Received: 30 January 2017
| Accepted: 20 February 2017
DOI: 10.1111/ijac.12678
ORIGINAL ARTICLE
Dielectric and ferroelectric properties of the sol-gel–derived
Zr-doped Ba0.7Sr0.3TiO3 polycrystalline ceramic systems
Asad Mahmood1 | Abdul Naeem1 | Yaseen Iqbal2 | Asad Ullah1
1
National Centre of Excellence in
Physical Chemistry, University of Abstract
Peshawar, Peshawar, Pakistan Ba0.7Sr0.3ZrxTi1xO3 (BSZT; where x=0.02, 0.04, 0.06, 0.08, 0.1) ceramics were
2
Materials Research Laboratory, processed through a sol-gel method at 1450°C for 6 h. All the samples showed a
Department of Physics, University of
diffuse phase transition which might be due to the presence of polar nanoregions,
Peshawar, Peshawar, Pakistan
those associated with the composition inhomogeneity in the BSZT ceramics. The
Correspondence sample with x=0.02 exhibited a dielectric constant (ɛ=23714) which successively
Abdul Naeem
Email: naeem@upesh.edu.pk decreased with increasing x up to 8569 for the sample with x=0.1 around Tc
measured at 10 kHz. Ceramic samples showed a ferroelectric hysteresis behavior
Funding information similar to relaxor materials.
International Research Support Initiative
Program (IRSIP)
KEYWORDS
dielectric materials, electroceramics, sol-gel
1 | INTRODUCTION calculated to be 475. Bera et al.10 reported the preparation
of Ba1xSrx(Ti0.5Zr0.5)O3 by conventional solid-state sinter-
Ba1xSrxTiO3 (BST) solid solution is used in electrical ing route and the resultant solid solution was reported to
device fabrication for microwave (MW) circuit technology have a cubic structure at x<0.6 and tetragonal structure at
due to its high tunability (g) and low dissipation factor x>0.6. Bulk density was reported to decrease from 5.75 g/
(tan d) around the room temperature. The dielectric behav- cm3 to 4.44 g/cm3 with an increase in x from 0.00 to 1.
ior of BST ceramics is unstable around Tc, which under- But they did not report the variation in the dielectric prop-
goes significant changes and results in the communication erties with temperature.
disruption.1,2 Therefore, the dielectric properties of the Detailed literature survey shows that few attempts have
BST ceramics are tuned by various methods, among which been reported to investigate the effect of Zr content on the
few studies reported the compositional gradation of the BST solid solution. Furthermore, in most cases the ceram-
BST ceramics according to the stoichiometric formula; ics were processed by the conventional solid-state sintering
such as Ba1xSrxTiO3.3 Other approaches include doping route.11 Limited attempts have been made to report the
of various elements, i.e., La, Nd, Pb, Mg, Mn, Zr, in the dielectric properties of the Ba0.7Sr0.3ZrxTi1xO3 (BSZT)
ABO3 perovskite structure. Among these Zr-doped BaTiO3 ceramics despite of the fact that extensive consideration
solid solution (BaZrTiO3 or BZT) has been reported to has been given to Zr-doped BaTiO3 or SrTiO3 polycrys-
bring out interesting alteration in Tc and dielectric response talline ceramic systems.8,12,13 This study reports, synthesis
at varying Zr content.4-7 An isovalent substitution of Zr4+ of the BSZT ceramics using stoichiometries formula
for Ti4+ on the B-site in BaTiO3-based ceramics results in Ba0.7Sr0.3Ti1xZrxO3. The dielectric properties were mea-
a pinching effect on the Tc.8 Recently, Wang et al.9 pro- sured in a broad temperature regime using a frequency
posed that replacing Ti4+ by Zr4+ doping in the BST crys- range from 1 kHz to 1 MHz. Zirconium as a dopant was
tal structure can enhance its dielectric response. They used because substitution of Zr4+ for Ti4+ ions significantly
synthesized Ba0.6Sr0.4(Zr0.2Ti0.8)O3 ceramics using spark alters the structural and electrical behavior due to the rela-
plasma sintering (SPS) at 1270°C for 5 min. They reported tively high chemical stability of Zr4+ than Ti4+. Further-
ɛr=850 at 100 kHz and Figure of Merit (FOM) were more, Zr4+ doping for Ti4+ results in a decrease in Tc.
Int J Appl Ceram Technol. 2017;1–7. wileyonlinelibrary.com/journal/ijac © 2017 The American Ceramic Society | 1
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| MAHMOOD ET AL.
