Materials Science and Engineering B 178 (2013) 1469– 1475
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Materials Science and Engineering B
jou rn al hom ep age: www.elsev ier .com/ locate /mseb
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nhanced dielectric, ferroelectric and optical properties of lead freeK0.17Na0.83)NbO3 ceramic with WO3 addition
yoti Rania, K.L. Yadava,∗, Satya Prakashb
Smart Materials Research Laboratory, Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, IndiaMetallurgical and Materials Engineering Department, Indian Institute of Technology Roorkee, Roorkee 247667, India
r t i c l e i n f o
rticle history:eceived 1 June 2013eceived in revised form 4 September 2013ccepted 23 September 2013vailable online 5 October 2013
a b s t r a c t
Polycrystalline lead-free ceramics (K0.17Na0.83)NbO3 + x wt.% WO3; (x = 0, 1, 3 and 5) have been synthesizedvia solid state reaction method. X-ray diffraction pattern at room temperature indicates the formation ofpure perovskite phase with monoclinic structure for all samples. Dielectric constant versus temperaturemeasurements shows an increase in dielectric constant with a shift in Curie temperature (TC) towardhigher 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 (K0.17Na0.83)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 gapof (K0.17Na0.83)NbO3 with increasing concentration of WO3.
Lead oxide based ceramics possess high dielectric, ferroelec-ric, electromechanical, piezoelectric and optical properties due tohich they are extensively used in various applications such as
erroelectric memories, electro mechanical systems (MEMS), sen-or, electro-optic devices and multilayer capacitors etc. [1–4]. Leadirconium titanate (PZT)-based compounds lose some of its leadontent during processing which is hazardous for environments well as for living being. Therefore, there is a great demand toevelop lead-free ceramics [5].
Various lead-free ceramics, such as bismuth sodium titanateBi0.5Na0.5TiO3) (BNT)-based ceramics, barium titanate (BaTiO3)-,ismuth ferrite (BiFeO3)-, potassium sodium niobate [(K,Na)NbO3]-ased ceramics [6], bismuth-layered structure ceramics [7,8] andungsten bronze-type ceramics [9] etc. have been extensively stud-ed for replacing the PZT-based ceramics. Among these lead-freeeramics, (KxNa1−x)NbO3 (KNN) is considered as a good can-idate due to its high Curie temperature, strong piezoelectricnd ferroelectric properties. Potassium sodium niobate (KNN) is
solid solution of ferroelectric potassium niobate (KNbO3) and
ntiferroelectric sodium niobate (NaNbO3). Both niobates haverthorhombic symmetry at room temperature and their Curie tem-erature (tetragonal-cubic phase transition) is 435 ◦C and 355 ◦C,
respectively [1,10]. Lead-free (KxNa1−x)NbO3 system shows threephase boundaries corresponding approximately to x = 0.17, 0.35and 0.5, respectively as reported by V.J. Tennery et al. [11]. In theabove suggested phase boundaries, most of the studies have beenfocused on the composition at about x = 0.5 [12–14] as it is com-monly accepted that piezoelectric properties appear to be optimumwhen 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 phaseboundary [15–21] and the literature related to the other two phaseboundaries i.e. x = 0.17 and x = 0.35 is scarce.
Recently, Mgbemere et al. [22] and Zang et al. [23] have reportedtheir work at phase boundary x = 0.35 of KNN i.e. (K0.35Na0.65)NbO3.The work on the other phase boundary of KNN i.e. x = 0.17 dopedwith Li has been reported by Zang et al. [24]. Very few studieson other phase boundary (i.e. x = 0.17) of KNN ceramic persuadedus to make some effort on this phase boundary with or withoutadditives.
Previous studies show that the electrical properties of KNNceramics get affected by additive constituents [25–27]. However,there are few reports available with WO3 incorporation in KNNceramic. Shelter et al. [28] reported the effect of WO3 additionon sintering and microstructure of (K0.5Na0.5)NbO3 ceramic. Zanget al. [29] doped W and Bi ion together in KNN with 48/52 ratioof K/Na. There is a dearth of literature that gives the informationabout electrical and optical properties of (K0.17Na0.83)NbO3 com-
position. Hence, in the present study, we report the synthesis of(K0.17Na0.83)NbO3 ceramic with the addition of 0, 1, 3 and 5 wt.%of WO3 and discussed its effect on microstructure, dielectric, ferro-electric, piezoelectric and optical properties.
