J. Am. Ceram. Soc., 90 [4] 1327–1330 (2007)
DOI: 10.1111/j.1551-2916.2007.01526.x r
2007 The American Ceramic Society
0.5
0.5
3
z Graduate School of the Chinese Academy of
Sciences, Beijing, China
w Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
Porous Ba
0.5
Sr
0.5
TiO
3
(BST) ceramics were fabricated by the traditional solid-state reaction process, and their structural, microstructural, dielectric, and tunability properties were systemically investigated. Compared with the fully dense BST samples, porous samples exhibit smaller grain sizes, a more uniform microstructure, and much lower dielectric constants, while at the same time, exhibiting little increase in tunability, which is beneficial to the development of microwave-tunable applications. At a frequency of 10 kHz and a temperature of
181C, as porosity increased from 0% to 28.8%, the dielectric constant of the BST ceramics (under zero bias field) decreased from e r(0)
B 1690 to e r(0)
B 990, while the dielectric losses were still less than 0.2%, and the tunability increased from 17.6% to 19.6%
(2.6 kV/mm).
not been achieved. Until now, the general trend that states that
‘‘the higher the dielectric constant, the higher the tunability’’ generally holds in ferroelectric materials.
It has been reported that the dielectric constant can be significantly reduced by increasing the porosity in porous lead zirconate titanate ceramics. However, until now, there has been little reported about porous BST ceramics. This paper describes a study of porous BST ceramics for tunable device applications. The structural, microstructural, dielectric, and tunable properties of the samples with different porosities have also been compared.
I. Introduction strontium titanate (BST, Ba
1
_ x
Sr x
TiO
3
) is a promising B ARIUM ferroelectric material for tunable microwave device applications such as voltage-tunable phase shifters, oscillators, filters, and delay lines.
1 These tunable devices are based on the large field-dependent dielectric constant, which results in a change in phase velocity in the device, allowing it to be tuned in real time for a particular application. From the viewpoint of practical applications, the critical parameters for tunable devices are a low dielectric constant for impedance matching, a low dielectric loss tangent, and a high dielectric tunability in certain electric field ranges. Unfortunately, the tradeoff for high tunabilities is a high dielectric constant. Recently, the authors have proposed approaches to improve the dielectric and tunable properties of BST materials. These approaches can be divided into two groups: (1) doping with different ions and (2) adding nonferroelectric materials to synthesize composites. The dopants, such as La + , Co + , Mg + , Mn + , In + , Fe + , and Sc 3+ , have been known to reduce dielectric loss, but have a dielectric constant that is still too high and/or more or less reduced the tunability.
5 – 7 BST composites, when synthesized with non-ferroelectric materials such as MgO, MgTiO
3
, or Mg
2
SiO
4
/MgO can dilute the dielectric constant and suppress the dielectric loss, but at the same time, reduce the dielectric tunability. 8–1 That is, the low dielectric constant high-tunability BST ceramics have
M. Cole—contributing editor
Manuscript No. 22309. Received September 29, 2006; approved November 13, 2006.
This work was supported partly by Shanghai Key Basic Research Project (06JC14070) and
Shanghai Qimingxing Project (06QA14055).
w Author to whom correspondence should be addressed.
e-mail: xldong@sunm.shcnc. ac.cn
z Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China.
1327
II. Experimental Procedure
The compositions prepared were Ba1_ x
Sr x
TiO3 (x = 0.5, this compound will be referred to hereafter as ‘‘BST’’) and poly(methyl methacrylate) (PMMA, as pore formers). The designed compositions of mixed powders were BST+a PMMA, where a =
0, 4, 8, and 12 wt%. The BST powder was prepared using a standard oxide mixture method, with BaCO
3
(99.18% purity),
SrCO
3
(99.00% purity), and TiO
2
(99.44% purity) as the raw materials. Stoichiometric amounts of the starting powders were thoroughly mixed by ball milling. The mixture was then calcined at 12201C for 2 h to obtain the BST compound. The calcined powder was then ball milled again to decrease the particle size.
