The Characteristics of (1-x) (Na 0.5 Bi 0.5 )TiO 3 -x BaTiO 3 Ceramics

Transcription

1 The Characteristics of (1-x) (Na 0.5 Bi 0.5 )TiO 3 -x BaTiO 3 Ceramics Chia-Cheng Huang 1, Fang-Hsing Wang 1, Chien-Chen Diao 2, Cheng-Fu Yang 3 *, and Chia-Ching Wu 4 1 Department of Electrical Engineering, National Chung Hsing University, Taiwan, R.O.C. 2 Department of Electronic Engineering, Kao Yuan University, Kaohsiung, Taiwan, R.O.C. 3 Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung, Taiwan, R.O.C. *Corresponding Author: cfyang@nuk.edu.tw 4 Department of Electrical Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan, R.O.C. Abstract The influences of sintering temperatures on the sintering and dielectric characteristics of (1-x) (Na 0.5 Bi 0.5 )TiO 3 -x BaTiO 3 ((1-x) NBT-x BT3, x= 0.03, 0.05 and 0.07) ferroelectric ceramics are developed in this study. The sintering process is carried out in air for 2h from 1120 o C to 1240 o C. From the surface observations, the grain sizes increase with the increase of sintering temperatures and decrease with the increase of BaTiO 3 content. The dielectric-temperature properties are investigated from 30 o C to 350 o C at 10 khz, 100 khz and 1 MHz. The Curie temperatures of (1-x) NBT-x BT3 ceramics slightly increase with the increase of sintering temperature and decrease with the increase of BaTiO 3 content. As the sintering temperatures are higher than 1160 o C, the maximum dielectric constant of (1-x) NBT-x BT3 ceramics increase with the increase of BaTiO 3 content and are almost unchanged as the measured frequencies increase. Keywords: piezoelectric ceramics, (1-x) (Na 0.5 Bi 0.5 )TiO 3 -x BaTiO 3 ceramics, Curie temperatures Introduction Although Pb(Ti,Zr)O 3 -based ceramics were playing a dominant role in piezoelectric materials, the waste of Pb during fabricating processes had caused a crucial environmental problem under ground water. The rhombohedral perovskite ABO 3 ferroelectric (Na 0.5 Bi 0.5 )TiO 3 (abbreviated as NBT) ceramic was discovered by Smolenskii et al. in 1960 [1]. It was considered to be a good candidate for lead-free piezoelectric ceramics because of the strong ferroelectricity at room temperature and the higher Curie temperature (T C ) of 320 C [2-3]. However, their electromechnical properties of NBT ceramic were much lower than those of Pb(Ti,Zr)O 3 -based piezoelectric ceramics and there was also some difficulty in poling NBT ceramic. To improve its properties, some modified compositions with the ABO 3 structure were added into the NBT composition and their dielectric and electrical characteristics had been well performed. For example, NBT-based compositions modified with K 0.5 Bi 0.5 TiO 3 and NaNbO 3 showed improved piezoelectric properties and ease of poling as compared with the pure NBT ceramic [4-5]. It also had been reported that NBT-based compositions modified with BaTiO 3 (BT3) showed improved piezoelectric properties and easier treatment in the poling process compared with pure NBT ceramic [6]. For that, the (Na 0.5 Bi 0.5 )TiO 3 -BaTiO 3 compositions were also developed as the piezoelectric materials in order to replace the Pb(Ti,Zr)O 3 -based piezoelectric ceramics [6]. In the past, we had developed the BaTiO 3 -based ceramics to be used as the capacitor materials. The developed BaTiO 3 -based ceramics were calcined to form the ABO 3 phase and then the addition of sintering aids to improve the sintering characteristics [7-8]. In this study, we were interested to investigate the materials based on (1-x) NBT-x BT3 ceramics to be used as the high dielectric constant capacitor materials, where x=0.03, 0.05, and 0.07, respectively. After the compositions were formatted, the (1-x) NBT-x BT3 ceramics were sintered at different temperatures and their crystal structures and grain growth were examined by X-ray diffraction (XRD) patterns and scanning electronic microscope (SEM). The relative dielectric characteristics were also investigated as a function of sintering temperatures, measured temperatures and measured frequencies. Experimental Procedure The ceramic materials of (1-x) (Na 0.5 Bi 0.5 )TiO 3 -x BaTiO 3 were prepared by the solid state reaction method. Reagent-grade raw materials of Na 2 CO 3, Bi 2 O 3, BaCO 3 and TiO 2 with purity higher than 99.5% were used as starting materials, mixed according to the compositions of (1-x) (Na 0.5 Bi 0.5 )TiO 3 -x BaTiO 3, where x=0.03, 0.05, and 0.07, respectively. After drying and grinding, the (1-x) NBT-x BT3 powders were calcined at 900 o C for 2h. After grinding

