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Effect of La-doping on dielectric properties and energy storage density of lead-free Ba(Ti0.95 Sn0.05 )O3 ceramics
Journal Pre-proof Effect of La-doping on dielectric properties and energy storage density of lead-free Ba(Ti0.95 Sn0.05 )O3 ceramics R. Kumar, I. Singh, R. Meena, K. Asokan, Balaji Birajdar, S. Patnaik
PII:
S0025-5408(19)31679-4
DOI:
https://doi.org/10.1016/j.materresbull.2019.110694
Reference:
MRB 110694
To appear in:
Materials Research Bulletin
Received Date:
4 July 2019
Revised Date:
1 October 2019
Accepted Date:
12 November 2019
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M.R.B. Materials Research Bulletin 00 (2019) 1–7
Effect of La-doping on dielectric properties and energy storage density of lead-free Ba(Ti0.95Sn0.05)O3 ceramics R. Kumara,c,∗ , I. Singha , R. Meenab , K. Asokanb , Balaji Birajdara,∗∗ , S. Patnaikc,∗∗∗ Centre for Nano Sciences, Jawaharlal Nehru University, Delhi, India
b Material
Science Division, Inter University Accelerator Centre, Delhi, India.
c School
of Physical Sciences, Jawaharlal Nehru University, Delhi, India
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Abstract
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Lead-free Ba1−z Laz Ti0.95 Sn0.05 O3 with z=0.00, 0.015, 0.025, and 0.035 samples were prepared using solid-state reaction route. X-ray diffraction measurement reveals the single phase formation of ceramic samples. Scanning electron microscopy shows the inhibited grain growth (≤ 0.5 microns) at low La-doping while the higher La-doping in BTS ceramics yielded the larger grain size (2.5 microns). Raman spectra indicate the substitution of La at Ba site sublattice. Dielectric properties improved with La-doping in BTS ceramic samples. 3.5LBTS sample exhibits energy storage density as high as (Jd ) ∼ 0.492 J/cm3 with an energy efficiency of 63 % at ∼ 40 kV/cm, which is 5.5 times more than that of pure BTS ceramics. Such significant increment in energy storage density could be attributed to reduced grain size due to La-doping. Thus, the selected composition could be a new method for enhancing the energy storage density for device applications.
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Keywords: Lead-free materials; La-doping; microstructure; dielectric properties; energy storage density
1. Introduction
considerable attention as a energy storage materials in place of Pb-based materials because of environment concern [12, 13]. Here in the present work, we study the structural, microstructural, ferroelectric, dielectric, and energy storage properties of lead-free Ba1−z Laz Ti0.95 Sn0.05 O3 with z=0.00, 0.015, 0.025, and 0.035 ceramic samples. Further, we investigated this composition from the viewpoint of energy storage applications. Significant achievements related to (i) significant improvement in dielectric constant and dielectric loss (ii) enhancement of energy storage density (iii) reduction in hysteresis loss are reported.
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The evolution of lead-free material with large electrical energy storage density is of significant importance for achieving the requirements of advanced power technologies [1]. Scientists have been searching the new dielectric materials for high energy storage capacitors [2–4]. The capacitor is a crucial component in electronic industries and has been continuously used for many practical applications such as pulse power and power conditioning technologies [5–9]. Ferroelectric capacitors are well-known for high dielectric permittivity, low loss characteristics, and high power density due to the fast discharge time that makes them suitable candidate for high storage energy density applications [10, 11]. Among various ferroelectric materials, BaTiO3 -based ceramic material is ideal candidate that exhibits relatively high dielectric permittivity and low loss characteristics but, these ceramics suffer from the significant drawback of low breakdown field strength, high hysteresis loss, limiting the high energy storage density for practical applications [4, 9]. The energy storage density of lead-free ceramics is much smaller than that of Pb-based materials due to smaller polarization. In the recent decades, lead-free materials have attracted
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2. Experimental procedure
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Lead-free Ba1−z Laz Ti0.95 Sn0.05 O3 with z=0.00, 0.015, 0.025, and 0.035 ceramic samples were fabricated using solid-state reaction route. BaCO3 (99%), La2 O3 (99%), TiO2 (99%) and SnO2 (99.9%) (Sigma-Aldrich) powders were used as the starting raw materials and mixed at ambient conditions. Powders were weighed using following formula in the range (z = 0.00 0.015, 0.025 and 0.035 abbreviated as BTS, 1.5LBTS, 2.5LBTS, and 3.5LBTS respectively). The compounds were ground (with acetone as a medium) for ∼ 15 hrs using agate mortar pestle. Mixed powders were pressed into a disk shape pellet of diameter ∼ 10 mm and thickness 1 mm using a pressure of 10 tons.
[email protected]
∗∗ [email protected],
and [email protected]∗∗∗
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The ceramic samples were calcined at ∼ 1250◦ C for 72 hrs. Calcined powders are reground again and pressed into disk shape pellets. Pure BTS sample was sintered at ∼ 1350◦ C for 2 hrs and La-doped BTS ceramics were sintered at ∼ 1350◦ C for 6 hrs. The relative density of BTS, 1.5LBTS, 2.5LBTS, and 3.5LBTS ceramic samples, calculated by measuring mass and volume of pellets, are 87, 60, 74, and 93 % of their theoretical values respectively. Single phase of samples were confirmed using X-ray diffraction (Model: MiniFlex600, Rigaku) using Cu-Kα radiation (λ=1.542 Å) in the diffraction angle range of 20 - 80◦ . The microstructural studies were carried out after gold coating on the as-grown surfaces using a scanning electron microscopy (Model: Zeiss Figure 1: (A) Room temperature XRD patterns of (a) BTS (b) 1.5LBTS EVO40). Raman spectra of ceramic samples were carried out at (c) 2.5LBTS and (d) 3.5LBTS ceramics sintered at 1350◦ C/6hrs; and (B) −1 room temperature in the wavenumber range of ∼ 40 cm - 950 shows the magnified view of XRD pattern in the range of 45 to 47◦ . cm−1 by using Raman spectrometer (Model: Enspectr R 532) with a wavelength of 532 nm. Before dielectric permittivity La3+ (1.32 Å) as compared to Ba2+ (1.61 Å). The similar peak and ferroelectric properties measurement, ceramic pellets were shifting patterns by partial substitution of lower ionic radius for polished to make smooth faces using sandpaper (# P2000) fola higher ionic radius was observed in La-doped BTS and BZT lowed by diamond lapping. The silver electrodes were prepared ceramics [16, 17]. The experimental accuracy of XRD limits by putting the silver paste on both faces of ceramic pellets. The its use to identify the mixed phases (T or C) of the materials dielectric properties measurement was performed in the freso that other techniques are required to resolve it. Raman quency range under ac voltage amplitude of 1 volt (∼ 1 kHz spectroscopy is a very versatile tool to probe the short range - 2 MHz) using precision LCR meter (Model: E4980A, Agilent order modes and also explain the mixed phases. [14, 18]. Technologies). The temperature was controlled using a proFigure 2 displays the grain structure of as-grown ceramic grammable temperature controller (Model: 325, LakeShore). pellets of BTS, 1.5LBTS, 2.5LBTS, and 3.5LBTS. Pure BTS Room temperature polarization versus electric field (P-E) hyssample (Figure 2 (a)) showed homogeneous, rounded and denteresis loops for all the compositions are measured at a fixed sified micro-structure consisting of 3-dimensional (3-D) large frequency of 50 Hz and applying maximum field of ∼ 40 kV/cm grains (grain size ∼ 5 to 30 microns). (Model: P-E Loop Tracer, Marine India). 3. Results and discussion
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Room temperature X-ray diffraction patterns after sintering of BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS ceramic samples are shown in Figure 1. X-ray diffraction study reveals the predominately single phase perovskite structure. The splitting of characteristic peaks (002)/(200) at ∼ 45◦ indicates that at room temperature pure BTS has a tetragonal structure (In agreement with Raman and dielectric properties data plot) and samples could be indexed to tetragonal structure (space group p4mm), while it turns into a single peak for Ladoped samples, suggesting a phase transformation (Figure 1 (B)). The Raman spectra shows similar results, the peak at 304 cm−1 characteristics peak of tetragonal barium titanate [14], diminishes with La-doping [15], and disappears for 3.5LBTS5 sample. Figure 1 shows dash lines corresponding to (111) reflection at peak position (2θ = 38.72◦ ) and corresponding to (002) reflection at peak position (2θ = 45◦ ) which is drawn as a guide to the eye to clearly identify the right shifting of peaks with La-doping. Peaks corresponding to (111) at 2θ ∼ 38.72◦ and (002) at 2θ ∼ 45◦ are clearly shifting towards higher angle side with increasing La concentration, which reveals the contraction of the lattice cell. This confirms the successful substitution of La3+ ions at Ba2+ ions in BTS lattice (Figure 1). The shifting in the diffraction peaks is expected due to smaller ionic radius of
Figure 2: SEM images on as-grown of (a) BTS (b) 1.5LBTS (c) 2.5LBTS (d) 3.5LBTS ceramic samples sintered at 1350◦ C/6hrs.
