New dielectric material system of Nd(Mg1/2Ti1/2)O3-CaTiO3 with ZnO addition at microwave frequencies

Article abstract of DOI:10.1016/j.jallcom.2008.12.019

The microwave dielectric properties and the microstructures of (1 - x)CaTiO₃-xNd(Mg_1/2Ti_1/2)O₃ ceramics prepared by the conventional solid-state route have been studied. Doping with 0.5 wt% ZnO can effectively p

Full text of DOI:10.1016/j.jallcom.2008.12.019

Journal of Alloys and Compounds 478 (2009) 781–784
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
New dielectric material system of Nd(Mg Ti )O –CaTiO with ZnO
1
/2 1/2
3
3
addition at microwave frequencies
∗
Yuan-Bin Chen
Department of Engineering and Management, Chang Jung Christian University, 396 Chang Jung Rd., 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 6 November 2008
Received in revised form
The microwave dielectric properties and the microstructures of (1 − x)CaTiO –xNd(Mg1/2Ti1/2)O3 ceram-
3
ics prepared by the conventional solid-state route have been studied. Doping with 0.5 wt% ZnO
can effectively promote the densification and the microwave dielectric properties of (1 − x)CaTiO3–
27 November 2008
xNd(Mg1/2Ti1/2)O3. The dielectric constant decreases from 145 to 30.5 as x varies from 0.1 to 1.0. In the
Accepted 10 December 2008
Available online 24 December 2008
(
1 − x)CaTiO3–xNd(Mg1/2Ti1/2)O3 system, the microwave dielectric properties can be effectively controlled
◦
by varying the x value. The dielectric constant of 44, a Q × f value of 43,800 GHz and a ꢀf value of 1.2 ppm/ C
◦
were obtained for 0.1CaTiO3–0.9Nd(Mg1/2Ti1/2)O3 ceramics sintered at 1325 C for 4 h. A band-pass filter
Keywords:
is designed and simulated using the proposed dielectric to study its performance.
Dielectric properties
Microwave dielectric properties
X-ray
©
2009 Elsevier B.V. All rights reserved.
1
. Introduction
tric materials [3–6]. However, this requires a flexible procedure
that increases the cost and time for fabrication of dielectric res-
onators. Liquid phase sintering by adding glass has been found to
lower the firing temperature of ceramics. In this work, ZnO was
selected as a sintering aid to lower the sintering temperature of
The development of microwave communication systems
requires materials which can be used as resonators in filters or
oscillators at microwave frequencies in radar detectors, cellular
telephones, and global positioning satellite (GPS) devices. To sat-
isfy this demand, a material should have a reasonably high dielectric
constant (εr > 20) to allow size reduction of the component, a low
dielectric loss (Q > 5000, where Q = 1/tan ı) in the microwave fre-
quency range, and temperature stability (ꢀ = 0) [1]. The use of at
least two compounds with negative and positive temperature coef-
ficients, employed to form a solid solution or in mixed phases, is
(1 − x)CaTiO –xNd(Mg1/2Ti1/2)O3 ceramics.
3
2. Experimental procedure
Samples of CaTiO3 and Nd(Mg1/2Ti1/2)O3 were individually synthesized by con-
ventional solid-state methods from high-purity oxide (>99.9%) powders: CaCO3,
TiO2, Nd2O3, and MgO. A small amount of ZnO (0.5 wt%) was added as a sin-
tering aid. The starting materials were mixed according to the stoichiometry of
Nd(Mg1/2Ti1/2)O3 and CaTiO3, and then ground in distilled water for 10 h in a balling
f
the most promising method for obtaining a zero ꢀ . Although most
dielectric ceramics with high dielectric constants have positive ꢀf
values, materials with a high dielectric constant, high Q and neg-
f
◦
mill with agate balls. Both mixtures were dried and calcined at 1300 C for 4 h.
The crystalline phases of the calcined powder were identified by X-ray powder
◦
◦
diffraction (XRD) analysis using Cu K␣ radiation from 20 to 60 in 2Â. The calcined
ative ꢀ are desired to achieve this goal. Kim and colleagues have
powder was mixed to the desired composition (1 − x)CaTiO –xNd(Mg1/2Ti1/2)O3 and
f
3
2+
4+
reported many complex perovskites A(B1/2 B1/2 )O3 with neg-
ative ꢀf [2]. Among them, Nd(Mg1/2Ti1/2)O3 has a high dielectric
constant (ε ∼ 27), a high quality factor (Q × f value ∼46,000 GHz)
re-milled for 5 h with PVA solution as a binder. Pellets of 11 mm diameter and 5 mm
thickness were uniaxially pressed. After debinding, the pellets were sintered at tem-
◦
◦
peratures of 1325 C for 4 h. The heating and cooling rates were both set at 10 C/min.
The crystalline phases of calcined powder was identified by X-ray diffraction
patterns. Microstructure observations of the sintered surface were made by scan-
ning electron microscopy (SEM, Philips XL-40FEG). The bulk densities of the sintered
pellets were measured using Archimedes method. The microwave dielectric prop-
erties were calculated from the sizes of the samples and the resonant frequency,
using the Hakki and Coleman’s dielectric resonant TE011 and TE01␦ methods [7]. A
HP8757D network analyzer and a HP8350 sweep oscillator were employed to make
the measurements. The same technique was used to measure the temperature coef-
ficient of resonant frequency (ꢀf). The test set was placed over a thermostat in the
◦
and a negative ꢀ value (−49 ppm/ C). CaTiO (εr > 200, Q × f < 1000,
f
3
◦
> 1100 ppm/ C) with a positive ꢀ value was introduced into the
f
ꢀ
f
mixture to form a solid solution (1 − x)CaTiO –xNd(Mg1/2Ti1/2)O3
3
to compensate for the ꢀ value.
f
Chemical processing and the use of small particles of starting
materials normally reduce the sintering temperature of dielec-
◦
temperature range of 25–80 C. The temperature coefficient of resonant frequency
(
ꢀf) was also measured by the same method associated using Eq. (1)
∗ Fax: +886 6 335981.
E-mail address: cubnck@yahoo.com.tw.
f2 − f1
ꢀ
f =
(1)
f1(T2 − T1)
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.12.019

