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Synthesis and piezoelectric properties of Li-doped BaTiO3 by a solvothermal approach
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Journal of the European Ceramic Society 33 (2013) 1009–1015
Synthesis and piezoelectric properties of Li-doped BaTiO3 by a solvothermal approach Takeshi Kimura a,∗, Qiang Dong a, Shu Yin a, Takatoshi Hashimoto b, Atsushi Sasaki b, Tsugio Sato a a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan b NEC Tokin Corporation, 6-7-1 Koriyama, Taihaku-ku, Sendai 980-8510, Japan Received 16 April 2012; received in revised form 6 November 2012; accepted 8 November 2012 Available online 7 December 2012
Abstract Li-doped BaTiO3 particles with the Li+ mole fraction, x, of 0–0.06 were synthesized by a solvothermal approach at 200 ◦ C. The products consisted of nanoparticles of 50–100 nm in diameter. The sinterability and piezoelectric property of Li-doped BaTiO3 were improved by doping with Li ion, i.e., the Li-doped BaTiO3 samples could be sintered to almost full theoretical density (>95%) at a low temperature such as 1100 ◦ C, and the highest piezoelectric constant, d33 (260 pC/N) and electromechanical coupling factor, kp (43.7%) could be realized at x value of 0.03. The Curie temperatures of all samples were around 130 ◦ C, and did not change very much depending on the amount of Li-doping. © 2012 Elsevier Ltd. All rights reserved. Keywords: Solvothermal approach; Perovskite; Piezoelectricity; Sintering; Ferroelectric properties
1. Introduction Lead-based perovskite solid solution of Pb(Zr,Ti)O3 (PZT) have been widely used for piezoelectric devices due to their large piezoelectric constants at the relatively high temperatures.1,2 Nowadays, because of the toxicity of lead oxide, it is desired to use lead-free piezoelectric materials in place of PZT for environmental protection, therefore, there is an increasing interest of investigating lead-free piezoelectric materials. BaTiO3 has become one of the most important ferroelectric materials used in the electronics ceramic industry. However, the piezoelectric properties of BaTiO3 were insufficient compared with PZT. Recently, it was reported that the addition of Li2 CO3 as a sintering additive, could improve both the sinterability and the piezoelectric properties, where Li+ was doped into the lattice of BaTiO3 during the sintering.3 Usually, BaTiO3 -based ceramics are fabricated by the conventional solid-state method. Recently, considerable research efforts have been devoted to the preparation of materials by various wet chemical methods, such as solvothermal process,4 citrate method,5 emulsion method,6 and composite-hydroxidemediated method.7 It was found that piezoelectric ceramics
∗
Corresponding author. Tel.: +81 22 217 5599; fax: +81 22 217 5598. E-mail address: [email protected] (T. Kimura).
0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.11.007
made from powders synthesized by alternative methods exhibit improved sinterability, poling property and piezoelectric performance, because of their higher homogeneity of chemical composition with well dispersed fine particles8 and/or unique morphologies such as plate-like9 and cube-like shape.10 In the present study, Li-doped BaTiO3 ceramics with x = 0–0.06 were produced by the hydrothermal process, and their piezoelectrical properties were examined. 2. Experimental 2.1. Synthesis Li-doped BaTiO3 particles with x = 0–0.06 were synthesized by a solvothermal approach. Ba(OH)2 ·8H2 O, Ti(i-PrO)4 and Li2 CO3 were used as starting materials. Firstly, 40 mmol Ti(iPrO)4 was dissolved in 30 ml isopropanol, and then 40 mmol Ba(OH)2 ·8H2 O, desired amount of Li2 CO3 and 17 ml NH3 aqueous solution (6.5 M) were added to precipitate amorphous gel. Then, the slurry solution was introduced into a Teflon® -lined stainless autoclave with a 100 cm3 of internal volume together with 10 Teflon® balls of 11 mm in diameter. The autoclave was sealed and heated at 200 ◦ C for 12 h in an electric oven, where the autoclave was rotated at 100 rpm during the reaction using a rotation type hydrothermal reaction apparatus (Fig. 1). After that, the autoclave was taken out to allow cooling to room
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Fig. 1. Hydrothermal apparatus with ball milling system.
