Microstructure tailoring for enhancing the energy storage performance of 0.98[0.6Ba(Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)TiO3]- 0.02BiZn1/2Ti1/2O3 ceramic capacitors

Journal of Science: Advanced Materials and Devices 5 (2020) 119e124

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Journal of Science: Advanced Materials and Devices
journal homepage: www.elsevier.com/locate/jsamd

Original Article

Microstructure tailoring for enhancing the energy storage
performance of 0.98[0.6Ba(Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)TiO3]0.02BiZn1/2Ti1/2O3 ceramic capacitors
A.R. Jayakrishnan a, Penna Venkata Karthik Yadav a, J.P.B. Silva b, **, K.C. Sekhar a, *
a
b

Department of Physics, School of Basic and Applied Sciences, Central University of Tamil Nadu, Thiruvarur, 610 005, India
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

a b s t r a c t

Article history:
Received 6 September 2019
Received in revised form
4 December 2019
Accepted 6 December 2019
Available online 13 December 2019

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 spectroscopy (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. These findings 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 ( />
Keywords:
Ceramics
Relaxor ferroelectrics
Lattice distortion
Grain size
Recoverable energy

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 therefore, have been also investigated for promising application as energy storage capacitors [1,2]. For instance, Puli et al. reported an
energy storage density of 0.94 J/cm3 with an efficiency of 72% in (1x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramic 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 (DP ¼ PmePr) between the remanent (Pr)

* 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.

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 BiMO3 with 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 bismuthbased perovskites in ferroelectric materials usually decreases the
tetragonal distortion and broadens the ferroelectric-paraelectric

/>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
( />

120

A.R. Jayakrishnan et al. / Journal of Science: Advanced Materials and Devices 5 (2020) 119e124

phase transition [10]. Moreover, it is observed that ferroelectricBiMO3 solid solutions are good relaxor ferroelectric materials.
Usually, the relaxor behavior arises when the translational symmetry, which is essential for the ferroelectric long-range order, is
disturbed by the doping [12] causing the non-homogenous stoichiometry 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)
TiO3eBiZn1/2Ti1/2O3 solid 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/
2Ti1/2O3 in 0.6Ba(Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)TiO3 can form the PNRs
due to the difference in the valence states and the ionic radii between Bi3þ and Ba2þ as well as Zn2þ and Ti4þ ions. 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.

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 1200 C for 3 h and allowed to cool down to room

temperature naturally. These single sintered pellets were sintered

for the second time at 1200 C 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 CuKa radiation 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 a field emission scanning electron microscope
(FESEM) (Tescan Mira 3, Australia) with a build-in energy dispersive x-ray spectroscopy (EDS) function.

2. Experimental

2.3. Ferroelectric measurements

2.1. Ceramic preparation

A highly conductive silver paste was coated on the smooth
surfaces of the pellets and the ferroelectric properties were investigated in the metal-insulator-metal (MIM) configuration. The
polarization-electric field (P-E loop) hysteresis curves were recorded using P-E loop tracer (Marine India), built on the modified
Sawyer and Tower circuit [18]. The P-E loops were recorded at
different electric fields in the range from 30 kV/cm to 150 kV/cm.

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%,



Fig. 1. (a) XRD patterns of BCZT-BZ ceramics sintered at different time periods; (b) Extended scan near 2q z 45 .


A.R. Jayakrishnan et al. / Journal of Science: Advanced Materials and Devices 5 (2020) 119e124

121


3. Results and discussion
3.1. Microstructural properties
Fig. 1(a) shows the XRD patterns of the BCZT-BZ ceramics sintered at various time periods from 3 to 13 h. All the samples exhibit
a single perovskite phase of BCZT without any impurity phase
[JCPDS file no: 05e0626, 85e0368]. This suggests the formation of
the solid solution of BCZT and BZ. Further, the reflection near 2q z

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
Table 1
Effect of sintering time on the lattice parameters, the crystallite size and the tetragonality of the BCZT-BZ ceramics.
Sintering
time
(hrs)

a¼b

(Å)

C
(Å)

c/a ratio

Crystallite
size
(nm)

3
5
7
9
11
13

3.9916
3.9941
3.9950
3.9972
3.9915
3.9967

4.0028
4.0081
4.0122
4.0291
4.0011

4.0251

1.0028
1.0035
1.0043
1.0079
1.0024
1.0071

41
43
49
57
34
38

c
n ¼( À 1)*100
a

0.28
0.35
0.43
0.79
0.24
0.71

sintered for 11 h. Furthermore, the reflection at 2q z 66 splits
into two corresponding to the (220) and (202) planes of the
rhombohedral phase of BCZT (JCPDS file no: 85e0368, R3m). This

is in good agreement with the fact that BCZT exhibits a morphotropic phase boundary such as the co-existence of both the rhombohedral and the tetragonal phases at room temperature [14,19].
The effect of sintering time on the lattice parameters, the tetragonality (i.e. the c/a ratio), the crystallite size and the tetragonal
distortion (n ¼ (ac À 1)*100) is given in Table 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 in Fig. 2. All the samples show
the dense microstructure without any pores. The variation of the
average grain size with the sintering time is shown in Fig. 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.


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A.R. Jayakrishnan et al. / Journal of Science: Advanced Materials and Devices 5 (2020) 119e124

30 kV/cm. It is found that the saturation polarization (Ps) and the
coercive field (Ec) follow a similar pattern as that of the tetragonal
distortion (n) and of the grain size (G) [ Fig. 4(b)] with the sintering time. This is in good agreement with the modified LGD

theory. It relates Ps, n, and G, as follows [22]:

P s 2 f Gfn

Fig. 3. EDS spectrum of the BCZT-BZ sample sintered for 11 h.

sintering (over 11 h) may lead to excessive grain growth, so that the
grains become unstable and decompose into tiny ones as virtually
evidenced in Fig. 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 in Fig. 3 confirms 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 sintered at different sintering time periods under an electric field of

(1)

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 pseudocubic 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 domains [18]. The size of these microdomains or PNRs is small
compared to the macroscopic domains. Hence, they respond much
faster to the external field 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 translational symmetry is disturbed with the increase of grain boundaries 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 Pr is 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. 4. (a) P-E loops of the BCZT-BZ ceramics sintered at different time periods; (b) Ps, n, and G as a function of the sintering time; (c) Schematic representation of the calculation of
energy storage properties; (d) Wr, Wl, and h as a function of the sintering time.


A.R. Jayakrishnan et al. / Journal of Science: Advanced Materials and Devices 5 (2020) 119e124

123

Fig. 5. (a) High field P-E loops of the BCZT-BZ ceramics sintered for 11 h; (b) Comparison of the recoverable energy density of our work with those of other reported BCZT ceramics.

hysteresis loops as shown in Fig. 4(c) based on the following
equations [25e27]:

Wr ¼

ð Pmax

E:dP

(2)

Pr

where Pr is the remanent polarization and Pmax is 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 calculated using the following formula [25e27]:

h¼



W
Wr þ Wl


 100

(3)

and the results are shown in Fig. 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 electric fields. The electric field
dependence of the P-E loops of the sample sintered for 11 h is
shown in Fig. 5(a). The values of the recoverable energy density and
the energy storage efficiency are found to be 2.61 J/cm3 and 91%,
respectively, at an electric field 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 in Fig. 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 capacitors, 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)O30.4(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 performance. The present 0.98[0.6Ba (Zr0.2Ti0.8)O3-0.4(Ba0.7Ca0.3)
TiO3]-0.02BiZn1/2Ti1/2O3 ceramics 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 ferroelectricBiMO3 solid 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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