The processed BST powder was then mixed with the pore former
PMMA at the designed compositions, and then milled for 6 h to promote a homogeneous mixing. After milling, the slurry was dried at room temperature and then the powder was mixed with 5 wt% PVA liquid. The mixed powder was then pressed at about
200 MPa in a disk 16 mm in diameter. From the thermogravimetric analysis of PMMA shown in Fig. 1, the optimum heating procedure was decided to begin with an initial heating rate of 21C/min up to 2401C, followed by 11C/min up to 4201C and then 21C/min up to 8501C to assure the complete burnout of the
PMMA and PVA. The compacts were sintered at 13501C for 2 h.
The sintered samples were ground to remove the surface layers, and evaporated with silver electrodes.
The phase of these specimens was confirmed by X-ray diffractometry (XRD; Model RAX-10, Rigaku, Tokyo, Japan) with
C u K OL radiation. Thermal analysis (NETZSCH STA-449C,
Bayern, Germany) was used to identify the decomposition temperature of the pore former PMMA. Scanning electron microscopy (SEM; JXA-8100, JEOL, Tokyo, Japan) was used to observe the microstructure of the PMMA and the ceramics. The dielectric properties of the ceramics body were measured at 181C with an impedance analyzer (HP4284A, Hewlett Packard, Palo
Alto, CA) at 10 kHz.
The dielectric tunability is usually defined by the following equation:
Tunability
— (C0 —
C E )/C0 X 100 %
(1)

1328 Communications of the American Ceramic Society Vol. 90, No. 4
Fig.1. (a) TG curve of poly(methyl methacrylate) (PMMA) at a heating rate of 101C/min and (b) scanning electron microscopy micrograph of the PMMA.
where C
0
and C
E
denote the capacitance without and with a dc bias field, respectively. The tunable properties were measured at
181C with an LCR meter (TH2816, Tonghui Electronic,
Changzhou, China) with a dc power supply as the external dc bias electric field.
III. Results and Discussion
The variation in porosity with the content of pore formers is shown in Fig. 2. The porosity increased with an increase in the content of pore former PMMA at a fixed sintering temperature.
When the content of PMMA increased from 0% to 12%, the porosity increased from 5.0% to 28.8%. This indicated that the porosity can be controlled by the content of the PMMA.
Figure 3 shows the XRD patterns of the BST, BST–PMMA ceramics sintered at 13501C for 2 h. All the samples show the typical XRD patterns of the perovskite structure and there is no second phase. Table I shows the peaks’ relative intensity of those samples with different porosity. It can be seen that the (100) peaks were reduced with increasing porosity, and the (110) peaks were enhanced. This shows that the ceramics have preferred orientations with increasing porosity, which may be attributed to the stress relaxation by the pores.
SEM micrographs of the samples are shown in Fig. 4. For all of the porous materials, macropores were formed in the ceramics after sintering. Generally, the pore sizes were equal to the initial size of the PMMA. This indicates that the pore size is
Fig.3. XRD patterns of the barium strontium titanate (BST) ceramics with different contents of poly(methyl methacrylate) (PMMA): (a)
Ba
0.5
Sr
0.5
TiO
3
; (b) Ba
0.5
Sr
0.5
TiO
3
14 wt% PMMA; (c) Ba
0.5
Sr
0.5
TiO
3
18 wt%
PMMA; (d) Ba
0.5
Sr
0.5
TiO
3
112 wt% PMMA.
determined by the size of the pore formers. The grain size is also observed to become slightly smaller and more uniform with increasing porosity.
Figure 5 shows the variation in relative dielectric constant and loss tangent of BST specimens as a function of temperature and porosity. The Curie temperature T
C
does not shift at all. The relative dielectric constant of BST specimens decreases more than
40% (from e r(0)
B 1690 (the density BST) to e r(0)
B 990 (porosity
28.8% BST)) at 10 kHz at 181C, while the dielectric losses were increased slightly, but were still less than 0.2%. The dominant factor responsible for the decreasing dielectric constant was the increase in porosity. The porous
Fig.2. Variation of porosity with the content of pore former poly(me-thyl methacrylate).
Table I. X-ray Diffraction (XRD) Peaks’ Relative Intensity of
Samples the Samples
(100) (110) (111) (200)
Ba
0.5
Sr
0.5
TiO
3
Ba
0.5
Sr
0.5
TiO
3
14 wt% PMMA
Ba
0.5
Sr
0.5
TiO
3
18 wt% PMMA
Ba
0.5
Sr
0.5
TiO
3
112 wt% PMMA
PowderofBa
0.5
Sr
0.5
TiO
3
PMMA, poly(methyl methacrylate).