1470 J. Rani et al. / Materials Science and Engineering B 178 (2013) 1469– 1475
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ig. 1. (a) X-ray diffraction pattern of (K0.17Na0.83)NbO3 + x wt.% WO3 (x = 0, 1, 3 andiew of X-ray diffraction pattern from angle 44 to 48◦ .
. Experimental
Polycrystalline (K0.17Na0.83)NbO3 + x wt.% WO3; (x = 0, 1, 3 and) ceramics were synthesized by conventional solid state reactionethod. All chemicals used for the synthesis were of analyti-
al grade. The starting chemicals were Na2CO3 (Qualigns, 99.9%),2CO3 (Himedia, 99%), Nb2O5 (Himedia, 99.9%), WO3 (Himedia,8%) and used without further purification.
For the synthesis of (K0.17Na0.83)NbO3 ceramic, above men-ioned chemical were taken in appropriate proportion and mixedhoroughly in an agate mortar with the addition of acetone mediaor better mixing. Approximately 3 wt.% excess of K2CO3 anda2CO3 were added initially to compensate the K and Na loss during
he calcination and sintering process. The mixed powder was cal-ined in an alumina crucible at 825 ◦C for 4 h in air atmosphere. Then
O3 was added according to the weight percentage in this calcinedowder and again mixed thoroughly for better homogeneity. Thebtained powders were pressed into cylindrical pellets of diameter–7 mm and thickness 0.8–1.2 mm by applying pressure of an orderf ∼ 6 × 107 Kg/m2 using hydraulic press. These compacted pelletsere finally sintered at 1160 ◦C for 4 h in air atmosphere. The heat-
ng rate of the furnace was 5 ◦C/min while the cooling occurred withts thermal inertia.
The sintered ceramics were characterized by X-ray diffractionXRD) analysis using Bruker AXS-D8 X-ray diffractometer (Cu-K�adiation, � = 1.540598 A) in the angle range 20–60◦ at a scanningate of 1◦ min−1. The density of the sample was measured by masser unit volume formula. Microstructure of the sintered pelletsere observed by field emission scanning electron microscope (FE-
EM) with the help of a FEI Quanta 200F microscope operating atn accelerating voltage of 20 kV coupled with an energy disper-ive X-ray analyser (EDAX). Grain size was calculated from FE-SEMicrographs using linear intercept method. For electrical mea-
urement electrode was formed using high purity silver paint asoating on the two parallel surfaces of sintered pellets and thenried at 250 ◦C for 30 min. The capacitance of the samples waseasured using HIOKI 3532-50 LCR meter at different frequencies
nd temperatures. The polarization-electric field (P-E) hysteresisoop measurement of the samples was accomplished at room tem-erature by using computer controlled modified Sawyer Tower
ircuit (Automatic P-E loop tracer system, Marine India Electr. Pvt.td.). For piezoelectric measurement the samples were poled bypplying direct current electric field of 3–4 kV/mm for 40 min in sil-con oil bath at room temperature. Piezoelectric charge coefficient
) enlarged view of X-ray diffraction pattern from angle 22 to 23.5◦ and (c) enlarged
(d33) measurement was performed by using piezometer systemof PIEZOTEST. The optical band gap energy of the ceramics wascalculated with the help of UV–Vis diffuse reflectance spectropho-tometer (Shimadzu UV-2450) in the wavelength range 200–800 nmusing BaSO4 as the reference.
