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Enhanced dielectric, ferroelectric and optical properties of lead free (K0.17Na0.83)NbO3 ceramic with WO3 addition
Materials Science and Engineering B 178 (2013) 1469–1475
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Short communication
Enhanced dielectric, ferroelectric and optical properties of lead free (K0.17 Na0.83 )NbO3 ceramic with WO3 addition Jyoti Rani a , K.L. Yadav a,∗ , Satya Prakash b a b
Smart Materials Research Laboratory, Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, India Metallurgical and Materials Engineering Department, Indian Institute of Technology Roorkee, Roorkee 247667, India
a r t i c l e
i n f o
Article history: Received 1 June 2013 Received in revised form 4 September 2013 Accepted 23 September 2013 Available online 5 October 2013 Keywords: Lead-free ceramics X-ray diffraction Dielectric properties Ferroelectricity Optical properties
a b s t r a c t Polycrystalline lead-free ceramics (K0.17 Na0.83 )NbO3 + x wt.% WO3 ; (x = 0, 1, 3 and 5) have been synthesized via solid state reaction method. X-ray diffraction pattern at room temperature indicates the formation of pure perovskite phase with monoclinic structure for all samples. Dielectric constant versus temperature measurements shows an increase in dielectric constant with a shift in Curie temperature (TC ) toward higher temperature side. Remnant polarization (Pr ) is found to be enhanced and reached upto 24 C/cm2 for x = 5 wt.% WO3 from 12.5 C/cm2 for pure (K0.17 Na0.83 )NbO3 ceramic. The value of coercive field (Ec ) decreases with increasing wt.% of WO3 . From optical band gap study, we found blue shift in the band gap of (K0.17 Na0.83 )NbO3 with increasing concentration of WO3 . © 2013 Elsevier B.V. All rights reserved.
1. Introduction Lead oxide based ceramics possess high dielectric, ferroelectric, electromechanical, piezoelectric and optical properties due to which they are extensively used in various applications such as ferroelectric memories, electro mechanical systems (MEMS), sensor, electro-optic devices and multilayer capacitors etc. [1–4]. Lead zirconium titanate (PZT)-based compounds lose some of its lead content during processing which is hazardous for environment as well as for living being. Therefore, there is a great demand to develop lead-free ceramics [5]. Various lead-free ceramics, such as bismuth sodium titanate (Bi0.5 Na0.5 TiO3 ) (BNT)-based ceramics, barium titanate (BaTiO3 )-, bismuth ferrite (BiFeO3 )-, potassium sodium niobate [(K,Na)NbO3 ]based ceramics [6], bismuth-layered structure ceramics [7,8] and tungsten bronze-type ceramics [9] etc. have been extensively studied for replacing the PZT-based ceramics. Among these lead-free ceramics, (Kx Na1−x )NbO3 (KNN) is considered as a good candidate due to its high Curie temperature, strong piezoelectric and ferroelectric properties. Potassium sodium niobate (KNN) is a solid solution of ferroelectric potassium niobate (KNbO3 ) and antiferroelectric sodium niobate (NaNbO3 ). Both niobates have orthorhombic symmetry at room temperature and their Curie temperature (tetragonal-cubic phase transition) is 435 ◦ C and 355 ◦ C,
∗ Corresponding author. Tel.: +91 1332 285744; fax: +91 1332 273560. E-mail addresses: [email protected], [email protected] (J. Rani), [email protected] (K.L. Yadav). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.09.014
respectively [1,10]. Lead-free (Kx Na1−x )NbO3 system shows three phase boundaries corresponding approximately to x = 0.17, 0.35 and 0.5, respectively as reported by V.J. Tennery et al. [11]. In the above suggested phase boundaries, most of the studies have been focused on the composition at about x = 0.5 [12–14] as it is commonly accepted that piezoelectric properties appear to be optimum when the ratio of Na/K at A-site of the perovskite structure is 50/50. Generally, most of the reports have concentrated on x = 0.5 phase boundary [15–21] and the literature related to the other two phase boundaries i.e. x = 0.17 and x = 0.35 is scarce. Recently, Mgbemere et al. [22] and Zang et al. [23] have reported their work at phase boundary x = 0.35 of KNN i.e. (K0.35 Na0.65 )NbO3 . The work on the other phase boundary of KNN i.e. x = 0.17 doped with Li has been reported by Zang et al. [24]. Very few studies on other phase boundary (i.e. x = 0.17) of KNN ceramic persuaded us to make some effort on this phase boundary with or without additives. Previous studies show that the electrical properties of KNN ceramics get affected by additive constituents [25–27]. However, there are few reports available with WO3 incorporation in KNN ceramic. Shelter et al. [28] reported the effect of WO3 addition on sintering and microstructure of (K0.5 Na0.5 )NbO3 ceramic. Zang et al. [29] doped W and Bi ion together in KNN with 48/52 ratio of K/Na. There is a dearth of literature that gives the information about electrical and optical properties of (K0.17 Na0.83 )NbO3 composition. Hence, in the present study, we report the synthesis of (K0.17 Na0.83 )NbO3 ceramic with the addition of 0, 1, 3 and 5 wt.% of WO3 and discussed its effect on microstructure, dielectric, ferroelectric, piezoelectric and optical properties.
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Fig. 1. (a) X-ray diffraction pattern of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5), (b) enlarged view of X-ray diffraction pattern from angle 22 to 23.5◦ and (c) enlarged view of X-ray diffraction pattern from angle 44 to 48◦ .
