Improved high Q value of MgTiO3-CaTiO3 microwave dielectric resonator using WO3-doped at lower sintering temperature for microwave applications

Article abstract of DOI:10.1016/j.jallcom.2008.11.102

The effect of WO₃ additive on the microstructures, the phase formation and the microwave dielectric properties of MgTiO₃-CaTiO₃ ceramics system were investigated. The sintering temperature of WO₃-doped 0.95MgTiO

Full text of DOI:10.1016/j.jallcom.2008.11.102

Journal of Alloys and Compounds 478 (2009) 657–660
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Improved high Q value of MgTiO –CaTiO microwave dielectric resonator using
3
3
WO -doped at lower sintering temperature for microwave applications
3
∗
Yuan-Bin Chen
Department of Engineering and Management, Chang Jung Christian University, 396 Chang Jung Road, Sec. 1, Kway Jen, Tainan 71101, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 October 2008
Received in revised form
The effect of WO3 additive on the microstructures, the phase formation and the microwave dielectric
properties of MgTiO3–CaTiO3 ceramics system were investigated. The sintering temperature of WO3-
doped 0.95MgTiO3–0.05CaTiO3 (95MCT) ceramics can be lowered to 1175 C. The microwave dielectric
◦
21 November 2008
properties are found strongly to correlate with the sintering temperature as well as the amount of WO3
Accepted 21 November 2008
Available online 30 November 2008
◦
addition. At 1250 C 95MCT ceramics with 0.5 wt% WO3 addition gives a dielectric constant εr of 20.3, a
◦
Q × f value of 66,000 (GHz) and a ꢀf value of −8.2 ppm/ C. A band-pass filter is designed and simulated
using the proposed dielectric to study its performance.
Keywords:
Sintering
©
2008 Published by Elsevier B.V.
X-ray diffraction
Dielectric properties
1
. Introduction
temperature. However, it also decreased the microwave dielectric
microwave dielectric properties of ceramics. The chemical pro-
cess often required a flexible process that increased the cost and
time required to fabricate dielectric devices. Since tungsten-based
dielectric materials are well-known low -sintering-temperature
ceramics and have been widely investigated, WO3 is chosen as a
sintering aid in this study.
MgTiO -based ceramics is finding wide applications as
3
dielectrics in resonators and filters for communication and radar
systems operating at microwave frequencies. The development of
microwave dielectric resonator for communication systems such
as cellular phones and global positioning systems has been rapidly
growing in the past decade [1,2]. The advantage of using dielec-
tric resonators is that it makes the size reduction of microwave
components possible. To achieve the requirements, these dielec-
tric resonators must be combined with a high dielectric constant
MgTiO –CaTiO3 (hereafter referred to as MCT) ceramics is
3
well known as the material for temperature compensating type
capacitor, dielectric resonator and patch antenna. The material
is made of a mixture of magnesium titanate (MgTiO : εr ∼17,
3
◦
(
εr > 15) for possible size miniaturization (size of a dielectric res-
Q × f value ∼160,000 at 7 GHz and ꢀ value ∼−55 ppm/ C) [3]
f
√
onator ∼1/ εr), a low dielectric loss (Q > 5000, where Q = 1/tan ı)
and calcium titanate (CaTiO : εr ∼170, Q ×f value ∼3600 at
3
◦
for high frequency selectivity and low signal attenuation, and a
7 GHz and ꢀ value ∼800 ppm/ C) [4]. With the ratio Mg:Ca = 95:5,
f
near-zero temperature coefficient of resonant frequency (ꢀ ) for
0.95MgTiO –0.05CaTiO (hereafter referred to as 95MCT) ceramics
f
3 3
temperature stable circuit. Magnesium titanate-based ceramics is
one of the popular microwave dielectric materials, and has been
widely applied in the coaxial type, large dimensional resonators
and GPS antenna when operating at higher frequencies. Since most
of the known commercial dielectric materials for high frequency
applications have high sintering temperatures, they would certainly
increase the manufacturing cost. Three methods are commonly
used for reducing the sintering temperature of dielectric ceram-
ics: low-melting glass addition, chemical processing, and using
starting materials with smaller particle sizes. The method involv-
ing liquid-phase sintering was found to effectively lower the firing
gives εr ∼21, Q ∼8000 at 7 GHz, and a zero ꢀf value. However, it
◦
required sintering temperatures as high as 1400–1500 C. Many
researchers made efforts to study the microstructures and the
microwave dielectric properties of the 95MCT ceramics by adding
various additives or varying processing. Ferreira et al. [5,6] had seri-
ous reports of microwave dielectric properties of MCT ceramics.
Doping with La O3 and Cr O3 enhanced the densification of MCT
2
2
◦
ceramics prepared by a mixed oxide route at 1400 C. It increased
the quality factor but decreased the dielectric constant. The ꢀf
value was not reported. MCT ceramics prepared by the chemi-
cal technique could be sintered at lower temperature, and gave
excellent dielectric properties (εr = 19.1, Q × f = 8400 at 8 GHz and ꢀf
unreported). However, the chemical process often required a flex-
ible procedure that would increase the cost and time required to
fabricate microwave dielectric components. The liquid phase sin-
∗ Fax: +886 6 3359815.
E-mail address: cubnck@yahoo.com.tw.
0
925-8388/$ – see front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2008.11.102