The larger grain size in pure BTS ceramic sample could be ascribed to enhanced mass transport phenomenon because of formation of oxygen vacancies. These oxygen vacancies in ceramics are known as a result of sintering at high temperatures (≥ 1350◦ C, due to oxygen loss) [19]. Subsequently, this mechanism helps in grain growth and densification of the ce2
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cm−1 and the sharp resonance dip at 178 cm−1 disappear, indicating a phase transformation from tetragonal to pseudo-cubic phase [14, 18, 23]. This indicates that at room temperature the tetragonal and cubic phases co-exist, volume fraction of cubic phases increases with increasing with La-doping. The most noticeable changes in this composition due to La substitution at Ba site in BTS ceramic sample is the appearance of a new peak at 838 cm−1 . The peak at 838 cm−1 is associated with the A1g stretching mode of TiO6 octahedra, which is the feature of polar nano regions [16, 17]. Pokorny et al. also observed the A1g mode at ∼ 830 cm−1 , and 838 cm−1 in Ca and La-doped BaTiO3 system, respectively [15]. Feteira et al. observed A1g mode at 823 cm−1 by the simultaneous incorporation of La3+ and Y3+ ions in (1-x)BaTiO3 -xLaYO3 system [23]. The discrepancy between X-ray diffraction data and Raman spectroscopy data can be explained in term of the difference in coherence length of two techniques. The Raman response is more sensitive to the short range (Unit cell level), while the information transfer by X-ray diffraction technique is averaged over 104 unit cells [18]. Thus, Raman spectroscopy is sensitive enough to display the small changes in crystal structure at the atomic scale while Xray diffraction technique provides the information of a complete unit cell. Figure 4 shows the temperature dependence of dielectric constant and loss tangent of BTS, 1.5LBTS, 2.5LBTS, and 3.5LBTS ceramic pellets. Dielectric data are acquired in the temperature range of 100 K - 500 K during the heating of ceramic pellets at the rate of 3 K/min in the frequency range of 1 kHz 2 MHz. For pure BTS ceramic sample, three structural phase transitions are observed; rhombohedral to orthorhombic (R-O) at Tr−o ∼ 256 K, orthogonal to tetragonal (O-T) at To−t ∼ 296 K and tetragonal to cubic (T-C) at Tt−c ∼ 350 K. These phase transitions in BTS (Figure 4 (a)) are similar to the one reported by Deluca et al. for BaTiO3 ceramic sample for low content of Sn [18]. For pure BTS sample, there is no frequency dispersion around dielectric maxima, indicating long-range ferroelectric order. On the other hand in La-doped samples, lowtemperature phase transitions are suppressed and dielectric maximum shifted to lower temperature, but the frequency dispersion is still negligible. The addition of La content (1.5 atomic %) in BTS, results into a single broadened peak at ∼ 250 K with enhanced dielectric constant, which resembles so-called diffuse phase transition. Interestingly, the magnitude of dielectric permittivity increases, while transition temperature is lowered with La-doping. In general, lowering of the transition temperature could be related to smaller grain size attained with La-doping [24]. The dielectric loss values is reduced with addition of La-doping compared to pure BTS sample (Figure 4 (a-d)). Much above the transition temperature (T > Tm ), there is an abrupt increase in the value of tan δ with increasing temperature which would be the direct consequence of higher electrical conductivity [25]. The dielectric constant of ceramic samples is observed to increased with La-doping. This could be attributed to distorted TiO6 octahedra due to Ti vacancies, which break long range order and assist in the reorientation of ferroelectric domain close
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ramics. From SEM micrographs, it is found that grain size for La-doped ceramics is much smaller than pure BTS ceramic sample. The 1.5LBTS ceramic pellet (Figure 2 (b)) reveals a dense microstructure with small grains of ∼ 0.5 to 1 micrometers. Similarly, the 2.5LBTS ceramic pellet (Figure 2 (c)) reveals denser microstructure with smaller grains of 0.25-0.50 microns. It has been reported that during the sintering process, overall diffusion rate is found to be reduced after the incorporation of La ions at Ba-site and it could suppress the oxygen vacancies [19, 20]. However, a further increase in La atomic % (3.5LBTS ceramic sample) reveals a micro-structure with grains of 0.5 to 2.5 microns. A systematic observation of microstructure formation of La-doped Ba(Ti0.8 Sn0.2 )O3 and La-doped Ba(Ti0.8 Zr0.2 )O3 ceramic samples has been discussed in details by Kumar et al. [16, 17]. Similar grain growth inhibition at low La-doping and enhanced grain growth at large La-doping in La x Ba1−x (Ti0.8 Zr0.2 )O3 system has been observed by Chou et al. [21]. Figure 3 shows the room temperature Raman spectra of asgrown ceramic pellets of BTS, 1.5LBTS, 2.5LBTS, and 3.5LBTS in the wavenumber range of 40 - 950 cm−1 . Raman spectroscopy characterization of as-grown samples is carried out for correlating structural and phase purity aspects. As seen in Figure 3, the Raman spectrum of pure BTS sample shows following dominant modes centered at 115, 180, 189, 253, 304, 515, 715 cm−1 and one anti-resonance dip at 178 cm−1 [15, 16].
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Figure 3: Room temperature Raman spectra of as-grown ceramic pellets of (a) BTS (b) 1.5LBTS (c) 2.5LBTS (d) 3.5LBTS sintered at 1350◦ C/6hrs.
The Raman spectrum of pure BTS ceramic sample shows the sharp silent mode at 304 cm−1 [E(TO+LO)] [14], and weak broadband at 715 cm−1 [E(LO)] which are usually considered as signatures of the tetragonal phase of BaTiO3 [22]. The peak at 253 cm−1 and 515 cm−1 [A1 (TO)] are strong, broad and asymmetric [15]. Several noticeable changes appear in the spectral region between 40 cm−1 - 350 cm−1 with an increase of Ladoping in BTS ceramics. For examples, the sharp resonance dip at ∼ 178 cm−1 and the intensity of the sharp band at 304 cm−1 starts to decrease at 1.5 atomic % of La-doping in BTS. Further, for 3.5LBTS sample, both the sharp band characteristic at 304 3
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Table 1: The summary of fitting parameters obtained from the plot of ln( 1 − 1m ) versus ln(T − T m ); Tm , m , ∆T m = Tm (1 MHz) - Tm (1 kHz), γ, C∗ (K), FWHM (K) and tan δRT at room temperature and at 100 kHz.
z Tm (K) C∗ (K) T 0 (K) m ∆T m (K) tan δRT γ FWHM (K)
Figure 4: Temperature dependence of dielectric constant and loss tangent of (a) BTS (b) 1.5LBTS (c) 2.5LBTS (d) 3.5LBTS ceramic samples sintered at 1350◦ C/6hrs.