2. Experimental Polycrystalline (K0.17 Na0.83 )NbO3 + x wt.% WO3 ; (x = 0, 1, 3 and 5) ceramics were synthesized by conventional solid state reaction method. All chemicals used for the synthesis were of analytical grade. The starting chemicals were Na2 CO3 (Qualigns, 99.9%), K2 CO3 (Himedia, 99%), Nb2 O5 (Himedia, 99.9%), WO3 (Himedia, 98%) and used without further purification. For the synthesis of (K0.17 Na0.83 )NbO3 ceramic, above mentioned chemical were taken in appropriate proportion and mixed thoroughly in an agate mortar with the addition of acetone media for better mixing. Approximately 3 wt.% excess of K2 CO3 and Na2 CO3 were added initially to compensate the K and Na loss during the calcination and sintering process. The mixed powder was calcined in an alumina crucible at 825 ◦ C for 4 h in air atmosphere. Then WO3 was added according to the weight percentage in this calcined powder and again mixed thoroughly for better homogeneity. The obtained powders were pressed into cylindrical pellets of diameter 6–7 mm and thickness 0.8–1.2 mm by applying pressure of an order of ∼ 6 × 107 Kg/m2 using hydraulic press. These compacted pellets were finally sintered at 1160 ◦ C for 4 h in air atmosphere. The heating rate of the furnace was 5 ◦ C/min while the cooling occurred with its thermal inertia. The sintered ceramics were characterized by X-ray diffraction (XRD) analysis using Bruker AXS-D8 X-ray diffractometer (Cu-K␣ ˚ in the angle range 20–60◦ at a scanning radiation, = 1.540598 A) ◦ −1 rate of 1 min . The density of the sample was measured by mass per unit volume formula. Microstructure of the sintered pellets were observed by field emission scanning electron microscope (FESEM) with the help of a FEI Quanta 200F microscope operating at an accelerating voltage of 20 kV coupled with an energy dispersive X-ray analyser (EDAX). Grain size was calculated from FE-SEM micrographs using linear intercept method. For electrical measurement electrode was formed using high purity silver paint as coating on the two parallel surfaces of sintered pellets and then dried at 250 ◦ C for 30 min. The capacitance of the samples was measured using HIOKI 3532-50 LCR meter at different frequencies and temperatures. The polarization-electric field (P-E) hysteresis loop measurement of the samples was accomplished at room temperature by using computer controlled modified Sawyer Tower circuit (Automatic P-E loop tracer system, Marine India Electr. Pvt. Ltd.). For piezoelectric measurement the samples were poled by applying direct current electric field of 3–4 kV/mm for 40 min in silicon oil bath at room temperature. Piezoelectric charge coefficient
(d33 ) measurement was performed by using piezometer system of PIEZOTEST. The optical band gap energy of the ceramics was calculated with the help of UV–Vis diffuse reflectance spectrophotometer (Shimadzu UV-2450) in the wavelength range 200–800 nm using BaSO4 as the reference. 3. Results and discussion 3.1. Structural and morphological analysis Fig. 1(a) indicates the X-ray diffraction pattern for polycrystalline (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics at room temperature. XRD pattern showed that (K0.17 Na0.83 )NbO3 + x wt.% WO3 ceramics have pure monoclinic perovskite phase and peaks were indexed as reported in literature [24]. The phase formation of compounds was confirmed by XRD analysis which is in good agreement with the following references [24,28–30]. No change in phase of (K0.17 Na0.83 )NbO3 ceramic was observed by the addition of WO3 . No evidence of extra phase and any other impurity was found for different compositions which have also been confirmed by EDAX analysis [Fig. 2(e)–(h)]. Hence, it is clear that WO3 has diffused in the parent lattice to form a homogeneous solid solution. This may be because of approxi˚ and Nb5+ [r6 = 0.64 A]. ˚ mately similar ionic radius of W6+ [r6 = 0.6 A] Fig. 1(b) and (c) shows the enlarged view of XRD pattern in the angle range 22–23.5◦ and 44–48◦ , which indicates the shifting of (1 0 0), (0 1 0) and (2 0 0), (0 0 2) peaks respectively, toward lower angle side for all compositions. Thus, an increase in the lattice parameters has been observed with increasing WO3 concentration. The calculated lattice parameters and volume of the unit cell are given in Table 1. Generally, doping of ions that have smaller ionic radius to that of the parent ion causes the decrease in lattice parameter. However, we obtained contrast result i.e. increase in lattice parameter with W6+ doping at the Nb5+ site. According to the theories of charge neutrality and crystal chemistry, the substitution of higher valent W6+ at the lower valent Nb5+ for the B-site of the ABO3 structure results in excess electron. To maintain overall charge neutrality, niobium vacancies will be created for compensation purposes. The generation of niobium vacancies results in the enlargement of the unit cells as suggested by H. Sun et al. [31] for 0.94(Bi0.5 Na0.5 )TiO3 –0.06BaTiO3 ceramic. Another possible reason for this change in lattice parameter is that W and/or Nb may undergo valence fluctuation for charge compensation which may lead to the large ionic radius and increases lattice parameter [32].
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Fig. 2. FE-SEM micrographs of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (a) x = 0, (b) x = 1, (c) x = 3 and (d) x = 5. EDAX spectra (e) x = 0, (f) x = 1, (g) x = 3 and (h) x = 5 ceramics.
