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Low-temperature sintering and piezoelectric properties of CuO-doped (K,Na)NbO3 ceramics
G Model MRB 9265 No. of Pages 5
Materials Research Bulletin xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Low-temperature sintering and piezoelectric properties of CuO-doped (K,Na)NbO3 ceramics J.H. Kima , J.S. Kima , S.H. Hanb , H.-W. Kangb , H.-G. Leeb , C.I. Cheona,* a b
Department of Materials Science and Engineering, Hoseo University, Baebang, Asan, Chungnam 336-795, Republic of Korea Electronic Materials and Device Research Center, Korea Electronics Technology Institute, Bundang-gu, Seongnam, Kyeonggi 463-816, Republic of Korea
A R T I C L E I N F O
Article history: Received 17 August 2016 Received in revised form 1 March 2017 Accepted 30 March 2017 Available online xxx Keywords: A. Ceramics B. Microstructure B. Piezoelectricity D. Dielectric properties
A B S T R A C T
The lead-free piezoelectric [(K0.485Na0.515)0.935Li0.065](Nb0.99Ta0.01)O3 (KNLNT) ceramics with a high mechanical quality factor (Qm) were prepared at low sintering temperature below 950 C for multilayered piezoelectric devices. Co-doping effect of 1 mol% Na2CO3 and 0–1 mol% CuO on the densification and piezoelectric properties were investigated. Co-doping effectively decreased the sintering temperature of KNLNT ceramics to 920 C. The co-doped KNLNT ceramics had a bimodal size distribution of large abnormal grains and small matrix grains above 900 C. When the sintering temperature increased above 900 C, a dielectric constant (er) and loss (tan d) decreased considerably and concurrently mechanical quality factor (Qm) greatly enhanced. The co-doped KNLNT ceramic with 1 mol% Na2CO3 and 0.5 mol% CuO had a high Qm and excellent piezoelectric properties when sintered at 940 C: a dielectric constant of 245 and loss (tan d) of 0.0034, an electromechanical coupling factor (kp) of 0.449, and Qm of 904. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Piezoelectric ceramics are widely applied in many electronic devices such as precise actuators and various sensors [1]. There have been considerable efforts to develop lead-free piezoelectric ceramics for replacing Pb-based piezoelectric ceramics due to a global environment issue [1–6]. (K,Na)NbO3 (KNN) is one of the highest potential candidates for a lead-free piezoelectric ceramic due to a large piezoelectric constant and a high Curie temperature [4–6]. KNN-based ceramics with a good thermal stability as well as the piezoelectric constant (d33) larger than 300 pC/N have been reported in recent years [7,8]. A demand for multilayered piezoelectric ceramics is increasing due to their advantages such as a low operating voltage and a high piezo-response [9]. One of the effective methods to reduce the manufacturing cost for multilayered piezoelectric devices is to decrease the amount of expensive noble metals like palladium being used as internal electrodes by sintering the piezoelectric ceramic layers at a low temperature [9,10]. Excess alkaline oxide like Na2O has been reported to reduce the sintering temperature below 1000 C and compensate for the alkali
* Corresponding author. E-mail address: [email protected] (C.I. Cheon).
metal that evaporates during the sintering process [11–13]. And KNN-based ceramics have been successfully fabricated at low sintering temperature below 1000 C by adding various sintering aids such as CuO, LiF, or mixture of Li2CO3 and Bi2O3 [14–17]. A hard piezoelectric material with a high Qm is required for high power transducers and resonators [1]. Im et al. demonstrated a high mechanical quality factor of 1500 in KNN ceramics by adding K4CuNb8O23 (KCN) [18]. They claimed that the Cu2+ ions substituted for Nb5+ ions created oxygen vacancies for charge compensation, and the associates of acceptor Cu2+ ions and oxygen vacancies increased Qm by prohibiting domain wall motion [18]. The sintering temperature in their study, however, was 1100 C which is too high for multi-layered devices. Recently, Huang et al. reported that CuO-doping to an alkali-excess KNN composition, 0.94(K0.48Na0.535)NbO3-0.06LiNbO3 (KNN-LN) reduced the sintering temperature to 890 C and somewhat increased the mechanical quality factor (Qm) [19]. J.J. Zhou et al. have also fabricated hard piezoelectric (Na0.53K0.48)0.98Nb0.8Ta0.2O3–0.05AgSbO3 (NKNTAS) ceramics by doping CuO [15]. However, the Qm in these KNN-LN or NKNT-AS ceramics was lower than 200 which was far below that of the KCN-doped KNN sample. In this work, we fabricated the hard KNN ceramics with a high Qm at a low sintering temperature below 950 C for multilayered piezoelectric devices. [(K0.485Na0.515)0.935Li0.065] (Nb0.99Ta0.01)O3 (KNLNT) ceramics were prepared by a conventional ceramic
http://dx.doi.org/10.1016/j.materresbull.2017.03.066 0025-5408/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: J.H. Kim, et al., Low-temperature sintering and piezoelectric properties of CuO-doped (K,Na)NbO3 ceramics, Mater. Res. Bull. (2017), http://dx.doi.org/10.1016/j.materresbull.2017.03.066
