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Electrical investigations of BaTiO3 ceramics with Ba/Ti contents under
influence of temperature
Article in Solid State Ionics · May 2017
DOI: 10.1016/j.ssi.2017.02.003
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Solid State Ionics 303 (2017) 16–23
Contents lists available at ScienceDirect
Solid State Ionics
journal homepage: www.elsevier.com/locate/ssi
Electrical investigations of BaTiO3 ceramics with Ba/Ti contents under
influence of temperature
Nasira Sareecha a,⁎, W. Ali Shah b, M. Anis-ur-Rehman c, M. Latif Mirza a, M.S. Awan d
a
Laboratory of Physical Chemistry, Department of Chemistry, Islamia University of Bahawalpur, Pakistan
Glass and Ceramics Research Centre, PCSIR Laboratories Complex, Ferozpur Road, Lahore 54600, Pakistan
Applied Thermal Physics Laboratory, COMSATS Institute of Information and Technology, Park Road, Islamabad 44000, Pakistan
d
Ibn-e-Sina Institute of Technology (ISIT), H-11/4, Islamabad, Pakistan
b
c
a r t i c l e
i n f o
Article history:
Received 4 December 2016
Received in revised form 3 February 2017
Accepted 4 February 2017
Available online xxxx
Keywords:
BaTiO3 ceramics
Ferroelectric materials
Curie temperature
Paraelectric regions
Ionic conduction
a b s t r a c t
Electrical properties of the solid state sintered BaTiO3 ceramics at 0.98 and 0.94 Ba/Ti molar ratio are investigated
perhaps for the first time in the wide range of temperatures (40–700 °C) at 1 kHz frequency. The studies were
focused to find the controlling parameters, resistance and understanding of the conduction process. BaTiO3 ceramics were perovskite ferroelectric materials, showed cubic and tetragonal crystal structures (Pm-3m, P4MM)
at room temperature. Thermogravimetric and differential scanning calorimetric analysis (TGA-DSC) revealed
melting temperatures, weight losses and crystallization of the as ground powders. Curie temperature (Tc) increased from120–130 °C with decreasing Ba/Ti contents. Dielectric anomalies with relaxor behavior were observed. Resistivity decreased with increasing temperature, all specimens showed semiconductor behavior with
negative temperature coefficient of resistivity (NTCR) characteristics. At 0.98 molar ratio, BaTiO3 ceramics
were found to be more conductive and less resistive. Conductivity followed Arrhenius Law with Ea = 0.2948–
0.3284 and 0.967–1.189 eV which can be attributed to ionic conduction followed by single ionized and doubly
ionized oxygen vacancies. Well defined hysteresis P-E loops under electric fields showed ferroelectric
characteristics.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
Since the discovery of ferroelectricity in single crystalline materials
(Rochelle Salt) in 1921 and its subsequent extension during early to
mid-1940s into the realm of polycrystalline ceramics (barium titanate)
led to investigation of new materials and technological development
with number of industrial and commercial applications, being continued to the present day [1]. Among ferroelectric materials, the perovskites (ABO3) are by far the most important and the widely studied,
well known BaTiO3 and PbTiO3 belong to this category [1,2].
BaTiO3 (BT) is the first ferroelectric ceramic [1,3–5] which is widely
used in the electronic industry due to its excellent dielectric, ferroelectric and piezoelectric properties. Ferroelectric to paraelectric phase
transitions occurs around 120 °C (Curie temperature, Tc), above which
BaTiO3 is cubic, below the curie temperature, the structure is slightly
distorted, taking a tetragonal symmetry [6]. Undoped BT is an electrical
insulator with its resistivity varying between 109 and 1012 Ω cm at room
temperature [7], it can be converted into a semiconductor after partial
substitution of Ba+2 by trivalent cations such as Y+3, La+3 or Ti+4 by
pentavalent Nb+5 and Ta+5 cations [8,9]. BaTiO3 exhibits interesting
⁎ Corresponding author.
