Ceramics International xx (xxxx) xxxx–xxxx
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Ceramics International
journal homepage: www.elsevier.com/locate/ceramint
B2O3 doping in 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 lead-free piezoelectric
ceramics
⁎
Metin Ozgul , Abdullah Kucuk
Afyon Kocatepe University, Department of Materials Science and Engineering, Afyonkarahisar, 03200 Turkey
A R T I C L E I N F O
A BS T RAC T
Keywords:
C. Dielectric properties
D. BaTiO3 and titanates
Piezoelectric properties
Perovskites
The effects of B2O3 doping on the dielectric and piezoelectric properties of 0.94(Bi0.5 Na0.5) TiO3-0.06BaTiO3
(BNT-6BT) ceramics were investigated. Pre-reacted BNT-6BT powders synthesized by solid state reaction
method were doped with B3+ ions in 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1, and 2 mol%. All samples were sintered at 1150 °
C for 12 h in air. The effects of boron doping on structure and electrical properties were characterized as a
function of dopant concentration. The results indicate a substantial increase in piezoelectric coefficient (d33),
dielectric constant (εr ), and dielectric loss (tan δ ) approximately 19%, 42%, and 95%, respectively. The positive
effects of doping were more significant in BNT-6BT samples with 0.3% and more B2O3 addition. The highest
piezoelectric coefficient was obtained as d33=173 pC/N in 1% doped BNT-6BT. In terms of dielectric properties,
both εr and tan δ values increased with B3+% reaching 1075 and 0.0421, respectively in 2% doped samples. Even
though the measured density values remain relatively unchanged, the increase of all these coefficients together
implies a donor dopant role of B3+ ions.
1. Introduction
Piezoelectric ceramics are very important for many electronic
applications and many researchers have shown great interest in the
synthesis and electrical properties of these materials. For more than a
half century lead-based piezoelectric ceramics, such as Pb(Zr1−xTix)O3
(PZT), have been the most commonly studied and used piezoelectric
materials in sensors, actuators, and transducers due to their excellent
piezoelectric properties [1–3]. The outstanding properties of PZT
ceramics are mostly obtained in the compositions near the morphotropic phase boundary (MPB) where Zr/Ti ratio is ~53/47. Due to
coexistence of both rhombohedral and tetragonal phases around MPB,
a high degree of alignment of ferroelectric dipoles takes place resulting
in striking enhancement in the piezoelectric properties [4]. However,
the strong toxicity and high volatility of lead oxide during synthesis and
processing at elevated temperatures cause severe environmental and
ecological problems. Therefore, synthesis and use of PZT ceramics were
started to be regulated in many countries [5–9]. For this reason
piezoelectric materials research started to focus on finding alternatives
to lead-based systems in recent years. Significant improvements have
been made in the development of lead-free piezoelectric ceramics with
good properties which are comparable with PZT. The search for
alternative piezoelectric ceramics has been concentrated on alkali
niobates (KNaNbO3; KNN), modified barium titanates (BaTiO3; BT),
and bismuth titanates (BiNaTiO3; BNT) and other systems with MPBs
⁎
[4–9]. Among these materials, a solid solution of 0.94(Bi0.5Na0.5)TiO30.06BaTiO3 (BNT-6BT) with a MPB has been considered as a potential
alternative for piezoelectric applications [10]. Similar with PZT, MPB
separates rhombohedral and tetragonal phases in BNT-6BT. BNT is a
perovskite ferroelectric material with a large remanent polarization
(Pr=38 μC/cm2). However, due to its relatively large coercive field
(Ec=73 kV/cm) pure BNT piezoelectric ceramics are difficult to pole
effectively and therefore provide low piezoelectric properties
(d33=83 pC/N), which is significantly lower than that of the Pb
(Zr0.52Ti0.48)O3 ceramic (d33=223 pC/N) [1,10,11]. Much higher piezoelectric properties (d33=155 pC/N) were reported for BNT-6BT in
contrast to pure BNT [9–13]. The coercive field values reported to be
much lower for BNT–BT compositions near the MPB in comparison to
pure BNT, providing substantially improved piezoelectric properties
[11–13]. On the other hand, in order to meet the strict requirements
for specific applications, the electrical properties need to be improved,
for example, by doping of various oxides [14–24]. Modifications of
BNT-BT include lithium [14], cerium [15], lanthanum [16], zirconium
[17], niobium [18], tantalum [19], antimony [20], hafnium [21], cobalt
[22], silver [23] and manganese [24]. All enhance some combination of
piezoelectric and dielectric properties by improving densification and/
or controlling microstructure and chemistry. In various lead-free
perovskite systems B2O3 was used as sintering aid and/or dopant to
improve properties [25–32]. Studies in BaTiO3 based compositions
have reported that both dielectric and piezoelectric coefficients increase
Correspondence to: Afyon Kocatepe University, Department of Materials Science and Engineering, ANS Kampusu, 03200 Afyonkarahisar, Turkey.
