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The effects of the phase structure of the polymorphic phase boundary on the piezoelectric properties of (K,Na)NbO3-based ceramics
Current Applied Physics 16 (2016) 593e598
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
The effects of the phase structure of the polymorphic phase boundary on the piezoelectric properties of (K,Na)NbO3-based ceramics Sun-A Yang a, Byung-Hoon Kim a, Min-Ku Lee a, Sang-Don Bu b, Gyoung-Ja Lee a, * a b
Nuclear Materials Development Division, Korea Atomic Energy Research Institute, Daejeon 34057, Republic of Korea Department of Physics and Research Institute of Physics and Chemistry, Chonbuk National University, Jeonju 54896, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2016 Received in revised form 24 February 2016 Accepted 3 March 2016 Available online 4 March 2016
We report the effects of the phase, i.e., the rhombohedral (R), orthorhombic (O) and tetragonal (T) phase, within a polymorphic phase boundary on the piezoelectric properties of (K,Na)NbO3 ceramics doped with Bi(Na,K,Li)ZrO3 and (Bi,Na)TiO3. For the R-O-T phase boundary, the formation of an R-T phase boundary by O phase shrinkage is clearly beneficial to enhance the piezoelectric performance, whereas the enrichment of the T phase in the R-T phase boundary negatively affects the piezoelectric activity. Electrical poling in relation to the piezoelectric property strongly depends on the nature of the phase boundary, requiring the optimization of temperatures corresponding to the R-T phase boundary without the O phase. © 2016 Elsevier B.V. All rights reserved.
Keywords: Piezoelectric ceramics Polymorphic phase transformation Phase structure Piezoelectricity
1. Introduction (K,Na)NbO3 (KNN)-based ceramics have become the most overwhelmingly investigated lead-free piezoelectric material system in the past ten years owing to a large piezoelectric coefficient d33 comparable to that of PZT and a higher Curie temperature TC than PZT [1e7]. From the expectations of superior piezoelectric properties owing to the formation of a phase boundary, a great amount of attention has been directed toward the construction of a polymorphic phase boundary (PPB) of the KNN system near the working temperature, usually room temperature [3e5,8]. It is known that pure KNN undergoes a series of structural phase transitions, as the temperature increases. These are the rhombohedral-orthorhombic (R-O) transition at 123 C (TR-O), the orthorhombic-tetragonal (O-T) transition at 210 C (TO-T), and the tetragonal-cubic (T-C) transition at 410 C (TC) [1]. Such PPBs do not form in the vertical direction with regard to compositions, instead showing strong temperature-dependence, clearly different from a traditional morphotropic phase boundary (MPB) that is only dependent on the composition. However, the underlying physical origins of the enhanced piezoelectricity observed at both
* Corresponding author. Nuclear Materials Development Division, Korea Atomic Energy Research Institute, Yuseong-gu, Daejeon 34057, Republic of Korea. E-mail address: [email protected] (G.-J. Lee). http://dx.doi.org/10.1016/j.cap.2016.03.003 1567-1739/© 2016 Elsevier B.V. All rights reserved.
boundaries are considered to be identical. In KNN-based ceramics, two different PPBs are considered as the intrinsic characteristics, i.e., R-O and O-T, and their electrical properties are very sensitive to not only the compositions but also the temperatures. The most popular research strategy involves constructing the R-O or O-T phase boundary around room temperature, particularly by shifting the intrinsic TR-O and/or TO-T values close to room temperature by chemically modifying the relevant components [2,3,6,7,9e15]. Various additives, including Sb5þ, Ta5þ, AZrO3 (A ¼ Ba2þ, Sr2þ, Ca2þ), and BiScO3, have been used to increase TR-O, but rather poor piezoelectricity is common for the R-O phase boundary [9,10]. Great progress with regard to d33 has been made through the formation of the O-T phase boundary via the ion substitution of Liþ, Sb5þ, and Ta5þ, and the addition of ABO3 multicomponents [2,11e13]. More recently, it was demonstrated that a new phase boundary consisting of R and T is more effective to promote the piezoelectric activity of KNN-based ceramics with respect to others [3,6,7,14,15], further increasing research interest in KNN-based piezoelectrics. However, such studies have mainly focused on the relationship between the type of phase boundary and the piezoelectric activity, while there are few studies [13,14] which attempt to clarify the respective roles of the R, O, and T phases formed in the associated phase boundaries on the piezoelectric properties. In this study, we explore the influence of each phase within the phase boundary on the ferroelectric and piezoelectric properties of
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KNN-based ceramics through compositional engineering. To design the ceramics with the coexistence of the R, O and T phases or a phase boundary around room temperature, we introduced an ABO3-type additive, i.e., Bi(Na,K,Li)ZrO3 (BNKLZ), which is the most commonly investigated perovskite material to increase the TR-O value and decrease the TO-T value of KNN simultaneously [7,16e18]. For further modification of the phase boundary, we also added (Bi,Na)TiO3 (BNT) as a third component. Finally, the (0.97-x) [(K0.5Na0.5)NbO3]-0.03[Bi0.5(Na0.7K0.2Li0.1)0.5ZrO3]-x[(Bi0.5Na0.5) TiO3] ternary composition (x ¼ 0e0.02) was investigated in a case study in this work. In addition, because electrical poling is crucial for enhancing d33 without greatly sacrificing TC, the phasedependent poling behaviors in relation to the piezoelectric properties were investigated at different temperatures.
