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Dielectric, piezoelectric properties of MnO2-doped (K0.5Na0.5)NbO3–0.05LiNbO3 crystal grown by flux-Bridgman method
Journal of Alloys and Compounds 603 (2014) 95–99
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Dielectric, piezoelectric properties of MnO2-doped (K0.5Na0.5) NbO3–0.05LiNbO3 crystal grown by flux-Bridgman method Ying Liu a,b, Guisheng Xu a,⇑, Jinfeng Liu a, Danfeng Yang a, Xiaxia Chen a,b a Key Laboratory of Transparent Opto-Functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Shanghai 201899, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
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Article history: Received 25 December 2013 Received in revised form 3 March 2014 Accepted 4 March 2014 Available online 20 March 2014 Keywords: Piezoelectric KNN–LN Defect Single crystal
a b s t r a c t Lead-free potassium sodium niobate piezoelectric single crystals substituted with lithium and then doped with MnO2 (K0.5Na0.5)NbO3–0.05LiNbO3–yMnO2 (y = 0%, 1.0% and 1.5%) (abbreviated as KNN– 0.05LN–yMnO2) have been grown by flux-Bridgman method using KCl–K2CO3 eutectic composition as the flux. Their actual composition as well as the dielectric and piezoelectric properties were studied. Their actual composition deviated from the ratio of the raw materials due to different segregation coefficients of K and Na. The orthorhombic–tetragonal (To–t) and tetragonal–cubic phase transition temperature (the Curie temperature Tc) of the single crystal appears at 186 °C and 441 °C, respectively, for KNN–0.05LN– 1.0%MnO2, shift to higher temperatures compared with that of pure KNN–0.05LN crystals, according to the dielectric permittivity versus temperature loops. The KNN–0.05LN–1.0%MnO2 (001) plate shows higher piezoelectric coefficient d33 and dielectric permittivity er when compared with pure KNN– 0.05LN crystal, being on the order of 226 pC/N and 799 (161 pC/N and 530 for KNN–0.05LN), respectively. These excellent properties show that MnO2 dopant is effective in improving KNN–0.05LN based piezoelectric crystals. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Lead-based piezoelectric and ferroelectric materials such as PbTiO3–PbZrO3 (PZT) and PZT-based ternary-component [1,2] are widely used in transducers, actuators, sensors, resonators as well as other electromechanical fields due to their excellent properties. However, the usage of lead-based materials has caused serious environmental problems due to their high toxicity of lead oxide and their high vapor pressure during fabrication. Thus, there is an urgent demand to develop some lead-free piezoelectric materials with good piezoelectric properties to replace the lead-containing materials in various applications. In the quest to find suitable replacements for PZT, many ceramic systems, e.g. (KxNa1x)NbO3 (KNN) based material systems, (Na0.5Bi0.5)TiO3–BaTiO3 and BiFeO3–BaTiO3 have been explored [3]. Among these systems, KNN-based materials are the most promising candidates, which show not only an excellent piezoelectric property but also a relatively high Curie temperature (Tc 420 °C) [4–9]. Alkaline niobate (KxNa1x)NbO3 (KNN)-based composition is a solid solution of ferroelectric potassium niobate ⇑ Corresponding author. Tel.: +86 21 69987753; fax: +86 21 59927184. E-mail address: [email protected] (G. Xu). http://dx.doi.org/10.1016/j.jallcom.2014.03.006 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
KNbO3 and anti-ferroelectric potassium niobate NaNbO3 [10,11]. Substantial improvements in KNN-based materials have been achieved in recent years. Some previous researches about KNN systems with additions of SrTiO3 [3], LiNbO3, LiTaO3, LiSbO3 [4,9,12– 16], YbMnO3 [17], as well as Mn [12,18–21], Er [22,23], and Ag [14,15] have been made. However, only a few reports about KNN-based single crystals have been published [20,21,24–27]. It is anticipated that KNN-based single crystals might have piezoelectricity that is even better than their textured ceramics counterpart since the optimal crystallographic orientation and domain engineering can be realized in single crystals. Therefore, more fundamental researches on KNN-based single crystals are urgently needed to be carried out. In the previous study, KNN single crystals doped with Ta [4,16], Sb and Sr [3] were studied, but the dopants may lower the Curie temperature. (K0.5Na0.5) NbO3–0.05LiNbO3 single crystals were successfully grown by Bridgman Method [4]. The single crystals show a high d33 on the order of 205–405 pC/ N, with a Curie temperature of 424 °C. But with a relatively high degree of oxide vacancies, the ferroelectric polarization hysteresis loop is round shape and not saturated, which suggests that the leakage current induced by oxide vacancies is large. MnO2 is one of the most useful additives and be widely used, because Mn ions have multi-valences: Mn2+, Mn3+, and Mn4+.
