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Phase structure of Li0.058(Na0.51K0.49)0.942NbO3 lead-free piezoelectric ceramics
Materials Letters 84 (2012) 52–55
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Phase structure of Li0.058(Na0.51K0.49)0.942NbO3 lead-free piezoelectric ceramics Yongjie Zhao n, Rongxia Huang n, Rongzheng Liu, Heping Zhou State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 March 2012 Accepted 9 June 2012 Available online 20 June 2012
Several kinds of phase structure change behavior in Li0.058(Na0.51K0.49)0.942NbO3 (KNNLN) ceramics were presented in this work. XPS measurement was employed to determine Na volatilization in the KNNLN samples, and this result explains the difference of the phase structure between the surface and interior of the samples. The phase structure of lead-free KNNLN piezoceramics are studied based on the measurements of the analysis of X-ray diffraction pattern and dielectric properties. The poled samples exhibit orthorhombic structure whereas the surface and interior for unpoled samples exhibit tetragonal and tetragonal-orthorhombic coexistent structures, respectively. An explanation to the mechanism of the structure transformation during poling processing was proposed. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ceramics Ferroelectrics Phase structure XPS
1. Introduction Due to the outstanding piezoelectric properties at the composition of the so-called morphotropic phase boundary (MPB), [1] KNN based piezoceramics have been regarded as a promising lead-free candidate. In recent years more and more researchers have reported that the so-called MPB is actually a structure originating from the polymorphic phase transition (PPT) around room temperature.[2–5] Therefore, the outstanding piezoelectric properties of KNN-based ceramics are thermally unstable owing to the narrow temperature region for the coexistence of the tetragonal and orthorhombic structure.[6] Thus, it can be seen that the phase structure (room temperature) of KNN-based ceramics is of great importance for the practical performance of piezoceramics. Previous studies have found that the phase structure of KNN-based ceramics were sensitive to processing parameters, especially the composition lying in the vicinity of polymorphic phase transition. The more attention should be paid on the research of the conventionally prepared processing. In this work, we present several kinds of phase structure change behavior in KNNLN ceramics and interpret the underlying mechanism of these phase structure changes.
K2CO3 (99.0%), Li2CO3 (99.0%) and Nb2O5 (99.9%). The powder were mixed in a nylon jar with agate balls for 24 h and dried. The dried powders were calcinated at 760 1C for 5 h. After the calcination, the powders re-milled for 24 h. Then the powders were dried and pressed into discs under a pressure of 60 MPa using polyvinyl butyral (PVB) as a binder. After PVB was burnt out, the pellets were, respectively, sintered in air at 980 1C, 1040 1C and 1080 1C for 2 h. Silver paste was fired on both sides of the samples at 600 1C for 20 min to form electrodes for the electrical measurements after polishing. XRD to identify the crystal structures was performed with a Rigaku D/MAX-2500 diffractometer with CuKa1 radiation (l ¼0.15406 nm). The as-sintered sample was used to obtain the surface XRD pattern. Then, the as-sintered samples were polished to get the interior XRD pattern. And the poling depth almost was several hundred micrometers. The temperature dependence of the dielectric constant was measured in a temperature-controllable container which was connected to an Agilent 4284 m (Hewlett–Packard, Palo Alto, CA) at 1 kHz. The X-ray photoemission spectroscopy (XPS) measurement was carried out on a ESCALAB 250Xi scanning X-ray microprobe instrument employing monochromatic Al-Ka radiation as the excitation source.
2. Experimental 3. Results and discussions The ceramics samples with a composition Li0.058(Na0.51K0.49)0.942 NbO3 were prepared using analytical grades of Na2CO3 (99.8%), n
Corresponding authors. Tel./fax: þ86 10 62772549. E-mail addresses: [email protected] (Y. Zhao), [email protected] (R. Huang). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.06.030
Fig. 1 shows the XRD patterns for the surface and interior of the KNNLN ceramics for 1040 1C. All samples exhibit a single perovskite structure, while the surface and interior patterns present tetragonal and tetragonal-orthorhombic polymorphic, respectively. That is, the surface and interior phase structures for the sample are different. We
Y. Zhao et al. / Materials Letters 84 (2012) 52–55
attribute this transition to the compositional change which may be resulted in by the extent of volatilization of alkali metal oxides during sintering process. Although lots of researchers consider that Na2O evaporation results in the above phase structure difference, these are no specific measurement to certify this point. To further confirm the above hypothesis, the X-ray photoemission spectroscopy (XPS)
Fig. 1. XRD patterns for the surface and interior of KNNLN samples for 1040 1C.