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Table 1 Band gap, density, lattice parameters and volume of (K0.17 Na0.83 )NbO3 + x wt.% WO3 for x = 0, 1, 3 and 5. x wt.% WO3
0 1 3 5
Band gap (eV)
3.35 3.37 3.43 3.50
Density (g/cm3 )
4.10 4.13 4.18 4.24
Volume (Å3 )
Lattice parameters a (Å)
b (Å)
c (Å)
ˇ
3.9301 3.9304 3.9318 3.9471
3.8939 3.8941 3.8984 3.9133
3.9634 3.9650 3.9892 4.0334
90.5802 90.6206 90.7924 90.9966
The density of (K0.17 Na0.83 )NbO3 ceramic was found to increase (Table 1) with increasing WO3 content. The possible reason for the increase in density is as follows: during the synthesis of WO3 doped (K0.17 Na0.83 )NbO3 ceramics, K2 O and/or Na2 O are present in the system which may react with WO3 and form K2 WO4 and/or Na2 WO4 below the sintering temperature. The melting point of K2 WO4 and Na2 WO4 are 912 ◦ C [33] and 698 ◦ C, respectively, which are very low as compared to the sintering temperature (1160 ◦ C) of the WO3 doped (K0.17 Na0.83 )NbO3 ceramics. Therefore they might behave as a liquid phase during sintering process of the ceramics. This liquid phase may speed up the material transportation and lead to the densification of the system. As the constituent element of the liquid may either vaporize or enter in the lattice of crystal during sintering, it will result in a transient liquid phase sintering. In this case the secondary phase is avoided and the final product consists of a single phase as suggested by Nielsen et al. [32] in the case of PZT ceramics. Fig. 2(a)–(d) shows the FE-SEM micrographs of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics. It reveals that grains have cuboidal shape. Homogenous microstructure with minor increase in grain size was observed with WO3 addition. The average grain size was calculated from linear intercept method and was found to vary from 3 to 4 m. Fig. 2(e)–(f) shows the EDAX analysis of all compositions. We did not find any secondary/impurity phases in FE-SEM and EDAX analysis which is in confirmation with many reports available with different additives like CuO [34], MnO2 [35] and Fe2 O4
60.650 60.682 61.142 62.291
[36] in KNN in which single phase was observed without any segregation. 3.2. Dielectric study Fig. 3(a)–(d) shows the variation of dielectric constant (εr ) with temperature from room temperature (RT) to 450 ◦ C at 1, 10, 100 kHz and 1 MHz frequencies of (K0.17 Na0.83 )NbO3 + x wt.% WO3 ceramics for x = 0, 1, 3 and 5. Pure as well as WO3 added samples show two phase transitions temperature: one is monoclinic to tetragonal transition temperature (TM-T ) and other is tetragonal to cubic phase transition temperature (TC ) with different value of TM-T and TC temperatures. It is observed that Curie temperature (TC ) is independent of frequency and sharp phase transition is observed at TC at all frequencies for all compositions. Fig. 4(a) shows the variation of dielectric constant (εr ) as a function of temperature (RT to 450 ◦ C) for (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics at 1 kHz frequency. It can be seen that there is no significant change in TM-T and it lies near ∼223 ◦ C for all compositions. The variations of maximum dielectric constant (εr-max ) and TC with x wt.% WO3 are shown in Fig. 4(b). The shift in TC toward high temperature side can be clearly observed with increasing WO3 content. This shift of TC toward higher temperature may be due to the distortion in the structure i.e. increase in volume of unit cell as given in Table 1 [37,38]. Dielectric constant (εr ) at room temperature and at TC was found to be higher for higher value of x and it is maximum for x = 5 wt.% WO3 . This may
Fig. 3. Temperature dependence of dielectric constant of (K0.17 Na0.83 )NbO3 + x wt.% WO3 at frequency 1 kHz, 10 kHz, 100 kHz and 1 MHz (a) x = 0, (b) x = 1, (c) x = 3 and (d) x = 5.
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Fig. 4. (a) Dielectric response of (K0.17 Na0.83 )NbO3 + x wt.% WO3 with temperature for all composition at frequency 1 kHz. (b) Variation of εmax and TC with x value. (c) Dielectric loss (tan ı) versus temperature for all compositions at 1 kHz frequency.
be either due to increase in “rattling space” or increase in density (given in Table 1). In ABO3 structure, when B-site ion is replaced by an ion of smaller ionic radius, large rattling space is available for the smaller ion. In such a structure when an ac signal is applied the smaller ion may easily move, hence increases the polarizability of the system [39,40]. In (K0.17 Na0.83 )NbO3 + x wt.% WO3 ceramic, the unit cell volume is increasing with increase of WO3 concentration which causes the increase in rattling space for the B-site cations. Therefore, the value of dielectric constant increases with increasing WO3 content. Fig. 4(c) illustrates the dielectric loss (tan ı) versus temperature for all composition at 1 kHz frequency. The value of dielectric loss is low at room temperature but increases at high temperature for all the compositions which may be due to thermally
activated conduction process [41]. Smeltere et al. [28] obtained εr-max ∼ 8000 for 1 wt.% WO3 doped (K0.5 Na0.5 )NbO3 at frequency 1 kHz while we obtained better dielectric constant εr-max ∼ 8535 for (K0.17 Na0.83 )NbO3 + 5 wt.% WO3 at same frequency. Our sample also shows better dielectric property than that reported by Zang et al. [29]. 3.3. Polarization versus electric field hysteresis loop (PE loop) Fig. 5(a) shows the polarization versus electric field plot for (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics at room temperature. The variation of Pr and Ec with respect to the value of x is shown in Fig. 5(b). The plot indicates that remnant
Fig. 5. (a) PE hysteresis loops of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) at room temperature. (b) Variation of Pr and Ec with x wt.% WO3 .