2 | EXPERIMENTAL PROCEDURE
The precursor materials, barium acetate (≥99%), strontium
acetate (99%), zirconium (IV) oxynitrate hydrate (≥99%),
titanium isopropoxide (97%), and acetylacetone (Aldrich,
99.5%), were received from Sigma-Aldrich and were used
without further purification. Deionized water, acetic acid,
and isopropanol were used as solvents. First, barium acet-
ate, strontium acetate, and zirconium (IV) oxynitrate
hydrate were added to glacial acetic acid according to the
composition formula Ba0.7Sr0.3Ti1xZrxO3 to make a solu-
tion of metal salts at 80°C. Titanium isopropoxide was dis-
solved in isopropanol at room temperature. Acetylacetone
was added to the Ti solution in order to form a stable che- F I G U R E 1 TG/DTA for the BSZT (x=0.02) precursor gel
late with Ti to avoid any premature precipitation and was synthesized by sol-gel method
added to salt solution (Ba, Sr, and Zr) while stirring. Fur-
thermore, 10 mL of water was added to assist clear solu-
tion formation. The solution was converted into sol and first two steps were attributed to the removal of water,14
subsequently into gel at 80°C after 2 hours. The gel was low boiling point solvent (acetic acid and acetylacetone),
dried at 120°C overnight to make xerogel. The xerogel was and thermal decomposition of carboxylate-alkoxides precur-
calcined at selected temperatures from 450°C to 850°C for sors, respectively. Endothermic (120°C) and exothermic
4 h. The Ba0.7Sr0.3Ti1xZrxO3 powders along with 5% (200°C) peaks on the DTA curve were observed for the
polyvinyl butyral (PVB) as binder were mix milled for first step. The second step was accompanied by two
20 min and subsequently pressed into pellets which were exothermic peaks on the DTA curve around 330°C and
treated at 600°C for 30 min and 1450°C for 6 h. Gold 410°C.15 The third weight loss region corresponded to the
paste was deposited on the polished surfaces of the pellets temperature range 620–750°C. A broad exothermic peak
and cooked at 800°C for 2 h to make electrodes. around 680°C on the DTA curve with a corresponding
The phase development, purity, and the crystalline nature weight loss of around 9.8% on the TG curve was close to
of the BSZT samples were studied by a SIEMENS D500 X- the theoretical weight loss expected during intermediate
ray diffractometer (Munich, Germany). Thermal analysis [(Ba,Sr)2Ti2ZrxO5CO3]x-r phase formation. The weight loss
was carried out on xerogel using Perkin Elmer, TG/DTA. A associated with the solid-state reaction between metal car-
JEOL, Tokoyo, Japan (JSM-6330F), scanning electron bonate and amorphous TiO2 was reported as 17.4%.16
microscope (SEM) operating at 15 keV was used to investi- Therefore, the current weight loss suggested that the reac-
gate the microstructure of sintered ceramics. Polarization (P) tion followed the carboxylate complex mechanism.
versus electric field (E) was measured using an aix-ACCT Figure 2A shows XRD profiles for the BSZT (x=0.02)
(PES) ferroelectric test unit at room temperature up to gel. The as-prepared gel was amorphous at room tempera-
20 kV/cm. A Hewlett Packard 4284A LCR meter connected ture and no crystalline phases were detected. The increase
to a computer through CP-IB interface was used for measur- in temperature on dried gel resulted in physical and chemi-
ing the capacitance and tan d. All the measurements were cal changes. The formation of oxides and titanate crystal-
carried out from room temperature, i.e., 20–200°C at vari- lization was observed at 550°C. Further increase in
ous frequencies (1 kHz to1 MHz). The samples were put in annealing temperature confirmed the formation of the crys-
a tube furnace connected with a thermocouple, and heating talline perovskite phase at 750°C and 850°C, where the
rate was kept at 1°C per minute. The impedance analysis complete crystallization was achieved and single-phase
was carried out using a LF-Hewlett Packard 4192A (Agi- BSZT powders were produced. Figure 2B presents the
lent, Santa Clara, CA) impedance analyzer connected with a XRD patterns for the BSZT ceramic samples sintered in air
tube furnace. at 1450°C which were matched to the tetragonal (p4mm)
symmetry (PDF-00-044-0093) due to the peak splitting
around 45° in (200) and (002) planes. Highly crystallized
3 | RESULTS AND DISCUSSION ceramic samples were produced as evidenced from the
high-intensity XRD peaks. Secondary phases were not
TG-DTA curve for the BSZT (x=0.02) dried gel is given in observed in the resultant ceramics which confirmed Zr4+
Figure 1. TG curve showed weight loss in temperature substitution for Ti4+ in the Ba0.7Sr0.3TiO3 crystal structure.
ranges 50–250°C, 270–460°C, and 620–750°C, where the The peak around 45° shifted toward relatively low angles
MAHMOOD ET AL. | 3
sample x=0.08 shows considerable porosity compared to rest
of compositions. No liquid phase and grains could be
observed in the bulk microstructures of Ba0.7Sr0.3Ti1-xZrxO3
sintered samples.