7
82
Y.-B. Chen / Journal of Alloys and Compounds 478 (2009) 781–784
where fT is the resonant frequency of the dielectric resonator at temperature
◦
T ( C)
3. Results and discussion
Fig. 1 shows the room temperature XRD patterns recorded
for (1 − x)CaTiO –xNd(Mg1/2Ti1/2)O3 ceramic system sintered at
3
◦
1
325 C for 4 h. It includes peaks that indicate the presence of
Nd(Mg1/2Ti1/2)O3 and CaTiO3 as crystalline phases. The mixed
phases in the (1 − x)CaTiO –xNd(Mg1/2Ti1/2)O3 ceramic system
3
were formed because the structures were the same, and the solid
solution system could be obtained. CaTiO3 is a GdFeO -type per-
3
ovskite structure and Nd(Mg1/2Ti1/2)O is also perovskite structure.
3
All the peaks were indexed based on the perovskite unit cell.
(
1 − x)CaTiO –xNd(Mg1/2Ti1/2)O3 solid solution exhibited a per-
3
ovskite structure. The perovskite structure was identified without
any second phase for all compositions tested in the experiment.
However, non-linear variation with composition is clearly observed
in the shift of XRD peak positions (Fig. 1). One can see that the com-
positions over x of 0.5 demonstrate a different variation in the peak
shift, compared with the others (x < 0.5). Microwave dielectric prop-
erties (Q × f and ꢀ ) also show a non-linearity in this compositional
f
range. Seabra et al. [8] investigated in detail the crystal structure
(space group and cell parameter) of a solid solution formed in CT-
LaMT, and related B-site ordering with the non-linear properties in
the microwave dielectric properties. The symmetry changes when
x is between 0.3 and 0.5 and is supported by the sudden change
in the sign of ꢀ values, dielectric properties and densities (see in
f
Figs. 3–6).
The SEM micrographs of (1 − x)CaTiO –0.9Nd(Mg1/2Ti1/2)O3
3
◦
ceramics with 0.5 wt% ZnO additive sintering at 1325 C for 4 h are
illustrated in Fig. 2. The ceramics were already dense at 1325 C. The
◦
grain growth was uniform and increased with x value. Because of
Nd(Mg1/2Ti1/2)O3 have the lower sintering temperature.
The lattice parameters were calculated for different values of
x from the X-ray diffraction patterns and the theoretical densities
obtained using the lattice parameters are plotted in Fig. 3. Fig. 3
shows the observed variation of the bulk densities with composi-
tion. It is expected that the density should increase with increasing
x because of the larger molecular weight of NMT. But the bulk
density varies non-linearly in the region between 0.3 < x < 0.5. The
Fig. 2. SEM photographs of (1 − x)CaTiO3–xNd(Mg1/2Ti1/2)O3 ceramics with 0.5 wt%
◦
ZnO additive sintered at 1325 C: (a) x = 0.1, (b) x = 0.5, and (c) x = 0.9.
Fig. 1. X-ray diffraction patterns of the (1 − x)CaTiO3–xNd(Mg1/2Ti1/2)O3 system
Fig. 3. Bulk density of (1 − x)CaTiO3–xNd(Mg1/2Ti1/2)O3 ceramics with 0.5 wt% ZnO
◦
◦
with 0.5 wt% ZnO additive sintered at 1325 C for 4 h.
additive system sintered at 1325 C for 4 h.