E-mail address: metinozgul@aku.edu.tr (M. Ozgul).
http://dx.doi.org/10.1016/j.ceramint.2016.09.073
Received 6 August 2016; Received in revised form 8 September 2016; Accepted 10 September 2016
Available online xxxx
0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Ozgul, M., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.073
Ceramics International xx (xxxx) xxxx–xxxx
M. Ozgul, A. Kucuk
3. Results and discussion
with B2O3 addition mainly due to the lowered sintering temperature
and formation of boron interstitials acting as donors [25,26]. Recently
the influence of B2O3 addition in K0.5Na0.5NbO3 (KNN) ceramics has
also been investigated and improved dielectric and piezoelectric
properties reported [29,30,32]. However dielectric and piezoelectric
properties of B2O3 added Bi0.5Na0.5TiO3 (BNT) ceramics have not been
studied previously. In this work, 0.94(Bi0.5Na0.5)-0.06BaTiO3 (BNT6BT) was selected as a representative of MPB compositions in the
BNT–BT system to study. BNT-6BT ceramics were doped with varied
amounts of B2O3 and the influence of B3+ on the structure and
electrical properties was examined.
Fig. 1(a) shows the XRD diffraction pattern of ceramics as a
function of boron (B) concentration (mol%) in BNT-6BT+xB ceramics,
measured at room temperature and in the 2θ range of 20–70°. All
samples possess a pure perovskite structure and no secondary phase
was detected, implying that both Ba2+ and B3+ have diffused into BNT
lattice to form a homogeneous solid solution. Fig. 1(b) shows the
expanded XRD patterns of ceramics in the 2θ range of 39–47°. It is
known that the rhombohedral symmetry of BNT at room temperature
is characterized by a (003)/(021) peak splitting of between 38° and 42°
and a single peak of (202) between 45° and 48°. For tetragonal
symmetry, it is characterized by the profile of the (111) XRD line with
a single peak in the range of 38.5–41° and splitting of the (002) and
(200) peaks in the range of 44.5–47.5° [34]. As observed in Fig. 1(b),
for samples of all compositions (x=0–2 mol% B) both (111) and (200)
peaks were found to be split, demonstrating that these compositions
were located in the MPB region, with the coexistence of rhombohedral
and tetragonal phases. It is also worth to note that a substantial shift to
higher 2θ angles was observed when boron (B3+) doping is 0.4 mol%
and over. This may be due to a reduced lattice constant as new
vacancies form in the perovskite crystal as a function of doping. There
may be at least two different sources of vacancies in BNT-BT system as
shown in the following defect chemistry Eqs. (1) and (2) based on
Kröger-Vink notation:
2. Experimental
Bi2O3 (% 99.9, Sigma-Aldrich), Na2CO3 (% 99.9 Merck), BaCO3 (%
99, Sigma-Aldrich), TiO2 (% 99.8, Sigma-Aldrich), and B2O3 (%99.999,
Alfa Aesar) were used as raw materials to prepare 0.94(Bi0.5Na0.5)TiO30.06BaTiO3+(x mol%)B2O3 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 2.0)
ceramics by the solid-state reaction method. The choice of the B3+
dopant concentrations in this study was made in light of earlier
research for similar perovskite materials reported in the literature
[25–32]. The amount of B2O3 was systematically increased from 0.1 to
2 mol% step by step to determine the optimum dopant concentration
based on primary piezoelectric coefficient (d33) measurements for every
composition. First, BNT-6BT powders were synthesized by using high
purity Bi2O3, Na2CO3, BaCO3, and TiO2 based on the stoichiometric
formula. The carbonates were dried at 150 °C for 4 h prior to use. The
weighed batch was ball-milled in ethanol with 3 mm zirconia balls in a
polyethylene bottle for 24 h, then dried, and calcined at 900 °C for 4 h.