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polarization (Pr ) increases and the coercive field (Ec ) decreases with increasing percentage of WO3 . This indicates that addition of WO3 improves the ferroelectric properties of the ceramic i.e. the compound transform into soft ferroelectric material with WO3 addition. The presence of WO3 along with (K0.17 Na0.83 )NbO3 affect the structure i.e. increase in the dimension of the unit cells, that facilitates the displacement of Nb5+ ion, leading to the enhanced remnant polarization [42]. The decrease in Ec with increasing wt.% of WO3 may be due to decrease in oxygen vacancies. Because oxygen vacancies affect domain wall motion (domain wall pinning) by screening of the charge polarization [43]. WO3 act as a donor for (K0.17 Na0.83 )NbO3 system and reduces oxygen vacancies concentration to maintain the charge neutrality [44]. As the concentration of oxygen vacancies decreases, there is a reduction in domain wall pinning and domain wall move easily and hence the coercive field decreases [37]. Zang et al. [24] reported Ec = 23 kV/cm and Pr = 21 C/cm2 for 2 mol % of Li in (K0.17 Na0.83 )NbO3 . In the present study, (K0.17 Na0.83 )NbO3 + 5 wt.% WO3 shows better ferroelectric properties with Pr = 24.1 C/cm2 and lower Ec = 12.16 kV/cm for 5 wt.% WO3 .
Fig. 6. Variation of d33 value of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) at room temperature.
3.4. Piezoelectric properties Piezoelectric charge coefficient d33 (pC/N) for (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramic at room temperature is shown in Fig. 6. It is observed that d33 value increases with increasing concentration of WO3 and have maximum value for x = 5 wt.% WO3 . The increase in d33 value may be explained on the basis of three possible reasons. First, the d33 value is sensitive to the densification of the system. The increase in density of the sample with increasing WO3 content may increase d33 value, as it is reported that the higher density may lead higher piezoelectric properties [45,46]. Second, WO3 addition may create
some A-site vacancies to maintain the charge neutrality in the system [47]. These vacancies may relax the strain which is originated due to the reorientation of non-180◦ domains. Therefore, non-180◦ domains may be more sufficiently reorientated and improve the piezoelectric properties [44]. Third, the d33 value is related to dielectric constant and polarization via a general equation d33 = 2εo εr Q11 Ps [48], where εo , εr , Ps and Q11 are relative dielectric constant, absolute dielectric constant, spontaneous polarization and electrostrictive coefficient, respectively. We found increasing value of dielectric constant and polarization with increasing WO3 concentration that may be responsible for increasing value of d33 .
Fig. 7. The absorption spectra for the (K0.17 Na0.83 )NbO3 + x wt.% WO3 at room temperature (a) x = 0, (b) x = 1, (c) x = 3 and (d) x = 5.
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3.5. Optical band gap The optical band gap of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics have been obtained from absorption spectra, which is recorded by UV–vis diffuse reflectance spectroscopy at room temperature. The absorption as a function of photon energy (h) is shown in Fig. 7 for all compositions. The absorption in the ceramics takes place due to transition of electron from valance band to conduction band. Extrapolation of linear region of absorption edge, which is observed in all samples, gives the value of optical band gap [49]. The estimated value of optical band gap is 3.35 eV for pure (K0.17 Na0.83 )NbO3 and it increases up to 3.50 eV for 5 wt.% WO3 . The value of optical band gap is given in Table 1 for all the compositions. The band gap in (K0.17 Na0.83 )NbO3 ceramic may correspond to the transition from the top of the valence band, which is engaged by 2p electrons of oxygen, to the bottom of conduction band that is dominated by the empty Nb4d electron states [50]. The blue shift in the band gap energy with increasing WO3 content may be due to structural modification that can easily be observed from X-ray diffraction pattern (Fig. 1). In ABO3 -type perovskite materials, the optical properties are determined by the oxygen-octahedral [51]. In WO3 added (K0.17 Na0.83 )NbO3 ceramic the unit cell volume increases that may expand the oxygen octahedral which may be responsible for blue shift in the band gap energy of ceramic. 4. Conclusions In summary, polycrystalline (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics was synthesized by solid state reaction method. WO3 addition causes the increase in the volume of unit cell of (K0.17 Na0.83 )NbO3 ceramic and promotes densification. The value of dielectric constant at room temperature and at Curie temperature was found to be increasing with increasing WO3 concentration. Dielectric constant was found to be maximum for x = 5 wt.% WO3 . There is a shift in TC toward higher temperature side with WO3 addition. The value of Pr improves and the coercive field decreases with increasing WO3 content. The optical band gap increases from 3.35 to 3.50 eV with increasing wt.% of WO3 . Therefore, (K0.17 Na0.83 )NbO3 + 5 wt.% WO3 gives the optimum value of dielectric constant and remnant polarization, hence a good candidate for memory device application. Acknowledgements Authors would like to acknowledge the financial help from Council of Scientific and Industrial Research, New Delhi India under the research grant no. 03(1272)/13/EMR-II dated 12.04.2013. And also we acknowledge Dr. K.K. Maurya, National Physical Laboratory Delhi, India, for providing the d33 meter facility. References [1] [2] [3] [4]