temperature. After removing the solution using a rotary evaporator, the precipitates were calcined at 600 ◦ C for 1 h. 2.2. Characterization The particle morphology was observed by a transmission electron microcopy (JEOL, TEM-2000EX). The X-ray diffraction (XRD) analysis of the obtained powder samples and sintered ceramics was carried out using Cu K␣ radiation with a pyrolytic graphite monochromater mounted powder diffractometer (Bruker, D2 PHASER). The sample powders were uniaxially pressed at 20 MPa in a steel die to form pellets of 20 mm in diameter and 3 mm in thickness, and then isostatically pressed at 200 MPa. The pellets were sintered at 1000–1300 ◦ C for 2 h. The fracture surfaces of the sintered bodies were observed by a scanning electron microscope (SEM, Hitachi S-4800). The densities of the sintered pellets were measured by the Archimedes’ method. The lattice parameters were evaluated by X-ray diffraction analysis using the Rietveld refinement with the RIETAN-FP program.11 Measurements for Rietveld refinements were made using a tube power of 40 kV and 30 mA over the 2θ range of 10–130◦ with a 0.02◦ 2θ step size, an around 0.3 s count time and the maximum intensity of
XRD patterns was set to 10,000 counts (Bruker, D2 PHASER). The chemical compositions of the ceramics were determined by inductively coupled plasma-atomic emission spectroscopy (ICP, Perkin Elmer, Optima 3300XL) after dissolving the 0.5 mg samples in 50 ml mixed aqueous solutions (2.0 mol/l HCl and 3.1 wt% H2 O2 ) at 25 ◦ C for 5 min. The elemental TOF-SIMS map of fracture surfaces of the sintered bodies were observed by a Time of Flight Secondary Ion Mass Spectrometer (ION-TOF, TOF-SIMS 5-100). The upper and bottom surfaces of specimens were coated with gold paint and fired at 800 ◦ C for 20 min for the electrical measurements. Prior to piezoelectric measurements, the sintered bodies were shaped as a disk of size 15φ × 1.0 mm for the Qm and kp determination, then the poling treatment was performed in a silicon oil bath at 80 ◦ C by applying a DC electric field of 3.0 kV/mm for 30 min (Withstanding Voltage Tester, TOS 5101). The piezoelectric constant, d33 , of the samples was measured by means of a quasi-static d33 meter (IACAS, ZJ-3BN) based on the Berlincourt method at 110 Hz. The dielectric properties and piezoelectric properties were determined using an Agilent 4294A precision impedance analyzer. 3. Results and discussion Fig. 2 shows the X-ray diffraction patterns of the sample powders before and after calcination at 600 ◦ C for 1 h. The samples both before and after calcinations consisted of the single phase of BaTiO3 (PDF#01-072-0138) structure, and no impurity peak could be found. In Fig. 2, the lattice volumes V of calcination samples are plotted as a function of Li content. It can be seen that the lattice volume of calcined samples slightly increased by an increase in doping concentration of Li+ , indicating the incorporation of Li ions in the lattice of BaTiO3 . Fig. 3 shows the TEM images of as-prepared powders. The Li-doped BaTiO3 samples consisted of the well dispersed
Fig. 2. XRD patterns of undoped BaTiO3 powder before calcination and Li-doped BaTiO3 powders with the Li molar fractions of x = 0–0.06 after calcinations at 600 ◦ C for 1 h in the 2θ ranges of (a) 10–70◦ and (b) 43.5–47◦ . (c) Plot of lattice volume V vs. lithium content.
T. Kimura et al. / Journal of the European Ceramic Society 33 (2013) 1009–1015
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Fig. 3. TEM images of (a) undoped BaTiO3 before calcination and Li-doped BaTiO3 powders with the Li molar fractions of (b) 0, (c) 0.01, (d) 0.03, (e) 0.04 and (f) 0.06 after calcinations at 600 ◦ C for 1 h.
spherical and near cube nanoparticles of 50–100 nm in diameter. The particle size of undoped BaTiO3 increased from 30 nm to 100 nm after calcinations (Fig. 3(a) and (b)). The size of Lidoped BaTiO3 (x = 0–0.06) did not change very much with an increment of Li content. The samples were sintered at various temperatures (1000–1300 ◦ C) for 2 h with heating rate of 10 ◦ C/min. Fig. 4 shows the densities of the sintered bodies as a function of the sintering temperature. With increasing the sintering temperature, the density of both samples increased at first, and then decreased. Undoped BaTiO3 prepared by the hydrothermal reaction and commercial one (Sakai Chemical Industry Co.: BT05) showed almost identical relative density of ca. 90% at 1200 ◦ C. Although BT05 showed 72.8% of theoretical density at 1100 ◦ C, Li-doped BaTiO3 samples of x = 0.01 and 0.03 could be sintered to more than 95% of theoretical density, indicating that the sinterability of BaTiO3 could be greatly improved by doping with Li+ . The promotion of the sintering by Li-doping might be related with the formation of well dispersed fine particles with high crystallinity as well as the liquid phase sintering mechanism.12–15 Fig. 5 shows the scanning electron micrographs (SEM) of the fracture surfaces of the Li-doped BaTiO3 (x = 0–0.04) ceramics
sintered at 1100 ◦ C for 2 h. The samples consisted of the grains of 0.2–2 m in diameter without noticeable large pores. The average grain size increased with an increase in the amount of Li.
Fig. 4. Density versus sintering temperature of Li-doped BaTiO3 powders with the Li molar fractions of 0, 0.01, and 0.03 calcined at 600 ◦ C for 1 h together with those of commercial BaTiO3 (Sakaikagaku BT05) powder.
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Fig. 5. SEM images of the fracture surfaces of Li-doped BaTiO3 sintered at 1100 ◦ C for 2 h. The Li molar fraction: (a) 0.01, (b) 0.02, (c) 0.03, and (d) 0.04.
Fig. 6 shows the XRD patterns of Li-doped BaTiO3 (x = 0–0.06) ceramics sintered at 1300 ◦ C (x = 0) and 1100 ◦ C (x = 0.02–0.06). All ceramics consisted of the single phase of BaTiO3 (PDF#01-072-0138) structure, and no impurity phase could be found by XRD. Although the sample powders showed only one peak around 45◦ corresponding to the (2 0 0) peak (Fig. 2(b)), the sintered bodies showed the peak splitting with the intensity ratio of the lower angle peak to the higher angle peak is about 2.16,17 It was reported that the intensity ratio of the lower angle peak to the higher angle peak XRD patterns for the tetragonal structure and is 0.5, whereas that of the orthorhombic phase is
2.18 Generally, the orthorhombic BaTiO3 shows two-step phase transformation as orthorhombic → tetragonal → cubic, and the Curie temperature decreases by forming the orthorhombic phase. The present samples showed one-step phase transformation and no noticeable change in the Curie temperature as shown in Fig. 11, therefore, we considered that the sintered bodies consisted of the mixture of tetragonal phase and cubic phase. Further studies are required to conclude whether the samples consist of the orthorhombic phase or the mixture of tetragonal and cubic phases. Fig. 7 and Table 1 show the lattice parameters and lattice volumes of the sintered bodies for tetragonal ferroelectric phase. The lattice parameter of c did not change very much with the increment of Li content. On the other hand, the lattice parameter of a decreased a little at first with increasing Li+ content up to x = 0.03, and then increased. The similar tendency was also shown in the change in lattice volume. By taking into consideration of ionic sizes of 12 coordination of Ba2+ (0.161 nm) and Li+ (0.118 nm), and 6 coordination of Ti4+ (0.061 nm) and Li+ (0.076 nm), the decrease in the lattice parameter might be due to Table 1 Lattice parameters, volumes and tetragonality (c/a) of undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.01–0.06) sintered at 1100 ◦ C for 2 h.