0.0 350 1.48×104 354 1230 2 0.0549 1.1 43
0.015 290 9.82×105 310 3671 17 0.0516 1.46 125
0.025 259 3.31×106 265 4117 19 0.0381 1.59 137
0.035 250 5.88×105 250 5219 6 0.0085 1.24 120
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for relaxors. A linear relationship was observed for all ceramic samples in high-temperature regime (Above Tm ). The experimental data points were fitted using Equation (2), and value of m , C∗ (K), T0 (K), γ, Tm (K), ∆T m (K), FWHM (K), and tan δRT at 100 kHz are listed in Table 1 for all ceramic samples.
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to transition temperature at low La-doping levels resulting in improved dielectric properties. The results obtained in this study are well-matched with those obtained by Morrison et al. [26, 27] and West et al. [28]. In their reports, they found that samples annealed at 1350 - 1400◦ C shows the magnitude of dielectric permittivity increases up to 6 atomic % of La-doped Ba1−x La x Ti1−x/4 O3 beyond which decreasing trend is observed [26, 27]. The sharper dielectric peak close to 250 K in 3.5LBTS sample could be attributed to the larger grain size (Figure 2 (d)) in this sample in comparison to that of 1.5LBTS (Figure 2 (b)) and 2.5LBTS (Figure 2 (c)). Like in the present case, the full width of half maxima of 4 atomic % La-doped BaTiO3 was reported by Morrison et al. [26, 27] and West et al. [28] to be smaller than that of 2.5 atomic % La-doped BaTiO3 . In paraelectric regions, dielectric permittivity peak usually follows Curie-Weiss law at T > Tm : [29]
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(T − T 0 ) 1 = r C
(1)
where T0 is Curie temperature, r is dielectric constant, and C is the Curie constant. The temperature dependence of the inverse dielectric constant is plotted for selected frequency of 100 kHz for BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS ceramics ( Figure 5). It is observed that La-doped BTS ceramics show diffuse phase transition (Figure 4). The above feature can be understood qualitatively by analyzing the dielectric data using empirical formula known as the modified Curie-Weiss law: [29]
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Figure 5: The plot of inverse dielectric permittivity ( 1r ) at a selected frequency of 100 kHz as a function of temperature for (a) BTS (b) 1.5LBTS (c) 2.5LBTS (d) 3.5LBTS ceramics sintered at 1350◦ C/6hrs. Inset shows the corresponding temperature dependence of inverse dielectric constant plot of ln ( 1r − 1m ) versus ln (T − T m ).
1 1 (T − T m )γ − = r m C∗
The γ factor for pure BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS ceramics were observed to be 1.10, 1.46, 1.59, and 1.24 respectively. The value of γ = 1.1 for pure BTS sample indicates the normal ferroelectric behavior while La-doped samples revealed diffuse phase transition with negligible frequency dispersion. The calculated value of γ gradually increases and reaches a value 1.59 for 2.5LBTS sample, which indicates that the diffuseness in the phase transition (FWHM =137 K) increases with doping of La. This increase in diffuseness may be due to decrease in the degree of ordering as seen in the Raman spectra where the intensity of 304 cm−1 decreases with La-doping. The parameter ∆T m , which describe the deviation from Curie-
(2)
where γ is the degree of relaxation, Tm is the temperature corresponding to the m , and C∗ is modified Curie-Weiss constant. Inset in each panels in Figure 5 shows the plot of ln ( 1r − 1m ) versus ln (T-Tm ) at a selected frequency of 100 kHz for all samples, the slope of which determined the value of γ. The value of γ is assigned to quantify the characteristics of the phase transition. The value of γ lies in the range of 1 ≤ γ ≤ 2. The value of γ is assigned to be 1 for normal ferroelectric and 2 4
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Table 2: The summary of various P-E loop parameters; remnant polarization, 2Pr (µC/cm2 ), maximum polarization, 2Pm (µC/cm2 ), coercive electric field, 2Ec (kV/cm), Pm -Pr (µC/cm2 ), charge energy density, Jc (J/cm3 ), discharge energy density, Jd (J/cm3 ) and energy efficiency, η(%).
z 0.00 0.015 0.025 0.035
2Ec 12.4 10.2 9.46 3.63
2Pr 28.7 6.68 11.54 6.30
2Pm 47.8 29.0 45.2 47.5
Pm -Pr 9.5 11.16 16.83 20.5
Jc 1.15 0.31 0.57 0.29
Jd 0.092 0.165 0.210 0.492
η(%) 7.5 35 30 63
Weiss law, defined as: ∆T m = TCW − T m
(3)
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where TCW is temperature at which the dielectric constant begins to deviate from the Curie-Weiss law and Tm is the temperature of dielectric constant maxima (m ). The value of ∆T m is observed to be 2 K, 17 K, 19 K and 6 K for pure BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS, respectively (Table 1). It is deducible from Table 1 that the value of ∆T m first increases up to 2.5 atomic % of La-doping and then decreases. This indicates that 2.5 atomic % of La-doped sample has a highest diffuse phase transition among all samples. Figure 4 shows Tm is almost independent to applied frequency indicating that LBTS system does not show dielectric dispersion in the composition range studied. Therefore, it is concluded that the system is not a relaxor ferroelectrics in the composition range investigated. It has been well described in the literature that substitution of lower ionic radius La3+ (1.32 Å) ions in place of higher ionic radius Ba2+ (1.61 Å) introduce the Ti vacancies. This disorder enhances the random field, because of which it serve as a nucleation sites for polar nano regions [20, 21, 24]. Figure 6 represents the room temperature ferroelectric polarization versus electric field (P-E) loops of BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS ceramic pellets at a frequency of 50 Hz under an applied electric field of ∼ 40 kV/cm. The values of remnant polarization (2Pr ), maximum polarization (2Pm ), coercive electric field (2Ec ), Pm -Pr and discharge energy density (Jd ) for BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS are listed in Table 2 and discussed below. Pure BTS sample exhibits a well-develop ferroelectric polarization hysteresis loop. P-E loop plot indicates that both negative Ec and positive Ec are shifted toward the right and left, on the horizontal axis (axis of applied electric field) with La-doping. The magnitude of this shift is however different leading to the unsymmetrical shape of the P-E loop, which can be attributed to the presence of an internal bias field [30]. The remnant polarization strongly depends on La-doping concentration while Pm remains more or less unchanged. The gradual increase of La-doping in BTS ceramics results in narrowing of P-E hysteresis loops, which correspond to lesser energy loss [31]. This could be attributed to the existence of pinned polar nano-regions, leading to a slim P-E loop [32]. Discharge energy density (Jd ) of ceramic samples are calculated using the integral area of P-E loops curve and the y-axis [33, 34], as shown in the shaded area of Figure 6 (b). The en-
Figure 6: The room temperature polarization versus electric filed loop of (a) BTS (b) 1.5LBTS (c) 2.5LBTS (d) 3.5LBTS ceramic samples sintered at 1350◦ C/6hrs.; and (b) energy density calculation for 1.5LBTS sample.
