Room temperature lead-free relaxor antiferroelectric electroceramics for energy storage applications

Room temperature lead-free relaxor –

antiferroelectric electroceramics for energy storage applications ?

Hitesh Borkar,a V.N.Singh,a B.P.Singh,a M.Tomar,b Vinay Gupta c and Ashok Kumar *a

Round the globe,scienti ?c communities have been searching for new materials for “green ”energy,producing e ?ciently both high power as well as high energy density.Relaxor ferroelectrics (RFEs)have shown immense potential to achieve this goal.We report fabrication of [Na 0.42Bi 0.44Al 0.06Ba 0.08)TiO 3(NBAT –BT)],a lead-free-relaxor antiferroelectric ceramic,via a conventional solid-state reaction method.A small fraction of trivalent cations (Al 3+)doping at Na 1+/Bi 3+sites develop anti-polar phase in the ferroelectric matrix which in turn changes its functional properties.Rietveld re ?nement suggests the existence of both tetragonal and rhombohedral phases which is well supported by d -spacing values obtained in high resolution transmission electron microscopy (HRTEM)studies.Elemental analysis con ?rms the stoichiometry of the system and matches the starting composition well within the experimental uncertainty (?10%)of secondary electron microscopy (SEM)and HRTEM data.Raman spectra suggest the substitution of Al 3+cation at an A-site sublattice.Temperature-dependent dielectric spectra show frequency dependent dielectric dispersion near 80–110 C,high dielectric loss at high probing frequency,and a non-linear Vogel –Fulcher relation,substantiating the relaxor –antiferroelectric (r-AFE)nature of NBAT –BT.A second order di ?use anti/ferro-electric to paraelectric phase transition near 230–240 C was observed which follows a modi ?ed Curie –Weiss law.The energy density was calculated from polarization –electric ?eld (P –E )loops and dielectric –electric ?eld (3–E )plot.The values were in the range of 0.4–0.6J cm à3,which is reasonably good for bulk polar material.NBAT –BT shows a much thinner AFE hysteresis above its relaxor FE phase transition;that favors the enhanced energy storage capacity at elevated temperature in the depolarized paraelectric region.

Introduction

Nowadays green energy generation and storage materials and devices are a major growth area of research and technology.Dielectric capacitors are well-known for their high power density and discharge capability (time constant $m s).Among the dielectrics,ferroelectrics usually possess higher dielectric constants and fast discharge time ($ns)that make them suit-able candidates for high energy density storage applications.1,2At present,the energy density of dielectrics/ferroelectrics materials is poor compared to supercapacitors and the Li-ion batteries.3The low energy density of polar dielectrics is due to low breakdown eld,high dielectric saturation,square hyster-esis (dielectric loss)and large leakage current under the appli-cation of an electric eld.4,5Short range order (SRO)relaxors

embedded in an anti-ferroelectric matrix have an enormous potential in designing new polar dielectrics for high energy storage applications due to their slim polarization –electric eld (P –E )hysteresis.Anti-ferroelectrics have linear and non-linear hysteresis (in E )that yield the largest energy storage area in P –E hysteresis in any polar dielectrics.6This indicates that an anti-ferroelectric matrix embedded with polar-nano-regions (PNRs)(relaxor –antiferroelectric)are suitable candidates for moderate power and energy-density applications.Such a system may allow us to understand new capacitor materials having high dielectric constant and energy storage capacity.Lead-based relaxor antiferroelectric PLZT (lead –lanthanum –zirconate –tita-nate)thick lms show a high energy density (up to 58.1J cm à3at 2.8MV cm à1).However,the environmental issues and regulations to ban toxic materials by several countries are forcing the development of lead-free (non-toxic)research.7,8

The direct way to calculate the energy density and energy storage capacity per unit volume of material is as follows:

U ?DE d D ?D

E d P

(1)

a

CSIR-National Physical Laboratory,Dr.K.S.Krishnan Marg,New Delhi 110012,India.E-mail:ashok553@https://www.wendangku.net/doc/a015923407.html,

b Department of Physics,Miranda House,University of Delhi,Delhi 110007,India

c

Department of Physics and Astrophysics,University of Delhi,Delhi 110007,India

?Electronic supplementary information

(ESI)

available.

