Dielectric and tunable characteristics of Ba0.4Sr 0.6TiO3-BaWO4 composite ceramics for microwave applications

Article abstract of DOI:10.1016/j.materresbull.2011.03.009

Phase compositions, microstructure and microwave dielectric properties, of BaWO₄ (BW)-Ba_0.4Sr_0.6TiO₃ (BST) composite ceramics, prepared by the traditional solid-state route, were systematically characterized. Meanwhile, mechanism of dielectric tunability of those materials was discussed. Dielectric properties of the BW-BST composites at a DC bias field near the phase transition temperature could be interpreted by using Johnson's phenomenological equation. The sample with x = 0.60 exhibited a tunability of 29.5%, a dielectric permittivity of 192 and a Q value of 231 (at 2.700 GHz), which make it a promising candidate for applications in electrically tunable microwave devices.

Full text of DOI:10.1016/j.materresbull.2011.03.009

Materials Research Bulletin 46 (2011) 1045–1050
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Dielectric and tunable characteristics of Ba0.4Sr0.6TiO3–BaWO4 composite
ceramics for microwave applications
*
Mingwei Zhang, Jiwei Zhai , Bo Shen, Xi Yao
Functional Materials Research Laboratory, Tongji University, 1239 Siping Road, Shanghai 200092, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Phase compositions, microstructure and microwave dielectric properties, of BaWO4 (BW)–Ba0.4Sr0.6TiO3
(BST) composite ceramics, prepared by the traditional solid-state route, were systematically
characterized. Meanwhile, mechanism of dielectric tunability of those materials was discussed.
Dielectric properties of the BW–BST composites at a DC bias field near the phase transition temperature
could be interpreted by using Johnson’s phenomenological equation. The sample with x = 0.60 exhibited
a tunability of 29.5%, a dielectric permittivity of 192 and a Q value of 231 (at 2.700 GHz), which make it a
promising candidate for applications in electrically tunable microwave devices.
Received 7 October 2010
Received in revised form 10 December 2010
Accepted 11 March 2011
Available online 21 March 2011
Keywords:
A. Ceramics
ß 2011 Elsevier Ltd. All rights reserved.
A. Composites
B. X-ray diffraction
D. Dielectric properties
1. Introduction
permittivity was still higher than the requirement of real
applications. BST–Mg2SiO4 composite ceramics displayed relative-
Barium strontium titanate (Ba1ꢀxSrxTiO3 or BST) ferroelectric
materials have attracted significant interests due to their
electrically tunable dielectric properties for potential application
in tunable microwave devices, such as oscillators, tunable mixers,
delay lines, steerable antennas, parametric amplifiers, capacitors,
varactors, resonators, voltage-controlled oscillators, tunable filters
and phase shifters [1–8]. All these applications require materials
ly high tunability, adjustable dielectric permittivity and relatively
low dielectric loss. However, these properties were still not
satisfactory for microwave applications [18]. Similarly, Ba1ꢀxSrx-
TiO3–Mg2TiO4 based composite ceramics also had similar pro-
blems for microwave applications due to their relatively high loss
tangent at microwave frequencies. Introduction of MgO into BST–
Mg2SiO4 binary composites resulted in BST–Mg2SiO4–MgO ternary
composites, which had readily adjustable dielectric permittivity
without decreasing their tunability but increasing their dielectric
loss at microwave frequencies [13,19]. The reduced dielectric
permittivities of BST as a result of the addition of low dielectric
permittivity non-ferroelectric phases were at the expense of
increase in dielectric losses as well as decrease in dielectric
tunability.
with low dielectric loss (tan d), high tunability (T) and appropriate
value of dielectric permittivity (<500) [9]. BST is one of the most
promising candidates owing to its high tunability and low loss
[10,11]. However, pure BST has relatively high dielectric permit-
tivity (e) and thus is not suitable for practical tunable microwave
device applications.