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process. 1 mol% of excess-Na2CO3 were added to KNLNT ceramics in order to reduce the sintering temperature, and 0–1 mol% of CuO was also added to facilitate densification and increase Qm. Codoping effect of Na2CO3 of CuO on the densification and piezoelectric properties were investigated. 2. Experimental Raw powders of K2CO3 (Aldrich, 99%), Na2CO3 (Aldrich, 99%), Li2CO3 (Aldrich, 99%), Nb2O5 (Kojundo Chemical Lab., 99.9%), and Ta2O5 (Kojundo Chemical Lab., 99.9%) were weighed in an appropriate mole ratio for KNLNT. These were mixed by ballmilling for 24 h with anhydrous ethanol and yttria-stabilized zirconia (YSZ) balls in plastic bottles. After drying, the mixed powder was calcined at 800 C for 4 h. The calcined powder was ball-milled again for 24 h with 1 mol% Na2CO3 and 0–1 mol% CuO (Aldrich, 99%) after adding 0.5 wt% polyvinyl butyral (PVB). The disk-type compacts with a diameter of 9 mm were formed by uniaxial pressing at 150 MPa and heat-treated at 600 C for 2 h for binder burn-out. The KNLNT samples were sintered at 860–960 C for 3 h. The phases of the calcined and sintered samples were identified by X-ray diffraction analysis (XRD-6100, Shimadzu). The surface morphologies were observed by a scanning electron microscope (SEC, SNE-4500E). The densities of the sintered samples were measured using Archimedes’ principle. Silver paste was printed on the sintered sample and fired at 800 C for 10 min for electrical measurements. A DC electric field of 4–5 KV/mm was applied to the samples for 30 min at 100 C for dipole alignment (poling). The piezoelectric constant d33 was measured using a Berlincourt-type d33 meter (YE2730A, APC International Ltd.). The capacitance, the impedance and the phase angle were measured as a function of the frequency with an impedance analyzer (HP 4294A). The relative dielectric constant (er) was calculated from the capacitance and the sample dimension. The kp and Qm for a radial vibration mode were calculated according to the following equations; 1 2 kp
2
¼ 0:395
fr fa ; þ 0:574; Q m ¼ 2 2 fa fr 2 pf r Z r C f a f r
where fr, fa, Zr, and C are the resonant frequency, the anti-resonant frequency, the impedance at fr, and the capacitance at 1 kHz, respectively.
3. Results The X-ray diffraction patterns of the KNLNT ceramics are shown in Fig. 1. The samples sintered at 920 C with various dopants are represented in Fig. 1(a). The samples co-doped with 1 mol% NaCO3 and 0.5 mol% CuO sintered at various temperatures are shown in Fig. 1(b). All samples have orthorhombic structures without any impurity phase. The XRD pattern of the sample showed no significant change with either CuO-doping or increasing the sintering temperature to 940 C. When the sintering temperature increased from 860 C to 940 C, the density initially increased to reach the maximum at 920 C, and then decreased a little in KNLNT ceramics as shown in Fig. 2. The samples partially melted at the sintering temperature above 940 C. Fig. 2 shows that the samples co-doped with Na2CO3 and CuO have higher densities than 1 mol% Na2CO3 single-doped sample throughout the whole sintering temperature range. The highest density was about 4.257 g/cm3 (95% of the theoretical density) at the sintering temperature of 920 C in the sample co-doped with 1 mol% Na2CO3 and 0.5 mol% CuO. Increasing the CuO content above 0.5 mol% had little effect on the density. In contrast to the single doping case, the co-doping with Na2CO3 and CuO facilitates the densification of a KNLNT ceramic at low temperature around 900 C. Surface morphologies of KNLNT ceramics sintered at 880– 940 C are displayed in Fig. 3. KNLNT ceramics sintered at 880 C consist of fine grains of around 1 mm size in every composition. The grain size increased slowly in the KNLNT ceramic doped with 1 mol % Na2CO3 when the sintering temperature increased from 880 C to 940 C. On the other hand, the samples co-doped with Na2CO3 and CuO showed an abnormal grain growth. Some grains started to grow up to a few micrometers at 900 C and grew abnormally to a few tens of micrometers, resulting in a bimodal size distribution at the sintering temperature above 900 C. In the single-doped KNLNT ceramic with 1 mol% Na2CO3, the dielectric constant showed a modest increase as the sintering temperature increased from 880 C to 940 C, and the loss factor decreased as the temperature increased to 900 C and then remained the same at the temperature above 900 C Fig. 4. The change of the dielectric constant with the sintering temperature displays a similar tendency with that of the density. The density would have played an important role on the increase of dielectric constant and the increase of the grain size with the sintering temperature could contribute to the increase additionally. In the
Fig. 1. X-ray diffraction patterns of (a) the KNLNT ceramics doped with Na2CO3 and CuO and sintered at 920 C and (b) the samples co-doped with 1 mol% Na2CO3 and 0.5 mol% CuO and sintered at various temperatures.