Fig. 6. XRD patterns of undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.02–0.06) sintered at 1100 ◦ C for 2 h in the 2θ ranges of (a) 10–70◦ and (b) 43.5–47.0◦ .
x
a
c
V
c/a
0.000 0.010 0.025 0.030 0.035 0.040 0.060
4.0026(1) 4.0014(2) 3.9977(2) 3.9967(1) 3.9987(1) 4.0026(2) 4.0027(3)
4.0327(2) 4.0329(3) 4.0341(1) 4.0330(2) 4.0329(1) 4.0356(3) 4.0345(5)
64.606(5) 64.571(6) 64.470(5) 64.421(6) 64.484(5) 64.653(8) 64.64(1)
1.0075 1.0079 1.0091 1.0091 1.0086 1.0082 1.0079
T. Kimura et al. / Journal of the European Ceramic Society 33 (2013) 1009–1015
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Fig. 7. (a) and (b) Lattice parameters and (c) volume of undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.01–0.06) sintered at 1100 ◦ C for 2 h.
Fig. 8. (a) Ba/Ti and (b) Li/Ti molar ratio variation as a function of Li contents in undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.01–0.06) sintered at 1100 ◦ C for 2 h.
the substitution of Ba2+ with Li+ , while the increase might be due to the substitution of Ti4+ with Li+ . However, the changes in the lattice constants shown in Fig. 7(a) and (b) were much smaller than those estimated according to the Vegard’s law for both Ba2+ site substitution and Ti4+ site substitution, indicating that the amount of Li+ doped in the lattice is quite low. In addition, the lattice volumes of samples increased a little after sintering (see Fig. 2(c)), probably because of increased the tetragonality (c/a). Fig. 8 shows the Ba/Ti and Li/Ti molar ratios of Li-doped BaTiO3 ceramics as a function of x added to the sample. It can be seen that the Ba/Ti molar ratio was around 1 for all samples, and the Li/Ti molar ratio determined was almost identical to that added in the starting solution, indicating that the loss of both Ba2+ and Li+ by evaporation during sintering was quite small. From the results shown in Figs. 7 and 8, it may be concluded
that most of Li+ locates at the grain boundary as a glassy phase to play an important role to promote the sintering of the sample. Fig. 9 shows the Elemental TOF-SIMS map on the fracture surface of Li-doped BaTiO3 (x = 0.06) sintered at 1100 ◦ C for 2 h. The profiles of Ba and Ti maps shown in Fig. 9(a) and (b) are exactly identical. In contrast, the map of Li also indicates a similar profile, but quite bright places are seen at the grain boundary (Fig. 9(c)). Therefore, we considered that the small amounts of Li+ are incorporated into the lattice of BaTiO3 , but the Li+ ion mainly locates at the grain boundary. Table 2 and Fig. 10 summarize the sintering temperature, bulk density, ρ, relative density, ρ , relative dielectric constant, εT33 /ε0 , volume resistivity, R, piezoelectric constant, d33 , electromechanical coupling factor, kp , mechanical quality factor, Qm , and phase of Li-doped BaTiO3 (x = 0–0.06) ceramics. With
Fig. 9. Elemental TOF-SIMS map of the fracture surface of Li-doped BaTiO3 (x = 0.06) sintered at 1100 ◦ C for 2 h. The element: (a) Ba, (b) Ti, and (c) Li.
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Table 2 (a) Sintering temperature, (b) density, ρ, (c) relative density, ρ , (d) relative dielectric constant, εT33 /ε0 , (e) volume resistivity, R, (f) piezoelectric constant, d33 , (g) electromechanical coupling factor, kp , (h) mechanical quality factor, Qm , and (i) phase of Li-doped BaTiO3 ceramics (x = 0–0.06) sintered for 2 h.
(a) (b) (c) (d) (e) (f) (g) (h) (i)
(◦ C)
Sint. temp. ρ (g/cm3 ) ρ (%) εT33 /ε0 R (108 ohm cm) d33 (pC/N) kp (%) Qm Phase (◦ )
0.000
0.010
0.020
0.025
0.030
0.035
0.040
0.060
1300 5.56 92.4 3270 4.73 106 15.5 32 −51.0
1100 5.84 97.0 3125 35.0 123 13.9 128 −0.6
1100 5.71 94.9 2716 50.6 110 14.3 137 5.0
1100 5.76 95.7 2480 61.0 197 29.6 186 71.8
1100 5.83 96.9 1744 48.3 260 43.7 357 84.6
1100 5.79 96.2 3229 46.3 161 21.6 99.4 29.5
1100 5.73 95.2 2759 53.6 93 13.4 162 −1.2
1100 5.77 95.9 2475 53.5 148 20.6 189 53.2
increasing the molar fraction of Li, the d33 , kp and Qm firstly increased, reached to a maximum value and then decreased, but εT33 /ε0 decreased and then increased. The values of εT33 /ε0 , d33 , kp , and Qm for Li-doped BaTiO3 with x = 0.03 were 1744, 260 pC/N, 43.7%, and 357, respectively. The relation between the amount of doped Li and the piezoelectric constant d33 in the present study resembles that reported by Yang et al.,18 i.e., they reported that the maximum d33 of 270 pC/N was obtained by adding 4 mol% LiF. But the relationship between the relative dielectric constant and Li content was different. In addition, the Curie temperature of BaTiO3 –xLiF decreased with an increase
in LiF content, while that of our samples did not change very much regardless the mole fraction of Li (Fig. 11). This difference may be due to the difference in the chemical composition, i.e., our sample did not contain F. The high d33 and low εT33 /ε0 are desirable in the application as piezoelectric materials. In general d33 is proportional to 0.5 (εT33 /ε0 ) as shown by Eq. (1). d33 ∝ kp
εT33 ε0
0.5 (1)
However, it can be seen that the volume resistivity of Li-doped BaTiO3 was greatly increased by doping with Li+ . It may be due to the control of the lattice defect generation caused by the stoichiometric mismatch of Ba2+ and Ti4+ in BaTiO3 by doping with Li+ . The increase in the volume resistivity by doping with the adequate amount of Li+ might contribute to improve the poling efficiency and to decrease the dielectric constant by decreasing leaking charge, while the doping with the excess amount of Li+ resulted to depress the poling efficiency and increase the dielectric constant by increasing Li-impurity at the grain boundary.19,20 This may be the reason why the sample of x = 0.03 showed the highest d33 and lowest εT33 /ε0.
