Composite dielectrics (1 - Y)(Mg1-xZnx) 1.8Ti1.1O4-yCaTiO3 suitable for microwave applications

Article abstract of DOI:10.1016/j.jallcom.2012.03.091

Composite dielectrics, (1 - y)(Mg_1-xZn_x) _1.8Ti_1.1O₄-yCaTiO₃, were prepared by the conventional solid-state route. The results of the X-ray diffraction and field emission scanning electron microscopy (FESEM) indicated the formation of solid solutions. Microwave dielectric properties of the composites were investigated systematically. For (Mg_1-xZn_x) _1.8Ti_1.1O₄ system, superior dielectric properties (ε_r = 15.03, Q × f = 185,000 GHz, τ_f = -45.1 × 10^-6/°C) were achieved with (Mg_0.95Zn_0.05)_1.8Ti_1.1O₄ sintered at 1375 °C. CaTiO₃, as a τ_f compensator, was added to form a temperature-stable ceramic system. For (1 - y)(Mg _0.95Zn_0.05)_1.8Ti_1.1O ₄-yCaTiO₃ system, 0.93(Mg_0.95Zn _0.05)_1.8Ti_1.1O₄-0.07CaTiO ₃ ceramic sintered at 1375 °C had optimal dielectric properties (ε_r = 18.26, Q × f = 96,000 GHz, τ_f = -4.6 × 10^-6/°C) which satisfied microwave applications in resonators, filters and antenna substrates.

Full text of DOI:10.1016/j.jallcom.2012.03.091

Journal of Alloys and Compounds 531 (2012) 18–22
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Composite dielectrics (1 − y)(Mg Zn ) Ti O –yCaTiO suitable
1
−x
x 1.8 1.1
4
3
for microwave applications
∗
Lingxia Li , Mingming Zhang, Qingwei Liao, Wangsuo Xia, Xiang Ding
School of Electronic and Information Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
−x
Composite dielectrics, (1 − y)(Mg Znx)1.8Ti1.1O –yCaTiO , were prepared by the conventional solid-
1
4
3
Received 13 November 2011
Received in revised form 17 March 2012
Accepted 19 March 2012
state route. The results of the X-ray diffraction and field emission scanning electron microscopy
FESEM) indicated the formation of solid solutions. Microwave dielectric properties of the compos-
ites were investigated systematically. For (Mg1−xZnx)1.8Ti1.1O4 system, superior dielectric properties
(
Available online 28 March 2012
−6 ◦
(
εr = 15.03, Q × f = 185,000 GHz, ꢀf = −45.1 × 10 / C) were achieved with (Mg0.95Zn0.05)1.8Ti1.1O4 sintered
◦
at 1375 C. CaTiO3, as a ꢀf compensator, was added to form a temperature-stable ceramic system. For
Keywords:
(
1 − y)(Mg0.95Zn0.05)1.8Ti1.1O4–yCaTiO3 system, 0.93(Mg0.95Zn0.05)1.8Ti1.1O4–0.07CaTiO3 ceramic sintered
(
1 − y)(Mg1−xZnx)1.8Ti1.1O4–yCaTiO3
◦
−6 ◦
at 1375 C had optimal dielectric properties (εr = 18.26, Q × f = 96,000 GHz, ꢀf = −4.6 × 10 / C) which
satisfied microwave applications in resonators, filters and antenna substrates.
© 2012 Elsevier B.V. All rights reserved.
Crystal structure
Dielectric property
Microwave application
−
6
◦
1
. Introduction
Mg TiO4 (ε ∼ 14, Q × f ∼ 150,000 GHz, ꢀ ∼ −50 × 10 / C) were
2
r
f
reported by Wakino and Belous [5,6]. Moreover, researchers
made effort to study their microstructures and dielectric
properties by various methods. Different low-loss microwave
dielectric materials were investigated such as Mg(Zr Ti )O3,
[(Mg1 Znx)0.95Co0.05]2TiO [7,8]. Lately, the tetragonal-structured
The rapid growth of recent wireless communication systems
led to an increasing demand for small-scale high-frequency res-
onators, filters and antennas. When using in microwave devices
such as resonators, filters, three primary factors are required: a
high dielectric constant to reduce the component size, a high
quality factor (Q) to increase the frequency selectivity, and a near-
zero ꢀf to ensure high-temperature stability. Research of new
materials that satisfy such a requirement has been ongoing and
varieties of microwave dielectric ceramics have been intensively
investigated, such as ZnTa O , ZnTiNbTaO , (Mg0.95M0.05)Ta O ,
x
1−x
−x
4
Mg1.8Ti1.1O4 as a new microwave dielectric material was reported
and its dielectric properties were improved by doping CoO [9,10].
