples are given in Fig. 3. The microstructures of these ceramics exhibit dense and uniform microstructure. Pure KNN usually has square- or rectangular shaped grains. But it is known from literature that Li addition promotes grain growth and densification [10]. Density measurements were done and the measured density values of samples are above %94 of theoretical density. Impedance spectroscopy analyses were conducted to investigate the dielectric properties of the samples and effect of Li and Ta modifications in the structure. Impedance spectroscopy is an experimental method for explaining the contribution of resistivity from bulk and grain boundaries of samples. Real (Z ) and imaginary parts (Z ) of impedance curves, as a function of frequency (f) were measured for three samples using impedance analyzer at various temperatures. All measurements were taken up to 798 K due to the limitation of the measurement system. Figure 4 shows the variation of the real ( Z ) and imaginary ( Z ) parts of the impedance of 7% modified KL7 sample with respect to frequency and temperature. The real part of impedance (presented as an inset in Fig. 4) exhibits a sigmoidal variation with frequency in the lower frequency regions but saturates and merges together around 10 khz for all temperatures. This clearly indicates the presence of space charge polarization and the release of space charges as a result of lowering of the barrier properties of the material with

6 Lithium Modified Potassium Sodium Niobate Ceramics [227]/99 Figure 3. SEM micrographs of (a) KL4T (b) KL4 and (c) KL7 samples. temperature [14, 15]. This process may also be responsible for the enhancement of AC conductivity of material with temperature at higher frequencies where the Z values were found to consistently decrease with increasing temperature [7]. Li is a monovalent cation and replaces K + or Na + in the perovskite structure as an isovalent A site substitution. Thus, Li doping is not expected to create additional point defects. The major point defects in the case of Li-doped KNN is expected to be K vacancies [V K ]due to the high volatility of K at sintering temperatures, and the oxygen vacancies [ ] V O that arise to electrically compensate the [V K ]. Since, oxygen vacancies are the main mobile defects in the perovskite structure, the space charges are expected to arise in the form of [V O ]. The variation of the imaginary part of impedance at higher temperatures ( K) where Z first increases and then decreases after reaching a peak, is characteristic and indicative of an dielectric relaxation process. The presence of these relaxation peaks and their nature provides insight into the type and strength of the relaxation processes occurring in the material. A significant broadening of the peak with increasing temperature was also observed in these curves, which is a further indication of the presence of a temperature dependent relaxation phenomenon in the material [7]. The maxima of the peaks were very clear at higher temperatures (T > 673 K), however, they were not clearly observed at lower temperatures. These results indicate that the relaxation frequency shifted to lower values with decreasing temperature and that it is far stronger at high temperatures. When the temperature is increased, the space charge will have lesser time to relax and the recombination will be faster at the high frequencies.

7 100/[228] E. M. Mensur-Alkoy et al. Figure 4. Imaginary and real (inset) parts of impedance of KL7 sample as a function of frequency at different temperatures (Color figure available online). The magnitude of both Z and Z decreases with increasing temperature for KL7, as expected. This temperature dependent behavior is due to the increasing AC conductivity with increasing temperatures as a result of released space charges [7]. Similar behaviors were also observed for KL4 and KL4T samples as given in Figs. 5 and 6, respectively. When the levels of Z and Z are compared in these three samples, it is clearly seen that increasing the Li content from 4 to 7 mol% caused an increase in the conductivity of the material. Whereas, co-doping KL4 with Ta caused a decrease in the conductivity. Ta is a pentavalent cation and replaces Nb 5+ in the perovskite structure as an isovalent B site substitution. Thus, similar to Li doping, Ta doping is also not expected to create additional point defects. However, as it is clearly seen in the results discussed above, Li and Ta content do clearly led to changes in the conductivity behavior of KNN, possibly through the changes they induce in the crystal lattice size and mobility of the space charges. When compared to the impedance behavior of undoped and CuO-added KNN discussed in our previous report [16], the Z and Z values reported in the current work are lower than undoped KNN and much higher compared to CuO-added KNN. The relaxation times (τ) were also calculated [5 8] from Z f graphs using appropriate values and (2πf max τ = 1) relationship. The values and parameters were obtained from the impedance spectra of samples using fitting procedure. Ln τ is plotted against 1000/T in Fig. 7 for all three samples. In such plots, a linear fit can be applied to the data and from the slope of this line, the thermal activation energy for relaxation of charge carriers (E g ) can

