High-porosity Ba1−xSrxTiO3 ceramics from particle

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High-porosity Ba1−xSrxTiO3 ceramics from particle
Available online at www.sciencedirect.com
CERAMICS
INTERNATIONAL
Ceramics International 40 (2014) 10401–10405
www.elsevier.com/locate/ceramint
High-porosity Ba1 xSrxTiO3 ceramics from particle-stabilized emulsions
Xiang Wang, Jin-hong Lin, Hong-yao Zhang, Wei-min Guan
National Laboratory of Mineral Materials, China University of Geosciences, Beijing 100083, China
Received 11 January 2014; received in revised form 27 February 2014; accepted 28 February 2014
Available online 12 March 2014
Abstract
Macroporous barium strontium titanate ceramics consisting of closed cells with an average size of approximately 10 μm and exhibiting
porosities in the range of 65–80% were synthesized using particle-stabilized emulsions. These ceramics, after sintering at 1450 1C for 2 h,
exhibited high crystallinity and a cubic perovskite structure. The synthesized high-porosity macroporous ceramics exhibited a significantly lower
dielectric constant ( 315, when the porosity was 80%) than fully dense ceramics. The dielectric loss of the ceramics increased slightly with an
increase in porosity, yet remained less than 0.2%.
& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Porosity; C. Dielectric properties; D. BaTiO3 and titanates
1. Introduction
Barium strontium titanate (BST) (Ba1 xSrxTiO3) is a ferroelectric material that is used widely in tunable microwave devices
such as phase shifters, filters, varactors, and delay lines. BST has
a large field-dependent dielectric constant, and this allows it to
change its phase velocity in devices in real time [1–4]. Tunable
devices require ferroelectric materials that simultaneously exhibit
high tunability, a relatively low dielectric constant, and a small
dielectric loss tangent [5,6]. However, high tunability is always
accompanied by a high dielectric constant.
A number of approaches have been proposed to solve this
issue. Two of them most commonly used are: (i) doping with
different ions such La3 þ , Mg2 þ , Fe2 þ , and Mn2 þ ; and (ii)
adding nonferroelectric materials such as MgO, MnO2, and
Al2O3 [7–9]. These approaches lower the dielectric constant,
though only partially, and suppress dielectric loss; however,
despite this suppression, the dielectric constant remains too high
to be tunable. Hence, it is currently difficult to fabricate lowdielectric-constant BST ceramics that also exhibit high tunability.
Recently, porous ferroelectric ceramics have generated
significant interest owing to their desirable dielectric and
pyroelectric properties. In contrast to doping with various
n
Corresponding author. Tel.: þ86 10 82323201; fax: þ 86 10 82322974.
E-mail address: [email protected] (J.-h. Li).
http://dx.doi.org/10.1016/j.ceramint.2014.02.126
0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
types of ions or adding non-ferroelectric materials to solid
materials, increasing their porosity can decrease their dielectric
constant without adversely affecting their tunability. Zhang
et al. [10] reported that, as the porosity of BST ceramics was
increased from 0% to 28.8%, the dielectric constant, εr(0), of
the ceramics decreased from 1690 to 990; however, the
dielectric loss remained less than 0.2%.
Usually, pores can be introduced in a material by subjecting
it to incomplete sintering or by introducing into it sacrificial
pore formers such as starch, graphite, and various polymers
[11–14]. However, when porous ceramics are prepared by
these methods, either the porosity is too low or the pores
formed are similar in size to the grains.
In this study, we attempted to vary the dielectric constant of
BST ceramics by controlling their porosity and the structure of
their pores. A novel approach involving the use of particlestabilized emulsions was employed to fabricate the porous
BST ceramics [15,16]. The porosities, microstructural characteristics, and dielectric properties of the synthesized ceramics
were all investigated.