It has been shown that adding low dielectric permittivity non-
ferroelectric phase (such as MgO, Al2O3, Mg2SiO4, and Mg2TiO4)
[11–19] into ferroelectric material is an effective method to reduce
its dielectric permittivity and loss while maintaining acceptable
dielectric tunabilities. In most cases, low dielectric permittivity is
readily achievable, but microwave loss is usually increased while
tunability is decreased. Addition of MgO was found to suppress
dielectric permittivity, losses and tunability of BST ceramics. Wu
et al. [17] reported that 1 wt% Al2O3-doped Ba0.6Sr0.4TiO3 ceramics
had very low loss and relatively high tunability, but their dielectric
In this study, we tried to further improve microwave
dielectric properties of BST-based composite ceramics. BaWO4
microwave dielectric material [20] with low dielectric permit-
tivity (
e = 8.27) and high quality factor (Q ꢁ f = 30,229) was used
to form composites with Ba0.4Sr0.6TiO3 ferroelectric ceramics.
The effect of BaWO4 on the dielectric properties of BST40
ceramics was systematically investigated to evaluate their
suitability for microwave device applications. This study was
aimed at finding a composite BW–BST system that has high Q
value, high dielectric tunability and appropriate level of
dielectric permittivity. At the same time, mechanisms that
govern dielectric properties of the BW–BST composite ceramics
will be discussed in detail.
* Corresponding author. Tel.: +86 216 598 0544; fax: +86 216 598 5179.
E-mail address: apzhai@tongji.edu.cn (J. Zhai).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.03.009

1046
M. Zhang et al. / Materials Research Bulletin 46 (2011) 1045–1050
2. Experimental
respectively. Then, the two powders were mixed according to the
stoichiometric ratios of xBW–(1 ꢀ x)BST and ball-milled with
zirconia media in ethanol for 24 h. The dried powders, mixed with
8 wt% polyvinyl alcohol (PVA), were pressed into pellets. Samples
for low frequency dielectric measurement are 10 mm in diameter
and 1 mm in thickness, while those for microwave frequency
measurements have dimensions of 10/5 mm, 12/6 mm, 15/7 mm
xBaWO4(BW)–(1 ꢀ x)Ba0.4Sr0.6TiO3(BST) (x = 15, 30, 45, 60 wt%)
composite ceramics were prepared by the conventional solid-state
reaction route. High purity BaCO3 (99.8%), SrCO3 (99.0%), TiO2
(99.9%) and WO3 (99.8%) were used as the starting materials. BST
and BaWO4 powers were synthesized at 1200 8C and 1100 8C,
[()TD$FIG]
Fig. 1. BEI micrographs of the xBaWO4–(1 ꢀ x)Ba0.4Sr0.6TiO3 composite ceramics with (a) x = 15, (b) x = 30, (c) x = 45 and (d) x = 60 wt% and EDS spectra of the sample (d).

M. Zhang et al. / Materials Research Bulletin 46 (2011) 1045–1050
1047
and 17/8 mm in diameter/thickness, respectively. The green
pellets were kept at 550 8C for 6 h in air to remove the solvent
and the binder and then sintered at 1320–1340 8C in air for 4 h.
Phase identification of the sintered ceramics was conducted by
using X-ray diffraction (XRD, Bruker D8 Advanced, Germany) with
all a dense and uniform microstructure. The two component
phases can be observed distinctively in the composite ceramics.
Energy dispersive spectroscopy (EDS) result of the sample with
x = 60 wt% is also shown in Fig. 1. The dark phase consists of
primarily Ba, Sr, Ti and O, while the light phase consists of primarily
Ba, W, O and small amount of Sr. It suggests that the dark grains are
BST (marked as ‘‘1’’) while the light grains are BW (marked as ‘‘2’’).
This EDS result indicated that inter-diffusion between BST and BW
occurred during the sintering process. BST grains of the composite
ceramics are smaller than those of pure BST40 (not shown here).
