Ferroelectric-Dielectric Solid Solution and Composites for Tunable Microwave Application

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Ferroelectric-Dielectric Solid Solution and
Composites for Tunable Microwave Application
Yebin Xu and Yanyan He
Huazhong University of Science and Technology
China
1. Introduction
Electric field tunable ferroelectric materials have attracted extensive attention in recent years
due to their potential applications for tunable microwave device such as tunable filters,
phased array antennas, delay lines and phase shifters (Maiti et al. 2007a; Rao et al. 1999;
Romanofsky et al. 2000; Varadan et al 1992.; Zhi et al. 2002). Ba1-xSrxTiO3 and BaZrxTi1−xO3
have received the most attention due to their intrinsic high dielectric tunability. However,
the high inherent materials loss and high dielectric constant has restricted its application in
tunable microwave device. Various methods have been investigated to lower the dielectric
constant and loss tangent of pure ferroelectrics.
Forming ferroelectric-dielectric composite is an efficient method to reduce material dielectric
constant, loss tangent and maintain tunability at a sufficiently high level. For binary
ferroelectric-dielectric composite (such as BST+MgO) (Chang & Sengupta 2002; Sengupta &
Sengupta 1999), with the increase of dielectrics content, the dielectric constant and tunability
of composites decrease. In order to decrease the dielectric constant of binary composite, it is
necessary to increase the content of linear dielectric, and the tunability will decrease
inevitably due to ferroelectric dilution. Replacing one dielectric by the combination of
dielectrics with different dielectric constants and forming ternary ferroelectric-dielectric
composite can decrease the dielectric constant of composite and maintain or even increase
the tunability. This is beneficial for tunable application. The Ba0.6Sr0.4TiO3-Mg2SiO4-MgO
and BaZr0.2Ti0.8O3-Mg2SiO4-MgO composites exhibited relatively high tunability in
combination with reduced dielectric permittivity and reduced loss tangent (He et al. 2010,
2011). With the increase of Mg2SiO4 content and the decrease of MgO content in
Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composite, the dielectric constant decrease and the tunability
remain almost unchanged. For BaZr0.2Ti0.8O3-Mg2SiO4-MgO composite, an anomalous
relation between dielectric constant and tunability was observed: with the increase of
Mg2SiO4 content (>30 wt%), the dielectric constant of composite decreases and the tunability
increases. The anomalous increased tunability can be attributed to redistribution of the
electric field. Ba1-xSrxTiO3-Mg2TiO4-MgO can also form ferroelectric (Ba1-xSrxTiO3)-dielectric
(Mg2TiO4-MgO) ternary composite and the dielectric constant can be decreased. With the
increase of Mg2TiO4 content and the decrease of MgO content, the tunability of Ba1-xSrxTiO3-
Mg2TiO4-MgO composite increase. The multiple-phase composites might complicate
method to effectively deposit films, particularly if the dielectrics and ferroelectric are not
compatible for simultaneous deposition or simultaneous adhesion with a substrate or with

