The sample sintered for 11 h with pseudocubic nature and small grain size shows a slim P-E loop owing to a high recoverable energy density 2.61 J/cm3 and a high efficiency 91% at an elect
Trang 1Original Article
Microstructure tailoring for enhancing the energy storage
A.R Jayakrishnana, Penna Venkata Karthik Yadava, J.P.B Silvab,**, K.C Sekhara,*
a Department of Physics, School of Basic and Applied Sciences, Central University of Tamil Nadu, Thiruvarur, 610 005, India
b Centro de Fısica das Universidades do Minho e do Porto (CF-UM-UP), Campus de Gualtar, 4710-057, Braga, Portugal
a r t i c l e i n f o
Article history:
Received 6 September 2019
Received in revised form
4 December 2019
Accepted 6 December 2019
Available online xxx
Keywords:
Ceramics
Relaxor ferroelectrics
Lattice distortion
Grain size
Recoverable energy
a b s t r a c t
In this work, we introduce a new approach to enhance the energy storage properties of 0.98 [0.6Ba(Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)TiO3]-0.02BiZn1/2Ti1/2O3[BCZT-BZ] ceramic capacitors via tuning the microstructure through the sintering time The x-ray diffraction (XRD) analysis confirmed the formation
of BCZT-BZ solid solution without any secondary phase It is observed that the rhombohedral and tetragonal phases co-exist in BCZT-BZ samples sintered at different time periods, except the one sintered for 11 h, where the rhombohedral and pseudocubic phases co-exist The energy dispersive x-ray spec-troscopy (EDS) revealed the presence of the elements constituting the BCZT-BZ samples The variation of the grain size with the sintering time is explained based on the coalescence process and the Ostwald ripening mechanism A strong correlation was observed between the ferroelectric properties and the microstructure The sample sintered for 11 h with pseudocubic nature and small grain size shows a slim P-E loop owing to a high recoverable energy density (2.61 J/cm3) and a high efficiency (91%) at an electric field of 150 kV/cm The observed recoverable energy density was found 3 to 13 times higher than that reported for bulk BCZT Thesefindings suggest that the present BCZT-BZ ceramics are attractive materials for the energy storage capacitor applications
© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Recently, (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics with
outstanding dielectric, ferroelectric and piezoelectric properties are
considered a typical lead free electroceramic material and
there-fore, have been also investigated for promising application as
en-ergy storage capacitors [1,2] For instance, Puli et al reported an
energy storage density of 0.94 J/cm3with an efficiency of 72% in
(1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3ceramic capacitors at an electric
field of 170 kV/cm [3] However, real-world applications actually
require materials with an energy storage efficiency greater than
90% [4] For enhancing the energy storage efficiency, we need to
improve the difference (DP¼ PmePr) between the remanent (Pr)
and the maximum polarization (Pm), and the dielectric breakdown strength (DBS) [5,6] as well Thus, it may be an appropriate strategy
to induce a dielectric relaxor behavior in conventional ferroelectric materials as the relaxor ferroelectrics possess slim P-E loops [5,6
Different approaches, such as doping of rare earth metals in ferroelectric materials, the formation of ferroelectric-paraelectric (SrTiO3) solid solutions and the coupling of ferroelectrics with semiconductors (ZnO, MgO) have been adopted to improve the relaxor behavior of the conventional ferroelectric materials [7,8,9
On the other hand, the bismuth-based perovskites BiMO3(with M
¼ Sn, In, Fe, Yb, Zn1/2Ti1/2, Ni1/2Ti1/2, Zn1/2Sn1/2,Mg1/2Ti1/2, Li1/2Nb1/
2, etc.) have a large ferroelectric polarization, a large dielectric constant and great piezoelectric coefficients, but they are very unstable under ambient conditions [10,11] Therefore, researchers have made an attempt to form solid solutions of BiMO3with the conventional ferroelectric materials in the form of binary or ternary oxides in order to utilize their outstanding ferro- and piezoelectric properties [10,11] The inclusion of a small amount of bismuth-based perovskites in ferroelectric materials usually decreases the tetragonal distortion and broadens the ferroelectric-paraelectric
* Corresponding author.