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When Mn ions are used to substitute the metal cations in perovskite-type ABO3 ferroelectric materials, they usually substitute cations with similar ionic radii in A or B site [28]. MnO2 additive can be recognized as an acceptor or ‘‘hard’’ doping element, but it can also cause ‘‘soft’’ doping [29]. When Mn ions doped as a donor, it could absorb the surplus positive charge. The purpose of this paper is to grow MnO2-doped (KxNa1x)NbO3–LiNbO3 single crystals by using flux-Bridgman method and to investigate the influence of MnO2 doping on dielectric and piezoelectric properties. 2. Experimental details (KxNa1x)NbO3–LiNbO3 and MnO2-doped (KxNa1x) NbO3–LiNbO3 single crystals were grown by flux-Bridgman method. The raw materials were high-purity (99.99%) powders of Na2CO3, K2CO3, Li2CO3, Nb2O5 and MnO2. These materials were weighed according to the composition formula of (K0.5Na0.5) NbO3–0.05LiNbO3– yMnO2 (y = 0%, 1.0%, 1.5%) and mixed thoroughly. The powders were calcined in air at 800 °C for 2 h. KCl–K2CO3 (molar ratio of 63.4%:36.6%) flux was added to the calcined powders in a ratio of 1:4 by weight and combined as the starting material. The starting material was pressed at cold isostatic of 200 MPa, and then, they were charged in the platinum crucibles, which were sealed to suppress the volatilization of melts. In the flux-Bridgman method growth of (KxNa1x)NbO3–0.05LiNbO3–yMnO2 crystals, the platinum crucibles were maintained at 1200 °C for more than 10 h and then descended at a rate of 0.2–0.4 mm/h. The temperature gradient of 30–50 °C/cm was used near the solid–liquid interface in the growth furnace. After the end of the crystal growth, the temperature of the growth furnace was cooled down at a rate of 100–200 °C/h to room temperature. The crystal blanks were washed with distilled water to obtain KNN-based crystals. The crystals actual composition was detected by Electron Probe Microanalysis (EPMA, JXA-8100). The crystal structures were determined by X-ray powder diffraction analysis using a Cu Ka filter. For electrical characteristics, samples were polished and painted with silver paste on the opposite two surfaces. Their dielectric properties were measured using an HP4284A precision impedance analyzer in the temperature range of 25–600 °C. Before piezoelectric measurements, the samples were poled by 30–40 kV/cm dc field at 130 °C for 30 min within silicon oil. The piezoelectric constant d33 was measured with Berlicourt-type quasistatic meter of ZJ-4A type made by Institute of Acoustics, Chinese Academy of Sciences. The ferroelectric hysteresis loops were measured at room temperature by using a ferroelectric tester (TF Analyzer-2000) at 10 Hz.
3. Results and discussion 3.1. Composition of the crystals The Electron Probe Microanalysis (EPMA) was used to determine quantitatively the amount of each element present as well as the crystal morphologies. A mean value was obtained and the element relative amounts in moles were shown in Table 1 (Mn is not detected due to its small content). The crystal composition is deviated from its nominal composition. This deviation is due to the different segregation coefficients of K and Na. During the analysis process, we found that Cl appears in the dark area of the invisible surface. It means that Cl from the flux composition KCl may gather at grain boundaries or form inclusions inside the crystals.
Fig. 1. X-ray powder diffraction patterns of KNN–0.05LN, KNN–0.05LN–1.0%MnO2 and KNN–0.05LN–1.5%MnO2.