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Enhanced dielectric, ferroelectric and optical properties of lead free (K0.17 Na0.83 )NbO3 ceramic with WO3 addition Jyoti Rani a , K.L. Yadav a,∗ , Satya Prakash b a b
Smart Materials Research Laboratory, Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, India Metallurgical and Materials Engineering Department, Indian Institute of Technology Roorkee, Roorkee 247667, India
a r t i c l e
i n f o
Article history: Received 1 June 2013 Received in revised form 4 September 2013 Accepted 23 September 2013 Available online 5 October 2013 Keywords: Lead-free ceramics X-ray diffraction Dielectric properties Ferroelectricity Optical properties
a b s t r a c t Polycrystalline lead-free ceramics (K0.17 Na0.83 )NbO3 + x wt.% WO3 ; (x = 0, 1, 3 and 5) have been synthesized via solid state reaction method. X-ray diffraction pattern at room temperature indicates the formation of pure perovskite phase with monoclinic structure for all samples. Dielectric constant versus temperature measurements shows an increase in dielectric constant with a shift in Curie temperature (TC ) toward higher temperature side. Remnant polarization (Pr ) is found to be enhanced and reached upto 24 C/cm2 for x = 5 wt.% WO3 from 12.5 C/cm2 for pure (K0.17 Na0.83 )NbO3 ceramic. The value of coercive field (Ec ) decreases with increasing wt.% of WO3 . From optical band gap study, we found blue shift in the band gap of (K0.17 Na0.83 )NbO3 with increasing concentration of WO3 . © 2013 Elsevier B.V. All rights reserved.
1. Introduction Lead oxide based ceramics possess high dielectric, ferroelectric, electromechanical, piezoelectric and optical properties due to which they are extensively used in various applications such as ferroelectric memories, electro mechanical systems (MEMS), sensor, electro-optic devices and multilayer capacitors etc. [1–4]. Lead zirconium titanate (PZT)-based compounds lose some of its lead content during processing which is hazardous for environment as well as for living being. Therefore, there is a great demand to develop lead-free ceramics [5]. Various lead-free ceramics, such as bismuth sodium titanate (Bi0.5 Na0.5 TiO3 ) (BNT)-based ceramics, barium titanate (BaTiO3 )-, bismuth ferrite (BiFeO3 )-, potassium sodium niobate [(K,Na)NbO3 ]based ceramics [6], bismuth-layered structure ceramics [7,8] and tungsten bronze-type ceramics [9] etc. have been extensively studied for replacing the PZT-based ceramics. Among these lead-free ceramics, (Kx Na1−x )NbO3 (KNN) is considered as a good candidate due to its high Curie temperature, strong piezoelectric and ferroelectric properties. Potassium sodium niobate (KNN) is a solid solution of ferroelectric potassium niobate (KNbO3 ) and antiferroelectric sodium niobate (NaNbO3 ). Both niobates have orthorhombic symmetry at room temperature and their Curie temperature (tetragonal-cubic phase transition) is 435 ◦ C and 355 ◦ C,
∗ Corresponding author. Tel.: +91 1332 285744; fax: +91 1332 273560. E-mail addresses: [email protected], [email protected] (J. Rani), [email protected] (K.L. Yadav). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.09.014
respectively [1,10]. Lead-free (Kx Na1−x )NbO3 system shows three phase boundaries corresponding approximately to x = 0.17, 0.35 and 0.5, respectively as reported by V.J. Tennery et al. [11]. In the above suggested phase boundaries, most of the studies have been focused on the composition at about x = 0.5 [12–14] as it is commonly accepted that piezoelectric properties appear to be optimum when the ratio of Na/K at A-site of the perovskite structure is 50/50. Generally, most of the reports have concentrated on x = 0.5 phase boundary [15–21] and the literature related to the other two phase boundaries i.e. x = 0.17 and x = 0.35 is scarce. Recently, Mgbemere et al. [22] and Zang et al. [23] have reported their work at phase boundary x = 0.35 of KNN i.e. (K0.35 Na0.65 )NbO3 . The work on the other phase boundary of KNN i.e. x = 0.17 doped with Li has been reported by Zang et al. [24]. Very few studies on other phase boundary (i.e. x = 0.17) of KNN ceramic persuaded us to make some effort on this phase boundary with or without additives. Previous studies show that the electrical properties of KNN ceramics get affected by additive constituents [25–27]. However, there are few reports available with WO3 incorporation in KNN ceramic. Shelter et al. [28] reported the effect of WO3 addition on sintering and microstructure of (K0.5 Na0.5 )NbO3 ceramic. Zang et al. [29] doped W and Bi ion together in KNN with 48/52 ratio of K/Na. There is a dearth of literature that gives the information about electrical and optical properties of (K0.17 Na0.83 )NbO3 composition. Hence, in the present study, we report the synthesis of (K0.17 Na0.83 )NbO3 ceramic with the addition of 0, 1, 3 and 5 wt.% of WO3 and discussed its effect on microstructure, dielectric, ferroelectric, piezoelectric and optical properties.
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Fig. 1. (a) X-ray diffraction pattern of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5), (b) enlarged view of X-ray diffraction pattern from angle 22 to 23.5◦ and (c) enlarged view of X-ray diffraction pattern from angle 44 to 48◦ .