Fig. 10. (a) Relative dielectric constant, εT33 /ε0 , (b) volume resistivity, R, (c) piezoelectric constant, d33 , (d) electromechanical coupling factor, kp , and (e) phase of undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.01–0.06) sintered at 1100 ◦ C for 2 h as a function of Li concentration.
Fig. 11. Temperature dependence of dielectric constant undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.01–0.06) sintered at 1100 ◦ C for 2 h.
T. Kimura et al. / Journal of the European Ceramic Society 33 (2013) 1009–1015
Fig. 11 shows the temperature dependence of dielectric constant (r ) for Li-doped BaTiO3 ceramics at the frequency of 100 kHz. For each specimen, an abnormal dielectric peak is observed during heating process. The temperature corresponding to the peak dielectric constant is denoted as Curie temperature (Tc ). All samples showed the Curie temperatures around 130 ◦ C regardless of the mole fractions of Li. Generally, piezoelectric properties and Curie temperature are in a relation of the trade-off. However, it is notable that the piezoelectric properties were successfully improved by doping with Li+ without decreasing the Curie temperature, i.e., the piezoelectric properties changed depending on the tetragonality (c/a) as shown in Fig. 10, but the Curie temperature did not change very much. 4. Conclusion The piezoelectric properties and sinterability of BaTiO3 were successfully improved by doping with a small amount of Li+ . The powders synthesized by the solvothermal approach consisted of homogeneous fine particles. The Li-doped BaTiO3 ceramics exhibited not only the tetragonality (c/a) superior to pure BaTiO3 , but also the superior polarizability. It might lead to excellent piezoelectric properties. The piezoelectric constant attained a maximum value of d33 = 260 pC/N at x = 0.03. The Curie temperatures of all samples were around 130 ◦ C, regardless of the different mole fractions of Li+ . Acknowledgment This research was supported in part by the Management Expenses Grants for National Universities Corporations from the Ministry of Education, Culture, Sports and Science for Technology of Japan (MEXT). References 1. Haertling HG. Ferroelectric ceramics: history and technology. J Am Ceram Soc 1999;82:797–818. 2. Luo HL, Zhu H, Zhao SC, Wang XH, Luo SH. Cylinder-shaped ultrasonic motors 4.8 mm in diameter using electroactive piezoelectric materials. Appl Phys Lett 2007;90:052904–52910. 3. Xie HY, Yin S, Takatoshi T, Tokano Y, Sasaki A, Sato T. Sintering and dielectric properties of BaTiO3 prepared by a composite-hydroxide-mediated approach. Mater Res Bull 2010;45:1345–50.
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4. Kiss K, Magder J, Vukasovich MS, Lockhart RJ. Ferroelectrics of ultrafine particle size: I, synthesis of titanate powders of ultrafine particle size. J Am Ceram Soc 1966;49:291–5. 5. Xu Q, Chen S, Chen W, Huang D, Zhou J, Sun H, et al. Synthesis of (Na0.5 Bi0.5 )TiO3 and (Na0.5 Bi0.5 )0.92 Ba0.08 TiO3 powders by a citrate method. J Mater Sci 2006;41:6146–9. 6. Sakabe Y, Yamashita Y, Yamamoto H. Dielectric properties of nanocrystalline BaTiO3 synthesized by micro-emulsion method. J Eur Ceram Soc 2005;25:2739–42. 7. Xie Y, Yin S, Hashimoto T, Tokano Y, Sasaki A, Sato T. Low temperature synthesis of tetragonal BaTiO3 by a novel composite-hydroxide-mediated approach and its dielectric properties. J Eur Ceram Soc 2010;30: 699–704. 8. Yoon SM, Khansur HN, Choi KB, Lee GY, Ur CS. The effect of nano-sized BNBT on microstructure and dielectric/piezoelectric properties. Ceram Int 2009;35:3027–36. 9. Hong HS, McKinstry TS, Messing LG. Dielectric and electromechanical properties of textured niobium-doped bismuth titanate ceramics. J Am Ceram Soc 2000;83:113–8. 10. Yang Z, Hou Y, Liu B, Wei L. Structure and electrical properties of Nd2 O3 -doped 0.82 Bi0.5 Na0.5 TiO3-0.18 Bi0.5 K0.5 TiO3 ceramics. Ceram Int 2009;35:1423–7. 11. Izumi F, Ikeda T. A Rietveld-analysis program RIETAN-98 and its applications to zeolites. Mater Sci Forum 2000;321:198–205. 12. Jerome B, Florent B, David H, Jean-Marie H. Low sintering temperature of MgTiO3 for type I capacitors. J Eur Ceram Soc 2005;25:2779–83. 13. Yumin Z, Yunlong Z, Jiecai H, Luyang H. The effect of sintering additive on fracture behavior of carbon-whisker-reinforced silicon carbide composites. Mater Sci Eng 2008;A480:62–7. 14. Bernd B, Omer BV, R¨a tzer S, Hans-Joachim. Lowering the sintering temperature for EPD coatings by applying reaction bonding. J Eur Ceram Soc 2008;28:1793–9. 15. Wang L, Sui W, Mu K, Luan S. Effect of silica sintering additive on the sintering behavior and dielectric properties of strontium barium niobate ceramics. J Eur Ceram Soc 2009;29:1427–32. 16. Yamaji A, Enomoto Y, Kinoshita K, Murakami T. Preparation, characterization, and properties of Dy-doped small-grained BaTiO3 Ceramics. J Am Ceram Soc 1977;60:97–101. 17. Buessem RW, Cross EL, Goswami K. A. Phenomenological theory of high permittivity in fine-grained barium titanate. J Am Ceram Soc 1966;49: 33–6. 18. Yang W-G, Zhang B-P, Ma N, Zhao L. High piezoelectric properties of BaTiO3 –xLiF ceramics sintered at low temperature. J Eur Ceram Soc 2012;32:899–904. 19. Kang SB, Park HB, Bu DS, Kang HS, Noh WT. Different fatigue behaviors of SrBi2 Ta2 O9 and Bi3 TiTaO9 films: Role of perovskite layers. Appl Phys Lett 1999;75:2644–50. 20. Noguchi Y, Matsumoto T, Miyayama M. Impact of defect control on the polarization properties in Bi4 Ti3 O12 ferroelectric single crystals. Jpn J Appl Phys 2005;44:L570–2.