Table 2 The crystals’ lattice parameters calculated according to XRD patterns. Sample (Å)
KNN– 0.05LN
KNN–0.05LN– 1.0%MnO2
KNN–0.05LN– 1.5%MnO2
a b c
3.9862 3.9473 3.9865
3.9864 3.9385 3.980
3.980 3.9375 3.9764
(0 0 2) peak splitting at 2h = 45° are characteristics of single orthorhombic phase. Compared with KNN–0.05LN, peaks were found to shift to a little higher 2h values because of their smaller lattice volume due to the MnO2 doping. Lattice parameters calculated according to the XRD pattern are listed in Table 2 for KNN– 0.05LN and KNN–0.05LN–yMnO2. There is only little difference between them. This is different from MnO2-doped K0.5Na0.5NbO3 single crystals, in which the XRD diffraction peaks shift to lower 2h due to the Mn doping [25]. When compare the ionic radius of Mn ions and Nb ions, the ionic radius of Mn2+ (low spin: 0.67 Å) and Mn3+ (high spin: 0.64 Å, low spin: 0.58 Å) are a bit smaller than that of Nb5+ (0.69 Å), while the radius of Mn2+ (high spin: 0.83 Å) is larger than that of Nb5+ [30]. The little smaller lattice parameters due to the Mn substitute and ionic size consideration suggest that the Mn ions are substituted at the Nb5+ site as Mn2+ with low-spin configuration or Mn3+ with high-spin configuration. To some extent, this is consistent with the substitution of Mn2+ with lowspin configuration for K0.5Na0.5NbO3 powders, as well as the Mn3+ substitution with high-spin configuration for K0.14Na0.86NbO3 single crystals [19,30]. 3.3. Dielectric properties
3.2. Crystal structure The X-ray diffraction (XRD) results (Fig. 1) show that the crystals both KNN–0.05LN and KNN–0.05LN–yMnO2 all have pure perovskite structures and an orthorhombic phase without other phases. The (110)/(001) peak splitting at 2h = 22° and the (2 2 0)/
Table 1 Composition of the crystals derived from EPMA spectra. Element
K
Na
Li
Nb
O
Mn
KNN–0.05LN KNN–0.05LN–1.0%MnO2 KNN–0.05LN–1.5%MnO2
12.11 13.92 13.69
5.53 5.35 4.44
– – –
21.09 20.31 20.80
61.45 60.42 61.07
– – –
Fig. 2 shows the temperature dependence of dielectric permittivity (er ) and dielectric loss (tan d) measured at 1, 10, 100 kHz of unpoled crystals. As to KNN–0.05LN, two dielectric peaks show up at 179 °C and 421 °C at all frequencies (1 kHz, 10 kHz, and 100 kHz), which is consistent with the previous research results [4,31]. The peak at 179 °C should be attributed to the phase transition of orthorhombic to tetragonal, while the peak at 421 °C (Tc) to the phase transition of tetragonal to cubic (ferroelectric to paraelectric phase at Curie temperature Tc). When the KNN–0.05LN based crystals were doped with MnO2, temperatures of the two phase transitions shift to higher temperatures. According to Fig. 2(a), the two peaks show up at 186 °C and 441 °C for KNN–0.05LN– 1%MnO2, while 195 °C and 446 °C for KNN–0.05LN–1.5%MnO2.
Y. Liu et al. / Journal of Alloys and Compounds 603 (2014) 95–99
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Fig. 2. Dielectric properties of pure and MnO2-doped KNN–0.05LN crystals (a) temperature dependence of the dielectric permittivity for KNN–0.05LN and MnO2-doped KNN– 0.05LN single crystals at 10 kHz, (b) temperature dependence of the dielectric permittivity for KNN–0.05LN crystal, (c) temperature dependence of the dielectric permittivity for KNN–0.05LN–1%MnO2, (d) temperature dependence of the dielectric permittivity for KNN–0.05LN–1.5MnO2 and (e) temperature dependence of the dielectric loss at 10 kHz.
Table 3 listed the dielectric properties of [001]-oriented KNN– 0.05LN and MnO2-doped KNN–0.05LN crystals. The results indicate that MnO2 doping enhanced the dielectric properties obviously, dielectric permittivity increases from 530 to 899 while the dielectric loss decreases from 0.35 to 0.09. On the other hand, the dielectric and piezoelectric properties in the tetragonal state play an
important role in practical use [7,32]. Our research shows that MnO2-doping is effective for decreasing dielectric loss in the tetragonal state as shown in Fig. 2(e). It is beneficial to its applications.In addition, when compare the temperature dependence of dielectric permittivity of KNN–0.05LN and MnO2-doped KNN–0.05LN, there exists an abnormal dielectric peak at about
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Y. Liu et al. / Journal of Alloys and Compounds 603 (2014) 95–99
Table 3 Dielectric properties of pure and MnO2-doped KNN–0.05LN crystals. Sample
To–t (°C)
Tc (°C)
tan d
er
c
KNN–5LN KNN–0.05LN–1.0%MnO2 KNN–0.05LN–1.5%MnO2
179 186 195
421 441 446
0.35 0.14 0.09
530 614 899
1.08 1.22 1.49
loss for MnO2-doped KNN–0.05LN was deemed to be correlated to the decrease of the electron–hole (h) due to the MnO2 doping. The introduction of Mn2+/Mn3+ can absorb the h by the valence increase of Mn, which could be expressed by: 00 Mn000 Nb þ h ! MnNb
ðMn2þ to Mn3þ Þ
ð1Þ
Mn00Nb þ h ! Mn0Nb
ðMn3þ to Mn4þ Þ
ð2Þ
h acts as detrimental carrier for the leakage current of KNN based crystals. With the decreasing of h, leakage current diminishes, and thus leads to lower dielectric loss. In addition, it is known that the electron–hole polarization exists under frequency in the range of DC 1 kHz. This proves that the abnormal dielectric peak at 275 °C of KNN–0.05LN is induced by h. Thus the Mn ions doping could absorb h and enhance dielectric properties of KNN–0.05LN based crystal. All the crystals behave like relaxor ferroelectrics that the dielectric constants decrease with the increase of frequencies. On the other hand, they are not typical relaxor ferroelectrics because of the unchanged position and half-width of the transition peak with frequencies at Tc. Therefore, it can be said that KNN–0.05LN– yMnO2 crystals are transitional ferroelectrics between typical normal ones and typical relaxor ones. In order to determine which one is predominant, an empirical expression is employed,
1 Fig. 3. The log [1/e 1/emax] as a function of log (T Tm) at 10 kHz.
Table 4 Dielectric and piezoelectric properties of KNN and Mn-doped KNN based crystals.
