Structure and electrical properties of the Ho2O3 doped ... - Springer Link

J Mater Sci: Mater Electron (2012) 23:2167–2172 DOI 10.1007/s10854-012-0734-5

Structure and electrical properties of the Ho2O3 doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 lead-free piezoelectric ceramics Peng Fu • Zhijun Xu • Ruiqing Chu • Xueyan Wu Wei Li • Huimin Zhang

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Received: 9 March 2012 / Accepted: 16 April 2012 / Published online: 12 May 2012 Ó Springer Science+Business Media, LLC 2012

Abstract Ho2O3 (0–0.7 wt%)-doped 0.82Bi0.5Na0.5TiO3– 0.18Bi0.5K0.5TiO3 (BNKT18) lead-free piezoelectric ceramics were synthesized by a conventional solid-state reaction method. The effects of Ho2O3 on the microstructure and electrical properties were investigated. X-ray diffraction data shows that Ho2O3 in an amount of 0.1–0.7 wt% can diffuse into the lattice of the BNKT18 ceramics and form the pure perovskite phase. Scanning electron microscope (SEM) images indicate that the grain sizes of BNKT18 ceramics decrease with the increase of Ho2O3 content; in addition, the modified ceramics have the clear grain boundary and a uniformly distributed grain size. At room temperature, the electrical properties of the BNKT18 ceramics have been improved with the addition of Ho2O3, and the BNKT18 ceramics doped with 0.3wt.% Ho2O3 have the highest piezoelectric constant (d33 = 137 pC/N), the highest remnant polarization (Pr = 26.9 lC/cm2), the higher relative dielectric constant (er = 980) and lower dissipation factor (tand = 0.046) at a frequency of 10 kHz. The BNKT18 ceramics doped with 0.1 wt% Ho2O3 have the highest planar coupling factor (kp = 0.2426).

P. Fu (&) Z. Xu R. Chu W. Li School of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of China e-mail: [email protected] P. Fu X. Wu School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China H. Zhang Liaocheng People’s Hospital, Liaocheng 252000, People’s Republic of China

1 Introduction Bismuth sodium titanate Bi0.5Na0.5TiO3 (BNT) and bismuth potassium titanate Bi0.5K0.5TiO3 (BKT) are important as end members for a variety of solid solutions. However, the properties of the pure materials themselves are not sufficiently good for applications [1]. Bi0.5Na0.5TiO3– x Bi0.5K0.5TiO3 (BNKT), synthesized in 1996 by Elkechai et al. [2], has attracted considerable attention owing to its excellent piezoelectric properties in the composition being close to the tetragonal-rhombohedral morphotropic phase boundary (MPB) near x = 0.18 [3–5], which can provide substantially improved piezoelectric properties [6–9]. This directive gave a boost to the development of BNKT system piezoelectric ceramics, and a variety of BNKT systems have developed in recent decades. However, BNKT18 ceramics are less dense and more difficult to pole because of higher leakage currents [10]. Therefore, for practical applications, the electrical properties of BNKT18 ceramics need to be further enhanced. In order to further enhance the properties of BNKT18 ceramics, some rare earth oxides such as Sm2O3 [11], Nd2O3 [12], CeO2 [13, 14], Eu2O3 [15] have been used as additives of BNKT18 ceramics, and the results show that those rare earth elements were effective additives in enhancing the electrical properties of BNKT18 ceramics. However, the research of Ho2O3 doped BNKT18 ceramics has not been conducted so far. As we all know, the radius ˚ ) is very close to the radii of Bi3? of Ho3? (0.901 A ? ˚ ) and Na (1.02 A ˚ ). In view of the radius, it is (1.03 A possible for Ho3? to enter into the A-sites of the BNKT18 lattice and affect the properties of BNKT18 ceramics. Therefore, in present study, Ho2O3 doped BNKT18 ceramics were prepared by conventional solid-state reaction method firstly and the effects of Ho2O3 dopant on the

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3 Results and discussions

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Figure 1 shows XRD patterns of the Ho2O3 doped BNKT18 ceramics sintered at 1,150 °C. It can be seen that all samples have formed the pure perovskite phase, as