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each other. But ferroelectric-dielectric composite bulk ceramics show promising application,
especially in accelerator: bulk ferroelectrics composites can be used as active elements of
electrically controlled switches and phase shifters in pulse compressors or power
distribution circuits of future linear colliders as well as tuning layers for the dielectric based
accelerating structures (Kanareykin et al. 2006, 2009a, 2009b).
Forming ferroelectric-dielectric solid solution is another method to reduce material dielectric
constant and loss tangent. Ferroelectric Ba0.6Sr0.4TiO3 can form solid solution with dielectrics
Sr(Ga0.5Ta0.5)O3, La(Mg0.5Ti0.5)O3, La(Zn0.5Ti0.5)O3, and Nd(Mg0.5Ti0.5)O3 that have the same
perovskite structure as the ferroelectrics (Xu et al. 2008, 2009). With the increase of the
dielectrics content, the dielectric constant, loss tangent and tunability of solid solution
decrease. Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3 shows better dielectric properties than other solid
solutions. Compared with ferroelectric-dielectric composite, forming solid solution can
decrease the dielectric constant more rapidly when the doping content is nearly the same,
and can also improve the loss tangent more effectively. On the other hand, ferroelectric-
dielectric solid solution shows lower tunability than composites. The advantage of
ferroelectric-dielectric solid solution is that single phase materials is favorable for the thin
film deposition. The high dielectric field strength can be obtained easily in thin film to get
high tunability.
In this chapter, we summarize the microstructures, dielectric tunable properties of
ferroelectric-dielectric solid solution and composites, focusing mainly on our recent works.
2. Ferroelectric-dielectric composite
2.1 Ba1-xSrxTiO3 based composites
Various non-ferroelectric oxides, such as MgO, Al2O3, ZrO2, Mg2SiO4 and MgTiO3, were
added to Ba1-xSrxTiO3 to reduce the dielectric constant and loss tangent and maintain the
tunability at sufficient high level (Chang & Sengupta 2002; Sengupta & Sengupta 1997,
1999). It is better that non-ferroelectric oxide doesn’t react with ferroelectric Ba1-xSrxTiO3.
MgO has low dielectric constant and loss tangent, can form ferroelectric (Ba1-xSrxTiO3)-
dielectric (MgO) composite. BST-MgO composite shows better dielectric properties. Mg2SiO4
is also a linear dielectrics with low dielectric constant, but it can react with Ba1-xSrxTiO3 to
form Ba2(TiO)(Si2O7), as shown in Fig. 1. For 10 mol% Mg2SiO4 mixed Ba0.6Sr0.4TiO3, the
major phase is Ba0.6Sr0.4TiO3, and no Mg2SiO4 phase can be found except for two
unidentified peaks at 27.6o and 29.7o (relative intensity: ~1%). As the content of Mg2SiO4
increases from 20 to 60 mol%, the impurities phase of Ba2(TiO)(Si2O7) is observed obviously
and the relative content is increased with respect to the content of Mg2SiO4. For 60 mol%
Mg2SiO4 mixed Ba0.6Sr0.4TiO3 ceramics sintered at 1220oC, the strongest diffraction peak is
the (211) face of Ba2(TiO)(Si2O7) (not shown in Fig. 1). Therefore, for Mg2SiO4 added
Ba0.6Sr0.4TiO3, it is not as we expected that the ferroelectric (Ba0.6Sr0.4TiO3)-dielectric
(Mg2SiO4) composite formed. The dielectric constants and unloaded Q values at microwave
frequency were measured in the TE01δ dielectric resonator mode using the Hakki and
Coleman method by the network analyzer. Table 1 summarizes εr and the quality factor
(Q×f=f0/tanδ, where f0 is the resonant frequency) at microwave frequencies for some
Ba0.6Sr0.4TiO3-Mg2SiO4 ceramics. Increasing the Mg2SiO4 content results in a decrease of
dielectric constant but has no obvious effect on the Q×f value. The low Q×f of Ba0.6Sr0.4TiO3-
Mg2SiO4 ceramics restricts their microwave application, and so the tunability has not been
measured furthermore. The low Q×f is due to Ba2(TiO)(Si2O7) which is a ferroelectrics with
promising piezoelectric uses.

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Ferroelectric-Dielectric Solid Solution and Composites for Tunable Microwave Application

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10 20 304050 60 7080
60 mol%
50 mol%
20 mol%
30 mol%
40 mol%
10 mol%
Ba0.6Sr0.4TiO3
Ba2(TiO)(Si2O7)
Mg2SiO4


2θ(deg.)

Fig. 1. The XRD patterns of Ba0.6Sr0.4TiO3-Mg2SiO4 ceramics. The Mg2SiO4 content is 10-
60mol%.