E-mail address: nsareecha@hotmail.com (N. Sareecha).
http://dx.doi.org/10.1016/j.ssi.2017.02.003
0167-2738/© 2017 Elsevier B.V. All rights reserved.
electrical properties like positive temperature coefficient of resistance
(PTCR) effect, a jump in the resistivity of few orders around the curie
temperature (Tc). The PTCR effect and other electrical properties find
applications for electronic devices such as multilayer ceramic capacitors
(MLCC), thermistors, varistors as well as sensors and piezoelectric actuators [10–12]
BT based ceramics have been tailored with variety of dopants in
order to modify or optimize properties such high or low resistivity
and temperature of the ferroelectric Curie transition. Curie temperature
(Tc) is important parameter for PTCR applications but BaTiO3 ceramics
have curie temperature around 120 °C. Pb+2 has been substituted for
Ba+2 to increase the curie temperature over 120 °C [13]. Owing to
high toxicity of lead oxide, Pb-based materials have been replaced
with lead-free BT-based PTCR ceramics [14]. BT based, Bi0.5 Na0.5 TiO3
(BT-BNT), Bi0.5 K0.5 TiO3 (BT-BKT) and Bi0.5 Li0.5 TiO3 (BT-BLT) ceramics
have been used as lead free PTCR materials with Curie temperature over
130 °C [15–17].
BT is a definite chemical compound possessing relatively high stability components that make it easier to sinter while maintaining good
chemical stoichiometry [1]. Both stoichiometry and composition control
are important parameters for controlling the ferroelectric properties
[18]. W.P. Chen et al. [19] studied correlation of crystal structure and
Curie point with Ba/Ti ratio (0.96–1.04). Curie temperature ranging
N. Sareecha et al. / Solid State Ionics 303 (2017) 16–23
17
from 98 to 120 °C was attained; dielectric studies were restricted to only
150 °C; while conduction process remained unexplored.
In the present study, BT powders were fabricated using solid state
sintering reaction method [20,21]. Since it has long been believed that
very small amounts of both BaO and TiO2 can be dissolved in BT [22],
b100 ppm for BaO [23,24] and about 300 ppm for TiO2 [24,25] as excess
of the precursor composition can lead to the formation of secondary
phases [26]. Hence, studies were attempted with 0.94 and 0.98 Ba/Ti
molar ratio to find the controlling parameters such as Curie temperature
(TC) and novelty resistance of the obtained ceramics for their manipulation to PTCR effects and the understanding of conduction process at
higher temperatures. Studies are accomplished for the first time with
emphasis to investigate electrical properties of undoped BT in the
wide range of temperatures (40–700 °C) at 1 kHz frequency; crystal
structure, morphology, thermal and dielectric behavior coupled with
ferroelectric measurements were explored as well.
2. Experimental
Fig. 1. XRD Patterns of BaTiO3 ceramics (0.94) sintered at 1300 °C/2 h, pre fired at 1100–
1200 °C/4 h (a–b), 1300 °C/1-4 h (c–f).
2.1. Preparation of BaTiO3
BT ceramics were prepared from commercial grade precursors barium carbonate (BaCO3, Fluka 98.5% pure) and titanium oxide (TiO2,
sigma Aldrich 99% pure) by solid state sintering method. Two batches
in compositions with Ba/Ti ratio 0.94(a) and 0.98(b) were weighed accordingly and hand agate mortared intensively for 8 h. In the first step,
mixed powders (a) were fired at 1100 °C and 1300 °C for 4 h, in the second , powders of both batches (a) and (b) were fired at 1300 °C for 1–4 h
separately. The fired powders were milled in agate mortar for 2 h to obtain chemical homogeneity [1] and sieved in laboratory test sieve stainless steel with mesh no. 150. Sieved powders were granulated
and binded with 5 wt% polyvinyl alcohol (PVA), pressed into circular
pallets of 2 mm thickness and 12 mm diameters at 20 MPA pressure.
They were fired in an open air furnace at 1300 °C/2 h.
2.2. Characterization
The crystal structure and phase development in the powders were
studied using X-Ray diffractometer (XRD) (PANalytical, X'pert pro)
with Cu K α & radiation source, being operated at 40 kV and 30 mA.