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Ho2O3 doped BNKT18 ceramics were prepared by conventional solid-state reaction processes using high purity Bi2O3 (99.63 %), Na2CO3 (99.5 %), K2CO3 (99.5 %), TiO2 (99.5 %) and Ho2O3 (99.9 %) as starting raw materials. The starting materials were put in the oven at 120 °C for 12 h to dry, weighed according to the stoichiometric formula, and then mixed together by a planet mill in a nylon jar with agate ball for 12 h. Next, the mixed powders were dried and calcined at 900°C for 2 h. After calcining, the powders were ball-milled again by the planet mill with agate balls for 6 h, the dried powders were mixed with polyvinyl alcohol (PVA) and pressed into disks with a diameter of 12 mm, and then calcined at 800 °C to exclude binder (PVA). Finally, the pressed disks were sintered at 1,150 °C for 2 h in air. The sintered samples were polished and pasted with silver slurry on both faces, and then fired at 740 °C as electrodes. Specimens for piezoelectric measurements were poled for 20 min by silicone oil bath with the existence of a dc electric field of 4–6 kV/mm. After laying the polarized specimens for approximate 24 h to release the remnant stress, piezoelectric properties were measured subsequently. The crystal structures of the BNKT18 ceramics were determined by X-ray diffraction (XRD) using a Cu Ka ˚ ) (Ultima IV, Rigaku Co., LTD, radiation (k = 1.54178 A Japan). The surface microstructure of the sintered ceramic specimens was observed by scanning electron microscope (SEM; JSM-5900, Japan). The piezoelectric coefficient d33 was measured by a quasistatic d33-meter (YE2730, SINOCERA, China). The electromechanical coupling factors (kp) was determined by a resonance-antiresonance method on the basis of IEEE standards using a precision impedance analyzer (Agilent 4294A, America). The ferroelectric polarization versus electric field (P–E) measurements was conducted at 10 Hz using a standardized ferroelectric test system (TF2000, Germany). All measurements of electrical properties above were carried out at temperatures in the range of 20–25 °C. And the curve between relative dielectric constant and temperature were also measured by Agilent 4294A precision impedance analyzer.

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shown in Fig. 1, implying that Ho3? may have diffused into the BNKT18 ceramics lattice or the second phase cannot be detected because of the small doping amount of Ho2O3. These results indicate that the addition of Ho2O3 does not lead to an obvious change in the phase structure. Figure 2 shows the SEM micrographs of the Ho2O3 doped BNKT18 ceramics sintered at 1,150 °C. From Fig. 2, the grains with regular crystal shape and crystalline boundaries are clear in all samples. The grain size of BNKT18 ceramics decreases with the increase of Ho2O3 content, and the crystal grain becomes homogeneous; this result is similar to the Refs. [11, 12]. Piezoelectric ceramics usually require a high mechanical strength. A fine grain size and uniform grain microstructure is able to enhance the density and mechanical strength of piezoelectric ceramics [5, 16]. Therefore, the ceramics with a fine grain size and homogeneous microstructure are advantageous for piezoelectric ceramics applications. However, the crystal shape becomes irregular and crystalline boundaries become smeared-out as x further increasing to 0.7. The polarization versus electric field hysteresis loops for all samples were measured at room temperature at 10 Hz, and the results are presented in Fig. 3. The variations of the remnant polarization Pr and coercive field Ec with x are shown in Fig. 4. From Fig. 3, it is evident that the ferroelectric properties of the BNKT18 ceramics have significantly been affected by doping with Ho2O3. Compared with the pure BNKT18 ceramics, the remnant polarization Pr increases obviously while the coercive field Ec increases slightly with increasing of Ho2O3 contents. Pr reaches a maximum value of 26.9 lC/cm2 when Ho2O3 content is 0.3 wt% and the coercive field Ec reaches a maximum value of 4.92 kV/mm when Ho2O3 content is 0.1 wt%, as shown in Fig. 4. This result indicates that the BNKT18

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structure and electrical properties of the BNKT18 ceramics were investigated.