Cu K
to characterize microstructures of the samples. Temperature
dependent dielectric permittivity ( ) and loss tangent (tan ) of
a radiation. Back electron image (BEI, JSM EMP-800) was used
e
d
the ceramics were measured over 150–400 K at 10 kHz by using a
HP4284A precision LCR meter (Agilent, Palo Alto, CA). Room
temperature dielectric permittivity versus DC bias voltage was
measured at 10 kHz by using a Keithley model 2410 (Cleveland,
OH) high voltage source coupled with TH2816A LCR meter
(Changzhou, China). Microwave dielectric permittivity and loss
of the samples were measured by using the resonance method [21]
with a vector network analyzer (Agilent E5071C).
The grain sizes of pure BST40 are more than 15
mm, however, the
grain size in BST–BW composite is about 1 m. This indicates that
m
the addition of BW reduced the grain sizes of BST. The grain size of
the BW phase increased with increasing content of BW and its
distribution became more homogeneous, but grain size of the BST
phase kept almost unchanged.
XRD patterns of the BW–BST composite ceramics are shown in
Fig. 2. All samples can be indexed as two phases of cubic perovskite
BST and wolframite BaWO4. The absence of secondary crystalline
phases implies no obvious chemical reaction between BST and BW
phase. The (2 1 1) and (2 2 0) peaks of pure BST and the BW–BST
are enlarged in order to observe the shift of diffraction peaks as
shown in Fig. 2(b). Compared with pure BST, both (2 1 1) and
(2 2 0) peaks shift toward lower angle with increasing BW content,
indicating a lattice expansion of the BST. The variation of the cell
parameter obtained from XRD analysis by Jade 5.0 is plotted in
Fig. 3. This lattice expansion can be attributed to an increase of
Ba/Sr ratio in the BST phase due to the inter-diffusion, as
3. Results and discussion
BEI micrographs of natural surfaces of the BW–BST40 ceramics
are shown in Fig. 1. It is demonstrated that BW–BST ceramics have
[()TD$FIG]
[)(TD$FIG]
Fig. 3. Lattice parameter of the xBaWO4–(1 ꢀ x)Ba0.4Sr0.6TiO3 (x = 15, 30, 45 and
60 wt%) composite ceramics.
[)(TD$FIG]
Fig. 2. XRD patterns of the xBaWO4–(1 ꢀ x)Ba0.4Sr0.6TiO3 composite ceramics.
Fig. 4. Curie temperature (TC) of Ba1ꢀxSrxTiO3 as a function of x.

[
M. Zhang et al. / Materials Research Bulletin 46 (2011) 1045–1050
I
Fig. 5. Temperature dependences of dielectric permittivity and dielectric loss of the
xBaWO4–(1 ꢀ x)Ba0.4Sr0.6TiO3 composite ceramics.
Fig. 6. DC bias field dependences of dielectric permittivity of the xBaWO4–
(1 ꢀ x)Ba0.4Sr0.6TiO3 composite ceramics measured at 10 kHz and room
temperature.
demonstrated by the above EDS result, because the radius of Sr2+
Furthermore, TC increases with increasing content of BW
from 15 to 60 wt%, which should be attributed to the increase
of Ba/Sr ratio originating from the inter-diffusion as discussed
above [25].
˚
ion (coordination number N = 12, ionic radius = 1.44 A) is smaller
than that of Ba2+ ion (coordination number N = 12, ionic
˚
radius = 1.61 A) [22]. Fig. 3 also shows that the lattice expansion
tended to saturate at x = 45 wt%, which means that there has a limit
in ion s inter-diffusion at this composition. In our previous work,
we got a relationship between the value of Ba1ꢀxSrxTiO3 and TC, as
shown in Fig. 4. According to the fitting formula we can get a rough
Ba/Sr ratio of the samples as follows: 0.422/0.578, 0.433/0.567,
0.441/0.549, and 0.473/0.527.