Morphological features were observed using scanning electron microscope (JSM 6490) equipped with energy dispersive X-ray spectroscopy
(EDX). Thermal behavior of the as ground raw material was interpreted
by employing thermo gravimetric analysis (TGA) analysis and differential scanning calorimetry (DSC) (SDT Q600 V8.0 build 95 model). Capacitance and resistance were recorded in the temperature range of 40–
700 °C at 1 KHz frequency by using LCR meter (6440B) WYNE KERR
connected with homemade temperature programmable furnace at
heating rate of 5/°C min. The ac conductivity (σ ac) was also evaluated
from dielectric data by using relationship σ ac = 2πf Ɛ′ Ɛ0 tanδ. Ohmic
conductivity (σ dc) was fitted to Arrhenius equation in the temperature
range 200–360 °C and 480–700 °C to calculate Ea values. Drift mobility
was calculated by employing the formula μd = 1/neρ. Ferroelectric measurements are made on ferroelectric tester and keithly (8512 A)
electrometer.
barium and oxygen in the surface layer generally contribute to the initial formation of Ba2TiO4 and TiO2 that further result in BT formation. Increasing sintering time contributed to increase in crystalinity, however
minor peaks of Ba2TiO4 were detected analogous to the other reports
[19,27]. With composition 0.94, besides Ba2TiO4, minor peaks of TiO2
were observed around 2θ = 43° (fig.2, a–f).
All the peaks were labeled by comparing the XRD data with JCPD
card No. 01-074-1964, 01-075-2120, 01-083-1880, the perovskite barium titanate synthesis. Ceramics with both the precursor composition
showed tetragonality N 1 and were indexed to the perovskite (ABO3)
ferroelectric materials [28]. Cubic (Pm-3m) and tetragonal (P4mmm,
P4MM) crystal structures were indicated. Splitting of 002/200 peaks
around 2θ = 45° that supported the tetragonal symmetry (Fig.1, e and
Fig. 2, c–d). The widely used indirect method based on XRD data was
employed for calculations, lattice parameters c and a were taken into account. Lattice parameter c increased with increasing temperature and
time that increased tetragonality of both compositions (Fig. 3a).
However, cubic perovskite BT was obtained with composition 0.94
(Fig.1 f) owning to increase in lattice parameters ‘a, with increasing
sintering time. Chekcell software was used to calculate the lattice parameters. The crystallite size of the BT ceramics were calculated using
Scherrer formula
t¼
0:9λ
β cosθ
3. Results and discussion
3.1. Preparation analysis and structural properties
Phase formation, tetragonality and crystallite size of the obtained ceramics were determined from the indexed XRD diffraction patterns.
Ba2TiO4 was detected as secondary phase in both compositions 0.94
and 0.98 (Fig.1 a–f, Fig. 2, a–b). An excess of BaO or TiO2 in the precursor
compositions usually result in the formation of extra phases like
Ba2TiO4, Ba2Ti5O12 [26]. Beauger et al. [20] suggested that the excess of
Fig. 2. XRD Patterns of BaTiO3 ceramics (0.98) sintered at 1300 °C/2 h, pre fired at 1300 °C/
1–4 h (a–d).
18
N. Sareecha et al. / Solid State Ionics 303 (2017) 16–23
of the BT powders, prefired at 1100–1200 °C (Fig. 4 a, b). Needle like
grains revealed the existence of Ba2TiO4 phase in accordance with
XRD results (Fig .1 a, b). Significant densification and growth formation
progressed on increasing the prefiring temperature and duration to
1300 °C /4 h. Grain size varied from 0.5 μm- 1 μm and 5 μm- 10 μm
(Fig.4 c–d). With 0.98 Ba/Ti contents, microstructure of typical polycrystalline materials was observed with almost interconnected angular and
somewhat rounded grains (Fig.4d). However, the grains were coarse
and appeared to be denser and surrounded by the randomly oriented
neighboring grains. They were closely adhered at the grain boundaries;
fractured surfaces were also revealed. On decreasing the Ba/Ti contents
to 0.94, less dense specimens (Fig. 3b) with polygonal, rounded and
plate like grains were obtained (Fig.4 c). Difference in the grain sizes
and shapes indicates varying Ba/Ti contents and the lack of uniformity
among the specimens.
3.3. Thermal analysis
Thermal behavior, crystallization temperatures of the as ground raw
materials were studied using high temperature differential scanning
calorimeter (DSC) well equipped with thermogravimetric
analysis(TGA). The samples were scanned from room temperature to
1300 °C at the heating rate of 10 °C/min under the dynamic atmosphere
of insert (Ar) Gas. Fig. 5 a, b describes the DCS/TGA thermograms of the
as ground raw powders for both precursor compositions. The powders
exhibited broad endothermic curves in the entire temperature range
of about 1170–1190 °C and then responded to exothermic behavior.