** Corresponding author.
E-mail addresses: josesilva@fisica.uminho.pt (J.P.B Silva), sekhar.koppole@
gmail.com (K.C Sekhar).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
https://doi.org/10.1016/j.jsamd.2019.12.001
2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ).
Trang 2phase transition [10] Moreover, it is observed that
ferroelectric-BiMO3 solid solutions are good relaxor ferroelectric materials
Usually, the relaxor behavior arises when the translational
sym-metry, which is essential for the ferroelectric long-range order, is
disturbed by the doping [12] causing the non-homogenous
stoi-chiometry in nanoscale regions, ascribed as polar nano regions
(PNRs) [12] It has been also shown that the translational symmetry
can be disturbed by modifying the microstructure, especially by the
reduction of the grain size to nanoscale and thus, the same relaxor
behavior can be induced by the microstructure control [12,13]
In view of this, we proposed a new kind of relaxor ferroelectric
material based on the 0.6Ba (Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)
TiO3eBiZn1/2Ti1/2O3solid solution We have chosen BiZn1/2Ti1/2O3
because of its very high ionic polarization of 153 mC/cm2 and
0.6Ba(Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)TiO3 due to its optimum energy
storage properties [11,14] The addition of a small amount of BiZn1/
2Ti1/2O3in 0.6Ba(Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)TiO3can form the PNRs
due to the difference in the valence states and the ionic radii
be-tween Bi3þ and Ba2þ as well as Zn2þand Ti4þions These PNRs
hinder the long-range ferroelectric order and improve the relaxor
behavior [15] Further, we have made an attempt to improve the
relaxor behavior of 0.6Ba(Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)TiO3eBiZn1/
2Ti1/2O3 ceramics through the grain size tuning by varying the
sintering time, thus enhancing their energy storage performance
2 Experimental
2.1 Ceramic preparation
A conventional solidestate reaction was utilized to prepare the
0.98[0.6Ba(Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)TiO3]-0.02BiZn1/2Ti1/2O3
[BCZT-BZ] ceramics High purity BaCO3(99%, SigmaeAldrich), TiO2
(99%, SigmaeAldrich), CaCO3(98.5%, Merck - Emplura), ZrO2(97%,
LobaChemie), Bi2O3 (99%, SigmaeAldrich) and ZnO (99%,
SigmaeAldrich) were used as the starting materials and weighed according to stoichiometric ratio The reagents were then mixed and grinded using a ball miller and subsequently calcined at 1000
C for 2 h The calcined powder was pressed into pellets and then, sintered at 1200C for 3 h and allowed to cool down to room temperature naturally These single sintered pellets were sintered for the second time at 1200C by varying the sintering time from 3
to 13 h
2.2 Structural characterization
An Empyrean X-ray diffractometer (Malvern Panalytical, The Netherlands) with Ni-filtered CuKaradiation of wavelength 1.54 Å was utilized to record the x-ray diffraction patterns (XRD) of the ceramic samples The lattice parameters of the different samples were determined using a lattice refinement program called CellCalc [16,17] The surface morphology and the chemical composition were analyzed using afield emission scanning electron microscope (FESEM) (Tescan Mira 3, Australia) with a build-in energy disper-sive x-ray spectroscopy (EDS) function
2.3 Ferroelectric measurements
A highly conductive silver paste was coated on the smooth surfaces of the pellets and the ferroelectric properties were inves-tigated in the metal-insulator-metal (MIM) configuration The polarization-electricfield (P-E loop) hysteresis curves were recor-ded using P-E loop tracer (Marine India), built on the modified Sawyer and Tower circuit [18] The P-E loops were recorded at different electricfields in the range from 30 kV/cm to 150 kV/cm
Fig 1 (a) XRD patterns of BCZT-BZ ceramics sintered at different time periods; (b) Extended scan near 2qz 45
.