Mg2SiO4 content
(mol%)
20
40
Sintering
temperature (oC)
1260
1240
f0(GHz)
ε
tanδ
Q×f(GH
z)
112
124
1.79
2.98
683.7
169.2
0.016
0.024
Table 1. Microwave dielectric properties of Ba0.6Sr0.4TiO3-Mg2SiO4 ceramics
For Mg2SiO4-MgO added Ba0.6Sr0.4TiO3, ferroelectric (Ba0.6Sr0.4TiO3)-dielectric (Mg2SiO4-
MgO) composite is formed, as shown in Fig. 2 (He et al., 2010). With the decrease of MgO
content and the increase of Mg2SiO4 content, the diffraction peaks from MgO decrease
gradually and the diffraction peaks from Mg2SiO4 increase. Therefore, Mg2SiO4-MgO
combination can prohibit the formation of Ba2(TiO)(Si2O7) phase.
Fig. 3 shows the FESEM images of Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composites sintered at
1350oC for 3h. The FESEM image and element mapping of 40Ba0.6Sr0.4TiO3-12Ba0.6Sr0.4TiO3-
48MgO as determined by energy dispersive spectroscopy (EDS) are shown in Fig. 4. Three
kind of different grains can be found clearly: light grains with average grain size of about
2μm, nearly round larger grains and dark grains with sharp corners. The element mapping
of Si Kα1 and Ti Kα1 in Fig. 4 can show the distribution of Mg2SiO4 and Ba0.6Sr0.4TiO3 grains
clearly. Therefore, we can identify that light grains are Ba0.6Sr0.4TiO3, the dark, larger grains
are MgO, and dark grains with sharp corners are Mg2SiO4. With the decrease of MgO
content and the increase of Mg2SiO4 content, more and more Mg2SiO4 grains with different
size can be found (Fig. 4). It is consistent with the XRD results. We can conclude that
Mg2SiO4 and MgO were randomly dispersed relative to ferroelectric Ba0.6Sr0.4TiO3 phase.

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102030 4050 60 7080
Ba0.6Sr0.4TiO3
Mg2SiO4
MgO

2θ(deg.)

Fig. 2. The XRD patterns of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) composite ceramics sintered
at 1350oC for 3h. From bottom to top, the MgO content is 48 wt%, 36 wt%, 30 wt%, 24 wt%
and 12 wt%, respectively.


(a) (b) (c)

(d) (e)
Fig. 3. FESEM images of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) composites ceramics sintered at
1350°C for 3h. From (a) to (e), the MgO content is 48 wt%, 36 wt%, 30 wt%, 24 wt% and 12
wt%, respectively.

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FESEM Mg Kα1_2 Si K α1


Ti K α1 Ba L α1 Sr K α1
Fig. 4. FESEM image and element mapping of 40Ba0.6Sr0.4TiO3-12Mg2SiO4-48MgO as
determined by energy dispersive spectroscopy (EDS).
Because of the relatively low dielectric constant and loss tangent of Mg2SiO4 and MgO, it is
expected that Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composites have lower dielectric constant and
loss tangent. Fig. 5 shows the dielectric constant and loss tangent of Ba0.6Sr0.4TiO3-Mg2SiO4-
MgO composite ceramics at 1MHz. The dielectric constant of composites is much smaller
than that of Ba0.6Sr0.4TiO3 (ε~5160 at 1MHz) (Chang & Sengupta, 2002; Sengptal & Sengupta
1999;). The loss tangent of Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composites sintered at 1350oC is
~0.0003-0.0006, but the loss tangent of Ba0.6Sr0.4TiO3 is ~0.0096 (Sengptal et al. 1999).
Therefore, the composites have much smaller loss tangent than Ba0.6Sr0.4TiO3.
The temperature dependence of dielectric properties for various Ba0.6Sr0.4TiO3-Mg2SiO4-
MgO composites (sintering temperature: 1350oC) measured at 100kHz is illustrated in Fig. 6.
Broadened and suppressed dielectric peaks and shifts of Curie temperature TC are observed.
For 40Ba0.6Sr0.4TiO3-12Mg2SiO4-48MgO ceramics, its εmax is ~ 176.5 at Tc ~224K. As the
relative content of Mg2SiO4 increase, Tc is shifted slightly to lower temperatures, thus
resulting in a decrease in dielectric constant at a given temperature; at the meantime, εmax
decreases also. For 40Ba0.6Sr0.4TiO3-30Mg2SiO4-30MgO, εmax is ~140.1 at ~216K and for
40Ba0.6Sr0.4TiO3-48Mg2SiO4-12MgO, εmax is ~126.8 at ~214K. With the decrease of
temperature, the loss tangent increase.
Fig. 6 shows the effect of applied field on the tunability of the Ba0.6Sr0.4TiO3-Mg2SiO4-MgO
composites at 100kHz. The tunability of 40Ba0.6Sr0.4TiO3-12Mg2SiO4-48MgO at 100kHz under
at 2kV/mm is 10.5%. With the increase of Mg2SiO4 content, the tunability of 40Ba0.6Sr0.4TiO3-
24Mg2SiO4-36MgO decreases slightly to 9.2%. Further increasing Mg2SiO4 content results in
a slight increase of tunability: 40Ba0.6Sr0.4TiO3-48Mg2SiO4-12MgO composite has tunability
of 10.2%.

