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Characterization of (Na0.47K0.47Li0.06)(SbxNb1−x)O3 ceramics prepared by molten salt synthesis method
Solid State Communications 149 (2009) 581–584
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Characterization of (Na0.47 K0.47 Li0.06 )(Sbx Nb1−x )O3 ceramics prepared by molten salt synthesis method Jianhua Li a,∗ , Qingchi Sun b a
School of Electronic and Information Engineering, Tianjin University, Tianjin 300072, PR China
b
Key Laboratory for Advanced Ceramic and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, PR China
article
info
Article history: Received 23 September 2008 Received in revised form 3 October 2008 Accepted 3 February 2009 by P. Sheng Available online 11 February 2009 PACS: 7000 Keywords: A. Piezoelectric ceramics B. Molten salts synthesis method C. X-ray diffraction D. Piezoelectric properties
a b s t r a c t In this study, (Na0.47 K0.47 Li0.06 )(Sbx Nb1−x )O3 (NKL(Sbx Nb1−x )O3 ) powder is synthesized by molten salt synthesis method (MSS). The resulting products were characterized by the X-ray diffraction analysis (XRD) and scanning electron microscope (SEM) method. The (XRD) result of the corresponding ceramics proved the existence of the tetragonal phase symmetry near the morphotropic phase boundary. The SEM images indicated that the crystalline powder was well separated and no aggregate was present. The NKL(Sbx Nb1−x )O3 exhibited a strong compositional dependence and enhanced electrical properties while maintaining high Curie temperature. The results show that (NKL)(Sbx Nb1−x )O3 ceramics made by MSS method are promising candidates for lead-free piezoelectric materials. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Potassium–sodium niobate (Na0.5 K0.5 )NbO3 (NKN) [1–3] which is a kind of perovskite structure with superior piezoelectric properties [4] has been studied as an excellent candidate for leadfree piezoelectric ceramics. It has been reported that NKN-based compositions modified with a small amount of additives, such as BaTiO3 [5], SrTiO3 [6], LiNbO3 [7], LiTaO3 [2], CuO [8] and ZnO [9] shown improved piezoelectric properties compared with pure KNK ceramics. Moreover, it is generally accepted that the addition of Sb2 O3 can enhance stability and compactness, and will also offer a high electromechanical coupling factor [10–12]. Among these materials, (Na0.5 K0.5 )LiNbO3 with different LiNbO3 content (KNLN) system has attracted considerable attention. Guo and Kakimoto et al. have reported that the morphortropic phase boundary (MPB) of (NKLN) existed with about 5–7 mol% of LiNbO3 confirmed by XRD and Raman scattering study [13,14]. The material constants of NKN-based ceramics have been evaluated by Ringgaard et al. with respect to the application of industrial applications in pulse–echo measurement devices [3]. KNN systems, however, have difficulties in densification, which is one of the main obstacles for the development of KNN as a commercial piezoceramic material.
∗ Corresponding address: School of Electronic and Information Engineering, Tianjin University, NO. 92, Weijin Street, Nankai District, Tianjin, PR China. Tel.: +86 22 27891990; fax: +86 22 26733563. E-mail address: [email protected] (J. Li). 0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.02.003
Research efforts have been devoted to the preparation of the material by various simple and reproducible low temperature synthesis techniques, such as sol–gel method [15], citrate method [16], hydrothermal process [17] and MSS method [18]. The MSS method is a simple and cost-effective technique for preparing single crystalline particles with the desired composition in a low-melting point flux. In the present paper, the phase transition, the microstructure, the piezoelectric and dielectric properties of (NKL)(Sbx Nb1−x )O3 lead-free piezoelectric ceramics prepared by a MSS method have been studied. The results indicate that (NKL)(Sbx Nb1−x )O3 ceramics possessing significantly improved piezoelectric properties when small amount of Sb2 O3 is doped. 2. Experimental The powder with the nominal composition of (Na0.47 K0.47 Li0.06 ) (Sbx Nb1−x )O3 (x = 0, 0.02, 0.08, 0.16) was prepared by the MSS method. The starting materials used in this study were K2 CO3 , Na2 CO3 , Nb2 O5 , Sb2 O3 and Li2 CO3 of 99.9% purity. K2 CO3 –Na2 CO3 salt with a 0.45:0.55 eutectic composition was used; its melting point is about 710 ◦ C. The weight ratio (W) of salt to oxide was 1:1.3 mol%. Firstly, oxide sources and the Na2 CO3 – K2 CO3 flux were co-mixed and ball-milled in ethanol for 6 h. Then, the dried slurry was calcined at 800 ◦ C for 2 h. The calcined powder was washed using hot distilled water. After dried, powder was mixed with 5
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Fig. 2. SEM of (NKL)(Sbx Nb1−x )O3 powders calcined at 800 ◦ C for 2 h. Fig. 1. X-ray diffraction diagrams of the (NKL)(Sbx Nb1−x )O3 powders.
