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Synthesis and characterizations of NaNbO3 modified BNT–BT–BKT ceramics for energy storage applications
Physica B 497 (2016) 59–66
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Synthesis and characterizations of NaNbO3 modified BNT–BT–BKT ceramics for energy storage applications M. Chandrasekhar, Sonia, P. Kumar n Department of Physics, National Institute of Technology, Rourkela, Odisha 769008, India
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2016 Received in revised form 14 June 2016 Accepted 15 June 2016 Available online 16 June 2016
Lead-free (1 x)[0.884BNT–0.036BT–0.08BKT]–xNaNbO3 samples (with x ¼0, 0.04, 0.08, 0.12 and 0.16) were synthesized in single perovskite phase by solid state reaction route. Decrease of grain size with the increase of NaNbO3/NN content was related to the grain boundary pinning effect. Dielectric study confirmed the relaxor nature of x ¼ 0.08, 0.12 and 0.16 samples. Energy storage density and efficiency were calculated from polarization vs. electric field hysteresis loops. Value of energy storage efficiency increased with the increase of NaNbO3 content. A relatively large energy storage density of 0.721 J/cm3 was obtained in the x ¼0.08 samples. Electric field induced polarization and strain loops, respectively suggested the increase of AFE ordering with the increase of NN content. & 2016 Elsevier B.V. All rights reserved.
Keywords: Grain boundary pinning effect Energy storage density Relaxor Non-polar phase
1. Introduction High energy storage density functional materials are used in compact devices like electrical vehicles, mobile electronics and different types of pulsed power technologies [1]. Recently, Bi0.5Na0.5TiO3 (BNT)-based ferroelectric (FE) functional materials have drawn attention of research community for their promising use in energy storage applications. In comparison to conventional dielectrics, antiferroelectric (AFE) materials have small remnant polarization (Pr) and large charge–discharge speed, and are preferred in energy storage applications [2,3]. Better energy storage density and efficiency of BNT system appear near depolarization temperature (Td) [4]. On the other hand, though FE materials with square shaped Polarization–Electric field (P–E) hysteresis loops have high energy storage density than those of the double like P–E hysteresis loops of the AFE materials, yet they cannot withstand the large number of charge–discharge cycles [2]. Therefore, double smaller area P–E hysteresis loops are desirable for achieving large number of charge–discharge cycles [5,6] in the energy storage device applications. Energy storage density (W1) of a FE material is given by the following equation [7] Pmax
W1 =
∫
EdP…. .
0
n
Corresponding author. E-mail addresses: [email protected] (M. Chandrasekhar), [email protected] (P. Kumar). http://dx.doi.org/10.1016/j.physb.2016.06.015 0921-4526/& 2016 Elsevier B.V. All rights reserved.
(A.1)
where, E is the applied electric field and P is the polarization. According to above equation, materials having large saturated polarization (Ps), smaller remnant polarization (Pr) and moderate breakdown field, are suitable for having high energy storage density. On the other hand, energy storage efficiency (η) of a functional material is calculated by using the following equation [7]:
η = W1/(W1 + W2)…. .
(A.2)
where, W1 is the stored energy density; W2 is the energy loss density and calculated by the numerical integration of closed area of the P–E curves of the hysteresis loops. Sodium niobate NaNbO3/NN, strontium titanate SrTiO3/ST are some of the prominent lead-free AFE systems [8]. Chia-Ching Wu and Cheng-Fu Yang [9] studied the effects of NN modification on the dielectric properties and relaxor nature of lead-free BNT ceramics. Wang et al. [10] studied the temperature dependent electrical properties of BNT–BKT–SrTiO3 piezoceramics. In the literature, NaNbO3 and BNT–BT–BKT are reported as the promising lead free AFE and FE systems respectively [11,12]. Para electric phase transition temperature of NN system lies well below room temperature (RT). Therefore, modification of effective BNT–BT–BKT piezoelectric system with NN AFE system will not only lower down the Td temperature but also increase the energy storage density and energy storage efficiency. In the present work, dielectric, energy-storage density and efficiency properties of NaNbO3 modified BNT–BT–BKT system are studied and discussed in detail.
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Fig. 1. XRD patterns of the samples.