2. Material and methods
Commercially available BaCO3 (AR, Sinopharm Chemical
Reagent Co., Ltd.), SrCO3 (AR, Sinopharm Chemical Reagent
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X. Wang et al. / Ceramics International 40 (2014) 10401–10405
Co., Ltd.) and TiO2 (AR, Sinopharm Chemical Reagent Co.,
Ltd.) were used as the starting materials for synthesizing BST
ceramics. The composition of the prepared BST sample was
Ba0.5Sr0.5TiO3. A powder of BST with a mean grain size of
2 μm was obtained by subjecting the precursor materials to
high-energy ball-milling (HEBM) for 1.5 h and subsequent
calcination at 1000 1C for 2 h [17]. The short amphiphile
propyl gallate (C10H12O5) (AR), which exhibits high solubility
and a high critical micelle concentration (CMC) in the aqueous
phase, was used to modify the BST particles in situ [18]. A
dispersant (the ammonium salt of poly (acrylic acid); 0.5 wt%)
was added to the suspension to prevent the BST particles from
agglomerating. The other chemicals used in the experiments
were deionized water, ammonia (4 M), hydrochloric acid
(2 M), and ethanol. The amount of solid loaded in the
suspension was initially fixed at 45 vol%; the pH of the
suspension was adjusted to 11.0 using ammonia and hydrochloric acid. The homogenization and deagglomeration of the
suspension were performed by ball-milling for 24 h using
polyethylene milling bottles and agate balls (10 mm in
diameter; weight ratio of the balls to suspension ¼ 2:1).
The pH was readjusted to 11.0 after the completion of the
ball-milling process. Then, a certain amount of propyl gallate
was dissolved in ethanol and added dropwise to the ball-milled
suspension while constantly stirring the solution to prevent
particle agglomeration. Next, an appropriate volume of octane
(AR) was added as oil to the suspension, which was emulsified
using a household mixer at full power for 3 min. A number of
oil/water (o/w) emulsions were prepared in this manner. All
the emulsions were placed in a culture vessel (2 cm in height
and 9 cm in diameter) and left to dry at room temperature for 1
week to allow the oil and water to evaporate. After the
emulsions had dried, the BST samples were shaped. Finally,
the samples were sintered at 1150 1C to 1450 1C for 2 h; the
ramp rate from room temperature was 10 1C min 1. This
yielded sintered BST foam ceramics, which are shown in
Fig. 1.
The phases of the sintered samples were determined through
X-ray diffraction (XRD) analyses (D’Max-Ra12kW, Ouyatu,
Japan), performed using Cu-Kα radiation; the scanning rate
was 8.0 deg min 1. The morphologies of the samples were
investigated by scanning electron microscopy (SEM) (S-4800,
Hitachi, Japan). The dielectric properties of the porous BST
ceramics were measured at different temperatures using an
impedance analyzer (Agilent 4294A, USA) over frequencies
ranging from 100 Hz to 1 MHz.
3. Results and discussion
The porosities and bulk densities of the BST ceramics are
both plotted in Fig. 2 as functions of the oil/slurry volume
ratio. As expected, the porosity of the prepared BST samples
sintered at 1450 1C for 2 h increased with an increase in the oil
content in the initial emulsions. When the oil/slurry volume
ratio was changed from 1:8 to 1:1, the porosity increased from
65% to 80%. This indicated that the porosity can be controlled
by the varying the oil/slurry volume ratio in the initial
emulsions.
Fig. 3 shows the XRD patterns of the BST ceramics sintered
at 1450 1C for 2 h. The fact that the peaks are sharp and well
defined suggests that the BST ceramics were highly crystalline.
The Bragg reflection is attributable to the cubic perovskite
structure of BST (JCPDS File no. 39-1395). The single
diffraction peak observed between 451 and 471 corresponds
to the (2 0 0) Bragg reflection, which is characteristic of the
cubic lattice of BST [19]. The inset of Fig. 3 shows the XRD
patterns of the following samples for 2θ values of 44.01 to
48.01: (a) samples sintered at 1150 1C to 1450 1C and (b)
samples sintered at 1450 1C and having porosities ranging
from 65% to 80%. All the samples exhibited XRD patterns
characteristic of the perovskite structure; no other phases were
observed. The diffraction intensity of the (2 0 0) Bragg
reflection of the BST samples increased with an increase in
the sintering temperature from 1150 1C to 1450 1C, as shown
in Fig. 3(a). This change can be explained by the fact that
sintering at higher temperatures enhances the crystallinity of
the BST ceramics; an increase in the crystallinity was also
noticed from the SEM micrographs. On the other hand, as can
be seen from Fig. 3(b), the diffraction intensity decreased
slightly in the case of the samples with porosities of 65–80%;
this is because porous structures hinder the sintering process.
