Microwave dielectric properties of low temperature firing Bi 2Mo2O9 ceramic

Article abstract of DOI:10.1111/j.1551-2916.2008.02596.x

Preparation, phase stability, sintering behavior, microwave dielectric properties of single-phase Bi₂Mo₂O₉ ceramic and its chemical compatibility with Ag have been investigated. The single-phase Bi₂Mo₂O₉ ceramic can be well densified in the temperature range from 620° to 645°C with relative density about 96%. X-ray diffraction data show that Bi₂Mo₂O₉ has a monoclinic structure (P2₁/n), with lattice parameters a=11.9664 Aa, b=10.8089 Aa, and c=11.8871 Aa. When sintering temperature ≥650°C, Bi₂Mo₃O₁₂ appears as a secondary phase. Pure monoclinic Bi₂Mo₂O₉ ceramic sintered at 620°C for 2 h exhibits good microwave dielectric properties with permittivity about 38, Q_f value about 12 500 GHz and temperature coefficient of resonant frequency about +31ppm/°C. The permittivity of single-phase Bi₂Mo₂O₉ corrected for porosity is about 40.17. However, the reaction between Bi ₂Mo₂O₉ and Ag will affect its further application in low-temperature cofired ceramic.

Full text of DOI:10.1111/j.1551-2916.2008.02596.x

J. Am. Ceram. Soc., 91 [10] 3419–3422 (2008)
DOI: 10.1111/j.1551-2916.2008.02596.x
r 2008 The American Ceramic Society
ournal
J
Microwave Dielectric Properties of Low Temperature Firing
Bi2Mo2O9 Ceramic
Di Zhou, Hong Wang,w Xi Yao, and Li-Xia Pang
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education, Xi’an Jiaotong University,
Xi’an 710049, China
Preparation, phase stability, sintering behavior, microwave
dielectric properties of single-phase Bi2Mo2O9 ceramic and
its chemical compatibility with Ag have been investigated. The
single-phase Bi2Mo2O9 ceramic can be well densified in the
temperature range from 6201 to 6451C with relative density
about 96%. X-ray diffraction data show that Bi2Mo2O9 has a
monoclinic structure (P21/n), with lattice parameters
a 5 11.9664 A, b 5 10.8089 A, and c 5 11.8871 A. When sinte-
ring temperature ꢀ 6501C, Bi2Mo3O12 appears as a secondary
phase. Pure monoclinic Bi2Mo2O9 ceramic sintered at 6201C
for 2 h exhibits good microwave dielectric properties with per-
mittivity about 38, Qf value about 12500 GHz and temperature
coefficient of resonant frequency about 131ppm/1C. The per-
mittivity of single-phase Bi2Mo2O9 corrected for porosity is
about 40.17. However, the reaction between Bi2Mo2O9 and
Ag will affect its further application in low-temperature cofired
ceramic.
TiO2–TeO2 ternary system and Bi2W2O9 systems.17ꢁ23 TeO2-
rich compounds show relatively low sintering temperatures be-
tween 5001 and 8001C. However, they react with Ag electrode.
This problem was recently overcome for BaTe4O9 through the
use of Al electrodes.22 However, TeO2 is rather expensive and it
is poisonous. Recently Feteira and Sinclair23 had found that a
single phase of Bi2W2O9, which could be formed and sintered
below 8851C without any sintering aid addition, exhibited good
microwave dielectric properties with dielectric constant er lying
between 28 and 41, Q ꢂ f between 3900 and 8000 GHz and tem-
perature coefficient TCF between ꢁ42 to ꢁ63 ppm/1C. The
chemical compatibility of Bi2W2O9 with Au made it possible for
application in LTCC technology.23,24
The phase diagram and compounds of the Bi2O3–MoO3 sys-
tem have been studied by many researchers. Belyaev and Bleijen-
berg et al.25,26 reported two congruently melting compounds,
Bi2O3–3MoO3(a) and Bi2O3–MoO3(g), in the region 0–50 mol%
Bi2O3. Erman et al.27–29 found some other compounds, Bi2O3–
2MoO3(b) (not a congruently melting compound) and (e) phase
having solid solubility with a composition of xBi2O3–MoO3 (1.3
o ꢂ o1.4). 7Bi2O3–MoO3, 3Bi2O3–MoO3, low and high tem-
perature forms of 3Bi2O3–2MoO3 were also identified by X-ray
diffraction (XRD) results as reported by Egashira et al.29 in
Bi2O3-rich region. The MoO3 rich region in Bi2O3–MoO3 sys-
tem showed very low reaction temperature and low melting
temperature below 7001C. This system constitutes some poten-
tial compounds for LTCC applications. However, the micro-
wave dielectric properties of this system have not been reported.