Table 1 Phase fractions and lattice parameters of the (1 x)[BNT–BT–BKT] xNN samples. Sample
x¼0
x¼ 0.04
x¼ 0.08
x¼ 0.12
x ¼0.16
R.P phase fraction Lattice parameters (aR, α) T.P phase fraction Lattice parameters (aT, cT)
31
33.4
30.7
31.2
34.2
(3.8925 Å, 89.910°)
(3.8926 Å, 89.860°)
(3.8908 Å, 89.870°)
(3.8917 Å, 89.860°)
(3.8875 Å, 89.740°)
69
66.6
69.3
68.8
65.8
(5.7063, 13.4893) Å
(5.7032, 13.4820) Å
(5.6992, 13.5033) Å
(5.7024, 13.4974) Å
(5.7004, 13.5016) Å
sintered stages were confirmed by X-ray diffraction (XRD) (Rigaku Ultima IV, Tokyo, Japan) technique. Micrographs of the sintered pellets were taken using Field emission scanning electron microscope (FESEM NOVA nano SEM). Silver paste was applied on the polished surfaces of the sintered pellets and then fired at 400 °C for 30 min for good adhesion. Dielectric constant (εr) and dielectric loss (tan) were measured as a function of temperature using a computer interfaced HIOKI 3532-50 LCR-HITESTER. RT Polarizations vs. electric field (P–E) hysteresis loops were measured using Radiant precision premier II unit (P-HVI210k, Radiant Technologies INC. U.S.A). RT bipolar strain vs. electric field (S–E) hysteresis loops were measured using Radiant precision premier II attached with MTI-2100 Fotonic sensor (MTI Instruments INC, U.S.A).
3. Results and discussion 2. Material and methods 3.1. XRD and microstructure study Solid-state reaction route was used to synthesize the selected materials. Firstly, the 0.884Bi0.5Na0.5TiO3–0.036BaTiO3– 0.08BiKTiO3/BNT–BT–BKT (abbreviated as x¼ 0) ceramics were prepared by taking stoichiometric weights of Bi2O3, Na2CO3, TiO2, BaCO3 and K2CO3 (all are 99.9% purity) as the starting precursors. The mixture of these starting precursors was ball milled for 20 h (h) by using zirconia balls and acetone as the grinding media. The dried ball milled powder was calcined at 900 °C and sintered at 1050 °C for 4 h. NN was synthesized by taking stoichiometric weights of Na2CO3 and Nb2O5 precursors, ball milled under the same conditions mentioned above and calcined at 800 °C for 4 h. Finally, the (1 x)[BNT–BT–BKT]–xNN ceramics with x¼0.04, 0.08, 0.12 and 0.16 were synthesized by taking stoichiometric weights of the calcined BNT–BT–BKT and NN powders. 3 wt% polyvinyl alcohol binder solution was added to the calcined powders. Green pellets of 10 mm in diameter and 1.5 mm in thickness were compacted using a uniaxial hydraulic press with 10 MPa pressure. Optimized sintering temperature for x¼0 i.e BNT–BT–BKT samples was 1050 °C/4 h and for NN modified BNT–BT–BKT samples it was 1100 °C/4 h. Perovskite phase formation during calcination and
Fig. 1(a) shows the XRD patterns of samples, which confirmed the retention of single perovskite phase [13,14] nature except x¼ 0.04 samples. Secondary phase in x ¼0.04 sample is caused by reaction between sodium, titanium and oxygen ions (JCPDS no: 31-1329, 80-1282). In the x ¼0 i.e BNT–BT–BKT samples, sintered at 1100 °C, there develops secondary phase. The content of secondary phase of x ¼0.04 samples is less than that of x ¼0 samples, sintered at 1100 °C. It is further observed that XRD peak intensities of secondary phase diminish for x 40.04 samples. Therefore, NN addition enhances the sintering temperature and inhibits the secondary phase formation in the NN modified BNT–BT–BKT samples, sintered at 1100 °C. Deconvolution of XRD peak at 2θ 46° of the all (1 x)[BNT–BT–BKT]–xNN samples, shown in Fig. 1(b), consists of two tetragonal, (200) and (002), and one rhombohedral (200) peak. Based on integral intensities (I), tetragonal phase (T.P) and rhombohedral phase (R.P) fractions are calculated using the following relations [15]:
{
T.P.= I( 200) + I( 002) T
T
} / { I(
200)
T
+ I( 002) + I( 200) T
}…. .
R
(A.3)
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Fig. 2. FESEM micrographs of the samples.