SEM micrographs of the porous BST samples, which had a
porosity of 80% and were sintered for 2 h at different
temperatures, are shown in Fig. 4. In the case of all the
samples, the macroporous structure exhibited a total porosity
of 65–80%. Most of the foam cells were closed and ranged
from 5 μm to 30 μm in size, with the average size being
approximately 10 μm. The individual cells were separated by
walls with a minimum thickness of less than 1 μm (Fig. 4(e)).
Most of the foam cells were closed, because the oil droplets
were completely covered by the surface-modified BST particles [20]. It was confirmed that the pore size was affected by
the sintering temperature, with the pore size decreasing slightly
with an increase in the temperature (Fig. 4(a)–(d)). At the same
time, the grain size increased with an increase in the sintering
temperature, and the grains became more interconnected, as
shown in Fig. 4(e) and (f).
In order to determine the dielectric properties of the
prepared BST samples, we plotted their dielectric constants
and loss values for frequencies of 100 Hz–1 MHz as functions
of the porosity; the curves are shown in Fig. 5. It was found
that, when the porosity increased, the relative dielectric
constant of the BST samples decreased; however, the dielectric
loss increased only slightly. For the sample with a porosity of
80%, the relative dielectric constant decreased by more than
80%, changing from 1690 (in the case of highly dense BST)
to 315 at 10 kHz and 25 1C; however, the dielectric loss was
still less than 0.2% [10]. The primary factor responsible for the
decrease in the dielectric constant was the increase in porosity.
A porous dielectric ceramic can be considered a two-phase
system, with the two phases being the solid material and air.
The simplest model for porous materials is εeff ¼ v1ε1 þ v2ε2,
where v1 and v2 are the volumes of the BST phase and the
X. Wang et al. / Ceramics International 40 (2014) 10401–10405
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Fig. 1. Digital images of (a) a particle-stabilized emulsion, (b) a dried foam sample, and (c) a machined, sintered ceramic sample.
Fig. 2. Porosity and bulk density values of the BST ceramics as functions of
the oil content in the emulsions; the BST ceramics were formed after sintering
at 1450 1C for 2 h.
pores, respectively, and ε1 and ε2 are the dielectric constants of
BST and air, respectively. The dielectric constant of air is
much lower than that of BST. Hence, εeff decreased with the
increase in porosity [21–22]. The results obtained in this study
were analyzed using this model; the predicted dielectric
constant values were typically within 7% of the measured
values. This also indicated that the value of the dielectric
constant can be tailored by varying the porosity.
4. Conclusions
Highly porous barium strontium titanate ceramics were
prepared successfully by using particle-stabilized emulsions.
The effects of the porosity of the ceramics on their microstructural and dielectric properties were investigated. It was
found that porosity of the ceramics could be tailored by
varying the oil content in the emulsions. When the oil/slurry
volume ratio was varied from 1:8 to 1:1, the porosity increased
Fig. 3. XRD patterns of the BST ceramics sintered at 1450 1C for 2 h. Insets
show the splitting of the (200) peaks observed in the XRD patterns of the
samples with (a) different porosities and (b) those sintered at different
temperatures.
from 65% to 80%. After sintering at 1450 1C for 2 h, porous
barium strontium titanate ceramics with high crystallinity and a
cubic perovskite structure were obtained. When the porosity of
the ceramics was increased, their dielectric constant decreased
significantly; however, the dielectric loss did not increase
significantly. This phenomenon can be attributed mainly to the
introduction of a second phase, that is, air, in the ceramics, and
to the fact that the εr value of air is much lower than that of
barium strontium titanate.
Thus, using particle-stabilized emulsions is an effective way
of preparing macroporous barium strontium titanate ceramics
with high porosities and low dielectric constants. Such
ceramics should find application in tunable microwave
devices.
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X. Wang et al. / Ceramics International 40 (2014) 10401–10405
Fig. 4. SEM micrographs of BST ceramics that had a porosity of 80% and were sintered for 2 h at different temperatures: (a) 1150 1C, (b) 1250 1C, (c) 1350 1C,
and (d) 1450 1C; (e) and (f) are magnified images of (d).
Fig. 5. Dielectric constant and dielectric loss values of the BST ceramics
having different porosities.
Acknowledgments
This project was supported by the program for New Century
Excellent Talents in University (NCET-08-828) and by the
Fundamental Research Funds for the Central Universities
(Grant nos. 2011YXL003, 53200959641, and 53200959396).
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