Please cite this article in press as: J.H. Kim, et al., Low-temperature sintering and piezoelectric properties of CuO-doped (K,Na)NbO3 ceramics, Mater. Res. Bull. (2017), http://dx.doi.org/10.1016/j.materresbull.2017.03.066
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Fig. 4. The changes of the dielectric constants and loss factors at 100 kHz with the sintering temperature in KNLNT ceramics doped with Na2CO3 and CuO. Fig. 2. The changes of the densities with the sintering temperature in the KNLNT ceramics doped with Na2CO3 and CuO.
KNLNT co-doped with Na2CO3 and CuO, the dielectric constant showed a peak value at the sintering temperature of 900 C and reduced substantially with further increase of the temperature. The dielectric loss factor also decreased greatly when the sintering temperature increased above 900 C. The sintering temperature dependence of the dielectric constant and the loss factor could be attributed to the Cu ions acting as an acceptor in the co-doped KNLNT ceramics sintered at above 900 C. The changes of the electromechanical coupling factors (kp) with the sintering temperature are displayed in Fig. 5. The kp increased in the single-doped KNLNT ceramic with 1 mol% Na2CO3 when the sintering temperature increased from 880 C to 900 C, but decreased with further increase of the temperature. Some of the
single-doped samples sintered at temperature above 900 C showed electrical failure during the poling due to a low insulating resistance; others could be poled only at a lower electric field than the samples sintered at lower temperature. It led to the small kp at the sintering temperature above 900 C in the single-doped KNLNT ceramic with 1 mol% Na2CO3. In the co-doped KNLNT ceramics with Na2CO3 and CuO, the samples sintered at 900 C were partially poled due to low insulating resistances when compared to the samples sintered above 900 C. It resulted in the small kp at 900 C. Excepting the 900 C, the kp increased gently when the temperature increased from 880 C to 940 C. The degree of poling can be judged from the maximum phase angle (u max), which is 90 between the resonant frequency (fr) and anti-resonant frequency (fa) in the completely poled piezoelectric material [20]. The maximum phase angles of the samples are shown in Fig. 6. The samples with small kp display low umax with a large variation. It
Fig. 3. The changes of surface morphologies with the sintering temperature in the KNLNT ceramics doped with Na2CO3 and CuO.
Please cite this article in press as: J.H. Kim, et al., Low-temperature sintering and piezoelectric properties of CuO-doped (K,Na)NbO3 ceramics, Mater. Res. Bull. (2017), http://dx.doi.org/10.1016/j.materresbull.2017.03.066
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Fig. 5. The changes of electromechanical coupling factors with the sintering temperature in the KNLNT ceramics doped with Na2CO3 and CuO.
Fig. 7. The changes of a mechanical quality factors with the sintering temperature in the KNLNT ceramics doped with Na2CO3 and CuO.
indicates that poor electromechanical coupling factors resulted from incomplete poling due to low insulating resistances in singledoped samples sintered at temperature above 900 C and the codoped samples sintered below 920 C. Fig. 7 shows the change in the mechanical quality factor (Qm) with the sintering temperature. The Qm of the single-doped KNLNT ceramic with Na2CO3 was about 200 at the sintering temperature of 880–900 C and decreased to below 100 when the sintering temperature increased above 900 C. The low Qm above 900 C seems to be caused by insufficient degree of poling. On the other hand, Qm of the co-doped KNLNT ceramics drastically increased when the sintering temperature increased above 900 C. The Qm was about 900 at 940 C, which is much higher than the reported ones for the CuO-doped KNN ceramics sintered below 1000 C [16,19]. The KNLNT ceramic co-doped with 1 mol% Na2CO3 and 0.5 mol% CuO showed not only a high Qm but also excellent piezoelectric properties when sintered at 940 C: er of 245 and tan d of 0.0034 at 1 kHz after poling, kp of 0.449, d33 of 145 pC/N, and Qm of 904.