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10 15 2025
MgO content(%)
3035404550
0
10
20
30
40
50
60
70
80
90
100
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
0.0014
0.0016
0.0018
0.0020
0.0022
0.0024
0.0026
0.0028
0.0030

ε
1300
1320
1350
oC
oC
oC
tanδ



Fig. 5. Dielectric constant (solid) and loss tangent (open) of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-
MgO) composite ceramics sintered at different temperature (measure frequency: 1MHz).


100150 200250 300
60
80
100
120
140
160
180
0.000
0.005
0.010
0.015
0.020
0.025
0.030
48 MgO wt%
30 MgO wt%
12 MgO wt%

ε
Temperature(K)
tanδ



Fig. 6. Variation of dielectric constant (solid) and loss tangent (open) with temperature for
40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) ceramics measured at 100kHz.

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Ferroelectric-Dielectric Solid Solution and Composites for Tunable Microwave Application

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0500 1000 15002000
0
2
4
6
8
10
12
48 MgO wt%
36 MgO wt%
30 MgO wt%
24 MgO wt%
12 MgO wt%

Tunability(%)
Electric Field(V/mm)

Fig. 7. The tunability of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) composites at 100kHz (sintering
temperature: 1350oC).

MgO content (wt.%)
12
24
30
36
48
f0(GHz)
5.74
5.74
5.80
5.96
5.33
ε
tanδ
0.023
0.019
0.021
0.017
0.014
Q×f(GHz)
250
302
276
351
381
74.59
77.72
77.12
74.39
93.86
Table 2. Microwave Dielectric Properties of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) ceramics
The room temperature microwave dielectric properties of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO)
composites were summarized in Table 2. With the increase of Mg2SiO4 content, the dielectric
constant remain almost the same and the Q×f value decrease.
Mg2TiO4 is a low loss tangent linear dielectrics and Mg2TiO4 added Ba1-xSrxTiO3 shows
better tuanble dielectric properties (Chou et al. 2007; Nenasheva et al. 2010). The XRD
patterns of 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4 (Fig. 8) show that ferroelectric
(Ba0.6Sr0.4TiO3)-dielectric (MgO-Mg2TiO4) composite is formed. On the other hand, impurity
phase BaMg6Ti6O19 is found in Mg2TiO4 doped Ba0.6Sr0.4TiO3. The fomation of BaMg6Ti6O19
depends on Ba/Sr ratio. BaMg6Ti6O19 forms in Mg2TiO4 doped Ba0.6Sr0.4TiO3 and
Ba0.55Sr0.45TiO3 but not Ba0.5Sr0.5TiO3. Mg2TiO4-MgO combination can prohibit the formation
of BaMg6Ti6O19 phase. The FESEM images (Fig. 9) show clearly three kind of grains:
Ba0.6Sr0.4TiO3, Mg2TiO4 and MgO.
Table 3 shows the microwave dielectric properties of 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4
ceramics. With the increase of MgO content, the dielectric constant decrease due to lower

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Ferroelectrics – Material Aspects

218
dielectric constant of MgO. For x=0-36 wt%, the Q×f value remain unchanged. As a whole,
the loss tangent is too high to be used for tunable microwave application.

10203040 5060 7080
x=0%
x=12%
x=24%
x=36%
x=48%
x=60%
Ba0.6Sr0.4TiO3
MgO
Mg2TiO4
BaMg6Ti6O19

2θ(deg.)

Fig. 8. The XRD patterns of 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4 ceramics


x=0 x=12 x=24

x=36 x=48 x=60
Fig. 9. FESEM images of 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4 composites ceramics sintered
at 1400°C for 3h.