The phase formation of the calcined mixture was checked by x-ray
diffraction technique (XRD). At the second step, pre-reacted BNT-6BT
powders were mixed thoroughly with different amount of B2O3 by remilling to prepare compositions doped up to 2 mol%. After a second
milling, powders of all the compositions were further mixed with 5 wt%
polyvinyl alcohol (PVA) as a binder and then uniaxially pressed into
discs with a diameter of 12 mm at 100 MPa pressure. Pre-shaped
specimens were further densified by a cold isostatic press (CIP) at
150 MPa. After burning out PVA at 600 °C for 3 h with 1 °C/min
heating rate, the green samples were placed on a Pt foil in an alumina
crucible and sintered in air at 1150 °C for 12 h. Prolonged soaking time
was chosen to ensure the full crystallization of B2O3 containing
ceramics. Bulk density (ρb) of the sintered samples was measured by
the Archimedes method (ASTM C373)[33]. As one of the easy methods
for determining bulk density it involves measuring the dry weight (D)
of the sample, boiling it in distilled water for 5 h, cooling it in water for
24 h, and measuring the suspended weight in water (S) and wet weight
in air (W). The bulk density is then calculated as follows: ρb=D/V=D/
(W−S) where V represents the volume. Phase formation and the crystal
structure of all the samples with different boron content were analyzed
using an x-ray diffractometer (Bruker D8 Advance) with CuKα radiation (λ=1.5406 Å). In order to determine the influence of doping on
phase structure of the ceramics, fine scanning was also recorded with a
step interval of 0.02° in the range of 38–48°. The microstructure of the
thermally etched (900 °C for 12 h) surfaces of sintered ceramics was
analyzed by using a scanning electron microscope (LEO 1430VP). For
electrical characterization, the parallel surfaces of samples were
polished using 1200-grit SiC paper before applying a sputtered gold
electrode. The thickness of samples used in this study ranged from
0.824 to 1.193 mm. The ceramics were poled for piezoelectric measurement in silicone oil bath at room temperature for 20 min and DC
electric field strength of 30 kV/cm by using a high voltage supply
amplifier/controller (Trek Model 610 E, Lockport, NY). The room
temperature dielectric properties (εr and tan δ) were determined using
an LCR-meter (Instek LCR-816) at a frequency of 1 kHz. The piezoelectric coefficients d33 of the samples were measured using a piezoelectric d33-meter (APC YE2730A d33 METER).
Fig. 1. XRD patterns of undoped and doped BNT-6BT as a function of x=B3+ content
(mol%) in the 2θ range of (a) 20–70° and (b) 39–47°. (Note the shift in peaks with
increasing B3+% in (b)).
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M. Ozgul, A. Kucuk
BaO
B2 O3
Na2O •
→ BaNa + V/Na + O Ox
BNT − BT
→
2B•••
+ 6V/Na + 3O Ox
i
interstitial position in the BNT-6BT perovskite lattice [26]. As the
amount of B3+ mol% increases, higher concentration of positive
charges will be added to the system. These positively charged boron
interstitials, B•••
i , may suppress the negatively charged V′Na defects even
formed in undoped BNT-6BT due to volatilization of Na+ alkali ions.
However since each B•••
can neutralize three of V′Na defects, excessive
i
boron doping may also cause the formation of sodium vacancies in the
system. This is consistent with the shifting of (111) and (200) peaks
towards higher 2θ angles when the boron dopant concentration is
0.4 mol% and over. Also in the case of boron doping, since B•••
may
i
neutralize V′Na defects in BNT-6BT lattice, the need for the formation of
oxygen vacancies, V••
O , will be reduced. Suppression of oxygen vacancies, V••
O , is known to influence both sintering and properties of
piezoelectric ceramics [36–38].