Fig. 6 shows DC field dependences of dielectric permittivity of
all samples measured at 10 kHz and 20 8C. Loss tangents of the
samples were also measured at 10 kHz and room temperature (not
shown here). The BW–BST composite ceramics have higher
tunability than pure BST, which could be related to their relatively
high Ba/Sr ratio. Tunabilities of the samples decrease from 23.2 to
16.0% (at 30 kV) with increasing content of BW from x = 15 wt% to
x = 60 wt%, which is attributed to the increase of the non-
ferroelectric content. However, our BW–BST composite ceramics
still have sufficiently high tunabilities. The inter-diffusion between
Ba ions (in BW) and Sr ions (in BST) drove TC upwards and thus led
to high tunability at room temperature. Their high tunabilities
might also be related to their homogeneous microstructures
[26,27]. It is well known that tunability in a ferroelectric state is
larger than that in a paraelectric state and the maximum tunability
appears at the vicinity of phase transition temperature [28,29].
Specifically, the sample with x = 60 wt% has a dielectric permittiv-
ity of 249 and yet a tunability of 16% (at 30 kV/cm and 20 8C). This
result is very encouraging as compared with those of other BST-
based composites [30]. For instance, the tunability of MgO–BST
decreased abruptly with increasing content of MgO. The high
tunability and low dielectric permittivity make our BW–BST
composite ceramics attractive materials for tunable microwave
device applications.
Temperature dependences of dielectric permittivity of the BW–
BST composite ceramics are shown in Fig. 5. Microwave dielectric
properties of all samples measured at room temperature are listed
in Table 1. Dielectric peaks of the composite ceramics were
broadened and suppressed with increasing content of BW.
Meanwhile, Curie temperature (TC) of the composites increased
gradually. The decrease in dielectric permittivity can be readily
attributed to a dilution effect caused by BW and the diffuse phase
transition of the samples [10]. For example, room temperature
dielectric permittivity (10 kHz) is decreased from 1098 for pure
BST to 212 for the sample with x = 0.6. On the other hands, the
increase in fraction of grain boundary which is caused by the
decrease in BST grain size could be an additional contributor to the
reduction in dielectric permittivity with increasing content of BW.
Temperature dependent dielectric losses of all samples are also
presented in Fig. 5. The composite ceramics have significantly low
dielectric losses as a result of the addition of the low loss non-
ferroelectric BW [23]. Dielectric losses of all samples increase with
increasing temperature above 250 K, which may be attributed to
the increase of conduction loss at high temperatures [24].
It is well known that the effect on dielectric permittivity by DC
field of BST in paraelectric state originates from the anharmonic
Table 1
Microwave and dielectric properties of all the samples with different contents of BaWO4.
x
TC (K)
Dielectric properties (at 10 kHz)
Microwave properties
At about 20 8C
Resonant frequency (GHz)
e
(at resonant frequency)
Q (1/tand)
e0
tan
d
Tunability (%) at
30 kV/cm bias
0
212.2
217.9
221.8
228.3
236.0
1098
869
759
669
259
0.0009
0.0162
0.0077
0.0030
0.0010
8.8
23.2
20.1
17.4
16.0
1.409
1.520
1.478
1.755
2.700
1021
674
598
481
192
935
52
15
30
45
60
126
221
231

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M. Zhang et al. / Materials Research B
]
ulletin 46 (2011) 1045–1050
1049
Fig. 8. Tunability of the xBaWO4–(1 ꢀ x)Ba0.4Sr0.6TiO3 composite ceramics: (a)
x = 15, (b) x = 30, (c) x = 45 and (d) x = 60 wt%, measured at 10 kHz and room
temperature.