2. Experiment The (0.97-x)KNN-0.03BNKLZ-xBNT ceramics with x ¼ 0e0.02 were prepared using a conventional solid-state method with the starting materials of K2CO3 (99.0%, SigmaeAldrich), Na2CO3 (99.5%, SigmaeAldrich), Nb2O5 (99.9%, SigmaeAldrich), Bi2O3 (99.9%, SigmaeAldrich), Li2CO3 (99.997%, SigmaeAldrich), ZrO2 (99.0%, SigmaeAldrich), and TiO2 (99.9%, SigmaeAldrich). The weighed powders were ball-milled in ethanol for 24 h and then calcined at 850 C for 6 h. The calcined powders were pressed into disks with diameters of 10 mm using PVA as a binder. After burning off the PVA at 650 C, the pellets were sintered at 1100 C for 3 h. The investigated ceramics with different BNT concentrations had similar densities of 4.25e4.32 g/cm3 (greater than 95% of the theoretical density) as well as similar grain sizes between 1.17 and 1.43 mm. The sintered ceramics were ground down until their thicknesses were 500 mm. X-ray diffraction (XRD, D/Max-2500; Rigaku, Tokyo, Japan) was used with CuKa radiation at 40 kV and 30 mA. Temperaturedependent XRD investigations were also carried out in a temperature range between 150 C and 450 C. To investigate the phase structure, the peaks in the 2q range at 21 e24 and 44 e47 were fitted and analyzed by using the origin Pro 8.5 software and the Joint Committee on Power Diffraction Standards card Nos. 01-0710946 (Orthorhombic, Amm2), 01-071-0947 (Rhombohedral, R3m), and 01-071-0948 (Tetragonal, P4mm). For the electrical measurements, silver paste was coated onto two main surfaces of the ceramics, after which they were fired at 650 C for 10 min. The prepared samples were poled in silicon oil at set temperatures ranging from 27 C to 160 C. It was found through pre-test of poling that the poling time from 10 s to 60 min did not largely affect the value of d33, whereas an electric field significantly affected the value of d33. The value of d33 sharply increased with increasing electric field to 60 kV/cm, then dropped with a further increase in electric field to 100 kV/cm, and finally the ceramic sample is electrically broken at electric field larger than 100 kV/cm. Therefore, the poling field and poling time were set to 60 kV/cm and 10 min, respectively. The dielectric properties were measured at a frequency of 100 kHz in the temperature range of 30 Ce500 C using a Solartron 1260 Impedance Analyzer. Polarization versus electric field (PeE) hysteresis loops and electric-field-induced unipolar strain curves were determined at a probing frequency of 10 Hz using a commercial aixPES setup (aixACCT Systems GmbH, Germany), i.e., a computer-based measurement tool which characterizes ferroelectric ceramics. The converse piezoelectric coefficient (d*33) was determined from the ratio of the maximum strain to the peak electric field, d*33 ¼ Smax/Emax using the obtained unipolar strain curves. The piezoelectric coefficient (d33) was measured using a piezo-d33 m (ZJ-6B, China).
3. Results and discussion Fig. 1(a) shows the XRD patterns of (0.97-x)KNN-0.03BNKLZxBNT ceramics measured at room temperature as a function of the BNT concentration (x). A pure perovskite structure without secondary phases was obtained in all of the samples, indicating the growth of a stable solid solution among the KNN, BNKLZ and BNT. The evolution of the phase structure of the ceramics with the BNT concentration is shown in Fig. 1(b). The concentration-dependence of the phase structure is clearly visible. The simulated data reveal that the ceramics with 0 x 0.01 show the coexistence of rhombohedral (R), orthorhombic (O), and tetragonal (T) phases, with their relative phase compositions changing with the BNT concentration: the O phase gradually decreases, while the T phase increases. At x 0.015, the O phase completely disappears and finally only the R and T phases remain. It is clear that an addition of BNT induces the shrinkage of the O phase. To explore the temperature-dependence of the phase structure of the ceramics with different BNT concentrations, XRD experiments were carried out in a temperature range of 150 C and 450 C for ceramics with the selected BNT concentrations. As shown in Fig. 2, for the ceramics with 0 x 0.01, a common trend was noted in that the R-O, R-O-T, R-T, T, and C phases are formed in sequence with an increase in the temperature. The only notable difference is a gradual shift of the phase-transition temperature in each case as well as a shift in the ferroelectric Curie temperature (TC) to a lower level with an increase in the amount of BNT. It should be noted that for the ceramics with the R-O-T phase boundary around room temperature, the R-T phase boundary is formed by the shrinkage of the O phase prior to the formation of the T phase region; for the ceramics with x ¼ 0.015, the R-T phase boundary is formed near room temperature. Interestingly, a triphasic R-O-T at 0 x 0.01 and a diphasic R-T at 0.015 x 0.02 appear as the
Fig. 1. (a) XRD patterns of (0.97-x)KNN-0.03BNKLZ-xBNT ceramics with x ¼ 0e0.02 measured at room temperature and (b) the simulated patterns in the 2q range of 21 e24 and 44 e47. The terms “R”, “O” and “T” indicate the rhombohedral, orthorhombic and tetragonal phases, respectively.
