Scholarly article on topic 'Enhanced energy storage performance of glass added 0.715Bi0.5Na0.5TiO3-0.065BaTiO3-0.22SrTiO3 ferroelectric ceramics'

Enhanced energy storage performance of glass added 0.715Bi0.5Na0.5TiO3-0.065BaTiO3-0.22SrTiO3 ferroelectric ceramics Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Satyanarayan Patel, Aditya Chauhan, Rahul Vaish, P. Thomas

Abstract In the present work, lead-free ferroelectric 0.715Bi0.5Na0.5TiO3-0.065BaTiO3-0.22SrTiO3 (BNT-BT-ST) bulk ceramics with 3BaO-3TiO2-B2O3 (BTBO) glass additive were fabricated by conventional solid state reaction route. The effect of glass content on microstructure and energy storage properties of BNT-BT-ST ceramics was investigated. The maximum energy storage of ∼203kJ/m3 was achieved for BNT-BT-ST ceramic with addition of 4wt.% glass. The 4wt.% glass addition improves energy storage density and energy storage efficiency by ∼15% and ∼52% higher than that of the pure BNT-BT-ST, respectively. The effect of temperature on the energy storage was also estimated. It was observed the temperature has similar effect on energy storage improvement in all compositions. The energy storage density (U) dependent scaling behavior on remnant polarization (P r ), maximum polarization (P max ), electric field (E) and temperature (T) was also studied. The results of this study are expected to largely benefit the field of ferroelectric based capacitors in discerning the dependency of U on hysteresis parameters (P r , P max , and E) and T.

Academic research paper on topic "Enhanced energy storage performance of glass added 0.715Bi0.5Na0.5TiO3-0.065BaTiO3-0.22SrTiO3 ferroelectric ceramics"

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Enhanced energy storage performance of glass added 0.715Bio.5Nao.5TiO3-0.065BaTiO3-0.22SrTiO3 ferroelectric ceramics

Satyanarayan Patela, Aditya Chauhana, Rahul Vaish3 *, P. Thomasb

a School of Engineering, Indian Institute of Technology Mandi, 175 001 Himachal Pradesh, India b Dielectric Materials Division, Central Power Research Institute, Bangalore 560 080, India

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Article history: Received 29 March 2015 Received in revised form 19July 2015 Accepted 29 July 2015 Available online 19 August 2015

Keywords: Energy storage Glass addition Lead-free ferroelectric Scaling

ABSTRACT

In the present work, lead-free ferroelectric 0.715Bi0.5Na0.5Ti03 -0.065BaTi03 -0.22SrTi03 (BNT-BT-ST) bulk ceramics with 3Ba0-3Ti02-B203 (BTBO) glass additive were fabricated by conventional solid state reaction route. The effect of glass content on microstructure and energy storage properties of BNT-BT-ST ceramics was investigated. The maximum energy storage of ~203 kJ/m3 was achieved for BNT-BT-ST ceramic with addition of 4wt.% glass. The 4wt.% glass addition improves energy storage density and energy storage efficiency by ~15% and ~52% higher than that of the pure BNT-BT-ST, respectively. The effect of temperature on the energy storage was also estimated. It was observed the temperature has similar effect on energy storage improvement in all compositions. The energy storage density (U) dependent scaling behavior on remnant polarization (Pr), maximum polarization (Pmax), electric field (E) and temperature (T) was also studied. The results of this study are expected to largely benefit the field of ferroelectric based capacitors in discerning the dependency of U on hysteresis parameters (Pr, Pmax, and E) and T.

© 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by

Elsevier B.V. All rights reserved.

