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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 applications [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 temperature (Tc ) and good temperature dependent dielectric behavior
[12–14]. BiMeO3 compounds, where Me can be Sc3+ , Y3+ ,
Al3+ , (Mg1/2 Ti1/2 )3+ or (Zn1/2 Ti1/2 )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(Mg1/2 Ti1/2 )O3
(BT–BMT) system was reported to possess lower dielectric
maximum temperature Tm with diffused characteristics as BMT
∗
Corresponding authors. Tel.: +86 27 87885811; fax: +86 27 87885811.
E-mail addresses: [email protected] (H. Hao), [email protected]
(H. Liu).
increased, exhibiting relaxor-like behavior [23,24]. Analog to
this system, BiAlO3 was thought to be a promising lead-free
ferroelectric with a Curie temperature (Tc ) of 800 K [25] and
a solubility limit of BiAlO3 into BaTiO3 about 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 Tc and increased the relaxor behavior in
xBiAlO3 –(1 − x)BaTiO3 [xBA–(1 − x)BT] system [27]. Meanwhile, it was observed the dielectric constant became flatter
above the maximum dielectric constant temperature (Tm ) as
BiAlO3 increased, improving the temperature-stable characteristics 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
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Please cite this article in press as: Liu M, et al. Temperature stability of dielectric properties for xBiAlO3 –(1 − x)BaTiO3 ceramics. J Eur Ceram
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chemical homogeneity and low-temperature processing to produce high-purity and phase-pure powders in comparison with
solid state method [32–34]. In this paper, xBA–(1 − x)BT ceramics 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. Moreover, 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 synthesized using Bi2 O3 (99%), Al2 O3 (99.99%), BaCO3 (99.0%),
TiO2 (99.0%) as starting materials, 5 mol% excess Bi2 O3 was
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(C4 H9 O)4 was 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 solution (named sol I) was obtained after water bathing at 80 ◦ C for
3 h. Then, Bi(NO3 )3 ·5H2 O(99.0%), Al(NO3 )3 ·9H2 O(99.0%)
and Ba(CH3 COO)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 treatment about 30 min to obtain 0.2BA–0.8BT slurry. Nb(OH)5 was
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 thermogravimetry (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, PerkinElmer, 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 Å), operating 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 exothermic peak were showed at about 70 ◦ C and 300 ◦ C in Fig. 2a,
perhaps relating to the loss of the hydration water and the evaporation 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 → ␤-BaCO3 and 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)BaTiO3 ceramics. J Eur Ceram
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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 second 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 compounds. 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 calcined 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 BaAl2 O4 , was detected when
x > 0.1. The solubility of BiAlO3 in BaTiO3 was 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, respectively. 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, probably 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 representative, 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 process, 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 accordance with the secondary phase BaAl2 O4 from XRD patterns.
It is obvious that the secondary phase was greatly decreased in
Fig. 5b, associated with the chemical uniform of the ceramics
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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%
Bi
Al
Ba
Ti
15.65
2.12
39.40
14.65
W(B), wt%
n(B) (mol)
21.79
2.96
54.86
20.39
0.10
0.11
0.40
0.42
Ratio
n(Bi):n(Al)
n(Ba):n(Ti)
n(Bi):n(Ba)
n(Al):n(Ti)
Actual ratio
Theoretical ratio
0.91:1
1:1
0.95:1
1:1
0.25:1
0.25:1
0.26:1
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
0.2
0.3
1200 ◦ C-2 h
1100 ◦ C-2 h
1050 ◦ C-2 h
93.81
94.15
96.47
295
610
790
24
19
−25
3250
2303
1558
0.019
0.010
0.006
−6 to 73
−28 to 75
−55 to 124
Wet chemical
method
0.1
0.2
0.3
1150 ◦ C-2 h
1050 ◦ C-2 h
1050 ◦ C-2 h
96.89
97.56
98.01
230
520
675
28
15
5
3183
1238
660
0.011
0.006
0.012
−3 to 82
−55 to 145
−55 to 440
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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.
Please cite this article in press as: Liu M, et al. Temperature stability of dielectric properties for xBiAlO3 –(1 − x)BaTiO3 ceramics. J Eur Ceram
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Fig. 6. Dielectric constant and loss as a function of temperature at frequencies from 1 kHz to 100 kHz for xBA–(1 − x)BT (a1 , b1 ) x = 0.1, (a2 , b2 ) x = 0.2, (a3 , b3 )
x = 0.3.
produced by sol–gel method. Besides, the ceramics prepared
by sol–gel method showed smaller grain sizes, lower porosity
and more uniform grains in comparison with those by solid state
reaction method, owing to the lower calcining temperature (seen
from TG–DSC).
3.4. Dielectric properties of xBA–(1 − x) BT ceramics
The dielectric constant and loss as a function of temperature
at 1 kHz, 10 kHz, and 100 kHz for xBA–(1 − x)BT (x = 0.1–0.3)
ceramics were shown in Fig. 6. As BA contents increased, the
maximum dielectric constant (εm ) decreased drastically and
a broad and diffuse phase transition took place in all samples, leading to smooth dielectric temperature characteristics
for a relatively broad temperature range. Moreover, as seen in
Fig. 6, the effect of flattening dielectric curve was enhanced in
sol–gel method possibly as a result of smaller grain sizes effect.
The temperature corresponding to the maximum of the diffuse
phase transition (Tm ) shifted toward a lower temperature with
increasing x value of the xBA–(1 − x)BT ceramics, which were
consistent with the literature [26].