J Mater Sci: Mater Electron (2012) 23:2167–2172

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Fig. 1 X-ray diffraction patterns of the BNKT18–x (wt%) Ho2O3 ceramics


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Fig. 2 SEM micrographs of surfaces of the BNKT18– x (wt%) Ho2O3 ceramics

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Fig. 4 The remnant polarization Pr and coercive field Ec of the BNKT18–x (wt%) Ho2O3 ceramics as a function of x (f = 10 Hz)

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0:82Bi0:5 Na0:5 TiO3 0:18Bi0:5 K0:5 TiO3

Ho2 O3 ! 2HoBi þ 3OO 0:82Bi0:5 Na0:5 TiO3 0:18Bi0:5 K0:5 TiO3

! 2HoNa þ 4V0Na þ 3OO Ho2 O3 ð2Þ When Ho3? occupies Bi-site, as shown in Eq. (1), the substitution of Bi3? by Ho3? may cause the slack of BNKT18 lattice. The lattice deformation can make the ferroelectric domains reorientation more easily during electrical poling and leads to the enhancement of piezoelectric properties. Additionally, Ho3? can also occupy the A-site of Na?, as shown in Eq. (2). In this case, the valence of Ho3? ion is higher than that of Na? ion. To maintain an overall electrical neutrality, Ho3? acts as a donor leading to some Na-site vacancies [V0Na ], which can relax the strain caused by reorientation of domains. Therefore, the movement of the domains becomes easier and thus the piezoelectric properties of the BNKT18 ceramics are improved significantly [18, 19]. Furthermore, easier domain reorientation increases the degree of modulating spontaneous polarization, which makes the remnant polarization Pr increase. This result agrees with the discussions in Fig. 4. However, if the amount of Ho2O3 is excessive, the distortion of crystal cell would be enlarged, the difficulty of polarization would be increased. As a result, the piezoelectric and ferroelectric properties of BNBK18 ceramics doped with 0.7 wt% Ho2O3 decreases subsequently. Figure 6 shows temperature dependence of the relative dielectric constant and the loss tangent of BNKT18– x (wt.%) Ho2O3 ceramics at a frequency of 10 kHz. The

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ceramics doped with appropriate Ho2O3 exhibit a larger remnant polarization Pr and coercive field Ec compared with the pure BNKT18 ceramics. Enhanced remnant polarization shows that ferroelectric properties of the BNKT18 ceramics have been improved with the addition of Ho2O3. Moreover, even if the maximum voltage allowed by the ferroelectric test instrument was added, the polarization versus electric field hysteresis loop of the pure BNKT18 ceramics has not reached saturation because the pure BNKT18 ceramics is not easily polarized [10], which makes the Ec of the pure BNKT18 ceramics smaller. But the curves of the Ho2O3 modified BNKT18 ceramics are relatively saturated, the Ec spreads outward in the hysteresis loops which makes the Ho2O3 modified BNKT18 ceramics have larger Ec, as discussed in Figs. 3 and 4. Figure 5 shows the variations of the room-temperature piezoelectric properties of BNKT18–x (wt%) Ho2O3 ceramics with x. From Fig. 5, with the increase of x, the piezoelectric constant d33 and planar coupling factor kp increase firstly and reaches peak values at the Ho2O3 doping level of 0.3 and 0.1 wt% respectively: d33 = 137 pC/N, kp = 0.2426, and then the two values decrease with the further increase of Ho2O3. The results show that the addition of appropriate Ho2O3 improves the piezoelectric properties of the BNKT18 ceramics significantly. The maximum value of the piezoelectric constant is also larger than the reported values of some rare earth oxide modified BNKT ceramics [12, 14], but lower than that of other rare earth oxide modified BNKT ceramics [13, 15]. The variation of piezoelectric properties can be explained with the ‘‘soft’’ and ‘‘hard’’ additive model. According to ‘‘soft’’ and ‘‘hard’’ additive model [17], the ˚ ) is very close to the radii of Bi3? radius of Ho3? (0.901 A ? ˚ ˚ ). Accordingly, Ho3? is consid(1.03 A) and Na (1.02 A ered to be a substitute occupying the A sites of BNKT18

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Fig. 5 The piezoelectric coefficient d33 and planar electromechanical coefficient kp of the BNKT18–x (wt%) Ho2O3 ceramics as a function of x

Fig. 6 The relative dielectric constants and loss tangent of the BNKT18–x (wt%) Ho2O3 ceramics as a function of temperature (f = 10 kHz)