Table 2 Dielectric and ferroelectric properties of the samples. Sample
Grain size (μm)
εr (RT) at 1 kHz
εm at 1 kHz
Td (°C) at 1 kHz
Tc (°C) at 1 kHz
Ec (kV/ cm)
Pr (μC/ cm2)
Ps (μC/ cm2)
x¼0 x ¼ 0.04 x ¼ 0.08 x ¼ 0.12 x ¼ 0.16
1.4 0.47 0.70 0.69 0.71
1375 1529 1583 1618 1445
3360 3220 3488 3050 2799
70 112 110 107 106
299 306 260 255 245
22 11.6 8.1 7.8 7.3
22 7 4.1 3.5 2.9
36.8 30.69 30.66 25.80 22.78
R.P=1−T.P =
{ I( ) } / { I( 200
R
200)
T
}
+ I( 002) + I( 200) …. .. T R
(A.4)
Lattice parameters along with phase fractions of rhombohedral and tetragonal structures are listed in the (Table 1). Both the phases exist in all the samples, which confirm the morphotrophic phase boundary (MPB) nature of these samples. Fig. 2 shows the microstructure of the (1 x)[BNT–BT–BKT] xNN samples. Average grain size, calculated using linear intercept method, is given in (Table 2). It is observed that with the modification of NN phase, grain size of (1 x)[BNT–BT–BKT] xNN samples decreases, which may be attributed to the pinning effect of defect dipoles. Sung et al. [16,17] reported that in oxide perovskites, sintered at high temperatures, defect dipoles, denoted below, are generated
′ − VO•• − VNa ′ …. VNa
(A.5)
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Fig. 3. Temperature dependence of dielectric constant and loss of the samples.
′ is sodium-ion and VO•• is oxygen-ion vacancies (KroWhere, VNa ger–Vink notation). Generation of defect dipoles in these samples can cause grain boundary pinning effect, which can account the development of smaller grains [16,17]. In x ¼0.04 samples, grain size decreases drastically, which may attributed to combined effect of presence of secondary phase and the grain boundary pinning effect. Higher average grain size, which is almost constant, is observed in x ¼0.08, 0.12 and 0.16 samples compared to x ¼0.04 samples. In x 4 0.04 samples, due to the absence of secondary phase, the grain size is higher than the x ¼ 0.04 samples. Generally, in Na based perovskite oxides, liquid phase sintering is prominent, which favors grain growth [18]. Therefore, increased grain size of x ¼0.08, 0.12 and 0.16 samples, compared to x ¼ 0.04 samples, can be the effect of liquid phase sintering in these samples. 3.2. Dielectric study Fig. 3 shows the temperature (T) dependence of εr and tan δ , measured at different frequencies of the samples. Decrease of εr with the increase of frequency can be related with the decrease of
net polarization [19]. Whereas, increase of εr with the increase of temperature can be related with the increase of dipole orientation. Value of εr increases up to a particular temperature called maximum dielectric constant temperature (Tm), and beyond it decreases with the further increase of temperature. Dielectric spectra show two anomalies; one near Td and other near Tm [20]. In x¼ 0.08, 0.12 and 0.16 samples, diffused phase transition and shift of Tm to higher temperature side with the increase of frequency is observed, which confirms the relaxor nature of these samples. Relaxor behavior in complex perovskite FE materials is accounted in terms of generation of cation-ordered clusters [21]. Therefore, relaxor nature of x¼ 0.08, 0.12 and 0.16 samples suggests the onset of generation of cation-ordered clusters in xZ0.08 samples. Value of tan δ at different frequencies of all the NN modified samples is nearly constant up to Td. At a particular frequency, tanδ is constant up to Td. Occurrence of tanδ peaks at higher temperatures suggests the presence of oxygen vacancies in the samples [22,23]. The tan δ (at low frequency) at high temperatures drastically increases for x¼ 0.12 samples, which may be attributed to increase of electrical conductivity due to the increase of space charge effect [24]. The
M. Chandrasekhar et al. / Physica B 497 (2016) 59–66
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Fig. 4. ln (1/εr 1/εm) vs. ln (T Tm) plots of the samples.
dielectric parameters at 1 kHz frequency of the samples are given in (Table 2). Temperature zone from Td to Tm gradually decreases with the increase of NN content, which suggests the increased antiferroelectric ordering at RT [25,26]. Tm deceases with the increase of NN content, which can be related to the stabilization of tetragonal phase [27]. Diffuseness of the samples was studied using the modified
Curie–Weiss law, given as: γ
(1/εr –1/εm)=( T − Tm) /C…. .
