RARE METALS Vol. 30, No. 1, Feb 2011, p. 72 DOI: 10.1007/s12598-011-0200-1

Corresponding author: HUANG Xinyou E-mail: huangxy@ujs.edu.cn

Influence of Sb2O3 doping on the properties of KBT-NBT-BT lead-free piezoelectric ceramics HUANG Xinyou, GAO Chunhua, WEI Minxian, CHEN Zhigang, and CUI Yongzhen School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China

Received 20 November 2009; received in revised form 20 January 2010; accepted 30 January 2010 © The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2011

Abstract

0.144(K0.5Bi0.5)TiO3-0.85(Na0.5Bi0.5)TiO3-0.006BaTiO3 (KBT-NBT-BT) lead-free piezoelectric ceramics were prepared using a conventional solid state method. The influence of Sb2O3 doping on the crystal phase, surface microstructure and properties of the KBT-NBT-BT lead-free piezoelectric ceramics were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM) and other analytical methods. The results show that all compositions are of pure perovskite structure solid states. Sb2O3 doping does not influence the microstructure of KBT-NBT-BT lead-free piezoelectric ceramics obviously in the Sb2O3 doping range of 0.1-0.5 wt.%. Sb2O3 functions as a donor when doped small amount, while functions as a acceptor when doped large amount. The piezoelectric strain constant (d33) increases first and then decreases; the dielectric constant (εT

33/ε0) and the dielectric loss (tanδ) decrease continuously when the amount of Sb2O3 dopant increases. When the doping amount of Sb2O3 is 0.1 wt.%, the KBT-NBT-BT piezoelectric ceramics with good comprehensive properties are obtained, whose d33, εT

33/ε 0 and tanδ are 147 pC/N, 1510 and 4.2%, respectively.

Keywords: nonmetallic materials; piezoelectric ceramics; doping; piezoelectricity; dielectric properties

1. Introduction

Piezoelectric ceramics are a kind of important functional material that can transform mechanical energy into electrical energy. They are widely used in transforming from supersonic to energy, transducer, detection without damage, communica-tion technology and so on. However, for a long time, re-searchers have focused on lead zirconate titanate, ternary sys-tem and quaternary system piezoelectric ceramics. The lead content of this kind of materials is more than 60 wt.%, and the lead oxide (PbO) evaporation formed in sintering and produc-tion processes can pollute the environment and cause public nuisance. Therefore, the urgent task in the electronic materials field is to find a new kind of material that can replace lead zirconate titanate, ternary system and quaternary system pie-zoelectric ceramics. Lead-free piezoelectric ceramics pose no danger to the environment and have been paid great attention by many countries [1-3]. (Na0.5Bi0.5)TiO3 (NBT) piezoelectric ceramics have many excellent properties such as strong ferroelectricity (Pr=38 μC/cm2), high piezoelectric constant, low permittivity, good acoustics property, and low sintering temperature. It has an excellent potential as lead-free piezo-electric ceramic materials [4]. The NBT-KBT- BT ternary system is based on NBT and modified with (K0.5Bi0.5)TiO3

(KBT) and BaTiO3 (BT), which can enhance piezoelectric properties well. Many scholars have researched this ternary system [5-9], but they were not satisfied with its piezoelec-tric properties. Thus, it is necessary for more intensive re-search. There are few reports about the study of Sb2O3 dop-ing on the structure and properties of NBT-KBT- BT ce-ramics. On the basis of ternary piezoelectric ceramics (Na0.5Bi0.5)0.85(K0.5Bi0.5)0.144Ba0.006TiO3 (0.85NBT-0.144KBT- 0.006BT) (NBT-KBT-BT) whose piezoelectric properties are good [10], the influence of Sb2O3 doping on the structure and properties of NBT-KBT-BT ceramics was systematically studied and their properties were further improved. The char-acter and law of influence of Sb2O3 doping on the structure and properties of NBT-KBT-BT lead-free piezoelectric ce-ramics was studied, and provided a basis for producing high property lead-free piezoelectric ceramics.

2. Experimental

Lead-free piezoelectric ceramics were prepared by con-ventional techniques. The starting materials (reagent grade) were Na2CO3, K2CO3, BaCO3, Bi2O3, and TiO2, prepared according to theoretical chemical formula 0.85NBT- 0.144KBT-0.006BT. These oxide and carbonate powders

Huang X.Y. et al., Influence of Sb2O3 doping on the properties of KBT-NBT-BT lead-free piezoelectric ceramics 73

were mixed in absolute alcohol with silica balls by planetary ball-milling for 8 h, and then they were dried and sifted out with 100 mesh sifter. Powders obtained were calcined at 890°C for 2 h in a crucible. After that the powders were doped with Sb2O3 (0, 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, 0.4 wt.%, 0.5 wt.%) and ball-milled for 8 h in distilled water with silica balls by planetary ball-milling. The crystalline phase of the calcined powders was identified by X-ray dif-fraction (XRD). The XRD patterns indicated that a pure sin-gle phase of perovskite structure was synthesized. The dried planetary ball-milled powders were added with 6 wt.% PVA solution (the concentration of PVA solution was 5 wt.%) and pressed into disc shaped samples (φ20 mm×1.5 mm). The samples were heat-treated at 500°C to release cohesive material at a heating-up rate of 50-100°C/h. Then, the sam-ples were sintered at 1120-1200°C for 2 h at a heating-up rate of 200-250°C/h. After the samples were polished and coated with Ag electrodes, they were polarized in silicone under the condition of the polarization voltage of 3-4 kV/mm, the polarization time of 10-15 min and the polariza-tion temperature of 60-80°C.