8 Lithium Modified Potassium Sodium Niobate Ceramics [229]/101 Figure 5. Imaginary and real (inset) parts of impedance of KL4 sample as a function of frequency at different temperatures (Color figure available online). be calculated using the τ = τ 0 e Eg/kT relationship. This activation energy is responsible for the migration of charge carriers through hopping between the adjacent lattice sites. Oxygen vacancies are known to be the only highly mobile ionic defects in the perovskite structure. Thus, with increasing mobility of the defects that create the space charge, they are expected to comply with the ac field up to higher frequency levels, and have a shorter relaxation time. In this study, a linear fit was not applied to the plot in Fig. 7 due to the scattering of the data points. However, when guide lines are drawn, a higher slope can be observed for the KL4T samples. Normalized imaginary parts (Z /Z max ) of impedance as a function of normalized frequency (f/f max ) was also compared for all three samples at different temperatures and given in Fig. 8(a) 8(c). This type of presentation of the data is known as a master curve, and allows an evaluation of the dielectric behaviour of the material as a function of temperature [17]. From these figures, the curves for different temperatures have similar shape and pattern with slight variation in full width at half maximum (FWHM) with increasing temperature, and they seem to overlap into a single master curve. This is a further indication of occurrence of the same relaxation process at different temperatures. The complex impedance behavior can also be represented by a plot of Z vs. Z,the so-called Nyquist or Cole-Cole plot. The Nyquist plots of all three samples are presented in Fig. 9 for various temperatures. The firs observation is on the nature of the curves in

9 102/[230] E. M. Mensur-Alkoy et al. Z'' (kω) Z' (kω) 320 KL4T ,E+02 1,E+04 1,E+06 1,E+08 Frequency (Hz) C 723K 500C 773K 798K 525C 0 1,E+02 1,E+04 1,E+06 Frequency (Hz) Z'' (kω) Z' (kω) ,E+02 1,E+04 1,E+06 1,E ,E+02 1,E+04 1,E+06 1,E+08 Frequency (Hz) Frequency (Hz) Figure 6. Imaginary and real (inset) parts of impedance of KL4T sample as a function of frequency at different temperatures (Color figure available online). these graphs. All of the curves have the arc shape and a single semicircle can be traced. The presence of a single semicircle suggests that the bulk (grain interior) property of the material is dominant in the impedance behavior and this can be modeled as an equivalent circuit comprising of a parallel combination of bulk resistance and bulk capacitance. Normally, 573 K 623 K 673 K Figure 7. Temperature dependence of relaxation times (Color figure available online).

10 Lithium Modified Potassium Sodium Niobate Ceramics [231]/103 Figure 8. Impedance scaling behavior of KNN ceramics in the master curves. (a) 7 mol% Li-doped, (a) 4 mol% Li-doped, and (a) 4 mol% Li & 10 mole% Ta-doped. a grain boundary effect was also observed for 1 mol% copper oxide added KNN in our previous study [16], as indicated by the presence of a second semicircle. However, a grain boundary phase has not been observed in the microstructure of the Li and Li+Ta doped KNN samples in the current study, as clearly seen in the micrographs given in Fig. 3. Thus, in agreement with this result, this grain boundary effect was not observed in the current study. The intercept of the semicircular arc on the Z axis is the bulk DC resistance (R b )ofthe sample. Since the sample sizes are comparable in the current study, direct conclusions can

11 104/[232] E. M. Mensur-Alkoy et al. 120 KL7 300 KL Z'' (kω) K Z'' (kω) K 773K 773K 0 798K Z' (kω) Z'' (kω) K 0 773K 798K Z' (kω) KL4T 723K Z' (kω) Figure 9. Cole-Cole plots of KNN samples at different temperatures (Color figure available online). be drawn from the values of R b. Thus, the second observation on these results is that the R b decreased with rising temperature for all samples indicating the negative temperature coefficient of resistivity (NTCR) nature of these materials [7, 9]. Variations of AC conductivities of samples with frequency at various temperatures were investigated in detail. The log-log plots of AC conductivity versus frequency are given in Fig. 10. It was clearly seen from these figures that there is frequency independent region at low frequencies, i.e. the DC conductivity, and a region where the conductivity is sensitive to frequency, as well as the temperature. The increasing trend of AC conductivity may be attributed to the presence of space charges that vanish at higher temperatures and frequencies [14]. Additionally, it was found that the onset (the change in slope and switching from frequency independent to frequency dependent region) takes place at a particular frequency known as the hopping frequency and it shifts towards higher frequencies with increasing temperature.

12 Lithium Modified Potassium Sodium Niobate Ceramics [233]/105 Figure 10. Variation of the AC conductivity of the samples with frequency at different temperatures.

