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Effects of doping Eu2O3 on the phase transformation and piezoelectric properties of Na0.5Bi0.5TiO3-based ceramics
Materials Science and Engineering B99 (2003) 449 /452 www.elsevier.com/locate/mseb
Effects of doping Eu2O3 on the phase transformation and piezoelectric properties of Na0.5Bi0.5TiO3-based ceramics Yuanhua Lin *, Shujin Zhao, Ning Cai, Junbo Wu, Xisong Zhou, Ce Wen Nan Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China Received 14 June 2002; received in revised form 20 September 2002
Abstract (Na0.5Bi0.5)(1 1.5x )Eux TiO3 ceramics have been prepared by solid state reaction at 1120 8C for 2 h. Scanning electronic micrograph results indicate that the doped Eu2O3 can prohibit the grain growth. With increasing the doped amount of Eu2O3, the phase transformation (rhombohedral 0/cubic) can be observed in the X-ray diffraction patterns, and correlated with a strong decrease in the piezoelectric properties d33. The dielectric constants of poled and unpoled Na0.5Bi0.5TiO3 (NBT)-0 samples vary greatly as compared to NBT-5 sample due to the change of crystal structure. # 2002 Elsevier Science B.V. All rights reserved. Keywords: (Na0.5Bi0.5)(1 1.5x )Eux TiO3; Phase transformation; Dielectric constant
1. Introduction Sodium bismuth titanate Na0.5Bi0.5TiO3 (NBT) and NBT-based ceramics are the most useful materials for the piezo- and pyro-electric applications. They are perovskite ferroelectrics and were discovered by Smolenskii et al. and have been studied continually by several researchers [1 /4]. The NBT system is of great interest as a new candidate to replace the widely used lead-based perovskite material because of free control atmosphere and lack of lead pollution. Two phase transformations, the cubic 0/tetragonal and tetragonal 0/rhombohedral phase transitions, exist in this system. The cubic /tetragonal transition corresponds to an electric order transformation from a paraelectric state to an antiferroelectric state, whereas the tetragonal /rhombohedral transition corresponds to a transformation from an antiferroelectric state to a ferroelectric state [5]. Soukhojak et al. [6] found recently that the NBT doped with BaTiO3 (BT) has promising piezoelectric
* Corresponding author. Tel.: /86-10-62773741; fax: /86-1062773587. E-mail address: [email protected] (Y. Lin).
properties in single crystal form. The piezoelectric properties appear to be highly dependent on BT concentration, with compositions close to the morphotropic phase boundary (:/6% BT) having the highest piezoelectric coefficients. Elkechai et al. [7] investigated the Na0.5Bi0.5TiO3 /K0.5Bi0.5TiO3 (NBT /KBT) system using X-ray diffraction (XRD) and dielectric measurements, and determined the limits of rhombohedral and orthorhombic solid solutions as well as the evolution of their lattice parameters as a function of composition and temperature. In this work, different (Na0.5Bi0.5)(1 1.5x )Eux TiO3 (x /0, 0.01, 0.02, 0.03, 0.05, 0.10) samples are prepared by the solid state reaction. The morphology, particle size and phase composition of the NBT-based ceramics are fully characterized, and the dielectric properties have been also measured.
2. Experimental procedure To synthesize the NBT-based ceramic compounds, stoichiometric amounts of pure Na2CO3, Eu2O3, Bi2O3, TiO2 as the composition of (Na0.5Bi0.5)(1 1.5x )Eux TiO3 shown in Table 1 were ground thoroughly by ballmilling. The powders were pre-calcined at 900 8C for 2
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00465-8
Y. Lin et al. / Materials Science and Engineering B99 (2003) 449 /452
450 Table 1 The nominal composition of samples Samples
Nominal composition
NBT-0 NBT-1 NBT-2 NBT-3 NBT-4 NBT-5
Na0.5Bi0.5TiO3 (Na0.5Bi0.5)0.985Eu0.01TiO3 (Na0.5Bi0.5)0.97Eu0.02TiO3 (Na0.5Bi0.5)0.955Eu0.03TiO3 (Na0.5Bi0.5)0.925Eu0.05TiO3 (Na0.5Bi0.5)0.85Eu0.1TiO3
h. The compounds were once again thoroughly crushed and then pressed the pre-sintered powder as ¥10 / 1.5 /2.5 mm plates employed polyvinyl alcohol as binder. The samples were finally sintered around 1120 8C for 2 h.
