Abstract: Lead-based complex perovskite ferroelectric materials have been widely used as electromechanical sensors, actuators, and transducers. Among them, Pb(Ni1/3Nb2/3)O3-PbTiO3 (PNN-PT) based solid solution has been drawn much attentions of scientists for its excellent dielectric and piezoelectric properties near morphotropic phase boundary (MPB) region. However, the relatively high dielectric loss and low Curie temperature near MPB region limited its application in high temperature and high power devices. In this work, Pb(In1/2Nb1/2)O3 (PIN) was introduced into PNN-PT ceramics for improving their electrical properties and Curie temperature. The ternary ferroelectric ceramics Pb(In1/2Nb1/2)O3-Pb(Ni1/3Nb2/3)O3-PbTiO3 were successfully prepared by a two-step synthesis process. All samples exhibited pure perovskite phase without any secondary phase. The structure transferred from rhombohedral to tetragonal phase with increasing PT content. The MPB phase diagram of ternary system at room temperature was established based on XRD results. The values of Curie temperature were improved significantly after PIN added into PNN-PT system. Importantly, the introduction of PIN into PNN-PT system can effectively reduce dielectric loss and conductivity. The ceramics in the MPB region exhibited excellent properties. 0.30PIN-0.33PNN-0.37PT ceramic was found to have optimal properties with d33=417 pC/N, TC=200 ℃, ε′= 3206, tanδ=0.033, Pr=33.5 μC/cm2 and Ec=14.1 kV/cm at room temperature, respectively. The Curie temperature and piezoelectric coefficient were improved while dielectric loss and conductivity were reduced after the introduction of PIN into PNN-PT. The enhancements of piezoelectric properties and high Curie temperature make this ternary system a promising material for high power and high temperature transducer applications.
Key words: ferroelectric ceramics; morphotropic phase boundary; Curie temperature; piezoelectric properties
Lead-based complex perovskite ferroelectric materials Pb(B1,B2)O3-PbTiO3 (B1=Mg2+, Zn2+, Ni2+, Sc3+, In3+,Lu3+, …, B2=Nb5+, Ta5+, W6+, …) have been widely used as electromechanical sensors, actuators, and transducers[1-2]. These systems near the morphotropic phase boundary (MPB) display outstanding piezoelectric and dielectric properties[3-4]. As a typical relaxor ferroelectrics, Pb(Ni1/3Nb2/3)O3-PbTiO3 (PNN-PT) have attracted great attention due to its excellent dielectric constant (ε' > 4000) and piezoelectric coeffiecient (d33=450 pC/N) near MPB region[5-6]. On this basis, PNN-based binary and ternary systems, such as Pb(Ni1/3Nb2/3)O3-PbTiO3 (PNN-PT), Pb(Mg1/3Nb2/3)-Pb(Ni1/3Nb2/3)O3-PbTiO3(PMN-PNN-PT), Pb(Zn1/3Nb2/3)O3-Pb(Ni1/3Nb2/3)O3-PbTiO3(PZN-PNN-PT), which exhibit excellent electrical properties near the MPB compositions, have been reported. Although PNNPT binary system has outstanding electrical properties, it has some drawbacks. Fistly, the relatively low electric polarization intensity and high leakage current induced by the variable valence of Ni ions, following inferior piezoelectric coefficients. Secondly, PNN-PT binary system exhibit relatively low Curie temperature near MPB region (120 ℃), which limit its application in high temperature[7-8,11]. Therefore, the addition of other compo-nent with high Curie temperature to PNN-PT system can improve electrical properties and Curie temperature, such as Pb(Lu1/2Nb1/2)O3-Pb(Ni1/3Nb2/3)O3-PbTiO3(PLN-PNN-PT), Pb(Ni1/3Nb2/3)O3-PbHfO3-PbTiO3(PNN-PH-PT) and
Pb(Ni1/3Nb2/3)O3-Pb(Sc1/2Nb1/2)O3-PbTiO3(PNN-PSN-PT). These ternary systems have been confirmed to have high Curie temperature and excellent piezoelectric properties[12-14].