3. Results and discussion
3.1. Structural and morphological analysis
Fig. 1(a) indicates the X-ray diffraction pattern for poly-crystalline (K0.17Na0.83)NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5)ceramics at room temperature. XRD pattern showed that(K0.17Na0.83)NbO3 + x wt.% WO3 ceramics have pure monoclinicperovskite phase and peaks were indexed as reported in litera-ture [24]. The phase formation of compounds was confirmed byXRD analysis which is in good agreement with the following refer-ences [24,28–30]. No change in phase of (K0.17Na0.83)NbO3 ceramicwas observed by the addition of WO3. No evidence of extra phaseand any other impurity was found for different compositions whichhave also been confirmed by EDAX analysis [Fig. 2(e)–(h)]. Hence,it is clear that WO3 has diffused in the parent lattice to forma homogeneous solid solution. This may be because of approxi-mately similar ionic radius of W6+ [r6 = 0.6 A] and Nb5+ [r6 = 0.64 A].Fig. 1(b) and (c) shows the enlarged view of XRD pattern in the anglerange 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 angleside for all compositions. Thus, an increase in the lattice param-eters has been observed with increasing WO3 concentration. Thecalculated lattice parameters and volume of the unit cell are givenin Table 1. Generally, doping of ions that have smaller ionic radiusto that of the parent ion causes the decrease in lattice parame-ter. However, we obtained contrast result i.e. increase in latticeparameter with W6+ doping at the Nb5+ site. According to the the-ories of charge neutrality and crystal chemistry, the substitutionof higher valent W6+ at the lower valent Nb5+ for the B-site ofthe ABO3 structure results in excess electron. To maintain overallcharge neutrality, niobium vacancies will be created for compen-sation purposes. The generation of niobium vacancies results inthe enlargement of the unit cells as suggested by H. Sun et al. [31]
for 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 ceramic. Another possible rea-son for this change in lattice parameter is that W and/or Nb mayundergo valence fluctuation for charge compensation which maylead to the large ionic radius and increases lattice parameter [32].
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Fig. 2. FE-SEM micrographs of (K0.17Na0.83)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.
1472 J. Rani et al. / Materials Science and Engineering B 178 (2013) 1469– 1475
Table 1Band gap, density, lattice parameters and volume of (K0.17Na0.83)NbO3 + x wt.% WO3 for x = 0, 1, 3 and 5.
x wt.% WO3 Band gap (eV) Density (g/cm3) Lattice parameters Volume (Å3)
The density of (K0.17Na0.83)NbO3 ceramic was found to increaseTable 1) with increasing WO3 content. The possible reason forhe increase in density is as follows: during the synthesis of WO3oped (K0.17Na0.83)NbO3 ceramics, K2O and/or Na2O are present inhe system which may react with WO3 and form K2WO4 and/ora2WO4 below the sintering temperature. The melting point of2WO4 and Na2WO4 are 912 ◦C [33] and 698 ◦C, respectively, whichre very low as compared to the sintering temperature (1160 ◦C) ofhe WO3 doped (K0.17Na0.83)NbO3 ceramics. Therefore they mightehave as a liquid phase during sintering process of the ceramics.his liquid phase may speed up the material transportation andead to the densification of the system. As the constituent elementf the liquid may either vaporize or enter in the lattice of crystaluring sintering, it will result in a transient liquid phase sintering.
n this case the secondary phase is avoided and the final productonsists of a single phase as suggested by Nielsen et al. [32] in thease of PZT ceramics.
Fig. 2(a)–(d) shows the FE-SEM micrographs ofK0.17Na0.83)NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics. Iteveals that grains have cuboidal shape. Homogenous microstruc-ure with minor increase in grain size was observed with
O3 addition. The average grain size was calculated from lin-ar intercept method and was found to vary from 3 to 4 �m.
ig. 2(e)–(f) shows the EDAX analysis of all compositions. Weid not find any secondary/impurity phases in FE-SEM and EDAXnalysis which is in confirmation with many reports availableith different additives like CuO [34], MnO2 [35] and Fe2O4
ig. 3. Temperature dependence of dielectric constant of (K0.17Na0.83)NbO3 + x wt.% WO3
[36] in KNN in which single phase was observed without anysegregation.
3.2. Dielectric study
Fig. 3(a)–(d) shows the variation of dielectric constant (εr) withtemperature from room temperature (RT) to 450 ◦C at 1, 10, 100 kHzand 1 MHz frequencies of (K0.17Na0.83)NbO3 + x wt.% WO3 ceram-ics for x = 0, 1, 3 and 5. Pure as well as WO3 added samples showtwo phase transitions temperature: one is monoclinic to tetrago-nal transition temperature (TM-T) and other is tetragonal to cubicphase transition temperature (TC) with different value of TM-T andTC temperatures. It is observed that Curie temperature (TC) is inde-pendent of frequency and sharp phase transition is observed at TCat all frequencies for all compositions.
