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this article in press as: Liu M, et al. Temperature stability of dielectric properties for xBiAlO3–(1 − x)BaTiO3ceramics. J Eur Ceram
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Journal of the European Ceramic Society xxx (2015) xxx–xxx
Temperature stability of dielectric properties for xBiAlO3–(1 − x)BaTiO3
ceramics
Mengying Liu, Hua Hao ∗, Yichao Zhen, Ting Wang, Dongdong Zhou, Hanxing Liu ∗,
Minghe
Cao, Zhonghua Yao
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Material Science and Engineering, Wuhan University of
Technology,
Wuhan 430070, People’s Republic of China
Received
1 December 2014; received in revised form 12 February 2015; accepted 14 February 2015
Abstract
xBiAlO3–(1
− x)BaTiO3[xBA–(1 − x)BT] polycrystalline ceramics were prepared by solid-state reaction method and sol–gel method, respectively.
The
ceramics were in tetragonal phase when x ≤ 0.1, transformed to pseudocubic at x > 0.1. The temperature stability of the dielectric constant for
BA–BT
was improved with BA contents increasing. Of particular interest was that the ceramics prepared by sol–gel method had less secondary
phase,
exhibiting much improved dielectric behavior. Among them, 0.3BA–0.7BT met the requirement of capacitance variation (C/C ≤ ±15%)
in
the temperature range of −55 to 440 ◦C with moderate dielectric constant (ε = 660) and low dielectric loss (tan δ = 1.2%) at room temperature
(1
kHz). Nb-doped 0.2BA–0.8BT ceramics were investigated to improve the dielectric temperature stability at high-temperature end. It is found
that
all samples with 0.01–0.04 mol Nb addition satisfied the X8R specification.
©
2015 Elsevier Ltd. All rights reserved.
Keywords: Temperature stability; Sol–gel method; Dielectric properties
1.
Introduction
Perovskite materials operating under harsh environments
(≥200 ◦C)
are of great interest for electronic device appli-
cations
[1–5]. Current researches are focused on lead-free
materials
in view of health and environmental concerns [6–11].
Among
them, the perovskite solid solutions BiMeO3–BaTiO3
have been extensively studied due to their high Curie tempera-
ture
(Tc) and good temperature dependent dielectric behavior
[12–14]. BiMeO3compounds, where Me can be Sc3+, Y3+,
Al3+,
(Mg1/2Ti1/2)3+ or (Zn1/2Ti1/2)3+, etc., will effectively
improve
the temperature-dependent behavior taking advantage
of
the combinatory substitutions of Bi3+ and Me3+ in BaTiO3
[15–22]. For example, the (1 − x)BaTiO3–xBi(Mg1/2Ti1/2)O3
(BT–BMT) system was reported to possess lower dielectric
maximum
temperature Tmwith diffused characteristics as BMT
∗Corresponding
authors. Tel.: [phone]; fax: [phone].
E-mail
addresses:
[email protected] (H. Hao), [email protected]
(H.
Liu).
increased, exhibiting relaxor-like behavior [23,24]. Analog to
this
system, BiAlO3was thought to be a promising lead-free
ferroelectric
with a
Curie temperature (Tc) of 800 K [25] and
a
solubility
limit of BiAlO3into BaTiO3about 0.12 [26].
Based
on the preliminary results, it was confirmed that the
combined
substitutions of Bi3+ and Al3+ played a dominant
role
to decrease the Tcand increased the relaxor behavior in
xBiAlO3–(1
− x)BaTiO3[xBA–(1 − x)BT] system [27]. Mean-
while,
it was observed the dielectric constant became flatter
above
the maximum dielectric constant temperature (Tm) as
BiAlO3increased,
improving the temperature-stable character-
istics
for capacitor applications. Furthermore, it is believed that
Nb
and many other oxides-doped BT systems could achieve
good
dielectric properties for advanced X8R capacitors with
improved
dielectric temperature stability at the cost of dielectric
constant
value [28–31]. Thus it is a key technical requirement
to
choose Nb as dopant in order to improve the temperature
stability.
Undesired
phases were easily appeared in xBA–(1 − x)BT
synthesized
by solid state method [26,27]. However, sol–gel
method
was an excellent technique for offering a better
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.02.015
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Please cite
this article in press as: Liu M, et al. Temperature stability of dielectric properties for xBiAlO3–(1 − x)BaTiO3ceramics. J Eur Ceram
Soc
(2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.02.015
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chemical homogeneity and low-temperature processing to pro-
duce
high-purity and
phase-pure powders in comparison with
solid
state method [32–34]. In this paper, xBA–(1 − x)BT ceram-
ics
were prepared by solid-state reaction method and sol–gel
method,
respectively.