Two melting regions were observed around 449 °C and 810 °C, the second region corresponds to melting point of BaCO3. First crystallization
was initiated at 965 °C, the other at 1100–1170 °C (Fig. 5.b) and then
responded to exothermic region at 1300 °C. DCS/TGA thermo gram of
both samples were almost similar with the exception of small endothermic peak at 1100–1170 °C that reveals better crystallization of the powders in accordance with XRD patterns Fig. 2. (d)
Fig. 3. (Panel a) Tetragonality (A) and crystallite size inset (B) of BaTiO3 ceramics sintered
at 1300 °C/2 h, pre fired at their respective temperatures (a and b) 1300 °C/1–4 h and (c) at
1100–1300 °C/4 h. (Panel b) Density of BaTiO3 ceramics (A) sintered at 1300 °C/2 h, pre
fired at the temperatures (a and b) 1300 °C/1–4 h, Inset B shows the powders (a) pre
fired at 1100–1300 °C/4 h.
where, t is the crystallite size, λ is the wavelength of the incident X-rays
(1.5406 Å), θ is the Bragg's diffraction angle and β is the full width at half
maximum (FWHM). All the peaks were employed in the calculations.
The density was obtained by measuring the dimensions and mass of
the samples and calculations were made by employing the eq. [29]
ρ¼
4m
2
d
hπ:
where, m is the mass, d is the average diameter and h is the height of the
sintered samples. Density increased owning to decrease in porosity with
increasing sintering temperature and sintering time for both compositions (Fig. 3b). Maximum densities were 4.67 and 5.02 g/cm3 with considerable corresponding porosities 22.87 and 16.82%. Values of density
are relatively less due to the ununiformed coarse large grain structure
[30] and to the presence of porosity in the obtained specimens (Fig. 4
c–d). Crystallite size increased as well owing to coalescence of crystallites [31] (Fig. 3b).
3.2. Morphology
Morphological features of the ceramics were studied using scanning
electron microscopy (SEM); the sintered samples were coated with
gold. Sintering temperatures and Ba/Ti molar ratio obviously influenced
grain formation and densification. Large irregular agglomerated grains
coupled with needle like formation were observed in the micrographs
3.4. Electrical properties
3.4.1. Dielectric studies
The dielectric constant (ԑ′) and loss tangent (δ) of the BT ceramics
were measured by parallel plate technique at 1 kHz frequency. The dielectric calculations were made by using the relation.
ε¼
Cd
εo A
where d is the thickness of pallet, C is the capacitance, ԑo is the permittivity of free space and A is the cross-sectional area of the pallet. Specimens were sintered at 1300 °C /2 h. In Fig. 6 insets A (a–f) and B (c–d)
describe the effect of varying sintering temperature and sintering time
on the dielectric constant of BT ceramics. Increasing sintering temperature and prolongation of sintering time resulted in the crystallization
(Fig.1 a–f and Fig.2 a–d) and densification of specimens with conspicuous grain growth (Fig.4 a–d) [32–34]; dielectric constant enhanced
accordingly.
Temperature dependence of dielectric constant (ԑ′) can be divided
into two temperature regions, region (I) from 40 to 200 °C corresponding to structural phase transition from ferroelectric to paraelectric. Region (II) from 200 to 700 °C correspond to high temperature region
where enhanced dielectric response was observed. In region (I), dielectric constant increased with increasing temperature, maximum dielectric constant was observed at phase transition (TC = Curie
temperature). In general, for any material, ԑ′ at different frequencies increases with increase in temperature and is influenced by dipolar, interfacial, ionic and electronic polarizations [35]. In the ferroelectric phase,
below TC, BT has tetragonal symmetry with permanent electric dipole.
These dipoles are ordered with a domain structure. Near TC, the domain
N. Sareecha et al. / Solid State Ionics 303 (2017) 16–23
19
Fig. 4. SEM Micrographs of BaTiO3 powders and BT pallets. Powders a and b (0.94) were pre fired at 1100 °C–1200 °C/4 h , BT pallets c (0.94) and d (0.98) at 1300 °C/4 h. Inset shows
magnified version of grains at 1 μm, headed arrows reveal the fractured surfaces.
structure breaks up and carriers become free. At Curie temperature,
ionic and electronic polarizability are at the peak levels and dielectric
constant attains the maximum value. Among all specimens, f (0.94)
and d (0.98) showed maximum dielectric constant at TC, 1110 and
1520 that are considerably low as compared to other studies [19,36]
due to stresses generated owning to large grain structures [37]. Moreover, as fully dense materials were not obtained; porosity presence
also lowered the permittivity values and resulted in the increase of
loss tangent [38,39]. TC was shifted from 120 to 130 °C with decreasing
Ba/Ti contents in accordance with other results [19]. Specimen f and d
were employed for all further investigations in the wide temperature
range of 40–700 °C.