2 again, the polyvinylalcohol (PVA) was added into the powders as a binder. Then the mixing powders were uniaxially pressed into pellets in a steel die with the size of 1mm in thickness and 12mm in diameter. After debindering, the sintering of (1-x) NBT-x BT3 ceramics was carried out from 1120 o C to 1240 o C for 4h. The morphologies of sintered ceramics were observed using SEM and the crystal structures were also investigated using XRD patterns. The sintered ceramics were painted with Ag-Pd paste and sintered at 700 o C for 15min. Temperature-dependent dielectric characteristics were measured with oscillating amplitude (50mV) at 10 khz~1 MHz using an impedance analyzer, HP4294, by putting the sintered ceramics in a temperature-programmable testing chamber. Results and Discussion The XRD patterns of (1-x) NBT-x BT3 ceramics are revealed in Fig.1 as a function of sintering temperatures. Using 1120 o C and 1200 o C as the sintering temperatures, no unknown or satellite phases are observed and only the ABO 3 phase is revealed in the (1-x) NBT-x BT3 ceramics. The crystal peaks revealed in the XRD patterns of (1-x) NBT-x BT3 ceramics are similar with those of NBT ceramic. However, as the XRD patterns shown in Fig.2 are compared, the different sintering temperature and different BaTiO 3 content will lead the (1-x) NBT-x BT3 ceramics to have different results. For 1200 o C-sintered ceramics, the crystal intensity of (200) plane decreases with the increase of BaTiO 3 content. The crystal intensities of other planes have no apparent variation as the sintering temperatures and BaTiO 3 content increase. Sintered at 1120 o C, the 2θ values of main crystal peak have no apparent shift as the BaTiO 3 content increase. Using 1200 o C as the sintering temperature, the crystal peaks are apparently shifted to lower 2θ values as the BaTiO 3 content increases. From the XRD standard patterns of JCPD card, the a-axis of unit cell of BaTiO 3 ceramic (in the cubic structure) is larger than that of (Na 0.5 Bi 0.5 )TiO 3 ceramic. This result suggests that the lattice constants of (1-x) NBT-x BT3 ceramics will increase with the increase of BaTiO 3 content, and the shift of 2θ values of (1-x) NBT-x BT3 ceramics in Fig.2 have proved that. Compared the XRD patterns of 1120 o C- and 1200 o C- sintered ceramics, the main peak, (110) plane, of 1200 o C-sintered ceramics have sharper full width at half maximum (FWHM) values than that of 1120 o C-sintered ceramic does. The wider FWHM values in the 1120 o C-sintered ceramics may be caused by that the BT3 and NBT compositions in the calcined 0.93 NBT-0.07 BT3 powder has not formed the solid solution of (Na 0.5 Bi 0.5 )TiO 3 -BaTiO 3, completely. Sintered at 1200 o C, the BT3 and NBT compositions will form the solid solution and only one ABO 3 phase is revealed. The surface observations of (1-x) NBT-x BT3 ceramics are investigated and the results are shown in Fig.2. Sintered at 1120 C, the (1-x) NBT-x BT3 ceramics show a porous structure and the grain growth is not observed. Sintering 0.97 NBT-0.03 BT3 ceramic at 1160 o C, as shown in Fig.2(a), the grain growth is observed and almost no pores are observed. Sintering 0.93 NBT-0.07 BT3 ceramic at 1160 o C, as shown in Fig.2(b), the grain growth is not apparently observed and the pores are observed. The needed sintering temperature (1350 o C~1400 o C) of BT3 ceramic is higher than that (1150 o C~ 1200 o C) of NBT ceramic, for that the increase of BaTiO 3 content will increase the needed sintering temperature. Further increasing the sintering temperature to 1200 o C, the grain size of 0.97 NBT-0.03 BT3 ceramic increases apparently and the 0.93 NBT-0.07 BT3 ceramic reveals a normal grain growth, as shown in Figs. 2(c) ~2(d). Fig. 2 suggests that the BaTiO 3 content has large influence on sintering characteristics of (1-x) NBT-x BT3 ceramics, and furthermore, the BaTiO 3 content will have large influences on dielectric characteristics of (1-x) NBT-x BT3 ceramics. Figure 3(a) shows the Curie temperatures (Tc, the temperatures to reveal the maximum dielectric constants) of (1-x) NBT-x BT3 ceramics. As Fig.3(a) shows, the Tc values increase with the increase of sintering temperatures and decrease with the increase of BaTiO 3 content. And the Tc values will reach a saturation value as the sintering temperatures are higher than 1160 o C ~1200 o C. The Tc values of (1-x) NBT-x BT3 ceramics can be predicted by Eq.(1): Tc = (1-x) * x * 120 (1) Where the 320 o C and 120 o C are the Curie temperatures of (Na 0.5 Bi 0.5 )TiO 3 and BaTiO 3 ceramics, and Tc is the predicted Curie temperature. The predicted Tc values of (1-x) NBT-x BT3 ceramics for x=0.03, 0.05, and 0.07 are 314 o C, 310 o C, and 306 o C, respectively. As Fig.4 shows, the measured Tc values of (1-x) NBT-x BT3 ceramics are lower than those of predicted values. Fig.3(b) shows the Curie temperatures of 1200 o C-sntered (1-x) NBT-x BT3 ceramics as a function of measured frequencies. As the measured frequencies increase from 10 khz to 1 MHz, the Curie temperatures are unchanged. As the results shown in Figs.1 and 3, that the higher sintering temperatures cause the solid reaction of NBT and BT3 compositions and improve the grain growth of (1-x) NBT-x BT3 ceramics will be the reason. The maximum dielectric constants (ε max, revealed at Curie temperature) of (1-x) NBT-x BT3 ceramics are shown in Fig.4(a) as a function of sintering temperatures, and the measured frequency is 1MHz. Sintered at 1120 o C, the ε max values decrease with the increase of BaTiO 3 content, it is caused by that the pores increase with the increase of BaTiO 3 content. The ε max values critically increase as the sintering temperatures increase from 1120 o C to 1160 o C and then the ε max values slightly increase as the sintering temperatures are further increased. From the SEM micrographs in Fig.2, the pores eliminated and the grain growth improved are the reasons. (1-x) NBT-x BT3 ceramics had a morphotropic phase boundary (MPB) point existing around 0.95 NBT-0.05 BT3 ceramic, which would reveal a larger dielectric constant. In this research, as the sintering temperatures are higher than 1160 o C, the ε max values increase with the