wt% poly vinyl alcohol (PVA) solution, and then was pressed into pellets with a diameter of 22 mm and 2 mm in thickness under 100 MPa pressure. After binder burnout, the green plates were sintered at 1100 ◦ C for 2 h. Relative densities (for samples with x = 0–0.08) higher than 96% of the theoretical density were obtained. Silver electrodes were formed on both surfaces of each sintered disk, and the samples were immersed in silicon oil and poled in a 30 kV cm−1 DC field for 30 min at 150 ◦ C. All electrical measurements were carried out about 24 h after poling. The calcined powder was examined by X-ray diffraction (XRD, Model D/MAX-2500X, Japan). To ensure phase purity, the microstructure of calcined powder and sintered bodies was observed using a scanning electron microscope (SEM, Model XL30-DX-4, Philips, Holland). Apparent sintered densities were measured by the Archimedes method with distilled water. Dielectric properties were obtained by measuring the capacitance and loss at 1 kHz using an LCR meter (HP 4294A). The Curie
temperature (Tc ) was determined by temperature dependence of the dielectric constant at 10 kHz. The piezoelectric constant (d33 ) was measured using a quasi-static piezoelectric d33 meter (Model ZJ-3d, China). The planar electromechanical coupling coefficient (Kp ), the thickness electromechanical coupling factor (Kt ) and the mechanical quality factor (Qm ) were determined by the resonance and antiresonance technique on the basis of IEEE standards using an impedance analyzer (HP4294A). 3. Results and discussion Fig. 1 displays the X-ray diffraction patterns for (NKL)
(Sbx Nb1−x )O3 calcined at 800 ◦ C in the 2θ range of 20◦ –60◦ . The phase structure in all samples expect x = 0 is pure perovskite
phase and no any secondary impurity could be certified. At the same time, all the (NKL)(Sbx Nb1−x )O3 possess tetragonal structure. The result indicates that the Sb2 O3 have completely diffused into the (NKL)Nb lattice to form a new solid solution. This mainly attributed to the enhanced diffusivity of the constituent oxides
Fig. 3. SEM micrographs of (NKL)(Sbx Nb1−x )O3 ceramics sintered at 1100 ◦ C for 2 h: (a) x = 0.02, (b) x = 0.08, (c) x = 0.16.
