Research Article www.acsami.org
Relationship between Poling Characteristics and Phase Boundaries of Potassium−Sodium Niobate Ceramics Ting Zheng and Jiagang Wu* Department of Materials Science, Sichuan University, 610064, Chengdu, People’s Republic of China S Supporting Information *
ABSTRACT: The controversy about the optimum poling conditions of (K,Na)NbO3 (KNN)-based lead-free ceramics was still unresolved and the relationships between poling characteristics and phase boundary types were rarely mentioned. Here, we tried to unveil the relationships between poling characteristics and phase boundary types of these ceramics. The optimum poling temperatures should be chosen near their corresponding phase transition temperatures. In addition, a large piezoelectricity can be attained in the ceramics with a multiphase coexistence under a lower poling electric field (EC) can guarantee the sufficient poling for KNN-based ceramics,9−12 while a large d33 can be achieved by applying an electric field far below EC in the KNN-based ceramics with R− © 2016 American Chemical Society
Received: February 12, 2016 Accepted: March 23, 2016 Published: March 23, 2016 9242
DOI: 10.1021/acsami.6b01796 ACS Appl. Mater. Interfaces 2016, 8, 9242−9246
Research Article
ACS Applied Materials & Interfaces
2. EXPERIMENTAL PROCEDURE
shows the room-temperature XRD patterns of nine kinds of material systems, measured at 2θ = 20−70°. All ceramics possess typical ABO3-type perovskite structure without impurity phases. In addition, the enlarged XRD patterns in 2θ = 44−47° were selected to determine the phase structures of all ceramics. According to the XRD patterns and previous results, a pure KNN ceramic belongs to single orthorhombic (O) phase,17 a KNNS ceramic belongs to the rhombohedralorthorhombic (R−O) phase boundary,18 the KNNS-0.02BNZ/ 0.02BNH/0.02BNZH ceramics are the orthorhombic-tetragonal (O−T) phase boundary,19 and the KNNS-0.04BNZ/ 0.035BNH/0.035BNZH ceramics are rhombohedral-tetragonal (R−T) phase boundary.19 In order to attain the ceramics with a single tetragonal (T) phase, we utilized the CaTiO3 additive to modify the KNNS-0.04BNZ ceramics by shifting its TR−T below room temperature.20 As a result, the O, T, R−O, O−T, and R− T phase compositions of KNN-based ceramics were designed by the chemical modifications. To further identify the phase structure, we chose the KNNS-0.04BNZ, KNNS-0.02BNZ, KNNS, and KNNS-BNZ-CT ceramics as representatives of R− T, O−T, R−O, and T, respectively. Thus, the temperature dependence of dielectric constant (εr-T) of four representative materials was exhibited in Figure S1 (Supporting Information), measured at −150 to 200 °C and f = 100 kHz. The phase transition temperature (TR−T, TO−T, and TR−O) of all ceramics can be clearly detected in the investigated temperature range, and then the relationships between phase transition temperatures and optimum poling temperatures will be discussed in detail. Table 1 shows some parameters of nine kinds of the compositions, including density, average grain size, dielectric properties, phase transition temperature, and phase boundary. We can find from Table 1 that the density of all modified compositions was enhanced compared with that of the pure KNN ceramics. In addition, the average grain sizes and their dielectric constant have a big difference. For example, large average grain sizes and high dielectric constant can be attained in the KNNS-0.04BNZ and KNNS-0.035BNH ceramics with an R−T phase boundary. In order to clearly exhibit the microstructure evolution, the surface microstructure of all ceramics is shown in Figure S2. A bimodal grain size distribution and dense microstructure can be found in all ceramics. However, the grain sizes can be greatly affected by chemical compositions. For example, the KNNS-0.04BNZ/ 0.035BNH ceramics with R−T phase boundary have a larger grain size as compared with others (Table 1). As is known, the piezoelectricity of a ferroelectric material can be induced by the poling process because the randomly reoriented domains in as-sintered samples will align toward the direction of the external electric fields.21 Therefore, the optimized poling conditions (poling temperature and poling electric field) is closely related to the property enhancement. Figure 2 shows the d33 of nine kinds of material systems with different phase structures as a function of poling temperatures (Tp) under the poling electric field (E) of 3 kV/mm. As shown in Figure 2a, the d33 of the ceramics with R−T almost decreases linearly with the increase of Tp due to the Tp deviating from TR−T (∼30 °C). A similar changing trend can also be found in other KNN-based ceramics with R−T phase boundary.13 It was reported that R−T phase coexistence can induce a lattice stress associated with the high degree of the polarization directions, which is beneficial to the domain mobility and the enhancement of piezoelectric properties.22,23 Therefore, we can conclude that the poling temperatures located at the phase