See

DOI:

10.1039/c4ra00094c

Cite this:RSC Adv.,2014,4,22840

Received 6th January 2014Accepted 3rd April 2014DOI:

10.1039/c4ra00094c https://www.wendangku.net/doc/a015923407.html,/advances

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where D ?30E +P ,since polarization of ferroelectric dielectrics is very high so D z P .

U ?12CV 2volume ?12330AE b 2t die 2die die ?1

2

330h 2E b 2?J cm à3

?(2)

where C is capacitance;E ,applied electric eld;P ,polarization;V ,voltage;3,dielectric constant of the material,30dielectric constant of vacuum,and E b breakdown eld.The dielectric is susceptible to breakdown under the application of constant high external eld.To avoid any premature breakdown of the capacitor,it is advisable to use an external eld almost half of the breakdown eld,i.e.,V ?h V b (0

The lead free (1àx )(Na 0.50Bi 0.50)TiO 3àx BaTiO 3(NBT –BT)system is well known for its excellent piezoelectric properties,several structural phase transitions,large variation in func-tional properties due to composition tunability,and coexistence of relaxor –polar and antipolar phases.10–14The NBT –BT system illustrates depolarization properties [that is,transition of ferroelectric FE to AFE-like phases]in the temperature range of 100to 190 C depending on the composition of BT near the morphotropic phase boundary (MPB).15–17The rhombohedral (FE)to high-temperature tetragonal di ?use phase transition (220–320 C)is achieved via an intermediate AFE-like phase transition.Ma et al.showed the presence of nano-size short-range polar-regions for the compositions (0.07#x #0.09)that lead to the relaxor –AFE phase boundary.Recently they also established a new phase boundary in NBT –BT measuring the precise positions of oxygen octahedral tilting in multi-domain perovskite ferroelectrics using electron di ?raction analysis.18Another aspect,non-stoichiometry,is equally important in determining piezoelectric properties and depolarization temperatures.Many research groups have extensively studied the non-stoichiometry –microstructure –property relation-ship.19–25In general,it has been accepted that Na +cation de -ciencies and excess Bi cations lead to a decrease in depolarization temperature and develop the AFE-like micro-polar regions in the ferroelectric matrix.The ratio of A-site cations (Na +and Bi 3+)was always considered as unity;however,in reality it is very hard to maintain such a ratio,due to the volatile nature of both Na-and Bi-cations.Guo et al.have carried out an extensive study on the compositional modulation of the NBT –BT system and have shown that an AFE-like phase can be induced at an ambient temperature.26

Lead-based relaxor/anti-ferroelectrics have been thoroughly investigated for high energy density applications and have shown a high energy storage capacity.27Polymer relaxor ferro-electric thin lms show a high energy density (10–25J cm à3)with a very fast discharge capacity.3Ceramic –polymer and ceramic –glass composite systems are also potential candidates for high energy density capacitors due to their high breakdown strength and low dielectric saturation.28–30Polar or non-polar multilayers and superlattice thin lms have been investigated by most material science techniques and have also shown tremendous potential in energy applications with a suitable design of the polar –nonpolar layers.3

In this paper,we report the e ?ect of Al 3+cation (valance state (+3)of A-site)substitution on the relaxor –AFE properties of

NBAT –BT.A minimum Al 3+cation percentage is incorporated in

the system to avoid any pyrochlore or impurity phase.The microstructure –property relationship is discussed in the context of its ability to store energy.