The properties of samples were measured after aging for 48 h. The capacitance and dielectric loss(tanδ) were meas-ured at 1 kHz at room temperature using a YY2814 digital electric bridge instrument; the dielectric constant (εT

33/ε0) was calculated. Piezoelectric strain constant (d33) was meas-ured by ZJ-3A quasistatic instrument. The crystalline struc-ture was analyzed by D/max2500PC type XRD instrument. The microstructure of the ceramic surface was analyzed by scanning electron microscope (SEM, Joel JXA-840A), and the SEM microstructure photographs of samples were ob-tained by electronic probe scanning microscope.

3. Results and discussion 3.1. Influence of Sb2O3 doping on the properties of NBT-KBT-BT ceramics

The relations between Sb2O3 doping amount and d33, tanδ, εT

33/ε0 are listed in Fig. 1, Fig. 2 and Fig. 3, respectively. As can be seen from Fig. 1, with the increase of Sb2O3 doping amount, d33 of the ceramics first increases and then reaches the maximum of 147 pC/N while the Sb2O3 doping amount is 0.1 wt.%. After that point d33 begin decreasing and it drop sharply when the Sb2O3 doping amount is over 0.3wt.%. For example, when the sintering temperature is 1160°C, d33 is 72 pC/N when the Sb2O3 doping amount is 0.4 wt.%, which is half the value of 0.1 wt.%. tanδ and εT

33/ε0 increase gradu-ally when the Sb2O3 doping amount increases.

Sb ion is a kind of element that has different valences, and exists in a crystal structure with Sb3+ or Sb5+. With re-spect to ion valences, Sb3+ occupying the A site of Bi3+ is

Fig. 1. d33 of ceramic samples as a function of Sb2O3 doping amount.

Fig. 2. tan δ of ceramic samples as a function of Sb2O3 doping amount.

Fig. 3. εT

33/ε0 of ceramic samples as a function of Sb2O3 doping amount.

very possible. However, the radius of Sb3+ is 0.076 nm and the radius of Bi3+ is 0.103 nm. There is a certain degree of difference between the radiuses of the two ions. When Sb3+

is substituted for Bi3+, the crystal cell is shrunk, the structure is deformed and domain movement becomes easier. With an external electric field, it is easier to change direction for the domain, and the number of domains along the electric field direction increases, which increase piezoelectric and dielec-tric properties.

With respect to ion radius, the ion radius of Sb5+ is 0.062 nm, the ion radius of the B site of Ti4+ is 0.064 nm. The ion radius of Sb3+ and Sb5+ are all similar to the B site of Ti4+ and have possibility to occupy Ti4+. When Sb3+ occupies the B site of Ti4+, there will be oxygen vacancies produced. The

74 RARE METALS, Vol. 30, No. 1, Feb 2011

deformation of the perovskite structure octahedron makes domain moving difficult. When oxygen vacancies diffuse to the domain wall, they will nail the domain easily and block the movement of domain which makes it difficult for do-main to change direction enough in poling. Therefore, ce-ramic piezoelectric and dielectric properties will decrease. Here, Sb3+ takes the effect of acceptor doping materials. When Sb5+ occupies the B site of Ti4+, there will be A va-cancies produced and compensating superfluous positive charge. The A vacancies make domain moving easy. The piezoelectric and dielectric properties increase and Sb5+ works as a donor material. The influence of Sb2O3 doping on the piezoelectric and dielectric properties of ternary sys-tem NBT-KBT-BT piezoelectric ceramics is simultaneously affected by many of the above mentioned factors [11].

3.2. Influence of Sb2O3 doping on the microstructure and phase composition

Fig. 4 shows XRD patterns for Sb2O3-doped NBT-KBT- BT ceramics sintered at 1160°C for 2 h. As can be seen from

Fig. 4, the perovskite structure is formed in studied ceramics. Sb2O3 doping does not change crystal structure. Sb ions en-ter the crystal lattice entirely and form a solid solution.

Fig. 5 shows the microstructure for Sb2O3 doping NBT-

Fig. 4. XRD patterns of Sb2O3-doped 0.85NBT-0.144KBT- 0.006BT ceramics: (a) w(Sb2O3) = 0; (b) w(Sb2O3) = 0.1%; (c) w(Sb2O3) = 0.2%; (d) w(Sb2O3) = 0.3%; (e) w(Sb2O3) = 0.4%; (f) w(Sb2O3) = 0.5%.