1. Introduction

Power electronics are widely used in a number of applications such as power distribution, spacecraft, transportation, weapons, X-ray and medical devices [1-5]. In these applications, all the components are used for power conditioning in pulsed circuit and capacitors are indispensable components of pulse power devices [4,6,7]. Capacitors are used to deliver large amount of energy in a very short time. However, owing to the low energy density, a capacitor forms the major bulk of the any device [5,8]. Hence, compact size and lightweight capacitor are desirable with accompanying high energy storage densities. High energy storage capacity can be achieved by using materials with high dielectric constant along with large electric breakdown strength [4,7,9]. Usually high dielectric constant is associated with ferroelectric materials due to dipolar polarizations and hence these materials are extensively investigated for electrostatic capacitors [4,9,10]. However, these materials have lower electric breakdown strength. Hundreds of materials of this family have already been explored for such appli-

* Corresponding author. Tel.: +91 1905 237921; fax: +91 1905 237945. E-mail address: rahul@iitmandi.ac.in (R. Vaish).

Peer review under responsibility ofThe Ceramic Society ofJapan and the Korean Ceramic Society.

cations. These include the conventional lead-based and the novel lead-free ceramics such as Bi0.5Na0.5TiO3, K0.5Na0.5NbO3 (KNN), BaTiO3 (BT), Bi0.5Na0.5TiO3-BaTiO3 (BNT-BT) and their solid solutions [11-16]. Other materials' properties, which are important for capacitor applications, are low dielectric loss and low piezoelectric constant. Recently, a number of studies have reported that the dielectric breakdown strength of BT and barium strontium titanate ceramics can be improved by addition of glass [6,7,9,17]. The results indicate that higher energy densities are thus possible without altering the material composition. We have performed materials selection studies and reported that BNT family is superior for capacitor applications [18,19].These materials are generally used in polycrystalline form and have low electric breakdown strength due electrically fragile grain boundaries. Few studies have reported negative statistical correlation between dielectric constant and electric breakdown strength in ferroelectric ceramics [20-22]. Hence, it is important to attempt to increase electricbreak-down strength while retaining large dielectric constant. One of the handy solutions in this direction is incorporation of glass additives in ferroelectric matrix. Glasses have been reportedly added to ferroelectric ceramics in order to facilitate their reduced sintering temperature using viscous sintering [5,7,9,23]. Glasses have very high electric breakdown strength and careful selection and addition of glasses in ferroelectric ceramics could possibly enhance their energy storage properties [17,23-25]. This is especially

http://dx.doi.org/10.1016/jjascer.2015.07.004

2187-0764 © 2015 The Ceramic Society ofJapan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.

effective if glasses reside in grain boundaries, as they then play active role in fortifying electric breakdown strength. However, glass addition can also drastically affect shape and size of grains in the ceramics which affects the ferroelectric characteristics such as maximum and remnant polarization and coercive field values [4,6,7,9,23,25]. The present study deals with energy density enhancement by means of glass addition in ferroelectric compositions.

Bio.5Nao.5TiO3-BaTiO3-SrTiO3 is a lead-free ferroelectric composition which has been extensively studied by a number of researchers [24,26,27]. It possesses a morphotropic phase boundary (MPB) for (1 - x)Bi05Na05TiO3-0.065BaTiO3-xSrTiO3 compositions. Pure BNT has almost square shaped hysteresis loops which shrinks with addition of SrTiO3 (ST) [24,26,27]. We have selected 0.715Bi0 5Na0 5TiO3-0.065BaTiO3-0.22SrTiO3 (BNT-BT-ST) composition for present investigation as it has been reported for its high dielectric constant while possessing low hysteresis. Thus, it forms an ideal candidate for enhancement of energy storage density. In this work, we have also evaluated the energy storage density (U) scaling behavior dependence on remnant polarization (Pr), maximum polarization (Pmax), electric field (E) and temperature (T).

2. Materials and method

Glass composition of 3BaO-3TiO2-B2O3 (BTBO) was used in the present study. It was fabricated via conventional melt-quench technique. For this purpose, reagent grade chemicals (99.8% pure) BaCO3, TiO2 and H3BO3 were used as starting materials. The raw powders were weighed according to their stoichiometric ratio (in moles) and melted in a Pt crucible at 1300°C for 1 h. The melt was quenched between stainless plates to obtain transparent glass samples [25]. The amorphous nature of as-quenched glass was confirmed using X-ray powder diffraction (XRD) at room temperature.