53
measurement was employed. XPS measurement is known to be an effective probe for the exact determination of element composition. Fig. 2 shows the XPS analysis of KNNLN sintered at 1040 1C along the thickness direction from the surface to the interior at an interval of 10–100 nm. The location of characteristic peak is closely related with the valence states of the elements. It is evidently observed that the location of Na1s and Nb3d change very little with the thickness direction. However, it is not the same case for K2p. There exists big difference between the K’s valence states of the surface and interior. It considers that this clear difference of the surface and interior is ascribed to two reasons. The first one is the surface charge effect. However, if this case works and the location of Na1s and K2p would also be affected, which is not observed however in Fig. 2. Therefore, the first hypothesis does not come into force. The other reason is the change of the existence form of K element. Meanwhile, the Na content of the sample surface is lower than that of the interior and heavier volatilization of Na on the sample surface should have occurred during sintering process. So we could consider that the secondary phase rich in K element would come into existence on the surface of KNNLN samples. And this consideration could interpret above result of the shift of the location of K2p. The lase figure indicates the atomic percent of Na, K and Nb along the thickness of the sample. Fig. 3 shows the XRD patterns of the KNNLN ceramics sintered at different temperatures. All samples exhibit a single perovskite structure, while the intensity ratio of each split peak changes obviously. The symmetry of the phase structures for
Fig. 2. XPS analysis of KNNLN sample along the thickness direction.
54
Y. Zhao et al. / Materials Letters 84 (2012) 52–55
Fig. 3. XRD patterns and temperature dependence of relative permittivity of KNNLN samples for different temperatures.
Fig. 4. XRD patterns and temperature dependence of relative permittivity of unpoled and poled KNNLN samples.
the KNN-based ceramics can be distinguished from the intensity ratio of the (0 0 2)/(2 0 0). It is well known that, for the KNNbased systems, there are only two peaks in XRD patterns corresponding to the {2 0 0} planes of either pure orthorhombic phase (a¼c 4b) or pure tetragonal phase (c4a ¼b). [7] There are obvious differences among these XRD patterns, which indicate that a transition from tetragonal to orthorhombic phase occurs in the samples sintered at 980–1080 1C. The similar phase transition in Li modified KNN ceramics synthesized at 1020–1080 1C have been reported by Zhao et al.[8] and they attributed this transition to the compositional change which may be resulted in by the extent of volatilization of alkali metal oxides during high temperature sintering. The other effective method to determine the phase transition of KNN-based ceramics is the temperature dependence of dielectric constant. Fig. 3 also shows the temperature dependence of dielectric constant for KNNLN ceramics measured at 1 kHz. An orthorhombic to tetragonal phase transition, To t, is identified near room temperature by the presence of a maximum dielectric constant between 30 and 80 1C. Meanwhile the peak of the coexistence of two phase’s shifts to a higher temperature, gradually becomes flattened, and disappears with sintering temperature increasing. It comes to the conclusion that the sintering temperature affected the To t and the To t increased with the sintering temperature increasing. This result corresponds well with the room temperature XRD patterns of these samples. Crystalline structure changes were induced by the poling process as has been commonly observed in commercial PZT-based ceramics, where the main rhombohedral phase of as-sintered samples evolved
toward a T phase after poling [9,10]. Detailed studies of phase content with respect to applied electric field has also been carried out on some lead-free systems, e.g., sodium bismuth titanate–potassium sodium niobate and sodium bismuth titanate–barium titanate (NBT-BT) [11]. Meanwhile, the electric-field-induced phase transformation was also found in our work. The XRD patterns and the temperature of the dielectric constant for KNNLN ceramics before and after poling are shown in Fig. 4. The To t transitions of the poled samples undergo changes with respect to those of unpoled samples. The To t transitions of the samples after poling shifted toward the higher temperature compared with those of the samples before poling. It is easy to conclude that the poling process induced a phase transition and favored the appearance of the O (M) phase. An explanation as to the mechanism of the structure transformation from tetragonal to mixed phase (rather than single phase) is as follows. Prior to the application of a poling electric field, the tetragonal domains are randomly orientated. Upon application of the electric field, tetragonal domains with 101-axies (in pseudocubic setting) orientated close to the direction of the electric field transform to orthorhombic, as the free energy for this phase becomes lower than for tetragonal. That is to say an occurrence of deformation induced by electric field. Domains where the 001-axies are orientated parallel to the electric field remain tetragonal, as the free energy for this phase remains lower than for orthorhombic. 4. Conclusion In conclusion, beside that the phase structure of KNN-based ceramics could be affected by the conventional composition variation.