Journal of the European Ceramic Society 33 (2013) 1009–1015
Synthesis and piezoelectric properties of Li-doped BaTiO3 by a solvothermal approach Takeshi Kimura a,∗, Qiang Dong a, Shu Yin a, Takatoshi Hashimoto b, Atsushi Sasaki b, Tsugio Sato a a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan b NEC Tokin Corporation, 6-7-1 Koriyama, Taihaku-ku, Sendai 980-8510, Japan Received 16 April 2012; received in revised form 6 November 2012; accepted 8 November 2012 Available online 7 December 2012
Abstract Li-doped BaTiO3 particles with the Li+ mole fraction, x, of 0–0.06 were synthesized by a solvothermal approach at 200 ◦ C. The products consisted of nanoparticles of 50–100 nm in diameter. The sinterability and piezoelectric property of Li-doped BaTiO3 were improved by doping with Li ion, i.e., the Li-doped BaTiO3 samples could be sintered to almost full theoretical density (>95%) at a low temperature such as 1100 ◦ C, and the highest piezoelectric constant, d33 (260 pC/N) and electromechanical coupling factor, kp (43.7%) could be realized at x value of 0.03. The Curie temperatures of all samples were around 130 ◦ C, and did not change very much depending on the amount of Li-doping. © 2012 Elsevier Ltd. All rights reserved. Keywords: Solvothermal approach; Perovskite; Piezoelectricity; Sintering; Ferroelectric properties
1. Introduction Lead-based perovskite solid solution of Pb(Zr,Ti)O3 (PZT) have been widely used for piezoelectric devices due to their large piezoelectric constants at the relatively high temperatures.1,2 Nowadays, because of the toxicity of lead oxide, it is desired to use lead-free piezoelectric materials in place of PZT for environmental protection, therefore, there is an increasing interest of investigating lead-free piezoelectric materials. BaTiO3 has become one of the most important ferroelectric materials used in the electronics ceramic industry. However, the piezoelectric properties of BaTiO3 were insufficient compared with PZT. Recently, it was reported that the addition of Li2 CO3 as a sintering additive, could improve both the sinterability and the piezoelectric properties, where Li+ was doped into the lattice of BaTiO3 during the sintering.3 Usually, BaTiO3 -based ceramics are fabricated by the conventional solid-state method. Recently, considerable research efforts have been devoted to the preparation of materials by various wet chemical methods, such as solvothermal process,4 citrate method,5 emulsion method,6 and composite-hydroxidemediated method.7 It was found that piezoelectric ceramics
∗
Corresponding author. Tel.: +81 22 217 5599; fax: +81 22 217 5598. E-mail address: [email protected] (T. Kimura).
0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.11.007
made from powders synthesized by alternative methods exhibit improved sinterability, poling property and piezoelectric performance, because of their higher homogeneity of chemical composition with well dispersed fine particles8 and/or unique morphologies such as plate-like9 and cube-like shape.10 In the present study, Li-doped BaTiO3 ceramics with x = 0–0.06 were produced by the hydrothermal process, and their piezoelectrical properties were examined. 2. Experimental 2.1. Synthesis Li-doped BaTiO3 particles with x = 0–0.06 were synthesized by a solvothermal approach. Ba(OH)2 ·8H2 O, Ti(i-PrO)4 and Li2 CO3 were used as starting materials. Firstly, 40 mmol Ti(iPrO)4 was dissolved in 30 ml isopropanol, and then 40 mmol Ba(OH)2 ·8H2 O, desired amount of Li2 CO3 and 17 ml NH3 aqueous solution (6.5 M) were added to precipitate amorphous gel. Then, the slurry solution was introduced into a Teflon® -lined stainless autoclave with a 100 cm3 of internal volume together with 10 Teflon® balls of 11 mm in diameter. The autoclave was sealed and heated at 200 ◦ C for 12 h in an electric oven, where the autoclave was rotated at 100 rpm during the reaction using a rotation type hydrothermal reaction apparatus (Fig. 1). After that, the autoclave was taken out to allow cooling to room
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Fig. 1. Hydrothermal apparatus with ball milling system.