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Fig. 2. Temperature dependence of XRD patterns for the (0.97-x)KNN-0.03BNKLZ-xBNT ceramics.
near room-temperature-phase in this work. The role of BNKLZ in KNN-based ceramics is considered not only to decrease the O-T phase-transition temperature TO-T of KNN, but also to increase its RO phase transition temperature TR-O [18]. Moreover, the movement of such phase transitions towards room temperature is often observed depending on the composition, usually showing separate TO-T and TR-O values [19,20]. It was also reported that BNT causes a simultaneous increase of TR-O and a decrease of TO-T, as does BNKLZ [7,11,17]. With the addition of BNT as a third component, it is thus believed that TO-T can be decreased further to or below room temperature, resulting in the formation of a triphasic R-O-T or diphasic R-T structure at room temperature. This behavior is obviously promoted by the proximity of TR-O and TO-T owing to the combined effect of BNKLZ and BNT. Fig. 3(a) shows the temperature-dependent dielectric constants (εr) of the (0.97-x)KNN-0.03BNKLZ-xBNT ceramics with 0 x 0.02 as measured at 100 kHz in the temperature range of 20e500 C. A gradual decline in TC corresponding to a sharp anomaly of εr is observed with the BNT concentration: TC decreases from 360 C to 328 C when x increases from 0 to 0.02. Supporting the above XRD results, no other dielectric peaks indicate that the phase-transition temperatures (usually, R-O or O-T) are below or near room temperature. As shown in the inset of Fig. 3(a), the slight and broad decrease of εr at compositions below x ¼ 0.01 is thought to be due to the formation of the R-O-T phase (see Figs. 1 and 2). Well-saturated P-E hysteresis loops are confirmed for all of the ceramics in Fig. 3(b). The room-temperature εr values dramatically increase with x, but the increase of εr is insignificant above x ¼ 0.015 (Fig. 3(c)). The remanent polarization (Pr) obtained from Fig. 3(b) increases to x ¼ 0.015 and then drops with a further increase in x (Fig. 3(c)). The maximum Pr value of ~18.4 mC/cm2 is obtained for the ceramic material when x ¼ 0.015, consistent with the R-T phase boundary region. Fig. 3(d) shows the variation of the d33 and εrPr values as a function of the BNT concentration (x). The d33 measurements were taken for the ceramics poled at room
temperature for 10 min under an electric field of 60 kV/cm. The d33 value increases, reaches the maximum at x ¼ 0.015, and finally drops, as the x increases. The measured d33 changes from 149 to 234 pC/N. Because the piezoelectric properties are considered to be closely related to the dielectric and ferroelectric properties [3e7,17e20], i.e., d33 z aεrPr (where a is a parameter which does not depend on the composition or temperature), the εrPr values obtained from Fig. 3(c) are also plotted in Fig. 3(d). These results show that the profiles of d33 and εrPr continue to change analogously with the BNT concentration, suggesting a contribution of εrPr to d33. The coherent peaks of d33 and εrPr at x ¼ 0.015 also confirm that the enhanced εrPr strongly affects the larger value of d33. Based on the XRD results shown in Figs. 1 and 2, the origin of the largest piezoelectricity observed at x ¼ 0.015 is considered to be the coexistence of R and T phases near room temperature. It is well accepted that the formation of the R-T phase boundary close to room temperature, similar to the classic MPB in the PZT system, leads to superior piezoelectric properties due to the improved polarizability induced by the coupling between the two equivalent energy states of the R and T phases [21]. However, it should be noted that the existence of the O phase in the R-T phase (x < 0.015) negatively affects the value of d33. Moreover, it cannot be overlooked that d33 is decreased by a more enriched T phase (x ¼ 0.02). Such results strongly suggest that the change in the piezoelectric property is attributed to not only the involvement of different phase structures but also their relative phase compositions. Recently, Cheng et al. reported that the formation of the R-O-T phase boundary in KNN-BNKLZ ceramics leads to higher piezoelectric properties compared to the formation of the O-T phase boundary, further emphasizing the need for a new phase boundary through the removal of the O phase in the event of the coexistence of R-O-T phase [18]. Our experimental results manifest the role of the O phase in the R-O-T phase boundary on the ferroelectric and piezoelectric properties of KNN-based ceramics, also showing that the construction of the R-T phase boundary through the shrinkage
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Fig. 3. (a) Temperature dependence of the εr of (0.97-x)KNN-0.03BNKLZ-xBNT ceramics with x ¼ 0e0.02 measured at 100 kHz in the temperature range of 30 Ce500 C (the insets are the variation of εr from 30 to 160 C), (b) P-E hysteresis loops of the ceramics measured at a frequency of 10 Hz under an applied field of 60 kV/cm, (c) the variation in εr and Pr values, and (d) the d33 and εrPr values of the ceramics as a function of the BNT concentration.