Y. Zhao et al. / Materials Letters 84 (2012) 52–55
The electric field, sintering temperature and volatilization could also induce phase transition in Li-modified KNN ceramics. XPS measurement was employed to certify the difference between the phase structure of the surface and interior of the ceramics.
Acknowledgment This work was supported by National Natural Science Foundation of China.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Guo YP, Kakimoto K, Hitoshi O. Appl PhysLett 2004;85:4121. Zang GZ, et al. Appl Phys Lett 2006;88:212908. Zhang SJ, Xia R, Shrout T. Appl Phys Lett 2007;91:132913. Dai YJ, Zhang XW, Zhou GY. Appl Phys Lett 2007;90:362903. Du J, et al. Chin Phys Lett 2008;25:1446. Zhang SJ, et al. Solid State Commun 2007;141:675. Dai YJ, Zhang XW, Chen KP. Appl Phys Lett 2009;94:042905. Zhao P, Zhang BP, Li JF. Appl Phys Lett 2007;90:342909. Liao JH, Cheng SY, Wang CM. Ferroelectrics 1990;106:357. Liang CK, Wu L, Wu TS. Ferroelectrics 1991;120:185. Daniels JE, Jo W, Rodel J, Jones JL. Acta Mater 2010;58:2103.
55
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Phase structure of Li0.058(Na0.51K0.49)0.942NbO3 lead-free piezoelectric ceramics Yongjie Zhao n, Rongxia Huang n, Rongzheng Liu, Heping Zhou State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 March 2012 Accepted 9 June 2012 Available online 20 June 2012
Several kinds of phase structure change behavior in Li0.058(Na0.51K0.49)0.942NbO3 (KNNLN) ceramics were presented in this work. XPS measurement was employed to determine Na volatilization in the KNNLN samples, and this result explains the difference of the phase structure between the surface and interior of the samples. The phase structure of lead-free KNNLN piezoceramics are studied based on the measurements of the analysis of X-ray diffraction pattern and dielectric properties. The poled samples exhibit orthorhombic structure whereas the surface and interior for unpoled samples exhibit tetragonal and tetragonal-orthorhombic coexistent structures, respectively. An explanation to the mechanism of the structure transformation during poling processing was proposed. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ceramics Ferroelectrics Phase structure XPS
1. Introduction Due to the outstanding piezoelectric properties at the composition of the so-called morphotropic phase boundary (MPB), [1] KNN based piezoceramics have been regarded as a promising lead-free candidate. In recent years more and more researchers have reported that the so-called MPB is actually a structure originating from the polymorphic phase transition (PPT) around room temperature.[2–5] Therefore, the outstanding piezoelectric properties of KNN-based ceramics are thermally unstable owing to the narrow temperature region for the coexistence of the tetragonal and orthorhombic structure.[6] Thus, it can be seen that the phase structure (room temperature) of KNN-based ceramics is of great importance for the practical performance of piezoceramics. Previous studies have found that the phase structure of KNN-based ceramics were sensitive to processing parameters, especially the composition lying in the vicinity of polymorphic phase transition. The more attention should be paid on the research of the conventionally prepared processing. In this work, we present several kinds of phase structure change behavior in KNNLN ceramics and interpret the underlying mechanism of these phase structure changes.