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ergy density of dielectric materials is defined by the integral Equation: Jd =
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EdP
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where P is polarization and E is applied electric filed. The maximum discharge density at ∼ 40 kV/cm is found to be 0.492 J/cm3 for 3.5LBTS sample, which is 5.5 times more than that of pure BTS sample (0.09 J/cm3 ). The enhancement in energy density could be due to a decrease in the slope of P-E loops and reduced grain size achieved by La-doping. In addition, the increase in energy storage density could be attributed to large difference in the values of remnant polarization (Pr ) and maximum polarization (Pm ) i.e. (Pm -Pr is large). Apart from higher Jd value, larger energy efficiency (η) is also required in practical applications. The energy storage efficiency is calculated using following Equation [35]: η=
Jd × 100% Jd + Jc
(5)
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Based on the literature survey knowledge, P-E loop measurements and discharge energy density calculation are very limited for La-doped BTS ceramics. For example, Wang et al. reported the discharge energy density of 0.85BT-0.15BA ceramics system is 0.16 J/cm3 under an electric field of 50 kV/cm [36]. Puli et al. obtained the energy density of Ba(Zr0.30 Ti0.70 )O3 (BZT30) ceramics is 0.71 J/cm3 at a maximum electric field of 150 kV/cm [37]. In this study, a moderate filed value of ∼ 40 to 50 kV/cm is applied to the samples to investigate the ferroelectric loops to avoid the large leakage current. However in the present study, authors could not measured the leakage current. The leakage current for above mentioned field is found to be a few of mA [38]. From the above discussion, considering that 5
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ics. 3.5LBTS5 sample exhibits energy storage density as high as (Jd ) ∼ 0.492 J/cm3 with an energy efficiency of 63 % at ∼ 40 kV/cm, which is 5.5 times more than that of pure BTS ceramics. Such significant increment in energy storage density could be attributed to reduced grain size due to La-doping. Thus, the material selected in this study could be a new method for enhancing the energy storage density for device applications. Acknowledgments Dr. R. Kumar acknowledges Inter University Accelerator Centre, New Delhi for fellowship support (Project Ref. UFR58303) and research facilities. Authors thanks Dr. Satyendra Singh from Special Centre for Nano Sciences, Jawaharlal Nehru University, Delhi, for access to P-E loop and AIRF and DBT grant No. (BT/PR3130/INF/22/139/2011), JNU, New Delhi for access to SEM. B. B. acknowledges financial support via UGC startup grant and DST Purse grant.
the applied field was limited to 40 kV/cm, one can conclude that the samples in this study exhibit comparable or better energy storage density as reported in the literature.
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Figure 7: Discharge energy density as a function of applied electric field of BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS ceramics samples sintered at 1350◦ C/6hrs.
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The discharge energy density of current work is higher than that of 0.85BT-0.15BA ceramics [36] and lower than BZT30 ceramics [37]. The high energy density of BZT30 sample could be due to high applied electric field to the ceramic samples. Figure 7 shows the discharge energy densities as a function of applied electric field of Ba1−z Laz Ti0.95 Sn0.05 O3 with z=0.00, 0.015, 0.025, and 0.035 ceramic samples. The increase in discharge energy density is due to enhanced hysteresis caused by the occurrence of ferroelectric state at higher electric field [39]. One can see from the plot, the discharge energy density of 3.5 atomic % of La-doped sample is significantly high and ferroelectric hysteresis loops are very slim corresponding to less energy loss [30]. 3.5LBTS sample show the enhanced energy storage density of ∼ 0.492 J/cm3 with an efficiency of 63 % at ∼ 40 kV/cm. Thus, 3.5LBTS sample in the present work is a strong candidate for high energy storage applications. 4. Conclusions
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Lead-free Ba1−z Laz Ti0.95 Sn0.05 O3 with z=0.00, 0.015, 0.025, and 0.035 ceramic samples were prepared using solid state reaction route. X-ray diffraction study reveals single phase formation of ceramic samples. The scanning electron microscopy shows the inhibited grain growth (0.5 microns) at low La-doping (1.5 and 2.5 atomic %) while the higher La-doping in BTS ceramics yielded the larger grain size (2.5 microns). Raman peak at 838 cm−1 indicates Ti-vacancies and successful La-doping at Ba-site in BTS system. Dielectric constant and dielectric loss improved with an increase of La-doping in BTS ceramic samples. 1.5LBTS and 2.5LBTS samples showed a broad phase transition with negligible frequency dispersion indicating diffuse phase transition. Interestingly 3.5LBTS sample showed a sharp transition. Well-developed polarization-electric field (PE) hysteresis loops confirmed the ferroelectric nature of ceram6
R. Kumar et al / Materials Research Bulletin 00 (2019) 1–7 [15] J. Pokorny, U. Pasha, L. Ben, O. Thakur, D. Sinclair, I. Reaney, Use of Raman spectroscopy to determine the site occupancy of dopants in BaTiO3, Journal of Applied Physics, 109 (2011) 114110. [16] R. Kumar, K. Asokan, S. Patnaik, B. Birajdar, Evolution of microstructure and relaxor ferroelectric properties in (LazBa1 z)(Ti0. 80Sn0. 20) O3, Journal of Alloys and Compounds, 687 (2016) 197-203. [17] R. Kumar, K. Asokan, S. Patnaik, B. Birajdar, Combined effect of oxygen annealing and La-doping in broadening the phase transition of Ba (Zr 0.2 Ti 0.8) O3 ceramics, Journal of Alloys and Compounds, 737 (2018) 561567. [18] M. Deluca, L. Stoleriu, L.P. Curecheriu, N. Horchidan, A.C. Ianculescu, C. Galassi, L. Mitoseriu, High-field dielectric properties and Raman spectroscopic investigation of the ferroelectric-to-relaxor crossover in BaSn x Ti1x O3 ceramics, Journal of Applied Physics, 111 (2012) 084102. [19] I. Souza, L. Cavalcante, J. Sczancoski, F. Moura, C. Paiva-Santos, J.A. Varela, A.Z. Simes, E. Longo, Structural and dielectric properties of Ba0. 5Sr0. 5 (SnxTi1 x) O3 ceramics obtained by the soft chemical method, Journal of Alloys and Compounds, 477 (2009) 877-882. [20] S. Singh, P. Singh, O. Parkash, D. Kumar, Structural and relaxor behavior of (Ba1 xLax)(Ti0. 85Sn0. 15) O3 ceramics obtained by solid state reaction, Journal of Alloys and Compounds, 493 (2010) 522-528. [21] X. Chou, J. Zhai, X. Yao, Relaxor behavior and dielectric properties of La2O3-doped barium zirconium titanate ceramics for tunable device applications, Materials Chemistry and Physics, 109 (2008) 125-130. [22] U.D. Venkateswaran, V.M. Naik, R. Naik, High-pressure Raman studies of polycrystalline BaTiO 3, Physical Review B, 58 (1998) 14256. [23] A. Feteira, D.C. Sinclair, J. Kreisel, Average and local structure of (1 x) BaTiO3 xLaYO3 (0 x 0.50) ceramics, Journal of the American Ceramic Society, 93 (2010) 4174-4181. [24] W. Cai, C. Fu, J. Gao, Z. Lin, X. Deng, Effect of hafnium on the microstructure, dielectric and ferroelectric properties of Ba [Zr0. 2Ti0. 