(A.6)
where, γ and C are constants and value of γ lies between 1 and 2 and gives information about the characteristic of the phase transition. γ ¼1 for normal ferroelectrics and γ ¼ 2 for ideal relaxor ferroelectrics [8]. Fig. 4 shows the ln (1/εr 1/εm) vs. ln (T Tm)
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3.3. P–E hysteresis loop and Energy storage study
Fig. 5. P–E hysteresis loops of the samples.
Fig. 5 shows the P–E hysteresis loops of the samples. P–E hysteresis loop study suggest the dominant AFE nature of all the compositions (except x¼ 0). Coercive field (Ec), Pr and Ps values of the samples are given in (Table 2). Fig. 6 shows the variation of Pr and Ps with the variation of NN content. Decrease of Ec and Pr values with the increase of NN content suggest the decrease of FE nature of these sample. Appreciable decrement in Pr value is observed for x Z0.08 samples, whereas, very little compositional influence was observed on Ps value. This indicates that the FE order is perturbed by NN and the free energy of the ferroelectric phase appears competitive enough with that of the “non-polar” phase, which leads to ergodic relaxor state [28]. Therefore, for x¼ 0.08, 0.121 and 0.16 ceramic samples can exhibit relaxor behavior, which also was confirmed from dielectric study. With the increase of NN content, Ec value decreases, which further suggest the decrease of FE ordering and increase of AFE ordering at RT [29]. Fig. 7 shows the stored energy density (W1) and energy storage efficiency (η%) as a function of NN content in the samples. Due to decrease of Pr and retention of same Ps, W1 increases up to x ¼0.08 samples. In x 40.08 samples, W1 decreases due to the large decrease of Ps than the Pr values. Initially, value of η% increases up to x¼ 0.08, which can be related to drastic decrease of Pr values. Beyond x ¼0.08, increment in η% is small, which can be related with reduction of polar phase (confirmed by P–E hysteresis loop study) [27]. To the best of our knowledge, the sample with x ¼0.08 shows the maximum energy storage density 0.721 J/cm3, which is larger than the earlier reported 0.37 J/cm3 at 120 kV/mm of Ba0.4Sr0.6TiO3 [30], 0.598 J/cm3 at 52 kV/cm of 0.93BNT– 0.06BT3–0.01KNN [20] and 0.4 J/cm3 at 90 kV/mm of BaTiO3– glass composites [31], and many other lead based systems [5,32,33]. Therefore, x ¼0.08 samples are suitable for energy storage applications. 3.4. S–E hysteresis loop study
Fig.6. Variation of Pr and Ps with NN content of the samples.
Fig. 8 shows the RT S–E hysteresis loops of the samples. The sample x ¼0 shows butterfly shaped S–E loop, which suggests its piezoelectric nature. Non butterfly shape of S–E loops of all the NN modified samples suggests their non-piezoelectric nature. This non-piezoelectric nature in these samples can be associated with the AFE nature of NN phase [18]. Asymmetry in S–E loops in the NN modified samples can be attributed to the back switching of domains during bipolar cycles of the applied external electric field [34]. Maximum electric field induced strain of 0.63, 0.1, 0.13, 0.07 and 0.08 % were observed in x ¼0, 0.04, 0.08, 0.12 and 0.16 samples, respectively. Drastic decrease of maximum strain% for the x¼ 0.04 samples can be associated with grain boundary pinning effect, which reduces the switching and movement of domain walls [35–37].
4. Conclusions
Fig. 7. Energy storage density and energy storage efficiency of the samples.
plots of the samples. In these plots, slope of linearly fitted curves give γ factor. γ values are found to be 1.69, 1.79, 1.91, 1.75 and 1.84 for the x ¼0, 0.04, 0.08, 0.12 and 0.16 samples, respectively, which confirm the diffusive nature of phase transition.
Except x¼ 0.04 sample, XRD patterns of the samples show the retention of single perovskite phase nature. Increase of grain boundary pinning and liquid phase sintering effects were accounted for the variation of grain size with the increase of NN content of the samples. Dielectric study confirmed the relaxor behavior of x ¼0.08, 0.12 and 0.16 samples. Highest value of energy storage density suggested the suitability of x ¼0.08 samples for energy storage applications. P–E and S–E hysteresis loop study confirmed the reduction of FE, piezoelectric and increase of AFE phase nature with the increase of NN content.