(1)
(2)
The ionic radii of Ba2+, Na+, Bi3+, Ti4+, and B3+ are 0.160, 0.139,
0.128, 0.061, and 0.020 nm, respectively, so Ba2+ ions likely to
substitute Na+ alkali ions with similar size creating negatively charged
cation vacancies, V′Na [35]. Such excess negative charges will have to be
balanced by positively charged defects in the system. One strong
possibility is the formation of oxygen vacancies,V••
O , which would
consequently increase total vacancy concentration in the crystal lattice.
Another source of vacancies in the BNT-BT crystals in this study can be
the boron impurity ion as a dopant. Due to its extremely small ionic
radius of approximately 0.020 nm, B3+ ion is expected to occupy
Fig. 2. SEM images of undoped and doped BNT-6BT ceramics as a function of B3+ content (mol%): (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4, (f) 0.5, (g) 1.0, (h) 2.0. (Scale bar is for 1 µm).
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Fig. 2 shows the SEM micrographs of boron doped BNT-6BT
ceramics as a function of dopant concentration. As can be seen from
the figures, undoped and boron doped samples exhibit a dual microstructure containing large size grains (up to 6 µm) with smaller grains
(as small as 1 µm). BNT-6BT samples doped with B in 0.3, 0.4, 1.0, and
2.0 mol% exhibits much more uniform microstructures. It can be
concluded, with the exception of 0.5 mol%, that the average grain size
of the ceramics is reduced when the content of boron is 0.3 mol% and
over.
Table 1 shows the density values and electrical properties as a
function of dopant concentration in BNT-6BT ceramics sintered at
1150 °C for 12 h. To compare with the measured bulk densities,
theoretical densities are calculated as a function of dopant percent by
using reported theoretical densities of BNT-6BT and B2O3 given as 5.99
and 2.46 g/cm3, respectively [39,40]. Relative density values were also
calculated from the ratio of measured bulk density to calculated
theoretical density. As presented in Table 1 and shown in Fig.3, the
density increases from 5.57 g/cm3 for the undoped samples to 5.67 g/
cm3 for the 0.1 mol% B doped samples. The equally highest density was
obtained in 1 and 2 mol% doped samples as 5.71 g/cm3 (~96% relative
density). Electrical properties (d33 and εr ) were also observed to be
higher for these relatively high density samples. On the other hand, in
contrast to the lower density value of 5.51 g/cm3, 0.5 mol% doped
samples also exhibit improved electrical properties. B2O3 is usually
reported to act as a glass-forming agent in barium titanate based
ceramics. With its relatively low melting temperature, ~450 °C, B2O3
can form a liquid phase and promote densification during sintering
Fig. 4. Piezoelectric coefficient (d33) values of undoped and doped BNT-6BT as a
function of B3+ content (mol%).
[25,26,30]. However, the purpose of B2O3 addition into BNT-6BT
ceramics in the current study is not merely obtaining better densification. Besides improving the densification behavior, B3+ ions can also
play a different role as a dopant. In terms of improving electrical
properties both high density and donor doping can contribute.
Fig. 4 indicates the room temperature piezoelectric coefficient (d33)
values as a function of dopant concentration in BNT-6BT ceramics. If
the dopant concentration is less than 0.3 mol%, a decrease in d33 values
in contrast to undoped samples was observed. Measured d33 value of
145 pC/N for undoped samples dropped down to 136 pC/N for 0.2 mol
% B doped samples. When samples doped with 0.3 mol% B, a
substantial increase measured in d33 values reaching 156 pC/N. And
then, with further increasing B addition, the d33 remarkably increased,
giving a maximum value of 173 pC/N at 1 mol% B. Thereafter, d33
values started to decrease when B amount increased up to 2 mol%. It is
clearly observed that d33 coefficient values were higher for all the
compositions doped with 0.3 mol% and more B in contrast to undoped
BNT-6BT. The effect of B doping can be explained in two ways; first one
is its role as a sintering aid by providing liquid phase and the second
one is its role as a dopant modifying crystal chemistry. The highest d33
value of 173 pC/N was obtained in the composition (1 mol% B) having
the highest density of 5.71 g/cm3. For the 0.3 mol% B doped ceramics,
the density and the d33 values were measured as 5.67 g/cm3 and
156 pC/N, respectively. However in contrast to their lower density
values than 0.3 mol% B doped composition, higher d33 values of 161
and 163 pC/N were observed in 0.4 and 0.5 mol% B doped samples,
respectively. This may be explained by the modified crystal chemistry
of the samples as higher amount of B interstitials, B•••
, formed in
i
heavily doped ceramics. These excessive positively charged ions will act
as donors for the BNT-6BT system and improve the domain wall
contribution to piezoelectric properties [36–38]. Since the higher
amount of positively charged boron interstitials, B•••
i , would form in
the lattice structure, the need for oxygen vacancies, V••
O , will be
diminished in 1 and 2 mol% B-doped BNT-6BT ceramics providing
higher piezoelectric coefficients. This result also weakens the possibility
of B3+ ions to substitute with Ti4+ ions,BT/i , which would create more
oxygen vacancies, V••
O , instead of suppression.