density of co-doped sample increased rapidly when the sintering temperature increased to 900 C and showed the saturated at about 95% of the theoretical density at 920 C. The addition of NaCO3 and CuO in KNN-based ceramics has been reported to facilitate densification by the formation of a liquid phase at low temperatures in KNN-based ceramics [15,19]. The co-doping with NaCO3 and CuO in KNLNT ceramics of this work is considered also to produce a liquid phase which facilitates the densification at low temperature around 900 C. No second phase was detected in the X-ray diffraction patterns for the co-doped KNLNT ceramics. This is consistent with the previous reports in which CuO impurity phase could be observed only when the CuO content exceeded 1.0 mol% [15,19]. It is assumed that the liquid phase formed during the sintering remained as glassy phase at grain boundaries and/or diffused into grains in these KNLNT ceramics. The single-doped KNLNT ceramic with Na2CO3 had a uniform microstructure with grain size smaller than a few micrometers at the sintering temperature of 880–940 C and showed a normal grain growth behavior. On the other hand, the co-doped samples with NaCO3 and CuO display an early stage of an abnormal grain growth (AGG) at the sintering temperature of 900 C. These samples also exhibit a bimodal size distribution above 900 C, which consist of abnormally grown grains of a few tens of micrometers in small-grained matrix. The KNN-based ceramics have been reported to have faceted grain boundaries and develop AGG through two-dimensional (2D) nucleation and growth during liquid phase sintering [11,13,21,22]. The formation of the interfacial liquid phase has been reported to facilitate the AGG [11–13,23–28]. Sufficient amount of liquid phase for initiating AGG seems to be formed at the sintering temperature of 900 C in the co-doped samples. The accelerated AGG at the temperature above 900 C can be explained by the higher growth rate at the higher temperature [21]. The co-doped KNLNT ceramics have similar dielectric and piezoelectric properties with the single-doped sample when sintered at 880 C, but the former show poor electromechanical coupling factors at 900 C as shown in Fig. 5. It seems that the liquid phase formed during the sintering facilitated the densification below 900 C, but did not influence the electrical properties of the co-doped samples probably because Cu ions did not diffuse into the lattice. A large amount of liquid phase is supposed to form at the sintering temperature of 900 C and remained as glassy phase at grain boundaries. The continuous glassy phase was observed in some part of the fractured surface of the co-doped samples sintered at 900 C (not shown). Low insulating resistances of the
4. Discussion Co-doped KNLNT ceramics with Na2CO3 and CuO had higher densities than the Na2CO3-doped sample as shown in Fig. 2. The
Fig. 6. The changes of maximum phase angles with the sintering temperature in the KNLNT ceramics doped with Na2CO3 and CuO.
Please cite this article in press as: J.H. Kim, et al., Low-temperature sintering and piezoelectric properties of CuO-doped (K,Na)NbO3 ceramics, Mater. Res. Bull. (2017), http://dx.doi.org/10.1016/j.materresbull.2017.03.066
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co-doped samples sintered at 900 C are considered to be due to the continuous glassy phase at grain boundaries. The co-doped samples show an abrupt decrease in the dielectric constant and the sharp increase in the mechanical quality factor at the sintering temperature above 900 C; these results indicate that Cu ions acting as acceptor had diffused into grains from the glassy phase and substituted Nb and/or Ta ions in the co-doped KNLNT ceramics. The continuous glassy phase at grain boundaries was not observed at the sintering temperature above 900 C. The similar densities and electrical properties of 0.5 mol% and 1.0 mol% CuO-doped samples suggest that the solubility of CuO in a KNLNT ceramic is around 0.5 mol%. 5. Conclusion Co-doping with Na2CO3 and CuO effectively decreased the sintering temperature of KNLNT ceramics to about 900 C and the highest density was about 4.257 g/cm3 (95% of the theoretical density) at the sintering temperature of 920 C in the co-doped sample with 1 mol% Na2CO3 and 0.5 mol% CuO. The co-doped KNLNT ceramics had uniform microstructures with grain size around 1 mm at 880 C, and some grains grew abnormally to a few micrometers at 900 C, and these samples exhibit a bimodal size distribution above 900 C, which consist of abnormally grown grains of a few tens of micrometers in small-grained matrix. The co-doped KNLNT ceramics had poor piezoelectric properties at 900 C due to a glassy phase with a low insulating resistance at grain boundaries. A dielectric constant (er) and loss (tan d) decreased considerably and a mechanical quality factor (Qm) greatly enhanced when the sintering temperature increased above 900 C. These changes suggest that Cu2+ ions had diffused into grains and substituted Nb5+ and/or Ta5+ ions at the temperature above 900 C in the co-doped KNLNT ceramics with NaCO3 and CuO. The physical and electrical properties of the co-doped KNLNT ceramic little changed when the CuO content increased above 0.5 mol%. The co-doped KNLNT ceramic with 1 mol% Na2CO3 and 0.5 mol% CuO had not only a high Qm but also excellent piezoelectric properties when sintered at 940 C: er of 245 and tan d of 0.0034 at 1 kHz, kp of 0.449, d33 of 145 pC/N, and Qm of 904. Acknowledgement
5
Korea [10050958, Development of 1 mW self-powered module for driving system prognosis sensor using piezoelectric composite].