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Fig. 8. S–E loops of the samples.
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Physica B journal homepage: www.elsevier.com/locate/physb
Synthesis and characterizations of NaNbO3 modified BNT–BT–BKT ceramics for energy storage applications M. Chandrasekhar, Sonia, P. Kumar n Department of Physics, National Institute of Technology, Rourkela, Odisha 769008, India
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2016 Received in revised form 14 June 2016 Accepted 15 June 2016 Available online 16 June 2016
Lead-free (1 x)[0.884BNT–0.036BT–0.08BKT]–xNaNbO3 samples (with x ¼0, 0.04, 0.08, 0.12 and 0.16) were synthesized in single perovskite phase by solid state reaction route. Decrease of grain size with the increase of NaNbO3/NN content was related to the grain boundary pinning effect. Dielectric study confirmed the relaxor nature of x ¼ 0.08, 0.12 and 0.16 samples. Energy storage density and efficiency were calculated from polarization vs. electric field hysteresis loops. Value of energy storage efficiency increased with the increase of NaNbO3 content. A relatively large energy storage density of 0.721 J/cm3 was obtained in the x ¼0.08 samples. Electric field induced polarization and strain loops, respectively suggested the increase of AFE ordering with the increase of NN content. & 2016 Elsevier B.V. All rights reserved.
Keywords: Grain boundary pinning effect Energy storage density Relaxor Non-polar phase
1. Introduction High energy storage density functional materials are used in compact devices like electrical vehicles, mobile electronics and different types of pulsed power technologies [1]. Recently, Bi0.5Na0.5TiO3 (BNT)-based ferroelectric (FE) functional materials have drawn attention of research community for their promising use in energy storage applications. In comparison to conventional dielectrics, antiferroelectric (AFE) materials have small remnant polarization (Pr) and large charge–discharge speed, and are preferred in energy storage applications [2,3]. Better energy storage density and efficiency of BNT system appear near depolarization temperature (Td) [4]. On the other hand, though FE materials with square shaped Polarization–Electric field (P–E) hysteresis loops have high energy storage density than those of the double like P–E hysteresis loops of the AFE materials, yet they cannot withstand the large number of charge–discharge cycles [2]. Therefore, double smaller area P–E hysteresis loops are desirable for achieving large number of charge–discharge cycles [5,6] in the energy storage device applications. Energy storage density (W1) of a FE material is given by the following equation [7] Pmax
W1 =
∫
EdP…. .
0
n
Corresponding author. E-mail addresses: [email protected] (M. Chandrasekhar), [email protected] (P. Kumar). http://dx.doi.org/10.1016/j.physb.2016.06.015 0921-4526/& 2016 Elsevier B.V. All rights reserved.
(A.1)
where, E is the applied electric field and P is the polarization. According to above equation, materials having large saturated polarization (Ps), smaller remnant polarization (Pr) and moderate breakdown field, are suitable for having high energy storage density. On the other hand, energy storage efficiency (η) of a functional material is calculated by using the following equation [7]:
η = W1/(W1 + W2)…. .
(A.2)
where, W1 is the stored energy density; W2 is the energy loss density and calculated by the numerical integration of closed area of the P–E curves of the hysteresis loops. Sodium niobate NaNbO3/NN, strontium titanate SrTiO3/ST are some of the prominent lead-free AFE systems [8]. Chia-Ching Wu and Cheng-Fu Yang [9] studied the effects of NN modification on the dielectric properties and relaxor nature of lead-free BNT ceramics. Wang et al. [10] studied the temperature dependent electrical properties of BNT–BKT–SrTiO3 piezoceramics. In the literature, NaNbO3 and BNT–BT–BKT are reported as the promising lead free AFE and FE systems respectively [11,12]. Para electric phase transition temperature of NN system lies well below room temperature (RT). Therefore, modification of effective BNT–BT–BKT piezoelectric system with NN AFE system will not only lower down the Td temperature but also increase the energy storage density and energy storage efficiency. In the present work, dielectric, energy-storage density and efficiency properties of NaNbO3 modified BNT–BT–BKT system are studied and discussed in detail.
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Fig. 1. XRD patterns of the samples.
