Low-temperature sintering and microwave dielectric properties of Ba 3(VO4)2-BaWO4 ceramic composites

Article abstract of DOI:10.1111/j.1551-2916.2008.02672.x

New low-temperature co-fired microwave dielectric composites with compositions of (1-x)Ba₃(VO₄)₂-xBaWO ₄ (x=0-1) were prepared by firing mixtures of Ba₃(VO ₄)₂ and BaWO₄. Thermal mechanical analysis indicated the ceramic composites had relatively low densification temperatures and could be sintered in the temperature range 925°～950°C. X-ray diffraction patterns showed that Ba₃(VO₄)₂ and BaWO₄ coexisted and no secondary phase was detected in the sintered bodies, implying good chemical compatibility between the two phases. Near-zero temperature coefficients of the resonant frequency (τ_f) could be achieved by changing the relative content of the two phases, due to their positive and negative τ_f values, respectively. With increasing BaWO₄ (x from 0.50 to 0.65), the τ_f value of the composites decreased from +10.9 to -1.9 ppm/°C, and the quality factor (Q × f value) increased from 66 700 to 79 100 GHz. The best microwave dielectric properties were obtained for x=0.65 samples sintered at 925°C with a dielectric constant of 11.1, a Q × f value of 79 100 GHz, and a τ_f value of -1.9 ppm/°C. Chemical compatibility experiments showed the ceramics are compatible with silver during cofiring process.