J. Li, Q. Sun / Solid State Communications 149 (2009) 581–584
in the liquid state of the melt salt. It is clear that the crystal structure of (NKL)(Sbx Nb1−x )O3 ceramics is changed by adding a small amount of Sb2 O3 . It can be seen that the diffraction peaks become sharper and move subtly to high ‘‘2θ ’’ as the content of Sb2 O3 increases. In another word, the grain size (D) which determined by the formula D = kλ/(β cos θ ) becomes smaller. According to the principle of crystal chemistry, the substitution ion tends to replace the ion which has the close radius when the valence is equal. XRD reveal that the Sb5+ doping have influenced the structure of Sb-doped KNLN. Fig. 2 shows SEM image of crystalline (NKL)(Sbx Nb1−x )O3 powder synthesized at 800 ◦ C for 2 h. It indicates that the particles are cubic morphology and have uniform distribution. And then, the crystalline powder was separated well. It is because that the calcined powder made by the MSS method does not require calcination at high temperature to make the dried powder transform into final powder. The powder made by MSS method was aggregate free powder, which is detrimental to the sintering of powder. Thus, the synthesized powder has high sintering activity and can be sintered at relative low temperature. Fig. 3 displays typical microstructure of NKLN doped with different amount of Sb2 O3 (x = 0.02, 0.08, 0.16). The SEM observation confirms that grain size first increases then decreases with the increase of Sb2 O3 concentration. The grain size of the sample doped with 8 mol% Sb2 O3 is largest, with homogeneous distribution. The radius of Sb5+ (0.62 Å) ion is smaller than that of K+ and Na+ (1.33 and 0.95Å), but is bigger than that of Li+ (0.60 Å). In view of the radius, Li+ ions entering to the pattern may cause distortion of lattice [19]. The loosening structure can be sintered easily; this then makes the grain size increase [20]. A consecutive increase of Sb2 O3 , the growth of grains was significantly inhibited by the excessive content of Sb2 O3 segregating at grain boundaries which exerted a drag force against the grain boundary movement. The temperature dependence of dielectric constant (εr ) for (NKL)(Sbx Nb1−x )O3 ceramics measured at 10 kHz is shown in Fig. 4. It indicates two dielectric anomalies which correspond to the phase transitions of paraelectric (cubic) ferroelectric (Tc ) and tetragonal–orthorhombic (TT –O ). For pure NKN, Tc and TT –O are reported at 420 and 200 ◦ C, respectively [21]. For (NKL)(Sbx Nb1−x )O3 , similar to the pure NKN ceramics, two peaks of the dielectric constant are still observed. However, Tc shifts to lower temperature with the increase of Sb2 O3 content and TT –O shifts to room temperature. When Sb2 O3 content is 16 mol%, TT –O has disappeared. The frequency dispersion and diffuseness of transition increased as Sb2 O3 content in the compositions increased. And no liner decrease of Tc happens, which has also been reported by researchers [22]. Fig. 4 indicates (NKL)(Sbx Nb1−x )O3 maintain high Tc with small amount of Sb2 O3 . The Curie temperature, Tc , of the (NKL)(Sb0.08 Nb0.92 )O3 ceramics is about 397 ◦ C, which is higher than the reported values of NKN-based ceramics with doped Sb2 O3 content [23,24]. High Curie temperature of (NKL)(Sbx Nb1−x )O3 ceramics in this work is attributed to the existence of LiNbO3 , which has high Curie temperature (Tc = 1210 ◦ C) Dielectric loss for (NKL)(Sbx Nb1−x )O3 ceramics measured at 10 kHz as a function of temperature is shown in Fig. 5. It was found that the tan δ for (NKL)(Sbx Nb1−x )O3 ceramics is lower than 5% except x = 0 indicating that ceramics have no significant conductivity appearance even at temperature as high as 400 ◦ C. It can be concluded that the Sb2 O3 -doped NKL ceramics are suitable for higher temperature application [2]. Fig. 6 shows the effects of εr and tan δ at 1 kHz of different Sb-doped compositions sintered at 1100 ◦ C for 2 h. It can be seen that tan δ increases with the increase of Sb2 O3 content. εr increases first and reach the highest value of 614 (x = 0.08) then sharply decreases. This result is related to the domain wall
583
Fig. 4. Temperature dependence of dielectric constant for (NKL)(Sbx Nb1−x )O3 ceramics at 10 kHz as a function of x.
Fig. 5. Temperature dependence of dielectric loss for (NKL)(Sbx Nb1−x )O3 ceramics at 10 kHz as a function of x.
Fig. 6. εr and tan δ at 1 kHz as a function of x sintered at 1100 ◦ C for 2 h.
motion contribution, which causes energy loss because of the interaction of domain walls with other domain walls, domain walls crystallites, and domain walls with defects. Fig. 7 shows that the maximum values of d33 , Kp and Kt , which is 230 pC N−1 , 37% and 47%, were obtained at x = 0.08, and then these values decrease with increased x. Unlike εr d33 ,
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Fig. 7. d33 , Qm, Kp and Kt of (NKL)(Sbx Nb1−x )O3 sintered at 1100 ◦ C for 2 h.