◦
But its sintering temperature is as high as 1450 C.
According to previous researches, ZnO, as a sintering aid, can
effectively lower the sintering temperature of ceramic materials
via liquid-phase sintering and improve the dielectric properties
2
6
8
2
6
(
1 − x)BaTi O –xBaZn Ti O
[1–4]. Nowadays, the microstrip
[11,12]. In this work, (Mg Znx)1.8Ti1.1O4 ceramics with differ-
4
9
2
4
11
1−x
antenna substrates are mostly made by FR4. In comparison with
FR4, ceramic substrates show a tremendous reduction in the inser-
tion loss. However, high permittivity ceramics are limited in actual
applications of microstrip patch antennas because of their nar-
row frequency bandwidths. In order to get a miniaturization, high
gain and broad bandwidth microstrip antenna substrate, appropri-
ate permittivity, high Q × f value and temperature-stable ceramic
system is necessary.
ent ZnO addition concentration were synthesized. Relationships
between crystal structures and microwave dielectric properties
were analyzed based on densification, X-ray diffraction (XRD)
patterns, field emission scanning electron microscopy (FESEM)
associated with Energy Dispersive X-ray Detector (EDX). Fur-
−
6
◦
thermore, CaTiO3 (ε ∼ 170, Q × f ∼ 3600 GHz, ꢀ ∼ 800 × 10 / C),
r
f
which was reported as a ꢀ compensator [11], was selected as a
f
further addition to optimize the temperature coefficient of the reso-
nant frequency of the selected ceramic for practical application. The
Recently, the intense development of high-Q microwave mate-
rials based on low-cost raw oxides from the MgO–TiO2 system has
(1 − y)(Mg0.95Zn0.05)1.8Ti1.1O –yCaTiO3 ceramics were composed
4
begun. In this binary system, ilmenite-structured MgTiO (εr ∼ 16,
and optimal dielectric properties (ε = 18.26, Q × f = 96,000 GHz,
3
r
Q × f ∼ 160,000 GHz, ꢀf ∼ −50 × 10− / C) and spinel-structured
6
◦
−6 ◦
ꢀ = −4.6 × 10 / C) were achieved when y = 0.07 for microwave
f
application.
2
.
Experimental procedures
∗ Corresponding author. Tel.: +86 22 27402838/13752079365;
fax: +86 22 27402838.
The samples involved in this study were synthesized by the solid-state reac-
E-mail address: lilingxia@tju.edu.cn (L. Li).
tion technique using high-purity MgO (≥99%), TiO2 (≥99%, rutile), ZnO (≥99%), CaO
0
925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2012.03.091

L. Li et al. / Journal of Alloys and Compounds 531 (2012) 18–22
19
Table 1
The EDX data of the (Mg0.95Zn0.05)1.8Ti1.1O4 ceramics for spots A and B shown in
Fig. 2.
Spots
At.(%) (K)
Mg
Zn
Ti
O
A
B
43.52
37.7
2.49
1.86
19.12
24.41
34.86
36.03
dimension due to the substitution of larger ionic radius of
2
+
2+
Zn (0.74 A˚ , CN = 6) into the Mg (0.72 A˚ , CN = 6) site of the
Mg1.8Zn1.1O4 spinal tetragonal.