K2CO3 (99.0%), Li2CO3 (99.0%) and Nb2O5 (99.9%). The powder were mixed in a nylon jar with agate balls for 24 h and dried. The dried powders were calcinated at 760 1C for 5 h. After the calcination, the powders re-milled for 24 h. Then the powders were dried and pressed into discs under a pressure of 60 MPa using polyvinyl butyral (PVB) as a binder. After PVB was burnt out, the pellets were, respectively, sintered in air at 980 1C, 1040 1C and 1080 1C for 2 h. Silver paste was fired on both sides of the samples at 600 1C for 20 min to form electrodes for the electrical measurements after polishing. XRD to identify the crystal structures was performed with a Rigaku D/MAX-2500 diffractometer with CuKa1 radiation (l ¼0.15406 nm). The as-sintered sample was used to obtain the surface XRD pattern. Then, the as-sintered samples were polished to get the interior XRD pattern. And the poling depth almost was several hundred micrometers. The temperature dependence of the dielectric constant was measured in a temperature-controllable container which was connected to an Agilent 4284 m (Hewlett–Packard, Palo Alto, CA) at 1 kHz. The X-ray photoemission spectroscopy (XPS) measurement was carried out on a ESCALAB 250Xi scanning X-ray microprobe instrument employing monochromatic Al-Ka radiation as the excitation source.
2. Experimental 3. Results and discussions The ceramics samples with a composition Li0.058(Na0.51K0.49)0.942 NbO3 were prepared using analytical grades of Na2CO3 (99.8%), n
Corresponding authors. Tel./fax: þ86 10 62772549. E-mail addresses: [email protected] (Y. Zhao), [email protected] (R. Huang). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.06.030
Fig. 1 shows the XRD patterns for the surface and interior of the KNNLN ceramics for 1040 1C. All samples exhibit a single perovskite structure, while the surface and interior patterns present tetragonal and tetragonal-orthorhombic polymorphic, respectively. That is, the surface and interior phase structures for the sample are different. We
Y. Zhao et al. / Materials Letters 84 (2012) 52–55
attribute this transition to the compositional change which may be resulted in by the extent of volatilization of alkali metal oxides during sintering process. Although lots of researchers consider that Na2O evaporation results in the above phase structure difference, these are no specific measurement to certify this point. To further confirm the above hypothesis, the X-ray photoemission spectroscopy (XPS)
Fig. 1. XRD patterns for the surface and interior of KNNLN samples for 1040 1C.
53
measurement was employed. XPS measurement is known to be an effective probe for the exact determination of element composition. Fig. 2 shows the XPS analysis of KNNLN sintered at 1040 1C along the thickness direction from the surface to the interior at an interval of 10–100 nm. The location of characteristic peak is closely related with the valence states of the elements. It is evidently observed that the location of Na1s and Nb3d change very little with the thickness direction. However, it is not the same case for K2p. There exists big difference between the K’s valence states of the surface and interior. It considers that this clear difference of the surface and interior is ascribed to two reasons. The first one is the surface charge effect. However, if this case works and the location of Na1s and K2p would also be affected, which is not observed however in Fig. 2. Therefore, the first hypothesis does not come into force. The other reason is the change of the existence form of K element. Meanwhile, the Na content of the sample surface is lower than that of the interior and heavier volatilization of Na on the sample surface should have occurred during sintering process. So we could consider that the secondary phase rich in K element would come into existence on the surface of KNNLN samples. And this consideration could interpret above result of the shift of the location of K2p. The lase figure indicates the atomic percent of Na, K and Nb along the thickness of the sample. Fig. 3 shows the XRD patterns of the KNNLN ceramics sintered at different temperatures. All samples exhibit a single perovskite structure, while the intensity ratio of each split peak changes obviously. The symmetry of the phase structures for
Fig. 2. XPS analysis of KNNLN sample along the thickness direction.
54
Y. Zhao et al. / Materials Letters 84 (2012) 52–55
Fig. 3. XRD patterns and temperature dependence of relative permittivity of KNNLN samples for different temperatures.