Table 1 Phase fractions and lattice parameters of the (1 x)[BNT–BT–BKT] xNN samples. Sample
x¼0
x¼ 0.04
x¼ 0.08
x¼ 0.12
x ¼0.16
R.P phase fraction Lattice parameters (aR, α) T.P phase fraction Lattice parameters (aT, cT)
31
33.4
30.7
31.2
34.2
(3.8925 Å, 89.910°)
(3.8926 Å, 89.860°)
(3.8908 Å, 89.870°)
(3.8917 Å, 89.860°)
(3.8875 Å, 89.740°)
69
66.6
69.3
68.8
65.8
(5.7063, 13.4893) Å
(5.7032, 13.4820) Å
(5.6992, 13.5033) Å
(5.7024, 13.4974) Å
(5.7004, 13.5016) Å
sintered stages were confirmed by X-ray diffraction (XRD) (Rigaku Ultima IV, Tokyo, Japan) technique. Micrographs of the sintered pellets were taken using Field emission scanning electron microscope (FESEM NOVA nano SEM). Silver paste was applied on the polished surfaces of the sintered pellets and then fired at 400 °C for 30 min for good adhesion. Dielectric constant (εr) and dielectric loss (tan) were measured as a function of temperature using a computer interfaced HIOKI 3532-50 LCR-HITESTER. RT Polarizations vs. electric field (P–E) hysteresis loops were measured using Radiant precision premier II unit (P-HVI210k, Radiant Technologies INC. U.S.A). RT bipolar strain vs. electric field (S–E) hysteresis loops were measured using Radiant precision premier II attached with MTI-2100 Fotonic sensor (MTI Instruments INC, U.S.A).
3. Results and discussion 2. Material and methods 3.1. XRD and microstructure study Solid-state reaction route was used to synthesize the selected materials. Firstly, the 0.884Bi0.5Na0.5TiO3–0.036BaTiO3– 0.08BiKTiO3/BNT–BT–BKT (abbreviated as x¼ 0) ceramics were prepared by taking stoichiometric weights of Bi2O3, Na2CO3, TiO2, BaCO3 and K2CO3 (all are 99.9% purity) as the starting precursors. The mixture of these starting precursors was ball milled for 20 h (h) by using zirconia balls and acetone as the grinding media. The dried ball milled powder was calcined at 900 °C and sintered at 1050 °C for 4 h. NN was synthesized by taking stoichiometric weights of Na2CO3 and Nb2O5 precursors, ball milled under the same conditions mentioned above and calcined at 800 °C for 4 h. Finally, the (1 x)[BNT–BT–BKT]–xNN ceramics with x¼0.04, 0.08, 0.12 and 0.16 were synthesized by taking stoichiometric weights of the calcined BNT–BT–BKT and NN powders. 3 wt% polyvinyl alcohol binder solution was added to the calcined powders. Green pellets of 10 mm in diameter and 1.5 mm in thickness were compacted using a uniaxial hydraulic press with 10 MPa pressure. Optimized sintering temperature for x¼0 i.e BNT–BT–BKT samples was 1050 °C/4 h and for NN modified BNT–BT–BKT samples it was 1100 °C/4 h. Perovskite phase formation during calcination and
Fig. 1(a) shows the XRD patterns of samples, which confirmed the retention of single perovskite phase [13,14] nature except x¼ 0.04 samples. Secondary phase in x ¼0.04 sample is caused by reaction between sodium, titanium and oxygen ions (JCPDS no: 31-1329, 80-1282). In the x ¼0 i.e BNT–BT–BKT samples, sintered at 1100 °C, there develops secondary phase. The content of secondary phase of x ¼0.04 samples is less than that of x ¼0 samples, sintered at 1100 °C. It is further observed that XRD peak intensities of secondary phase diminish for x 40.04 samples. Therefore, NN addition enhances the sintering temperature and inhibits the secondary phase formation in the NN modified BNT–BT–BKT samples, sintered at 1100 °C. Deconvolution of XRD peak at 2θ 46° of the all (1 x)[BNT–BT–BKT]–xNN samples, shown in Fig. 1(b), consists of two tetragonal, (200) and (002), and one rhombohedral (200) peak. Based on integral intensities (I), tetragonal phase (T.P) and rhombohedral phase (R.P) fractions are calculated using the following relations [15]:
{
T.P.= I( 200) + I( 002) T
T
} / { I(
200)
T
+ I( 002) + I( 200) T
}…. .
R
(A.3)
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Fig. 2. FESEM micrographs of the samples.