a b
Sample
To–t (°C)
Tc (°C)
tan d
er
d33 (pC/N)
KNN–0.05LN KNN–0.05LNa KNN–0.05LN–1.0%MnO2 Mn-doped KNN crystalb
179 192 186 194
421 426 441 416
0.35 1.1 0.14 0.11
530 – 614 590
189 240–405 226 230
Ref. [5]. Ref. [12].
e
1
emax
¼
ðT T m Þc C
ð3Þ
where c and C are constants, and c is between 1 and 2, which is correspond to the Curie–Weiss law valid for the case of normal ferroelectric and to the quadratic law valid for an ideal relaxor ferroelectric, respectively [33]. The logarithm of (1/e 1/emax) was plotted against the logarithm of (T Tm) as shown in Fig. 3 and the c values are listed in Table 3. The result indicates that with MnO2 doping, c increases from 1.08 to 1.49, the KNN–0.05LN based crystal is likely to be more relaxor ferroelectrics with the increasing amount of MnO2. And its Tc shifts to higher temperature. This is different from PMNT crystals [34]. 3.4. Piezoelectric and ferroelectric properties
275 °C for KNN–0.05LN crystal at 1 kHz, but for KNN–0.05LN– 1%MnO2 and KNN–0.05LN–1.5%MnO2, this peak disappeared. This disappeared dielectric peak together with the decreased dielectric
After the (0 01)-oriented sample plates were poled at 35 kV/cm, the piezoelectric constant of KNN–0.05LN and KNN–0.05LN–1%MnO2
Fig. 4. Polarization hysteresis loops of (a) KNN–0.05LN and (b) KNN–0.05LN–1.0%MnO2 crystals.
Y. Liu et al. / Journal of Alloys and Compounds 603 (2014) 95–99
were 161 pC/N and 190 pC/N. It is obviously noted that after doping with MnO2, the piezoelectric constant d33 is increased. After the sample plates were annealed at 850 °C for 5 h, d33 increased to 189 pC/N and 226 pC/N respectively for KNN–0.05LN and KNN–0.05LN– 1%MnO2. KNN–0.05LN single crystals contain a large amount of h and other lattice defects. These defects may interfere with the polarization switching and poling process, and induce a poor insulting property. This will lead to a lower piezoelectric constant [35]. The process of doping MnO2 and annealing will absorb the oxygen vacancies and h, leading the relatively higher piezoelectric constant of KNN–0.05LN– 1.0%MnO2. The piezoelectric constant is a little smaller than that of previous study of K0.5Na0.5NbO3–5%LiNbO3 single crystal [4]. This may be due to the composition deviation and not in the MPB composition region. Table 4 listed dielectric and piezoelectric properties of MnO2 doped KNN and KNN–LN systems cited from several previous reports. It shows that MnO2-doped KNN–0.05LN single crystals perform well with a high piezoelectric constant and a much higher Tc. Fig. 4 shows the polarization hysteresis loops measured at 25 °C. Ferroelectric measurement is carried out at a maximum electric field of 2.5 kV/mm applied using an amplified bipolar wave form at 10 Hz. KNN–0.05LN crystal exhibited a coercive electric field (Ec) of 31.7 kV/cm and a remnant polarization (Pr) of 16 lC/cm2, while the KNN–0.05LN–1.0%MnO2 crystal exhibited a smaller coercive electric field of 17.5 kV/cm and a relatively large Pr of 18 lC/cm2. Compared with 0.95KNN–0.05LN single crystal in previous research which shows a round shape and not saturated ferroelectric polarization hysteresis loop [4], we have get well saturated hysteresis loops, and this shows good ferroelectricity. 4. Conclusions KNN–0.05LN–yMnO2 single crystals were grown directly from their melt by flux-Bridgman method. Their (0 0 1) plates undergo the orthorhombic to tetragonal phase transition and the phase transition from tetragonal to cubic (ferroelectric to paraelectric transition). Their transition temperatures are higher than that of KNN–0.05LN. The leakage current decreased due to the MnO2doping, Mn ions substitutes Nb5+ as Mn2+/Mn3+ which could absorb h during the valence increase process, this also leads to a higher dielectric constant of 226 pC/N and better ferroelectric property with Pr of 18 lC/cm2 and Ec of 17.5 kV/cm. Acknowledgements This work was supported by China’s manned space programmaterial research in space, the science and technology innovation
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project of Shanghai Institute of Ceramics, Chinese Academy of Science (Y39ZC6160G) and the opening program of the Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal Education (KFKT20130003). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Dielectric, piezoelectric properties of MnO2-doped (K0.5Na0.5) NbO3–0.05LiNbO3 crystal grown by flux-Bridgman method Ying Liu a,b, Guisheng Xu a,⇑, Jinfeng Liu a, Danfeng Yang a, Xiaxia Chen a,b a Key Laboratory of Transparent Opto-Functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Shanghai 201899, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 25 December 2013 Received in revised form 3 March 2014 Accepted 4 March 2014 Available online 20 March 2014 Keywords: Piezoelectric KNN–LN Defect Single crystal