The phase structures, dielectric properties
of
BA–BT with different BA contents were investigated. More-
over,
Nb-modified xBA–(1 − x)BT ceramics were also studied in
an
attempt to achieve low temperature coefficient of capacitance
over
a
wide temperature range.
2. Experimental procedure
2.1. Synthesis of xBA–(1 − x)BT powders by solid-state
reaction
method
The
xBA–(1 − x)BT (x = 0.1–0.3) ceramics were synthe-
sized
using Bi2O3(99%), Al2O3(99.99%), BaCO3(99.0%),
TiO2(99.0%)
as starting materials, 5 mol% excess Bi2O3was
added
to compensate the bismuth oxide volatility. They were
mixed
by ball milling in alcohol for 24 h. Then, the slurry was
dried,
and calcined at 900 ◦C for 2 h.
2.2. Synthesis
of
xBA–(1
− x)BT powders by sol–gel
method
The key of sol–gel method was to prepare xBA–(1 − x)BT
(x
= 0.1–0.3) sol. Firstly, Ti(C4H9O)4was dissolved in citric acid
solution
with
the pH value of 5–6, where the molar ratio of citric
acid
to
Ti4+ ions was 2:1. A transparent yellow organic solu-
tion
(named
sol I) was obtained after water bathing at 80 ◦C for
3
h. Then, Bi(NO3)3·5H2O(99.0%), Al(NO3)3·9H2O(99.0%)
and
Ba(CH3COO)2(99.0%) were batched stoichiometrically
according
to the nominal compositions of xBA–(1 − x)BT and
dissolved
in the acetic acid respectively. Thirdly, the solutions
were
mixed, followed by adding citric acid, where the molar
ratio
of citric acid to metallic ions was 2:1. After that, ammonia
solution
was added to adjust the pH value of the mixed slurry to
5.5.
The mixed slurry was water bathing to achieve a transparent
aqueous
solution (named sol II). Finally, the sol I and sol II were
mixed
to obtain a stable homogeneous sol(xBA–(1 − x)BT sol).
After
drying, the sol turned into a gel. xBA–(1 − x)BT powders
were
obtained by calcining the gel at 700 ◦C for 5 h. The detail
process
was illustrated in Fig. 1.
Fig. 1. The processing scheme of preparing xBA–(1 − x)BT sol.
2.3. Synthesis of Nb-doped 0.2BA–0.8BT powders and
ceramics
For
Nb-doping, firstly, the 0.2BA–0.8BT powders, prepared
by
sol–gel,
were dispersed in isopropanol by ultrasonic treat-
ment
about 30 min to obtain 0.2BA–0.8BT slurry. Nb(OH)5was
dissolved
in 0.3 mol/L oxalic acid solution to produce oxalic acid
of
Nb, which was added to the slurry. And then the pH value of
the
slurry was
adjusted to 5.5 with ammonia solution. The dried
powders
were calcined at 550 ◦C for 2 h to burn out the organics.
The
calcined powders mentioned above were subsequently
granulated
with the polyvinyl
alcohol (PVA) binder and pressed
into
pellets with 12 mm in diameter and 1.5 mm in thickness. The
pellets
were first heated at 600 ◦C for 2 h to drive off PVA, and
then
sintered at temperatures between 1050 and 1200 ◦C for 2 h
to
produce xBA–(1 − x)BT ceramics or Nb-doped 0.2BA–0.8BT
ceramics.
2.4.
Characterization
The obtained
gel was
characterized using thermogravime-
try
(TG) and differential scanning calorimetry (DSC)
(STA449c/3/G,
NETZSCH, Germany) in air to investigate the
thermal
behavior of the precursor. The chemical composition
of
the resulting 0.2BA–0.8BT powders synthesized by sol–gel
method
was analyzed by inductively coupled plasma optical
emission
spectrometry
(ICP-OES, Optima 4300DV, Perkin-
Elmer,
USA). Phase purity and structure of the ceramics were
determined
using XRD (PANalytical X’ Pert PRO, Eindhoven,
the
Netherlands) with CuK␣ radiation (λ = 1.54056 ˚ A), oper-
ating
at 40 kV and 40 mA. In addition, the microstructures of
ceramics
were characterized by scanning electron microscopy
(SEM,
JEOL JSM-5610LV). The surfaces of the ceramics were
polished
and painted with fire-on silver paste as electrodes for
electrical
measurements.
