Microstructure and microwave dielectric properties of Ba5Nb4O15-BaWO4 composite ceramics

Article abstract of DOI:10.1016/j.jallcom.2008.04.073

Microwave dielectric composite ceramics with compositions of (1 - x)Ba₅Nb₄O₁₅-xBaWO₄ (x = 0-1) were prepared by firing the mixture of Ba₅Nb₄O₁₅ and BaWO₄. The sinterin

Full text of DOI:10.1016/j.jallcom.2008.04.073

Journal of Alloys and Compounds 472 (2009) 411–415
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Microstructure and microwave dielectric properties of Ba5Nb4O15–BaWO4
composite ceramics
Hao Zhuang, Zhenxing Yue∗, Fei Zhao, Jing Pei, Longtu Li
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Microwave dielectric composite ceramics with compositions of (1 − x)Ba5Nb4O15 –xBaWO4 (x = 0–1) were
prepared by firing the mixture of Ba5Nb4O15 and BaWO4. The sintering temperature of the composite
ceramics was reduced from 1400 to 1100 ◦C with increasing BaWO4 content. XRD patterns indicate that
Ba5Nb4O15 and BaWO4 coexisted and no secondary phase was detected in sintered bodies. The compos-
ite ceramics have the finer grained microstructure compared with the pure Ba5Nb4O15 ceramics. TEM
observation and EDS analysis reveal that a very thin interfacial area (∼60 nm in width) was formed due
to the limited solid solution between Ba5Nb4O15 and BaWO4 grains. With increasing BaWO4 (from x = 0
to x = 0.65), the dielectric constants (εr) and the temperature coefficient of resonant frequency (ꢀf) of the
ceramics decrease from 39.2 to 16.9 and from +78 to −4.3 ppm/◦C, respectively, while the quality factors
(Q × f value) increase from 26,000 to 56,700 GHz. The samples with x = 0.54–0.65 sintered at 1100 ◦C exhibit
good microwave dielectric properties: εr = 21.0–16.9, Q × f = 49,500–56,700 GHz, and ꢀf = 8.9–−4.3 ppm/◦C.
© 2008 Elsevier B.V. All rights reserved.
Received 18 January 2008
Received in revised form 15 April 2008
Accepted 23 April 2008
Available online 6 June 2008
Keywords:
Ceramics
Electriconic properties
TEM
1. Introduction
ers [13]. The vacant octahedra layer results in the strong lattice
anharmonicity, causing the higher permittivity and the higher
In the past decades, microwave dielectric ceramics have received
much attention because of the rapid growth of wireless commu-
nication industry. A large number of ceramic dielectric materials
have been developed. A high dielectric constant (εr), a high qual-
ity factor (Q value) and a low temperature coefficient of resonant
frequency (ꢀf) are three key required properties for microwave res-
onator materials [1].
Recently, some hexagonal perovskite-like oxides have been
investigated and they have been found to be the candidate mate-
rials for dielectric resonators in base station of telecommunication
systems [2–10]. The structure and microwave dielectric proper-
ties of A5B4O15 (A = Ba, Sr, Mg, Ca, Zn; B = Nb, Ta) type ceramics
have been investigated [11,12]. Of them, Ba5Nb4O15 is one of the
most promising materials due to its good dielectric properties of
εr ∼39–40, Q × f ∼26,000 GHz and ꢀf ∼78 ppm/◦C [9,10]. However,
the high sintering temperature (1400–1450 ◦C) and the large ꢀf
value limit its application in microwave devices. Therefore, the fur-
ther improvement on dielectric properties and sintering behavior
is required. Ba5Nb4O15 is a five layers hexagonal perovskite com-
intrinsic dielectric loss. Kim et al. [7,8] modified the ꢀf value of
Ba5Nb4O15 toward 0 ppm/◦C with BaNb2O6 additions and lowered
the sintering temperature to 900 ◦C using B2O3 doping. Unfortu-
nately, the quality factor was reduced largely to 18,700 GHz.