6
58
Y.-B. Chen / Journal of Alloys and Compounds 478 (2009) 657–660
tering by adding glass or low melting point materials was found to
effectively lower the firing temperature, as in our previous reports
[
7–10]. In this paper, WO3 was added to 0.95MgTiO –0.05CaTiO3
3
ceramic system as a sintering aid to lower its sintering tempera-
ture. The resultant microwave dielectric properties were analyzed
based upon the densification, the X-ray diffraction patterns and the
microstructures of the ceramics. In addition, the low-temperature
sintered microstructures of MCT ceramics were also correlated with
its microwave dielectric properties. The correlation between the
microstructure and the Q × f value was also investigated.
2.
Experimental procedures
Samples of MgTiO3 and CaTiO3 were individually synthesized by conventional
solid-state methods from high-purity oxide (>99.9%) powders: MgO, CaCO3 and TiO2.
The starting materials were mixed according to the stoichometery of MgTiO3 and
CaTiO3, and ground in distilled water for 10 h in a balling mill with agate balls. Both
◦
mixtures were dried and calcined at 1100 C for 4 h. Impurities containing plastic
steel would burn out during firing. The calcined powders were mixed as desired
composition 0.95MgTiO3–0.05CaTiO3 with different amounts of WO3 additions as-
sintering aids and re-milled for 5 h with PVA solution as a binder. The powders were
sieved with dimension of 11 m in diameter and 5 mm in thickness. The pellets were
Fig. 1. X-ray diffraction patterns of 0.95MgTiO3–0.05CaTiO3 ceramics sintered at
1300 C with (a) 0.5 wt% (b) 2 wt% (c) 5 wt% WO3 additions. (+) MgTiO3, (ꢁ) CaTiO3,
(ꢀ) MgTi2O5, (*) CaWO4).
◦
◦
sintered at temperatures of 1170–1350 C for 4 h in air. The heating and cooling rates
◦
of the samples were 10 C/min.
The microstructure observation of surfaces of the sintered ceramics was
performed by scanning electron microscopy (SEM, JEOL JSM 6400, Japan). The crys-
talline phases of the sintered ceramics were identified using an X-ray diffraction
The SEM photographs of 95MCT ceramics with 0.5 wt% WO3
(
XRD, D5000 Diffractometer, Seimens, Germany) pattern. The bulk densities of the
addition at different sintering temperatures for 4 h are illustrated in
sintered pellets were measured using the Archimedes method. The dielectric con-
stant (εr) and the quality factor values (Q) at microwave frequencies were measured
using the Hakki and Coleman [11] dielectric resonator method as modified and
improved by Courtney [12]. A system combining a HP8757D network analyzer and
a HP8350B sweep oscillator was employed in the measurement. The disks were, in
turn, placed between two parallel brass plates. The TE011 and TE012 modes were mea-
sured using a HP8510B network analyzer. The dielectric constant was calculated from
the resonant frequency of the TE011 mode of the cylindrical disk. For the unloaded Q
measurement, the conductor loss resulting from the eddy currents around the con-
ductive plate surfaces must be subtracted to obtain the dielectric quality factor. For
this purpose, two disks with different heights were prepared, one for the TE011 mode
and the other is for the TE012 mode, where the disk for the TE012 mode measurement
is twice the height of the disk for the TE011 mode. An identical technique was applied
in measuring the temperature coefficient of resonant frequency ((f). The test set was
Fig. 2. The 95MCT ceramics was not dense and the grain did not grow
◦
◦
at 1170 C. However, rapid grain growth was observed at 1250 C and
◦
the pores were almost eliminated for specimen sintered at 1300 C
due to the effect of liquid phase.
Fig. 3 shows the bulk densities of 95MCT ceramics with various
amounts of WO3 additions at different sintering temperature for
4
h. The densities increased with increasing sintering temperature
due to enlarged grain size as observed in Fig. 2. They seemed to satu-
◦
rate at 1250 C despite of the amount of WO addition. A maximum
3
3
density of 3.8 g/cm was obtained for 95MCT ceramics with 5 wt%
WO3 1300 C for 4 h. It implies that WO3 addition can effectively
◦
◦
lower the sintering temperature of 95MCT ceramics.
placed over a thermostat in the temperature range from +20 to +80 C. The (f value
◦
(
ppm/ C) can be calculated by noting the change in resonant frequency (ꢁf),
Fig. 4 shows the dielectric constants of 95MCT ceramics with dif-
ferent amount of WO3 as functions of their sintering temperatures.
The relationships between εr values and sintering temperatures
revealed the same trend with those between densities and sinter-
ing temperatures since higher density means lower porosity. The
dielectric constant slightly increased with increasing sintering tem-
perature. Moreover, higher WO3 content also increased εr value
could be explained as the result of higher density. The εr values
of the 95MCT ceramics with 0.5 wt% WO3 addition increased from
19.89 to 20.23 as the sintering temperature increased from 1170 to
f2 − f1
ꢀ
f =
,
(1)
f1(T2 − T1)
where f1 and f2 represent the resonant frequencies at T1 and T2, respectively.
3. Results and discussion
Fig. 1 shows the X-ray diffraction patterns of as-sintered 95MCT
ceramics with various amounts of WO3 additions. MCT ceramics
showed mixed phases of MgTiO3 as the main phase associated
◦
1250 C.
with CaTiO3 and MgTi O5 as the minor phases. It is understood
that the crystal of MgTiO3 and CaTiO3 are trigonal (JCPDS #06-
The quality factor values (Q × f) of 95MCT ceramics with various
2
WO additions at different sintering temperatures are demon-
3
0
494) and orthorhombic (JCPDS #22-0153), respectively. MgTi O5,
strated in Fig. 5. With increasing sintering temperature, the Q × f
value was found to increase to a maximum value and thereafter
decreased. A maximum Q × f value of 60,000 GHz was obtained
2
usually formed as an intermediate phase, was identified and diffi-
cult to completely eliminate from the sample prepared by mixed
oxide route. The formation of CaTiO3 in MCT ceramics was due
to the structure difference and the larger ionic size difference
between Ca (1.00 Å) and Mg (0.64 Å). More amount of WO3
addition seemed to inhibit the formation of CaTiO , but slightly
◦
for 95MCT ceramics with 0.5 wt% WO3 addition at 1250 C. The
decrease of Q × f value was attributed to abnormal grain growth as
observed in Fig. 2. Since the liquid phase would enhance the densi-
fication of the specimen, individual maximum Q × f value appeared
2
+
2+
3
enhance the formation of MgTi O5 in MCT ceramics. Result of the
at lower sintering temperature for higher WO -doped 95MCT. The
2
3
X-ray diffraction pattern in Fig. 1, however, was obtained for WO -
microwave dielectric loss is mainly caused not only by the lat-
tice vibrational modes, but also by the pores, the second phases,
the impurities, or the lattice defect. Relative density also plays an
important role in controlling the dielectric loss, and has been shown
for other microwave dielectric materials. The Q × f value of 95MCT
ceramics decreased with increasing WO3 addition, which was not
consistent with the variation of density. Instead of the density, the
decrease in Q × f value was due to the formation of second phase
3
◦
doped specimens at 1300 C in comparison to that of pure 95MCT
◦
at 1300 C. With 2 wt% WO3 addition, both CaTiO3 and MgTi O5
2
◦
phases were observed and stable at temperatures 1300 C. Although
a very small amount of CaWO was identified, it became significant
4
when the WO3 addition was increased to 2 wt%. With WO3 addi-
tion, the secondary phase of CaWO4 was enhanced, and this which
lower the Q × f value as we can see in Fig. 5.