Kp and Kt , Qm increases and exhibits a maximum value when x is 0.02, then decrease as increasing Sb3 O2 fraction. However the Qm value of 520 when x = 0.08 is high enough for a wide range of electromechanical transducer applications [1, 2]. As a conclusion, (NKL)(Sb0.08 Nb0.92 )O3 reaches the optimum values. The appropriate amount of Sb2 O3 can lead to sufficient densification, the reduction of the microstructural discontinuity, and achieve optimum piezoelectric properties. This can be ascribed to the introduction of Sb5+ ions, which remarkably influenced the piezoelectric prosperities and sinterability during the sintering process. The piezoelectric properties decrease rapidly due to excessive Sb2 O3 which may result in inhibiting grain growth, existence of secondary phase and poor microstructural homogeneity. The radius of Sb5+ (0.62 Å) is smaller than that of K+ and Na+ (1.33 and 0.95 Å) but very close to that of Nb5+ (0.69 Å), so Sb5+ substitutes on the perovskite ‘‘B’’ site easily. However, the difference between Nb5+ and Sb5+ limits the substitution to a small amount. The superfluous amount of Sb2 O3 into (NKL)(Sbx Nb1−x )O3 deteriorated piezoelectric properties due to the excessive Sb5+ segregating at grain boundaries. 4. Conclusions A Molten salt synthesis method is an effective route to synthesizing (NKL)(Sbx Nb1−x )O3 powders, which have high sintering activity. Effects of NKLN near the MPB doped a different amount of Sb2 O3 of (NKL)(Sbx Nb1−x )O3 with perovskite structures calcined at 800 ◦ C for 2 h upon single phase formation, homogeneous microstructure, higher piezoelectric and dielectric properties were further studied in this work. For (NKL)(Sb0.08 Nb0.92 )O3 ceramics sintered at 1100 ◦ C for 2 h, the optimum piezoelectricity and dielectricity of Tc , ε r, tan δ , d33 , Kp , Kt and Qm are 397, 612, 0.026, 230 pC N-1, 37%, 7% and 520, respectively. The excellent piezoelectric and electromechanical properties indicate that this system might be a promising lead-free material for a wide range of electromechanical transducer applications.
Acknowledgements The authors would like to thank for the supports of Chinese National Natural Science Foundation, grant number: 10232030 and Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education of Tianjin University, grant number: x06050. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
L. Egerton, D.M. Dillon, J. Am. Ceram. Soc. 42 (1959) 438. Y. Guo, K. Kakimoto, H. Ohsato, Mater. Lett. 59 (2005) 241. E. Ringgaard, T. Wurlitzer, J. Eur. Ceram. Soc. 25 (2005) 2701. H.L. Du, F.S. Tang, F. Luo, D.M. Zhu, S.B. Qu, Z.B. Pei, W.C. Zhou, Mater. Res. Bull. 42 (2007) 1594. Y. Guo, K. Kakimoto, H. Ohsato, Japan. J. Appl. Phys. 43 (9B) (2004) 6662. Y. Guo, K. Kakimoto, H. Ohsato, Solid State Commun. 129 (2004) 279. Y. Guo, K. Kakimoto, H. Ohsato, Appl. Phys. Lett. 85 (2004) 4121. M. Matsubara, T. Yamaguchi, W. Sakamoto, K. Kikuta, T. Yogo, S. Hirano, J. Am. Ceram. Soc. 88 (2005) 1190. S.H. Park, C.W. Ahn, S. Nahm, J.S. Song, Japan. J. Appl. Phys. 43 (2004) 1072. K.S. Kon, K.C. Su, Mater. Sci. Semicond. Proc. 5 (2002) 115. J.G. Wu, D.Q. Xiao, Appl. Phys. Lett. 91 (2007) 252907. J.G. Wu, D.Q. Xiao, Y.Y. Wang, J.G. Zhu, P. Yu, Y.H. Jiang, J. Appl. Phys. 102 (2007) 114113. X.X. Wang, X.G. Tang, H.L.W. Chan, Appl. Phys. Lett. 85 (2004) 91. K. Kakimoto, K. Akao, Y. Guo, H. Ohsato, Japan. J. Appl. Phys. 44 (2005) 7064. J. Hou, Y. Qu, W. Ma, J. Mater. Sci. 42 (2007) 6787. D.L. West, D.A. Payne, J. Am. Ceram. Soc. 