Generally, tungstates have low sintering temperature and excel-
lent dielectric properties at microwave frequency [14]. Among
them, BaWO4 was reported to have excellent microwave dielectric
properties: εr ∼8, Q × f ∼57,500 GHz, and ꢀf ∼−78 ppm/◦C, sintered
at 1100 ◦C [15]. Yoon et al. [16] investigated the effect of BaWO4
on the microwave dielectric properties of Ba(Mg1/3Ta2/3)O3 ceram-
ics. The ꢀf value was shifted to 0 ppm/◦C when the content of
BaWO4 is 0.09 mol%, but the sintering temperature is still very high
(1650 ◦C). The sintering behavior of BaWO4 doped Ba(Zn1/3Ta2/3)O3
ceramics were investigated by Kima et al. [17], and BaWO4 was
found to have an positive effect on improving the sinterability of
Ba(Zn1/3Ta2/3)O3 ceramics. They also investigated the low temper-
ature cofiring ceramics: (1 − x)(Li, RE)W2O8–xBaWO4 (RE = Y, Yb).
Tian et al. [18] found that in Ba(Mg1/3Nb2/3)O3 system, the B-site
ordering parameter and the lattice constant values increase with
BaWO4 addition. As BaWO4 has large negative ꢀf (∼78 ppm/◦C),
we can expect that a dielectric material with near zero ꢀf value
and high Q × f value might be obtained by combining Ba5Nb4O15
with BaWO4. In this work, (1 − x)Ba5Nb4O15–xBaWO4 (x = 0–0.65)
composite ceramics were prepared and their microstructure and
microwave dielectric properties were investigated systematically.
pound belonging to the general family of compounds AnBn−1O3n
,
with corner-sharing octahedra separated by vacant octahedra lay-
∗
Corresponding author. Tel.: +86 10 62784579; fax: +86 10 62771160.
E-mail address: yuezhx@tsinghua.edu.cn (Z. Yue).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.04.073

412
H. Zhuang et al. / Journal of Alloys and Compounds 472 (2009) 411–415
phase increase. As x further increases up to 0.65, BaWO4 phase
becomes dominant. The XRD patterns indicate that Ba5Nb4O15 and
BaWO4 can coexist in the sintered bodies and no secondary phase
was detected.
The relative densities of (1 − x)Ba5Nb4O15–xBaWO4 ceramics as
a function of sintering temperature are shown in Fig. 2, through
which the optimized sintering temperatures of the ceramics can
be determined. For all compositions, with increasing temperature
the relative density increases initially, and then drops at a certain
temperature. It can be observed that the sintering temperatures
for samples x = 0.07 and 0.23 are about 1150 ◦C. With the increase
of BaWO4 content, the sintering temperature decreases to about
1100 ◦C. The maximal relative density of each sample increases from
∼92% to above 95% with x increasing. This result implies that the
sinterability of Ba5Nb4O15 can be improved by BaWO4. The sin-
tering temperatures of all the samples are much lower than pure
Ba5Nb4O15 (1400 ◦C) [9,10]. The existence of BaWO4 with low sin-
tering temperature (1100 ◦C) may be responsible for the reduction
of sintering temperature.
Fig. 3 shows the typical SEM micrographs for sintered surfaces
of the (1−x)Ba5Nb4O15–xBaWO4 ceramics with maximal densities.
Abnormal grain growth is observed in pure Ba5Nb4O15, the grain
size of which is about 25 ␮m in Fig. 3(a). The abnormal grain growth
causes the relative density of the sintered Ba5Nb4O15 reaches only
90%, which is in well agreement with Lee et al. [22] and our pre-
vious work [4]. With the addition of BaWO4, the grain size sharply
decreases to about ∼2 ␮m. It can be seen that there exists many
pores in the samples with x = 0.07 and 0.23 sintered at 1150 ◦C,
which causes the relative densities of the samples reach only ∼92%
as shown in Fig. 2. With x further increases up to 0.65, as shown in
Fig. 3(d), fully densified ceramics can be prepared at 1100 ◦C, which
is in agreement with the relative density measurement results. Fur-
thermore, two different grain sizes can be observed in the sample
with x = 0.65: one is less than 1 ␮m and the other is about 5 ␮m. The
SEM observation suggests that the addition of BaWO4 can improve
the densification behavior of Ba5Nb4O15. It lowered the sintering
temperature to 1100–1150 ◦C, which caused the smaller grain size
of Ba5Nb4O15 grains than that of the pure Ba5Nb4O15 ceramics. It
leads to the increasing of the relative density of samples owing
to the decrease of inevitable pores caused by abnormal grown
grains (see Fig. 2). As a result, the dense ceramics with fine-grained
microstructures could be obtained with BaWO4 addition. This may
Fig. 1. The XRD patterns of (1 − x)Ba5Nb4O15 –xBaWO4 (x = 0.07–0.65) ceramics sin-
tered at 1100 ◦C for 4 h.