Fig. 2. SEM graphs of different samples sintered at 1120 8C for 2 h. (a) NBT-0 sample (b) NBT-5 sample.
3. Results and discussion XRD patterns shown in Fig. 1 indicate that all peaks are ascribed to the special peaks of NBT phase sintered at 1120 8C for 2 h. The microstructure of NBT-0 and NBT-5 samples are shown in Fig. 2, the results imply that the grain size of doped Eu2O3 sample is smaller than that of undoped NBT sample, which proves that Eu2O3 can act as the grain growth inhibitor and prohibit the further growth of NBT grain. Piezoelectric properties of different doped amount of Eu are shown in Fig. 3. With increasing the doped amount of Eu2O3, the piezoelectric properties d33 increase firstly, and then decrease greatly. As the Aree Herabut’s previous work [8], they observed the phase transformation (rhombohedral 0/pseudocubic 0/cubic) can be observed in the (Na0.5Bi0.5)(1 1.5x )Lax TiO3
Fig. 3. Piezoelectric properties of different doped amount of Eu.
system with increasing the amount of La addition. In fact, besides the sintered temperature, the phase transi-
Fig. 1. XRD patterns of different NBT-based samples sintered at 1120 8C for 2 h.
Y. Lin et al. / Materials Science and Engineering B99 (2003) 449 /452
tion of NBT can also be caused by the composition, which can also result the transformation between the ferroelectric rhombohedral and paraelectric cubic phase. Probably, due to the different valence and ionic radius between Eu3 and the exchangeable sites of Na and Bi3 cations, the lattice distortion in the modified NBT compounds can be expected, which influence the phase composition of NBT directly. As shown in Fig. 4, it can be observed that increasing the doped amount of Eu would produce a paraelectric cubic phase. It’s obviously to observe the splitting of (1 1 1) plane while doped above 0.02 mol. Therefore, the piezoelectric constant d33 for NBT-1 specimen (rhombohedral phase) is more than that of the NBT-5 specimen (cubic phase), which proves that the Eu addition can induce the lattice distortion and changes in phase composition. With further increasing the amount of Eu addition, the cubic phase of NBT grow, and the piezoelectric properties diminish gradually as shown in Fig. 3. In addition, the photoluminescent properties of Eu ions in the NBT-based ceramics have been measured. The results shown in Fig. 5 indicate the main emission peaks are different each other, especially to the NBT-0 sample and NBT-5 sample. It’s well known that the peak positions in the emission spectra depend strongly on the nature of the Eu3 surroundings, and therefore, Eu3 ions can emit different visible lights in the various crystal fields [9]. Due to the different crystal structure of NBT-0 (rhombohedral) and NBT-5 (cubic) samples, the difference resulted the various emission peaks. It is also proved that the phase transformation can be existed in the (Na0.5Bi0.5)(1 1.5x )Eux TiO3 system, which resulted in the deteriorate of piezoelectric properties of NBTbased ceramics. The frequency dependence of dielectric constant of different NBT-based ceramics has been measured shown in Fig. 6. The results imply that the dielectric constant of NBT-0 sample vary greatly between before and after
451
Fig. 5. Emission spectra of different samples.
poling. With increasing the doped amount of Eu2O3, this difference decreases and almost same for the NBT-5 sample. The reason resulted this phenomenon is unknown, which could be correlated with the phase transformation of NBT-based ceramics.