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This work was supported by the Materials and Components Technology Development Program of MOTIE/KEIT, Republic of
Please cite this article in press as: J.H. Kim, et al., Low-temperature sintering and piezoelectric properties of CuO-doped (K,Na)NbO3 ceramics, Mater. Res. Bull. (2017), http://dx.doi.org/10.1016/j.materresbull.2017.03.066
Materials Research Bulletin xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Low-temperature sintering and piezoelectric properties of CuO-doped (K,Na)NbO3 ceramics J.H. Kima , J.S. Kima , S.H. Hanb , H.-W. Kangb , H.-G. Leeb , C.I. Cheona,* a b
Department of Materials Science and Engineering, Hoseo University, Baebang, Asan, Chungnam 336-795, Republic of Korea Electronic Materials and Device Research Center, Korea Electronics Technology Institute, Bundang-gu, Seongnam, Kyeonggi 463-816, Republic of Korea
A R T I C L E I N F O
Article history: Received 17 August 2016 Received in revised form 1 March 2017 Accepted 30 March 2017 Available online xxx Keywords: A. Ceramics B. Microstructure B. Piezoelectricity D. Dielectric properties
A B S T R A C T
The lead-free piezoelectric [(K0.485Na0.515)0.935Li0.065](Nb0.99Ta0.01)O3 (KNLNT) ceramics with a high mechanical quality factor (Qm) were prepared at low sintering temperature below 950 C for multilayered piezoelectric devices. Co-doping effect of 1 mol% Na2CO3 and 0–1 mol% CuO on the densification and piezoelectric properties were investigated. Co-doping effectively decreased the sintering temperature of KNLNT ceramics to 920 C. The co-doped KNLNT ceramics had a bimodal size distribution of large abnormal grains and small matrix grains above 900 C. When the sintering temperature increased above 900 C, a dielectric constant (er) and loss (tan d) decreased considerably and concurrently mechanical quality factor (Qm) greatly enhanced. The co-doped KNLNT ceramic with 1 mol% Na2CO3 and 0.5 mol% CuO had a high Qm and excellent piezoelectric properties when sintered at 940 C: a dielectric constant of 245 and loss (tan d) of 0.0034, an electromechanical coupling factor (kp) of 0.449, and Qm of 904. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Piezoelectric ceramics are widely applied in many electronic devices such as precise actuators and various sensors [1]. There have been considerable efforts to develop lead-free piezoelectric ceramics for replacing Pb-based piezoelectric ceramics due to a global environment issue [1–6]. (K,Na)NbO3 (KNN) is one of the highest potential candidates for a lead-free piezoelectric ceramic due to a large piezoelectric constant and a high Curie temperature [4–6]. KNN-based ceramics with a good thermal stability as well as the piezoelectric constant (d33) larger than 300 pC/N have been reported in recent years [7,8]. A demand for multilayered piezoelectric ceramics is increasing due to their advantages such as a low operating voltage and a high piezo-response [9]. One of the effective methods to reduce the manufacturing cost for multilayered piezoelectric devices is to decrease the amount of expensive noble metals like palladium being used as internal electrodes by sintering the piezoelectric ceramic layers at a low temperature [9,10]. Excess alkaline oxide like Na2O has been reported to reduce the sintering temperature below 1000 C and compensate for the alkali
* Corresponding author. E-mail address: [email protected] (C.I. Cheon).