Fig. 7. Electric field dependences of dielectric permittivity of the xBaWO4–
(1 ꢀ x)Ba0.4Sr0.6TiO3 composite ceramics: (a) x = 15, (b) x = 30, (c) x = 45 and (d)
x = 60 wt%, measured at 10 kHz and room temperature.
interaction of Ti4+ ions. An anharmonic interaction is attributed to
the crystal field exerted in a material. An anharmonic potential of
Ti4+ is generated in a material with perovskite structure due to the
symmetry group. The anharmonic interaction can be visualized as
an inelastic feature of the covalent bond constructed by Ti4+ and
O2ꢀ. Therefore, the magnitude of the dipole moment will be
suppressed at DC biasing [31], which causes the reduction of
dielectric permittivity. As is well known, dielectric tunability of a
given material is correlative with strength of the external field.
Generally, the higher the external field is applied, the higher the
tunability will be.
As shown in Fig. 6, dielectric permittivity of the BST–BW
composite ceramics decrease with increasing external field, which
is attributed to the anharmonic interaction of Ti4+ ions. It can be
explained by the phenomenological theory of Devonshire [32,33].
Johnson proposed an expression for permittivity under DC field as:
which leads to a reduction in dielectric permittivity. The
congelation and consolidation of polar micro-regions tend to
saturate with further increasing DC field, whose contribution to
tunability is dying down. For the samples with x = 0.45 and 0.60,
since TC is getting closer to measuring temperature, polar micro-
region contribution to the change of dielectric permittivity is
increasing, which is the reason why the slope dose not depress
with increasing DC field. In Fig. 7, the decline slope means a
decrease in tunability with increasing content of BW, which can be
attributed to the increase of the non-ferroelectric content.
Shown in Fig. 8, we measured tunabilities at different DC biases
(7 points). An increase in tenability with DC bias can also be
observed, which is consistent with the conclusion drawn above.
The change in slopes of the samples with x = 15 and x = 30 means a
reduction in change rate of their dielectric permittivity as a
function of DC electric field. Slopes of the samples with x = 45 and
x = 60 are constant, indicating a steady change rate of decrease in
their dielectric permittivity.
Microwave dielectric properties of all samples measured at
room temperature are listed in Table 1. Dielectric permittivities of
the composite ceramics are evidently decreased at microwave
frequencies as compared with those at low frequencies (below
1 MHz). Q value decreases with the content of BW at first, reaches a
minimum vale at x = 15 wt% and then increases with increasing
content of BW. This variation can be understood by taking into
account both intrinsic and extrinsic dielectric responses [36,37].
Extrinsic loss contributors include charged defect, residual
ferroelectric polarization, and local polar-regions. The high
microwave losses of the samples with low contents of BW could
be attributed to the main extrinsic loss due to their poor
microstructure homogeneities [37]. At high concentrations of
BW, the high Q value of BW became dominate factor. On the other
hand, good compositional homogeneity of the samples with high
contents of BW also favored to increase their Q factors [38]. It is
worth mentioning that the sample with x = 0.6 has an acceptable
level of Q value of 231 at ꢃ2.700 GHz, a high tunability (ꢃ29.5% at
60 kV/cm) and an appropriate value of dielectric permittivity
(ꢃ192 at zero biasing), which makes this composite a promising
candidate for applications in tunable microwave devices.
erða p pÞ
erð0Þ
1
¼
(1)
1=3
2
3
½1 þ aerð0ÞE ꢂ
where
e
r(0) and
e
r(app) are the dielectric permittivity at zero DC field
is the anharmonic
is determined by both the anharmonic interaction
and an applied field, respectively, and
coefficient.
a
a
and the dielectric permittivity of the sample. Larger dielectric
permittivity and stronger anharmonic interaction would yield a
higher value of field coefficient. In this case, DC field will be more
effective in reducing the dielectric permittivity. Eq. (1) can be
simplified by using Eq. (2):
y ¼ 1 þ bx
where y = (
(2)
e
/
r(0) er(app))3, x = E2 and b ¼ ae30. It was reported that all
experimental curves in the intermediate range of electric fields can
be well fitted by the bias equation [34]. Fig. 7 shows linear fittings
of experimental data of the BW–BST composite ceramics.