0.715Bi0 5Na0 5TiO3-0.065BaTiO3-0.22SrTiO3 (BNT-BT-ST) composition was fabricated using solid state reaction route. Reagent grade (99.8% pure) powders of Bi(OH)3, Na2CO3, TiO2, BaCO3 and SrCO3 were used as starting raw materials. The powders were weighed in stoichiometric proportions and thoroughly mixed and calcined at 900 °C for 2.5 h. Glass was added in calcined BNT-BT-ST powder with 2, 4, 6, 10% (by weight) and mixed thoroughly using mortar pestle. The mixture was then added with 2 wt.% polyvinyl alcohol (PVA) binder and subsequently pressed into the green pellet disks of size 12 mm x 1 mm (diameter x thickness). The pure samples (without glass addition) were sintered at 1200 °C for 2.5 h, while glass containing samples were sintered at 1150 °C for 2.5 h.To minimize evaporation of volatile elements, samples were embedded within the powder of same composition. Silver paste was coated on the circular faces of sintered pellets after proper grinding and polishing, to form electrodes. X-ray diffraction patterns (XRD) were recorded in order to confirm single phase materials at room temperature. Density of all sintered samples was measured using Archimedes principle. The polarization versus electric field (P-E) hysteresis loops were obtained for evaluating energy density characteristics of fabricated samples using a modified Sawyer-Tower circuit at different magnitude of electric field and temperatures. The electrical measurements were carried out as a function of frequency using an impedance analyzer.

3. Results and discussion

Fig. 1 shows XRD patterns of sintered pellets of BNT-BT-ST ceramics with BTBO glass (2,4, 6,10% by weight) addition. It can be clearly observed from Fig. 1 that all compositions under consideration have formed a proper single phase perovskite structure. The

-°0/^lassJ I 1 I . I A _i_i_,_i_,_i_,_i_i_i_i_i_,_i_

10 20 30 40 50 60 70 80

29 (")

Fig. 1. XRD patterns for the BNT-BT-ST ceramics with different glass addition. The inset shows XRD patterns for as-quench glass.

nature of XRD indicates that upon addition of 2% glass also reaction can take place between BNT-BT-ST and glass during the sintering process. However, we are unable to detect the exact nature of chemical reaction due to limitation of XRD. However, in the 4 and 6% glass added composition two small unidentified peaks are observed which confirms chemical reaction between glass and ceramics. BNT-BT-ST with 10% glass addition shows a number of extra peaks indicating the presence of large amount of unknown phases. Thus, it can be concluded that more than 6% of BTBO glass addition in BNT-BT-ST induced largely unknown phases. Therefore, in this work up to 6% glass added compositions have been used for analysis. Furthermore, the inset of Fig. 1 shows XRD of as-quenched glass at room temperature. It indicates that no peak is observed which confirms the amorphous nature of glass. For the present work BTBO (3BaO-3TiO2-B2O3) glass composition was selected for two reasons. First, the chemical composition of glass is similar to that of ceramic composition. Secondly, it has been reported for high dielectric constant, low loss and a frequency invariant behavior [28,29]. Moreover, recently our group has published an article on the effect of BTBO glass addition for improving energy storage density in BaTiO3-V2O5 ferroelectric material [25]. Therefore, it is selected as a suitable glass composition for addition into BNT-BT-ST ceramic. Furthermore, recently our group has also published an article on temperature dependent scaling behavior of the dynamic hysteresis in BNT-BT-ST, whereas the current work deals with the glass added energy storage scaling behavior in the under-study compositions [30].