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curves of temperature dependence of relative dielectric constant for different samples show two-phase transitions, as shown in Fig. 6. From Fig. 6, Td is the depolarization temperature which corresponds to the transition from a ferroelectric state to so-called ‘‘anti-ferroelectric’’ state which is defined as one in which lines of ions in the crystal are spontaneously polarized, but with neighboring lines polarized in antiparallel directions [20], while Tm is the maximum temperature at which relative dielectric constant er reaches a maximum value and corresponds to a transition from an ‘‘anti-ferroelectric’’ state to a paraelectric state [21]. The inflexion of the curves was called ‘‘a shoulder’’ on the curve in some reports and indicated an intermediate phase transition. An opinion about this shoulder is that the temperature region between the shoulder and Tm is an antiferroelectric phase [22]. Figure 7 shows the variations of the room-temperature relative dielectric constants and loss tangent for the BNKT18–x (wt%) Ho2O3 ceramics with x. At room temperature, compared with the pure BNKT18 ceramics, the relative dielectric constant er of Ho2O3 modified BNKT18 ceramics increases obviously, and the BNKT18 doped with 0.3 wt% Ho2O3 ceramics has the higher relative dielectric constant: er = 980, as shown in Fig. 7. The maximum value of the relative dielectric constant at room temperature is also higher than that of some other rare earth oxide modified BNKT ceramics [12–14]. Furthermore, compared with pure BNKT18 ceramics, the dielectric maxima of 0.3 wt% Ho2O3 modified BNKT18 ceramics attains the maximum value. It is 3665 for the BNKT18 ceramics doped with 0.3 wt% Ho2O3. However, the dielectric maxima become lower again when the volume of Ho2O3 continue to increase, as shown in Fig. 6. From Fig. 6, the loss tangent tand gradually increases with temperature up to Td where it reaches its maximum due

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to the ferroelectric to anti-ferroelectric phase transition, and then decreases with the temperature because of the less distortion in the crystalline structure after depolarization until Tm [23]. Moreover, it is found that the loss curves show a sharp Td peak for the pure BNKT18 ceramics, but the peaks were diffused for Ho2O3 modified BNKT18 ceramics, as shown in Fig. 6. The sharp peak in the loss curves of the pure BNKT18 ceramic is due to formation of macrodomains in its poled sample. However, the diffused character in the Ho2O3 modified BNKT18 ceramics is due to Ho ions substitutions, which are the sources of random fields that break the long range order of the BNKT18 and stabilize the polar nanoregions at low temperature. It is similar to Zr-modified Bi0.5(Na0.78K0.22)0.5TiO3 ceramics studied by H. Ali et al. [24]. At room temperature, low dielectric loss is obtained in all samples and it changes slightly with the increase of Ho2O3, and the BNKT18 ceramics doped with 0.3 wt% Ho2O3 have a lower dielectric factor (tand = 0.046) at a frequency of 10 kHz.

4 Conclusions The Ho2O3 doped BNKT18 ceramics has been investigated. All BNKT18 ceramics form the pure perovskite phase structure, and no obvious change in the crystal structure is observed with the addition of Ho2O3. The grain size of the BNKT18 ceramics decreases with the increase of Ho2O3 content, and Ho2O3 modified ceramics have the clear grain boundary and uniformly distributed grain size. The addition of appropriated Ho2O3 improves the piezoelectric and dielectric properties of BNKT18 ceramics significantly. At room temperature, the piezoelectric constant d33 and planar coupling factor kp reaches the peak values at the doping level of 0.3 and 0.1 wt% Ho2O3 respectively: d33 = 137 pC/N, kp = 0.2426, the relative dielectric constant er attains 980 and tand = 0.046 (at a frequency of 10 kHz) at the Ho2O3 doping level of 0.3 wt. %. The P–E hysteresis loops of BNKT18–x (wt%) Ho2O3 ceramics show that the proper Ho2O3 addition results in the increase of the remnant polarization Pr, and it reaches a maximum value at the Ho2O3 doping level of 0.3 wt%: Pr = 26.9 lC/cm2. Acknowledgments This work was supported by the Ph. D. Programs Foundation of Shandong Province of China (No. BS2010CL010) and the Natural Science Foundation of Shandong Province of China (No. ZR2011EMQ015).

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