Raw materials including K2CO3 (99%), Na2CO3 (99.8%), Nb2O5 (99.5%), Sb2O3 (99.99%), Bi2O3 (99.999%), CaCO3 (99%), ZrO2 (99%), HfO2 (99%), and TiO2 (98%) were utilized to fabricate the K 0 . 4 8 Na 0 . 5 2 NbO 3 (KNN) ceramics with single O phase, K0.48Na0.52Nb0.92Sb0.08O3 (KNNS) ceramics with R−O phase boundary, 0.98K0.48Na 0.52Nb 0.96Sb 0.04−0.02 Bi 0.5Na0.5ZrO3 (KNNS− 0.02BNZ) ceramics with O−T phase boundary, 0.98K0.48Na0.52Nb0.96Sb0.04-0.02 Bi0.5Na0.5HfO3 (KNNS−0.02BNH) ceramics with O−T phase boundary, 0.98K0.45Na0.55Nb0.96Sb0.04 −0.02 Bi0.5Na0.5Zr0.85Hf0.15O3 (KNNS−0.02BNZH) ceramics with O−T phase boundary, 0.96K0.48Na0.52Nb0.96Sb0.04-0.04 Bi0.5Na0.5ZrO3 (KNNS−0.04BNZ) ceramics with R−T phase boundary, 0.965K 0.48 Na 0.52 Nb 0.96 Sb 0.04 −0.035 Bi 0.5 Na 0.5 HfO 3 (KNNS− 0.035BNH) ceramics with R−T phase boundary, 0.965K0.45Na0.55Nb0.96Sb0.04 −0.035 Bi0.5Na0.5Zr0.85Hf0.15O3 (KNNS− 0.035BNZH) ceramics with R−T phase boundary, and 0.95K 0.48 Na 0.52 Nb 0.96 Sb 0.04 −0.04 Bi 0.5 Na 0.5 ZrO 3 −0.01CaTiO 3 (KNNS−BNZ−CT) ceramics with single T phase by the conventional sintering method. All raw materials were weighted and ball milled for 24 h with alcohol. After that, the mixing slurries were dried and calcined at 850 °C for 6 h. These calcined powders were mixed with a binder of 8 wt % poly(vinyl alcohol) (PVA) and then were pressed into pellets with 10 mm diameter and 1 mm thickness under a pressure of 10 MPa. The pellets were sintered for 3 h in air after burning of PVA at 850 °C. The sintering temperatures for KNNS− 0.02BNH and KNNS−0.035BNH ceramics are 1080 and 1075 °C, respectively, and other samples are sintered at 1085 °C in air. For electrical measurement, both sides of the sintered pellets were pasted on silver and then fired at 600 °C for 10 min. X-ray diffraction (XRD) (Bruker D8 Advanced XRD, Bruker AXS Inc., Madison, WI, Cu Kα) was used to identify the phase structures of the ceramics. The temperature dependence of capacitance was measured using an LCR analyzer (HP 4980, Agilent, Santa Clara, CA) from −150 to 200 °C and f = 100 kHz. The polarization-electric (P−E) hysteresis loops were attained by Radiant Precision Workstation (USA) under f = 100 Hz and room temperature, where the hysteresis offset is zero, and the preloop delay is 1000 ms. At last, the d33 was measured by piezo-d33 meter (ZJ-3 A, China) with a DC electric field and by Radiant Precision Workstation (USA) with a AC electric field. The poling process with a DC electric field is realized by using the fresh samples at each voltage with a poling time of 20 min, and the poling process with an AC electric field is conducted by using the increased electric fields.
3. RESULTS AND DISCUSSION To investigate the relationships among phase boundaries, poling conditions, and electrical properties in KNN-based ceramics, we must first identify the phase structure. Figure 1
Figure 1. XRD patterns of KNN-based ceramics with nine components. 9243
DOI: 10.1021/acsami.6b01796 ACS Appl. Mater. Interfaces 2016, 8, 9242−9246
Research Article
ACS Applied Materials & Interfaces Table 1. Some Parameters of the Nine Kinds of Compositions materials
density (g/cm3)
grain size (μm)
εr (100 kHz)
tan δ (100 kHz)
KNN KNNS KNNS−BNZ−CT KNNS−0.02BNZ KNNS−0.02BNH KNNS−0.02BNZH KNNS−0.04BNZ KNNS−0.035BNH KNNS−0.035BNZH
4.19 4.29 4.27 4.33 4.23 4.25 4.34 4.29 4.35
1.49 1.70 7.71 3.19 3.36 2.56 9.75 13.53 3.26
310 565 1320 984 954 877 2067 2192 1759
0.0256 0.0680 0.0456 0.0429 0.0484 0.0564 0.0434 0.0524 0.0526
TR−O (°C)
TO−T (°C)
−10
200 100
−55 −55 −50
80 80 85
TR−T (°C)
−10
30 30 40
phase boundary O R−O T O−T O−T O−T R−T R−T R−T
Figure 3. d33 of KNN-based ceramics with nine components as a function of poling electric fields: (a) R−T phase boundary, (b) O−T phase boundary, (c) R−O phase boundary, and (d) pure O and T phases.