Table 2 Dielectric and ferroelectric properties of the samples. Sample
Grain size (μm)
εr (RT) at 1 kHz
εm at 1 kHz
Td (°C) at 1 kHz
Tc (°C) at 1 kHz
Ec (kV/ cm)
Pr (μC/ cm2)
Ps (μC/ cm2)
x¼0 x ¼ 0.04 x ¼ 0.08 x ¼ 0.12 x ¼ 0.16
1.4 0.47 0.70 0.69 0.71
1375 1529 1583 1618 1445
3360 3220 3488 3050 2799
70 112 110 107 106
299 306 260 255 245
22 11.6 8.1 7.8 7.3
22 7 4.1 3.5 2.9
36.8 30.69 30.66 25.80 22.78
R.P=1−T.P =
{ I( ) } / { I( 200
R
200)
T
}
+ I( 002) + I( 200) …. .. T R
(A.4)
Lattice parameters along with phase fractions of rhombohedral and tetragonal structures are listed in the (Table 1). Both the phases exist in all the samples, which confirm the morphotrophic phase boundary (MPB) nature of these samples. Fig. 2 shows the microstructure of the (1 x)[BNT–BT–BKT] xNN samples. Average grain size, calculated using linear intercept method, is given in (Table 2). It is observed that with the modification of NN phase, grain size of (1 x)[BNT–BT–BKT] xNN samples decreases, which may be attributed to the pinning effect of defect dipoles. Sung et al. [16,17] reported that in oxide perovskites, sintered at high temperatures, defect dipoles, denoted below, are generated
′ − VO•• − VNa ′ …. VNa
(A.5)
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Fig. 3. Temperature dependence of dielectric constant and loss of the samples.
′ is sodium-ion and VO•• is oxygen-ion vacancies (KroWhere, VNa ger–Vink notation). Generation of defect dipoles in these samples can cause grain boundary pinning effect, which can account the development of smaller grains [16,17]. In x ¼0.04 samples, grain size decreases drastically, which may attributed to combined effect of presence of secondary phase and the grain boundary pinning effect. Higher average grain size, which is almost constant, is observed in x ¼0.08, 0.12 and 0.16 samples compared to x ¼0.04 samples. In x 4 0.04 samples, due to the absence of secondary phase, the grain size is higher than the x ¼ 0.04 samples. Generally, in Na based perovskite oxides, liquid phase sintering is prominent, which favors grain growth [18]. Therefore, increased grain size of x ¼0.08, 0.12 and 0.16 samples, compared to x ¼ 0.04 samples, can be the effect of liquid phase sintering in these samples. 3.2. Dielectric study Fig. 3 shows the temperature (T) dependence of εr and tan δ , measured at different frequencies of the samples. Decrease of εr with the increase of frequency can be related with the decrease of
net polarization [19]. Whereas, increase of εr with the increase of temperature can be related with the increase of dipole orientation. Value of εr increases up to a particular temperature called maximum dielectric constant temperature (Tm), and beyond it decreases with the further increase of temperature. Dielectric spectra show two anomalies; one near Td and other near Tm [20]. In x¼ 0.08, 0.12 and 0.16 samples, diffused phase transition and shift of Tm to higher temperature side with the increase of frequency is observed, which confirms the relaxor nature of these samples. Relaxor behavior in complex perovskite FE materials is accounted in terms of generation of cation-ordered clusters [21]. Therefore, relaxor nature of x¼ 0.08, 0.12 and 0.16 samples suggests the onset of generation of cation-ordered clusters in xZ0.08 samples. Value of tan δ at different frequencies of all the NN modified samples is nearly constant up to Td. At a particular frequency, tanδ is constant up to Td. Occurrence of tanδ peaks at higher temperatures suggests the presence of oxygen vacancies in the samples [22,23]. The tan δ (at low frequency) at high temperatures drastically increases for x¼ 0.12 samples, which may be attributed to increase of electrical conductivity due to the increase of space charge effect [24]. The
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Fig. 4. ln (1/εr 1/εm) vs. ln (T Tm) plots of the samples.
dielectric parameters at 1 kHz frequency of the samples are given in (Table 2). Temperature zone from Td to Tm gradually decreases with the increase of NN content, which suggests the increased antiferroelectric ordering at RT [25,26]. Tm deceases with the increase of NN content, which can be related to the stabilization of tetragonal phase [27]. Diffuseness of the samples was studied using the modified
Curie–Weiss law, given as: γ
(1/εr –1/εm)=( T − Tm) /C…. .