2. Experimental procedure
The used starting materials were high-purity (more than 99.9%) powders of
BaCO3, Nb2O5 and WO3. The Ba5Nb4O15 and BaWO4 powders were prepared using
conventional mixed oxide method by calcining at 1000 ◦C for 4 h and 750 ◦C for
5 h, respectively. Then (1 − x)Ba5Nb4O15 –xBaWO4 (x = 0–1) mixtures were prepared
by pure Ba5Nb4O15 and BaWO4 by weight radio. The mixtures were ball milled for
5 h using zirconia balls in alcohol medium. The slurries were dried, mixed with an
appropriate amount of PVA (5 wt.%) as a binder, and then screened by a 60 mesh.
The screened powders were pressed into cylindrical disks of diameter 10 mm and
height about 5 mm at a pressure of about 2000 kg/cm2. These pellets were preheated
at 600 ◦C for 4 h to expel the binder and then sintered at temperatures from 1050 to
1250 ◦C for 4 h in air.
The bulk densities of the samples were measured by the Archimedes method.
The relative densities were obtained from the bulk densities and the theoretical
densities. The theoretical densities were calculated from the following equation:
W1 + W2
ꢁ =
(1)
W1/ꢁ1 + W2/ꢁ2
where ꢁ1 and ꢁ2 are the theoretical density of Ba5Nb4O15 and BaWO4, 6.32 g/cm3 for
Ba5Nb4O15 phase and 6.38 g/cm3 for BaWO4 phase [17]. W1 and W2 are the weight
fraction of Ba5Nb4O15 and BaWO4.
The crystalline phases of the sintered samples were determined by X-ray
diffraction (XRD), using Cu K␣ radiation (Rigaku D/Max-2500, Tokyo, Japan).
The microstructures of sintered ceramics were observed by transmission elec-
tron microscope (TEM) and scanning electron microscopy (SEM). An HP8720ES
network analyzer (Hewlett–Packard, Santa Rosa, CA) was used in the measure-
ment of microwave dielectric properties. The microwave dielectric properties were
measured at room temperature using Hakki–Coleman method and cavity method
[19–21]. The temperature coefficients of resonant frequency (ꢀf) were obtained
in the temperature range from 20 to 80 ◦C. The ꢀf value can be calculated by the
following relationship:
f2 − f1
ꢀf
=
(2)
f1(T2 − T1)
where f1 and f2 represent the resonant frequencies at T1 and T2, respectively.
3. Results and discussions
Fig. 1 shows the XRD patterns of (1 − x)Ba5Nb4O15–xBaWO4
(x = 0.07–0.65) ceramic powders which were crushed from the sam-
ples sintered at 1100 ◦C for 4 h. All the main peaks could be indexed
in terms of the Ba5Nb4O15 (᭹) and BaWO4 (↓). For x = 0.07, the
main crystalline phase is Ba5Nb4O15 with hexagonal perovskite
structure and minor one is BaWO4 with scheelite structure. With
increasing BaWO4 content, the intensities of the diffraction peaks
of Ba5Nb4O15 phase decrease, whereas the intensities of BaWO4
Fig. 2. The relative densities of (1 − x)Ba5Nb4O15 –xBaWO4 ceramics as a function of
sintering temperature.

H. Zhuang et al. / Journal of Alloys and Compounds 472 (2009) 411–415
413
Fig. 3. SEM photographs of the surfaces of the (1 − x)Ba5Nb4O15 –xBaWO4 ceramics (a) pure Ba5Nb4O15 sintered at 1400 ◦C, (b) x = 0.07 sintered at 1150◦C, (c) x = 0.23 sintered
at 1150 ◦C and (d) x = 0.65 sintered at 1100 ◦C.
be beneficial to the improvement on the dielectric properties of
Ba5Nb4O15 ceramics.
and BaWO4 grains exists a thin interfacial area with about 60nm in
width, instead of grain boundary. The compositions of the interfa-
cial area (see inset of Fig. 4) was determined by EDS spectra, which
shows the change in W and Nb atomic percentage across the interfa-
cial area. In the interfacial area, the Nb content increases while the
W content decreases, implying that Ba5Nb4O15 and BaWO4 solid
solution can be formed. As the interfacial area is narrow (∼60 nm)
compared with the grain size of Ba5Nb4O15 and BaWO4 (see Fig. 3
SEM photograph), however, the solubility of Ba5Nb4O15 and BaWO4
solid solution is limited. Such limited solution was also observed
in W, Ti co-doping Ba5Nb4O15 system in our previous work [29].