4. Conclusion (Na0.5Bi0.5)(1 1.5x )Eux TiO3 ceramics have been prepared by solid state reaction at 1120 8C for 2 h. The phase transformation (rhombohedral 0/cubic) can be observed in the (Na0.5Bi0.5)(1 1.5x )Eux TiO3 system while increasing the doped amount of Eu2O3, which resulted the piezoelectric properties d33 decrease greatly. The dielectric constant of NBT-0 sample is great different between before and after poling as compared to NBT-5 sample. The Eu ions emission peaks are varied between NBT-2 and NBT-5 samples due to the changes of crystal structure of NBT.
Fig. 4. XRD patterns of different samples after poling at 2u between 38 and 428. (a) NBT-0 sample (b) NBT-2 sample (c) NBT-5 sample.
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Y. Lin et al. / Materials Science and Engineering B99 (2003) 449 /452
Fig. 6. Frequency dependence of dielectric constant of different samples.
References [1] G.A. Smolenskii, V.A. Isupv, A.I. Afranovskaya, N.N. Kainik, Soviet Journal of Physics and Solid State 2 (11) (1961) 2651 / 2654. [2] T.V. Kruzina, V.M. Duda, J. Suchanicz, Materials Science and Engineering B 87 (1) (2001) 48 /52. [3] I.G. Siny, E. Husson, J.M. Beny, S.G. Lushnikov, E.A. Rogacheva, P.P. Syrnikov, Physica B 293 (3 /4) (2001) 382 /389. [4] J. Suchanicz, M.G. Gavshin, A.Y. Kudzin, C. Kus, Journal of Materials Science 36 (8) (2001) 1981 /1985.
[5] S.E. Park, S.J. Chung, Journal of American Ceramic Society 79 (5) (1996) 1290 /1296. [6] A.N. Soukhojak, H. Wang, G.W. Farrey, Y.M. Chiang, Journal of Physics and Chemistry of Solids 61 (2) (1999) 301 /304. [7] O. Elkechai, M. Manier, J.P. Mercurio, Physica Status Solidi (A) Applied Research 157 (2) (1996) 499 /506. [8] H. Aree, S. Ahmad, Journal of American Ceramic Society 80 (11) (1997) 2954 /2958. [9] L. Boyer, B. Piriou, J. Carpena, J.L. Lacout, Journal of Alloys and Compounds 311 (2) (2000) 143 /152.
Effects of doping Eu2O3 on the phase transformation and piezoelectric properties of Na0.5Bi0.5TiO3-based ceramics Yuanhua Lin *, Shujin Zhao, Ning Cai, Junbo Wu, Xisong Zhou, Ce Wen Nan Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China Received 14 June 2002; received in revised form 20 September 2002
Abstract (Na0.5Bi0.5)(1 1.5x )Eux TiO3 ceramics have been prepared by solid state reaction at 1120 8C for 2 h. Scanning electronic micrograph results indicate that the doped Eu2O3 can prohibit the grain growth. With increasing the doped amount of Eu2O3, the phase transformation (rhombohedral 0/cubic) can be observed in the X-ray diffraction patterns, and correlated with a strong decrease in the piezoelectric properties d33. The dielectric constants of poled and unpoled Na0.5Bi0.5TiO3 (NBT)-0 samples vary greatly as compared to NBT-5 sample due to the change of crystal structure. # 2002 Elsevier Science B.V. All rights reserved. Keywords: (Na0.5Bi0.5)(1 1.5x )Eux TiO3; Phase transformation; Dielectric constant
1. Introduction Sodium bismuth titanate Na0.5Bi0.5TiO3 (NBT) and NBT-based ceramics are the most useful materials for the piezo- and pyro-electric applications. They are perovskite ferroelectrics and were discovered by Smolenskii et al. and have been studied continually by several researchers [1 /4]. The NBT system is of great interest as a new candidate to replace the widely used lead-based perovskite material because of free control atmosphere and lack of lead pollution. Two phase transformations, the cubic 0/tetragonal and tetragonal 0/rhombohedral phase transitions, exist in this system. The cubic /tetragonal transition corresponds to an electric order transformation from a paraelectric state to an antiferroelectric state, whereas the tetragonal /rhombohedral transition corresponds to a transformation from an antiferroelectric state to a ferroelectric state [5]. Soukhojak et al. [6] found recently that the NBT doped with BaTiO3 (BT) has promising piezoelectric
* Corresponding author. Tel.: /86-10-62773741; fax: /86-1062773587. E-mail address: [email protected] (Y. Lin).