temperature. After removing the solution using a rotary evaporator, the precipitates were calcined at 600 ◦ C for 1 h. 2.2. Characterization The particle morphology was observed by a transmission electron microcopy (JEOL, TEM-2000EX). The X-ray diffraction (XRD) analysis of the obtained powder samples and sintered ceramics was carried out using Cu K␣ radiation with a pyrolytic graphite monochromater mounted powder diffractometer (Bruker, D2 PHASER). The sample powders were uniaxially pressed at 20 MPa in a steel die to form pellets of 20 mm in diameter and 3 mm in thickness, and then isostatically pressed at 200 MPa. The pellets were sintered at 1000–1300 ◦ C for 2 h. The fracture surfaces of the sintered bodies were observed by a scanning electron microscope (SEM, Hitachi S-4800). The densities of the sintered pellets were measured by the Archimedes’ method. The lattice parameters were evaluated by X-ray diffraction analysis using the Rietveld refinement with the RIETAN-FP program.11 Measurements for Rietveld refinements were made using a tube power of 40 kV and 30 mA over the 2θ range of 10–130◦ with a 0.02◦ 2θ step size, an around 0.3 s count time and the maximum intensity of
XRD patterns was set to 10,000 counts (Bruker, D2 PHASER). The chemical compositions of the ceramics were determined by inductively coupled plasma-atomic emission spectroscopy (ICP, Perkin Elmer, Optima 3300XL) after dissolving the 0.5 mg samples in 50 ml mixed aqueous solutions (2.0 mol/l HCl and 3.1 wt% H2 O2 ) at 25 ◦ C for 5 min. The elemental TOF-SIMS map of fracture surfaces of the sintered bodies were observed by a Time of Flight Secondary Ion Mass Spectrometer (ION-TOF, TOF-SIMS 5-100). The upper and bottom surfaces of specimens were coated with gold paint and fired at 800 ◦ C for 20 min for the electrical measurements. Prior to piezoelectric measurements, the sintered bodies were shaped as a disk of size 15φ × 1.0 mm for the Qm and kp determination, then the poling treatment was performed in a silicon oil bath at 80 ◦ C by applying a DC electric field of 3.0 kV/mm for 30 min (Withstanding Voltage Tester, TOS 5101). The piezoelectric constant, d33 , of the samples was measured by means of a quasi-static d33 meter (IACAS, ZJ-3BN) based on the Berlincourt method at 110 Hz. The dielectric properties and piezoelectric properties were determined using an Agilent 4294A precision impedance analyzer. 3. Results and discussion Fig. 2 shows the X-ray diffraction patterns of the sample powders before and after calcination at 600 ◦ C for 1 h. The samples both before and after calcinations consisted of the single phase of BaTiO3 (PDF#01-072-0138) structure, and no impurity peak could be found. In Fig. 2, the lattice volumes V of calcination samples are plotted as a function of Li content. It can be seen that the lattice volume of calcined samples slightly increased by an increase in doping concentration of Li+ , indicating the incorporation of Li ions in the lattice of BaTiO3 . Fig. 3 shows the TEM images of as-prepared powders. The Li-doped BaTiO3 samples consisted of the well dispersed
Fig. 2. XRD patterns of undoped BaTiO3 powder before calcination and Li-doped BaTiO3 powders with the Li molar fractions of x = 0–0.06 after calcinations at 600 ◦ C for 1 h in the 2θ ranges of (a) 10–70◦ and (b) 43.5–47◦ . (c) Plot of lattice volume V vs. lithium content.
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Fig. 3. TEM images of (a) undoped BaTiO3 before calcination and Li-doped BaTiO3 powders with the Li molar fractions of (b) 0, (c) 0.01, (d) 0.03, (e) 0.04 and (f) 0.06 after calcinations at 600 ◦ C for 1 h.
spherical and near cube nanoparticles of 50–100 nm in diameter. The particle size of undoped BaTiO3 increased from 30 nm to 100 nm after calcinations (Fig. 3(a) and (b)). The size of Lidoped BaTiO3 (x = 0–0.06) did not change very much with an increment of Li content. The samples were sintered at various temperatures (1000–1300 ◦ C) for 2 h with heating rate of 10 ◦ C/min. Fig. 4 shows the densities of the sintered bodies as a function of the sintering temperature. With increasing the sintering temperature, the density of both samples increased at first, and then decreased. Undoped BaTiO3 prepared by the hydrothermal reaction and commercial one (Sakai Chemical Industry Co.: BT05) showed almost identical relative density of ca. 90% at 1200 ◦ C. Although BT05 showed 72.8% of theoretical density at 1100 ◦ C, Li-doped BaTiO3 samples of x = 0.01 and 0.03 could be sintered to more than 95% of theoretical density, indicating that the sinterability of BaTiO3 could be greatly improved by doping with Li+ . The promotion of the sintering by Li-doping might be related with the formation of well dispersed fine particles with high crystallinity as well as the liquid phase sintering mechanism.12–15 Fig. 5 shows the scanning electron micrographs (SEM) of the fracture surfaces of the Li-doped BaTiO3 (x = 0–0.04) ceramics
sintered at 1100 ◦ C for 2 h. The samples consisted of the grains of 0.2–2 m in diameter without noticeable large pores. The average grain size increased with an increase in the amount of Li.