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Ferroelectric-Dielectric Solid Solution and Composites for Tunable Microwave Application

219
MgO content (wt.%)
0
12
24
36
48
60
f0(GHz)
2.83
2.67
2.96
3.00
3.53
4.80
ε
tanδ
0.034
0.034
0.035
0.036
0.034
0.013
Q×f(GHz)
83
79
85
83
104
369
193.40
226.76
220.25
207.66
199.71
109.63
Table 3. Microwave dielectric properties of 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4 ceramics
Increasing Sr/Ba ratio can decrease the dielectric constant and loss tangent of Ba1-xSrxTiO3.
40Ba0.5Sr0.5TiO3-xMgO-(60-x)Mg2TiO4 will has lower dielectric constant and loss tangent
than 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4.
x)Mg2TiO4 ceramics and measured the tunability (Fig. 10). With the increase of Mg2TiO4
content, the tunabity of composite increases. The tunability of 40Ba0.5Sr0.5TiO3-12MgO-
48Mg2TiO4 is 16.6% at 2kV/mm and 28.5% at 3.9kV/mm, respectively. The corresponding
value of 40Ba0.5Sr0.5TiO3-60Mg2TiO4 is 13.6% and 24.0% respectively. The higher tunability of
40Ba0.5Sr0.5TiO3-12MgO-48Mg2TiO4 is due to its higer dielectric constant (ε=150.2) than
40Ba0.5Sr0.5TiO3-60Mg2TiO4 (ε=127.8).

We prepared 40Ba0.5Sr0.5TiO3-xMgO-(60-
050010001500 20002500300035004000
0
5
10
15
20
25
30


Tunability(%)
E(V/mm)
60% MgO
48% MgO
36% MgO
30% MgO
24% MgO
12% MgO
0% MgO

Fig. 10. The tunability of 40Ba0.5Sr0.5TiO3-xMgO-(60-x)Mg2TiO4 composites at 10kHz.
2.2 BaZrxTi1-xO3 based composites
BaZrxTi1-xO3 can form ferroelectric-dielectric composite with MgO (Maiti et al. 2007b, 2007c,
2008). High tunability and low loss tangent of the BaZrxTi1-xO3: MgO composites are

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Ferroelectrics – Material Aspects

220
102030 4050607080
(e)
(d)
(c)
(b)
(a)
222 220
200
111
113
013
134122322
233022
062
004
043
133
112
222
012
150
042
002
211
122
140
111
112
131
130
110
002
120
101021
001
020
Ba(Zr0.2Ti0.8)O3
Mg2SiO4
MgO

2θ(deg.)

Fig. 11. The XRD patterns of 40BaZr0.2Ti0.8O3-(60-x)Mg2SiO4-xMgO composites ceramics
sintered at 1350oC for 3h. (a) x=48wt%, (b) x=36wt%, (c) x=30wt%, (d) x=24wt%, (e)
x=12wt%.


(a) (b) (c)

(d) (e)
Fig. 12. FESEM images of 40BaZr0.2Ti0.8O3-(60-x)Mg2SiO4-xMgO composites ceramics
sintered at 1350°C for 3h. From (a) to (e), x=48 wt%, 36 wt%, 30 wt%, 24 wt% and 12 wt%,
respectively.

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Ferroelectric-Dielectric Solid Solution and Composites for Tunable Microwave Application

221
reported, but the sintering temperature is as high as 1500oC. We prepared BaZr0.2Ti0.8O3-
Mg2SiO4-MgO composite ceramics at 1350oC (He et al. 2011). The formation of ferroelectric
(BaZr0.2Ti0.8O3)-dielectric (Mg2SiO4-MgO) composite was proved by XRD patterns (Fig. 11).
Similar to Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composites, three kind of grains: BaZr0.2Ti0.8O3,
Mg2SiO4 and MgO, can be identified (Fig. 12 and Fig. 13).


FESEM Mg Kα1_2

Si Kα1 Ba Lα1
Fig. 13. FESEM image and element mapping of 40BaZr0.2Ti0.8O3-12Mg2SiO4-48MgO as
determined by energy dispersive spectroscopy (EDS).

101520 25
MgO content(%)
3035 404550
0
20
40
60
80
100
120
140
160
180
200
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
ε
1300
1320
1350
oC
oC
oC
tanδ

Fig. 14. Dielectric constant (solid) and loss tangent (open) of 40BaZr0.2Ti0.8O3-(60-x)Mg2SiO4-
xMgO composites ceramics sintered at various temperature (measure frequency: 1MHz).

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222
Fig. 14 shows the dielectric constant and loss tangent of BaZr0.2Ti0.8O3-Mg2SiO4-MgO
composite ceramics at 1MHz. With the increase of sintering temperature from 1300oC to
1350oC, the dielectric constant of the composites increase and the loss tangent decrease.