Since the AWO4 compounds have two structure types: wolframite
and scheelite structures which depend on the ion radii of A2+ cation.
In the wolframite structure, the coordination number (CN) of W6+
is 6, while the CN of W6+ is 4 in the scheelite structure[23]. BaWO4
tends to be the scheelite structures due to the large ion radii of Ba2+
cation. In the perovskite structure, however, the CN of Nb5+ tends
to be 6 [24]. The differences in the crystal structure and the CN of W
and Nb limit the formation of solid solution between BaWO4 and
Ba5Nb4O15, thus Ba5Nb4O15 and BaWO4 can coexist as shown in
XRD patterns (see Fig. 1).
Fig. 4 shows the TEM image of sample with x = 0.65 sintered at
1100 ◦C. The two different types of grains were observed too. EDS
analysis show that the larger grain is composed of pure BaWO4
and the smaller one is composed of pure Ba5Nb4O15, which may
be due to much lower sintering temperature of BaWO4 (1100 ◦C)
compared with that of Ba5Nb4O15 (1400 ◦C). At sintering tempera-
ture of 1100 ◦C, BaWO4 grains grow superiorly. Between Ba5Nb4O15
The
microwave
dielectric
properties
of
(1 − x)Ba5
Nb4O15–xBaWO4 ceramics with maximal densities are shown
in Table 1 and Fig. 5. With x increasing from 0.07 to 0.65, the
dielectric constants of (1 − x)Ba5Nb4O15–xBaWO4 decrease from
36.4 to 16.9 and the ꢀf values decrease from 45.1 to −4.3 ppm/◦C,
while the Q × f values increase from 37,800 to 56,700 GHz. The
variations of microwave dielectric properties are related to the
change of Ba5Nb4O15 and BaWO4 compositions in the mixtures.
The well-known general empirical model for multiphase ceramics
was employed to confirm the above results. The calculated dielec-
Fig. 4. TEM photographs of sample with x = 0.65 sintered at 1100 ◦C. Inset shows
the W and Nb atomic percentage across the interfacial area between Ba5Nb4O15 and
BaWO4 grains.

414
H. Zhuang et al. / Journal of Alloys and Compounds 472 (2009) 411–415
Table 1
Dielectric properties of sintered (1 − x)Ba5Nb4O15 –xBaWO4 ceramics
x
εr
Q × f (GHz)
ꢀf (ppm/◦C)
x = 0 (pure Ba5Nb4O15
x = 0.07
x = 0.23
x = 0.41
x = 0.54
)
39.2
36.4
29.7
24.8
21.0
16.9
8.3
25,200
37,800
44,600
44,100
49,500
56,700
67,500
78.1
45.1
27.5
15.0
8.9
x = 0.65
x = 1.00 (pure BaWO4)
−4.3
−54.4
tric constant and ꢀf of the samples were obtained by the following
equations [25,26]:
ε˛r = x1ε˛ + x2ε˛
(3)
(4)
r1
r2
ꢀf = x1ꢀf1 + x2ꢀf2
Fig. 6. Microwave dielectric properties of the (1 − x)Ba5Nb4O15 –xBaWO4 ceramics
as a function of sintering temperature: (a) dielectric constant and (b) Q × f values.
where x1, x2 are the volume fraction of the Ba5Nb4O15 and BaWO4,
respectively; εr1, εr2 are the dielectric constant of the Ba5Nb4O15
and BaWO4, respectively; ꢀf1, ꢀf2 are the ꢀf of the Ba5Nb4O15 and
BaWO4, respectively and ˛ is a constant. The value of ˛ determines
the type of mixing rule: ˛ = −1, 1 and 0 represents serial, parallel
and logarithmic mixing model, respectively. The volume fraction
was calculated from the weight fraction of each phase. We also
prepared pure Ba5Nb4O15 and pure BaWO4 and characterized the
microwave dielectric properties, which were also listed in Table 1.
The calculated results of dielectric constant through logarithmic
law are shown in Fig. 5(a) (open symbols). The calculated results
of ꢀf using Eq. (4) is shown in Fig. 5(b) (open symbols). The calcu-
lated results are in agreement with the experimental results in the
aspect of the change trend, which is emphasized in this article. This
phenomenon implies that the variations of microwave dielectric
properties are related to the composition of Ba5Nb4O15 and BaWO4.