a b s t r a c t Lead-free potassium sodium niobate piezoelectric single crystals substituted with lithium and then doped with MnO2 (K0.5Na0.5)NbO3–0.05LiNbO3–yMnO2 (y = 0%, 1.0% and 1.5%) (abbreviated as KNN– 0.05LN–yMnO2) have been grown by flux-Bridgman method using KCl–K2CO3 eutectic composition as the flux. Their actual composition as well as the dielectric and piezoelectric properties were studied. Their actual composition deviated from the ratio of the raw materials due to different segregation coefficients of K and Na. The orthorhombic–tetragonal (To–t) and tetragonal–cubic phase transition temperature (the Curie temperature Tc) of the single crystal appears at 186 °C and 441 °C, respectively, for KNN–0.05LN– 1.0%MnO2, shift to higher temperatures compared with that of pure KNN–0.05LN crystals, according to the dielectric permittivity versus temperature loops. The KNN–0.05LN–1.0%MnO2 (001) plate shows higher piezoelectric coefficient d33 and dielectric permittivity er when compared with pure KNN– 0.05LN crystal, being on the order of 226 pC/N and 799 (161 pC/N and 530 for KNN–0.05LN), respectively. These excellent properties show that MnO2 dopant is effective in improving KNN–0.05LN based piezoelectric crystals. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Lead-based piezoelectric and ferroelectric materials such as PbTiO3–PbZrO3 (PZT) and PZT-based ternary-component [1,2] are widely used in transducers, actuators, sensors, resonators as well as other electromechanical fields due to their excellent properties. However, the usage of lead-based materials has caused serious environmental problems due to their high toxicity of lead oxide and their high vapor pressure during fabrication. Thus, there is an urgent demand to develop some lead-free piezoelectric materials with good piezoelectric properties to replace the lead-containing materials in various applications. In the quest to find suitable replacements for PZT, many ceramic systems, e.g. (KxNa1x)NbO3 (KNN) based material systems, (Na0.5Bi0.5)TiO3–BaTiO3 and BiFeO3–BaTiO3 have been explored [3]. Among these systems, KNN-based materials are the most promising candidates, which show not only an excellent piezoelectric property but also a relatively high Curie temperature (Tc 420 °C) [4–9]. Alkaline niobate (KxNa1x)NbO3 (KNN)-based composition is a solid solution of ferroelectric potassium niobate ⇑ Corresponding author. Tel.: +86 21 69987753; fax: +86 21 59927184. E-mail address: [email protected] (G. Xu). http://dx.doi.org/10.1016/j.jallcom.2014.03.006 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
KNbO3 and anti-ferroelectric potassium niobate NaNbO3 [10,11]. Substantial improvements in KNN-based materials have been achieved in recent years. Some previous researches about KNN systems with additions of SrTiO3 [3], LiNbO3, LiTaO3, LiSbO3 [4,9,12– 16], YbMnO3 [17], as well as Mn [12,18–21], Er [22,23], and Ag [14,15] have been made. However, only a few reports about KNN-based single crystals have been published [20,21,24–27]. It is anticipated that KNN-based single crystals might have piezoelectricity that is even better than their textured ceramics counterpart since the optimal crystallographic orientation and domain engineering can be realized in single crystals. Therefore, more fundamental researches on KNN-based single crystals are urgently needed to be carried out. In the previous study, KNN single crystals doped with Ta [4,16], Sb and Sr [3] were studied, but the dopants may lower the Curie temperature. (K0.5Na0.5) NbO3–0.05LiNbO3 single crystals were successfully grown by Bridgman Method [4]. The single crystals show a high d33 on the order of 205–405 pC/ N, with a Curie temperature of 424 °C. But with a relatively high degree of oxide vacancies, the ferroelectric polarization hysteresis loop is round shape and not saturated, which suggests that the leakage current induced by oxide vacancies is large. MnO2 is one of the most useful additives and be widely used, because Mn ions have multi-valences: Mn2+, Mn3+, and Mn4+.