Full text of DOI:10.1111/j.1551-2916.2008.02672.x

J. Am. Ceram. Soc., 91 [11] 3738–3741 (2008)
DOI: 10.1111/j.1551-2916.2008.02672.x
r 2008 The American Ceramic Society
ournal
J
Low-Temperature Sintering and Microwave Dielectric Properties of
Ba (VO ) –BaWO Ceramic Composites
3
4 2
4
w
Hao Zhuang, Zhenxing Yue, Siqin Meng, Fei Zhao, and Longtu Li
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering,
Tsinghua University, Beijing 100084, China
3
–7
New low-temperature co-fired microwave dielectric composites
with compositions of (1–x)Ba (VO ) –xBaWO (x 5 0–1) were
been studied as glass-free LTCCs.
Bi O - or TeO -rich
2 3 2
compounds show relatively low sintering temperatures;
however, they have poor chemical compatibility with silver
electrodes, which limits their use in multilayer devices.
Tungsten- and vanadium-rich compounds have both low
sintering temperatures and excellent dielectric properties at
3
4 2
4
prepared by firing mixtures of Ba (VO ) and BaWO . Thermal
3
4 2
4
3
–6
mechanical analysis indicated the ceramic composites had
relatively low densification temperatures and could be sintered
in the temperature range 9251B9501C. X-ray diffraction
patterns showed that Ba (VO ) and BaWO coexisted and
8
,9
microwave frequency. Among them, BaWO
4
, which can be
3
4 2
4
no secondary phase was detected in the sintered bodies, implying
good chemical compatibility between the two phases. Near-zero
sintered at 11001C, was reported to have microwave dielectric
9
properties of e
Ba (VO has microwave dielectric properties of e
B40 000 GHz, and t
limited their applications in microwave devices. It is noted that
Ba (VO has a large positive t value (B160 ppm/1C), and
BaWO has a large negative t
r
B8, Q ꢁ fB57500 GHz, and t
f
Bꢀ78 ppm/1C.
B11, Q ꢁ f
B160 ppm/1C. However, large t values
temperature coefficients of the resonant frequency (s ) could be
3
)
4 2
r
f
8
achieved by changing the relative content of the two phases, due
to their positive and negative s values, respectively. With
f
f
f
increasing BaWO (x from 0.50 to 0.65), the s value of the
3
)
4 2
f
4
f
composites decreased from 110.9 to ꢀ1.9 ppm/1C, and the
quality factor (Q ꢁ f value) increased from 66700 to 79 100
GHz. The best microwave dielectric properties were obtained for
x 5 0.65 samples sintered at 9251C with a dielectric constant of
4
f
value (Bꢀ78 ppm/1C). One can
expect that a dielectric material with near-zero t value and
f
high Q ꢁ f value might be obtained by combining Ba (VO )
3
4 2
4 3 4 2 4
with BaWO . In the present work, (1–x)Ba (VO ) –xBaWO
1
1.1, a Q ꢁ f value of 79100 GHz, and a s value of ꢀ1.9 ppm/
(x 5 0–1) ceramic composites were prepared and their
microstructure and microwave dielectric properties were
investigated systematically.
f
C. Chemical compatibility experiments showed the ceramics
1
are compatible with silver during cofiring process.
I. Introduction
II. Experimental Procedure
ECENTLY, extensive attention has been paid to multilayer
microwave devices because of the rapid progress of mobile
The starting materials used were high-purity (499.9%) powders
of BaCO , V O , and WO . Ba (VO ) and BaWO powders
3 2 5 3 3 4 2 4
with stoichiometric compositions were synthesized using a
conventional mixed oxide route by calcining at 8001C for 4 h
R
1
communication systems. The development of low-temperature
cofired ceramics (LTCC) has been stimulated by the benefits
offered for the fabrication of miniature multilayer devices in-
volving the cofiring of dielectrics and highly conductive metals,
and 7501C for 5 h, respectively. (1–x)Ba
(x 5 0–1) mixtures were then prepared by pure Ba
BaWO by weight ratio. The mixtures were ball milled in alcohol
3
(VO
4
)
2
–xBaWO
4
3
(VO and
4 2
)
2
such as silver and copper. The sintering temperatures of dielec-
4
tric ceramics must be lower than the melting point of the elec-
trode metals, for example, 9611C for silver or 10641C for copper;
furthermore, chemical compatibility between the ceramic and
metal electrodes must be satisfied. However, most of the com-
mercial microwave dielectric ceramics available exhibiting high
for 5 h using zirconia balls. The slurries were dried, mixed with
an appropriate amount of PVA (5 wt%) as a binder, and then
screened with a 60 mesh. The screened powders were pressed
into cylindrical pellets with a diameter of 10 mm and a height of
about 5 mm under a pressure of B200 MPa. These pellets were
preheated at 6001C for 4 h to expel the binder and then sintered
at temperatures from 9001 to 10001C for 4 h in air.
quality factor (Q), high dielectric constant (e
r
), and near-zero
) are sintered at
temperature coefficient of resonant frequency (t
f
high temperatures. In order to lower the sintering temperature
of dielectric ceramics, some low melting point oxides or glasses
are generally added to promote the densification process by
liquid-phase sintering. Unfortunately, the presence of a glassy
phase in the sintered ceramics increases dielectric loss and de-
creases dielectric constant, due to the high loss and low permit-
tivity behavior of the glassy phases. Therefore, exploring new
ceramics with low firing temperature has been receiving increas-
ing attention. Some low-temperature firing ceramic compounds
such as bismuth- or tellurium-containing material systems have
The crystalline phases of the sintered samples were deter-
mined by X-ray diffraction (XRD), using CuKa radiation
(Rigaku D/Max-2500, Rigaku, Tokyo, Japan). Shrinkage
curves were measured using thermal mechanical analysis
(TMA) (SETARAM TGA TMA DSC, Caluire, France) at a
heating rate of 101C/min. The microstructures of sintered ce-
ramics were observed using a scanning electron microscope
(SEM, FEI QUANTA 200F, FEI Deutschland GmbH, Kassel,
Germany). The microwave dielectric properties were measured
1
0–12
by the Hakki–Coleman method and cavity method
using an
HP8720ES network analyzer (Hewlett-Packard, Santa Rosa,
CA). The temperature coefficient of the resonant frequency
N. Alford—contributing editor
f
(t ) was obtained in the temperature range from 201 to 801C.
Manuscript No. 24726. Received May 23, 2008; approved July 23, 2008.
This work was supported by the Natural Science Foundation of China (Grant Nos.
III. Results and Discussions
50672043, 50621201, and 50632030), and the Ministry of Science and Technology of China
through 973-Project under 2002CB613307.
3 4 2 4
Figure 1 shows the XRD patterns for (1–x)Ba (VO ) –xBaWO
(x 5 0.50–0.65) ceramics sintered at 9251C for 4 h. All the main
wAuthor to whom correspondence should be addressed. e-mail: yuezhx@tsinghua.
edu.cn
peaks could be indexed in terms of BaWO with the scheelite
4
3
738