properties in single crystal form. The piezoelectric properties appear to be highly dependent on BT concentration, with compositions close to the morphotropic phase boundary (:/6% BT) having the highest piezoelectric coefficients. Elkechai et al. [7] investigated the Na0.5Bi0.5TiO3 /K0.5Bi0.5TiO3 (NBT /KBT) system using X-ray diffraction (XRD) and dielectric measurements, and determined the limits of rhombohedral and orthorhombic solid solutions as well as the evolution of their lattice parameters as a function of composition and temperature. In this work, different (Na0.5Bi0.5)(1 1.5x )Eux TiO3 (x /0, 0.01, 0.02, 0.03, 0.05, 0.10) samples are prepared by the solid state reaction. The morphology, particle size and phase composition of the NBT-based ceramics are fully characterized, and the dielectric properties have been also measured.
2. Experimental procedure To synthesize the NBT-based ceramic compounds, stoichiometric amounts of pure Na2CO3, Eu2O3, Bi2O3, TiO2 as the composition of (Na0.5Bi0.5)(1 1.5x )Eux TiO3 shown in Table 1 were ground thoroughly by ballmilling. The powders were pre-calcined at 900 8C for 2
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00465-8
Y. Lin et al. / Materials Science and Engineering B99 (2003) 449 /452
450 Table 1 The nominal composition of samples Samples
Nominal composition
NBT-0 NBT-1 NBT-2 NBT-3 NBT-4 NBT-5
Na0.5Bi0.5TiO3 (Na0.5Bi0.5)0.985Eu0.01TiO3 (Na0.5Bi0.5)0.97Eu0.02TiO3 (Na0.5Bi0.5)0.955Eu0.03TiO3 (Na0.5Bi0.5)0.925Eu0.05TiO3 (Na0.5Bi0.5)0.85Eu0.1TiO3
h. The compounds were once again thoroughly crushed and then pressed the pre-sintered powder as ¥10 / 1.5 /2.5 mm plates employed polyvinyl alcohol as binder. The samples were finally sintered around 1120 8C for 2 h.
Fig. 2. SEM graphs of different samples sintered at 1120 8C for 2 h. (a) NBT-0 sample (b) NBT-5 sample.
3. Results and discussion XRD patterns shown in Fig. 1 indicate that all peaks are ascribed to the special peaks of NBT phase sintered at 1120 8C for 2 h. The microstructure of NBT-0 and NBT-5 samples are shown in Fig. 2, the results imply that the grain size of doped Eu2O3 sample is smaller than that of undoped NBT sample, which proves that Eu2O3 can act as the grain growth inhibitor and prohibit the further growth of NBT grain. Piezoelectric properties of different doped amount of Eu are shown in Fig. 3. With increasing the doped amount of Eu2O3, the piezoelectric properties d33 increase firstly, and then decrease greatly. As the Aree Herabut’s previous work [8], they observed the phase transformation (rhombohedral 0/pseudocubic 0/cubic) can be observed in the (Na0.5Bi0.5)(1 1.5x )Lax TiO3
Fig. 3. Piezoelectric properties of different doped amount of Eu.
system with increasing the amount of La addition. In fact, besides the sintered temperature, the phase transi-
Fig. 1. XRD patterns of different NBT-based samples sintered at 1120 8C for 2 h.