3 increase of BaTiO 3 content. For that, the grain sizes and the MPB point are not the important factors that will influence the ε max values of (1-x) NBT-x BT3 ceramics, the BaTiO 3 content will the most important factor that will influence the ε max values. Fig. 4(b) shows the ε max values of 1200 o C-sntered (1-x) NBT-x BT3 ceramics as a function of measured frequencies. For the most ferroelectric dielectric, the Tc values would shift to higher values and the ε max values would critically decrease as the measured frequencies increase [9, 10]. However, as the sintering temperatures are higher than 1160 o C, the (1-x) NBT-x BT3 ceramics will reveal the stable dielectric characteristics in the range of 10 khz to 1 MHz. For that the (1-x) NBT-x BT3 ceramics can be developed as a dielectric for the wide- frequency-used capacitor. The dielectric constants-temperature (ε r -T) curves of 1200 o C-sintered 0.93 NBT BT3 ceramics are investigated at 10kHz~1MHz, and the results are shown in Fig.5. For all (1-x) NBT-x BT3 ceramics, the ε r -T curves will show the ferroelectric characteristics rather than the relaxor characteristics. Because the ε r values of all (1-x) NBT-x BT3 ceramics first increase with the increasing temperature, reach a maximum at Curie temperature, and then decrease apparently. However, (1-x) NBT-x BT3 ceramics have diffusion dielectric constant-temperature effect because the Curie temperatures are not apparently observed and the temperature ranges to reveal the maximum dielectric constant have been broaden. Conclusions In this study, the sintering and dielectric characteristics of (1-x) (Na 0.5 Bi 0.5 )TiO 3 -x BaTiO 3 (x= 0.03, 0.05, and 0.07) ceramics are well developed. The Curie temperatures of (1-x) NBT-x BT3 ceramics are unchanged or slightly increase with the increase of sintering temperatures, and the measured Curie temperatures are lower than predicted values. As the sintering temperatures of (1-x) NBT-x BT3 ceramics are higher than 1160 o C, even the grain sizes decrease with the increase of BaTiO 3 content, the maximum dielectric constants increase with the increase of BaTiO 3 content. For all (1-x) NBT-x BT3 ceramics, the ε r -T curves will show the ferroelectric characteristics rather than the relaxor characteristics. Acknowledgment The authors will acknowledge to the financial support NSC E References [1] G.A. Smolenskii, V.A. Isupov, A.I. Agranovskaya, N.N. Krainik, Sov. Phys. Solid State 2 (1961) [2] J. Suchanicz, K. Roleder, A. Kania, and J. Handerek, Ferroelectrics 77 (1988) 107. [3] M. S. Hagiyev, I. H. Ismaizade, and A. K. Abiyev, Ferroelectrics 56 (1984) 215. [4] Y.M. Li, W. Chen, Q.Xu, J.Zhou, H.J. Sun, and M.S. Liao, J. Electroceramics 14 (2005) 53. [5] T. Takenaka, T. Okuda, K. Takegahara, Ferroelectrics 196 (1997) 175. [6] T. Takenaka, K. Maruyama, K. Sakata, Jpn. J. Appl. Phys. 30 (9B) (1991) [7] C.F.Yang, Jpn.J.Appl.Phys.35 (1996) [8] C.F. Yang, L. Wu, and T.S. Wu, J. Mater. Sci. 11 (1992) [9] C.H Chung, C.F. Yang, Y.C. Chung, and C.Y. Kung, Key Eng. Mater (2007) 188. [10] C.C. Chan, C.C. Su, C.C. Diao, and C.F. Yang, Key Eng. Mater (2007) 62.