Experimental

Polycrystalline ceramic samples of (Na 0.42Bi 0.44Al 0.06Ba 0.08)TiO 3(initial precursor compositions (Na 0.43Bi 0.43Al 0.06)TiO 3–Ba 0.08TiO 3)were prepared by conventional solid state reaction techniques.The high purity ($99.9%)initial precursors Na 2CO 3,BaCO 3,Bi 2O 3,TiO 2and Al 2O 3(Sigma Aldrich)were mixed at ambient conditions and then homogenously blended with IPA (isopropyl alcohol),using agate mortar –pestle grinding for 2h.The mixed precursors were rst calcined at an optimized temperature of 1000 C for about 4h in a high purity alumina crucible.Calcined powder was reground and then mixed with a binder (polyvinyl alcohol)in order to prepare the circular disc-shaped pellets.The average diameter and thickness of the pellets were 13mm and 1–1.5mm,prepared under uniaxial pressure of 5–6tons per square inch.The prepared pellets were subsequently sintered at an optimized temperature of 1200 C for 8h to achieve the 92–95%of theoretical density.A room temperature X-ray di ?raction pattern was taken using CuK a

radiation (l ?1.5460?A,

Bruker D8Advance)over a wide range of Bragg angle (20

Grain growth,surface morphology,and elemental analysis on sintered pellets were carried out using a scanning electron microscope (SEM,Zeiss EVO MA-10).Crystal structure,atom positions in lattice planes,grain orientations,and size were determined by using a Technai G20-stwin,300kV,High Reso-lution Transmission Electron Microscope (HRTEM).To measure the electrical properties,metal –insulator –metal capacitor struc-tures were prepared using the silver paint coating on both sides of the ceramic pellets.Temperature-dependent dielectric measurements were carried out on metallized pellets at various frequencies (1kHz to 1MHz)and temperatures using an LCR meter (4200-SCS Analyzer)at an oscillating amplitude of 0.5V.Ferroelectric P –E hysteresis loops and leakage current were measured by using a Radiant Ferroelectric Tester.Room temperature Raman studies were carried out using a Renishaw inVia Re ex Raman spectrometer,UK (with an excitation source of 514.5nm)with a resolution of less than 1.0cm à1.

Results and discussion

A.

Crystal structure

Rietveld re nement on the unpoled X-ray di ?raction data were tested for the determination of the amount of both the tetrag-onal (space group P 4mm )and rhombohedral (space group R 3c )crystal structures as reported earlier for the MPB of the NBT –BT system.31(Na,Bi,Al,Ba)and Ti ions were added to A and B sites of ABO 3perovskite,respectively,using the occupancy constraint relationship n A2+?1àn B4+.The observed di ?raction peaks were simulated with a pseudo-Voigt pro le function.The normal procedures for the Rietveld re nement have been

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followed for the simulation of experimental X-ray data.Aniso-tropic values of atomic (thermal)displacement (Debye –Waller factor)parameters were obtained in the stable and best t.32The reliability factors for the rhombohedral crystal structure are R p ?9.43%,R wp ?12.1%,c 2?2.89,R exp ?6.98%,R Bragg ?

5.46%and lattice constants (a ? 5.5173?A,

b ? 5.5173?A,

c ?13.4722?A)

and for tetragonal are R p ?7.61%,R wp ?10.4%,c 2

?2.09,R exp ?6.97%,R Bragg ?3.94%,and lattice constants

(a ?3.8964?A,

b ?3.8964?A,

c ?3.9255?A).The reliability factors were in goo

d agreement with thos

e reported by Wook et al.31Fig.1(a)&(b)show a very good agreement between the experi-mental and simulated pro le data for the above mentioned reliability factor values.All the prominent Bragg peaks were successfully indexed for both the crystal structures.The incor-poration o

f a very small fraction of Al 3+cations hardly a ?ects the lattice parameters,and it is not easy to detect the elemental compositions and crystallographic positions with low resolu-tion XRD and HRTEM results.Fractional atomic coordinates,positions,occupancies and other tted parameters are pre-sented in Table 1.The d -spacin