Y.-B. Chen / Journal of Alloys and Compounds 478 (2009) 657–660
659
Fig. 2. SEM photographs of 0.95MgTiO3–0.05CaTiO3 ceramics with 0.5 wt% WO3 additions at different sintering temperatures.
−
4
MgTi O5 (tan ı ∼6310 ) as well as the increase in WO content. As
tric materials. Interfacial polarization is also thought to play a role
in porous materials at lower sintering temperature. Because the
secondary phases CaWO4 with 2 wt% and 5 wt% addition, but not
find with 0.5 wt% addition MCT ceramics, the secondary phases was
suggested to dominate the Q × f values of MCT ceramics with WO3
additions at lower sintering temperatures.
2
3
◦
comparing to pure 95MCT (Q × f ∼55,000 sintering above 1400 C)
ceramics with 0.5 wt% WO addition not only lowered the sintering
3
temperature but also possessed comparable Q × f value (∼66,000
at 7 GHz). The Q × f value seemed not to obey the mixing rules,
because higher formation of CaTiO3 in MCT ceramics with 0.5 wt%
WO3 addition gave higher Q × f value than that with 5 wt% addi-
tion. According to previous reports, the microwave dielectric loss
was simultaneously affected by many factors, mainly caused not
only by the lattice vibration modes, but also by densification, pores,
grain sizes and boundaries, and secondary phases. Relative den-
sity and secondary phases are important factors in controlling the
dielectric loss, and has been shown for other microwave dielec-
Fig. 6 illustrates the temperature coefficients of resonant fre-
quency (ꢀf) of 95MCT ceramics with various WO3 additions at
different sintering temperatures. The temperature coefficient of
resonant frequency (ꢀf) was insensitive to the sintering temper-
◦
atures above 1170 C. The ꢀf values became more negative with
◦
increasing WO3 addition. It varied from −8.2 to −22.5 ppm/ C as
the amount of WO3 addition increased from 0.5 to 5 wt% at differ-