Figure 4 shows the ɛr versus temperature trend at differ-
ent frequency (1 kHz to 1 MHz). The ɛr increased with
increasing temperature reaching to a maximum (ɛm) and
then decreased with a further increase in temperature. The
Tm is the temperature associated with ɛm. The magnitude
of ɛm and Tm was observed to vary with Zr content as well
as frequency. The Tc shifted to low temperature with
increasing Zr content. The magnitude of er and tan d
around Tc for the BSZT ceramic samples is given in the
Table 1. All the samples examined in this study showed a
diffused phase transition. Various theories have been pro-
posed to explain the diffuse phase transition behavior in
perovskite, which are based on cation disorder, corresponds
to the fluctuations in the compositions of presence of polar
nanoregions (PNRs) where different kind of cations B0 and
B″ occupy the same crystallographic B-site in the ABO3-
type perovskite structure. Thus, the diffuse phase transition
behavior in BSZT ceramics might be due to the composi-
tion heterogeneity at the microscopic level.18-20 Figure 5
presents the observed variation in the tan d with tempera-
ture at frequency from 1 kHz-1 MHz. All the compositions
exhibited a frequency dispersion which was high at low
frequencies which indicated its association with the pres-
F I G U R E 2 X-ray diffraction patterns; (A) BSZT (x=0.02) ence of PNRs in the ceramic samples. The tan d was
powders synthesized by sol-gel method at different calcination observed to decrease with increasing Zr content which
temperatures, (B) XRD for the BSZT (x=0.02, 0.04, 0.06, 0.08, 0.1) might be due to the fact that BZT ceramics has lower tan
ceramics sintered at 1450°C for 6 h d than the BST solid solution. An increase in the amount
of the BZT solid solution may lead to a decrease in the
with increasing Zr4+ content which may be due to the tan d.
substitutions resulted in an increase in the lattice Figure 6 shows the Cole-Cole plots for the BSZT
parameters and hence a shift in the XRD peaks toward low ceramics at 500, 515, and 530°C. Both the grain and grain-
2-values.17 Table 1 presents various parameters extracted boundary contributions were observed at high and low fre-
from the XRD data. quency, respectively. A polynomial fit to the semicircles
Figure 3 shows the SEM images of cracked pellets was used for calculating the total resistance. All the sam-
(Bulk) of Ba0.7Sr0.3Ti1-xZrxO3 ceramics. The density of cera- ples showed a decrease in corresponding resistance with
mic increased with an increase in Zr content. Better sinter- increasing temperature, which is a typical semiconductor
ability was obtained in higher Zr-content samples. The behavior contradictory to metals. The depressed nature of
the semicircular arcs showed deviation from the ideal semi-
circle which was attributed to the non-Debye type of relax-
T A B L E 1 Calculated cell parameters and dielectric properties of ation mechanism in the BSZT ceramic samples.21
the BSZT (x=0.02, 0.04, 0.06, 0.08, 0.1) ceramics Figure 7 shows the variation in Z″ and M″ with temper-
Vo er at Tc tan d at Tc ature. Two peaks were observed for Z″ at low and high fre-
Sample a (
A) c (
A) c/a (
A3) (10 kHz) (10 kHz) quency which are associated with grain boundary and bulk
x=0.02 3.9893 4.0185 1.007 63.9 23714 0.01 contributions, respectively. Only one peak was observed
x=0.04 3.9997 4.0216 1.005 64.4 13663 0.02 for M″ at high frequency. This behavior of Z″ and M″ sug-
x=0.06 3.9986 4.0268 1.007 64.3 10070 0.02 gested inhomogeneous electrical microstructure of the pre-
sent ceramic samples and thus, the ceramics cannot be
x=0.08 4.0093 4.0296 1.005 64.7 16428 0.01
represented by a single RC element. The Z″ and M″ peaks
x=0.1 4.0103 4.0310 1.005 64.8 8569 0.02
at high frequency suggested that ceramic samples exhibited
4
| MAHMOOD ET AL.