Dielectric
properties were measured
using
an LCR meter (4980A, Agilent, Santa Clara, CA) at 1 kHz,
10
kHz, and 100 kHz in the temperature range −55 to 200 ◦C or
−55
to 500 ◦C.
3. Results
and discussions
3.1. TG–DSC and XRD for 0.2BA–0.8BT powders
The TG–DSC plots for 0.2BA–0.8BT powders derived from
solid-state
reaction method and sol–gel method were given in
Fig. 2a and b, respectively. An endothermic peak and exother-
mic
peak were showed at about 70 ◦C and 300 ◦C in Fig. 2a,
perhaps
relating to the loss of the hydration water and the evap-
oration
of some humidity and impurities absorbed from the
atmosphere
during preparation process. The main mass loss
occurred
at around 830 ◦C, with a sharp endothermic peak in
the
DSC curve, which was due to the phase transformation ␥-
BaCO3→
␤-BaCO3and decomposition of BaCO3[35]. Given
that
no weight loss was taken places at >900 ◦C, it is reasonable
to
select 900 ◦C as the calcination temperature in solid-state
reaction
method. Similarly, it can be seen from Fig. 2b that two
stages
of weight loss occurred. The first stage (below 334 ◦C)

Please cite
this article in press as: Liu M, et al. Temperature stability of dielectric properties for xBiAlO3–(1 − x)BaTiO3ceramics. J Eur Ceram
Soc
(2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.02.015
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Fig. 2. The TG–DSC patterns of the xBA–(1 − x)BT pre-calcined powders prepared by solid-state reaction method (a) and sol–gel method (b).
with a weight loss of 42.86% was consistent with the elimination
of
structural water and the vaporization of organic acid. The sec-
ond
stage (334–600 ◦C) with a weight loss of about 32.62% and a
sharp
exothermic peak in the DSC curve accordingly, which may
be
related to the decomposition and oxidation of organic com-
pounds.
At
700 ◦C, there was no weight loss, indicating that the
powders
started
to
crystallize and the perovskite structure can
be
formed at around 700 ◦C for sol–gel method, which was a
considerable
decrease in the calcination temperature compared
with
solid-state reaction method.
The
X-ray diffraction patterns of 0.2BA–0.8BT powders cal-
cined
at different temperatures were used to check calcination
temperature
judging from TG–DSC. As seen in Fig. 3a, although
the
perovskite
phase could be formed at temperature as low as
850 ◦C,
the crystallinity was slightly low, as indicated by the
broad
diffraction peaks. Instead, the sample calcined at 900 ◦C
or
higher temperatures showed quite well defined diffraction
peaks.
Additionally, in Fig. 3b, it is easy to observe a broad
peak
located in the range of 25–35◦when calcined at 600 ◦C, as
a
result of incompletely crystallizing. But the perovskite phase
could
be observed
when
sample calcined at ≥700 ◦C. Therefore,
the
minimum calcination temperatures of solid-state reaction
method
and sol–gel method were 900 ◦C and 700 ◦C.
3.2. Phase structure of xBA–(1 − x) BT
Fig. 4a and b showed the XRD patterns of xBA–(1 − x)BT
(x
= 0.1–0.3) ceramics synthesized by solid-state reaction
method
and sol–gel method, respectively. It was observed that
the
samples with x = 0.1 exhibited a pure perovskite phase,
while
a second phase, identified as BaAl2O4, was detected when
x
> 0.1. The solubility of BiAlO3in BaTiO3was at about x = 0.1,
in
agreement with previously reported results [26]. Moreover,
the
split peak of the {2 0 0} diffraction gradually merged into
single
peak with increasing x, confirming that the sample was
in
tetragonal symmetry for x ≤ 0.1, transform to pseudo-cubic
symmetry
when x > 0.1, which was due to the smaller Al3+ ion
(r
= 54 pm) occupying the B-site of the Ti4+ (r = 60.5 pm) in the
BT
lattice, inducing a slight decrease in tolerance factor from
the
incorporation of BA (t = 1.016) into BT (t = 1.062), where
t
= 1.0571, 1.0527, and 1.0482 for x = 0.1, 0.2 and 0.3, respec-
tively.
And the decreasing tolerance factor corresponded to a
lattice
distortion, namely the structure transition from tetragonal
into the pseudocubic phase [36]. Compared with Fig. 4a, it was
obvious
that the impurity peaks were much less in Fig. 3b, prob-
ably
owing to the lower reaction temperature of sol–gel method
helping
to react easily. Actually, the finer particle size/better
chemical
mixing in sol–gel method accounted for improving
reaction
kinetics,
which in turn led to a decrease in reaction
temperature.