As a member of Pb(B1,B2)O3 system, Pb(In1/2 Nb1/2)O3 (PIN) has Curie temperature of about 90 ℃. Furthermore, the solid solution of (1–x)PIN-xPT near its MPB exhibits high Curie temperature TC ~ 300 ℃ and excellent piezoelectric properties (d33=395 pC/N)[16-18]. For above purposes, the introduction of PIN into PNN-PT system may improve its Curie temperature and piezoelectric properties,
and reduced conductivity. Therefore, in present work, PIN was introduced into the PNN-PT system to form a ternary system PIN-PNN-PT. Structure, MPB diagram, electrical properties of PIN-PNN-PT were investigated.
yPb(In1/2Nb1/2)O3-(1–x–y)Pb(Ni1/3Nb2/3)O3-xPbTiO3(y=0.10, 0.30, 0.50) ceramics were prepared using a twostep synthesis process and raw materials of PbO, In2O3, NiO, Nb2O5 and TiO2. First, the precursors of B-site ions were prepared using the columbite or wolframite method. InNbO4 (IN) was calcined according to the stoichiometric proportions at 900 ℃ for 4 h, and NiNb2O6 (NN) was calcined at 1000 ℃ for 6 h. Second, IN, NN, PbO, TiO2 were mixed and calcined at 800–850 ℃ for 2 h with addition of 4mol% PbO for compensating its evaporation during sintering. Third, the calcined powders were mixed with 5wt% polyvinyl alcohol as binder and then pressed into pellets. After burning out the binder at 550 ℃ for 2 h, the pellets were sintered at 1050–1150 ℃ for 2 h in a sealed Al2O3 crucible to obtain the desired ceramics.
For structural analysis, the sintered samples after being pulverized into powder were examined by X-ray diffraction technique with CuKα radiation (MiniFlex II, Rigaku, Japan). A scanning electron microscope (JSM6700F, JEOL Tokyo, Japan) was used to investigate the morphology and microstructure of the ceramics. The dielectric properties were analyzed using a computercontrolled Alpha-A broad band dielectric/impedance spectrometer (Novocontrol, GmbH, Germany), with an AC signal of 1.0 V (peak-to-peak) applied. An aix-ACCT2000 analyzer (f=4 Hz) was used to display the ferroelectric hysteresis loops at room temperature. All samples were
poled at 90 ℃ for 15 min in silicone oil immersed in silicone oil using a DC electric field which was 1.5 times higher than coercive field. The piezoelectric coefficients d33 were measured using a quasi-static d33 meter (Institute of Acoustics, CAS, model ZJ-4AN, China)
2 Results and discussion
2.1 Structural analysis
The XRD patterns of 0.10PIN-(0.90–x)PNN-xPT (x=0.35, 0.37, 0.39 and 0.41), 0.30PIN-(0.70–x)PNNxPT (x=0.33, 0.35, 0.37 and 0.39) and 0.50PIN-(0.50–x)PNN-xPT (x=0.31, 0.33, 0.35 and 0.37) are shown in Fig. 1. All samples exhibit pure perovskite phase without any secondary phase. It can be observed that the structure of the ceramics samples is transferred from rhombohedral to tetragonal phase with increasing PT content according to the XRD patterns of (200)/(002) reflections around 2θ=45°, identifying one peak in rhombohedral phase and two peaks in tetragonal phase.
Fig. 1 XRD patterns and enlarged patterns of (200)/(002) reflections of PIN-PNN-PT ceramics at room temperature
(a) 0.10PIN-(0.90–x)PNN-xPT; (b) 0.30PIN-(0.7–x)PNN-xPT; (c)0.50PIN-(0.5–x)PNN-xPT
The MPB was determined to be at x=0.37–0.39, 0.35–0.37, 0.33–0.35 for 0.10PIN, 0.30PIN and 0.50PIN series, respectively. According to the XRD results, the MPB phase diagram of PIN-PNN-PT ternary system at room temperature was delimited as shown in Fig. 2
Fig. 2 MPB region of PIN-PNN-PT ternary system at room temperature
SEM micrographs of fracture surface of selected 0.10PIN-(0.90–x)PNN-xPT ceramics are showed in Fig. 3, indicating high density and few pore. In addition, it was found that the average grain size vary slightly for samples with different contents of PNN.