Trang 33 Results and discussion
3.1 Microstructural properties
Fig 1(a) shows the XRD patterns of the BCZT-BZ ceramics
sin-tered at various time periods from 3 to 13 h All the samples exhibit
a single perovskite phase of BCZT without any impurity phase
[JCPDSfile no: 05e0626, 85e0368] This suggests the formation of
the solid solution of BCZT and BZ Further, the reflection near 2qz
45 splits into (002) and (200) reflection planes as shown in
Fig 1(b) This confirms the presence of the tetragonal phase (JCPDS
file no: 05e0626, P4mm) in all the samples except in the one
sintered for 11 h Furthermore, the reflection at 2qz 66 splits into two corresponding to the (220) and (202) planes of the rhombohedral phase of BCZT (JCPDSfile no: 85e0368, R3m) This
is in good agreement with the fact that BCZT exhibits a morpho-tropic phase boundary such as the co-existence of both the rhom-bohedral and the tetragonal phases at room temperature [14,19] The effect of sintering time on the lattice parameters, the tetragon-ality (i.e the c/a ratio), the crystallite size and the tetragonal distortion (n¼ (c
a 1)*100) is given inTable 1 The low value of tetragonal distortion and the absence of the peak splitting for the sample sintered for 11 h suggest the presence of a pseudocubic phase rather than the tetragonal one, possibly due to the small grain size [13]
In order to evaluate the surface morphology of the ceramics, field emission scanning electron microscopy (FESEM) images were recorded and the results are shown inFig 2 All the samples show the dense microstructure without any pores The variation of the average grain size with the sintering time is shown inFig 2(f) The variation of the grain size with the sintering time periods is the same as that of the crystallite size as evidenced from the XRD analysis (Table 1) The slight increase in grain size with the increase
of the sintering time up to 7 h can be attributed to the coalescence process With further increase of sintering time to 9 h, the sharp increase in grain size can be attributed to the dominance of the Ostwald ripening mechanism [20,21] Further, too long period of
Fig 2 FESEM images of BCZT-BZ ceramics sintered for (a) 3 h; (b) 7 h; (c) 9 h; (d) 11 h; (e) 13 h; (f) Average grain size as a function of the sintering time.
Table 1
Effect of sintering time on the lattice parameters, the crystallite size and the
tetra-gonality of the BCZT-BZ ceramics.
Sintering
time
(hrs)
a ¼ b
(Å)
C (Å)
c/a ratio Crystallite
size (nm)
n ¼(ca 1)*100
3 3.9916 4.0028 1.0028 41 0.28
5 3.9941 4.0081 1.0035 43 0.35
7 3.9950 4.0122 1.0043 49 0.43
9 3.9972 4.0291 1.0079 57 0.79
11 3.9915 4.0011 1.0024 34 0.24
13 3.9967 4.0251 1.0071 38 0.71
Trang 4sintering (over 11 h) may lead to excessive grain growth, so that the
grains become unstable and decompose into tiny ones as virtually
evidenced inFig 2(d) [20] With further increase of the sintering
time to 13 h, the coalescence process may be revisited and the grain
size slightly increases (Fig 2(e)) [21] The EDS spectrum of BCZT-BZ
sample sintered at 11 h shown inFig 3confirms the presence of all
the constituting elements, such as Ba, Ca, Zr, Ti, Bi, Zn, and O
3.2 Ferroelectric and energy storage properties
Fig 4(a) displays the P-E loops of the BCZT-BZ ceramics
sin-tered at different sintering time periods under an electricfield of
30 kV/cm It is found that the saturation polarization (Ps) and the coercivefield (Ec) follow a similar pattern as that of the tetragonal distortion (n) and of the grain size (G) [Fig 4(b)] with the sin-tering time This is in good agreement with the modified LGD theory It relates Ps, n, and G, as follows [22]:
From the XRD and FESEM results, it is clear that the ceramic sample sintered for 11 h has the lowest value of G and n, and consequently a low value of Ps Furthermore, the sample sintered for 11 h exhibited a slim P-E loop with a small value of the remanent polarization (Pr) This slim behavior can be correlated to the pseu-docubic nature of its structure and its smallest grain size The origin
of the pseudocubic structural nature is usually attributed to the existence of microdomains or PNRs rather than macroscopic do-mains [18] The size of these microdomains or PNRs is small compared to the macroscopic domains Hence, they respond much faster to the externalfield than the macroscopic domain resulting
in a slim P-E loop [23,24] Further, the decrease in the grain size weakens the long-range ferroelectric order and induces the relaxor behavior due to the short-range forces Furthermore, the trans-lational symmetry is disturbed with the increase of grain bound-aries due to the grain size reduction and the material thus behaves like relaxor ferroelectrics [12] Consequently, the P-E loop shows the slim behavior with low Pr Since the domain size is proportional
to the square of the grain size, the formation of single domains rather than multiple ones becomes more probable with the reduction of the grain size In this case, low Pris expected due to the low contribution of domain walls and domain switching due to pinning/clamping by grain boundaries [18] Thus, a slim behavior is expected in ceramics with the lowest grain size
The recoverable energy density (Wr), the energy loss (Wl) and the efficiency (h) of the samples were calculated from P-E
Fig 3 EDS spectrum of the BCZT-BZ sample sintered for 11 h.