Fig. 2 shows the FESEM micrographs of (Mg0.95Zn0.05)1.8Ti1.1O4
ceramics sintered at different temperatures for 4 h. The grain
size increases with the increase of sintering temperature. In
order to confirm the content of Zn2+ ionic that solubilized
in Mg1.8Ti1.1O4 ceramics, the elemental analysis for marked
spots was taken from the well-developed specimens. As shown
in Fig. 2(d) and Table 1, the grain morphology of well-
developed specimens could be grouped into two types. The
major phase with large grains, such as spot A in Fig. 2(d),
Fig. 1. X-ray diffraction patterns of (Mg1−xZnx)1.8Ti1.1O4 (x = 0.01–0.1) ceramics sin-
tered at 1375 C for 4 h with different x value.
◦
is a spinal tetragonal phase (Mg0.95Zn0.05)1.8Ti1.1O , and some
4
≥99%). MgO was additionally fired at 700 ◦C to remove water and CO2 due to its
small grains such as spot B, are identified as an ilmenite phase
(Mg0.95Zn0.05)TiO3.
(
hygroscopic property. (Mg1−xZnx)1.8Ti1.1O4 and CaTiO3 powders were prepared by
mixing all the raw materials according to the desired stoichiometry. Mixtures were
◦
milled in distilled water for 12 h with agate balls and then dried at 100–150 C to
3
.1.2. Microwave dielectric properties
Fig. 3 shows the dielectric constant (εr) of (Mg
1−x
obtain homogenized powders. The preliminary heat treatment (calcination) of the
◦
◦
Znx)1.8Ti1.1O4
powders was carried out at 900 C (2 h) and 1100 C (2 h) in air in alumina crucibles
respectively. After calcination, the powders were mixed according to the molar
fraction (1 − y)(Mg0.95Zn0.05)1.8Ti1.1O4–yCaTiO3 and then re-milled for 12 h. The fine
powders ((Mg1−xZnx)1.8Ti1.1O4, (1 − y)(Mg0.95Zn0.05)1.8Ti1.1O4–yCaTiO3) granulated
by sieving through an 80 mesh together with the organic binder were pressed
into pellets with 10 mm in diameter and 4–5 mm in thickness. All the pellets were
(x = 0.00–0.1) solid solutions sintered at different temperatures
for 4 h. At microwave frequencies, the εr is dependent on the
density, secondary phases, and the crystal structure [14]. In this
work, the dielectric constant of (Mg1 Znx)1.8Ti1.1O4 with differ-
ent sintering temperature is mainly dependent on the density of
samples. As shown in Fig. 3, the variation trend of dielectric con-
stants was in accordance with variation trend of corresponding
density. By increasing the sintering temperature, the dielectric
constant of the specimen increased to a maximum value because
higher density produces lower porosity and then decreased
slightly.
−x
◦
◦
sintered at 1210–1450 C for 4 h in air, using a heating rate of 5 C/min.
The phase identification and crystal structure analysis of the sintered ceramics
were carried out by X-ray diffraction (Rigaku D/max) using Cu K␣ radiation at a
.02 /0.5 s scanning speed. Microstructure of sintered surfaces were performed by
field emission scanning electron microscopy (FESEM, FEI Nanosem 430) associated
with Energy Dispersive X-ray Detector (EDX, Oxford INCA). The apparent densities
of the sintered pellets were measured using the Archimedes method (Mettler Toledo
XS64).
◦
0
The dielectric constant εr and the quality factor Q at microwave frequency were
measured by the modified Hakki–Coleman’s method and the cavity method in the
TE01ꢁ mode using Agilent 8720ES network analyzer [12,13]. The temperature coef-
ficient of resonant frequency (ꢀf) was calculated using the equation
Moreover, the εr of (Mg1 Znx)1.8Ti1.1O4 with different x value
sintering at 1375 C were dependent on the theoretical dielec-
tric polarizabilities (˛theo.). Fig. 4 shows the theoretical dielectric
−x
◦
polarizability (˛theo.) of (Mg Znx)1.8Ti1.1O4 obtained from the
1−x
f85 − f25
ꢀ
f =
(1)
additivity rule of molecular polarizability shown in Eq. (2) [15,16].
f25 × 60
◦
◦
˛theo.(A1.8B1.1O4) = 1.8˛A + 1.1˛B + 4˛O
(2)
where f85 and f25 are the TE01ꢁ resonant frequency of the samples at 85 C and 25 C,
respectively.
where ˛theo. is the theoretical polarizability; ˛ , ˛B, ˛O are
A
the ionic polarizability of A-, B-site ion, and oxygen. With
increase of x, the theoretical dielectric polarizability (˛theo.)