of the O phase is certainly beneficial. Next, to investigate the effect of phase-dependent poling on the piezoelectric properties, ceramics with different BNT concentrations (x ¼ 0e0.02) were poled as a function of the temperature in the range from room temperature to 160 C. After poling at different temperatures, the samples were subject to d33 measurements at room temperature. These results are presented in Fig. 4(a). It was found that d33 changes significantly with the poling temperature Tp. For x 0.01, the d33 value increases, reaches a maximum and then decreases, as the Tp increases. The Tp value showing a maximum d33 becomes lower with an increase in the BNT concentration x; ~80 C at x ¼ 0, ~60 C at x ¼ 0.005, and ~40 C at x ¼ 0.01. We note that these temperatures are in good agreement with those at which the R-T phase boundaries are formed in Fig. 2. On the other hand, it was found that the d33 value decreases as either Tp approaches the R-O-T phase region (i.e., the O phase composition in the R-O-T phase becomes richer), or as Tp moves toward the T phase-rich region. Accordingly, for ceramics with a RO-T phase at room temperature, the precise operation of Tp near the R-T phase boundary without an O phase is necessary to attain enhanced piezoelectric properties. For x 0.015, Tp with the maximum value of d33 is close to room temperature due to the establishment of the R-T phase boundary around room temperature (see Figs. 1 and 2). With a further increase in Tp, a gradual decline in d33 is observed, which is thought to be due to the increase of the T phase composition relative to the R phase. In the case of ceramics with x 0.01, the maximum d33 values are derived from the R-O-T phase boundary region at room temperature after poling at the R-T region. Thus, to observe the piezoelectric activity at temperatures corresponding to the R-T region and the effect of the associated phase transition when cooling from Tp to room temp, the converse piezoelectric coefficient d*33 was further measured in-situ at Tp with max d33 corresponding to the optimum R-T region (i.e., ~80 C at x ¼ 0, ~60 C at x ¼ 0.005, and ~40 C at x ¼ 0.01) and then re-measured at the same locations
after cooling to room temp (i.e., R-O-T region). As shown in Fig. 4(b), the d*33 values obtained at each Tp are larger than those at room temperature and the difference between the two values becomes smaller with an increase in x. This clearly shows the effect of the O phase in the event of the temperature change associated with the phase transition from R-T to R-O-T as well as the variation of the BNT composition. It was also found that the εrPr and d*33 values measured at Tp showed a consistent trend, similar to that in Fig. 3(d). We speculate that the origin of the observed change of d*33 with BNT concentration might be the different relative phase composition of R and T, because the present doping levels are too low to induce the intrinsic Bi effect, i.e., the well-known hybridization effect between Bi-6p and O-2p orbits [11]. In consideration of high temperature applications, it should also be stressed that the d*33 property at temperature near the working temperature will be important, particularly for piezoceramics with polymorphic phase boundaries such as in a KNN-based system. As a result, since the effect of poling on the piezoelectric property of the KNN-based ceramics is strongly dependent on the nature of the phase boundary, it is proposed that poling at the optimum phase composition of R and T as well as removal of the O phase is necessary through optimization of Tp.
4. Conclusion In summary, the influence of the R, O, and T phases within the phase boundary on the ferroelectric and piezoelectric properties of KNN-based ceramics with the (0.97-x)KNN-0.03BNKLZ-xBNT ternary composition (x ¼ 0e0.02) was investigated. As the nearroom-temperature phase, the triphasic R-O-T phase boundary appeared at 0 x 0.01, while the diphasic R-T was formed at 0.015 x 0.02. According to the temperature-dependent XRD results, the R-O-T phase boundary at room temperature was transformed into the R-T phase boundary due to the shrinkage of the O phase prior to the formation of the T phase as the
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Fig. 4. (a) The variation of d33 of the (0.97-x)KNN-0.03BNKLZ-xBNT ceramics with x ¼ 0e0.02 as a function of the poling temperature Tp (the temperatures corresponding to the RO-T and R-T phases are also indicated in the figures based on Fig. 2) and (b) the variation of d*33 and εrPr of the ceramics as a function of the BNT concentration. Red line represents the d*33 and εrPr values measured at the optimized Tp corresponding to the R-T phase boundary region, i.e., ~80 C at x ¼ 0, ~60 C at x ¼ 0.005, ~40 C at x ¼ 0.01, room temperature at x ¼ 0.015 and x ¼ 0.02, while blue line indicates the d*33 values measured at the R-O-T region after cooling down to room temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
temperature increased. The existence of the O phase as well as the enrichment of the T phase within the R-T phase boundary negatively affected the value of d33. The highest ferroelectric and piezoelectric performances were obtained for the ceramic with x ¼ 0.015, corresponding to the R-T phase boundary region. The electrical poling behavior on the piezoelectric property was also strongly dependent on the property of the phase boundary. For ceramics with the R-O-T phase boundary at room temperature (0 x 0.01), poling at temperatures corresponding to the R-T phase boundary without an O phase is required to improve the piezoelectric property. These results show that not only the involvement of different phase structures but also the relative phase compositions in the polymorphic phase boundary strongly affect the piezoelectric properties of KNN-based ceramics.