metal that evaporates during the sintering process [11–13]. And KNN-based ceramics have been successfully fabricated at low sintering temperature below 1000 C by adding various sintering aids such as CuO, LiF, or mixture of Li2CO3 and Bi2O3 [14–17]. A hard piezoelectric material with a high Qm is required for high power transducers and resonators [1]. Im et al. demonstrated a high mechanical quality factor of 1500 in KNN ceramics by adding K4CuNb8O23 (KCN) [18]. They claimed that the Cu2+ ions substituted for Nb5+ ions created oxygen vacancies for charge compensation, and the associates of acceptor Cu2+ ions and oxygen vacancies increased Qm by prohibiting domain wall motion [18]. The sintering temperature in their study, however, was 1100 C which is too high for multi-layered devices. Recently, Huang et al. reported that CuO-doping to an alkali-excess KNN composition, 0.94(K0.48Na0.535)NbO3-0.06LiNbO3 (KNN-LN) reduced the sintering temperature to 890 C and somewhat increased the mechanical quality factor (Qm) [19]. J.J. Zhou et al. have also fabricated hard piezoelectric (Na0.53K0.48)0.98Nb0.8Ta0.2O3–0.05AgSbO3 (NKNTAS) ceramics by doping CuO [15]. However, the Qm in these KNN-LN or NKNT-AS ceramics was lower than 200 which was far below that of the KCN-doped KNN sample. In this work, we fabricated the hard KNN ceramics with a high Qm at a low sintering temperature below 950 C for multilayered piezoelectric devices. [(K0.485Na0.515)0.935Li0.065] (Nb0.99Ta0.01)O3 (KNLNT) ceramics were prepared by a conventional ceramic
http://dx.doi.org/10.1016/j.materresbull.2017.03.066 0025-5408/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: J.H. Kim, et al., Low-temperature sintering and piezoelectric properties of CuO-doped (K,Na)NbO3 ceramics, Mater. Res. Bull. (2017), http://dx.doi.org/10.1016/j.materresbull.2017.03.066
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process. 1 mol% of excess-Na2CO3 were added to KNLNT ceramics in order to reduce the sintering temperature, and 0–1 mol% of CuO was also added to facilitate densification and increase Qm. Codoping effect of Na2CO3 of CuO on the densification and piezoelectric properties were investigated. 2. Experimental Raw powders of K2CO3 (Aldrich, 99%), Na2CO3 (Aldrich, 99%), Li2CO3 (Aldrich, 99%), Nb2O5 (Kojundo Chemical Lab., 99.9%), and Ta2O5 (Kojundo Chemical Lab., 99.9%) were weighed in an appropriate mole ratio for KNLNT. These were mixed by ballmilling for 24 h with anhydrous ethanol and yttria-stabilized zirconia (YSZ) balls in plastic bottles. After drying, the mixed powder was calcined at 800 C for 4 h. The calcined powder was ball-milled again for 24 h with 1 mol% Na2CO3 and 0–1 mol% CuO (Aldrich, 99%) after adding 0.5 wt% polyvinyl butyral (PVB). The disk-type compacts with a diameter of 9 mm were formed by uniaxial pressing at 150 MPa and heat-treated at 600 C for 2 h for binder burn-out. The KNLNT samples were sintered at 860–960 C for 3 h. The phases of the calcined and sintered samples were identified by X-ray diffraction analysis (XRD-6100, Shimadzu). The surface morphologies were observed by a scanning electron microscope (SEC, SNE-4500E). The densities of the sintered samples were measured using Archimedes’ principle. Silver paste was printed on the sintered sample and fired at 800 C for 10 min for electrical measurements. A DC electric field of 4–5 KV/mm was applied to the samples for 30 min at 100 C for dipole alignment (poling). The piezoelectric constant d33 was measured using a Berlincourt-type d33 meter (YE2730A, APC International Ltd.). The capacitance, the impedance and the phase angle were measured as a function of the frequency with an impedance analyzer (HP 4294A). The relative dielectric constant (er) was calculated from the capacitance and the sample dimension. The kp and Qm for a radial vibration mode were calculated according to the following equations; 1 2 kp
2
¼ 0:395
fr fa ; þ 0:574; Q m ¼ 2 2 fa fr 2 pf r Z r C f a f r
where fr, fa, Zr, and C are the resonant frequency, the anti-resonant frequency, the impedance at fr, and the capacitance at 1 kHz, respectively.
3. Results The X-ray diffraction patterns of the KNLNT ceramics are shown in Fig. 1. The samples sintered at 920 C with various dopants are represented in Fig. 1(a). The samples co-doped with 1 mol% NaCO3 and 0.5 mol% CuO sintered at various temperatures are shown in Fig. 1(b). All samples have orthorhombic structures without any impurity phase. The XRD pattern of the sample showed no significant change with either CuO-doping or increasing the sintering temperature to 940 C. When the sintering temperature increased from 860 C to 940 C, the density initially increased to reach the maximum at 920 C, and then decreased a little in KNLNT ceramics as shown in Fig. 2. The samples partially melted at the sintering temperature above 940 C. Fig. 2 shows that the samples co-doped with Na2CO3 and CuO have higher densities than 1 mol% Na2CO3 single-doped sample throughout the whole sintering temperature range. The highest density was about 4.257 g/cm3 (95% of the theoretical density) at the sintering temperature of 920 C in the sample co-doped with 1 mol% Na2CO3 and 0.5 mol% CuO. Increasing the CuO content above 0.5 mol% had little effect on the density. In contrast to the single doping case, the co-doping with Na2CO3 and CuO facilitates the densification of a KNLNT ceramic at low temperature around 900 C. Surface morphologies of KNLNT ceramics sintered at 880– 940 C are displayed in Fig. 3. KNLNT ceramics sintered at 880 C consist of fine grains of around 1 mm size in every composition. The grain size increased slowly in the KNLNT ceramic doped with 1 mol % Na2CO3 when the sintering temperature increased from 880 C to 940 C. On the other hand, the samples co-doped with Na2CO3 and CuO showed an abnormal grain growth. Some grains started to grow up to a few micrometers at 900 C and grew abnormally to a few tens of micrometers, resulting in a bimodal size distribution at the sintering temperature above 900 C. In the single-doped KNLNT ceramic with 1 mol% Na2CO3, the dielectric constant showed a modest increase as the sintering temperature increased from 880 C to 940 C, and the loss factor decreased as the temperature increased to 900 C and then remained the same at the temperature above 900 C Fig. 4. The change of the dielectric constant with the sintering temperature displays a similar tendency with that of the density. The density would have played an important role on the increase of dielectric constant and the increase of the grain size with the sintering temperature could contribute to the increase additionally. In the
Fig. 1. X-ray diffraction patterns of (a) the KNLNT ceramics doped with Na2CO3 and CuO and sintered at 920 C and (b) the samples co-doped with 1 mol% Na2CO3 and 0.5 mol% CuO and sintered at various temperatures.