Monoclinic phase of Bi2Mo2O9, which belongs to space
group P21/n,30 attracts our attention because of its easy prepa-
ration and empty report of dielectric characterization. In the
present work the synthesis, phase evolution, microstructure and
microwave dielectric properties of Bi2Mo2O9 ceramic have been
investigated. The chemical compatibility of Bi2Mo2O9 with Ag
has also been discussed.
˚
˚
˚
I. Introduction
APID development in wireless communication systems has
increased demand for the miniaturization of components
R
such as band pass filters and local oscillators. Low-temperature
cofired ceramic (LTCC) technology offers significant benefits
over other established packaging technologies for high density,
high RF, and fast digital applications requiring hermetical pack-
aging and good thermal management.1,2 Microwave dielectric
materials with low sintering temperature, high dielectric con-
stant er, low dielectric loss, and near-zero temperature coefficient
of resonant frequency TCF are needed to cofire with low loss,
low-melting-point conductors such as Ag, Cu, Au, or Al.
Many materials with good microwave dielectric properties
have high sintering temperature (above 10001C) such as ZnO–
Nb2O5 system,3,4 Bi(Nb,Ta,Sb)O4,5–7 BaO–TiO2–Nb2O5
system,8,9 Li2O–Nb2O5–TiO2 system,10,11 (Zr,Sn)TiO4,12,13 and
(A1A2)(B1B2)O3 complex perovskite system.14–16 To lower the
sintering temperature, addition or substitution of some sintering
aids, such as V2O5, CuO, Bi2O3, and B2O3, are a often used
method. Another approach has been devoted to the use of glass–
ceramic composites. However, most glasses are known to exhibit
low permittivity, which makes them parasitic phases in high-
permittivity ceramics. Recently, much attention has been
attracted by a third method, which is based on many low-
temperature firing ceramic compounds such as the Bi2O3–TeO2,
TiO2–TeO2, CaO–TeO2, BaO–TeO2 binary systems, BaO–
II. Experimental Procedure
Proportionate amounts of reagent-grade starting materials of
Bi2O3 (499%, Shu-Du Powders Co. Ltd., Chengdu, China)
and MoO3 (499%, Fuchen Chemical Reagents, Tianjin,
China) were prepared according to Bi2Mo2O9 composition.
Powders were mixed and milled for 4 h using a planetary mill
(Nanjing Machine Factory, Nanjing, China) by setting the run-
ning speed at 150 rpm with the zirconia balls (2 mm in diam-
eter) as milling media. Differential scanning calorimetry (DSC)
and thermogravimetry analysis of mixed powders were carried
out using a thermoanalyzer system (Netzsch STA-449C,
Netzsch, Selb, Germany) with heating rate of 101C/min. The
other mixed oxides were calcined at 6001C for 4 h. After being
crushed and remilled for 5 h using ZrO2 balls and deionized
water, powders were pressed into cylinders (10 mm in diameter
and 5 mm in height) in a steel die under uniaxial pressure of 20
kN/cm2 with 5 wt% PVA solution binder addition, amount of
A. Feteira—contributing editor
Manuscript No. 24571. Received April 22, 2008; approved June 3, 2008.