Acknowledgements This research was financially supported by the Korea Atomic Energy Research Institute (KAERI) R&D Program and in part supported by the Nuclear Power Core Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20131520000210).
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
The effects of the phase structure of the polymorphic phase boundary on the piezoelectric properties of (K,Na)NbO3-based ceramics Sun-A Yang a, Byung-Hoon Kim a, Min-Ku Lee a, Sang-Don Bu b, Gyoung-Ja Lee a, * a b
Nuclear Materials Development Division, Korea Atomic Energy Research Institute, Daejeon 34057, Republic of Korea Department of Physics and Research Institute of Physics and Chemistry, Chonbuk National University, Jeonju 54896, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2016 Received in revised form 24 February 2016 Accepted 3 March 2016 Available online 4 March 2016
We report the effects of the phase, i.e., the rhombohedral (R), orthorhombic (O) and tetragonal (T) phase, within a polymorphic phase boundary on the piezoelectric properties of (K,Na)NbO3 ceramics doped with Bi(Na,K,Li)ZrO3 and (Bi,Na)TiO3. For the R-O-T phase boundary, the formation of an R-T phase boundary by O phase shrinkage is clearly beneficial to enhance the piezoelectric performance, whereas the enrichment of the T phase in the R-T phase boundary negatively affects the piezoelectric activity. Electrical poling in relation to the piezoelectric property strongly depends on the nature of the phase boundary, requiring the optimization of temperatures corresponding to the R-T phase boundary without the O phase. © 2016 Elsevier B.V. All rights reserved.
Keywords: Piezoelectric ceramics Polymorphic phase transformation Phase structure Piezoelectricity
1. Introduction (K,Na)NbO3 (KNN)-based ceramics have become the most overwhelmingly investigated lead-free piezoelectric material system in the past ten years owing to a large piezoelectric coefficient d33 comparable to that of PZT and a higher Curie temperature TC than PZT [1e7]. From the expectations of superior piezoelectric properties owing to the formation of a phase boundary, a great amount of attention has been directed toward the construction of a polymorphic phase boundary (PPB) of the KNN system near the working temperature, usually room temperature [3e5,8]. It is known that pure KNN undergoes a series of structural phase transitions, as the temperature increases. These are the rhombohedral-orthorhombic (R-O) transition at 123 C (TR-O), the orthorhombic-tetragonal (O-T) transition at 210 C (TO-T), and the tetragonal-cubic (T-C) transition at 410 C (TC) [1]. Such PPBs do not form in the vertical direction with regard to compositions, instead showing strong temperature-dependence, clearly different from a traditional morphotropic phase boundary (MPB) that is only dependent on the composition. However, the underlying physical origins of the enhanced piezoelectricity observed at both
* Corresponding author. Nuclear Materials Development Division, Korea Atomic Energy Research Institute, Yuseong-gu, Daejeon 34057, Republic of Korea. E-mail address: [email protected] (G.-J. Lee). http://dx.doi.org/10.1016/j.cap.2016.03.003 1567-1739/© 2016 Elsevier B.V. All rights reserved.
boundaries are considered to be identical. In KNN-based ceramics, two different PPBs are considered as the intrinsic characteristics, i.e., R-O and O-T, and their electrical properties are very sensitive to not only the compositions but also the temperatures. The most popular research strategy involves constructing the R-O or O-T phase boundary around room temperature, particularly by shifting the intrinsic TR-O and/or TO-T values close to room temperature by chemically modifying the relevant components [2,3,6,7,9e15]. Various additives, including Sb5þ, Ta5þ, AZrO3 (A ¼ Ba2þ, Sr2þ, Ca2þ), and BiScO3, have been used to increase TR-O, but rather poor piezoelectricity is common for the R-O phase boundary [9,10]. Great progress with regard to d33 has been made through the formation of the O-T phase boundary via the ion substitution of Liþ, Sb5þ, and Ta5þ, and the addition of ABO3 multicomponents [2,11e13]. More recently, it was demonstrated that a new phase boundary consisting of R and T is more effective to promote the piezoelectric activity of KNN-based ceramics with respect to others [3,6,7,14,15], further increasing research interest in KNN-based piezoelectrics. However, such studies have mainly focused on the relationship between the type of phase boundary and the piezoelectric activity, while there are few studies [13,14] which attempt to clarify the respective roles of the R, O, and T phases formed in the associated phase boundaries on the piezoelectric properties. In this study, we explore the influence of each phase within the phase boundary on the ferroelectric and piezoelectric properties of
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KNN-based ceramics through compositional engineering. To design the ceramics with the coexistence of the R, O and T phases or a phase boundary around room temperature, we introduced an ABO3-type additive, i.e., Bi(Na,K,Li)ZrO3 (BNKLZ), which is the most commonly investigated perovskite material to increase the TR-O value and decrease the TO-T value of KNN simultaneously [7,16e18]. For further modification of the phase boundary, we also added (Bi,Na)TiO3 (BNT) as a third component. Finally, the (0.97-x) [(K0.5Na0.5)NbO3]-0.03[Bi0.5(Na0.7K0.2Li0.1)0.5ZrO3]-x[(Bi0.5Na0.5) TiO3] ternary composition (x ¼ 0e0.02) was investigated in a case study in this work. In addition, because electrical poling is crucial for enhancing d33 without greatly sacrificing TC, the phasedependent poling behaviors in relation to the piezoelectric properties were investigated at different temperatures.