Inductively
coupled plasma optical emission spectroscopy
(ICP-OES)
testing was employed for the powders produced
by
sol–gel method to confirm the chemical composition of
xBA–(1
− x)BT (x = 0.1–0.3) as we expected. Therefore the
Bi/Al,
Ba/Ti, Bi/Ba and Al/Ti molar ratios of the 0.2BA–0.8BT
powders
by sol–gel method, which was taken as a representa-
tive,
were shown in Table 1. It was seen that the actual ratio
was
nearly equal to the theoretical ratio, so the sol–gel method
was
confirmed to produce phase-pure powders with the target
chemical
composition.
3.3.
Density and microstructure of xBA–(1 − x) BT
ceramics
The densities as well as sintering temperatures of
xBA–(1
− x)BT(x = 0.1–0.3) ceramics were showed in Table 2.
The
relative density increased with x increasing in both two
methods
owing to the presence of a liquid phase related to the
Bi-containing
compound. Notably, sol–gel method was a useful
technique
to obtain higher density at lower sintering temperature
when
comparing with solid-state reaction method.
Fig. 5 showed the cross-section SEM micrographs of
xBA–(1
− x)BT(x = 0.1–0.3) ceramics synthesized by solid-state
reaction
method and sol–gel method, respectively. All samples
had
spherical-like grains and rather dense microstructure, in
accordance
with relative density analysis. Seen from Table 2,
as
the BA contents increased, a significant increase in grain
sizes
was observed. It was because that the liquid phase made
it
beneficial for a faster particle dissolving and diffusion pro-
cess,
consequently leading to accelerated grain growth. There
were
a few of big oblate spheroidal grains (pointed by arrows),
and
the EDS result of position 2 in Fig. 5a and b both indicated
that
it only contained the Ba, Al and O, which were in accor-
dance
with the secondary phase BaAl2O4from XRD patterns.
It
is obvious that the secondary phase was greatly decreased in
Fig. 5b, associated with the chemical uniform of the ceramics

Please cite
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Fig. 3. X-ray diffraction patterns of the 0.2BA–0.8BT powders calcined at different temperatures; (a) solid-state reaction method, (b) and sol–gel method.
Fig. 4. X-ray diffraction patterns of xBA–(1 − x)BT(x = 0.1–0.3) ceramics synthesized by solid-state reaction method (a) and sol–gel method (b).
Table
1
The
ICP-OES
analysis of 0.2BA–0.8BT powders synthesized by sol–gel method.
Element
(B) W(B),
wt% W(B), wt% n(B) (mol)
Bi
15.65 21.79 0.10
Al
2.12 2.96 0.11
Ba
39.40 54.86 0.40
Ti
14.65 20.39 0.42
Ratio
n(Bi):n(Al) n(Ba):n(Ti) n(Bi):n(Ba) n(Al):n(Ti)
Actual
ratio 0.91:1 0.95:1 0.25:1 0.26:1
Theoretical
ratio 1:1 1:1 0.25:1 0.25:1
Table 2
Properties
for xBA–(1 − x)BT ceramics prepared by solid-state reaction method and sol–gel method.
x Sintering
temperature
Relative
density
(%)
Average
grain
size
(nm)
Tm(◦C)
εm(25 ◦C) tan δ
(25 ◦C)
C/C25 ◦C≤
±15
(◦C)
Solid-state
reaction
method
0.1
1200 ◦C-2 h 93.81 295 24 3250 0.019 −6 to 73
0.2
1100 ◦C-2 h 94.15 610 19 2303 0.010 −28 to 75
0.3 1050 ◦C-2
h 96.47 790 −25 1558 0.006 −55 to 124
Wet chemical
method
0.1
1150 ◦C-2 h 96.89 230 28 3183 0.011 −3 to 82
0.2
1050 ◦C-2 h 97.56 520 15 1238 0.006 −55 to 145
0.3
1050 ◦C-2 h 98.01 675 5 660 0.012 −55 to 440

Please cite
this article in press as: Liu M, et al. Temperature stability of dielectric properties for xBiAlO3–(1 − x)BaTiO3ceramics. J Eur Ceram
Soc
(2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.02.015
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Fig.
5.
The cross-section SEM micrographs and EDS results of xBA–(1 − x)BT ceramics synthesized by solid-state reaction method (a) and sol–gel method (b); (a1,
b1)
x = 0.1, (a2, b2) x = 0.2, (a3, b3) x = 0.3; the sintering temperatures of ceramics were listed in Table 2.