86 (2003) 192. P. Pookmanee, G. Rujijanagul, S. Ananta, R.B. Heimann, S. Phanichphant, J. Eur. Ceram. Soc. 24 (2002) 517. K. Katayama, Y. Azuma, Y. Takahashi, J. Mater. Sci. 34 (1999) 301. L.T. Li, Y.J. Yao, Z.H. Mu, Ferroelectrics 28 (1980) 403. Y.D. Hou, M.K. Zhu, F. Gao, H. Wang, B. Wang, H. Yan, C.S. Tian, J. Am. Ceram. Soc. 87 (2004) 847. R.E. Jaeger, L. Egerton, J. Am. Ceram. Soc. 45 (1962) 209. R.W. Whatmore, O. Molter, C.P. Shaw, J. Eur. Ceram. Soc. 23 (2003) 721. J. Yoo, K. Lee, K. Chung, S. Lee, K. Kim, J. Hong, S. Ryu, C. Lhee, Japan. J. Appl. Phys. 45 (2004) 7444. S. Zhang, R. Xia, T.R. Shrout, G. Zang, J. Wang, Solid State Commun. 141 (2007) 675.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Characterization of (Na0.47 K0.47 Li0.06 )(Sbx Nb1−x )O3 ceramics prepared by molten salt synthesis method Jianhua Li a,∗ , Qingchi Sun b a
School of Electronic and Information Engineering, Tianjin University, Tianjin 300072, PR China
b
Key Laboratory for Advanced Ceramic and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, PR China
article
info
Article history: Received 23 September 2008 Received in revised form 3 October 2008 Accepted 3 February 2009 by P. Sheng Available online 11 February 2009 PACS: 7000 Keywords: A. Piezoelectric ceramics B. Molten salts synthesis method C. X-ray diffraction D. Piezoelectric properties
a b s t r a c t In this study, (Na0.47 K0.47 Li0.06 )(Sbx Nb1−x )O3 (NKL(Sbx Nb1−x )O3 ) powder is synthesized by molten salt synthesis method (MSS). The resulting products were characterized by the X-ray diffraction analysis (XRD) and scanning electron microscope (SEM) method. The (XRD) result of the corresponding ceramics proved the existence of the tetragonal phase symmetry near the morphotropic phase boundary. The SEM images indicated that the crystalline powder was well separated and no aggregate was present. The NKL(Sbx Nb1−x )O3 exhibited a strong compositional dependence and enhanced electrical properties while maintaining high Curie temperature. The results show that (NKL)(Sbx Nb1−x )O3 ceramics made by MSS method are promising candidates for lead-free piezoelectric materials. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Potassium–sodium niobate (Na0.5 K0.5 )NbO3 (NKN) [1–3] which is a kind of perovskite structure with superior piezoelectric properties [4] has been studied as an excellent candidate for leadfree piezoelectric ceramics. It has been reported that NKN-based compositions modified with a small amount of additives, such as BaTiO3 [5], SrTiO3 [6], LiNbO3 [7], LiTaO3 [2], CuO [8] and ZnO [9] shown improved piezoelectric properties compared with pure KNK ceramics. Moreover, it is generally accepted that the addition of Sb2 O3 can enhance stability and compactness, and will also offer a high electromechanical coupling factor [10–12]. Among these materials, (Na0.5 K0.5 )LiNbO3 with different LiNbO3 content (KNLN) system has attracted considerable attention. Guo and Kakimoto et al. have reported that the morphortropic phase boundary (MPB) of (NKLN) existed with about 5–7 mol% of LiNbO3 confirmed by XRD and Raman scattering study [13,14]. The material constants of NKN-based ceramics have been evaluated by Ringgaard et al. with respect to the application of industrial applications in pulse–echo measurement devices [3]. KNN systems, however, have difficulties in densification, which is one of the main obstacles for the development of KNN as a commercial piezoceramic material.