Microstructural homogeneity and characteristics of sintered pellets were observed by scanning electron micrograph (SEM). Fig. 2 shows the SEM images of 0,2,4 and 6% (BTBO) added BNT-BT-ST compositions. It is observed that as the glass content increases, grain size increases notably and the size distribution is observed to be more uniform. As shown in Fig. 2(a) pure BNT-BT-ST has small grain size and uneven distribution; however, the grain size increases with increasing glass content (Fig. 2(b) and (c)). However, at 6 wt.% glass addition, melting of the ceramic is observed which leads to unobvious grain boundaries (Fig. 2(d)). This effect of glass addition on grain size has been previously reported in the literature [31,32]. The glassy phase is usually located at grain boundaries as depicted in Fig. 2(b) and (c). The amorphous nature of the glassy phase can be confirmed by XRD (absence of extra peaks) as shown

(d) 6% glass

20 [_ii ————■

8/3/2014 I dwell j HV 1 pressure mag ffl spot I WD I det | - 20 pm -

4:21:24 PM 20 us 20.00 kV 1.00e-2 Pa 2 500 x 3.5 5.1mm ETD IfTMandi

in Fig. 1. It is important to mention that grain size is known to affect the piezoelectric, ferroelectric and dielectric properties.

Room temperature polarization-electric field hysteresis (P-E) loops of pure BNT-BT-ST and glass-added BNT-BT-ST samples are depicted in Fig. 3. The measurement frequency was kept constant at 50 Hz. Fig. 3 clearly shows that the hysteresis loop parameters decrease with increasing glass content. Pure BNT-BT-ST has a remnant polarization (Pr) of 8.15 ^C/cm2 which reduces significantly with 2, 4, 6% glass-addition to 7.03, 5.69 and 5.42 ^C/cm2, respectively. The reduction in remnant polarization is also accompanied with the decrease in maximum polarization (Pmax) and coercive

field. Therefore, to achieve an optimum polarization (Pr and Pmax) an appropriate amount of glass addition is the key factor [25].

Fig. 4 shows the P-E hysteresis loops for 0, 2, 4 and 6% compositions under the operating temperatures range of 25 °C to 125 °C. It clearly indicates that as the temperature increases Pr and Pmax decrease according to Curie-Weiss law. Increasing temperature imparts thermal agitation to the pervoskite structure and increases the symmetry of the system which affects polarization and dielectric properties. However, it is observed that glass addition improves the thermal stability of ferroelectric response as compared to pure ceramic. Addition of glass can also be credited with producing domain pining effect due to the difference in thermal expansion coefficient of glass and ceramic. Heating of the material generates internal stress between glass and ceramic phases at micro-level which ultimately leads to domain pinning and reduces Pmax. Further, these stresses are compressive in nature which helps to counter thermal depolarization by maintaining structural integrity. However, with increase in temperature a simultaneous reduction of remnant polarization, coercive electric field and hysteresis losses is also observed in all the compositions under study.

In order to compare the energy storage performance of BNT-BT-ST ceramics with different glass contents, the energy storage density has been calculated by using data generated from P-E loops and represented in Figs. 5 and 6. In general, the electrical energy storage capacity in ferroelectric materials can be estimated by integrating the area between the discharge curve of P-E loop and the polarization, within the interval of Pr to Pmax. The inset of Fig. 4(a) shows the area for the estimation of energy storage from P-E hysteresis loops. This can be mathematically expressed as [33-35]:

Fig.3. The room-temperature P-E hysteresis loops for different BNT-BT-STceramics compositions.

Here, U is the electrical energy storage density; E is the applied external electric field, Pr and Pmax are the remnant and maximum

-40 -30 -20 -10 0 10 20 30 40 -40 -30 -20 -10 0 10 20 30 40

Electric field (kV/cm) Electric field (kV/cm)

Fig. 4. P-E hysteresis loops as a function of temperature for (a) 0wt.%, (b) 2wt.%, (c) 4 wt.%, and (d) 6wt.% glass containing samples. The inset of figure (a) shows schematic of energy storage and loss.

polarization, respectively. However, Eq. (1) represents only the recoverable energy density, when the integration is done with respect to discharging curve. Furthermore, it can be stated that (from Eq. (1)), a high dielectric breakdown strength of material increases the energy density. The energy required to charge a

ferroelectric capacitor is a combination of the recoverable (U) energy and hysteresis losses (Uloss). The losses or unused energy density (Uloss) is defined as the area enclosed by charging and discharging curves (numerical integration of closed area under the

Fig. 5. Energy storage density and energy storage efficiency of BNT-BT-ST ceramics as a function of glass content.