8] O3 ceramics, Ceramics International, 38 (2012) 3367-3375. [25] R. Newnham, Structure-property relations in ceramic capacitors, Journal of Materials Education, 5 (1983) 941-982. [26] F.D. Morrison, D.C. Sinclair, J.M. Skakle, A.R. West, Novel doping mechanism for veryhighpermittivity barium titanate ceramics, Journal of the American Ceramic Society, 81 (1998) 1957-1960. [27] F.D. Morrison, D.C. Sinclair, A.R. West, Characterization of lanthanumdoped barium titanate ceramics using impedance spectroscopy, Journal of the American Ceramic Society, 84 (2001) 531-538.. [28] A.R. West, T.B. Adams, F.D. Morrison, D.C. Sinclair, Novel high capacitance materials:-BaTiO3: La and CaCu3Ti4O12, Journal of the European Ceramic Society, 24 (2004) 1439-1448. [29] K. Uchino, S. Nomura, Critical exponents of the dielectric constants in diffused-phase-transition crystals, Ferroelectrics, 44 (1982) 55-61. [30] L. Jin, F. Li, S. Zhang, Decoding the fingerprint of ferroelectric loops: comprehension of the material properties and structures, Journal of the American Ceramic Society, 97 (2014) 1-27. [31] Y.H. Huang, Y.J. Wu, W.J. Qiu, J. Li, X.M. Chen, Enhanced energy storage density of Ba 0.4 Sr 0.6 TiO 3MgO composite prepared by spark plasma sintering, Journal of the European Ceramic Society, 35 (2015) 1469-1476. [32] W. Kleemann, The relaxor enigmacharge disorder and random fields in ferroelectrics, in: Frontiers of Ferroelectricity, Springer, 2006, pp. 129136. [33] A. Zeb, S.J. Milne, Temperature-stable dielectric properties from 20 C to 430 C in the system BaTiO3Bi (Mg0. 5Zr0. 5) O3, Journal of the European Ceramic Society, 34 (2014) 3159-3166. [34] S. Liu, S. Xue, S. Xiu, B. Shen, J. Zhai, Surface-modified Ba (Zr 0.3 Ti 0.7) O 3 nanofibers by polyvinyl pyrrolidone filler for poly (vinylidene fluoride) composites with enhanced dielectric constant and energy storage density, Scientific reports, 6 (2016) 26198. [35] B. Chu, X. Zhou, K. Ren, B. Neese, M. Lin, Q. Wang, F. Bauer, Q. Zhang, A dielectric polymer with high electric energy density and fast discharge speed, Science, 313 (2006) 334-336. [36] Y. Wang, Y. Pu, X. Li, H. Zheng, Structural evolution, relaxation behaviors and dielectric properties of BaTiO3BiAlO3 perovskite solid solutions, Journal of Materials Science: Materials in Electronics, 27 (2016) 1156511571. [37] V.S. Puli, A. Kumar, D.B. Chrisey, M. Tomozawa, J. Scott, R.S. Katiyar, Barium zirconate-titanate/barium calcium-titanate ceramics via solgel pro-
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cess: novel high-energy-density capacitors, Journal of Physics D: Applied Physics, 44 (2011) 395403. [38] Y. Pu, M. Yao, L. Zhang, P. Jing, High energy storage density of 0.55 Bi0. 5Na0. 5TiO3-0.45 Ba0. 85Ca0. 15Ti0. 9 xZr0. 1SnxO3 ceramics, Journal of Alloys and Compounds, 687 (2016) 689-695. [39] J.E. Daniels, W. Jo, J. Rdel, V. Honkimki, J.L. Jones, Electric-fieldinduced phase-change behavior in (Bi0. 5Na0.5) TiO3BaTiO3(K0. 5Na0. 5) NbO3: A combinatorial investigation, Acta Materialia, 58 (2010) 21032111.
7
PII:
S0025-5408(19)31679-4
DOI:
https://doi.org/10.1016/j.materresbull.2019.110694
Reference:
MRB 110694
To appear in:
Materials Research Bulletin
Received Date:
4 July 2019
Revised Date:
1 October 2019
Accepted Date:
12 November 2019
Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
M.R.B. Materials Research Bulletin 00 (2019) 1–7
Effect of La-doping on dielectric properties and energy storage density of lead-free Ba(Ti0.95Sn0.05)O3 ceramics R. Kumara,c,∗ , I. Singha , R. Meenab , K. Asokanb , Balaji Birajdara,∗∗ , S. Patnaikc,∗∗∗ Centre for Nano Sciences, Jawaharlal Nehru University, Delhi, India
b Material
Science Division, Inter University Accelerator Centre, Delhi, India.
c School
of Physical Sciences, Jawaharlal Nehru University, Delhi, India
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Abstract
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Lead-free Ba1−z Laz Ti0.95 Sn0.05 O3 with z=0.00, 0.015, 0.025, and 0.035 samples were prepared using solid-state reaction route. X-ray diffraction measurement reveals the single phase formation of ceramic samples. Scanning electron microscopy shows the inhibited grain growth (≤ 0.5 microns) at low La-doping while the higher La-doping in BTS ceramics yielded the larger grain size (2.5 microns). Raman spectra indicate the substitution of La at Ba site sublattice. Dielectric properties improved with La-doping in BTS ceramic samples. 3.5LBTS sample exhibits energy storage density as high as (Jd ) ∼ 0.492 J/cm3 with an energy efficiency of 63 % at ∼ 40 kV/cm, which is 5.5 times more than that of pure BTS ceramics. Such significant increment in energy storage density could be attributed to reduced grain size due to La-doping. Thus, the selected composition could be a new method for enhancing the energy storage density for device applications.
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Keywords: Lead-free materials; La-doping; microstructure; dielectric properties; energy storage density
1. Introduction
considerable attention as a energy storage materials in place of Pb-based materials because of environment concern [12, 13]. Here in the present work, we study the structural, microstructural, ferroelectric, dielectric, and energy storage properties of lead-free Ba1−z Laz Ti0.95 Sn0.05 O3 with z=0.00, 0.015, 0.025, and 0.035 ceramic samples. Further, we investigated this composition from the viewpoint of energy storage applications. Significant achievements related to (i) significant improvement in dielectric constant and dielectric loss (ii) enhancement of energy storage density (iii) reduction in hysteresis loss are reported.
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The evolution of lead-free material with large electrical energy storage density is of significant importance for achieving the requirements of advanced power technologies [1]. Scientists have been searching the new dielectric materials for high energy storage capacitors [2–4]. The capacitor is a crucial component in electronic industries and has been continuously used for many practical applications such as pulse power and power conditioning technologies [5–9]. Ferroelectric capacitors are well-known for high dielectric permittivity, low loss characteristics, and high power density due to the fast discharge time that makes them suitable candidate for high storage energy density applications [10, 11]. Among various ferroelectric materials, BaTiO3 -based ceramic material is ideal candidate that exhibits relatively high dielectric permittivity and low loss characteristics but, these ceramics suffer from the significant drawback of low breakdown field strength, high hysteresis loss, limiting the high energy storage density for practical applications [4, 9]. The energy storage density of lead-free ceramics is much smaller than that of Pb-based materials due to smaller polarization. In the recent decades, lead-free materials have attracted
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2. Experimental procedure
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Lead-free Ba1−z Laz Ti0.95 Sn0.05 O3 with z=0.00, 0.015, 0.025, and 0.035 ceramic samples were fabricated using solid-state reaction route. BaCO3 (99%), La2 O3 (99%), TiO2 (99%) and SnO2 (99.9%) (Sigma-Aldrich) powders were used as the starting raw materials and mixed at ambient conditions. Powders were weighed using following formula in the range (z = 0.00 0.015, 0.025 and 0.035 abbreviated as BTS, 1.5LBTS, 2.5LBTS, and 3.5LBTS respectively). The compounds were ground (with acetone as a medium) for ∼ 15 hrs using agate mortar pestle. Mixed powders were pressed into a disk shape pellet of diameter ∼ 10 mm and thickness 1 mm using a pressure of 10 tons.