In the paraelectric phase, BT has cubic symmetry with large dipole
moments where the dielectric constant continued to decrease rapidly
with classical dielectric behavior. With increasing temperature, the randomness of dipoles enhances that contribute to decrease in dipolar polarization. However, in region (II) after 200 °C studied materials
depicted the typical broad dielectric relaxor behavior. Dielectric constant continued to increase gradually with rising temperature, dielectric
anomalies were observed for both materials (f and d) in the vicinity of
500 °C (Fig. 6 C) analogous to other studies for BaTiO3 system [40,41].
At 700 °C, dielectric constant approached to about three order jumps
as compared to that at TC. ԑ′ and δ increased with temperature that revealed the presence of thermally activated transport properties in the
materials. It is known, oxygen vacancies are major structural defects
in the barium titanate [42,43], generated due to loss of oxygen during
sintering at high temperature in accordance with following relation, a
process defined by Kronger-Vink notation [44].
VO →
1
O2 þ V″O þ 2e
2
Electrons created by ionization of thermally activated oxygen vacancies captured by Ti 4+ cause the hopping of electrons between Ti 4+ and
Ti 3 + that may attribute to the enhanced dielectric response in the
studied ceramics Fig. 6 (C). Kang et al. [40] related dielectric anomalies
to the competition phenomenon of the dielectric relaxation and the
electrical conduction by oxygen vacancies.
The ac conductivity was evaluated from dielectric constant (ԑ΄) and
dielectric loss tangent tan (δ) in the same frequency range (1 k Hz) by
employing the following relation [45]
σ ac ¼ ω0 ε0 tanðδÞ
where, σ ac is the ac conductivity and ω = 2 πf, the angular frequency, ԑ′
is the dielectric constant and tan (δ) is the loss tangent. Insets A and B
(Fig. 7) show ac conductivity from 40 to 120 °C–130 °C at varying processing parameters. Ac conductivity increased with increasing sintering
temperature and prolongation of sintering time in accordance with dielectric studies. Maximum value ac conductivity around the vicinity of
phase transition were 2.41 × 10−6S m− 1 S m− 1 and
2.57 × 10−6 S m−1, for f and d specimens respectively. As fully dense
materials were not obtained; morphology of the specimens indicated
the presence of porosity. The presence of pores might provide conduction path to electricity; could be a main contribution of tan (δ) and
thus to ac conductivity [38,39]. The values are almost in the pact of
other results [46]. However, conductivity continued to increase with increasing temperature, owning to increase in dielectric constant and dielectric losses. Fig. 7C shows the variance in conductivity with
temperature. At higher temperatures, like dielectric response, electrical
anomalies with a relaxor behavior were observed. Conductivity
approached
to
about
order
jump
of
1.9 × 10− 3S m−1 – 2.85 × 10−3S m−1 for specimen f and d at 700 °C,
high values indicate the long range movement of the charge on thermal
excitation. Ang et al. [47] described that doubly ionized oxygen vacancies can move due to thermal excitation. Conduction electrons created
by the ionization of oxygen vacancies can cause hopping of electrons between Ti 4+ and Ti 3+. Enhanced hopping of electrons at higher temperatures might have decreased the resistance of grains thereby increasing
the probability of electrons reaching the grain boundaries. Hence,
20
N. Sareecha et al. / Solid State Ionics 303 (2017) 16–23
Fig. 7. Variance of ac conductivity for BaTiO3 ceramics (C) with temperature at 1 k Hz
frequency. Insets (A and B) describe pre firing temperature of specimens Vs time.
eventually give rise to dc conductivity at high temperatures at lower
frequencies.
increased polarization had possibly elevated the ac conductivity of the
studied materials. Hence, conduction could occur through an electronhopping mechanism [48] at low temperatures and at higher temperatures, σ ac tends to the value of dc conductivity on thermal activation.