2. Experiment The (0.97-x)KNN-0.03BNKLZ-xBNT ceramics with x ¼ 0e0.02 were prepared using a conventional solid-state method with the starting materials of K2CO3 (99.0%, SigmaeAldrich), Na2CO3 (99.5%, SigmaeAldrich), Nb2O5 (99.9%, SigmaeAldrich), Bi2O3 (99.9%, SigmaeAldrich), Li2CO3 (99.997%, SigmaeAldrich), ZrO2 (99.0%, SigmaeAldrich), and TiO2 (99.9%, SigmaeAldrich). The weighed powders were ball-milled in ethanol for 24 h and then calcined at 850 C for 6 h. The calcined powders were pressed into disks with diameters of 10 mm using PVA as a binder. After burning off the PVA at 650 C, the pellets were sintered at 1100 C for 3 h. The investigated ceramics with different BNT concentrations had similar densities of 4.25e4.32 g/cm3 (greater than 95% of the theoretical density) as well as similar grain sizes between 1.17 and 1.43 mm. The sintered ceramics were ground down until their thicknesses were 500 mm. X-ray diffraction (XRD, D/Max-2500; Rigaku, Tokyo, Japan) was used with CuKa radiation at 40 kV and 30 mA. Temperaturedependent XRD investigations were also carried out in a temperature range between 150 C and 450 C. To investigate the phase structure, the peaks in the 2q range at 21 e24 and 44 e47 were fitted and analyzed by using the origin Pro 8.5 software and the Joint Committee on Power Diffraction Standards card Nos. 01-0710946 (Orthorhombic, Amm2), 01-071-0947 (Rhombohedral, R3m), and 01-071-0948 (Tetragonal, P4mm). For the electrical measurements, silver paste was coated onto two main surfaces of the ceramics, after which they were fired at 650 C for 10 min. The prepared samples were poled in silicon oil at set temperatures ranging from 27 C to 160 C. It was found through pre-test of poling that the poling time from 10 s to 60 min did not largely affect the value of d33, whereas an electric field significantly affected the value of d33. The value of d33 sharply increased with increasing electric field to 60 kV/cm, then dropped with a further increase in electric field to 100 kV/cm, and finally the ceramic sample is electrically broken at electric field larger than 100 kV/cm. Therefore, the poling field and poling time were set to 60 kV/cm and 10 min, respectively. The dielectric properties were measured at a frequency of 100 kHz in the temperature range of 30 Ce500 C using a Solartron 1260 Impedance Analyzer. Polarization versus electric field (PeE) hysteresis loops and electric-field-induced unipolar strain curves were determined at a probing frequency of 10 Hz using a commercial aixPES setup (aixACCT Systems GmbH, Germany), i.e., a computer-based measurement tool which characterizes ferroelectric ceramics. The converse piezoelectric coefficient (d*33) was determined from the ratio of the maximum strain to the peak electric field, d*33 ¼ Smax/Emax using the obtained unipolar strain curves. The piezoelectric coefficient (d33) was measured using a piezo-d33 m (ZJ-6B, China).
3. Results and discussion Fig. 1(a) shows the XRD patterns of (0.97-x)KNN-0.03BNKLZxBNT ceramics measured at room temperature as a function of the BNT concentration (x). A pure perovskite structure without secondary phases was obtained in all of the samples, indicating the growth of a stable solid solution among the KNN, BNKLZ and BNT. The evolution of the phase structure of the ceramics with the BNT concentration is shown in Fig. 1(b). The concentration-dependence of the phase structure is clearly visible. The simulated data reveal that the ceramics with 0 x 0.01 show the coexistence of rhombohedral (R), orthorhombic (O), and tetragonal (T) phases, with their relative phase compositions changing with the BNT concentration: the O phase gradually decreases, while the T phase increases. At x 0.015, the O phase completely disappears and finally only the R and T phases remain. It is clear that an addition of BNT induces the shrinkage of the O phase. To explore the temperature-dependence of the phase structure of the ceramics with different BNT concentrations, XRD experiments were carried out in a temperature range of 150 C and 450 C for ceramics with the selected BNT concentrations. As shown in Fig. 2, for the ceramics with 0 x 0.01, a common trend was noted in that the R-O, R-O-T, R-T, T, and C phases are formed in sequence with an increase in the temperature. The only notable difference is a gradual shift of the phase-transition temperature in each case as well as a shift in the ferroelectric Curie temperature (TC) to a lower level with an increase in the amount of BNT. It should be noted that for the ceramics with the R-O-T phase boundary around room temperature, the R-T phase boundary is formed by the shrinkage of the O phase prior to the formation of the T phase region; for the ceramics with x ¼ 0.015, the R-T phase boundary is formed near room temperature. Interestingly, a triphasic R-O-T at 0 x 0.01 and a diphasic R-T at 0.015 x 0.02 appear as the
Fig. 1. (a) XRD patterns of (0.97-x)KNN-0.03BNKLZ-xBNT ceramics with x ¼ 0e0.02 measured at room temperature and (b) the simulated patterns in the 2q range of 21 e24 and 44 e47. The terms “R”, “O” and “T” indicate the rhombohedral, orthorhombic and tetragonal phases, respectively.