Dielectric properties for xBA–(1 − x) BT ceramics at 1 kHz
were listed in Table 2. It is evident that the dielectric-temperature
stability was improved with BA contents increasing. Besides,
the ceramics prepared by sol–gel method exhibited much wider
temperature range (C/C25 ◦ C ≤ ±15%) due to homogeneous
grains. Of particular interest was that the 0.3BA–0.7BT ceramic
by sol–gel method possessed the best dielectric property, with
a moderate dielectric constant (ε = 660) and low dielectric loss
(tan δ = 1.2%) at room temperature and 1 kHz, and an ultra-broad
temperature range (−55 ◦ C to 440 ◦ C, C/C25 ◦ C ≤ ±15%).
3.5. Nb-doped 0.2BA–0.8BT
As shown in Table 2, the temperature variation of capacitance of 0.2BA–0.8BT was within ±15% from −55 ◦ C to
145 ◦ C, which still failed to meet the requirement of X8R.
Thus, Nb doping was chosen to improve the temperature
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stability. Ceramic samples with different Nb contents were prepared according to the method mentioned above. XRD patterns
of 0.2BA–0.8BT–xNb (x = 0.01, 0.02, 0.03, 0.04 mol) ceramics
all displayed a desired perovskite phase with little secondary
phases, as shown in Fig. 7. The EDS result proved the sheet
grains (position 3) only contained Ba, Bi, Ti, O and Nb,
which were consistent with the speculation from XRD patterns,
BaBi4 Ti4 O15 . While the secondary phase BaAl2 O4 was formed
in the matrix itself, which was confirmed by the EDS result
of big oblate spheroidal grains (position 2). Fig. 8 showed the
cross-section SEM micrographs of 0.2BA–0.8BT–xNb ceramics. A rather dense microstructure with low porosity was detected
for all samples. As showed in Table 3, the incorporation of
Nb into 0.2BA–0.8BT decreased the sintering temperature and
7
Fig. 7. X-ray diffraction patterns of Nb-modified 0.2BA–0.8BT ceramic samples prepared by precipitation method.
Fig. 8. The cross-section SEM micrographs and EDS results of 0.2BA–0.8BT–xNb ceramics (a) x = 0.01, (b) x = 0.02, (c) x = 0.03, (d) x = 0.04; the sintering
temperatures of ceramics were listed in Table 3.
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Fig. 9. Dielectric constant (a) and capacitance variation as a function of temperature (b) for 0.2BA–0.8BT–xNb ceramics.
Table 3
Properties for 0.2BA–0.8BT–xNb ceramics.
x
Sintering temperature
Relative density (%)
Average grain size (␮m)
0
0.01
0.02
0.03
0.04
1050 ◦ C-2 h
1050 ◦ C-2 h
1050 ◦ C-2 h
1030 ◦ C-2 h
1030 ◦ C-2 h
97.56
97.52
97.65
97.81
97.96
0.52
1.37
1.44
1.63
1.75
simultaneously influenced the grain growth slightly, corresponding to a denser microstructure.
Table 3 showed the dielectric properties of the samples. With
the increase of Nb concentration from 0 to 0.04 mol, the dielectric constant at room temperature decreased gradually from 1238
to 925, in part this may be due to secondary phase formation
(Fig. 7) [37]. Fig. 9 showed the temperature dependence of
dielectric constant and capacitance variation based on the capacitance at 25 ◦ C measured at 1 kHz for all samples. As Nb doping
concentrations increased, the dielectric curve became more flat
and the temperature stability was improved at high-temperature
end, besides, all samples doped with Nb compound met the
requirement of X8R. Among them, the 0.2BA–0.8BT ceramics
doped with 0.04 mol Nb achieved the best dielectric property,
with a moderate dielectric constant (ε = 925) and low dielectric
loss(tan δ = 0.4%) at room temperature and 1 kHz, and capacitance change being on the order of ±15% over the temperature
range from −55 to 178 ◦ C.
4. Conclusions
xBiAlO3 –(1 − x)BaTiO3 ceramics were synthesized by
solid-state reaction method and sol–gel method, respectively.
The results indicated that the calcination temperature was lower
in sol–gel method owing to the finer and more uniform powders.
The ceramics were in tetragonal phase when x ≤ 0.1, transformed to pseudocubic at x > 0.1. With BA contents increasing,
the maximum dielectric constant decreased and temperature stability of the dielectric constant was improved. The 0.3BA–0.7BT
ceramic prepared by sol–gel method was found to possess a
moderate dielectric constant (ε = 660) and low dielectric loss
(tan δ = 1.2%) at room temperature and 1 kHz, showing flat
dielectric behavior over the temperature range of −55 to 440 ◦ C.
εm (25 ◦ C)
1238
1206
1122
1068
925
tan δ (25 ◦ C)
C/C25 ◦ C ≤ ±15% (◦ C)
0.006
0.007
0.004
0.008
0.004
−55 to 145
−55 to 159
−55 to 167
−55 to 168
−55 to 178
Nb doped 0.2BA–0.8BT led to a more flat ε–T curve, with
deteriorated dielectric constant, achieved excellent temperature
stability meeting the X8R requirement.
Acknowledgments
This work was supported by Natural Science Foundation of
China (No. 51372191, No. 51102189), the Program for New
Century Excellent Talents in University (No. NCET-11-0685),
International Science and Technology Cooperation Program of
China (2011DFA52680), and the National Key Basic Research
Program of China (973 Program) (No. 2015CB654601).
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Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.02.015