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When Mn ions are used to substitute the metal cations in perovskite-type ABO3 ferroelectric materials, they usually substitute cations with similar ionic radii in A or B site [28]. MnO2 additive can be recognized as an acceptor or ‘‘hard’’ doping element, but it can also cause ‘‘soft’’ doping [29]. When Mn ions doped as a donor, it could absorb the surplus positive charge. The purpose of this paper is to grow MnO2-doped (KxNa1x)NbO3–LiNbO3 single crystals by using flux-Bridgman method and to investigate the influence of MnO2 doping on dielectric and piezoelectric properties. 2. Experimental details (KxNa1x)NbO3–LiNbO3 and MnO2-doped (KxNa1x) NbO3–LiNbO3 single crystals were grown by flux-Bridgman method. The raw materials were high-purity (99.99%) powders of Na2CO3, K2CO3, Li2CO3, Nb2O5 and MnO2. These materials were weighed according to the composition formula of (K0.5Na0.5) NbO3–0.05LiNbO3– yMnO2 (y = 0%, 1.0%, 1.5%) and mixed thoroughly. The powders were calcined in air at 800 °C for 2 h. KCl–K2CO3 (molar ratio of 63.4%:36.6%) flux was added to the calcined powders in a ratio of 1:4 by weight and combined as the starting material. The starting material was pressed at cold isostatic of 200 MPa, and then, they were charged in the platinum crucibles, which were sealed to suppress the volatilization of melts. In the flux-Bridgman method growth of (KxNa1x)NbO3–0.05LiNbO3–yMnO2 crystals, the platinum crucibles were maintained at 1200 °C for more than 10 h and then descended at a rate of 0.2–0.4 mm/h. The temperature gradient of 30–50 °C/cm was used near the solid–liquid interface in the growth furnace. After the end of the crystal growth, the temperature of the growth furnace was cooled down at a rate of 100–200 °C/h to room temperature. The crystal blanks were washed with distilled water to obtain KNN-based crystals. The crystals actual composition was detected by Electron Probe Microanalysis (EPMA, JXA-8100). The crystal structures were determined by X-ray powder diffraction analysis using a Cu Ka filter. For electrical characteristics, samples were polished and painted with silver paste on the opposite two surfaces. Their dielectric properties were measured using an HP4284A precision impedance analyzer in the temperature range of 25–600 °C. Before piezoelectric measurements, the samples were poled by 30–40 kV/cm dc field at 130 °C for 30 min within silicon oil. The piezoelectric constant d33 was measured with Berlicourt-type quasistatic meter of ZJ-4A type made by Institute of Acoustics, Chinese Academy of Sciences. The ferroelectric hysteresis loops were measured at room temperature by using a ferroelectric tester (TF Analyzer-2000) at 10 Hz.
3. Results and discussion 3.1. Composition of the crystals The Electron Probe Microanalysis (EPMA) was used to determine quantitatively the amount of each element present as well as the crystal morphologies. A mean value was obtained and the element relative amounts in moles were shown in Table 1 (Mn is not detected due to its small content). The crystal composition is deviated from its nominal composition. This deviation is due to the different segregation coefficients of K and Na. During the analysis process, we found that Cl appears in the dark area of the invisible surface. It means that Cl from the flux composition KCl may gather at grain boundaries or form inclusions inside the crystals.
Fig. 1. X-ray powder diffraction patterns of KNN–0.05LN, KNN–0.05LN–1.0%MnO2 and KNN–0.05LN–1.5%MnO2.