The samples with x = 15 wt% and x = 30 wt% have two linear
regions, with a large slope over 0–16 kV/cmꢀ1 and small slope over
16–30 kV/cmꢀ1. It means that the dielectric responses of the
composite materials at external DC fields cannot be solely
described by the phenomenological theory at different effective
Ba concentrations in BST, which result in the difference in TC. This
deviation could have other extrinsic contributions, such as polar
clusters, microcluster boundaries, defects, and space charge
polarization [35]. At a strong DC field, polar micro-domains in
nanoscale polar micro-regions would grow up, resulting in a
congelation and consolidation of polar micro-regions and a
reduction in the number of ferroelectric–paraelectric domain
walls. The contribution of reversible polarizations is reduced,
4. Conclusions
xBaWO4–(1 ꢀ x)Ba0.4Sr0.6TiO3 composite ceramics were fabri-
cated and characterized. All samples consisted of Ba0.4Sr0.6TiO3 and
BaWO4 phases. TC peaks of the composite ceramics were
suppressed, broadened and shifted to higher temperatures with
increasing content of BW. Dielectric tunability properties of the

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M. Zhang et al. / Materials Research Bulletin 46 (2011) 1045–1050
[8] W.T. Chang, L. Sengupta, J. Appl. Phys. 92 (2002) 3941.
[9] M.W. Cole, P.C. Joshi, M.H. Ervin, J. Appl. Phys. 9 (2001) 6336.
[10] J.J. Zhang, J.W. Zhai, M.W. Zhang, P. Qi, X. Yu, X. Yao, J. Phys. D: Appl. Phys. 42 (7)
(2009) 075414.
[11] R.H. Liang, X.L. Dong, Y. Chen, F. Cao, Mater. Chem. Phys. 95 (2006) 222.
[12] U.C. Chung, C. Elissalde, M. Maglione, C. Estournes, M. Pate, J.P. Ganne, Appl. Phys.
Lett. 92 (4) (2008) 042902.
[13] Y. Chen, X.L. Dong, R.H. Liang, J.T. Li, Y.L. Wang, J. Appl. Phys. 98 (2005) 064107.
[14] X.J. Chou, J.W. Zhai, X. Yao, Appl. Phys. Lett. 91 (2007) 122908.
[15] X.H. Wang, W.Z. Lu, J. Liu, Y.L. Zhou, D.X. Zhou, J. Eur. Ceram. Soc. 26 (10-11)
(2006) 1981.
[16] J.J. Zhang, J.W. Zhai, H.T. Jiang, X. Yao, J. Appl. Phys. 104 (8) (2008) 084102.
[17] L. Wu, Y.C. Chen, Y.P. Chou, Y.T. Tsai, S.Y. Chu, Jpn. J. Appl. Phys. 38 (1999) 5154.
[18] L.H. Chiu, X. Zhang, L.C. Sengupta, US Patent No. 6,514, 895 B1 (2003).
[19] L.C. Sengupta, X. Zhang, L.H. Chiu, US Patent No. 0,145,647 A1 (2007).
[20] K. Yang, Z.F. Gao, J.J. Bian, J. Chin. Ceram. Soc. 34 (2006) 251.
[21] B.W. Hakki, P.D. Coleman, IRE Trans. Microwave Theory Tech. 8 (1960) 402.
[22] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751.
BW–BST composites at a DC bias field near the phase transition
temperature were interpreted using Johnson’s phenomenological
equation. The sample with x = 0.60 had a dielectric permittivity of
192, a tunability of 29.5% and a Q factor of 231 (at 2.700 GHz). The
low dielectric permittivity composite ceramics with high tunabili-
ty and good microwave properties are promising candidates in
tunable device applications.