November 2008
3739
3 4 2 4
Fig. 1. X-ray diffraction patterns for (1–x)Ba (VO ) –xBaWO ceram-
ics (x 5 0.50–0.65) and the mixture of x 5 0.65 with 20 wt% Ag sintered
at 9251C for 4 h.
Ã
structure( ) and Ba (VO ) with a hexagonal structure(.). The
4 2
Fig. 2. Shrinkage (solid line, dL/L ) and shrinkage rate (dotted line,
0
3
XRD patterns indicate that Ba (VO ) and BaWO can coexist
3
3 4 2 4
dL/dT) of (1–x)Ba (VO ) –xBaWO ceramics as a function of temper-
4 2
4
in the sintered bodies and no secondary phase was detected,
suggesting a diphasic composite. WO compounds have two
structure types: wolframite and scheelite structures, which
ature.
4
of BaWO and Ba (VO ) with the different linkages of the
4
3
4 2
21
13
61
[WO ] and [VO ] units, and the different valences of W and
4 4
depend on the ion radius of the A cation. BaWO
4
tends to
21
be a scheelite structure due to the large ion radius of the Ba
51
V
and Ba (VO ) . Therefore, BaWO and Ba (VO ) can coexist in
has limited the formation of solid solution between BaWO4
1
3
cation. In the scheelite structure, the coordination number of
3
4 2
4
3
4 2
61
W
in the polyhedron is four, and tetrahedral [WO ] is linked
4
the sintered bodies, suggesting good chemical compatibility be-
tween BaWO4 and Ba (VO ) . A cofiring experiment of the
21
by eightfold coordinate Ba ions. However, Ba
3
(VO
hexagonal structure ðR32=mÞ in which the V ion is located
in the center of tetrahedral [VO ] units linked by sixfold and
tenfold coordinate Ba ions. The different crystal structures
4 2
) is a
3
4 2
51
ꢀ
composites with silver was performed by firing mixtures of
Ba (VO ) –BaWO and 20 wt% Ag powder, and the XRD
result is also shown in Fig. 1. It reveals that no secondary phases
4
3
4 2
4
21
14
3 4 2 4
Fig. 3. Scanning electron microscopic photographs of the as-fired surface of (1–x)Ba (VO ) –xBaWO ceramics sintered at 9251C, (a) x 50.50, (b)
x 5 0.55, (c) x 5 0.65, (d) Backscattered electron image of (c).