[email protected]
∗∗ [email protected],
and [email protected]∗∗∗
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The ceramic samples were calcined at ∼ 1250◦ C for 72 hrs. Calcined powders are reground again and pressed into disk shape pellets. Pure BTS sample was sintered at ∼ 1350◦ C for 2 hrs and La-doped BTS ceramics were sintered at ∼ 1350◦ C for 6 hrs. The relative density of BTS, 1.5LBTS, 2.5LBTS, and 3.5LBTS ceramic samples, calculated by measuring mass and volume of pellets, are 87, 60, 74, and 93 % of their theoretical values respectively. Single phase of samples were confirmed using X-ray diffraction (Model: MiniFlex600, Rigaku) using Cu-Kα radiation (λ=1.542 Å) in the diffraction angle range of 20 - 80◦ . The microstructural studies were carried out after gold coating on the as-grown surfaces using a scanning electron microscopy (Model: Zeiss Figure 1: (A) Room temperature XRD patterns of (a) BTS (b) 1.5LBTS EVO40). Raman spectra of ceramic samples were carried out at (c) 2.5LBTS and (d) 3.5LBTS ceramics sintered at 1350◦ C/6hrs; and (B) −1 room temperature in the wavenumber range of ∼ 40 cm - 950 shows the magnified view of XRD pattern in the range of 45 to 47◦ . cm−1 by using Raman spectrometer (Model: Enspectr R 532) with a wavelength of 532 nm. Before dielectric permittivity La3+ (1.32 Å) as compared to Ba2+ (1.61 Å). The similar peak and ferroelectric properties measurement, ceramic pellets were shifting patterns by partial substitution of lower ionic radius for polished to make smooth faces using sandpaper (# P2000) fola higher ionic radius was observed in La-doped BTS and BZT lowed by diamond lapping. The silver electrodes were prepared ceramics [16, 17]. The experimental accuracy of XRD limits by putting the silver paste on both faces of ceramic pellets. The its use to identify the mixed phases (T or C) of the materials dielectric properties measurement was performed in the freso that other techniques are required to resolve it. Raman quency range under ac voltage amplitude of 1 volt (∼ 1 kHz spectroscopy is a very versatile tool to probe the short range - 2 MHz) using precision LCR meter (Model: E4980A, Agilent order modes and also explain the mixed phases. [14, 18]. Technologies). The temperature was controlled using a proFigure 2 displays the grain structure of as-grown ceramic grammable temperature controller (Model: 325, LakeShore). pellets of BTS, 1.5LBTS, 2.5LBTS, and 3.5LBTS. Pure BTS Room temperature polarization versus electric field (P-E) hyssample (Figure 2 (a)) showed homogeneous, rounded and denteresis loops for all the compositions are measured at a fixed sified micro-structure consisting of 3-dimensional (3-D) large frequency of 50 Hz and applying maximum field of ∼ 40 kV/cm grains (grain size ∼ 5 to 30 microns). (Model: P-E Loop Tracer, Marine India). 3. Results and discussion
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Room temperature X-ray diffraction patterns after sintering of BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS ceramic samples are shown in Figure 1. X-ray diffraction study reveals the predominately single phase perovskite structure. The splitting of characteristic peaks (002)/(200) at ∼ 45◦ indicates that at room temperature pure BTS has a tetragonal structure (In agreement with Raman and dielectric properties data plot) and samples could be indexed to tetragonal structure (space group p4mm), while it turns into a single peak for Ladoped samples, suggesting a phase transformation (Figure 1 (B)). The Raman spectra shows similar results, the peak at 304 cm−1 characteristics peak of tetragonal barium titanate [14], diminishes with La-doping [15], and disappears for 3.5LBTS5 sample. Figure 1 shows dash lines corresponding to (111) reflection at peak position (2θ = 38.72◦ ) and corresponding to (002) reflection at peak position (2θ = 45◦ ) which is drawn as a guide to the eye to clearly identify the right shifting of peaks with La-doping. Peaks corresponding to (111) at 2θ ∼ 38.72◦ and (002) at 2θ ∼ 45◦ are clearly shifting towards higher angle side with increasing La concentration, which reveals the contraction of the lattice cell. This confirms the successful substitution of La3+ ions at Ba2+ ions in BTS lattice (Figure 1). The shifting in the diffraction peaks is expected due to smaller ionic radius of
Figure 2: SEM images on as-grown of (a) BTS (b) 1.5LBTS (c) 2.5LBTS (d) 3.5LBTS ceramic samples sintered at 1350◦ C/6hrs.
The larger grain size in pure BTS ceramic sample could be ascribed to enhanced mass transport phenomenon because of formation of oxygen vacancies. These oxygen vacancies in ceramics are known as a result of sintering at high temperatures (≥ 1350◦ C, due to oxygen loss) [19]. Subsequently, this mechanism helps in grain growth and densification of the ce2
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cm−1 and the sharp resonance dip at 178 cm−1 disappear, indicating a phase transformation from tetragonal to pseudo-cubic phase [14, 18, 23]. This indicates that at room temperature the tetragonal and cubic phases co-exist, volume fraction of cubic phases increases with increasing with La-doping. The most noticeable changes in this composition due to La substitution at Ba site in BTS ceramic sample is the appearance of a new peak at 838 cm−1 . The peak at 838 cm−1 is associated with the A1g stretching mode of TiO6 octahedra, which is the feature of polar nano regions [16, 17]. Pokorny et al. also observed the A1g mode at ∼ 830 cm−1 , and 838 cm−1 in Ca and La-doped BaTiO3 system, respectively [15]. Feteira et al. observed A1g mode at 823 cm−1 by the simultaneous incorporation of La3+ and Y3+ ions in (1-x)BaTiO3 -xLaYO3 system [23]. The discrepancy between X-ray diffraction data and Raman spectroscopy data can be explained in term of the difference in coherence length of two techniques. The Raman response is more sensitive to the short range (Unit cell level), while the information transfer by X-ray diffraction technique is averaged over 104 unit cells [18]. Thus, Raman spectroscopy is sensitive enough to display the small changes in crystal structure at the atomic scale while Xray diffraction technique provides the information of a complete unit cell. Figure 4 shows the temperature dependence of dielectric constant and loss tangent of BTS, 1.5LBTS, 2.5LBTS, and 3.5LBTS ceramic pellets. Dielectric data are acquired in the temperature range of 100 K - 500 K during the heating of ceramic pellets at the rate of 3 K/min in the frequency range of 1 kHz 2 MHz. For pure BTS ceramic sample, three structural phase transitions are observed; rhombohedral to orthorhombic (R-O) at Tr−o ∼ 256 K, orthogonal to tetragonal (O-T) at To−t ∼ 296 K and tetragonal to cubic (T-C) at Tt−c ∼ 350 K. These phase transitions in BTS (Figure 4 (a)) are similar to the one reported by Deluca et al. for BaTiO3 ceramic sample for low content of Sn [18]. For pure BTS sample, there is no frequency dispersion around dielectric maxima, indicating long-range ferroelectric order. On the other hand in La-doped samples, lowtemperature phase transitions are suppressed and dielectric maximum shifted to lower temperature, but the frequency dispersion is still negligible. The addition of La content (1.5 atomic %) in BTS, results into a single broadened peak at ∼ 250 K with enhanced dielectric constant, which resembles so-called diffuse phase transition. Interestingly, the magnitude of dielectric permittivity increases, while transition temperature is lowered with La-doping. In general, lowering of the transition temperature could be related to smaller grain size attained with La-doping [24]. The dielectric loss values is reduced with addition of La-doping compared to pure BTS sample (Figure 4 (a-d)). Much above the transition temperature (T > Tm ), there is an abrupt increase in the value of tan δ with increasing temperature which would be the direct consequence of higher electrical conductivity [25]. The dielectric constant of ceramic samples is observed to increased with La-doping. This could be attributed to distorted TiO6 octahedra due to Ti vacancies, which break long range order and assist in the reorientation of ferroelectric domain close
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ramics. From SEM micrographs, it is found that grain size for La-doped ceramics is much smaller than pure BTS ceramic sample. The 1.5LBTS ceramic pellet (Figure 2 (b)) reveals a dense microstructure with small grains of ∼ 0.5 to 1 micrometers. Similarly, the 2.5LBTS ceramic pellet (Figure 2 (c)) reveals denser microstructure with smaller grains of 0.25-0.50 microns. It has been reported that during the sintering process, overall diffusion rate is found to be reduced after the incorporation of La ions at Ba-site and it could suppress the oxygen vacancies [19, 20]. However, a further increase in La atomic % (3.5LBTS ceramic sample) reveals a micro-structure with grains of 0.5 to 2.5 microns. A systematic observation of microstructure formation of La-doped Ba(Ti0.8 Sn0.2 )O3 and La-doped Ba(Ti0.8 Zr0.2 )O3 ceramic samples has been discussed in details by Kumar et al. [16, 17]. Similar grain growth inhibition at low La-doping and enhanced grain growth at large La-doping in La x Ba1−x (Ti0.8 Zr0.2 )O3 system has been observed by Chou et al. [21]. Figure 3 shows the room temperature Raman spectra of asgrown ceramic pellets of BTS, 1.5LBTS, 2.5LBTS, and 3.5LBTS in the wavenumber range of 40 - 950 cm−1 . Raman spectroscopy characterization of as-grown samples is carried out for correlating structural and phase purity aspects. As seen in Figure 3, the Raman spectrum of pure BTS sample shows following dominant modes centered at 115, 180, 189, 253, 304, 515, 715 cm−1 and one anti-resonance dip at 178 cm−1 [15, 16].