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Fig. 2. Temperature dependence of XRD patterns for the (0.97-x)KNN-0.03BNKLZ-xBNT ceramics.
near room-temperature-phase in this work. The role of BNKLZ in KNN-based ceramics is considered not only to decrease the O-T phase-transition temperature TO-T of KNN, but also to increase its RO phase transition temperature TR-O [18]. Moreover, the movement of such phase transitions towards room temperature is often observed depending on the composition, usually showing separate TO-T and TR-O values [19,20]. It was also reported that BNT causes a simultaneous increase of TR-O and a decrease of TO-T, as does BNKLZ [7,11,17]. With the addition of BNT as a third component, it is thus believed that TO-T can be decreased further to or below room temperature, resulting in the formation of a triphasic R-O-T or diphasic R-T structure at room temperature. This behavior is obviously promoted by the proximity of TR-O and TO-T owing to the combined effect of BNKLZ and BNT. Fig. 3(a) shows the temperature-dependent dielectric constants (εr) of the (0.97-x)KNN-0.03BNKLZ-xBNT ceramics with 0 x 0.02 as measured at 100 kHz in the temperature range of 20e500 C. A gradual decline in TC corresponding to a sharp anomaly of εr is observed with the BNT concentration: TC decreases from 360 C to 328 C when x increases from 0 to 0.02. Supporting the above XRD results, no other dielectric peaks indicate that the phase-transition temperatures (usually, R-O or O-T) are below or near room temperature. As shown in the inset of Fig. 3(a), the slight and broad decrease of εr at compositions below x ¼ 0.01 is thought to be due to the formation of the R-O-T phase (see Figs. 1 and 2). Well-saturated P-E hysteresis loops are confirmed for all of the ceramics in Fig. 3(b). The room-temperature εr values dramatically increase with x, but the increase of εr is insignificant above x ¼ 0.015 (Fig. 3(c)). The remanent polarization (Pr) obtained from Fig. 3(b) increases to x ¼ 0.015 and then drops with a further increase in x (Fig. 3(c)). The maximum Pr value of ~18.4 mC/cm2 is obtained for the ceramic material when x ¼ 0.015, consistent with the R-T phase boundary region. Fig. 3(d) shows the variation of the d33 and εrPr values as a function of the BNT concentration (x). The d33 measurements were taken for the ceramics poled at room
temperature for 10 min under an electric field of 60 kV/cm. The d33 value increases, reaches the maximum at x ¼ 0.015, and finally drops, as the x increases. The measured d33 changes from 149 to 234 pC/N. Because the piezoelectric properties are considered to be closely related to the dielectric and ferroelectric properties [3e7,17e20], i.e., d33 z aεrPr (where a is a parameter which does not depend on the composition or temperature), the εrPr values obtained from Fig. 3(c) are also plotted in Fig. 3(d). These results show that the profiles of d33 and εrPr continue to change analogously with the BNT concentration, suggesting a contribution of εrPr to d33. The coherent peaks of d33 and εrPr at x ¼ 0.015 also confirm that the enhanced εrPr strongly affects the larger value of d33. Based on the XRD results shown in Figs. 1 and 2, the origin of the largest piezoelectricity observed at x ¼ 0.015 is considered to be the coexistence of R and T phases near room temperature. It is well accepted that the formation of the R-T phase boundary close to room temperature, similar to the classic MPB in the PZT system, leads to superior piezoelectric properties due to the improved polarizability induced by the coupling between the two equivalent energy states of the R and T phases [21]. However, it should be noted that the existence of the O phase in the R-T phase (x < 0.015) negatively affects the value of d33. Moreover, it cannot be overlooked that d33 is decreased by a more enriched T phase (x ¼ 0.02). Such results strongly suggest that the change in the piezoelectric property is attributed to not only the involvement of different phase structures but also their relative phase compositions. Recently, Cheng et al. reported that the formation of the R-O-T phase boundary in KNN-BNKLZ ceramics leads to higher piezoelectric properties compared to the formation of the O-T phase boundary, further emphasizing the need for a new phase boundary through the removal of the O phase in the event of the coexistence of R-O-T phase [18]. Our experimental results manifest the role of the O phase in the R-O-T phase boundary on the ferroelectric and piezoelectric properties of KNN-based ceramics, also showing that the construction of the R-T phase boundary through the shrinkage
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Fig. 3. (a) Temperature dependence of the εr of (0.97-x)KNN-0.03BNKLZ-xBNT ceramics with x ¼ 0e0.02 measured at 100 kHz in the temperature range of 30 Ce500 C (the insets are the variation of εr from 30 to 160 C), (b) P-E hysteresis loops of the ceramics measured at a frequency of 10 Hz under an applied field of 60 kV/cm, (c) the variation in εr and Pr values, and (d) the d33 and εrPr values of the ceramics as a function of the BNT concentration.