∗ Corresponding address: School of Electronic and Information Engineering, Tianjin University, NO. 92, Weijin Street, Nankai District, Tianjin, PR China. Tel.: +86 22 27891990; fax: +86 22 26733563. E-mail address: [email protected] (J. Li). 0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.02.003
Research efforts have been devoted to the preparation of the material by various simple and reproducible low temperature synthesis techniques, such as sol–gel method [15], citrate method [16], hydrothermal process [17] and MSS method [18]. The MSS method is a simple and cost-effective technique for preparing single crystalline particles with the desired composition in a low-melting point flux. In the present paper, the phase transition, the microstructure, the piezoelectric and dielectric properties of (NKL)(Sbx Nb1−x )O3 lead-free piezoelectric ceramics prepared by a MSS method have been studied. The results indicate that (NKL)(Sbx Nb1−x )O3 ceramics possessing significantly improved piezoelectric properties when small amount of Sb2 O3 is doped. 2. Experimental The powder with the nominal composition of (Na0.47 K0.47 Li0.06 ) (Sbx Nb1−x )O3 (x = 0, 0.02, 0.08, 0.16) was prepared by the MSS method. The starting materials used in this study were K2 CO3 , Na2 CO3 , Nb2 O5 , Sb2 O3 and Li2 CO3 of 99.9% purity. K2 CO3 –Na2 CO3 salt with a 0.45:0.55 eutectic composition was used; its melting point is about 710 ◦ C. The weight ratio (W) of salt to oxide was 1:1.3 mol%. Firstly, oxide sources and the Na2 CO3 – K2 CO3 flux were co-mixed and ball-milled in ethanol for 6 h. Then, the dried slurry was calcined at 800 ◦ C for 2 h. The calcined powder was washed using hot distilled water. After dried, powder was mixed with 5
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Fig. 2. SEM of (NKL)(Sbx Nb1−x )O3 powders calcined at 800 ◦ C for 2 h. Fig. 1. X-ray diffraction diagrams of the (NKL)(Sbx Nb1−x )O3 powders.
wt% poly vinyl alcohol (PVA) solution, and then was pressed into pellets with a diameter of 22 mm and 2 mm in thickness under 100 MPa pressure. After binder burnout, the green plates were sintered at 1100 ◦ C for 2 h. Relative densities (for samples with x = 0–0.08) higher than 96% of the theoretical density were obtained. Silver electrodes were formed on both surfaces of each sintered disk, and the samples were immersed in silicon oil and poled in a 30 kV cm−1 DC field for 30 min at 150 ◦ C. All electrical measurements were carried out about 24 h after poling. The calcined powder was examined by X-ray diffraction (XRD, Model D/MAX-2500X, Japan). To ensure phase purity, the microstructure of calcined powder and sintered bodies was observed using a scanning electron microscope (SEM, Model XL30-DX-4, Philips, Holland). Apparent sintered densities were measured by the Archimedes method with distilled water. Dielectric properties were obtained by measuring the capacitance and loss at 1 kHz using an LCR meter (HP 4294A). The Curie
temperature (Tc ) was determined by temperature dependence of the dielectric constant at 10 kHz. The piezoelectric constant (d33 ) was measured using a quasi-static piezoelectric d33 meter (Model ZJ-3d, China). The planar electromechanical coupling coefficient (Kp ), the thickness electromechanical coupling factor (Kt ) and the mechanical quality factor (Qm ) were determined by the resonance and antiresonance technique on the basis of IEEE standards using an impedance analyzer (HP4294A). 3. Results and discussion Fig. 1 displays the X-ray diffraction patterns for (NKL)
(Sbx Nb1−x )O3 calcined at 800 ◦ C in the 2θ range of 20◦ –60◦ . The phase structure in all samples expect x = 0 is pure perovskite
phase and no any secondary impurity could be certified. At the same time, all the (NKL)(Sbx Nb1−x )O3 possess tetragonal structure. The result indicates that the Sb2 O3 have completely diffused into the (NKL)Nb lattice to form a new solid solution. This mainly attributed to the enhanced diffusivity of the constituent oxides
Fig. 3. SEM micrographs of (NKL)(Sbx Nb1−x )O3 ceramics sintered at 1100 ◦ C for 2 h: (a) x = 0.02, (b) x = 0.08, (c) x = 0.16.