Fig 4 (a) P-E loops of the BCZT-BZ ceramics sintered at different time periods; (b) P s , n, and G as a function of the sintering time; (c) Schematic representation of the calculation of energy storage properties; (d) W r , W l , andhas a function of the sintering time.
Trang 5hysteresis loops as shown in Fig 4(c) based on the following
equations [25e27]:
ðPmax
P r
where Pris the remanent polarization and Pmaxis the maximum
polarization The area of the hysteresis loop represents the energy
loss (Wl) Further, the energy storage efficiency of the capacitor
based on ceramics sintered at different time periods were
calcu-lated using the following formula [25e27]:
h¼
W
and the results are shown inFig 4(d) Since, the sample sintered for
11 h possesses a high recoverable energy density, a low loss and a
high efficiency at 30 kV/cm, it has been chosen for the study of the
energy storage properties at higher electricfields The electric field
dependence of the P-E loops of the sample sintered for 11 h is
shown inFig 5(a) The values of the recoverable energy density and
the energy storage efficiency are found to be 2.61 J/cm3and 91%,
respectively, at an electricfield of 150 kV/cm The comparison of Wr
of the BCZT-BZ ceramics sintered for 11 h with that of other BCZT
based ceramic capacitors is given inFig 5(b) and it is found to be of
3e13 times higher than the reported values [3,26e30] Therefore,
the BCZT-BZ ceramics sintered for 11 h can be considered the most
promising candidate for the energy storage applications
Currently, most of the investigations are focused on the
improvement of the energy storage density and the efficiency
However, for the practical implementation of energy storage
ca-pacitors, other aspects like charging and discharging time, fatigue
behavior, temperature stability and lifetime are also important
which need to be addressed Therefore, studies on these aspects can
be taken as a future scope of the works
4 Conclusion
We have shown that the energy storage properties of 0.98
[0.6Ba(Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)TiO3]-0.02BiZn1/2Ti1/2O3 ceramic
capacitors can be tuned through the modification of the sample
microstructure The formation of the 0.98[0.6Ba(Zr0.2Ti0.8)O3 -0.4(Ba0.7Ca0.3)TiO3]-0.02BiZn1/2Ti1/2O3 solid solution, confirmed from the XRD and FESEM analyses, revealed that the sintering time has had a significant impact on the morphology The grain size variation with the sintering time was explained based on the coalescence and the Ostwald ripening mechanism The smallest grain size and the presence of the pseudocubic structural nature in the sample sintered for 11 h were found to weaken the long-range ferroelectric order and favor the relaxor behavior This led to a slim P-E loop and consequently the enhanced energy storage per-formance The present 0.98[0.6Ba (Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3) TiO3]-0.02BiZn1/2Ti1/2O3ceramics sintered for 11 h exhibit a very high recoverable energy density (2.61 J/cm3) and a high efficiency (91%) at 150 kV/cm Our study has shown that the ferroelectric-BiMO3solid solutions are a promising candidate for energy storage applications
Declaration of Competing Interest The authors declare that there is no conflict of interests Acknowledgements
The work was supported by (i) DST-SERB, Govt of India through Grant Nr ECR/2017/000068 and (ii) UGC through Grant Nr F.4e5(59-FRP/2014(BSR)) The author A R Jayakrishnan is thankful
to Central University of Tamil Nadu, India for his Ph.D fellowship J
P B S is grateful for the financial support by the Portuguese Foundation for Science and Technology in the framework of the Strategic Funding UID/FIS/04650/2019
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