Acknowledgements
This work was supported by the Ministry of Sciences and
Technology of China through 973-project under Grant
2009CB623302, the Cultivation Fund of the Key Scientific and
Technical Innovation Project, Ministry of Education of China (No.
707024) and the Shanghai Committee of Science and Technology
(No. 07DZ22302).
[23] Q. Xu, X.F. Zhang, Y.H. Huang, W. Chen, H.X. Liu, M. Chen, B.H. Kim, J. Alloys
Compd. 48 (2009) 448.
[24] Y. Chen, Y.Y. Zhang, X.L. Dong, G.S. Wang, F. Cao, J. Am. Ceram. Soc. 93 (2010) 161.
[25] P. Liu, J.L. Ma, L. Meng, J. Li, L.F. Ding, J.L. Wang, H.W. Zhang, Mater. Chem. Phys.
114 (2009) 624.
References
[26] R. Waser, Ferroelectrics 133 (1992) 109.
[27] A. Feteira, D.C. Sinclair, I.M. Reaney, J. Am. Ceram. Soc. 87 (2004) 1082.
[28] T. Maiti, R. Guo, A.S. Bhalla, Appl. Phys. Lett. 89 (2006) 122909.
[29] X. Wei, X. Yao, Mater. Sci. Eng. B 99 (2003) 74.
[30] L.C. Sengupta, S. Sengupta, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 44
(1997) 792.
[31] S.G. Lee, D.S. Kang, Mater. Lett. 57 (2003) 1629.
[32] K.M. Johnson, J. Appl. Phys. 33 (1962) 2826.
[33] H. Diamond, J. Appl. Phys. 32 (1961) 909.
[34] J.W. Liou, B.S. Chiou, J. Mater. Sci.: Mater. Electron. 11 (2000) 645.
[35] A. Chen, Y. Zhi, Phys. Rev. B 69 (2004) 174109.
[1] L.B. Kong, S. Li, T.S. Zhang, J.W. Zhai, F.Y.C. Boey, J. Ma, Prog. Mater. Sci. 55 (2010)
840.
[2] X.X. Xi, H.C. Li, W.D. Si, I.A. Akimov, J.R. Fox, A.M. Clark, J.H. Hao, J. Electroceram. 4
(2000) 393.
[3] R.E. Treece, J.B. Thompson, C.H. Mueller, T. Rivkin, M.W. Cromar, Appl. Supercond.
7 (1997) 2363.
[4] H.C. Li, W.D. Si, A.D. West, X.X. Xi, Appl. Phys. Lett. 73 (1998) 190.
[5] A.B. Kozyrev, T.B. Samoilova, A.A. Golovkov, E.K. Hollmann, D.A. Kalinikos, V.E.
Loginov, A.M. Prudan, O.I. Soldatenkov, D. Galt, C.H. Mueller, T.V. Rivkin, G.A.
Koepf, J. Appl. Phys. 84 (1998) 3326.
[36] A.K. Tagantsev, V.O. Sherman, K.F. Astafiev, J. Electroceram. 11 (2003) 199.
[37] R.H. Liang, X.L. Dong, Y. Chen, F. Cao, J. Am. Ceram. Soc. 89 (2006) 3273.
[38] X.H. Wang, W.Z. Lv, J. Liu, F. Liang, D.X. Zhou, J. Chin. Ceram. Soc. 32 (2004) 738.
[6] C.L. Chen, H.H. Feng, Z. Zhang, A. Brazdeikis, Z.J. Huang, W.K. Chu, C.W. Chu, F.A.
Miranda, F.W. Van Keuls, R.R. Romanofsky, Appl. Phys. Lett. 75 (1999) 412.
[7] A. Outzourhit, J.U. Trefny, T. Kito, B. Yarar, J. Mater. Res. 10 (1995) 1411.