13 106/[234] E. M. Mensur-Alkoy et al. IV. Conclusıons In this study, structural features and dielectric properties of KNN ceramics produced with lithium and tantalum modification were investigated. Li was added as an A-site dopant in 4 and 7 mol% ratios and Ta was added as a B-site dopant in 10 mol% ratio to the 4 mol% Li-doped KNN. An impedance spectroscopy study was conducted to evaluate the effect of Li and Ta as a dopant on the dielectric and AC conductivity behavior of KNN. Neither Li nor Li+Ta modification caused any secondary grain boundary phase, however both of them caused a densification of the ceramic. Increasing the lithium addition was found to decrease the impedance, while the Li+Ta addition drastically increased it. Finally, a temperature induced relaxation process was observed in the samples with the relaxation frequency shifting with temperature. Acknowledgment We kindly acknowledge the support for this work from The Scientific and Technological Research Council of Turkey (TUBITAK- 110M627 project). References 1. Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, and M. Nakamura, Lead-free piezoceramics, Nature. 432, (2004). 2. N. M. Hagh, K. Kerman, B. Jadidian, and A. Safari, Dielectric and piezoelectric properties of Cu 2+ -doped alkali niobates, J. Eur. Ceram. Soc. 29, (2009). 3. M. D. Maeder, D. Damjanovic, and N. Setter, Lead Free Piezoelectric Materials, J. Electroceram. 13, (2004). 4. J. T. S. Irvine, D. C. Sinclair, and A. R. West, Electroceramics : Characterization by Impedance Spectroscopy, Adv. Mater. 2, (1990). 5. M. A. L. Nobre and S. Lanfredi, Dielectric loss and phase transition of sodium potassium niobate ceramic investigated by impedance spectroscopy, Catalysis Today 78, (2003). 6. V. Petrovsky, T. Petrovsky, S. Kamlapurkar, and F. Dogan, Characterization of Dielectric Particles by Impedance Spectroscopy (Part I), J. Amer. Ceram. Soc. 91, (2008). 7. K. Lily, Kumari, K. Prasad, and R. Choudhary, Impedance spectroscopy of (Na 0.5 Bi 0.5 )(Zr 0.25 Ti 0.75 )O 3 lead-free ceramic, J. Alloys and Compounds 453, (2008). 8. S. Lanfredi and A. Rodrigues, Impedance spectroscopy study of the electrical conductivity and dielectric constant of polycrystalline LiNbO 3, J.Appl.Phys. 86, (1999). 9. D. P. Cann and C. A. Randall, The Thermochemistry and Non-Ohmic Electrical Contacts of a BaTiO 3 PTCR Ceramic, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 44, (1997). 10. S. Wongsaenmai, S. Ananta, and R. Yimnirun, Effect of Li addition on phase formation behavior an electrical properties of (K 0.5 Na 0.5 )NbO 3 lead free ceramics, Ceramics International 38, (2012). 11. Y. Guo, K. Kakimoto, and H. Ohsato, (K 0.5 Na 0.5 )NbO 3 LiTaO 3 lead-free piezoelectric ceramics, Materials Letters 59, (2009). 12. K. C. Singh, C. Jiten, R. Laishram, O. P. Thakur, and D. K. Bhattacharya, Structure and electrical properties of Li- and Ta-substituted K 0.5 Na 0.5 NbO 3 lead-free piezoelectric ceramics prepared from nanopowders, Journal of Alloys and Compounds 496, (2010). 13. E. Hollenstein, M. Davis, D. Damjanovic, and N. Setter, Piezoelectric properties of Li- and Ta-modified (K 0.5 Na 0.5 )NbO 3 ceramics, Applied Physics Letters 87, (2005). 14. P. Dhak, D. Dhak, M. Das, K. Pramanik, and P. Pramanik, Impedance spectroscopy study of LaMnO 3 modified BaTiO 3 ceramics, Materials Science and Engineering B 164, (2009).

14 Lithium Modified Potassium Sodium Niobate Ceramics [235]/ R. Rani, S. Sharma, R. Rai, and A. L. Kholkin, Investigation of dielectric and electrical properties of Mn doped sodium potassium niobate ceramic system using impedance spectroscopy, J. Appl. Phys. 110, (2011). 16. E. Mensur Alkoy, and A. Berksoy-Yavuz, Electrical Properties and Impedance Spectroscopy of Pure and Copper Oxide Added Potassium Sodium Niobate Ceramics, IEEE Trans. Ultrason. Ferroelectr. Freq. Control, in press. (2012). 17. R. Rai, I. Coondoo, R. Rani, I. Bdikin, S. Sharma, and A. L. Kholkin, Impedance spectroscopy and piezoresponse force microscopy analysis of lead-free (1-x) K 0.5 Na 0.5 NbO 3 -xlinbo 3 ceramics, Current Applied Physics. (2012) in press, doi: /j.cap