Y.-B. Chen / Journal of Alloys and Compounds 478 (2009) 781–784
783
Fig. 4. Dielectric constant of the (1 − x)CaTiO3–xNd(Mg1/2Ti1/2)O3 ceramics with
Fig. 6. Temperature coefficient of the resonant frequency of (1 − x)CaTiO3–
xNd(Mg1/2Ti1/2)O3 ceramics with 0.5 wt% ZnO additive sintered at 1325 ◦C for 4 h.
◦
0
.5 wt% ZnO additive sintered at 1325 C for 4 h.
Fig. 7. Layout of slow-wave band-pass filter using ring resonators.
Fig. 5. Q × f value of (1 − x)CaTiO3–xNd(Mg1/2Ti1/2)O3 ceramics with 0.5 wt% ZnO
◦
additive system sintered at 1325 C for 4 h.
modes, the pores and the secondary phases. Generally, a larger
grain size, i.e., a smaller grain boundary, indicates a reduc-
tion in lattice imperfection and the dielectric loss was thus
abrupt variation in the bulk density for the compositions with x
between 0.3 and 0.5 is due to phase transformation as indicated
by the microwave dielectric properties. Fig. 3 shows a plot of the
density of the ZnO-doped (1 − x)CaTiO –xNd(Mg1/2Ti1/2)O ceram-
reduced. As shown in Fig. 5, the Q × f values of (1 − x)CaTiO –
3
xNd(Mg Ti )O ceramics with different x value sintered at
3
3
1/2 1/2
3
◦
ics as a function of the x value. The figure reveals that densities of
1325 C, where the specimen possessed the highest density for
3
4
.2–6.24 g/cm were obtained for (1 − x)CaTiO –xNd(Mg1/2Ti1/2)O3
the corresponding composition. The Q × f value increase with the
3
◦
ceramics at sintering temperatures of 1325 C. Density was also
influenced by the composition and increased with x. The decrease
in density as the sintering temperature increased was attributable
to the pronounced grain boundary phases, indicating that increas-
ing the CaTiO3 content reduced the bulk density of the ceramics.
But the bulk density varies non-linearly in the region 0.3 < x < 0.5.
The abrupt variation in the bulk density of the compositions with x
between 0.3 and 0.5 is due to phase transformation as indicated by
the microwave dielectric properties. Yeo et al. [9] have observed
increase of Nd(Mg1 Ti )O content. It was expected since that
/2 1/2
3
the quality factor of Nd(Mg1/2Ti1/2)O3 is much higher than that
of CaTiO . But, the Q × f versus (x) plot shows a decrease in Q for
3
composition in the range x between 0.3 and 0.5. This is attributed
to the fact that the material undergoes a phase transition from
Pnma space group to Pmn1 space group where the atoms are in
a state of re-orientation to form the new structure. The maximum
Q × f ∼43,800 GHz for the investigated range (0.1 ≤ x ≤ 1) appeared
◦
at x = 0.9, where the specimen was sintered at 1325 C for 4 h. It
sharp variations in density for (1 − x)CaTiO − xLa(Zn1/2Ti1/2)O3
seems that the dielectric loss of (1 − x)CaTiO –xNd(Mg1/2Ti1/2)O3
3
3
ceramics system. They have attributed the decrease in density to
the numerous cracks and secondary phases. No such cracks or sec-
ondary phases were observed in the present system.
ceramics system was dominated by the phase transformation.
In Fig. 6 shows the temperature coefficients of resonant
frequency (ꢀ ) of (1 − x)CaTiO –xNd(Mg Ti1/2)O3 ceramics sin-
f
3
1/2
◦ ◦
tered at 1325 C. A ꢀ value of 1.2 ppm/ C was obtained for
f
The permittivity of (1 − x)CaTiO –xNd(Mg1/2Ti1/2)O3 ceramics
3
with 0.5 wt% ZnO additive sintered at various temperatures for 4 h
with different x value is shown in Fig. 4. The dielectric constant
of CaTiO3 and Nd(Mg1/2Ti1/2)O3 are 143 and 27, respectively. The
permittivity decreased with increasing x value owing to a lower
0.1CaTiO –0.9Nd(Mg Ti1/2)O3 ceramics with 0.5 wt% ZnO addi-
3
1/2
◦
tive sintered at 1325 C for 4 h. The temperature coefficient of the
resonant frequency is well known to be governed by the composi-
tion, the additives and the second phase of the material. A higher
CaTiO3 content seemed to make the ꢀf value more positive. The
temperature coefficient of the resonant frequency was found to
be related to the composition and the phase in ceramics. To ver-
ify the performance of the proposed material, a band-pass filter is
designed for a center frequency of 2.4 GHz and fabricated on FR4,
Al O and 0.1CaTiO –0.9Nd(Mg1/2Ti1/2)O . Fig. 7 shows the physi-
permittivity of Nd(Mg1/2Ti1/2)O ceramics. The dielectric constants
3
decreased from 145 to 27 as the x value increased from 0.1 to 1. The
relationship between εr values and sintering temperatures revealed
the same trend with those between densities and sintering temper-
atures since higher density means lower porosity.
The factors that are believed to affect the microwave
dielectric loss can be divided, such as the lattice vibrational
2
3
3
3
cal layout of the designed filter with a center frequency of 2.4 GHz.
Table 1
Simulation results of the band-pass filters using different dielectrics.
Substrate
tan ı
εr
Size (mm2)
Insertion loss (dB)
Return loss (dB)
fo
FR4
Al2O3
0.015
0.003
0.0018
4.5
9.8
44
19.87 × 18.3
9.13 × 8.12
7.18 × 6.2
1.8
0.87
0.69
15.8
21
20
2.39
2.4
2.4
0.1CaTiO3–0.9Nd(Mg1/2Ti1/2)O3 with ZnO addition