3
. Results and discussion
3
2+
increase due to the ionic polarizability (2.04 A˚ ) of Zn which
3
.1. (Mg1−xZnx)1.8Ti1.1O4
system
2+
˚ 3
is larger than the ionic polarizability of Mg (1.32 A )[17]. With
x increase from 0.00 to 0.1, a maximum εr value of 15.29 can
be obtained for specimen using (Mg0.9Zn0.1)1.8Ti1.1O4 sintered at
for 4 h.
In order to achieve optimal dielectric properties for microwave
application, we first studied the (Mg1 Znx)1.8Ti1.1O (x = 0.00–0.1)
system.
−x
4
Fig.
5
shows the Q × f values of (Mg1 Znx)1.8Ti1.1O4
−x
(
x = 0.00–0.1) solid solutions sintered at different temperatures for
3
.1.1. Phases and microstructure
Fig. 1 illustrates the room-temperature XRD patterns recorded
from the (Mg Znx)1.8Ti1.1O4 (x = 0.00–0.1) ceramics sintered at
4 h. Many factors believed to affect the microwave dielectric loss
and can be divided into two fields, the intrinsic loss and extrinsic
loss. The intrinsic losses are mainly caused by lattice vibration
modes while the extrinsic losses are dominated by second phases,
oxygen vacancies, grain sizes and densification or porosity [18].
The Q × f value with different sintering temperature is mainly
affected by density and the grain size of the ceramic here. As
apparent densities increased, the pores and the grain boundary
area decreased, the grain size increased, thus reducing the lattice
imperfections and increasing the Q × f value. The Q × f values
increase to a maximum and then decrease with the variation of
1−x
◦
1
375 C for 4 h. A spinal tetragonal phase of (Mg
Znx)1.8Ti1.1O4
1−x
(
indexed as Mg1.8Ti1.1O , ICDD-PDF#00-054-1226), belonging to
4
the space group P4122(91), was identified as the main phase.
A hexagonal ilmenite-structured (Mg1 Znx)TiO3 (indexed as
MgTiO , ICDD-PDF #00-079-0831) was assigned as a second
phase. Moreover, the reflection peaks of the tetragonal phases
tend to slightly shift toward the lower angle with increas-
ing Zn content, which suggests an increase in the unit-cell
−x
3

2
0
L. Li et al. / Journal of Alloys and Compounds 531 (2012) 18–22
Fig. 2. Field emission scanning electron microscopy of (Mg0.95Zn0.05)1.8Ti1.1O4 ceramics sintered at (a) 1300, (b) 1325, (c) 1350, (d) 1375, include EDX data of specimen for
◦
spot A and B, (e) 1400, and (f) 1425 C for 4 h.
−
6
◦
apparent densities that can be demonstrated in Figs. 3 and 5.
varied from −47.1 to −43.9 × 10 / C with different sintering
The degradation of Q × f value was attributed to abnormal grain
temperature.
growth as observed in Fig. 2 [19]. Compared with Mg1 Ti O ,
.8
1.1
4
a higher Q × f value of 185,000 GHz (measured at 9.4 GHz) was
3.2. (1 − y)(Mg1−xZnx)1.8Ti1.1O –yCaTiO system
4
3
◦
obtained for (Mg
for 4 h.