Table 2 The crystals’ lattice parameters calculated according to XRD patterns. Sample (Å)
KNN– 0.05LN
KNN–0.05LN– 1.0%MnO2
KNN–0.05LN– 1.5%MnO2
a b c
3.9862 3.9473 3.9865
3.9864 3.9385 3.980
3.980 3.9375 3.9764
(0 0 2) peak splitting at 2h = 45° are characteristics of single orthorhombic phase. Compared with KNN–0.05LN, peaks were found to shift to a little higher 2h values because of their smaller lattice volume due to the MnO2 doping. Lattice parameters calculated according to the XRD pattern are listed in Table 2 for KNN– 0.05LN and KNN–0.05LN–yMnO2. There is only little difference between them. This is different from MnO2-doped K0.5Na0.5NbO3 single crystals, in which the XRD diffraction peaks shift to lower 2h due to the Mn doping [25]. When compare the ionic radius of Mn ions and Nb ions, the ionic radius of Mn2+ (low spin: 0.67 Å) and Mn3+ (high spin: 0.64 Å, low spin: 0.58 Å) are a bit smaller than that of Nb5+ (0.69 Å), while the radius of Mn2+ (high spin: 0.83 Å) is larger than that of Nb5+ [30]. The little smaller lattice parameters due to the Mn substitute and ionic size consideration suggest that the Mn ions are substituted at the Nb5+ site as Mn2+ with low-spin configuration or Mn3+ with high-spin configuration. To some extent, this is consistent with the substitution of Mn2+ with lowspin configuration for K0.5Na0.5NbO3 powders, as well as the Mn3+ substitution with high-spin configuration for K0.14Na0.86NbO3 single crystals [19,30]. 3.3. Dielectric properties
3.2. Crystal structure The X-ray diffraction (XRD) results (Fig. 1) show that the crystals both KNN–0.05LN and KNN–0.05LN–yMnO2 all have pure perovskite structures and an orthorhombic phase without other phases. The (110)/(001) peak splitting at 2h = 22° and the (2 2 0)/
Table 1 Composition of the crystals derived from EPMA spectra. Element
K
Na
Li
Nb
O
Mn
KNN–0.05LN KNN–0.05LN–1.0%MnO2 KNN–0.05LN–1.5%MnO2
12.11 13.92 13.69
5.53 5.35 4.44
– – –
21.09 20.31 20.80
61.45 60.42 61.07
– – –
Fig. 2 shows the temperature dependence of dielectric permittivity (er ) and dielectric loss (tan d) measured at 1, 10, 100 kHz of unpoled crystals. As to KNN–0.05LN, two dielectric peaks show up at 179 °C and 421 °C at all frequencies (1 kHz, 10 kHz, and 100 kHz), which is consistent with the previous research results [4,31]. The peak at 179 °C should be attributed to the phase transition of orthorhombic to tetragonal, while the peak at 421 °C (Tc) to the phase transition of tetragonal to cubic (ferroelectric to paraelectric phase at Curie temperature Tc). When the KNN–0.05LN based crystals were doped with MnO2, temperatures of the two phase transitions shift to higher temperatures. According to Fig. 2(a), the two peaks show up at 186 °C and 441 °C for KNN–0.05LN– 1%MnO2, while 195 °C and 446 °C for KNN–0.05LN–1.5%MnO2.
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Fig. 2. Dielectric properties of pure and MnO2-doped KNN–0.05LN crystals (a) temperature dependence of the dielectric permittivity for KNN–0.05LN and MnO2-doped KNN– 0.05LN single crystals at 10 kHz, (b) temperature dependence of the dielectric permittivity for KNN–0.05LN crystal, (c) temperature dependence of the dielectric permittivity for KNN–0.05LN–1%MnO2, (d) temperature dependence of the dielectric permittivity for KNN–0.05LN–1.5MnO2 and (e) temperature dependence of the dielectric loss at 10 kHz.
Table 3 listed the dielectric properties of [001]-oriented KNN– 0.05LN and MnO2-doped KNN–0.05LN crystals. The results indicate that MnO2 doping enhanced the dielectric properties obviously, dielectric permittivity increases from 530 to 899 while the dielectric loss decreases from 0.35 to 0.09. On the other hand, the dielectric and piezoelectric properties in the tetragonal state play an
important role in practical use [7,32]. Our research shows that MnO2-doping is effective for decreasing dielectric loss in the tetragonal state as shown in Fig. 2(e). It is beneficial to its applications.In addition, when compare the temperature dependence of dielectric permittivity of KNN–0.05LN and MnO2-doped KNN–0.05LN, there exists an abnormal dielectric peak at about
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Table 3 Dielectric properties of pure and MnO2-doped KNN–0.05LN crystals. Sample
To–t (°C)
Tc (°C)
tan d
er
c
KNN–5LN KNN–0.05LN–1.0%MnO2 KNN–0.05LN–1.5%MnO2
179 186 195
421 441 446
0.35 0.14 0.09
530 614 899
1.08 1.22 1.49
loss for MnO2-doped KNN–0.05LN was deemed to be correlated to the decrease of the electron–hole (h) due to the MnO2 doping. The introduction of Mn2+/Mn3+ can absorb the h by the valence increase of Mn, which could be expressed by: 00 Mn000 Nb þ h ! MnNb
ðMn2þ to Mn3þ Þ
ð1Þ
Mn00Nb þ h ! Mn0Nb
ðMn3þ to Mn4þ Þ
ð2Þ
h acts as detrimental carrier for the leakage current of KNN based crystals. With the decreasing of h, leakage current diminishes, and thus leads to lower dielectric loss. In addition, it is known that the electron–hole polarization exists under frequency in the range of DC 1 kHz. This proves that the abnormal dielectric peak at 275 °C of KNN–0.05LN is induced by h. Thus the Mn ions doping could absorb h and enhance dielectric properties of KNN–0.05LN based crystal. All the crystals behave like relaxor ferroelectrics that the dielectric constants decrease with the increase of frequencies. On the other hand, they are not typical relaxor ferroelectrics because of the unchanged position and half-width of the transition peak with frequencies at Tc. Therefore, it can be said that KNN–0.05LN– yMnO2 crystals are transitional ferroelectrics between typical normal ones and typical relaxor ones. In order to determine which one is predominant, an empirical expression is employed,
1 Fig. 3. The log [1/e 1/emax] as a function of log (T Tm) at 10 kHz.
Table 4 Dielectric and piezoelectric properties of KNN and Mn-doped KNN based crystals.