Fig. 4. Density versus sintering temperature of Li-doped BaTiO3 powders with the Li molar fractions of 0, 0.01, and 0.03 calcined at 600 ◦ C for 1 h together with those of commercial BaTiO3 (Sakaikagaku BT05) powder.
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Fig. 5. SEM images of the fracture surfaces of Li-doped BaTiO3 sintered at 1100 ◦ C for 2 h. The Li molar fraction: (a) 0.01, (b) 0.02, (c) 0.03, and (d) 0.04.
Fig. 6 shows the XRD patterns of Li-doped BaTiO3 (x = 0–0.06) ceramics sintered at 1300 ◦ C (x = 0) and 1100 ◦ C (x = 0.02–0.06). All ceramics consisted of the single phase of BaTiO3 (PDF#01-072-0138) structure, and no impurity phase could be found by XRD. Although the sample powders showed only one peak around 45◦ corresponding to the (2 0 0) peak (Fig. 2(b)), the sintered bodies showed the peak splitting with the intensity ratio of the lower angle peak to the higher angle peak is about 2.16,17 It was reported that the intensity ratio of the lower angle peak to the higher angle peak XRD patterns for the tetragonal structure and is 0.5, whereas that of the orthorhombic phase is
2.18 Generally, the orthorhombic BaTiO3 shows two-step phase transformation as orthorhombic → tetragonal → cubic, and the Curie temperature decreases by forming the orthorhombic phase. The present samples showed one-step phase transformation and no noticeable change in the Curie temperature as shown in Fig. 11, therefore, we considered that the sintered bodies consisted of the mixture of tetragonal phase and cubic phase. Further studies are required to conclude whether the samples consist of the orthorhombic phase or the mixture of tetragonal and cubic phases. Fig. 7 and Table 1 show the lattice parameters and lattice volumes of the sintered bodies for tetragonal ferroelectric phase. The lattice parameter of c did not change very much with the increment of Li content. On the other hand, the lattice parameter of a decreased a little at first with increasing Li+ content up to x = 0.03, and then increased. The similar tendency was also shown in the change in lattice volume. By taking into consideration of ionic sizes of 12 coordination of Ba2+ (0.161 nm) and Li+ (0.118 nm), and 6 coordination of Ti4+ (0.061 nm) and Li+ (0.076 nm), the decrease in the lattice parameter might be due to Table 1 Lattice parameters, volumes and tetragonality (c/a) of undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.01–0.06) sintered at 1100 ◦ C for 2 h.
Fig. 6. XRD patterns of undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.02–0.06) sintered at 1100 ◦ C for 2 h in the 2θ ranges of (a) 10–70◦ and (b) 43.5–47.0◦ .
x
a
c
V
c/a
0.000 0.010 0.025 0.030 0.035 0.040 0.060
4.0026(1) 4.0014(2) 3.9977(2) 3.9967(1) 3.9987(1) 4.0026(2) 4.0027(3)
4.0327(2) 4.0329(3) 4.0341(1) 4.0330(2) 4.0329(1) 4.0356(3) 4.0345(5)
64.606(5) 64.571(6) 64.470(5) 64.421(6) 64.484(5) 64.653(8) 64.64(1)
1.0075 1.0079 1.0091 1.0091 1.0086 1.0082 1.0079
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Fig. 7. (a) and (b) Lattice parameters and (c) volume of undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.01–0.06) sintered at 1100 ◦ C for 2 h.
Fig. 8. (a) Ba/Ti and (b) Li/Ti molar ratio variation as a function of Li contents in undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.01–0.06) sintered at 1100 ◦ C for 2 h.
the substitution of Ba2+ with Li+ , while the increase might be due to the substitution of Ti4+ with Li+ . However, the changes in the lattice constants shown in Fig. 7(a) and (b) were much smaller than those estimated according to the Vegard’s law for both Ba2+ site substitution and Ti4+ site substitution, indicating that the amount of Li+ doped in the lattice is quite low. In addition, the lattice volumes of samples increased a little after sintering (see Fig. 2(c)), probably because of increased the tetragonality (c/a). Fig. 8 shows the Ba/Ti and Li/Ti molar ratios of Li-doped BaTiO3 ceramics as a function of x added to the sample. It can be seen that the Ba/Ti molar ratio was around 1 for all samples, and the Li/Ti molar ratio determined was almost identical to that added in the starting solution, indicating that the loss of both Ba2+ and Li+ by evaporation during sintering was quite small. From the results shown in Figs. 7 and 8, it may be concluded
that most of Li+ locates at the grain boundary as a glassy phase to play an important role to promote the sintering of the sample. Fig. 9 shows the Elemental TOF-SIMS map on the fracture surface of Li-doped BaTiO3 (x = 0.06) sintered at 1100 ◦ C for 2 h. The profiles of Ba and Ti maps shown in Fig. 9(a) and (b) are exactly identical. In contrast, the map of Li also indicates a similar profile, but quite bright places are seen at the grain boundary (Fig. 9(c)). Therefore, we considered that the small amounts of Li+ are incorporated into the lattice of BaTiO3 , but the Li+ ion mainly locates at the grain boundary. Table 2 and Fig. 10 summarize the sintering temperature, bulk density, ρ, relative density, ρ , relative dielectric constant, εT33 /ε0 , volume resistivity, R, piezoelectric constant, d33 , electromechanical coupling factor, kp , mechanical quality factor, Qm , and phase of Li-doped BaTiO3 (x = 0–0.06) ceramics. With
Fig. 9. Elemental TOF-SIMS map of the fracture surface of Li-doped BaTiO3 (x = 0.06) sintered at 1100 ◦ C for 2 h. The element: (a) Ba, (b) Ti, and (c) Li.