Fig. 6. Temperature dependences energy storage density with different glass additions.

hysteresis loop). Within the inset of Fig. 4(a), hatched area shows recoverable energy storage (U), whereas the area enclosed by the charging and discharging curves (hysteresis loop) displays losses (Uioss). Dielectric losses are generally manifested in the form of self-heating or piezoelectric noise.Thus, total energy (Utotai) required to charge a capacitor can be given as [33-35]:

Utotai = U + Uloss (2)

Further, electrical energy storage efficiency of the material can be evaluated by [33-35]:

It is clear from Eq. (3) that small value of ^ could be due to large hysteresis losses. Therefore, it can be improved by reducing the losses. The electrical energy storage density and storage efficiency calculated from Eqs. (1) and (3), for all compositions under study, have been plotted in Fig. 5. It is evident from Fig. 5 that the recoverable energy storage density first improves and then decreases with increasing glass content. Glass addition can be credited with increasing domain pining effect and consequently reduces Pr and Pmax ultimately resulting in a higher energy density. Among all the samples, 4wt.% glass sample possesses highest U of ~203kJ/m3, which is 15% higher than that of pure BNT-BT-ST ceramic. Consecutively, it also increases the energy storage efficiency by ~52% as compared to pure BNT-BT-ST. Addition of glass can also increase the dielectric breakdown strength which can be used to further improve the energy density [36]. However, this has not been investigated for the current study. Addition of glass is also credited with lowering the leakage current which helps to improve the energy efficiency.

Subsequently, the temperature dependent energy storage characteristics were also investigated and reported in Fig. 6. It is to be noted that as the temperature increases, energy storage also improves for all the compositions under study. This is due to the fact that Pr and Pmax decrease rapidly in pure BNT-BT-ST as compared to glass-added compositions (shown in Fig. 5). This behavior can be attributed to buildup of internal stress or domain pinning as discussed in above sections. Hence, the amount of energy storage improves with temperature. The improvement of energy storage is -20% in pure BNT-BT-ST and 15% in 4wt.% glass sample. However, in most of the devices maintaining a constant temperature is not feasible. It is also to be noted that with temperature energy storage efficiency also improves. Higher temperatures reduce the reversible part of polarization which ultimately leads to decrease in hysteresis [21,24,27,37,38].

Finally, an analysis of scaling of energy storage (U) versus Pr and Pmax for all the compositions was done. The results have been plotted in Fig. 7(a) and (b). In order to determine the scaling parameters, curves were plotted between logarithmic form of (U) versus Pr and Pmax. In Fig. 7(a) and (b), all the data for electric field of 17.5-35 kV/cm, a constant temperature and frequency of 25 °C and 50 Hz, respectively have been reported. Fig. 7(a) clearly indicates the U versus Pr possesses an increasing trend for all compositions. The dependence of U on ln Pr can be fitted very well by the linear least square-fitting method (with R2 —0.96-0.99). For simplification, the scaling relation between (U) versus Pr can be expressed as U a Pp, where n is estimated from slope of ln U/lnPr. The value of n is given in Table 1. It clearly indicates that energy storage in pure BNT-BT-ST is strongly dependent on Pr as compared to other compositions. Pure BNT-BT-ST is the most sensitive to changes in Pr with the maximum variation in U being observed to be almost 2-4 times higher than other compositions. However, 6% glass composition shows least dependence (almost linear) with respect to Pr. It can thus be concluded that a small reduction in Pr can significantly increase energy storage density in BNT-BT-ST. Therefore,

; (a) . • A ▼ 0% glass 2% glass 4% glass 6% glass , A /' A / •/' A

, 1 , 1 , 1 1 1 , 1 , 1 ..... ........ , i . i

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 In P (In nC/cm2)

5.6 6.4

^ 5.0 ™E

— 4.6 D

4.4 4.2 4.0 3.8

- (b) ■ 0% glass yk.