100150 200 250300
40
60
80
100
120
140
160
180
200
220
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
48 MgO wt%
30 MgO wt%
12 MgO wt%

ε
Temperature(K)
tanδ

Fig. 15. Variation of dielectric constant (solid) and loss tangent (open) with temperature for
40BaZr0.2Ti0.8O3-(60-x)Mg2SiO4-xMgO ceramics (sintering temperature: 1350oC) measured at
100kHz.

0 500
Applied Electric Field (V/mm)
10001500 2000
0
2
4
6
8
10
12
14
16
18
48 MgO wt%
36 MgO wt%
30 MgO wt%
24 MgO wt%
12 MgO wt%

Tunability(%)

Fig. 16. The tunability of 40BaZr0.2Ti0.8O3-(60-x)Mg2SiO4-xMgO composite ceramics at
100kHz at room temperature (sintering temperature: 1350oC).

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Ferroelectric-Dielectric Solid Solution and Composites for Tunable Microwave Application

223
Increasing Mg2SiO4 content tends to decrease the dielectric constant of composites. The
dielectric constant and loss tangent of composite sintered at 1350oC is ~125-183 and ~0.0010-
0.0016, respectively, which is smaller than that of BaZr0.2Ti0.8O3 (Maiti et al. 2007b).
The temperature dependence of dielectric properties for BaZr0.2Ti0.8O3-Mg2SiO4-MgO
composites (sintering temperature: 1350oC) measured at 100kHz is illustrated in Fig. 15.
Compared with pure BaZr0.2Ti0.8O3 bulk ceramic (Maiti et al. 2007b), broadened and
suppressed dielectric peaks and shifts of Curie temperature TC are observed with the
addition of Mg2SiO4 and MgO. The results are similar to that of Ba0.6Sr0.4TiO3-Mg2SiO4-MgO.
For 40BaZr0.2Ti0.8O3-12Mg2SiO4-48MgO ceramics, its εmax decreases to ~ 215.5 and Tc shifts to
lower temperature ~246K. For 40BaZr0.2Ti0.8O3-48Mg2SiO4-12MgO, εmax is ~157.7 at ~240K.
Fig. 16. shows the tunability of the BaZr0.2Ti0.8O3-Mg2SiO4-MgO composites at 100kHz at
room temperature. The tunability of 40BaZr0.2Ti0.8O3-12Mg2SiO4-48MgO under 2kV/mm is
15.6%. With the increase of Mg2SiO4 content, the tunability of 40BaZr0.2Ti0.8O3-30Mg2SiO4-
30MgO decreases slightly to 14.2%. Further increasing Mg2SiO4 content results in an
anomalous increase of tunability: 40BaZr0.2Ti0.8O3-48Mg2SiO4-12MgO composite has lower
dielectric constant than 40BaZr0.2Ti0.8O3-12Mg2SiO4-48MgO but slightly higher tunability
(17.9%).
3. Ferroelectric-dielectric solid solution
Forming ferroelectric-dielectric solid solution is another method to reduce material dielectric
constant and loss tangent. Some non-ferroelectric complex oxides with perovskite structures
have relatively low dielectric constant and low loss tangent. It is expected that they can be
combined with barium strontium titanate to reduce material dielectric constant and loss
tangent. Furthermore, it is possible for them to form solid solutions with barium strontium
titanate because they have the same perovskite structure as barium strontium titanate.
Single phase material is favorable for the thin film deposition. On the other hand, some
perovskite oxide has positive temperature coefficient of dielectric constant and it can
decrease the temperature coefficient of dielectric constant of barium strontium titanate
above Curie temperature.
3.1 Ba0.6Sr0.4TiO3-Sr(Ga0.5Ti0.5)O3 solid solution
Sr(Ga0.5Ta0.5)O3 has a comparatively small dielectric constant (27 at 1MHz), a positive
temperature coefficient of dielectric constant (120ppmK-1) and a low dielectric loss
(Q=8600 at 10.6 GHz) (Takahashi et al. 1997). The lattice constant (a=0.3949nm) of cubic
perovskite structure Sr(Ga0.5Ta0.5)O3 is very close to that of Ba0.6Sr0.4TiO3 (a=0.3965nm).
Therefore, Sr(Ga0.5Ta0.5)O3 will be possible to form solid solution with Ba0.6Sr0.4TiO3 and
reduce the dielectric constant of Ba0.6Sr0.4TiO3. The XRD results (Fig. 17.) prove that solid
solution can be formed between Ba0.6Sr0.4TiO3 and Sr(Ga0.5Ta0.5)O3 under the preparative
conditions (Xu et al. 2008).
Fig.18 shows the FESEM images of Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 ceramics sintered at 1600oC
for 3h. The effect of Sr(Ga0.5Ta0.5)O3 content on the average grain size in not very obvious.
We can also see that 0.9Ba0.6Sr0.4TiO3-0.1Sr(Ga0.5Ta0.5)O3 has higher porosity than other
compositions. The morphology of 0.5Ba0.6Sr0.4TiO3-0.5Sr(Ga0.5Ta0.5)O3 shows difference from
that of other three compositions.
The temperature dependence of dielectric properties for various Ba0.6Sr0.4TiO3-