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Table 2 (a) Sintering temperature, (b) density, ρ, (c) relative density, ρ , (d) relative dielectric constant, εT33 /ε0 , (e) volume resistivity, R, (f) piezoelectric constant, d33 , (g) electromechanical coupling factor, kp , (h) mechanical quality factor, Qm , and (i) phase of Li-doped BaTiO3 ceramics (x = 0–0.06) sintered for 2 h.
(a) (b) (c) (d) (e) (f) (g) (h) (i)
(◦ C)
Sint. temp. ρ (g/cm3 ) ρ (%) εT33 /ε0 R (108 ohm cm) d33 (pC/N) kp (%) Qm Phase (◦ )
0.000
0.010
0.020
0.025
0.030
0.035
0.040
0.060
1300 5.56 92.4 3270 4.73 106 15.5 32 −51.0
1100 5.84 97.0 3125 35.0 123 13.9 128 −0.6
1100 5.71 94.9 2716 50.6 110 14.3 137 5.0
1100 5.76 95.7 2480 61.0 197 29.6 186 71.8
1100 5.83 96.9 1744 48.3 260 43.7 357 84.6
1100 5.79 96.2 3229 46.3 161 21.6 99.4 29.5
1100 5.73 95.2 2759 53.6 93 13.4 162 −1.2
1100 5.77 95.9 2475 53.5 148 20.6 189 53.2
increasing the molar fraction of Li, the d33 , kp and Qm firstly increased, reached to a maximum value and then decreased, but εT33 /ε0 decreased and then increased. The values of εT33 /ε0 , d33 , kp , and Qm for Li-doped BaTiO3 with x = 0.03 were 1744, 260 pC/N, 43.7%, and 357, respectively. The relation between the amount of doped Li and the piezoelectric constant d33 in the present study resembles that reported by Yang et al.,18 i.e., they reported that the maximum d33 of 270 pC/N was obtained by adding 4 mol% LiF. But the relationship between the relative dielectric constant and Li content was different. In addition, the Curie temperature of BaTiO3 –xLiF decreased with an increase
in LiF content, while that of our samples did not change very much regardless the mole fraction of Li (Fig. 11). This difference may be due to the difference in the chemical composition, i.e., our sample did not contain F. The high d33 and low εT33 /ε0 are desirable in the application as piezoelectric materials. In general d33 is proportional to 0.5 (εT33 /ε0 ) as shown by Eq. (1). d33 ∝ kp
εT33 ε0
0.5 (1)
However, it can be seen that the volume resistivity of Li-doped BaTiO3 was greatly increased by doping with Li+ . It may be due to the control of the lattice defect generation caused by the stoichiometric mismatch of Ba2+ and Ti4+ in BaTiO3 by doping with Li+ . The increase in the volume resistivity by doping with the adequate amount of Li+ might contribute to improve the poling efficiency and to decrease the dielectric constant by decreasing leaking charge, while the doping with the excess amount of Li+ resulted to depress the poling efficiency and increase the dielectric constant by increasing Li-impurity at the grain boundary.19,20 This may be the reason why the sample of x = 0.03 showed the highest d33 and lowest εT33 /ε0.
Fig. 10. (a) Relative dielectric constant, εT33 /ε0 , (b) volume resistivity, R, (c) piezoelectric constant, d33 , (d) electromechanical coupling factor, kp , and (e) phase of undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.01–0.06) sintered at 1100 ◦ C for 2 h as a function of Li concentration.
Fig. 11. Temperature dependence of dielectric constant undoped BaTiO3 sintered at 1300 ◦ C and Li-doped BaTiO3 ceramics (x = 0.01–0.06) sintered at 1100 ◦ C for 2 h.
T. Kimura et al. / Journal of the European Ceramic Society 33 (2013) 1009–1015
Fig. 11 shows the temperature dependence of dielectric constant (r ) for Li-doped BaTiO3 ceramics at the frequency of 100 kHz. For each specimen, an abnormal dielectric peak is observed during heating process. The temperature corresponding to the peak dielectric constant is denoted as Curie temperature (Tc ). All samples showed the Curie temperatures around 130 ◦ C regardless of the mole fractions of Li. Generally, piezoelectric properties and Curie temperature are in a relation of the trade-off. However, it is notable that the piezoelectric properties were successfully improved by doping with Li+ without decreasing the Curie temperature, i.e., the piezoelectric properties changed depending on the tetragonality (c/a) as shown in Fig. 10, but the Curie temperature did not change very much. 4. Conclusion The piezoelectric properties and sinterability of BaTiO3 were successfully improved by doping with a small amount of Li+ . The powders synthesized by the solvothermal approach consisted of homogeneous fine particles. The Li-doped BaTiO3 ceramics exhibited not only the tetragonality (c/a) superior to pure BaTiO3 , but also the superior polarizability. It might lead to excellent piezoelectric properties. The piezoelectric constant attained a maximum value of d33 = 260 pC/N at x = 0.03. The Curie temperatures of all samples were around 130 ◦ C, regardless of the different mole fractions of Li+ . Acknowledgment This research was supported in part by the Management Expenses Grants for National Universities Corporations from the Ministry of Education, Culture, Sports and Science for Technology of Japan (MEXT). References 1. Haertling HG. Ferroelectric ceramics: history and technology. J Am Ceram Soc 1999;82:797–818. 2. Luo HL, Zhu H, Zhao SC, Wang XH, Luo SH. Cylinder-shaped ultrasonic motors 4.8 mm in diameter using electroactive piezoelectric materials. Appl Phys Lett 2007;90:052904–52910. 3. Xie HY, Yin S, Takatoshi T, Tokano Y, Sasaki A, Sato T. Sintering and dielectric properties of BaTiO3 prepared by a composite-hydroxide-mediated approach. Mater Res Bull 2010;45:1345–50.
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