J. Li, Q. Sun / Solid State Communications 149 (2009) 581–584
in the liquid state of the melt salt. It is clear that the crystal structure of (NKL)(Sbx Nb1−x )O3 ceramics is changed by adding a small amount of Sb2 O3 . It can be seen that the diffraction peaks become sharper and move subtly to high ‘‘2θ ’’ as the content of Sb2 O3 increases. In another word, the grain size (D) which determined by the formula D = kλ/(β cos θ ) becomes smaller. According to the principle of crystal chemistry, the substitution ion tends to replace the ion which has the close radius when the valence is equal. XRD reveal that the Sb5+ doping have influenced the structure of Sb-doped KNLN. Fig. 2 shows SEM image of crystalline (NKL)(Sbx Nb1−x )O3 powder synthesized at 800 ◦ C for 2 h. It indicates that the particles are cubic morphology and have uniform distribution. And then, the crystalline powder was separated well. It is because that the calcined powder made by the MSS method does not require calcination at high temperature to make the dried powder transform into final powder. The powder made by MSS method was aggregate free powder, which is detrimental to the sintering of powder. Thus, the synthesized powder has high sintering activity and can be sintered at relative low temperature. Fig. 3 displays typical microstructure of NKLN doped with different amount of Sb2 O3 (x = 0.02, 0.08, 0.16). The SEM observation confirms that grain size first increases then decreases with the increase of Sb2 O3 concentration. The grain size of the sample doped with 8 mol% Sb2 O3 is largest, with homogeneous distribution. The radius of Sb5+ (0.62 Å) ion is smaller than that of K+ and Na+ (1.33 and 0.95Å), but is bigger than that of Li+ (0.60 Å). In view of the radius, Li+ ions entering to the pattern may cause distortion of lattice [19]. The loosening structure can be sintered easily; this then makes the grain size increase [20]. A consecutive increase of Sb2 O3 , the growth of grains was significantly inhibited by the excessive content of Sb2 O3 segregating at grain boundaries which exerted a drag force against the grain boundary movement. The temperature dependence of dielectric constant (εr ) for (NKL)(Sbx Nb1−x )O3 ceramics measured at 10 kHz is shown in Fig. 4. It indicates two dielectric anomalies which correspond to the phase transitions of paraelectric (cubic) ferroelectric (Tc ) and tetragonal–orthorhombic (TT –O ). For pure NKN, Tc and TT –O are reported at 420 and 200 ◦ C, respectively [21]. For (NKL)(Sbx Nb1−x )O3 , similar to the pure NKN ceramics, two peaks of the dielectric constant are still observed. However, Tc shifts to lower temperature with the increase of Sb2 O3 content and TT –O shifts to room temperature. When Sb2 O3 content is 16 mol%, TT –O has disappeared. The frequency dispersion and diffuseness of transition increased as Sb2 O3 content in the compositions increased. And no liner decrease of Tc happens, which has also been reported by researchers [22]. Fig. 4 indicates (NKL)(Sbx Nb1−x )O3 maintain high Tc with small amount of Sb2 O3 . The Curie temperature, Tc , of the (NKL)(Sb0.08 Nb0.92 )O3 ceramics is about 397 ◦ C, which is higher than the reported values of NKN-based ceramics with doped Sb2 O3 content [23,24]. High Curie temperature of (NKL)(Sbx Nb1−x )O3 ceramics in this work is attributed to the existence of LiNbO3 , which has high Curie temperature (Tc = 1210 ◦ C) Dielectric loss for (NKL)(Sbx Nb1−x )O3 ceramics measured at 10 kHz as a function of temperature is shown in Fig. 5. It was found that the tan δ for (NKL)(Sbx Nb1−x )O3 ceramics is lower than 5% except x = 0 indicating that ceramics have no significant conductivity appearance even at temperature as high as 400 ◦ C. It can be concluded that the Sb2 O3 -doped NKL ceramics are suitable for higher temperature application [2]. Fig. 6 shows the effects of εr and tan δ at 1 kHz of different Sb-doped compositions sintered at 1100 ◦ C for 2 h. It can be seen that tan δ increases with the increase of Sb2 O3 content. εr increases first and reach the highest value of 614 (x = 0.08) then sharply decreases. This result is related to the domain wall
583
Fig. 4. Temperature dependence of dielectric constant for (NKL)(Sbx Nb1−x )O3 ceramics at 10 kHz as a function of x.