F I G U R E 3 SEM micrographs of the bulk of Ba0.7Sr0.3Ti1xZrxO3 ceramic samples sintered in air
increase in conduction losses because of the hoping mecha-
nism between the Ti4+ and Ti3+ (Ti4++e0 ↔Ti3+). Due to
the chemical stability of Zr4+ ion compared to Ti4+, elec-
tron hoping mechanism is depressed by the substitution of
Zr4+ for Ti4+ which is responsible for the corresponding
decrease in Ea.21-23
Figure 8 presents the hysteresis loops for the BSZT
ceramics measured at room temperature, which shows a
typical relaxor behavior. All samples showed well-saturated
hysteresis loops. The data extracted from the hysteresis
loops are given in Table 1. The hysteresis loops, XRD,
and temperature-dependant dielectric properties demon-
strated that the BSZT ceramics were ferroelectric at room
temperature. The coercive field (Ec) increased with increas-
ing Zr content from 1.33 kV/cm2 (x = 0.02) to 1.99 kV/cm2
F I G U R E 4 Temperature-dependant er of the BSZT (a=0.02),
(x = 0.1). The magnitude of saturated polarization (Ps)
(b=0.04), (c=0.06), (d=0.08), (e=0.1) ceramics
and remnant polarization (Pr) decreased gradually but non-
linearly with increasing Zr content. The x=0.04 composi-
tion exhibited the highest saturated polarization
homogeneous bulk and inhomogeneous grain-boundary (Ps=8.11 lC/cm2), whereas the highest remnant polariza-
response. The Ea values from the grain boundary of the tion (2.98 lC/cm2) was measured for the sample with
BSZT (x=0.02, 0.04, 0.06, 0.08, 0.1) compositions were x=0.1. There are two parameters related to the grain, which
calculated as 1.51, 1.21, 1.33, 1.68, and 1.01 eV, respec- can influence the polarization in ceramics. First, the grain
tively. The activation energy of the BSZT ceramics boundary is a lower permittivity region and hence the cor-
decreased nonlinearly with increasing Zr content from 1.51 responding ferroelectricity is poor, compared to the other
eV (x = 0.02) to 1.01 eV (x = 0.1). This may be due to an parts of the grain. Second, the formation of the depletion
MAHMOOD ET AL. | 5
F I G U R E 5 Temperature-dependant tan d of the BSZT (x=0.02, 0.04, 0.06, 0.08, 0.1) ceramics
F I G U R E 6 Cole-Cole plots for the
BSZT (x=0.02, 0.04, 0.06, 0.08, 0.1)
ceramics at selected temperatures
6
| MAHMOOD ET AL.
F I G U R E 7 Variation in Z″ and M″ with frequency for the BSZT (x=0.02, 0.04, 0.06, 0.08, 0.1) ceramics
analysis confirmed that the final ceramics were single
phase and exhibited tetragonal symmetry. Relaxor behavior
was observed in all the samples and BSZT ceramics
showed a diffused phase transition. Such behavior was
attributed to the presence of PNRs associated with the
composition fluctuations. Permittivity decreased from
23714 for the x=0.02 composition to 8569 for the x=0.1
composition around Tc at 10 kHz. The impedance spectro-
scopic analysis confirmed grain boundary and bulk contri-
butions in the conduction mechanisms of all the BSZT
samples. Ceramic samples showed ferroelectric hysteresis
behavior similar to relaxor materials. Zirconium doping is
an effective way to tune the dielectric properties of the
BaTiO3-based ceramics.
F I G U R E 8 Hysteresis loops for the BSZT (x=0.02, 0.04, 0.06,
0.08, 0.1) ceramics sintered at 1450°C for 6 h in air ACKNOWLEDGMENTS
The authors acknowledge the financial support of Higher
Education Commission of Pakistan under the 5000 Indige-
layer on the grain surface associated with the mechanism nous PhD Fellowship Program and International Research
by which the grain boundary possessing space charge Support Initiative Program (IRSIP). The authors acknowl-
excludes a polarization charge on the grain surface.24,25 edge the laboratory support extended by Prof. I. M. Reaney
These factors are responsible for the formation of depolar- and his group, Electroceramic Laboratory, Department of
ization field associated with the discontinuity in polariza- Material Science and Engineering, University of Sheffield
tion and hence polarization decreases. (UK).
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