This work was supported by the National 973-project of China (2002CB613302), Na-
tional 863-project of China (2006AA03Z0429), NSFC project of China (50572085) and
NCET-05-0840.
wAuthor to whom correspondence should be addressed. e-mail: hwang@mail.xjtu.edu.cn
3419

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Communications of the American Ceramic Society
Vol. 91, No. 10
which was about 8 wt% and burnt out at 5001C for 4 h. Green
samples were sintered at temperature range from 6001 to 6551C
for 2 h. To investigate the chemical compatibility of Bi2Mo2O9
with Ag, 20 wt% Ag was mixed with Bi2Mo2O9 and calcined at
6201C for 4 h.
The crystalline structures of samples were investigated using
XRD with CuKa radiation (Rigaku D/MAX-2400 X-ray
diffractometry, Tokyo, Japan). Microstructures of sintered ce-
ramic were observed on the as-fired surface and fractured sur-
face respectively with scanning electron microscopy (SEM)
(JSM-6460, JEOL, Tokyo, Japan) The apparent densities of
sintered ceramics were measured by Archimedes’ method. Di-
electric behaviors at microwave frequency were measured by the
TE01d shielded cavity method with a network analyzer (8720ES,
Agilent, Palo Alto, CA) and a temperature chamber (Delta
9023, Delta Design, Poway, CA). The (TCF) (tf ) was calculated
by the following formula:
0.2
0.0
297°C
656°C
100.2
99.9
99.6
99.3
604°C
–0.2
–0.4
–0.6
–0.8
–1.0
Loss=0.21%
692°C
200
400
600
800
Temperature (°C)
Fig. 2. Differential scanning calorimetry (DSC) and thermogravimetry
(TG) analysis of mixed Bi2O3–2MoO3 powders.
the compound, which is similar to that reported by Chen and
Sleight.28,30
f85 ꢁ f25
tf ¼
(1)
f25 ꢂ ð85 ꢁ 25Þ
The apparent density and relative density of Bi2Mo2O9 ce-
ramics sintered at different temperatures are shown in Fig. 3.
When sintering temperature is above 6201C, ceramics reach a
bulk density of about 6.25 g/cm3, which is equivalent to a rel-
ative density of about 96% of the theoretical density. The max-
imum density is reached at 6301C, and above this temperature a
slight decrease is observed, which should be attributed to the Bi
volatilization and phase decomposition. The SEM micrographs
of the as-fired surface and fractured surface of Bi2Mo2O9 ce-
ramics are shown in Fig. 4. Homogeneously fine microstructures
with a little pores existing can be revealed for Bi2Mo2O9 ceram-
ics sintered at 6301C for 2 h. Grain sizes lie between 2 and 8 mm
and no abnormal grains are observed.
Microwave dielectric properties of Bi2Mo2O9 ceramics as a
function of sintering temperature are shown in Fig. 5. Micro-
wave permittivity increases from about 25 to about 38 as sinte-
ring temperature increases from 6001 to 6201C. Then
permittivity reaches saturated and maintains stable, which
does not change as temperature increases. To eliminate the in-
fluence of pores on the permittivity of ceramics, the following
equation is used:
where f85 and f25 were the TE01d resonant frequencies at 851 and
251C respectively.