3
740
Journal of the American Ceramic Society—Zhuang et al.
Vol. 91, No. 11
Table I. Relative Density and Microwave Dielectric Proper-
ties of (1–x)Ba (VO ) –xBaWO Ceramics Sintered at
3
4 2
Optimized Temperature
4
Temperature
coefficient of
resonant
Quality
Factor
(Qꢁ f)
(GHz)
Sintering
Temperature
(1C)
Dielectric
Relative Constant
frequency
x
Density (%)
r
(e )
(tf) (ppm/1C)
x 5 0
(pure Ba3
1100
97.8
14.2
42,200
52.3
(
VO ) )
4
2
x 5 0.50
x 5 0.55
x 5 0.60
x 5 0.65
x 5 1
925
925
925
925
97.4
97.9
97.9
98.4
94.3
11.9
11.7
11.3
11.1
8.3
63,600
69,200
75,000
79,100
67,500
10.9
6.7
3.3
ꢀ1.9
1000
ꢀ54.4
(pure
BaWO4)
are detected, implying good chemical compatibility between
Ba (VO –BaWO and silver.
Figure 2 shows the shrinkage and shrinking rate as a function
3
)
4 2
4
Fig. 5. Relative density and microwave dielectric properties of the (1–x)
Ba (VO ) –xBaWO ceramics as a function of sintering temperature.
of temperature for the pressed samples. It demonstrates that the
maximum densification rate occurs at B11451C and B9001C
3
4 2
4
3 4 2 4
for Ba (VO ) and BaWO , respectively. The different sintering
V. Noticeably, almost all the large grains are composed of
BaWO , which may be due to the lower sintering temperature
behavior of BaWO and Ba (VO ) will influence the densificat-
4
3
4 2
4
ion process of the composites. From Figs. 2(b) and (c), it is
observed that the temperature at which the maximum densifi-
cation occurs, decreases from 10101 to 9901C when x increases
from 0.50 to 0.65. Additionally, the onset temperature of shrink-
4 3 4 2
of BaWO compared with Ba (VO ) .
The relative densities and microwave dielectric properties of
the Ba (VO ) –BaWO ceramics sintered at the optimized
3
4 2
4
temperature are shown in Table I and Fig. 4. With x increasing
from 0.50 to 0.65, the dielectric constant and t values of
Ba (VO –BaWO
ceramics decrease, whereas Q ꢁ f increases.
The variation in microwave dielectric properties is related to the
relative content of Ba (VO and BaWO in the mixtures, due
age decreases from 8301 to 8001C in this case. Thus, BaWO
improved the sintering behavior of the composites, which
may be due to the lower sintering temperature of BaWO
4
f
3
)
4 2
4
4
.
Considering the high heating rate (101C/min) during the TMA
measurement, the actual sintering temperature of the composites
will be lower than the temperature at which the maximum
densification occurs.
3
)
4 2
4
to the differences in dielectric constant and Q ꢁ f values of two
phases. It is noted that the Q ꢁ f value of x 5 0.65 is higher than
that of pure BaWO . An SEM photograph (not provided here)
4
Figure 3 shows SEM micrographs for the as-fired surface of
the composites sintered at 9251C. Dense ceramics with a bimod-
al grain size distribution were observed for all compositions
of the as-fired surface of BaWO
growth, which led to the low-relative density of BaWO
4
showed abnormal grain
4
9
(94.3%). This result is in agreement with Yoon et al.’s work.
In contrast to pure BaWO , fine grains are observed in the
composite ceramics as shown in Fig. 3. The existence of
investigated. As shown in Figs. 3(a)–(c), the increase of BaWO4
4
content significantly promoted the grain growth of the
Ba
ty of Ba
3
(VO
4
)
2
–BaWO
4
ceramics. This suggests that the sinterabili-
ceramics is improved with increasing
BaWO content, which is in good agreement with the TMA
3 4 2 4
Ba (VO ) suppresses the grain growth of BaWO , thus results
3
(VO –BaWO
)
4 2
4
in a higher relative density and higher Q ꢁ f values.
4
The relative density and microwave dielectric properties of
analyses (see Fig. 2). Figure 3(d) shows the backscattered
electron image of x 5 0.65. In accordance with the XRD
patterns, two different phases can be observed. Because of
the different atomic weight of W (183.85) and V (50.94), the
light-colored grains are rich in W and the dark ones are rich in
(
3 4 2 4
1–x)Ba (VO ) –xBaWO ceramics as a function of sintering
temperature are shown in Fig. 5. The relative density initially
increases with increasing temperature but then decreases around
9
251B9501C. Maximum density is achieved at 9501C for
x 5 0.50 and 0.55, and 9251C for x 5 0.60 and 0.65. The e in-
r
creases with increasing temperature and then saturates. Q ꢁ f
initially increases with increasing temperature but then decreases
around 9251B9501C. The trend in Q ꢁ f is in accordance with
that of the relative density.
IV. Conclusions
Ceramic composites with (1–x)Ba
obtained by sintering mixtures of Ba
around 9251–9501C. Ba (VO and BaWO
3
(VO
4
)
2
–xBaWO
4
could be
and BaWO at
coexist in the sin-
3
(VO
)
4 2
4
3
4 2
)
4
tered bodies. SEM reveals the composites have a fine-grained
microstructure. The (1–x)Ba (VO –xBaWO composite with
3
)
4 2
4
x 5 0.65 sintered at 9251C has good microwave dielectric prop-
erties of e 5 11.1, Q ꢁ f 5 79100 GHz, and t 5 ꢀ1.9 ppm/1C.
r
f
Chemical compatibility experiments show the ceramics to be
compatible with silver during the cofiring process. The present
LTCC composites are promising materials for microwave and
millimeter wave applications.
Fig. 4. Microwave dielectric properties of the (1–x)Ba
xBaWO ceramics as a function of x.
4
3 4 2
(VO ) –

November 2008
3741
8
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