Y. Lin et al. / Materials Science and Engineering B99 (2003) 449 /452
tion of NBT can also be caused by the composition, which can also result the transformation between the ferroelectric rhombohedral and paraelectric cubic phase. Probably, due to the different valence and ionic radius between Eu3 and the exchangeable sites of Na and Bi3 cations, the lattice distortion in the modified NBT compounds can be expected, which influence the phase composition of NBT directly. As shown in Fig. 4, it can be observed that increasing the doped amount of Eu would produce a paraelectric cubic phase. It’s obviously to observe the splitting of (1 1 1) plane while doped above 0.02 mol. Therefore, the piezoelectric constant d33 for NBT-1 specimen (rhombohedral phase) is more than that of the NBT-5 specimen (cubic phase), which proves that the Eu addition can induce the lattice distortion and changes in phase composition. With further increasing the amount of Eu addition, the cubic phase of NBT grow, and the piezoelectric properties diminish gradually as shown in Fig. 3. In addition, the photoluminescent properties of Eu ions in the NBT-based ceramics have been measured. The results shown in Fig. 5 indicate the main emission peaks are different each other, especially to the NBT-0 sample and NBT-5 sample. It’s well known that the peak positions in the emission spectra depend strongly on the nature of the Eu3 surroundings, and therefore, Eu3 ions can emit different visible lights in the various crystal fields [9]. Due to the different crystal structure of NBT-0 (rhombohedral) and NBT-5 (cubic) samples, the difference resulted the various emission peaks. It is also proved that the phase transformation can be existed in the (Na0.5Bi0.5)(1 1.5x )Eux TiO3 system, which resulted in the deteriorate of piezoelectric properties of NBTbased ceramics. The frequency dependence of dielectric constant of different NBT-based ceramics has been measured shown in Fig. 6. The results imply that the dielectric constant of NBT-0 sample vary greatly between before and after
451
Fig. 5. Emission spectra of different samples.
poling. With increasing the doped amount of Eu2O3, this difference decreases and almost same for the NBT-5 sample. The reason resulted this phenomenon is unknown, which could be correlated with the phase transformation of NBT-based ceramics.
4. Conclusion (Na0.5Bi0.5)(1 1.5x )Eux TiO3 ceramics have been prepared by solid state reaction at 1120 8C for 2 h. The phase transformation (rhombohedral 0/cubic) can be observed in the (Na0.5Bi0.5)(1 1.5x )Eux TiO3 system while increasing the doped amount of Eu2O3, which resulted the piezoelectric properties d33 decrease greatly. The dielectric constant of NBT-0 sample is great different between before and after poling as compared to NBT-5 sample. The Eu ions emission peaks are varied between NBT-2 and NBT-5 samples due to the changes of crystal structure of NBT.
Fig. 4. XRD patterns of different samples after poling at 2u between 38 and 428. (a) NBT-0 sample (b) NBT-2 sample (c) NBT-5 sample.
452
Y. Lin et al. / Materials Science and Engineering B99 (2003) 449 /452
Fig. 6. Frequency dependence of dielectric constant of different samples.
References [1] G.A. Smolenskii, V.A. Isupv, A.I. Afranovskaya, N.N. Kainik, Soviet Journal of Physics and Solid State 2 (11) (1961) 2651 / 2654. [2] T.V. Kruzina, V.M. Duda, J. Suchanicz, Materials Science and Engineering B 87 (1) (2001) 48 /52. [3] I.G. Siny, E. Husson, J.M. Beny, S.G. Lushnikov, E.A. Rogacheva, P.P. Syrnikov, Physica B 293 (3 /4) (2001) 382 /389. [4] J. Suchanicz, M.G. Gavshin, A.Y. Kudzin, C. Kus, Journal of Materials Science 36 (8) (2001) 1981 /1985.
[5] S.E. Park, S.J. Chung, Journal of American Ceramic Society 79 (5) (1996) 1290 /1296. [6] A.N. Soukhojak, H. Wang, G.W. Farrey, Y.M. Chiang, Journal of Physics and Chemistry of Solids 61 (2) (1999) 301 /304. [7] O. Elkechai, M. Manier, J.P. Mercurio, Physica Status Solidi (A) Applied Research 157 (2) (1996) 499 /506. [8] H. Aree, S. Ahmad, Journal of American Ceramic Society 80 (11) (1997) 2954 /2958. [9] L. Boyer, B. Piriou, J. Carpena, J.L. Lacout, Journal of Alloys and Compounds 311 (2) (2000) 143 /152.