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Figure 3: Room temperature Raman spectra of as-grown ceramic pellets of (a) BTS (b) 1.5LBTS (c) 2.5LBTS (d) 3.5LBTS sintered at 1350◦ C/6hrs.
The Raman spectrum of pure BTS ceramic sample shows the sharp silent mode at 304 cm−1 [E(TO+LO)] [14], and weak broadband at 715 cm−1 [E(LO)] which are usually considered as signatures of the tetragonal phase of BaTiO3 [22]. The peak at 253 cm−1 and 515 cm−1 [A1 (TO)] are strong, broad and asymmetric [15]. Several noticeable changes appear in the spectral region between 40 cm−1 - 350 cm−1 with an increase of Ladoping in BTS ceramics. For examples, the sharp resonance dip at ∼ 178 cm−1 and the intensity of the sharp band at 304 cm−1 starts to decrease at 1.5 atomic % of La-doping in BTS. Further, for 3.5LBTS sample, both the sharp band characteristic at 304 3
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Table 1: The summary of fitting parameters obtained from the plot of ln( 1 − 1m ) versus ln(T − T m ); Tm , m , ∆T m = Tm (1 MHz) - Tm (1 kHz), γ, C∗ (K), FWHM (K) and tan δRT at room temperature and at 100 kHz.
z Tm (K) C∗ (K) T 0 (K) m ∆T m (K) tan δRT γ FWHM (K)
Figure 4: Temperature dependence of dielectric constant and loss tangent of (a) BTS (b) 1.5LBTS (c) 2.5LBTS (d) 3.5LBTS ceramic samples sintered at 1350◦ C/6hrs.
0.0 350 1.48×104 354 1230 2 0.0549 1.1 43
0.015 290 9.82×105 310 3671 17 0.0516 1.46 125
0.025 259 3.31×106 265 4117 19 0.0381 1.59 137
0.035 250 5.88×105 250 5219 6 0.0085 1.24 120
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for relaxors. A linear relationship was observed for all ceramic samples in high-temperature regime (Above Tm ). The experimental data points were fitted using Equation (2), and value of m , C∗ (K), T0 (K), γ, Tm (K), ∆T m (K), FWHM (K), and tan δRT at 100 kHz are listed in Table 1 for all ceramic samples.
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to transition temperature at low La-doping levels resulting in improved dielectric properties. The results obtained in this study are well-matched with those obtained by Morrison et al. [26, 27] and West et al. [28]. In their reports, they found that samples annealed at 1350 - 1400◦ C shows the magnitude of dielectric permittivity increases up to 6 atomic % of La-doped Ba1−x La x Ti1−x/4 O3 beyond which decreasing trend is observed [26, 27]. The sharper dielectric peak close to 250 K in 3.5LBTS sample could be attributed to the larger grain size (Figure 2 (d)) in this sample in comparison to that of 1.5LBTS (Figure 2 (b)) and 2.5LBTS (Figure 2 (c)). Like in the present case, the full width of half maxima of 4 atomic % La-doped BaTiO3 was reported by Morrison et al. [26, 27] and West et al. [28] to be smaller than that of 2.5 atomic % La-doped BaTiO3 . In paraelectric regions, dielectric permittivity peak usually follows Curie-Weiss law at T > Tm : [29]
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(T − T 0 ) 1 = r C
(1)
where T0 is Curie temperature, r is dielectric constant, and C is the Curie constant. The temperature dependence of the inverse dielectric constant is plotted for selected frequency of 100 kHz for BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS ceramics ( Figure 5). It is observed that La-doped BTS ceramics show diffuse phase transition (Figure 4). The above feature can be understood qualitatively by analyzing the dielectric data using empirical formula known as the modified Curie-Weiss law: [29]
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Figure 5: The plot of inverse dielectric permittivity ( 1r ) at a selected frequency of 100 kHz as a function of temperature for (a) BTS (b) 1.5LBTS (c) 2.5LBTS (d) 3.5LBTS ceramics sintered at 1350◦ C/6hrs. Inset shows the corresponding temperature dependence of inverse dielectric constant plot of ln ( 1r − 1m ) versus ln (T − T m ).
1 1 (T − T m )γ − = r m C∗
The γ factor for pure BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS ceramics were observed to be 1.10, 1.46, 1.59, and 1.24 respectively. The value of γ = 1.1 for pure BTS sample indicates the normal ferroelectric behavior while La-doped samples revealed diffuse phase transition with negligible frequency dispersion. The calculated value of γ gradually increases and reaches a value 1.59 for 2.5LBTS sample, which indicates that the diffuseness in the phase transition (FWHM =137 K) increases with doping of La. This increase in diffuseness may be due to decrease in the degree of ordering as seen in the Raman spectra where the intensity of 304 cm−1 decreases with La-doping. The parameter ∆T m , which describe the deviation from Curie-
(2)
where γ is the degree of relaxation, Tm is the temperature corresponding to the m , and C∗ is modified Curie-Weiss constant. Inset in each panels in Figure 5 shows the plot of ln ( 1r − 1m ) versus ln (T-Tm ) at a selected frequency of 100 kHz for all samples, the slope of which determined the value of γ. The value of γ is assigned to quantify the characteristics of the phase transition. The value of γ lies in the range of 1 ≤ γ ≤ 2. The value of γ is assigned to be 1 for normal ferroelectric and 2 4
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Table 2: The summary of various P-E loop parameters; remnant polarization, 2Pr (µC/cm2 ), maximum polarization, 2Pm (µC/cm2 ), coercive electric field, 2Ec (kV/cm), Pm -Pr (µC/cm2 ), charge energy density, Jc (J/cm3 ), discharge energy density, Jd (J/cm3 ) and energy efficiency, η(%).