g of di ?erent planes calculated using the HRTEM data matches well wit

h the tetragonal –rhombohedral crystal structure.The d

i ?erence in the d -spacing for both the systems was detectable only a er two decimal points of d -spacing data (<1%).It would be unwise to report the

exact lattice planes for either system with the given HRTEM image resolution.XRD analysis was carried out on unpoled-polycrystalline samples to obtain the ratio of both crystal pha-ses.The results obtained from the re ned intensity of the coexisting rhombohedral (r-phase)and tetragonal phases (t-phase)suggests that NBAT –BT is composed of 48(?5)%r-phase and 52(?5)%t-phase.The existence of twin boundaries and their separate phases can also be seen in the HRTEM image of the (110)plane with their fast Fourier transformation (FFT)as depicted in Fig.3.The intensities of both phases are slightly di ?erent from each other over a large area of the crystal surface con rming the presence of both phases and its distribution throughout the matrix.B.

Morphology,size and d -spacing of crystal

The surface morphology and grain growth can be seen from the SEM image (inset of Fig.1(a))of the as-grown pellets.They indicate ne square grains with an average size of 1–5m m.These grains are highly packed with little porosity.EDAX anal-ysis for the appropriate elemental composition was carried out on large areas of the pellets and on individual grains.A reliable matching elemental ratio was obtained between precursor and the nal product.The nal elemental ratio is given in Table 2.To better understand the microstructure –property relation,electron di ?raction patterns were taken on various sintered ceramic grains.These grains were obtained from mechanically milled powder of the sintered pellets.The ultrasonicated dispersions of ne particles were put on carbon-coated copper grids for HRTEM study.The TEM results from the representa-tive grains are shown in Fig.2(a)–(f).Fig.2(a)shows the ne nano-grains surrounded by larger grains >100nm.To check the real distribution of the ne grains,TEM imaging was carried out on di ?erent particles of the NBAT –BT.Each time it showed an uneven distribution of nanoparticles in the matrix.Fig.2(b)elucidates the small ordered regions (di ?erent contrast compared to the surroundings)in the matrix;which

may

Fig.1

(a)shows the Rietveld ?tting of NBAT –BT XRD patterns with

tetragonal crystal structure (space group P 4mm )and (b)rhombohedral crystal structure (space group R 3c );SEM image is given in the inset (a).

Table 1

Simulation data of the Rietveld analysis for two space groups

(R 3c and P 4mm )and crystal systems a ,b Phases

Atoms X Y Z Tetragonal P 4mm

Bi 0.000.00 4.27949Ba 0.000.00 4.65655Na 0.000.00 4.95192Al 0.000.00 4.95192Ti 0.500.50à5.35526O10.500.50 6.46282O20.500.00 5.58466Rhombohedral R 3c

Bi 0.000.000.63378Ba 0.000.000.63378Na 0.000.000.63378Al 0.000.000.63378Ti 0.000.000.39033O

à0.05406

1.46715

1.11697

a

Reliability factors are R p ?7.61%,R wp ?10.4%,c 2? 2.09,R exp ?6.97%and R Bragg ?3.94%.b Reliability factors are R p ?9.43%,R wp ?12.1%,c 2?2.89,R exp ?6.98%and R Bragg ?5.46%.

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represent the PNRs,responsible for large dielectric dispersion near room temperature.The d -spacing for both the crystal systems were imaged for di ?erent parts of the grains in HRTEM studies.Fig.2(c)–(f)represents di ?erent lattice planes of the NBAT –BT crystal that matched well with the d -spacing obtained for the tetragonal crystal structure from the X-ray data (Rietveld analysis).The observed planes for both crystal systems vary only in the third decimal.This is hard to distinguish form the present HRTEM image resolutions.However,one point is clear from all these TEM images:these planes are not single crys-talline in nature;they possess polycrystalline contrast,chemical inhomogeneities,discontinuity,disorder,and a mixture of long and short range ordering.These observations support the relaxor –AFE character of the samples.Therefore,a relaxor –AFE phase and the coexistence of a rhombohedral –tetragonal phase at room temperature are proposed near the MPB with substi-tution of a small amount of trivalent cations.Fig.3(a)and (b)shows the FFT and inverse FFT (IFFT)images of NBAT –BT poly-crystals.FFT and IFFT analysis were carried out on the large area HRTEM image which shows the co-existence of both the phases with a slight di ?erence in their crystallographic plane intensities and positions.The presence of both t-phase and r-phase (101)and (110)planes can be seen from the Fig.3,