However, a certain difference between calculated and experimental
results is observed, which was also observed in our previous work
[27]. The significant decrease of ꢀf (from +78 to 45.1 ppm/◦C) with
the small addition of BaWO4 (x = 0.07) may be due to the limited for-
mation of the Ba5Nb4O15 and BaWO4 solid solution. Such result is
also observed in W, Ti co-doping Ba5Nb4O15 system in our previous
work [29]. According to our recent investigation [28], the octahe-
dral thickness variance of the B-site deficient hexagonal perovskite
plays an important role in the value of the temperature coefficient
of resonant frequency. Apparently, when the limited solid solu-
tion forms the substitution of W6+ for Nb5+ at B-site will affect the
octahedral thickness due to their different ion radii. Thus, the tem-
perature coefficient of resonant frequency decreases significantly
and is quite different from the calculated results. The formation
of solid solution may also influence the dielectric constants and
result in the difference between simulated εr and experimental
εr. Furthermore, the distribution of different phases is not perfect
uniformity in the ceramics may also cause the difference between
simulated results and experimental ones. Therefore, although the
microwave dielectric properties of the multiphase ceramics are
mainly determined by the phase compositions of ceramics, the
effects of solid solution phases in multiphase materials cannot be
neglected.
The
microwave
dielectric
properties
of
(1 − x)Ba5
Nb4O15–xBaWO4 ceramics as a function of sintering tempera-
ture are shown in Fig. 6. From Fig. 6(a) and (b), it is observed
that dielectric constant (εr) and Q × f values both increase with
increasing temperature initially, and then dropped around
1100 ∼ 1150 ◦C. This result is almost consistent with the change
trends in densities. The increases in εr and Q × f values with
sintering temperature result from the increases of densities. The
maximum Q × f values of (1 − x)Ba5Nb4O15–xBaWO4 ceramics
increase from 37,800 to 56,700 GHz due to the high Q × f value of
BaWO4 ceramics. Moreover, the increase of the relative density of
(1 − x)Ba5Nb4O15–xBaWO4 ceramics with the increasing BaWO4
content (see Fig. 2) would result in the improvement of the
samples’ Q × f value.
It is noted that the ꢀf of (1 − x)Ba5Nb4O15–xBaWO4 ceramics
were modified to nearly zero when x = 0.54–0.60. Furthermore the
ceramics still have high Q × f value (49,500–56,700 GHz) with εr
range from 21.0 to 16.9. The currently investigated ceramics with
εr of around 20 has either high sintering temperature or large
ꢀf value. It is well-known that the widely used ceramics is the
Mg0.95Ca0.05TiO3 ceramic. The microwave dielectric properties of
(1 − x)Ba5Nb4O15–xBaWO4 ceramics, however, are comparable to
those of the Mg0.95Ca0.05TiO3 ceramic, moreover, the sintering
temperature is much lower than that of Mg0.95Ca0.05TiO3(above
1250 ◦C).
4. Conclusions
The composite ceramics with (1 − x)Ba5Nb4O15–xBaWO4 com-
positions could be obtained by sintering the mixture of Ba5Nb4O15
and BaWO4 at around 1100–1150 ◦C. Ba5Nb4O15 and BaWO4 could
coexist in the sintered bodies. The composite ceramics have
fine-grained microstructure with very limited interfacial area
Fig. 5. The variations of experimental and calculated microwave dielectric proper-
ties with x for (1 − x)Ba5Nb4O15 –xBaWO4 ceramics.

H. Zhuang et al. / Journal of Alloys and Compounds 472 (2009) 411–415
415
between Ba5Nb4O15 and BaWO4 grains. The εr and the ꢀf values
of (1 − x)Ba5Nb4O15–xBaWO4 ceramics decrease with the increase
of BaWO4 content, and the quality factor (Q × f value) increase. The
(1 − x)Ba5Nb4O15–xBaWO4 ceramics with x = 0.54–0.65 sintered at
1100 ◦C have good microwave dielectric properties of εr = 21.0–16.9,
Q × f = 49,500–56,700 GHz, and ꢀf = 8.9 to −4.3 ppm/◦C. These com-
posite ceramics are promising materials for microwave and
millimeter wave applications.
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Acknowledgements
This work was supported by the Natural Science Foundation of
China (Grant Nos. 50672043, 50621201 and 50632030), and the
Ministry of Science and Technology of China through 973-Project
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