Fig. 4(a) shows the variation of dielectric constant (εr) as a func-tion of temperature (RT to 450 ◦C) for (K0.17Na0.83)NbO3 + x wt.%WO3 (x = 0, 1, 3 and 5) ceramics at 1 kHz frequency. It can be seenthat there is no significant change in TM-T and it lies near ∼223 ◦Cfor all compositions. The variations of maximum dielectric constant(εr-max) and TC with x wt.% WO3 are shown in Fig. 4(b). The shiftin TC toward high temperature side can be clearly observed withincreasing WO3 content. This shift of TC toward higher temper-
ature may be due to the distortion in the structure i.e. increasein volume of unit cell as given in Table 1 [37,38]. Dielectric con-stant (εr) at room temperature and at TC was found to be higher forhigher value of x and it is maximum for x = 5 wt.% WO3. This may
at frequency 1 kHz, 10 kHz, 100 kHz and 1 MHz (a) x = 0, (b) x = 1, (c) x = 3 and (d)
J. Rani et al. / Materials Science and Engineering B 178 (2013) 1469– 1475 1473
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ig. 4. (a) Dielectric response of (K0.17Na0.83)NbO3 + x wt.% WO3 with temperature fooss (tan ı) versus temperature for all compositions at 1 kHz frequency.
e either due to increase in “rattling space” or increase in densitygiven in Table 1). In ABO3 structure, when B-site ion is replacedy an ion of smaller ionic radius, large rattling space is available forhe smaller ion. In such a structure when an ac signal is applied themaller ion may easily move, hence increases the polarizability ofhe system [39,40]. In (K0.17Na0.83)NbO3 + x wt.% WO3 ceramic, thenit cell volume is increasing with increase of WO3 concentrationhich causes the increase in rattling space for the B-site cations.
herefore, the value of dielectric constant increases with increasing
O3 content. Fig. 4(c) illustrates the dielectric loss (tan ı) versus
emperature for all composition at 1 kHz frequency. The value ofielectric loss is low at room temperature but increases at high tem-erature for all the compositions which may be due to thermally
Fig. 5. (a) PE hysteresis loops of (K0.17Na0.83)NbO3 + x wt.% WO3 (x = 0, 1, 3 an
mposition at frequency 1 kHz. (b) Variation of εmax and TC with x value. (c) Dielectric
activated conduction process [41]. Smeltere et al. [28] obtainedεr-max ∼ 8000 for 1 wt.% WO3 doped (K0.5Na0.5)NbO3 at frequency1 kHz while we obtained better dielectric constant εr-max ∼ 8535for (K0.17Na0.83)NbO3 + 5 wt.% WO3 at same frequency. Our samplealso shows better dielectric property than that reported by Zanget al. [29].
3.3. Polarization versus electric field hysteresis loop (PE loop)
Fig. 5(a) shows the polarization versus electric field plot for(K0.17Na0.83)NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramics atroom temperature. The variation of Pr and Ec with respect to thevalue of x is shown in Fig. 5(b). The plot indicates that remnant
d 5) at room temperature. (b) Variation of Pr and Ec with x wt.% WO3.
1 Engineering B 178 (2013) 1469– 1475
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474 J. Rani et al. / Materials Science and
olarization (Pr) increases and the coercive field (Ec) decreasesith increasing percentage of WO3. This indicates that addition
f WO3 improves the ferroelectric properties of the ceramic i.e.he compound transform into soft ferroelectric material with WO3ddition. The presence of WO3 along with (K0.17Na0.83)NbO3 affecthe structure i.e. increase in the dimension of the unit cells, thatacilitates the displacement of Nb5+ ion, leading to the enhancedemnant polarization [42]. The decrease in Ec with increasing wt.%f WO3 may be due to decrease in oxygen vacancies. Because oxy-en vacancies affect domain wall motion (domain wall pinning) bycreening of the charge polarization [43]. WO3 act as a donor forK0.17Na0.83)NbO3 system and reduces oxygen vacancies concen-ration to maintain the charge neutrality [44]. As the concentrationf oxygen vacancies decreases, there is a reduction in domainall pinning and domain wall move easily and hence the coer-
ive field decreases [37]. Zang et al. [24] reported Ec = 23 kV/cm andr = 21 �C/cm2 for 2 mol % of Li in (K0.17Na0.83)NbO3. In the presenttudy, (K0.17Na0.83)NbO3 + 5 wt.% WO3 shows better ferroelectricroperties with Pr = 24.1 �C/cm2 and lower Ec = 12.16 kV/cm for
wt.% WO3.