• 2% glass A

A 4% glass

T 6% glass m /

- /Î /

2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 In Pmax (In nC/cm2)

Fig. 7. The energy storage density (U) dependency of BNT-BT-ST ceramic compositions at 25 °C with (a) remnant polarization (Pr) and (b) saturation polarization (Pmax) under the applied electric field of 17.5-35 kV/cm.

this scaling can be employed for quantification of energy storage dependency as a function of Pr. Similar calculations were also performed for U versus Pmax (shown in Fig. 7(b)). The relation between them can be expressed as U a Pmax> where m is estimated from slope of ln U/ln Pmax as listed in Table 1. Energy density of pure BNT-BT-ST again strongly depends on Pmax as compared to other compositions. In Fig. 7(a) and (b), symbols n and m denote the slopes for plots of energy storage (U) versus Pr and Pmax, respectively. It can be observed that n and m are almost similar for glass added compositions. Therefore, it can be assessed that U is strongly dependent on Pmax in samples containing 2% and 6% glass, whereas sample with 4% glass has a U dependency almost similar for both Pr and Pmax. However, for pure ceramics (without glass), n is almost 2 times larger than the m, indicating that U varies rapidly with variation in Pr and Pmax in pure BNT-BT-ST as compared to glass-ceramic composites. This scaling parameter of n and m signifies the energy storage variation with hysteresis parameters. Moreover, n and m can be used to estimate the energy storage behavior of under study compositions.

Furthermore, investigations to discern the dependence of energy density (U) on electric field strength (E) and temperature (T) were also performed. The results for the same are shown in Fig. 8(a) and (b), respectively for all compositions. The plot for ln U versus lnE at 25 °C is depicted in Fig. 8(a). These plots can be fitted very well by employing linear least square-fitting method (with Adj. R2 ~ 0.97-0.98) indicating a very good linearity of the U-E curves. The scaling relation between U versus E can be expressed as UaEk where k is estimated from slope of ln U/lnE as given

Table 1

Energy storage density scaling exponents with linear least square-fitting (Adj. R2) for glass added BNT-BT-ST compositions.

Glass content (wt.%) U «Prn U « Pmax U « Ek U « T

n ~R2 m ~R2 k ~R2 l ~R2

0 4.35 0.97 2.43 0.98 2.12 0.98 0.10 0.94

2 1.76 0.98 1.88 0.99 1.84 0.98 0.10 0.93

4 2.09 0.98 1.87 0.98 1.76 0.97 0.10 0.94

6 1.06 0.99 1.83 0.99 1.84 098 0.14 0.97

in Table 1. Fig. 8(a) shows that as electric field increases energy storage also increases. It also indicates that U of pure BNT-BT-ST is highly dependent on E as compared to other samples. It can be said that E influences domain-domain interaction in pure BNT-BT-ST ceramic to a larger extent. However, glass-ceramic composites possess less inter-domain interaction which untimely lowers their dependence on E. Estimation of energy storage (U) dependent scaling behavior with respect to remnant polarization (Pr), maximum polarization (Pmax) has also been attempted. For this purpose the electric field (E) was varied between 17.5 and 35 kV/cm. It could then be inferred that the scaling is ultimately an effect of the applied electric field intensity.

In the ferroelectric ceramics, oxygen vacancy is an important key factor to control domain growth and shrinking. The oxygen vacancy is mobile and generally trapped at the ferroelectric domain wall which results in domain wall clamping [37,39,40]. Therefore, at lower electric fields trapped oxygen vacancies are high and domain switching rarely occurs due to suppression of the polarization. It is a result of domain wall pinning. However, under the

applied higher electric field, less oxygen vacancy occurs due to higher dipole movement which allows bigger domain switching by moving oxygen vacancies aside which get accumulated at other boundaries [37,39,40]. This is the main reason of increase in coercive electric field (Ec), Pr and Pmax as the electric field increases which contributes to increasing the energy storage density.