Moreover, interpretation of different theoretical models [49–51] concludes that ac conductivity originates from migration of ions by hopping
between neighboring potential wells at lower temperatures which
3.4.2. Electrical dc resistivity and dc conductivity studies
Electrical dc resistivity (ρ) measurements of the sintered were made
using two probe method [52,53] at room temperature and from 40 to
700 °C by employing circular pallets of 2 mm thickness and 12 mm diameters. Pressure contacts equal to the pallet size were applied after
polishing the surfaces. In Fig. 8 insets A (a–f) and B (c–d) describe the
effect of varying sintering temperature and sintering time on the resistivity of BT ceramics. Increasing sintering temperature and prolongation
of sintering time resulted in the increase of resistivity [7,54]. Specimen f
and d showed the greater resistivity. Resistivity measured at the room
temperature for f and d were 4.5×109Ω cm , 3.5 × 109Ω cm respectively
in agreement with the reported literature [7].
Resistivity plots can be divided into regions. The first region from 40
to 120-130 °C, up to phase transition decrease in resistivity was slow
that may be due to the ordered state of ferroelectric phase. The second
region from 140 to 700 °C (Fig. 8C) where decrease in resistivity was
more rapid, semiconductor behavior was observed. Oxygen vacancies
are the most mobile charge carriers in oxide ferroelectrics and play an
important role in the conduction process in most dielectric ceramics
Fig. 6. Temperature dependence of dielectric constant (ԑ′) and loss tangent (δ) of the
BaTiO3 ceramics C (f and d) at 1 k Hz frequency. Insets (A and B) show their pre firing
temperature Vs time.
Fig. 8. Resistivity of BaTiO3 ceramics (C) Vs temperature in the ferroelectric and
paraelectric regions at 1 k Hz frequency. Insets (A and B) depict resistivity at 40–200 °C.
Fig. 5. a and b, TGA and DSC thermograms of the as ground BaTiO3 specimens, 5a (0.94)
and 5b (0.98).
N. Sareecha et al. / Solid State Ionics 303 (2017) 16–23
[55,56]. Oxygen vacancies can be neutral, single and doubly ionized respectively Vo, V o ⦁, V o ⦁⦁. Activation energies, Ea of 0.3–0.5 and 0.6–1.2 eV
are typically assigned to single ionized and doubly ionized oxygen vacancies [47,57] Conductivity of the specimens (f and d) was evaluated
from the impedance spectrum by using the relation [58]
σ dc
d
¼AR
where d is the sample thickness, A is the electrode area; R is the resistance of bulk ceramics. Fig. 9a and b show the variation in dc conductivity of both samples with inverse of absolute temperature, which
followed the Arrhenius Law [59]. Both specimen f and d showed semiconductor behavior with negative temperature coefficient of resistivity
(NTCR) characteristics.
Ea
σ ¼ σ 0 exp −
kB T
where σ₀ is the pre-exponential factor, Ea is the activation energy, kB is
the Boltzmann's constant and T is the absolute temperature in kelvin.
A close agreement between the experimental conductivity data and Arrhenius curves was obtained for f and d specimens in the range of 200–
360 °C and 480–700 °C (Fig. 9a and b); respective estimated Ea values
were 0.2948–0.3284 and 0.967–1.189 eV. Obtained Ea values were associated to singly ionized and doubly ionized oxygen vacancies V″O, V″O.
Hence, with increasing temperature, conduction seemed to be governed
by the single ionized oxygen followed by doubly ionized oxygen vacancies analogous to other studies for BaTiO3 ceramics [41]. We propose
21
that ionic conduction may be responsible for the conduction process
for the studied ceramics. Although both specimens depicted semiconเด่นv more resistive
ductor behavior, yet specimen, f was conspicuously
than d in the entire temperature range. Ceramics with more Ti contents
(specimen, f) showed greater resistivity as compared to that with more
Ba contents (specimen, d) in agreement with other studies [60].
3.4.3. The dc mobility studies
Drift mobility was calculated at temperatures (480–700 °C) using
underlying formula
μd ¼
1
neρ
where is ρ resistivity, e is the charge on electron and n is the carrier concentration. Value of n is calculated by following famous eq. [61]
n¼
NA Dm PBa
M
NA, Dm, M denote the Avogadro's no., mass density and molecular
weight of BT and PBa denotes the number of barium atoms in the formula unit of BT. Fig. 10 displays temperature dependence of drift mobility
for materials f and d. Our Ea values calculated at the temperatures
200–360 °C and 480–700 °C, were in the range of singly and doubly ionized oxygen vacancies V″O. Since doubly ionized oxygen vacancies can
move due to thermal activation [47], mobility of electrons increased
with rise in temperature due to hopping of charge carriers from one
site to another owning to decrease in resistivity for both materials. Ceramics with more Ba contents (specimen, d) showed greater drift mobility as compared to that with more Ti contents (specimen, f) in
accordance with conductivity studies (Figs. 7 and 8).