respectively.Atomic contrast on various atomic planes and small regions with di ?erent microstructure intensity (circled on the TEM image)can be seen in Fig.3(b)suggesting the existence of chemical inhomogeneity in the systems supporting the microstructure –property relationship.HRTEM images also revealed that some crystal areas having a large number of twin boundaries promote the formation of twin phases (as high-lighted by the transparent lines).C.

Raman spectroscopy

Raman spectroscopy is a versatile tool for the detection of subtle structural distortions,local defects,compositional inhomoge-neity,and di ?erent ordering states in perovskite structures.Raman spectra of the NBAT –BT are analyzed using the damped harmonic oscillator model (DHO).33The observed vibrational modes are analyzed and tted with the spectral response function.

S en T?

X

i

c 0i G i n 0i 2n

20i 2i n 2

F en ;T T

(3)

where F (n ,T )?[n (n )+1](Stokes scattering)and n (n )?[exp(h n /k T )à1]à1.The parameters in eqn (1)amplitude c 0(in

Table 2

Stoichiometric details of the A-site elements for the NBAT –BT crystal system

Elements at A-site Percentage of initial elemental compositions Observed elemental compositions

from SEM &HRTEM (?10%uncertainty)Na 0.430.42Bi 0.430.44Ba 0.080.08Al

0.06

0.06

Fig.2

(a –f)HRTEM images of NBAT –BT crystal and lattice planes,(a)HRTEM images of NBAT –BT nano-and micro-crystals,(b)ordered PNRs in

the AFE matrix,(c –f)d -spacing of crystal planes (001),(110),(220),(111)respectively.

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arbitrary units),the mode frequency n 0,the damping constant G ,and the temperature T ,describe each phonon mode as a damped harmonic oscillator.Fig.4(a)–(d)show the Raman spectra and their DHO model tting over large frequency regions of NBAT –BT ceramics.The intensities of low frequency vibrational modes are six times higher than the high frequency modes that make it di ?cult for DHO to t all the Raman modes with a single frequency window.Group theory predicts 4A 1(IR,R)+9E(IR,R)optical modes for the rhombohedral R 3c phase,and 7(IR +R)+1R modes for the tetragonal P 4mm phase.34Fig.4(a)illustrates the Raman spectra of NBAT –BT electro-ceramics which is in accordance with the previous reported data on NBT –BT.35,36In general,Raman bands are relatively broad due to chemical inhomogeneity,overlapping di ?erent Raman vibrational modes,and chemical potential of four cations with di ?erent atomic mass and radii at the A-site sublattice.Fig.4(b)and (c)shows three Raman active modes at 116cm à1,153cm à1,and 183cm à1below the 200wavenumber.A low frequency 96cm à1mode was observed,however,it cannot be considered trustworthy due to the experimental limitations of the Renishaw

inVia Re ex Raman spectrometer.The variation in the intensity of the various peaks below 200cm à1could be a result of the substitution of Al(3+)at an A-site sublattice.The presence of four cations (Al/Na/Bi/Ba)causes di ?culty in analyzing the mass and radii e ?ects on low frequency Raman modes.The overlapped Raman spectra of the specimens is mainly due to the random distribution of Na,Al,Bi and Ba cations breaks the k ?0selection rule and permits phonons from the entire Bril-louin zone to become Raman active.Four Raman active modes were observed for the medium frequency range (250–550cm à1)which represent the oxygen ion vibration with di ?erent A and B-site cations and also due to dielectric leakage.36The high frequency Raman modes in the region of 600–900cm à1have been attributed to B-site sublattice (Ti –O –Ti stretching modes),in which the vibration around 600–700cm à1corresponds to edge shared octahedral,748cm à1to corner sharing octahedral,and at 805/859cm à1short Ti –O bonds in the distorted TiO 6octahedral.These bands match the vibration of pure NBT –BT ceramics near the morphotropic phase boundaries (MPB).The band positions of high frequency Raman modes indicate the negligible e ?ect of the Al doping at the B-site.Low frequency band positions and intensity indicate a substantial in uence of the Al cation at the A-site sublattice.D.