Fig. 5. Temperature dependence of dielectric loss for (NKL)(Sbx Nb1−x )O3 ceramics at 10 kHz as a function of x.
Fig. 6. εr and tan δ at 1 kHz as a function of x sintered at 1100 ◦ C for 2 h.
motion contribution, which causes energy loss because of the interaction of domain walls with other domain walls, domain walls crystallites, and domain walls with defects. Fig. 7 shows that the maximum values of d33 , Kp and Kt , which is 230 pC N−1 , 37% and 47%, were obtained at x = 0.08, and then these values decrease with increased x. Unlike εr d33 ,
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J. Li, Q. Sun / Solid State Communications 149 (2009) 581–584
Fig. 7. d33 , Qm, Kp and Kt of (NKL)(Sbx Nb1−x )O3 sintered at 1100 ◦ C for 2 h.
Kp and Kt , Qm increases and exhibits a maximum value when x is 0.02, then decrease as increasing Sb3 O2 fraction. However the Qm value of 520 when x = 0.08 is high enough for a wide range of electromechanical transducer applications [1, 2]. As a conclusion, (NKL)(Sb0.08 Nb0.92 )O3 reaches the optimum values. The appropriate amount of Sb2 O3 can lead to sufficient densification, the reduction of the microstructural discontinuity, and achieve optimum piezoelectric properties. This can be ascribed to the introduction of Sb5+ ions, which remarkably influenced the piezoelectric prosperities and sinterability during the sintering process. The piezoelectric properties decrease rapidly due to excessive Sb2 O3 which may result in inhibiting grain growth, existence of secondary phase and poor microstructural homogeneity. The radius of Sb5+ (0.62 Å) is smaller than that of K+ and Na+ (1.33 and 0.95 Å) but very close to that of Nb5+ (0.69 Å), so Sb5+ substitutes on the perovskite ‘‘B’’ site easily. However, the difference between Nb5+ and Sb5+ limits the substitution to a small amount. The superfluous amount of Sb2 O3 into (NKL)(Sbx Nb1−x )O3 deteriorated piezoelectric properties due to the excessive Sb5+ segregating at grain boundaries. 4. Conclusions A Molten salt synthesis method is an effective route to synthesizing (NKL)(Sbx Nb1−x )O3 powders, which have high sintering activity. Effects of NKLN near the MPB doped a different amount of Sb2 O3 of (NKL)(Sbx Nb1−x )O3 with perovskite structures calcined at 800 ◦ C for 2 h upon single phase formation, homogeneous microstructure, higher piezoelectric and dielectric properties were further studied in this work. For (NKL)(Sb0.08 Nb0.92 )O3 ceramics sintered at 1100 ◦ C for 2 h, the optimum piezoelectricity and dielectricity of Tc , ε r, tan δ , d33 , Kp , Kt and Qm are 397, 612, 0.026, 230 pC N-1, 37%, 7% and 520, respectively. The excellent piezoelectric and electromechanical properties indicate that this system might be a promising lead-free material for a wide range of electromechanical transducer applications.
Acknowledgements The authors would like to thank for the supports of Chinese National Natural Science Foundation, grant number: 10232030 and Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education of Tianjin University, grant number: x06050. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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