21.5
100 29.3
73.9
15.6
100 30.0
45.1
11.4
100 25.9
43.6
10.3
100 28.0
45.2
38 100 64 74

April 2007 Communications of the American Ceramic Society 1329
Fig.4. Scanning electron microscopy micrograph of the barium strontium titanate (BST) ceramics with different contents of poly(methyl methacrylate)
(PMMA): (a) Ba
0.5
Sr
0.5
TiO
3
; (b) Ba
0.5
Sr
0.5
TiO
3
14 wt% PMMA; (c) Ba
0.5
Sr
0.5
TiO
3
18 wt% PMMA; (d) Ba
0.5
Sr
0.5
TiO
3
112 wt% PMMA.
ceramic is considered a two-phase system composed of bulk ceramic material and pores. The increase in pores can dramatically decrease the dielectric constant of ceramics.
12,13 This also indicates that the dielectric constant can be tailored by the degree of porosity. The grain size and the preferred orientation of (110) can also have an impact on the dielectric constant.
14,15 Nevertheless, the detailed information about the relationship is not very clear, and further work on this subject is in progress.
Figure 6 shows the applied dc field dependence of the relative dielectric constant for all specimens at 10 kHz. The fact that the relative dielectric constant decreases nonlinearly with increasing applied dc field can be explained by the phenomenological theory of Devonshire.
4 Based on the framework, Johnson gave an expression for the dielectric constant under the dc field as
8 r
=
8 r
(T,0) n 1 + [8
0
8 r
(T,0)] 3 B(T)E 2 o
1/3
(2) where e r
(T, 0) and e r
are the dielectric constants under 0 dc bias field and under the applied dc bias field, respectively; E is the dc bias field, and B(T) is the anharmonic coefficient that is both temperature and grain-size dependent.
9 From Eq. (2), we can see that the tunability increases with the value of [e
0 e r
(T, 0)] 3 B(T) at

-30 -15 0
Temperature (°C)
15 30 -60 -45 -30 -15 0
Temperature (°C)
Fig. 5. Temperature dependence of the dielectric constant and loss tangent measured at 10 kHz.

1330 Communications of the American Ceramic Society Vol. 90, No. 4
It also indicates that the value of B(T) increases with porosity.
This may be attributed to the smaller and more uniform grain size as a function of increasing porosity as shown in Fig. 4.
Fig. 6. DC field dependence of the dielectric constant measured at 18°C at 10 kHz: (
■
) Ba
0
.5Sr
0
.5TiO3; (□) Ba
0
.5Sr
0
.5TiO3+4 wt% poly(methyl methacrylate) (PMMA); (•) Ba
0
.5Sr
0
.5TiO
3
+ 8 wt% PMMA; (O)
Ba
0
.5Sr
0
.5TiO3+12 wt% PMMA.
Fig. 7. Tunability of the samples as a function of the strength of the external field at 10 kHz at 18°C: ( ■
) Ba
0
.5Sr
0
.5TiO
3
; (D) Ba0.5Sr0.5-
TiO
3
+4 wt% poly(methyl methacrylate) (PMMA); (•) Ba0.5Sr0.5-
TiO3 + 8 wt% PMMA; (O) Ba
0
.5Sr
0
.5TiO3+12 wt% PMMA.
a certain strength of E. The tunability of the samples as a function of the strength of the external field at 10 kHz at 181C is shown in
Fig. 7. It shows that the tunability is higher for the samples with a higher porosity when the e r
(T, 0) is much lower.
IV. Conclusions
Highly dense and porous BST ceramics were fabricated by conventional solid-state reactions, and their structural and dielectric properties were investigated. The ceramics developed preferred
(110) orientations, a more uniform microstructure, much lower dielectric constants with increasing porosity. At the same time, the tunability increased little. When the porosity reached 28.8%, the relative dielectric constant of the BST specimens decreased more than 42% (from e r
(0)~ 1690 to e r
(0)~990) at 10 kHz and at
18°C. The dielectric loss increased slightly but still remained less than 0.2%, while the tunability increased from 17.6% to 19.6%
(2.6 kV/mm, 10 kHz).
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