7
84
Y.-B. Chen / Journal of Alloys and Compounds 478 (2009) 781–784
The simulation results are listed in Table 1. Compared to FR4
to FR4 and alumina, the filter using 0.1CaTiO –0.9Nd(Mg1/2Ti1/2)O3
3
and alumina, the filter using the 0.1CaTiO –0.9Nd(Mg1/2Ti1/2)O3
ceramics shows a tremendous reduction in the insertion loss and
demonstrates a large reduction in its size.
3
ceramic shows a tremendous reduction in the insertion loss and
demonstrates a large reduction in its size. This design approach
enables one to use an EM simulator (IE3D) to complete the fil-
ter design in order to determine the physical dimensions of the
filters.
References
[1] K. Kageyama, J. Am. Ceram. Soc. 75 (1992) 1767.
[
[
2] S.-Y. Cho, C.-H. Kim, D.-W. Kim, J. Mater. Res. 14 (1999) 2484–2487.
3] R.-H. Liang, X.-L. Dong, Y. Chen, F. Cao, Y.-L. Wang, Mater. Res. Bull. 41 (2006)
4
. Conclusion
1
295–1302.
4] Q. Zeng, W. Li, J.-L. Shi, J.-k. Guo, H. Chen, M.-L. Liu, J. Eur. Ceram. Soc. 27 (2007)
61.
[
2
The dielectriccharacteristicsof (1 − x)CaTiO –xNd(Mg1/2Ti1/2)O3
3
[
[
5] W. Guoqing, W. Shunhua, S. Hao, Mater. Lett. 59 (2005) 2229.
6] V. Tolmer, G. Desgardin, J. Am. Ceram. Soc. 80 (1997) 1981.
ceramics with sintering aids ZnO were investigated. (1 − x)CaTiO –
3
xNd(Mg1 Ti )O ceramics exhibited perovskite structures. The
[7] B.W. Hakki, P.D. Coleman, IEEE Trans. Microwave Theory Tech., MTT-8 (1960)
402–410.
/2
1/2
3
Q × f varies non-linearly and increases for composition with
[
8] M.P. Seabra, M. Avdeev, V.M. Ferreira, R.C. Pullar, N.McN. Alford, J. Eur. Ceram.
Soc. 23 (2003) 2403–2408.
x ꢀ 0.5. The dielectric constant of 44, a Q × f value of 43,800 GHz
◦
and a ꢀ value of 1.2 ppm/ C were obtained for 0.1CaTiO –
0
f
3
[9] D.-H. Yeo, J.-B. Kim, J.-H. Moon, S.-J. Yoon, H.-J. Kim, Jpn. J. Appl. Phys. 35 (1996)
663.
◦
.9Nd(Mg1/2Ti1/2)O ceramics sintered at 1325 C for 4 h. Compared
3