3.5. Ferroelectric studies
Ferroelectric behavior was investigated at room temperature using
circular pallets of both the composition f (0.94) and d (0.98) sintered
at 1300 °C/2 h. Well defined (P-E) hysteresis loops under electric field
were obtained at room temperature (Fig.11) that indicate spontaneous
polarization of BT samples, a characteristic of typical ferroelectric materials [62]. Slight gape existed in the P-E loop that may be taken as an indication of incipient dielectric breakdown. Maximum polarization,
remnant polarization and coercive field were estimated to be Pm =
1.615 and 2.872 C/cm2, 2Pr = 0.794 and 1.408, EC = 8.394 and 8.449 respectively for f and d specimens. Low values of remnant polarization can
be related to the low internal polarizability and lower EC values may
Fig. 9. (a and b). Variation in conductivity (σ dc) with inverse of absolute temperature for
BT ceramics.
Fig. 10. Dependence of drift mobility with inverse of absolute temperature for BT ceramics.
22
N. Sareecha et al. / Solid State Ionics 303 (2017) 16–23
providing access to the work of analysis. Implausible thanks are to A.
Maqsood for her efforts at all grounds. Assistance of A.S. Saleemi (Ph.
D Scholar) to analytical measurements is incredible. Services of M.
Boota, Senior Technician and M. Shah Zaman, Furnace Operator, Glass
and Ceramics Research Centre PCSIR Laboratories Complex, Ferozpur
Road, Lahore-54600, Pakistan are acknowledged as well.
References
[1]
[2]
[3]
[4]
[5]
[6]
Fig. 11. P-E loops of BT ceramics sintered at 1300 °C/2 h, pre fired at 1300 °C/4 h.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
account to the large grain structures of the specimens [1] in accordance
with SEM results (Fig. 4 and d). The values of the spontaneous polarization are considerably low as compared to other results that may be due
to lack of homogeneity and uniformity among the grains of the studied
materials.
[16]
[17]
[18]
4. Conclusions
[19]
Polycrystalline BT ceramics were prepared through solid state
sintering. Ba/Ti ratio influenced both the crystal structure and Curie
temperature (TC). Specimens were crystallized to the perovskite materials with cubic and tetragonal structures (Pm-3m, P4MM). Curie temperature decreased with increasing Ba/Ti contents (TC 130–120 °C). All
studies including thermal analysis confirmed better performances of
BT ceramics with the 0.98 composition. Enhanced dielectric and electrical response was observed in the paraelectric regions. Resistivity decreased with increasing temperature, all specimens showed
semiconductor behavior with negative temperature coefficient of resistivity (NTCR) characteristics. Conductivity followed Arrhenius Law.
Ionic conduction was supposed to be responsible for the conduction
process; which can be attributed to the single ionized and doubly ionized oxygen vacancies. Hooping of conduction mechanism prevailed at
elevated temperatures with increasing dc mobility in the paraelectric
regions. Well defined P-E loops showed ferroelectric characteristics,
considerable polarization (Pm) and remnant polarization (2Pr) values
indicated ferroelectricity and internal polarizability of the obtained materials respectively. Studied materials with 0.94 composition had Curie
temperature 130 °C and were found to be more resistive; they could
be employed for high temperature positive temperature coefficient of
resistance (PTCR) application after manipulation with suitable dopants.
Ceramics with 0.98 composition showed relatively more dielectric behavior; all specimens showed semiconductor behavior with negative
temperature coefficient of resistivity (NTCR) characteristics at high
temperatures, which might be useful for the application for BT based
useful energy storage systems [58]. In addition with this composition,
high performance dielectric ceramics can be obtained by extending
milling time.
Acknowledgements
N. Sareecha is pleased to acknowledge Higher Education Commission, Islamabad, Pakistan (PIN:063-171039-Ps3-228) for awarding indigenous Ph. D fellowship; COMSATS Institute of Information and
Technology, Park Road, Islamabad 44000, Pakistan and National University of Science and technology (NUST) H-12, Islamabad, Pakistan for
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