III. Results and Discussions
The room temperature XRD patterns for Bi2Mo2O9 ceramics
sintered in temperature range from 6351 to 6551C are shown in
Fig. 1. When sintering temperature is below 6451C, all peaks can
be indexed as a single-phase monoclinic structure (P21/n). The
˚
lattice parameters were calculated as a 5 11.9664 A, b 5 10.8089
˚
˚
A, and c 5 11.8871 A, which agreed well with the results re-
ported by Chen, van den Elzen and et al.30,31 The theoretical
density of Bi2Mo2O9 ceramics calculated from XRD data was
about 6.5 g/cm3. When sintering temperature increases further,
Bi2Mo3O12 and Bi2MoO6 phases appear as the secondary phase
in ceramics sintered at temperatures ꢀ 6501C. Egashira et al.29
also gave the result that after DTA measurement (up to 8001C)
pure Bi2Mo2O9 would partially decompose to Bi2MoO6 and
Bi2Mo3O12 phase. These conclusions are also supported by DSC
results as shown in Fig. 2. There is one exothermic peak at about
2971C and three endothermic peaks at 6041, 6561 and 6921C in
the DSC curve respectively. The first endothermic peaks at
6041C indicates the formation monoclinic Bi2Mo2O9 and the
one at 6561C indicates the appearance of Bi2Mo3O12 phase. The
last endothermic peaks at 6921C corresponds to the melting of
er ¼ emeas ꢂ ð1 þ 1:5PÞ
(2)
where P is porosity.32 The calculated permittivity eliminating
influence of pores is about 40.17. Q ꢂ f value has the similar
trend with permittivity. When sintering temperature increases to
6201C, Q ꢂ f value gets a stable maximum about 12 500 GHz.
TCF value of Bi2Mo2O9 ceramics is also not sensitive to sinte-
ring temperature and stabilizes about 131ppm/1C as shown in
Fig. 5. In conclusion, Bi2Mo2O9 ceramic sintered at 6201C for
*--Bi2Mo3O12(PDF:21-0103)
m--Bi2Mo2O9(PDF:31-0196)
o--Bi2MoO6 (PDF:21-0102)
o
*
o
*
o
*
*
655°C
650°C
645°C
640°C
*
*
*
1.00
0.98
0.96
0.94
0.92
0.90
6.3
6.2
6.1
6.0
5.9
5.8
o
*
o
* *
o
*
*
m
m
m
m
m
m
m
m
m
m
m
m
m
mm
m
m
m
635°C
apparent density
relative density
0.88
0.86
5.7
5.6
10
20
30
2θ (°)
40
50
60
600
610
620
630
640
Sintering Temperature (°C)
Fig. 1. X-ray diffraction patterns of Bi2Mo2O9 ceramics sintered
at different temperatures for 2 h (m—monoclinic phase of Bi2Mo2O9,
—phase of Bi2Mo3O12, o—phase of Bi2MoO6).
Fig. 3. Apparent density and relative density of Bi2Mo2O9 ceramics
sintered at different temperatures.
Ã

October 2008
Communications of the American Ceramic Society
3421
Fig. 4. Scanning electron microscopy micrographs of as-fired surface (a) and fractured surface (b) of Bi2Mo2O9 ceramics sintered at 6301C for 2 h.
2 h exhibits good microwave dielectric properties with permit-
tivity about 38, Q ꢂ f value about 12 500 GHz and TCF about
131ppm/1C. The possible application of Bi2Mo2O9 ceramics in
microwave circuit is worth of consideration due to its low sinte-
ring temperature (about 6201C) and attractive dielectric prop-
erties. Nevertheless, to apply Bi2Mo2O9 ceramics in LTCC
technology, it must have chemical compatibility with some
metal electrodes, such as Ag, Cu, Au, and Al etc. In present
work we find that Bi2Mo2O9 are easy to react with Ag to form a
new phase of AgBi(MoO4)2 and other phases containing Ag and
Mo. This will limit the further application of Bi2Mo2O9 ceram-
ics. The several systems of microwave dielectric ceramics with
low sintering temperature, such as Bi2O3–TeO2, TiO2–TeO2,
CaO–TeO2, BaO–TeO2 and etc TeO2-based ceramics, all seem
to be easy to react with Ag17–23 because of the easy formation of
Ag6TeO6, Ag2TeO3, and Ag3Te2O7 phases.33 Because of the
limitation of our laboratory, the experiments on the chemical
compatibility of Bi2Mo2O9 ceramics with Cu, Au, and Al have
not been carried out.
temperature, the reactive between it and Ag will affect its com-
mercial application in LTCC.
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