4 0.07, 1200 o C 0.05, 1200 o C Intensity (a.u.) 0.03, 1200 o C 0.07, 1120 o C 0.05, 1120 o C (100) (110) (111) (200) 0.03, 1120 o C (210) (211) θ value Fig.1 XRD patterns of (1-x) NBT-x BT3 ceramics as a function of sintering temperatures. (a) (b) (c) (d) Fig.2 The micrographs of (1-x) NBT-x BT3 ceramics. (a) x=0.03, 1160 o C, (b) x=0.07, 1160 o C, (c) x=0.03, 1200 o C, (d) x=0.07, 1200 o C.

5 Curie temperature ( o C) Curie temperature ( o C) (a) Sintering temperature( o C) (b) Log (f) Fig.3 The Tc values of (1-x) NBT-x BT3 ceramics, as a function of (a) sintering temperatures and (b) frequencies (sintered at 1200 o C).

6 maximum dielectric constant (a) Sintering temperature( o C) maximum dielectric constant (b) Log (f) Fig.4 The ε max of (1-x) NBT-x BT ceramic, as a function of (a) sintering temperatures and (b) frequencies (sintered at 1200 o C). Dielectric constant kHz o 100kHz x 1MHz Measured temperature ( o C) Fig.5 The ε r -T curves of 0.93 (Na 0.5 Bi 0.5 )TiO BaTiO 3 ceramics, sintering temperature is 1200 o C.