Figure 2. d33 of KNN-based ceramics with nine components as a function of poling temperatures: (a) R−T phase boundary, (b) O−T phase boundary, (c) R−O phase boundary, and (d) pure O and T phases.
O−T, 80 °C for R−O, RT for T, and 100 °C for O). Here, the d33 was poled by a DC electric field. It can be seen that d33 can be significantly affected by the phase structure and poling electric field. For example, for R−T/O−T/R−O, d33 increases abruptly with the increase of E (≤0.5 kV/mm), and then slightly increases and quickly saturates, as shown in Figure 3a− c. However, d33 of the ceramics with O or T increases rapidly with the increase of E (≤1 kV/mm), and then gradually increases and slightly saturates in the E range of 1−4 kV/mm. The result indicates that the available domain alignment can be attained in KNN-based ceramics with multiphase coexistence under a lower poling electric field as compared with the ones with O or T. There are two questions we need to solve. The first one is what caused the rapid increase of d33 at low E and the saturated d33 at high E. The second one is why the saturated d33 can be achieved at a E lower than their corresponding EC for the ceramics with R−T/O−T/R−O, while d33 of the ceramics with other phase structures can be saturated just at E > EC. On the one hand, the intrinsic and extrinsic contributions of domain vibration, domain wall motion, and domain switching can devote to the improvement of d33 of a piezoelectric material.27,28 However, only the domain switching occurs for E > EC.27,28 Therefore, the rapid increased d33 at E < EC is partly assigned to the intrinsic piezoelectric contribution as well as the extrinsic contribution of domain vibration and domain wall motion. The piezoelectricity poled at E > EC is partly attributed to the intrinsic piezoelectrics and the extrinsic domain switching. On the other hand, the domain types also play an
boundary region will play a positive role in the enhancement of piezoelectricity. Such a conclusion can also be confirmed in the KNN-based ceramics with O−T.7,24 As shown in Figure 2b, the d33 increases first and then decreases with the increase of Tp, reaching a maximum value for Tp = 100 °C (near TO−T ∼ 80 °C). Consequently, the ceramics poled near TR−T and TO−T will exhibit the best piezoelectricity. In addition, relatively good d33 values can be achieved in KNNS ceramics for Tp < 100 °C (Figure 2c) because of the involvement of TR−O (∼−10 °C) and TO−T (∼100 °C).25 At last, for KNN-based ceramics with a single O or T phase, the variations of d33 as a function of Tp were presented in Figure 2d. For a pure KNN ceramic, due to the absence of phase boundary, the improvement of d33 with the increase of Tp can be interpreted by the easier switching of domains,9 while the linear decrease of d33 for KNNS-BNZ-CT is mainly because its TR−T is shifted to below room temperature.26 As a result, for KNN-based ceramics, selecting Tp near phase transition temperatures greatly promotes piezoelectricity regardless of phase structures. As discussed above, the piezoelectricity will be improved by optimizing the poling temperatures. Thus, we are considering that what effects will be caused by changing the poling electric fields. In this part, the relationships among phase structure, poling electric field, and piezoelectricity will be systematically explored. Figure 3 shows the d33 of nine material systems with different phase structure as a function of E under the condition of optimized poling temperature (e.g., RT for R−T, 100 °C for 9244
DOI: 10.1021/acsami.6b01796 ACS Appl. Mater. Interfaces 2016, 8, 9242−9246
Research Article
ACS Applied Materials & Interfaces
as shown in Figure 4. Here, their Pr and d33 were measured by Radiant Precision Workstation with an AC electric field (EAC).