Dielectric and tangent loss spectroscopy

Fig.5(a)and (b)show the dielectric constant (3)and tangent loss (tan d )data as functions of temperature at di ?erent frequencies.Two well-de ned 3and tan d anomalies can be seen near 80–120 C and 240 C,respectively.The rst phase transition shows a well-de ned 3and tan d dispersion with a shi ing of the dielectric maxima temperatures (T m )towards higher tempera-tures with increase in frequency.The T m values of the dielectric spectra were tted with a nonlinear Vogel –Fulcher (VF)relation (eqn (3))as shown in the inset of Fig.5(d).37–39

f ?f 0exp à

àE a k B àT m àT f áá(4)where f is the experimental frequency;f 0,the pre-exponential

factor;E a ,the activation energy;k B ,the Boltzmann constant;and T f ,the static freezing temperature.The tted parameters E a ?0.42eV,f 0?5.48?107Hz,and T f ?307K match the numerical values for other relaxor systems within the limit of their uncertainties.37The second phase transition is a di ?used phase transition that ts well with the modi ed Curie –Weiss laws,as shown in Fig.5(c).37The value of the di ?usivity expo-nent,g $1.78indicates that the system has broad dielectric relaxation and a higher level of disorder.The degree of disorder reveals that dynamic PNRs persist far above the lower phase transition temperature.The tangent loss illustrates a second anomaly below the second phase transition temperature that is normal for di ?use-type FE to PE phase transitions.It follows a similar trend of frequency response as that of the dielectric constant;however,the degree of tangent loss dispersion is very high (170–200 C)compared to dielectric data near the 2nd phase transition.These observations favor high crystal disorder until the 2nd phase transition.One should note that the dielectric loss data and their dispersion with frequency

and

Fig.3

(a)FFT image of the (101)and (110)planes of the r-phase and

t-phase,(b)IFFT image of the same FFT showing the presence of twin phase boundaries and various

SROs.

Fig.4

(a –d)Raman spectra of NBAT –BT system with DHO model

?tting (b)low (c)medium (d)high frequency regions.

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temperature provide more reliable information than the dielectric constant values alone.The functional properties,variation in T m and dielectric loss depends on the probe frequency which is mainly due to the presence of short range ordering (SROs)such as PNRs or chemical inhomogeneity due to cation disordering in the long range order matrix.HRTEM images provide a clear picture that a large number of SROs,twin boundaries,discontinuous crystal planes,inhomogeneous distribution of A-site cations,etc.are present in the NBAT –BT matrix which in turn favors the dielectric dispersion and anti-polar properties.

E.Polarization behavior under applied electric eld

It has been described in previous studies that di ?erent kinds of dielectric material and their energy density capacity can be calculated from the P –E loop.4–8Among all dielectrics,relaxor –antiferroelectrics have shown the largest integrated area within their P –E loops to store energy e ?ciently.The relaxor –anti-ferroelectrics is the only system that possesses both nonlinear and linear P –E regions (except for temperatures slightly above T c in the rst-order ferroelectrics);the linear antiferroelectric low- eld region,the non-linear antiferroelectric region,and linear dielectric saturated region at extremely high eld.The combi-nation of nonlinear and linear P –E loops in the rst quadrant of the P (E )hysteresis is suitable for high energy storage devices with a fast discharge capacity.Fig.6(a)shows the slim relaxor –antiferroelectric hysteresis (as-grown samples)of the NBAT –BT system for a frequency of 1Hz over a wide range of temperature.P –E loops for 1and 10Hz are given in the inset of Fig.6(a)for a comparative study.These P –E loops become narrower and centered with an increase in temperature.