z 0.00 0.015 0.025 0.035
2Ec 12.4 10.2 9.46 3.63
2Pr 28.7 6.68 11.54 6.30
2Pm 47.8 29.0 45.2 47.5
Pm -Pr 9.5 11.16 16.83 20.5
Jc 1.15 0.31 0.57 0.29
Jd 0.092 0.165 0.210 0.492
η(%) 7.5 35 30 63
Weiss law, defined as: ∆T m = TCW − T m
(3)
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where TCW is temperature at which the dielectric constant begins to deviate from the Curie-Weiss law and Tm is the temperature of dielectric constant maxima (m ). The value of ∆T m is observed to be 2 K, 17 K, 19 K and 6 K for pure BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS, respectively (Table 1). It is deducible from Table 1 that the value of ∆T m first increases up to 2.5 atomic % of La-doping and then decreases. This indicates that 2.5 atomic % of La-doped sample has a highest diffuse phase transition among all samples. Figure 4 shows Tm is almost independent to applied frequency indicating that LBTS system does not show dielectric dispersion in the composition range studied. Therefore, it is concluded that the system is not a relaxor ferroelectrics in the composition range investigated. It has been well described in the literature that substitution of lower ionic radius La3+ (1.32 Å) ions in place of higher ionic radius Ba2+ (1.61 Å) introduce the Ti vacancies. This disorder enhances the random field, because of which it serve as a nucleation sites for polar nano regions [20, 21, 24]. Figure 6 represents the room temperature ferroelectric polarization versus electric field (P-E) loops of BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS ceramic pellets at a frequency of 50 Hz under an applied electric field of ∼ 40 kV/cm. The values of remnant polarization (2Pr ), maximum polarization (2Pm ), coercive electric field (2Ec ), Pm -Pr and discharge energy density (Jd ) for BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS are listed in Table 2 and discussed below. Pure BTS sample exhibits a well-develop ferroelectric polarization hysteresis loop. P-E loop plot indicates that both negative Ec and positive Ec are shifted toward the right and left, on the horizontal axis (axis of applied electric field) with La-doping. The magnitude of this shift is however different leading to the unsymmetrical shape of the P-E loop, which can be attributed to the presence of an internal bias field [30]. The remnant polarization strongly depends on La-doping concentration while Pm remains more or less unchanged. The gradual increase of La-doping in BTS ceramics results in narrowing of P-E hysteresis loops, which correspond to lesser energy loss [31]. This could be attributed to the existence of pinned polar nano-regions, leading to a slim P-E loop [32]. Discharge energy density (Jd ) of ceramic samples are calculated using the integral area of P-E loops curve and the y-axis [33, 34], as shown in the shaded area of Figure 6 (b). The en-
Figure 6: The room temperature polarization versus electric filed loop of (a) BTS (b) 1.5LBTS (c) 2.5LBTS (d) 3.5LBTS ceramic samples sintered at 1350◦ C/6hrs.; and (b) energy density calculation for 1.5LBTS sample.
-p
ergy density of dielectric materials is defined by the integral Equation: Jd =
ZP
EdP
(4)
re
0
ur
na
lP
where P is polarization and E is applied electric filed. The maximum discharge density at ∼ 40 kV/cm is found to be 0.492 J/cm3 for 3.5LBTS sample, which is 5.5 times more than that of pure BTS sample (0.09 J/cm3 ). The enhancement in energy density could be due to a decrease in the slope of P-E loops and reduced grain size achieved by La-doping. In addition, the increase in energy storage density could be attributed to large difference in the values of remnant polarization (Pr ) and maximum polarization (Pm ) i.e. (Pm -Pr is large). Apart from higher Jd value, larger energy efficiency (η) is also required in practical applications. The energy storage efficiency is calculated using following Equation [35]: η=
Jd × 100% Jd + Jc
(5)
Jo
Based on the literature survey knowledge, P-E loop measurements and discharge energy density calculation are very limited for La-doped BTS ceramics. For example, Wang et al. reported the discharge energy density of 0.85BT-0.15BA ceramics system is 0.16 J/cm3 under an electric field of 50 kV/cm [36]. Puli et al. obtained the energy density of Ba(Zr0.30 Ti0.70 )O3 (BZT30) ceramics is 0.71 J/cm3 at a maximum electric field of 150 kV/cm [37]. In this study, a moderate filed value of ∼ 40 to 50 kV/cm is applied to the samples to investigate the ferroelectric loops to avoid the large leakage current. However in the present study, authors could not measured the leakage current. The leakage current for above mentioned field is found to be a few of mA [38]. From the above discussion, considering that 5
R. Kumar et al / Materials Research Bulletin 00 (2019) 1–7
References
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ics. 3.5LBTS5 sample exhibits energy storage density as high as (Jd ) ∼ 0.492 J/cm3 with an energy efficiency of 63 % at ∼ 40 kV/cm, which is 5.5 times more than that of pure BTS ceramics. Such significant increment in energy storage density could be attributed to reduced grain size due to La-doping. Thus, the material selected in this study could be a new method for enhancing the energy storage density for device applications. Acknowledgments Dr. R. Kumar acknowledges Inter University Accelerator Centre, New Delhi for fellowship support (Project Ref. UFR58303) and research facilities. Authors thanks Dr. Satyendra Singh from Special Centre for Nano Sciences, Jawaharlal Nehru University, Delhi, for access to P-E loop and AIRF and DBT grant No. (BT/PR3130/INF/22/139/2011), JNU, New Delhi for access to SEM. B. B. acknowledges financial support via UGC startup grant and DST Purse grant.
the applied field was limited to 40 kV/cm, one can conclude that the samples in this study exhibit comparable or better energy storage density as reported in the literature.
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Figure 7: Discharge energy density as a function of applied electric field of BTS, 1.5LBTS, 2.5LBTS and 3.5LBTS ceramics samples sintered at 1350◦ C/6hrs.
ur
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re
-p
The discharge energy density of current work is higher than that of 0.85BT-0.15BA ceramics [36] and lower than BZT30 ceramics [37]. The high energy density of BZT30 sample could be due to high applied electric field to the ceramic samples. Figure 7 shows the discharge energy densities as a function of applied electric field of Ba1−z Laz Ti0.95 Sn0.05 O3 with z=0.00, 0.015, 0.025, and 0.035 ceramic samples. The increase in discharge energy density is due to enhanced hysteresis caused by the occurrence of ferroelectric state at higher electric field [39]. One can see from the plot, the discharge energy density of 3.5 atomic % of La-doped sample is significantly high and ferroelectric hysteresis loops are very slim corresponding to less energy loss [30]. 3.5LBTS sample show the enhanced energy storage density of ∼ 0.492 J/cm3 with an efficiency of 63 % at ∼ 40 kV/cm. Thus, 3.5LBTS sample in the present work is a strong candidate for high energy storage applications. 4. Conclusions
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Lead-free Ba1−z Laz Ti0.95 Sn0.05 O3 with z=0.00, 0.015, 0.025, and 0.035 ceramic samples were prepared using solid state reaction route. X-ray diffraction study reveals single phase formation of ceramic samples. The scanning electron microscopy shows the inhibited grain growth (0.5 microns) at low La-doping (1.5 and 2.5 atomic %) while the higher La-doping in BTS ceramics yielded the larger grain size (2.5 microns). Raman peak at 838 cm−1 indicates Ti-vacancies and successful La-doping at Ba-site in BTS system. Dielectric constant and dielectric loss improved with an increase of La-doping in BTS ceramic samples. 1.5LBTS and 2.5LBTS samples showed a broad phase transition with negligible frequency dispersion indicating diffuse phase transition. Interestingly 3.5LBTS sample showed a sharp transition. Well-developed polarization-electric field (PE) hysteresis loops confirmed the ferroelectric nature of ceram6
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