Fig. 4. XRD patterns and temperature dependence of relative permittivity of unpoled and poled KNNLN samples.
the KNN-based ceramics can be distinguished from the intensity ratio of the (0 0 2)/(2 0 0). It is well known that, for the KNNbased systems, there are only two peaks in XRD patterns corresponding to the {2 0 0} planes of either pure orthorhombic phase (a¼c 4b) or pure tetragonal phase (c4a ¼b). [7] There are obvious differences among these XRD patterns, which indicate that a transition from tetragonal to orthorhombic phase occurs in the samples sintered at 980–1080 1C. The similar phase transition in Li modified KNN ceramics synthesized at 1020–1080 1C have been reported by Zhao et al.[8] and they attributed this transition to the compositional change which may be resulted in by the extent of volatilization of alkali metal oxides during high temperature sintering. The other effective method to determine the phase transition of KNN-based ceramics is the temperature dependence of dielectric constant. Fig. 3 also shows the temperature dependence of dielectric constant for KNNLN ceramics measured at 1 kHz. An orthorhombic to tetragonal phase transition, To t, is identified near room temperature by the presence of a maximum dielectric constant between 30 and 80 1C. Meanwhile the peak of the coexistence of two phase’s shifts to a higher temperature, gradually becomes flattened, and disappears with sintering temperature increasing. It comes to the conclusion that the sintering temperature affected the To t and the To t increased with the sintering temperature increasing. This result corresponds well with the room temperature XRD patterns of these samples. Crystalline structure changes were induced by the poling process as has been commonly observed in commercial PZT-based ceramics, where the main rhombohedral phase of as-sintered samples evolved
toward a T phase after poling [9,10]. Detailed studies of phase content with respect to applied electric field has also been carried out on some lead-free systems, e.g., sodium bismuth titanate–potassium sodium niobate and sodium bismuth titanate–barium titanate (NBT-BT) [11]. Meanwhile, the electric-field-induced phase transformation was also found in our work. The XRD patterns and the temperature of the dielectric constant for KNNLN ceramics before and after poling are shown in Fig. 4. The To t transitions of the poled samples undergo changes with respect to those of unpoled samples. The To t transitions of the samples after poling shifted toward the higher temperature compared with those of the samples before poling. It is easy to conclude that the poling process induced a phase transition and favored the appearance of the O (M) phase. An explanation as to the mechanism of the structure transformation from tetragonal to mixed phase (rather than single phase) is as follows. Prior to the application of a poling electric field, the tetragonal domains are randomly orientated. Upon application of the electric field, tetragonal domains with 101-axies (in pseudocubic setting) orientated close to the direction of the electric field transform to orthorhombic, as the free energy for this phase becomes lower than for tetragonal. That is to say an occurrence of deformation induced by electric field. Domains where the 001-axies are orientated parallel to the electric field remain tetragonal, as the free energy for this phase remains lower than for orthorhombic. 4. Conclusion In conclusion, beside that the phase structure of KNN-based ceramics could be affected by the conventional composition variation.
Y. Zhao et al. / Materials Letters 84 (2012) 52–55
The electric field, sintering temperature and volatilization could also induce phase transition in Li-modified KNN ceramics. XPS measurement was employed to certify the difference between the phase structure of the surface and interior of the ceramics.
Acknowledgment This work was supported by National Natural Science Foundation of China.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Guo YP, Kakimoto K, Hitoshi O. Appl PhysLett 2004;85:4121. Zang GZ, et al. Appl Phys Lett 2006;88:212908. Zhang SJ, Xia R, Shrout T. Appl Phys Lett 2007;91:132913. Dai YJ, Zhang XW, Zhou GY. Appl Phys Lett 2007;90:362903. Du J, et al. Chin Phys Lett 2008;25:1446. Zhang SJ, et al. Solid State Commun 2007;141:675. Dai YJ, Zhang XW, Chen KP. Appl Phys Lett 2009;94:042905. Zhao P, Zhang BP, Li JF. Appl Phys Lett 2007;90:342909. Liao JH, Cheng SY, Wang CM. Ferroelectrics 1990;106:357. Liang CK, Wu L, Wu TS. Ferroelectrics 1991;120:185. Daniels JE, Jo W, Rodel J, Jones JL. Acta Mater 2010;58:2103.
55