2. Experimental Polycrystalline (K0.17 Na0.83 )NbO3 + x wt.% WO3 ; (x = 0, 1, 3 and 5) ceramics were synthesized by conventional solid state reaction method. All chemicals used for the synthesis were of analytical grade. The starting chemicals were Na2 CO3 (Qualigns, 99.9%), K2 CO3 (Himedia, 99%), Nb2 O5 (Himedia, 99.9%), WO3 (Himedia, 98%) and used without further purification. For the synthesis of (K0.17 Na0.83 )NbO3 ceramic, above mentioned chemical were taken in appropriate proportion and mixed thoroughly in an agate mortar with the addition of acetone media for better mixing. Approximately 3 wt.% excess of K2 CO3 and Na2 CO3 were added initially to compensate the K and Na loss during the calcination and sintering process. The mixed powder was calcined in an alumina crucible at 825 ◦ C for 4 h in air atmosphere. Then WO3 was added according to the weight percentage in this calcined powder and again mixed thoroughly for better homogeneity. The obtained powders were pressed into cylindrical pellets of diameter 6–7 mm and thickness 0.8–1.2 mm by applying pressure of an order of ∼ 6 × 107 Kg/m2 using hydraulic press. These compacted pellets were finally sintered at 1160 ◦ C for 4 h in air atmosphere. The heating rate of the furnace was 5 ◦ C/min while the cooling occurred with its thermal inertia. The sintered ceramics were characterized by X-ray diffraction (XRD) analysis using Bruker AXS-D8 X-ray diffractometer (Cu-K␣ ˚ in the angle range 20–60◦ at a scanning radiation, = 1.540598 A) ◦ −1 rate of 1 min . The density of the sample was measured by mass per unit volume formula. Microstructure of the sintered pellets were observed by field emission scanning electron microscope (FESEM) with the help of a FEI Quanta 200F microscope operating at an accelerating voltage of 20 kV coupled with an energy dispersive X-ray analyser (EDAX). Grain size was calculated from FE-SEM micrographs using linear intercept method. For electrical measurement electrode was formed using high purity silver paint as coating on the two parallel surfaces of sintered pellets and then dried at 250 ◦ C for 30 min. The capacitance of the samples was measured using HIOKI 3532-50 LCR meter at different frequencies and temperatures. The polarization-electric field (P-E) hysteresis loop measurement of the samples was accomplished at room temperature by using computer controlled modified Sawyer Tower circuit (Automatic P-E loop tracer system, Marine India Electr. Pvt. Ltd.). For piezoelectric measurement the samples were poled by applying direct current electric field of 3–4 kV/mm for 40 min in silicon oil bath at room temperature. Piezoelectric charge coefficient
(d33 ) measurement was performed by using piezometer system of PIEZOTEST. The optical band gap energy of the ceramics was calculated with the help of UV–Vis diffuse reflectance spectrophotometer (Shimadzu UV-2450) in the wavelength range 200–800 nm using BaSO4 as the reference. 3. Results and discussion 3.1. Structural and morphological analysis Fig. 1(a) indicates the X-ray diffraction pattern for polycrystalline (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics at room temperature. XRD pattern showed that (K0.17 Na0.83 )NbO3 + x wt.% WO3 ceramics have pure monoclinic perovskite phase and peaks were indexed as reported in literature [24]. The phase formation of compounds was confirmed by XRD analysis which is in good agreement with the following references [24,28–30]. No change in phase of (K0.17 Na0.83 )NbO3 ceramic was observed by the addition of WO3 . No evidence of extra phase and any other impurity was found for different compositions which have also been confirmed by EDAX analysis [Fig. 2(e)–(h)]. Hence, it is clear that WO3 has diffused in the parent lattice to form a homogeneous solid solution. This may be because of approxi˚ and Nb5+ [r6 = 0.64 A]. ˚ mately similar ionic radius of W6+ [r6 = 0.6 A] Fig. 1(b) and (c) shows the enlarged view of XRD pattern in the angle range 22–23.5◦ and 44–48◦ , which indicates the shifting of (1 0 0), (0 1 0) and (2 0 0), (0 0 2) peaks respectively, toward lower angle side for all compositions. Thus, an increase in the lattice parameters has been observed with increasing WO3 concentration. The calculated lattice parameters and volume of the unit cell are given in Table 1. Generally, doping of ions that have smaller ionic radius to that of the parent ion causes the decrease in lattice parameter. However, we obtained contrast result i.e. increase in lattice parameter with W6+ doping at the Nb5+ site. According to the theories of charge neutrality and crystal chemistry, the substitution of higher valent W6+ at the lower valent Nb5+ for the B-site of the ABO3 structure results in excess electron. To maintain overall charge neutrality, niobium vacancies will be created for compensation purposes. The generation of niobium vacancies results in the enlargement of the unit cells as suggested by H. Sun et al. [31] for 0.94(Bi0.5 Na0.5 )TiO3 –0.06BaTiO3 ceramic. Another possible reason for this change in lattice parameter is that W and/or Nb may undergo valence fluctuation for charge compensation which may lead to the large ionic radius and increases lattice parameter [32].
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Fig. 2. FE-SEM micrographs of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (a) x = 0, (b) x = 1, (c) x = 3 and (d) x = 5. EDAX spectra (e) x = 0, (f) x = 1, (g) x = 3 and (h) x = 5 ceramics.