Polycrystalline

Fig.5

Dielectric spectra (a)and tangent loss (b)as functions of temperature in the frequency range of (1kHz –1MHz),(c)non-linear VF relation

?tting,(d)?tting of modi ?ed Curie –Weiss law near di ?use phase transition at 10

kHz.Fig.6

(a)P –E loop hysteresis at di ?erent temperatures and a

moderate applied electric ?eld (50kV cm à1),in inset,P –E loops at 1and 10kHz for comparison.(b)Leakage current as a function of the applied electric ?eld;right inset shows current density as a function of temperature.Shaded area (I)of P –E loop illustrates the capacity to store energy (left inset).Paper RSC Advances

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ceramic samples are very prone to electrical breakdown under high electric eld applications.We used a very moderate eld $50–60kV cm à1to investigate the P –E loops to avoid large leakage current.Leakage current was separately measured for the same electric elds and was found to be a few m A.The magnitudes of these currents were at least three orders less than the range of real breakdown current (ca.mA)for insulating ceramics.It can also be seen from Fig.5(b)that leakage current decreases with an increase in temperature which is a signature of the positive temperature coe ?cient of resistance (PTCR),extremely useful for power/energy based devices such as current-limiters.40

F.Energy density capacity

The linear –nonlinear part of the P –E loop provides an energy density of $0.4–0.6J cm à3at 50kV cm à1until 100 C then increasing with increase in temperature and persisting above the T m of the relaxor –AFE phase transition.The shaded area in the inset of the P –E loop (Fig.6(b))represents the energy stored by the system.The P –E loops become narrower with increase in temperature,suggesting a high potential to store energy density.The inset of Fig.6(b)shows two types (area I and II)of shaded areas (yellow and grey)that represent the charge and discharge https://www.wendangku.net/doc/a015923407.html,ing P –E data,the discharge (release)(U d ),and charge (U c )(storage)energy densities were calculated using the following relationship:U d ?area II,U c ?area I +area II.The charge –discharge e ?ciency (h )of the capacitors was calculated using the relationship:h ?U d /U c ?100%.The e ?-ciency of the NBAT –BT was 48%at 300K for an applied electric eld of 50kV cm à1.A similar range of e ?ciency was found for poly(vinylide uoride –hexa uoropropylene)(PVDF –HFP).41

Conclusions

In summary,relaxor-AFE lead-free NBAT –BT electroceramics were investigated for energy density applications.A small ($0.06%)substitution of Al 3+cations in the NBAT –BT system near the MPB leads to the development of short range order polar nano regions within an AFE matrix.XRD analysis eluci-dates the coexistence of mixed rhombohedral and tetragonal phases.HRTEM images suggest the presence of disordered crystals,chemical inhomogeneity,and conjunction of nano-size crystals with larger grains.Two step-like phase transitions were seen in the dielectric and tangent loss spectra,which is char-acteristic of the relaxor –AFE to mixed AFE –FE phase at 80–120 C,and the di ?use phase transition near 240 C.The lower phase transition shows a large dielectric dispersion and follows a non-linear VF relation with freezing of PNRs at 34 C.A pinched P –E hysteresis was observed over a wide range of temperature that clearly suggests the coexistence of the relaxor –AFE phase even far above the T m .A moderate energy density and a low leakage current of 0.4–0.6J cm à3and 1m A were obtained at 50kV cm à1,respectively.These features suggest that lead-free relaxor –AFE ceramics or thin lms have commercial potential for energy storage applications.

Acknowledgements

The authors would like to acknowledge to Prof.J.F.Scott (Univ.of Cambridge,UK)for his critical evaluations and suggestions to improve the manuscript and Mr K.N.Sood for his support in carrying out SEM.

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