of the O phase is certainly beneficial. Next, to investigate the effect of phase-dependent poling on the piezoelectric properties, ceramics with different BNT concentrations (x ¼ 0e0.02) were poled as a function of the temperature in the range from room temperature to 160 C. After poling at different temperatures, the samples were subject to d33 measurements at room temperature. These results are presented in Fig. 4(a). It was found that d33 changes significantly with the poling temperature Tp. For x 0.01, the d33 value increases, reaches a maximum and then decreases, as the Tp increases. The Tp value showing a maximum d33 becomes lower with an increase in the BNT concentration x; ~80 C at x ¼ 0, ~60 C at x ¼ 0.005, and ~40 C at x ¼ 0.01. We note that these temperatures are in good agreement with those at which the R-T phase boundaries are formed in Fig. 2. On the other hand, it was found that the d33 value decreases as either Tp approaches the R-O-T phase region (i.e., the O phase composition in the R-O-T phase becomes richer), or as Tp moves toward the T phase-rich region. Accordingly, for ceramics with a RO-T phase at room temperature, the precise operation of Tp near the R-T phase boundary without an O phase is necessary to attain enhanced piezoelectric properties. For x 0.015, Tp with the maximum value of d33 is close to room temperature due to the establishment of the R-T phase boundary around room temperature (see Figs. 1 and 2). With a further increase in Tp, a gradual decline in d33 is observed, which is thought to be due to the increase of the T phase composition relative to the R phase. In the case of ceramics with x 0.01, the maximum d33 values are derived from the R-O-T phase boundary region at room temperature after poling at the R-T region. Thus, to observe the piezoelectric activity at temperatures corresponding to the R-T region and the effect of the associated phase transition when cooling from Tp to room temp, the converse piezoelectric coefficient d*33 was further measured in-situ at Tp with max d33 corresponding to the optimum R-T region (i.e., ~80 C at x ¼ 0, ~60 C at x ¼ 0.005, and ~40 C at x ¼ 0.01) and then re-measured at the same locations
after cooling to room temp (i.e., R-O-T region). As shown in Fig. 4(b), the d*33 values obtained at each Tp are larger than those at room temperature and the difference between the two values becomes smaller with an increase in x. This clearly shows the effect of the O phase in the event of the temperature change associated with the phase transition from R-T to R-O-T as well as the variation of the BNT composition. It was also found that the εrPr and d*33 values measured at Tp showed a consistent trend, similar to that in Fig. 3(d). We speculate that the origin of the observed change of d*33 with BNT concentration might be the different relative phase composition of R and T, because the present doping levels are too low to induce the intrinsic Bi effect, i.e., the well-known hybridization effect between Bi-6p and O-2p orbits [11]. In consideration of high temperature applications, it should also be stressed that the d*33 property at temperature near the working temperature will be important, particularly for piezoceramics with polymorphic phase boundaries such as in a KNN-based system. As a result, since the effect of poling on the piezoelectric property of the KNN-based ceramics is strongly dependent on the nature of the phase boundary, it is proposed that poling at the optimum phase composition of R and T as well as removal of the O phase is necessary through optimization of Tp.
4. Conclusion In summary, the influence of the R, O, and T phases within the phase boundary on the ferroelectric and piezoelectric properties of KNN-based ceramics with the (0.97-x)KNN-0.03BNKLZ-xBNT ternary composition (x ¼ 0e0.02) was investigated. As the nearroom-temperature phase, the triphasic R-O-T phase boundary appeared at 0 x 0.01, while the diphasic R-T was formed at 0.015 x 0.02. According to the temperature-dependent XRD results, the R-O-T phase boundary at room temperature was transformed into the R-T phase boundary due to the shrinkage of the O phase prior to the formation of the T phase as the
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Fig. 4. (a) The variation of d33 of the (0.97-x)KNN-0.03BNKLZ-xBNT ceramics with x ¼ 0e0.02 as a function of the poling temperature Tp (the temperatures corresponding to the RO-T and R-T phases are also indicated in the figures based on Fig. 2) and (b) the variation of d*33 and εrPr of the ceramics as a function of the BNT concentration. Red line represents the d*33 and εrPr values measured at the optimized Tp corresponding to the R-T phase boundary region, i.e., ~80 C at x ¼ 0, ~60 C at x ¼ 0.005, ~40 C at x ¼ 0.01, room temperature at x ¼ 0.015 and x ¼ 0.02, while blue line indicates the d*33 values measured at the R-O-T region after cooling down to room temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
temperature increased. The existence of the O phase as well as the enrichment of the T phase within the R-T phase boundary negatively affected the value of d33. The highest ferroelectric and piezoelectric performances were obtained for the ceramic with x ¼ 0.015, corresponding to the R-T phase boundary region. The electrical poling behavior on the piezoelectric property was also strongly dependent on the property of the phase boundary. For ceramics with the R-O-T phase boundary at room temperature (0 x 0.01), poling at temperatures corresponding to the R-T phase boundary without an O phase is required to improve the piezoelectric property. These results show that not only the involvement of different phase structures but also the relative phase compositions in the polymorphic phase boundary strongly affect the piezoelectric properties of KNN-based ceramics.
Acknowledgements This research was financially supported by the Korea Atomic Energy Research Institute (KAERI) R&D Program and in part supported by the Nuclear Power Core Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20131520000210).
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