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Fig. 4. The changes of the dielectric constants and loss factors at 100 kHz with the sintering temperature in KNLNT ceramics doped with Na2CO3 and CuO. Fig. 2. The changes of the densities with the sintering temperature in the KNLNT ceramics doped with Na2CO3 and CuO.
KNLNT co-doped with Na2CO3 and CuO, the dielectric constant showed a peak value at the sintering temperature of 900 C and reduced substantially with further increase of the temperature. The dielectric loss factor also decreased greatly when the sintering temperature increased above 900 C. The sintering temperature dependence of the dielectric constant and the loss factor could be attributed to the Cu ions acting as an acceptor in the co-doped KNLNT ceramics sintered at above 900 C. The changes of the electromechanical coupling factors (kp) with the sintering temperature are displayed in Fig. 5. The kp increased in the single-doped KNLNT ceramic with 1 mol% Na2CO3 when the sintering temperature increased from 880 C to 900 C, but decreased with further increase of the temperature. Some of the
single-doped samples sintered at temperature above 900 C showed electrical failure during the poling due to a low insulating resistance; others could be poled only at a lower electric field than the samples sintered at lower temperature. It led to the small kp at the sintering temperature above 900 C in the single-doped KNLNT ceramic with 1 mol% Na2CO3. In the co-doped KNLNT ceramics with Na2CO3 and CuO, the samples sintered at 900 C were partially poled due to low insulating resistances when compared to the samples sintered above 900 C. It resulted in the small kp at 900 C. Excepting the 900 C, the kp increased gently when the temperature increased from 880 C to 940 C. The degree of poling can be judged from the maximum phase angle (u max), which is 90 between the resonant frequency (fr) and anti-resonant frequency (fa) in the completely poled piezoelectric material [20]. The maximum phase angles of the samples are shown in Fig. 6. The samples with small kp display low umax with a large variation. It
Fig. 3. The changes of surface morphologies with the sintering temperature in the KNLNT ceramics doped with Na2CO3 and CuO.
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Fig. 5. The changes of electromechanical coupling factors with the sintering temperature in the KNLNT ceramics doped with Na2CO3 and CuO.
Fig. 7. The changes of a mechanical quality factors with the sintering temperature in the KNLNT ceramics doped with Na2CO3 and CuO.
indicates that poor electromechanical coupling factors resulted from incomplete poling due to low insulating resistances in singledoped samples sintered at temperature above 900 C and the codoped samples sintered below 920 C. Fig. 7 shows the change in the mechanical quality factor (Qm) with the sintering temperature. The Qm of the single-doped KNLNT ceramic with Na2CO3 was about 200 at the sintering temperature of 880–900 C and decreased to below 100 when the sintering temperature increased above 900 C. The low Qm above 900 C seems to be caused by insufficient degree of poling. On the other hand, Qm of the co-doped KNLNT ceramics drastically increased when the sintering temperature increased above 900 C. The Qm was about 900 at 940 C, which is much higher than the reported ones for the CuO-doped KNN ceramics sintered below 1000 C [16,19]. The KNLNT ceramic co-doped with 1 mol% Na2CO3 and 0.5 mol% CuO showed not only a high Qm but also excellent piezoelectric properties when sintered at 940 C: er of 245 and tan d of 0.0034 at 1 kHz after poling, kp of 0.449, d33 of 145 pC/N, and Qm of 904.