Fig. 5. shows the dielectric constant (εr ) and dielectric loss (tan δ)
values as a function of dopant concentration in BNT-6BT ceramics,
measured at room temperature and a frequency of 1 kHz. Both
dielectric constant and loss values show an increase trend with the
increase of B-dopant concentrations. Highest εr and tan δ values were
recorded for 2 mol% B doped BNT-6BT ceramics. The significant
increase in both coefficients is consistent with the well known effect
of donor dopants in perovskite systems [1].
Table 1
Density values, piezoelectric, and dielectric property coefficients of undoped and doped
BNT-6BT as a function of B3+ content (mol%).
B3+
(mol
%)
ρth (g/
cm3)
ρb (g/
cm3)
ρr (=ρb/ρth)
d33
(pC/
N)
Ɛr (1 kHz)
tan δ
(1 kHz)
0.0
0.1
0.2
0.3
0.4
0.5
1.0
2.0
5.99
5.99
5.99
5.99
5.99
5.98
5.98
5.97
5.57
5.67
5.60
5.67
5.59
5.51
5.71
5.71
0.93
0.95
0.94
0.95
0.93
0.92
0.96
0.96
145
140
136
156
161
163
173
171
756
762
707
838
874
808
946
1075
0.0216
0.0264
0.0231
0.0272
0.0298
0.0269
0.0326
0.0421
ρth; Theoretical density, ρb; Measured bulk density, ρr; Relative density.
Fig. 3. Bulk density (ρb) values of undoped and doped BNT-6BT as a function of B3+
content (mol%).
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pressing of boron suboxide materials, J. Eur. Ceram. Soc. 28 (2008) 1613–1621.
Fig. 5. Dielectric constant (εr ) and dielectric loss (tan δ) values of undoped and doped
BNT-6BT as a function of B3+ content (mol%).
4. Conclusions
Phase structure, density, microstructure, dielectric and piezoelectric properties of undoped and up to 2 mol% B2O3 doped 0.94
(Bi0.5Na0.5)TiO3-0.06BaTiO3 (BNT-6BT) ceramics sintered at 1150 °C
for 12 h have been investigated as a candidate for lead-free piezoelectric ceramics. The incorporation of B3+ ions into the perovskite
structure resulted in an increase in piezoelectric coefficient (d33) from
145 to 173 pC/N. Dielectric constant (εr ) and loss (tan δ) values also
showed an enormous jump from 756 to 1075 and 0.0216–0.0421,
respectively with doping. The effect of doping on density values was
about 2.5% improvement for 1 and 2 mol% B2O3 containing ceramics.
The results indicate that, besides its well known role as a sintering
additive, B2O3 is a very effective dopant in improving the piezoelectric
properties of BNT-6BT by modifying the crystal defect chemistry. As
the density values are slightly affected, a remarkable increase of all the
dielectric and piezoelectric parameters measured in this study was
attributed to the donor dopant role of B3+ interstitial ions forming B•••
i .
Acknowledgments
This work was supported by the Department of Scientific Research
Projects (BAPK) of Afyon Kocatepe University, Turkey (Project funding
no. 15.FEN.BIL.09).
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