Page 14

Ferroelectrics – Material Aspects

224
10 203040506070 8090
311
310
221
220
211
210
200
111
110
100

2θ(deg.)

Fig. 17. The XRD patterns of Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 ceramics sintered at 1600
3h. From bottom to top, the Sr(Ga0.5Ta0.5)O3 content is 10, 20, 30 and 50mol%, respectively.
The intensity is plotted on a log scale.
oC for


(a) (b)

(c) (d)
Fig. 18. FESEM images of Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 ceramics sintered at 1600oC for 3h.
From (a) to (d), the Sr(Ga0.5Ta0.5)O3 content is 10, 20, 30 and 50mol%, respectively.

Page 15

Ferroelectric-Dielectric Solid Solution and Composites for Tunable Microwave Application

225
Sr(Ga0.5Ta0.5)O3 ceramics (sintering temperature: 1600oC) measured at 100kHz is illustrated
in Fig. 19. Broadened and suppressed dielectric peaks and shifts of Curie temperature TC are
observed with the addition of Sr(Ga0.5Ta0.5)O3. For 0.9Ba0.6Sr0.4TiO3-0.1Sr(Ga0.5Ta0.5)O3
ceramics, its εmax decreases to ~ 686 and Tc shifts to lower temperature ~250K. As more
Sr(Ga0.5Ta0.5)O3 is added to Ba0.6Sr0.4TiO3, Tc shifts to lower temperatures, thus resulting in a
decrease in dielectric constant at a given temperature and εmax. For 0.8Ba0.6Sr0.4TiO3-
0.2Sr(Ga0.5Ta0.5)O3, εmax is ~335 at ~200K and for 0.5Ba0.6Sr0.4TiO3-0.5Sr(Ga0.5Ta0.5)O3, εmax is
~95 at ~100K. On the other hand, loss tangent increases on cooling. For 0.9Ba0.6Sr0.4TiO3-
0.1Sr(Ga0.5Ta0.5)O3 ceramics, there is small peak around ~250K. The loss tangent of
0.5Ba0.6Sr0.4TiO3-0.5Sr(Ga0.5Ta0.5)O3 ceramics (not shown) is almost independent on
temperature and fluctuates around 0.004 at the temperature range of 60K-300K.

50100 150
Temperature(K)
200 250 300
0
100
200
300
400
500
600
700
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014

0.016
0.018
0.020
0.022
0.024
0.026
0.028
0.030
ε
(a)
(b)
(c)
tanδ

Fig. 19. Variation of dielectric constant (solid) and loss tangent (open) with temperature for
Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 ceramics (sintering temperature: 1600oC) measured at 100kHz:
From (a) to (c), the Sr(Ga0.5Ta0.5)O3 content is 10, 20, and 50 mol%, respectively.

0500 10001500200025003000
0
2
4
6
8
10
12
14
16
18
20
10 mol% Sr(Ga0.5Ga0.5)TiO3
30 mol% Sr(Ga0.5Ga0.5)TiO3


Tunability (%)
Applied Electric Field (V/mm)

Fig. 20. The tunability of 0.9Ba0.6Sr0.4TiO3-0.1Sr(Ga0.5Ta0.5)O3 and 0.7Ba0.6Sr0.4TiO3-
0.3Sr(Ga0.5Ta0.5)O3 at 100 kHz (sintering temperature: 1600oC).