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Table 1 Band gap, density, lattice parameters and volume of (K0.17 Na0.83 )NbO3 + x wt.% WO3 for x = 0, 1, 3 and 5. x wt.% WO3
0 1 3 5
Band gap (eV)
3.35 3.37 3.43 3.50
Density (g/cm3 )
4.10 4.13 4.18 4.24
Volume (Å3 )
Lattice parameters a (Å)
b (Å)
c (Å)
ˇ
3.9301 3.9304 3.9318 3.9471
3.8939 3.8941 3.8984 3.9133
3.9634 3.9650 3.9892 4.0334
90.5802 90.6206 90.7924 90.9966
The density of (K0.17 Na0.83 )NbO3 ceramic was found to increase (Table 1) with increasing WO3 content. The possible reason for the increase in density is as follows: during the synthesis of WO3 doped (K0.17 Na0.83 )NbO3 ceramics, K2 O and/or Na2 O are present in the system which may react with WO3 and form K2 WO4 and/or Na2 WO4 below the sintering temperature. The melting point of K2 WO4 and Na2 WO4 are 912 ◦ C [33] and 698 ◦ C, respectively, which are very low as compared to the sintering temperature (1160 ◦ C) of the WO3 doped (K0.17 Na0.83 )NbO3 ceramics. Therefore they might behave as a liquid phase during sintering process of the ceramics. This liquid phase may speed up the material transportation and lead to the densification of the system. As the constituent element of the liquid may either vaporize or enter in the lattice of crystal during sintering, it will result in a transient liquid phase sintering. In this case the secondary phase is avoided and the final product consists of a single phase as suggested by Nielsen et al. [32] in the case of PZT ceramics. Fig. 2(a)–(d) shows the FE-SEM micrographs of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics. It reveals that grains have cuboidal shape. Homogenous microstructure with minor increase in grain size was observed with WO3 addition. The average grain size was calculated from linear intercept method and was found to vary from 3 to 4 m. Fig. 2(e)–(f) shows the EDAX analysis of all compositions. We did not find any secondary/impurity phases in FE-SEM and EDAX analysis which is in confirmation with many reports available with different additives like CuO [34], MnO2 [35] and Fe2 O4
60.650 60.682 61.142 62.291
[36] in KNN in which single phase was observed without any segregation. 3.2. Dielectric study Fig. 3(a)–(d) shows the variation of dielectric constant (εr ) with temperature from room temperature (RT) to 450 ◦ C at 1, 10, 100 kHz and 1 MHz frequencies of (K0.17 Na0.83 )NbO3 + x wt.% WO3 ceramics for x = 0, 1, 3 and 5. Pure as well as WO3 added samples show two phase transitions temperature: one is monoclinic to tetragonal transition temperature (TM-T ) and other is tetragonal to cubic phase transition temperature (TC ) with different value of TM-T and TC temperatures. It is observed that Curie temperature (TC ) is independent of frequency and sharp phase transition is observed at TC at all frequencies for all compositions. Fig. 4(a) shows the variation of dielectric constant (εr ) as a function of temperature (RT to 450 ◦ C) for (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics at 1 kHz frequency. It can be seen that there is no significant change in TM-T and it lies near ∼223 ◦ C for all compositions. The variations of maximum dielectric constant (εr-max ) and TC with x wt.% WO3 are shown in Fig. 4(b). The shift in TC toward high temperature side can be clearly observed with increasing WO3 content. This shift of TC toward higher temperature may be due to the distortion in the structure i.e. increase in volume of unit cell as given in Table 1 [37,38]. Dielectric constant (εr ) at room temperature and at TC was found to be higher for higher value of x and it is maximum for x = 5 wt.% WO3 . This may
Fig. 3. Temperature dependence of dielectric constant of (K0.17 Na0.83 )NbO3 + x wt.% WO3 at frequency 1 kHz, 10 kHz, 100 kHz and 1 MHz (a) x = 0, (b) x = 1, (c) x = 3 and (d) x = 5.
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Fig. 4. (a) Dielectric response of (K0.17 Na0.83 )NbO3 + x wt.% WO3 with temperature for all composition at frequency 1 kHz. (b) Variation of εmax and TC with x value. (c) Dielectric loss (tan ı) versus temperature for all compositions at 1 kHz frequency.
be either due to increase in “rattling space” or increase in density (given in Table 1). In ABO3 structure, when B-site ion is replaced by an ion of smaller ionic radius, large rattling space is available for the smaller ion. In such a structure when an ac signal is applied the smaller ion may easily move, hence increases the polarizability of the system [39,40]. In (K0.17 Na0.83 )NbO3 + x wt.% WO3 ceramic, the unit cell volume is increasing with increase of WO3 concentration which causes the increase in rattling space for the B-site cations. Therefore, the value of dielectric constant increases with increasing WO3 content. Fig. 4(c) illustrates the dielectric loss (tan ı) versus temperature for all composition at 1 kHz frequency. The value of dielectric loss is low at room temperature but increases at high temperature for all the compositions which may be due to thermally
activated conduction process [41]. Smeltere et al. [28] obtained εr-max ∼ 8000 for 1 wt.% WO3 doped (K0.5 Na0.5 )NbO3 at frequency 1 kHz while we obtained better dielectric constant εr-max ∼ 8535 for (K0.17 Na0.83 )NbO3 + 5 wt.% WO3 at same frequency. Our sample also shows better dielectric property than that reported by Zang et al. [29]. 3.3. Polarization versus electric field hysteresis loop (PE loop) Fig. 5(a) shows the polarization versus electric field plot for (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics at room temperature. The variation of Pr and Ec with respect to the value of x is shown in Fig. 5(b). The plot indicates that remnant
Fig. 5. (a) PE hysteresis loops of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) at room temperature. (b) Variation of Pr and Ec with x wt.% WO3 .