(A.6)
where, γ and C are constants and value of γ lies between 1 and 2 and gives information about the characteristic of the phase transition. γ ¼1 for normal ferroelectrics and γ ¼ 2 for ideal relaxor ferroelectrics [8]. Fig. 4 shows the ln (1/εr 1/εm) vs. ln (T Tm)
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3.3. P–E hysteresis loop and Energy storage study
Fig. 5. P–E hysteresis loops of the samples.
Fig. 5 shows the P–E hysteresis loops of the samples. P–E hysteresis loop study suggest the dominant AFE nature of all the compositions (except x¼ 0). Coercive field (Ec), Pr and Ps values of the samples are given in (Table 2). Fig. 6 shows the variation of Pr and Ps with the variation of NN content. Decrease of Ec and Pr values with the increase of NN content suggest the decrease of FE nature of these sample. Appreciable decrement in Pr value is observed for x Z0.08 samples, whereas, very little compositional influence was observed on Ps value. This indicates that the FE order is perturbed by NN and the free energy of the ferroelectric phase appears competitive enough with that of the “non-polar” phase, which leads to ergodic relaxor state [28]. Therefore, for x¼ 0.08, 0.121 and 0.16 ceramic samples can exhibit relaxor behavior, which also was confirmed from dielectric study. With the increase of NN content, Ec value decreases, which further suggest the decrease of FE ordering and increase of AFE ordering at RT [29]. Fig. 7 shows the stored energy density (W1) and energy storage efficiency (η%) as a function of NN content in the samples. Due to decrease of Pr and retention of same Ps, W1 increases up to x ¼0.08 samples. In x 40.08 samples, W1 decreases due to the large decrease of Ps than the Pr values. Initially, value of η% increases up to x¼ 0.08, which can be related to drastic decrease of Pr values. Beyond x ¼0.08, increment in η% is small, which can be related with reduction of polar phase (confirmed by P–E hysteresis loop study) [27]. To the best of our knowledge, the sample with x ¼0.08 shows the maximum energy storage density 0.721 J/cm3, which is larger than the earlier reported 0.37 J/cm3 at 120 kV/mm of Ba0.4Sr0.6TiO3 [30], 0.598 J/cm3 at 52 kV/cm of 0.93BNT– 0.06BT3–0.01KNN [20] and 0.4 J/cm3 at 90 kV/mm of BaTiO3– glass composites [31], and many other lead based systems [5,32,33]. Therefore, x ¼0.08 samples are suitable for energy storage applications. 3.4. S–E hysteresis loop study
Fig.6. Variation of Pr and Ps with NN content of the samples.
Fig. 8 shows the RT S–E hysteresis loops of the samples. The sample x ¼0 shows butterfly shaped S–E loop, which suggests its piezoelectric nature. Non butterfly shape of S–E loops of all the NN modified samples suggests their non-piezoelectric nature. This non-piezoelectric nature in these samples can be associated with the AFE nature of NN phase [18]. Asymmetry in S–E loops in the NN modified samples can be attributed to the back switching of domains during bipolar cycles of the applied external electric field [34]. Maximum electric field induced strain of 0.63, 0.1, 0.13, 0.07 and 0.08 % were observed in x ¼0, 0.04, 0.08, 0.12 and 0.16 samples, respectively. Drastic decrease of maximum strain% for the x¼ 0.04 samples can be associated with grain boundary pinning effect, which reduces the switching and movement of domain walls [35–37].
4. Conclusions
Fig. 7. Energy storage density and energy storage efficiency of the samples.
plots of the samples. In these plots, slope of linearly fitted curves give γ factor. γ values are found to be 1.69, 1.79, 1.91, 1.75 and 1.84 for the x ¼0, 0.04, 0.08, 0.12 and 0.16 samples, respectively, which confirm the diffusive nature of phase transition.
Except x¼ 0.04 sample, XRD patterns of the samples show the retention of single perovskite phase nature. Increase of grain boundary pinning and liquid phase sintering effects were accounted for the variation of grain size with the increase of NN content of the samples. Dielectric study confirmed the relaxor behavior of x ¼0.08, 0.12 and 0.16 samples. Highest value of energy storage density suggested the suitability of x ¼0.08 samples for energy storage applications. P–E and S–E hysteresis loop study confirmed the reduction of FE, piezoelectric and increase of AFE phase nature with the increase of NN content.
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Fig. 8. S–E loops of the samples.
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