Fig. 8(b) indicates ln U versus ln T curves in the temperature range of 20-125 °C measured at 35kV/cm. In the same way U versus T can be expressed as U aTi, where l can be determined from ln U/ln T which is presented in Table 1, with good fitting. Fig. 8(b) also shows that T dependence of U is almost similar for all the compositions. Therefore, it can be concluded that temperature variation has similar effect on energy density for all compositions. Such scaling can be employed to help quantify the effect of remnant polarization (Pr), maximum polarization (Pmax), electric field (E) and temperature (T) on energy storage (U) in ferroelectric materials. Additionally, dielectric constant (e) and loss (tan 1) as a function of frequency have been measured at room temperature. Fig. 9(a) and (b) displays dielectric constant (e) and loss (tan 1)

5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0

ë- 5.2 =)

- (a) A

- • /

A ■ 0% glass

• 2% glass

T A 4% glass

■ 1 ....... ▼ I 6% glass ■

8 2.9 3.0 3.1 3.2 3.3 In E (In kV/cm) 3.4 3.5 3

A A A A

- A • ---- *-- * . ■ — * •

• ---—■ — ■

■ ▼ .................

- ■ 0% glass

.......T.............. T • 2% glass

i I I . I . I I A ▼ i 4% glass 6% glass I.I.

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 In T (In °C)

Fig. 8. The energy storage density dependency of glass-added compositions with (a) electric field (U-E) curves at 25 °Cand (b) temperature (U-T) curves at 35 kV/cm.

Fig. 9. (a) Relative dielectric permittivity and (b) dielectric loss plots of BNT-BT-ST ceramic compositions as a function of frequency.

respectively, for pure and glass added BNT-BT-ST ceramics. It indicates that dielectric constant decreases with increasing glass content. It is established that physical-confinement (compressive stress) can reduce dielectric constant due to domain pining [ 41-43]. In this context, a number of researchers have discussed this phenomenon in various ferroelectric and antiferroelectric materials using domain dynamics [44-47]. Similarly, glass also induces domain clamping/pining by means of internal-confinement. Hence, the dielectric constant is reduced with increasing glass content, as shown in Fig. 9(a). Similarly, dielectric loss also decreases with glass content, whereas addition of 4% glass increases dielectric loss as compared to pure BNT-BT-ST. Furthermore, dielectric constant and loss vary with frequency due to space-charge polarization contributions. The effect of frequency on dielectric constant and loss has been extensively studied by a number or researchers [ 17,27,44,48-51].

4. Conclusions

In this work, lead-free ferroelectric 0.715Bi0.5Na0.5TiO3-0.065BaTiO3-0.22SrTiO3 (BNT-BT-ST) bulk ceramics were fabricated with the addition of 3BaO-3TiO2-B2O3 (BTBO) glass to enhance the energy storage properties. A small secondary unidentified phase was detected in XRD indicating that some minor chemical reaction happened between the added glass and host ceramic during sintering process. It was found that addition of 4 wt.% glass improves energy storage density by ~15% as compared to pure BNT-BT-ST. Consecutively, it also increases the energy storage efficiency by ~52% higher than that of the pure BNT-BT-ST. The temperature dependent energy storage was also estimated which shows similar energy storage improvement behavior in all the compositions. The energy storage (U) dependency scaling behavior on remnant polarization (Pr), maximum polarization (Pmax), electric field (E) and temperature (T) was also evaluated.

Acknowledgments

One of the authors (Rahul Vaish) acknowledges support from the Indian National Science Academy (INSA), New Delhi, India, through a grant by the Department of Science and Technology (DST), New Delhi, under INSPIRE faculty award-2011 (ENG-01).

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