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Y.-B. Chen / Journal of Alloys and Compounds 478 (2009) 657–660
Table 1
Simulation results of the band-pass filters using different dielectrics.
Substrate
tan ı
εr
Size (mm2)
a × b
Insertion
loss (dB)
Return loss
(dB)
Fractional
bandwidth (%)
Al2O3
0.001
0.00012
9.8
20.3
15.25 × 21.7
14.1 × 16.1
2.2
1.9
15.1
20.0
8
8.1
9
5MCT ceramics with 0.5
wt% WO3
Fig. 3. Apparent density of 0.95MgTiO3–0.05CaTiO3 ceramics with different amount
of WO3 additions as function of their sintering temperature.
Fig. 7. Layout of the slow-wave band-pass filter.
Al O and 95MCT. Fig. 7 shows the physical layout of the designed
2
3
filter with a center frequency of 2 GHz. The simulation results are
listed in Table 1. Compared to alumina, the filter using the 95MCT
ceramic shows a tremendous reduction in the insertion loss and
demonstrates a large reduction in its size.
Fig. 4. εr value of 0.95MgTiO3–0.05CaTiO3 system with various WO3 addition sin-
tered at different temperatures.
4. Conclusion
WO3 addition to MgTiO –CaTiO3 ceramics not only effectively
3
lowered the sintering temperature but also promoted its microwave
dielectric properties due to the liquid phase effect. MCT ceramics
showed mixed phases of MgTiO as the main phase associated with
3
minor phases CaTiO and MgTi O5. With more WO additions, MCT
3
2
3
ceramics seemed to inhibit the formation of CaTiO3 but slightly
enhance the formation of MgTi O5 and second phase CaWO4 was
2
formed. The existence of CaWO phase caused decrease in the Q × f
4
value, which indicated that a greater WO3 addition would increase
the densification and increase the Q × f value of the ceramics. At
◦
Fig. 5. Q × f values of 0.95MgTiO3–0.05CaTiO3 system with various WO3 additions
1250 C, 0.95MgTiO –0.05CaTiO ceramics with 0.5 wt% WO addi-
3
3
3
sintered at different temperatures.
tion gave excellent microwave dielectric properties: εr ∼20.3, Q × f
◦
value ∼66,000 (at 7 GHz) and ꢀ value ∼−8.2 ppm/ C. Compared
f
to alumina, the filter using 95MCT ceramics shows a tremendous
reduction in the insertion loss and demonstrates a large reduction
in its size.
References
[
[
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3
◦
are −50 and 800 ppm/ C, respectively, it implies that zero ꢀ can be
f
[
[
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[
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