Zn
)
Ti
O
4
ceramics sintered at 1375 C
0
.95
0.05 1.8 1.1
Superior dielectric properties (ε = 15.03, Q × f = 185,000 GHz,
r
−
6
◦
Fig. 6 illustrates the temperature coefficients of resonant
ꢀ = −45.1 × 10 / C)
have
been
achieved
with
f
◦
)1.8Ti1.1O4 sintered at 1375 C. It can be clearly
0.05
frequency (ꢀf) of (Mg Znx)1.8Ti1.1O4 ceramics with differ-
(Mg
Zn
1−x
0.95
ent sintering temperature. The ꢀf is known to be governed
by the composition, the additives, and the second phase
of the material [20]. It is observed that the ꢀf value was
insensitive to the sintering temperatures. This was expected
because there was no extra phase detected throughout the
experiment. The ꢀf value of (Mg0.95Zn0.05)1.8Ti1.1O4 ceramic
seen that the main issue which limits the practical use of
(Mg0.95Zn0.05)1.8Ti1.1O4 ceramic is the large negative tempera-
ture coefficient of resonant frequency (ꢀ ). In order to achieve a
f
temperature-stable material, CaTiO was added to further improve
3
the microwave dielectric properties of the specimens, in particular,
the ꢀ value.
f

L. Li et al. / Journal of Alloys and Compounds 531 (2012) 18–22
21
Fig. 5. Qf values of (Mg1−xZnx)1.8Ti1.1O4 (x = 0.01–0.1) ceramics sintered at different
sintering temperatures for 4 h.
Fig. 3. Dielectric constant of sintered specimens with different x value.
◦
Fig. 6. ꢀf values of (Mg1−xZnx)1.8Ti1.1O4 ceramics with different x sintered at 1375 C.
Fig. 4. The theoretical dielectric polarizability (˛theo.) with different x value.
Fig.
7
shows the XRD patterns of (1 − y)(Mg
0.95
◦
Zn0.05)1.8Ti1.1O –yCaTiO3 ceramics sintered at 1375 C for 4 h.
4
The XRD patterns showed that (Mg0.95Zn0.05)1.8Ti1.1O4 presented
as the main crystalline phase, in association with minor phase
CaTiO3 and second phase (Mg0.95Zn0.05)TiO3.
Table 2 demonstrates the microwave dielectric properties of
(
1 − y)(Mg0.95Zn0.05)1.8Ti1.1O –yCaTiO3 ceramics system sintered
4
◦
at 1375 C for 4 h. As the y value increased from 0.03 to 0.1, the
−6
◦
ꢀf values varied from −20.8 to 5.5 × 10 / C. It indicates that zero
ꢀf value can be obtained by appropriately adjusting the y value
Fig. 7. X-ray diffraction patterns of (1 − y)(Mg0.95Zn0.05)1.8Ti1.1O –yCaTiO3 ceramics
4
◦
of (1 − y)(Mg0.95Zn0.05)1.8Ti1.1O –yCaTiO3 ceramics. With y = 0.07,
sintered at 1375 C for 4 h.
4
0.93 (Mg
Zn0.05)1.8Ti1.1O –0.07CaTiO3 ceramic showed good
.95 4
0

2
2
L. Li et al. / Journal of Alloys and Compounds 531 (2012) 18–22
Table 2
Acknowledgments
The microwave dielectric properties of (1 − y)(Mg0.95Zn0.05)1.8Ti1.1O4–yCaTiO3
◦
ceramics system sintered at 1375 C for 4 h.
This work was supported by Program for New Century Excellent
Talents in University (NCET) and 863 Program (2007AA03Z423) and
China Postdoctoral Science Foundation.
ꢀf (×10− / C)
6 ◦
y value
Apparent densities
εr
Qf (GHz)
3
(g/cm )
0.03
0.05
0.07
0.1
3.564
3.672
3.684
3.696
16.74
17.34
18.26
19.45
130,120
118,830
96,000
55,800
−20.8
−11.7
−4.6
5.5
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−x
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[
[
FESEM (EDX) analysis. By addition of Zn ionic, the sintering
◦
temperature decreased to 1375 C compared with Mg1.8Ti1.1O .
4
The Q × f value increased and then decreased with increasing
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◦
with 0.93(Mg
Zn
)
Ti O –0.07CaTiO sintered at 1375 C.
0
.95
0.05 1.8 1.1
4
3
[
It is a potential candidate material for application in microwave
dielectric resonators, filters and antenna substrate.