Fig. 5. SEM images of Sb2O3-doped NBT-KBT-BT ceramics sintered at 1160°C for 2h: (a) w(Sb2O3) = 0; (b) w(Sb2O3) = 0.1%; (c) w(Sb2O3) = 0.2%; (d) w(Sb2O3) = 0.3%; (e) w(Sb2O3) = 0.4%; (f) w(Sb2O3) = 0.5%.

Huang X.Y. et al., Influence of Sb2O3 doping on the properties of KBT-NBT-BT lead-free piezoelectric ceramics 75

KBT-BT ceramics sintered at 1160°C for 2 h. The average grain size was measured from Fig. 5, and the relationship between average grain size of ceramics and Sb2O3 doping amount is listed in Table 1.

As shown in Fig. 5 and Table 1, the crystal grain size in-

creases first and then decreases while the Sb2O3 doping amount increases, the crystal grain size reaches maximum while the Sb2O3 doping amount is 0.4 wt.%, but the influ-ence of Sb2O3 doping on the microstructure is not obvious.

Table 1. Average grain size of samples doped with various Sb2O3 amounts

Sb2O3 doping amount /wt.% 0 0.1 0.2 0.3 0.4 0.5 Average grain size/μm 1.3 1.6 1.75 1.83 2.1 1.8

4. Conclusions

The influence of Sb2O3 doping amount on the properties and structure of NBT-KBT-BT lead-free piezoelectric ce-ramics were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM) and other analytical methods, and the conclusions were obtained as follows.

The piezoelectric strain constant (d33) of materials in-crease first and then decrease while the Sb2O3 doping amount increases, but the dielectric constant (εT

33/ε0) and di-electric loss increases while the Sb2O3 doping amount in-creases. The comprehensive properties of NBT-KBT-BT lead-free piezoelectric ceramics are excellent, whose d33 is 147 pC/N, εT

33/ε0 is 1510, and tanδ is 4.2% when the sinter-ing temperature is 1160°C. Sb2O3 has a donor effect when doped small amount while has an acceptor effect when doped large amount. The material phase of all composition ceramics is single perovskite structure, meaning that Sb ions enter the crystal lattice and form a solid solution, thus Sb2O3 doping does not change the crystal structure. The crystal grain size increases first and reaches the maximum when the Sb2O3 doping amount is 0.4 wt.%, and then decreases, but the influence of Sb2O3 doping on the microstructure is not obvious.

References

[1] Xiao D.Q. and Wan Z., Environmentally conscious piezo-and ferroelectric ceramics, Piezoelectr. Acoustoopt., 1999, 21 (5): 363.

[2] Takenanka T., Piezoelectric properties of some lead-free ferroelectric ceramics, Ferroelectrics, 1999, 230 (1): 87.

[3] Xiao D.Q., Environmentally conscious ferroelectric research. J. Korean Phys. Soc., 1998, 32 (94): 1798.

[4] Takenaka T., Maruyama K.I., and Sakata K., (Bi1/2Na1/2)TiO3 system for lead-free piezoelectric ceramics, Jpn. J. Appl. Phys., 1991, 30 (9B): 2236.

[5] Nagata H.,Yoshida M., Makiuchi Y., and Takenaka T., Large piezoelectric constant and high curie temperature of lead-free piezoelectric ceramic system based on bismuth sodium titan-ate-bismuth potassium titanate-barium titanate near mor-photropic phase boundary, Jpn. J. Appl. Phys., 2003, 42 (12): 7401.

[6] Chen W., Li Y.M., Zhou J., Xu Q., Wang Y., Sun H.J., and Xu R.X., Study on dielectric and piezoelectric properties of Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3-BaTiO3 lead-free piezoceramics system, Electron. Compon. Mater., 2004, 23 (11): 24.

[7] Chen Z.W., Development of Bi0.5Na0.5TiO3-BaTiO3 and Bi0.5Na0.5TiO3- Bi0.5K0.5TiO3 based lead-free piezoelectric ce-ramics, Mater. Rev., 2006, 20 (1): 14.

[8] Shieh J., Wu K.C., and Chen C.S., Switching characteristics of MPB compositions of (Bi0.5Na0.5)TiO3-BaTiO3-(Bi0.5K0.5)TiO3 lead-free ferroelectric ceramics, Acta Mater., 2007, 55(9): 3081.

[9] Li Y.M., Chen W., Xu Q., Zhou J., Gu X.Y., and Fang S.Q., Electromechanical and dielectric properties of Na0.5Bi0.5TiO3- K0.5Bi0.5TiO3-BaTiO3 lead free ceramics, Mater. Chem. Phys., 2005, 94: 328.

[10] Wei M. X., Huang X.Y., Gao C.H., and Cui Y.Z., Study on NBT-KBT-BT ternary system lead-free piezoelectric ceram-ics, Piezoelectr. Acoustoopt., 2008, 30 (5): 608.

[11] Xu R.X., Chen W., Xu Q., and Li Y.M., Study on the piezo-electric properties of (Na0.88K0.12)0.5Bi0.5TiO3 lead-free piezoelectric ceramics mixed with CeO2 and Sb2O3, Chin. Ceram., 2005, 41(6): 15.