.4. Piezoelectric properties
Piezoelectric charge coefficient d33 (pC/N) forK0.17Na0.83)NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) ceramic atoom temperature is shown in Fig. 6. It is observed that d33alue increases with increasing concentration of WO3 and haveaximum value for x = 5 wt.% WO3. The increase in d33 value may
e explained on the basis of three possible reasons. First, the d33
alue is sensitive to the densification of the system. The increase inensity of the sample with increasing WO3 content may increase33 value, as it is reported that the higher density may lead higheriezoelectric properties [45,46]. Second, WO3 addition may create
Fig. 7. The absorption spectra for the (K0.17Na0.83)NbO3 + x wt.% WO
Fig. 6. Variation of d33 value of (K0.17Na0.83)NbO3 + x wt.% WO3 (x = 0, 1, 3 and 5) atroom temperature.
some A-site vacancies to maintain the charge neutrality in the sys-tem [47]. These vacancies may relax the strain which is originateddue to the reorientation of non-180◦ domains. Therefore, non-180◦
domains may be more sufficiently reorientated and improvethe piezoelectric properties [44]. Third, the d33 value is relatedto dielectric constant and polarization via a general equationd33 = 2εoεrQ11Ps [48], where εo, εr, Ps and Q11 are relative dielectricconstant, absolute dielectric constant, spontaneous polarization
and electrostrictive coefficient, respectively. We found increasingvalue of dielectric constant and polarization with increasing WO3concentration that may be responsible for increasing value of d33.
3 at room temperature (a) x = 0, (b) x = 1, (c) x = 3 and (d) x = 5.
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J. Rani et al. / Materials Science and
.5. Optical band gap
The optical band gap of (K0.17Na0.83)NbO3 + x wt.% WO3 (x = 0,, 3 and 5) ceramics have been obtained from absorption spectra,hich is recorded by UV–vis diffuse reflectance spectroscopy at
oom temperature. The absorption as a function of photon energyh�) is shown in Fig. 7 for all compositions. The absorption in theeramics takes place due to transition of electron from valance bando conduction band. Extrapolation of linear region of absorptiondge, which is observed in all samples, gives the value of opticaland gap [49]. The estimated value of optical band gap is 3.35 eVor pure (K0.17Na0.83)NbO3 and it increases up to 3.50 eV for 5 wt.%
O3. The value of optical band gap is given in Table 1 for all theompositions. The band gap in (K0.17Na0.83)NbO3 ceramic may cor-espond to the transition from the top of the valence band, whichs engaged by 2p electrons of oxygen, to the bottom of conductionand that is dominated by the empty Nb4d electron states [50]. Thelue shift in the band gap energy with increasing WO3 content maye due to structural modification that can easily be observed from-ray diffraction pattern (Fig. 1). In ABO3-type perovskite materi-ls, the optical properties are determined by the oxygen-octahedral51]. In WO3 added (K0.17Na0.83)NbO3 ceramic the unit cell volumencreases that may expand the oxygen octahedral which may beesponsible for blue shift in the band gap energy of ceramic.
. Conclusions
In summary, polycrystalline (K0.17Na0.83)NbO3 + x wt.% WO3x = 0, 1, 3 and 5) ceramics was synthesized by solid state reac-ion method. WO3 addition causes the increase in the volumef unit cell of (K0.17Na0.83)NbO3 ceramic and promotes densifi-ation. The value of dielectric constant at room temperature andt Curie temperature was found to be increasing with increasingO3 concentration. Dielectric constant was found to be maximum
or x = 5 wt.% WO3. There is a shift in TC toward higher temper-ture side with WO3 addition. The value of Pr improves and theoercive field decreases with increasing WO3 content. The opti-al band gap increases from 3.35 to 3.50 eV with increasing wt.%f WO3. Therefore, (K0.17Na0.83)NbO3 + 5 wt.% WO3 gives the opti-um value of dielectric constant and remnant polarization, hence
good candidate for memory device application.
cknowledgements
Authors would like to acknowledge the financial help fromouncil of Scientific and Industrial Research, New Delhi India underhe research grant no. 03(1272)/13/EMR-II dated 12.04.2013. Andlso we acknowledge Dr. K.K. Maurya, National Physical Laboratoryelhi, India, for providing the d33 meter facility.
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