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polarization (Pr ) increases and the coercive field (Ec ) decreases with increasing percentage of WO3 . This indicates that addition of WO3 improves the ferroelectric properties of the ceramic i.e. the compound transform into soft ferroelectric material with WO3 addition. The presence of WO3 along with (K0.17 Na0.83 )NbO3 affect the structure i.e. increase in the dimension of the unit cells, that facilitates the displacement of Nb5+ ion, leading to the enhanced remnant polarization [42]. The decrease in Ec with increasing wt.% of WO3 may be due to decrease in oxygen vacancies. Because oxygen vacancies affect domain wall motion (domain wall pinning) by screening of the charge polarization [43]. WO3 act as a donor for (K0.17 Na0.83 )NbO3 system and reduces oxygen vacancies concentration to maintain the charge neutrality [44]. As the concentration of oxygen vacancies decreases, there is a reduction in domain wall pinning and domain wall move easily and hence the coercive field decreases [37]. Zang et al. [24] reported Ec = 23 kV/cm and Pr = 21 C/cm2 for 2 mol % of Li in (K0.17 Na0.83 )NbO3 . In the present study, (K0.17 Na0.83 )NbO3 + 5 wt.% WO3 shows better ferroelectric properties with Pr = 24.1 C/cm2 and lower Ec = 12.16 kV/cm for 5 wt.% WO3 .
Fig. 6. Variation of d33 value of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) at room temperature.
3.4. Piezoelectric properties Piezoelectric charge coefficient d33 (pC/N) for (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramic at room temperature is shown in Fig. 6. It is observed that d33 value increases with increasing concentration of WO3 and have maximum value for x = 5 wt.% WO3 . The increase in d33 value may be explained on the basis of three possible reasons. First, the d33 value is sensitive to the densification of the system. The increase in density of the sample with increasing WO3 content may increase d33 value, as it is reported that the higher density may lead higher piezoelectric properties [45,46]. Second, WO3 addition may create
some A-site vacancies to maintain the charge neutrality in the system [47]. These vacancies may relax the strain which is originated due to the reorientation of non-180◦ domains. Therefore, non-180◦ domains may be more sufficiently reorientated and improve the piezoelectric properties [44]. Third, the d33 value is related to dielectric constant and polarization via a general equation d33 = 2εo εr Q11 Ps [48], where εo , εr , Ps and Q11 are relative dielectric constant, absolute dielectric constant, spontaneous polarization and electrostrictive coefficient, respectively. We found increasing value of dielectric constant and polarization with increasing WO3 concentration that may be responsible for increasing value of d33 .
Fig. 7. The absorption spectra for the (K0.17 Na0.83 )NbO3 + x wt.% WO3 at room temperature (a) x = 0, (b) x = 1, (c) x = 3 and (d) x = 5.
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3.5. Optical band gap The optical band gap of (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics have been obtained from absorption spectra, which is recorded by UV–vis diffuse reflectance spectroscopy at room temperature. The absorption as a function of photon energy (h) is shown in Fig. 7 for all compositions. The absorption in the ceramics takes place due to transition of electron from valance band to conduction band. Extrapolation of linear region of absorption edge, which is observed in all samples, gives the value of optical band gap [49]. The estimated value of optical band gap is 3.35 eV for pure (K0.17 Na0.83 )NbO3 and it increases up to 3.50 eV for 5 wt.% WO3 . The value of optical band gap is given in Table 1 for all the compositions. The band gap in (K0.17 Na0.83 )NbO3 ceramic may correspond to the transition from the top of the valence band, which is engaged by 2p electrons of oxygen, to the bottom of conduction band that is dominated by the empty Nb4d electron states [50]. The blue shift in the band gap energy with increasing WO3 content may be due to structural modification that can easily be observed from X-ray diffraction pattern (Fig. 1). In ABO3 -type perovskite materials, the optical properties are determined by the oxygen-octahedral [51]. In WO3 added (K0.17 Na0.83 )NbO3 ceramic the unit cell volume increases that may expand the oxygen octahedral which may be responsible for blue shift in the band gap energy of ceramic. 4. Conclusions In summary, polycrystalline (K0.17 Na0.83 )NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics was synthesized by solid state reaction method. WO3 addition causes the increase in the volume of unit cell of (K0.17 Na0.83 )NbO3 ceramic and promotes densification. The value of dielectric constant at room temperature and at Curie temperature was found to be increasing with increasing WO3 concentration. Dielectric constant was found to be maximum for x = 5 wt.% WO3 . There is a shift in TC toward higher temperature side with WO3 addition. The value of Pr improves and the coercive field decreases with increasing WO3 content. The optical band gap increases from 3.35 to 3.50 eV with increasing wt.% of WO3 . Therefore, (K0.17 Na0.83 )NbO3 + 5 wt.% WO3 gives the optimum value of dielectric constant and remnant polarization, hence a good candidate for memory device application. Acknowledgements Authors would like to acknowledge the financial help from Council of Scientific and Industrial Research, New Delhi India under the research grant no. 03(1272)/13/EMR-II dated 12.04.2013. And also we acknowledge Dr. K.K. Maurya, National Physical Laboratory Delhi, India, for providing the d33 meter facility. References [1] [2] [3] [4]
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