Fig. 3 SEM micrographs of fracture surface of 0.10PIN-xPNN-yPT ceramics
(a) 0.10PIN-0.49PNN-0.41PT; (b) 0.10PIN-0.51PNN-0.39PT; (c)0.10PIN-0.53PNN-0.37PT; (d) 0.10PIN-0.55PNN-0.35PT
2.2 Dielectric properties
The temperature dependence of dielectric constant (ε') and dielectric loss (tanδ) of 0.50PIN-(0.50–x)PNN-xPT are shown in Fig. 4. It can be seen that the Curie temperature TC increases with increasing PT content. The values of TC are improved significantly compared with PNN-PT binary systems. In addition, the Curie temperature TC is independent of frequency for 0.50PIN-(0.50–x)PNN-xPT, indicating normal ferroelectric behavior. The dielectric loss increased significantly when temperature above the Curie temperature, which was caused by leakage conductance loss.
Fig. 4 Temperature dependence of dielectric constant (ε') and dielectric loss (tanδ) of 0.50PIN-(0.50-x)PNN-xPT ceramics
(a) 0.50PIN-0.13PNN-0.37PT; (b) 0.50PIN-0.15PNN-0.35PT; (c) 0.50PIN-0.17PNN-0.33PT; (d) 0.50PIN-0.19PNN-0.31PT
Besides, as shown in Fig. 5(a), the values of tanδ decrease with increasing PIN level, indicating that the introduction of PIN into PNN-PT system can effectively reduce dielectric loss and conductivity, which is a shortcoming of PNN-PT. The Curie temperature TC as a function of PT contents is displayed in Fig. 5(b). The Curie temperatures TC increase abruptly from 109 to 191 ℃ for 0.10PIN series, from 171 to 222 ℃ for 0.30PIN series and from 221 to 228 ℃ for 0.50PIN series with increasing PT, respectively. Detailed parameters of dielectric properties measured at 1 kHz are listed in Table 1.
Fig. 5 (a) tanδ, (b) TC and (c) d33 as a function of PT contents for yPIN-(1–x–y)PNN-xPT ceramics
Table 1 The values of ε′, tanδ, d33, TC, Pr and EC of yPIN-(1–x–y)PNN-xPT ternary ceramics
2.3 Piezoelectric properties
Fig. 5(c) shows piezoelectric constant d33 as a function of PT content for the PIN-PNN-PT ternary ceramics. It is clearly observed that with increasing PT content, the piezoelectric constant d33 increases initially, reaching the maximum value at MPB region, and then decreases. The values of d33 vary from 317 to 364 pC/N for 0.10PIN series, from 375 to 417 pC/N for 0.30PIN series, from 362 to 401 pC/N for 0.50PIN series. The optimal electrical properties appear in the MPB composition of 0.30PIN-0.33PNN-0.37PT with the d33=417 pC/N. Detailed d33 of all samples were listed in Table 1.
2.4 Ferroelectric properties
The ferroelectric hysteresis loops of the PIN-PNN-PT ceramics were characterized as shown in Fig. 6, exhibiting well-saturated loops. The value of remnant polarization (Pr) and coercive field (Ec) of PIN-PNN-PT ceramics are displayed in Table 1. With increasing PT content, the remnant polarization Pr was found to increase firstly, reaching the maximum at MPB composition, and then decrease, which is caused by the coexistence of rhombohedral and tetragonal phases at the MPB region. On the contrary, the coercive field Ec was found to decrease at first, reaching the minimum at MPB region, and then increase with increasing of PT content, which was due to that free energy profile flatten at MPB region and then the reduced energy barrier causes the polarization easy to switch[19-21].
Fig. 6 Ferroelectric hysteresis of (a) 0.10PIN-xPNN-yPT, (b) 0.30PIN-xPNN-yPT, and (c) 0.50PIN-xPNN-yPT ceramics
In conclusion, the PIN-PNN-PT ternary ceramics with compositions near MPB region were synthesized using a two-step method and characterized by X-ray diffraction, dielectric, ferroelectric and piezoelectric measurements. The MPB region of the ternary system were obtained. The optimized composition was found to be the composition of 0.30PIN-0.33PNN-0.37PT with the d33=417 pC/N, TC= 200 ℃, ε′=3206, tanδ=0.033, Pr=33.5 μC/cm2 and EC=14.1 kV/cm. The PIN-PNN-PT ternary ferroelectric ceramics had the advantage of PIN-PT and PNN-PT, shows larger piezoelectric performance, higher TC and lower dielectric loss than those of PNN-PT ceramics, makes it a potential candidate utilize in transducer and actuator applications.
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