a b
Sample
To–t (°C)
Tc (°C)
tan d
er
d33 (pC/N)
KNN–0.05LN KNN–0.05LNa KNN–0.05LN–1.0%MnO2 Mn-doped KNN crystalb
179 192 186 194
421 426 441 416
0.35 1.1 0.14 0.11
530 – 614 590
189 240–405 226 230
Ref. [5]. Ref. [12].
e
1
emax
¼
ðT T m Þc C
ð3Þ
where c and C are constants, and c is between 1 and 2, which is correspond to the Curie–Weiss law valid for the case of normal ferroelectric and to the quadratic law valid for an ideal relaxor ferroelectric, respectively [33]. The logarithm of (1/e 1/emax) was plotted against the logarithm of (T Tm) as shown in Fig. 3 and the c values are listed in Table 3. The result indicates that with MnO2 doping, c increases from 1.08 to 1.49, the KNN–0.05LN based crystal is likely to be more relaxor ferroelectrics with the increasing amount of MnO2. And its Tc shifts to higher temperature. This is different from PMNT crystals [34]. 3.4. Piezoelectric and ferroelectric properties
275 °C for KNN–0.05LN crystal at 1 kHz, but for KNN–0.05LN– 1%MnO2 and KNN–0.05LN–1.5%MnO2, this peak disappeared. This disappeared dielectric peak together with the decreased dielectric
After the (0 01)-oriented sample plates were poled at 35 kV/cm, the piezoelectric constant of KNN–0.05LN and KNN–0.05LN–1%MnO2
Fig. 4. Polarization hysteresis loops of (a) KNN–0.05LN and (b) KNN–0.05LN–1.0%MnO2 crystals.
Y. Liu et al. / Journal of Alloys and Compounds 603 (2014) 95–99
were 161 pC/N and 190 pC/N. It is obviously noted that after doping with MnO2, the piezoelectric constant d33 is increased. After the sample plates were annealed at 850 °C for 5 h, d33 increased to 189 pC/N and 226 pC/N respectively for KNN–0.05LN and KNN–0.05LN– 1%MnO2. KNN–0.05LN single crystals contain a large amount of h and other lattice defects. These defects may interfere with the polarization switching and poling process, and induce a poor insulting property. This will lead to a lower piezoelectric constant [35]. The process of doping MnO2 and annealing will absorb the oxygen vacancies and h, leading the relatively higher piezoelectric constant of KNN–0.05LN– 1.0%MnO2. The piezoelectric constant is a little smaller than that of previous study of K0.5Na0.5NbO3–5%LiNbO3 single crystal [4]. This may be due to the composition deviation and not in the MPB composition region. Table 4 listed dielectric and piezoelectric properties of MnO2 doped KNN and KNN–LN systems cited from several previous reports. It shows that MnO2-doped KNN–0.05LN single crystals perform well with a high piezoelectric constant and a much higher Tc. Fig. 4 shows the polarization hysteresis loops measured at 25 °C. Ferroelectric measurement is carried out at a maximum electric field of 2.5 kV/mm applied using an amplified bipolar wave form at 10 Hz. KNN–0.05LN crystal exhibited a coercive electric field (Ec) of 31.7 kV/cm and a remnant polarization (Pr) of 16 lC/cm2, while the KNN–0.05LN–1.0%MnO2 crystal exhibited a smaller coercive electric field of 17.5 kV/cm and a relatively large Pr of 18 lC/cm2. Compared with 0.95KNN–0.05LN single crystal in previous research which shows a round shape and not saturated ferroelectric polarization hysteresis loop [4], we have get well saturated hysteresis loops, and this shows good ferroelectricity. 4. Conclusions KNN–0.05LN–yMnO2 single crystals were grown directly from their melt by flux-Bridgman method. Their (0 0 1) plates undergo the orthorhombic to tetragonal phase transition and the phase transition from tetragonal to cubic (ferroelectric to paraelectric transition). Their transition temperatures are higher than that of KNN–0.05LN. The leakage current decreased due to the MnO2doping, Mn ions substitutes Nb5+ as Mn2+/Mn3+ which could absorb h during the valence increase process, this also leads to a higher dielectric constant of 226 pC/N and better ferroelectric property with Pr of 18 lC/cm2 and Ec of 17.5 kV/cm. Acknowledgements This work was supported by China’s manned space programmaterial research in space, the science and technology innovation
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project of Shanghai Institute of Ceramics, Chinese Academy of Science (Y39ZC6160G) and the opening program of the Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal Education (KFKT20130003). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
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