important role in the enhancement of piezoelectricity. From the crystallographic point of view, there are six domain states in perovskite structure with tetragonal symmetry (90° and 180° domains), eight domain states in pervoskite structure with rhombohedral symmetry (71°, 90°, and 180° domains), twelve domain states in perovskite structure with orthorhombic symmetry (60°, 90°, 120°, and 180° domains). During the poling process, the 180° domains can increase their polarization under a low electric field, while non-180° domain switching helps the enhancement of piezoelectric activity during a high electric field.9,10,17,29 Therefore, the increase of d33 can partly originate from the enhancement of polarization due to the existence of 180° domain under low E (≤0.5 kV/mm). In addition, the switching of non-180° domain plays a dominated role in the piezoelectricity during high E (1−4 kV/mm). At last, for Pb-based ceramics, the rapid increase of d33 may be due to the polarization rotation from rhombohedral to monoclinic domains,30 the transformation of cubic structure to ferroelectric structure of BNT-based ceramics was proved under a poling electric field,31 and an electric field-induced phase transition was also observed in KNN-based ceramics.32 As a result, we can conclude that the rapid increase of d33 at E < EC mainly originates from the combined effects of the intrinsic piezoelectrics, domain vibration, domain wall motion, the enhancement of polarization due to the existence of 180° domain, and the appearance of electric field-induced phase transitions. The slight increase or saturation of d33 at E ≥ EC is mainly attributed to the intrinsic piezoelectric contribution and the non-180° domain switching. After understanding the different piezoelectric contribution factors with EC as the dividing line, we will continue to explain the second problem. It was reported that only a high E (>EC) can guarantee the sufficient poling behavior for most KNN-based ceramics.9−12 This point of view can be also proved in the KNN-based ceramics with single O/T phase, as shown in Figure 3d. However, a large piezoelectricity can be still observed at E < EC for the ones with R−T/O−T/ R−O, as shown in Figure 3a−c. This phenomenon was also reported in other KNN-based ceramics with R−T,13 BNT-BT ceramics with P4bm phase,14 and (Pb0.92La0.08) (Zr0.6Ti0.4)O3 ceramics with R−T.15 In the past, Guo et al. interpreted that the unusual behavior was due to the polarization alignment of polar nanodomains in the nonergodic relaxor phase. 14 However, we cannot use this theory to explain the current results in KNN-based ceramics due to the absence of experimental evidence. An interesting phenomenon was mentioned in BNT-BT ceramics, that is, this unusual behavior was not observed in the R3c and P4mm ferroelectric phase regions and was only shown in the compositions with P4bm phase.14 Combined with our results, a similar phenomenon can also be found, that is, the unusual behavior was observed only for KNN-based ceramics with R−T/O−T/R−O phase boundary. Therefore, we can deduce that the saturation of domain reorientation degree can be achieved at a lower motivated electric field due to the decreased energy barrier originated from the multiphase coexistence. As a result, the construction of multiphase coexistence is effective to attain high piezoelectric response under a low E. Throughout the researches about the poling effects on d33 in ferroelectric materials, there are few reports about the piezoelectric response after the measurement of P−E loops. In this work, the AC electric field dependence of remanent polarization (Pr) and piezoelectric constant (d33) of KNNbased ceramics with different phase structures was investigated,
Figure 4. Pr and d33 of KNN-based ceramics with nine components as a function of AC electric fields.
Compared with the effects of DC electric fields on d33 in KNNbased ceramics, different results can be attained by AC electric field. For O or T, there is almost no ferroelectric and piezoelectric response in EAC = 0−1 kV/mm, and their Pr and d33 values increase gradually with the further increase of EAC. However, for R−O/R−T/O−T, their Pr and d33 values increase quickly for EAC < 2 kV/mm and then slightly increase with the further increase of EAC. As a result, the domains of KNN-based ceramics with multiphase coexistence are easier to switch with respect to the ones with O or T. This results stimulated by EAC are similar to those stimulated by EDC. However, there is a big difference about the threshold electric field of domain switching under EAC and EDC. To clearly exhibit the different poling effects on d33 by applying EAC and EDC, their P−E loops and d33 as a function of EAC and EDC of KNNS-0.04BNZ, KNNS0.02BNZ, KNNS, KNN, and KNNS-BNZ-CT ceramics were shown in Figure 5. It can be seen that the threshold EAC (∼2 kV/mm) of all samples is much higher than the threshold EDC (0.5−1 kV/mm). That is, to get saturated piezoelectricity, the EAC of KNN-based ceramics should be higher than the corresponding EC regardless of phase structures, while EDC is lower or roughly equal to EC, which depends on phase structure. The higher threshold EAC may be due to the shorter poling time and the different mechanisms of domain switching.
4. CONCLUSIONS The comprehensive investigations between poling characteristics and phase boundary in KNN-based ceramics were carried out. The phase structure of KNN-based ceramics can greatly influence the poling characteristics. The largest piezoelectric response can be attained at a poling temperature near its phase transition temperature. A lower threshold DC electric field (