density of co-doped sample increased rapidly when the sintering temperature increased to 900 C and showed the saturated at about 95% of the theoretical density at 920 C. The addition of NaCO3 and CuO in KNN-based ceramics has been reported to facilitate densification by the formation of a liquid phase at low temperatures in KNN-based ceramics [15,19]. The co-doping with NaCO3 and CuO in KNLNT ceramics of this work is considered also to produce a liquid phase which facilitates the densification at low temperature around 900 C. No second phase was detected in the X-ray diffraction patterns for the co-doped KNLNT ceramics. This is consistent with the previous reports in which CuO impurity phase could be observed only when the CuO content exceeded 1.0 mol% [15,19]. It is assumed that the liquid phase formed during the sintering remained as glassy phase at grain boundaries and/or diffused into grains in these KNLNT ceramics. The single-doped KNLNT ceramic with Na2CO3 had a uniform microstructure with grain size smaller than a few micrometers at the sintering temperature of 880–940 C and showed a normal grain growth behavior. On the other hand, the co-doped samples with NaCO3 and CuO display an early stage of an abnormal grain growth (AGG) at the sintering temperature of 900 C. These samples also exhibit a bimodal size distribution above 900 C, which consist of abnormally grown grains of a few tens of micrometers in small-grained matrix. The KNN-based ceramics have been reported to have faceted grain boundaries and develop AGG through two-dimensional (2D) nucleation and growth during liquid phase sintering [11,13,21,22]. The formation of the interfacial liquid phase has been reported to facilitate the AGG [11–13,23–28]. Sufficient amount of liquid phase for initiating AGG seems to be formed at the sintering temperature of 900 C in the co-doped samples. The accelerated AGG at the temperature above 900 C can be explained by the higher growth rate at the higher temperature [21]. The co-doped KNLNT ceramics have similar dielectric and piezoelectric properties with the single-doped sample when sintered at 880 C, but the former show poor electromechanical coupling factors at 900 C as shown in Fig. 5. It seems that the liquid phase formed during the sintering facilitated the densification below 900 C, but did not influence the electrical properties of the co-doped samples probably because Cu ions did not diffuse into the lattice. A large amount of liquid phase is supposed to form at the sintering temperature of 900 C and remained as glassy phase at grain boundaries. The continuous glassy phase was observed in some part of the fractured surface of the co-doped samples sintered at 900 C (not shown). Low insulating resistances of the
4. Discussion Co-doped KNLNT ceramics with Na2CO3 and CuO had higher densities than the Na2CO3-doped sample as shown in Fig. 2. The
Fig. 6. The changes of maximum phase angles with the sintering temperature in the KNLNT ceramics doped with Na2CO3 and CuO.
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co-doped samples sintered at 900 C are considered to be due to the continuous glassy phase at grain boundaries. The co-doped samples show an abrupt decrease in the dielectric constant and the sharp increase in the mechanical quality factor at the sintering temperature above 900 C; these results indicate that Cu ions acting as acceptor had diffused into grains from the glassy phase and substituted Nb and/or Ta ions in the co-doped KNLNT ceramics. The continuous glassy phase at grain boundaries was not observed at the sintering temperature above 900 C. The similar densities and electrical properties of 0.5 mol% and 1.0 mol% CuO-doped samples suggest that the solubility of CuO in a KNLNT ceramic is around 0.5 mol%. 5. Conclusion Co-doping with Na2CO3 and CuO effectively decreased the sintering temperature of KNLNT ceramics to about 900 C and the highest density was about 4.257 g/cm3 (95% of the theoretical density) at the sintering temperature of 920 C in the co-doped sample with 1 mol% Na2CO3 and 0.5 mol% CuO. The co-doped KNLNT ceramics had uniform microstructures with grain size around 1 mm at 880 C, and some grains grew abnormally to a few micrometers at 900 C, and these samples exhibit a bimodal size distribution above 900 C, which consist of abnormally grown grains of a few tens of micrometers in small-grained matrix. The co-doped KNLNT ceramics had poor piezoelectric properties at 900 C due to a glassy phase with a low insulating resistance at grain boundaries. A dielectric constant (er) and loss (tan d) decreased considerably and a mechanical quality factor (Qm) greatly enhanced when the sintering temperature increased above 900 C. These changes suggest that Cu2+ ions had diffused into grains and substituted Nb5+ and/or Ta5+ ions at the temperature above 900 C in the co-doped KNLNT ceramics with NaCO3 and CuO. The physical and electrical properties of the co-doped KNLNT ceramic little changed when the CuO content increased above 0.5 mol%. The co-doped KNLNT ceramic with 1 mol% Na2CO3 and 0.5 mol% CuO had not only a high Qm but also excellent piezoelectric properties when sintered at 940 C: er of 245 and tan d of 0.0034 at 1 kHz, kp of 0.449, d33 of 145 pC/N, and Qm of 904. Acknowledgement
5
Korea [10050958, Development of 1 mW self-powered module for driving system prognosis sensor using piezoelectric composite].
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This work was supported by the Materials and Components Technology Development Program of MOTIE/KEIT, Republic of
Please cite this article in press as: J.H. Kim, et al., Low-temperature sintering and piezoelectric properties of CuO-doped (K,Na)NbO3 ceramics, Mater. Res. Bull. (2017), http://dx.doi.org/10.1016/j.materresbull.2017.03.066