Microwave dielectric properties and sintering behaviors of (Mg0.95Ni0.05)TiO3-CaTiO3 ceramic system

Article abstract of DOI:10.1016/j.jallcom.2008.04.099

The microwave dielectric properties and sintering behaviors of the new ceramic system (1 - x)(Mg_0.95Ni_0.05)TiO₃-xCaTiO₃ (x ≤ 0.1) prepared by conventional solid state route were investigated. The compositions wi

Full text of DOI:10.1016/j.jallcom.2008.04.099

Journal of Alloys and Compounds 472 (2009) 451–455
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Microwave dielectric properties and sintering behaviors of
(Mg0.95Ni0.05)TiO3–CaTiO3 ceramic system
Chun-Hsu Shen, Cheng-Liang Huang∗
Department of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
The microwave dielectric properties and sintering behaviors of the new ceramic system
(1 − x)(Mg0.95Ni0.05)TiO3–xCaTiO3 (x ≤ 0.1) prepared by conventional solid state route were investi-
gated. The compositions with x = 0.02–0.1 resulted in the mixture of two main phases, (Mg0.95Ni0.05)TiO3
and CaTiO3, and a second phase (Mg0.95Ni0.05)Ti2O5, which was favorable at high temperatures. The
two-phased system was confirmed by both XRD and EDS analysis. Zero ꢀf can be achieved by appro-
priately adjusting the compositional ratio. Specimen with x = 0.06 possessed an excellent combination
of microwave dielectric properties: εr ∼ 20.8, Q × f ∼ 79,200 GHz and ꢀf ∼ 1.2 ppm/◦C. It is proposed as a
candidate material for GPS patch antennas and ISM band filters.
Received 4 February 2008
Received in revised form 20 April 2008
Accepted 24 April 2008
Available online 17 June 2008
Keywords:
Crystal growth
Dielectric response
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
The ilmenite-type structured MgTiO3, belonging to the trigo-
nal space group R3, is one of the leading dielectric materials for
¯
Regarding materials for microwave use, requirements for these
candidates must satisfy three major criteria: a high dielectric con-
stant for component size reduction, a low dielectric loss for high
selectivity and a near-zero temperature coefficient of resonant fre-
quency (ꢀf) for stable frequency stability. However, increasing the
carrier frequencies from 900 MHz to 2.4, 5.2, 5.8 GHz or even to
millimeter regime, would render materials with high dielectric
constant a less of interest. Low dielectric loss, on the other hand,
would play a more prominent role instead. For instance, low loss
dielectrics with dielectric constants in the 20 s have become most
popular materials used for today’s GPS patch antennas, Wireless
LAN and 5.8 GHz ISM band filters [1,2]. Still, zero ꢀf remains as
one of the primary requirements for high frequency materials and
becomes more and more critical as the operating frequency going
higher.
Two conventional approaches are usually employed in the
development of excellent dielectric ceramics; one is to create a
new material and the other one is to combine two or more materi-
als to achieve characteristic compensation. The latter one, mixing
two or more compositions with different dielectric properties, is
more popular due to its simplicity. In other words, combining two
compounds having negative and positive ꢀf values to form a solid
solution or mixed phases is the most convenient and promising way
to achieve a zero ꢀf [3,4].
microwave applications. At microwave frequency range, it exhibits
a good quality factor Q × f ∼160,000 at 8 GHz, a dielectric con-
stant εr ∼ 17, and a temperature coefficient of resonant frequency
ꢀf ∼ −50 ppm/◦C [5]. MgTiO3-based ceramics has been widely
applied as dielectric materials for resonators, filters and antennas
for communication, radar, and global positioning systems operated
at microwave frequencies. For instance, 0.95MgTiO3–0.05CaTiO3
ceramic is well known as the material (εr ∼ 20, Q × f ∼ 56,000 GHz
and a zero ꢀf [5]) for temperature compensating type capacitor,
dielectric resonator and patch antenna. With partial replacement of
Mg by Ni, (Mg0.95Ni0.05)TiO3 (hereafter referred to as MNT) ceramic
with a ilmenite-type structure was reported to possess a better
combination of dielectric properties (εr ∼ 17.2, Q × f ∼180,000 GHz,
ꢀf ∼ −45 ppm/◦C) [6]) in comparison with that of MgTiO3. That
makes it a good candidate as a dielectric material for microwave
applications.
In this paper, CaTiO3 (hereafter referred to as CT) was
added to (Mg0.95Ni0.05)TiO3 to form
a new ceramic system
(1 − x)(Mg0.95Ni0.05)TiO3–xCaTiO3, which demonstrated an effec-
tive compensation in its ꢀf value and a lower dielectric loss.
The microwave dielectric properties were discussed based upon
the obtained densification, X-ray diffraction patterns and the
microstructures of the ceramics. The correlation between the
microstructures and the Q × f value was also investigated.
2. Experimental procedure
∗
Corresponding author. Tel.: +886 6 2757575x62390; fax: +886 6 2345482.
E-mail address: huangcl@mail.ncku.edu.tw (C.-L. Huang).
Samples of (Mg0.95Ni0.05)TiO3 and CaTiO3 were individually synthesized by
conventional solid-state methods from high-purity oxide powders (>99.9%): MgO,
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.04.099

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Fig. 2. X-ray diffraction patterns of 0.94(Mg0.95Ni0.05)TiO3–0.06CaTiO3 ceramics sin-
tered at (a) 1250 ◦C, (b) 1275 ◦C, (c) 1300 ◦C, (d) 1325 ◦C, and (e) 1350 ◦C for 4 h.
Fig. 1. X-ray diffraction patterns of (1 − x)(Mg0.95Ni0.05)TiO3–xCaTiO3 ceramics as a
function of the x value, sintered at 1300 ◦C/4 h, (a) x = 0.02, (b) x = 0.04, (c) x = 0.06,
(d) x = 0.08 and (e) x = 0.1. (*) Ilmenite, (+) perovskite, (ꢀ) (Mg0.95Ni0.05)Ti2O5.
sintered at temperatures of 1250–1350 ◦C for 4 h in air. Both the heating rate and the
cooling rate were set at 10 ◦C/min.
The densities of the sintered ceramics were measured using the Archimedes
method. The crystalline phases were analyzed using the X-ray powder diffrac-
tion method with Cu K␣ radiation from 20◦ to 60◦ in 2Â. The scanning rate was
NiO, CaCO3, and TiO2. The starting materials were mixed according to the stoi-
chiometry: (Mg0.9Ni0.05)TiO3 and CaTiO3. They were then ground in distilled water
for 24 h in a ball mill with agate balls. Both mixtures were dried and calcined
at 1100 ◦C for 4 h. The calcined reagents were mixed to the desired composition
(1 − x)(Mg0.95Ni0.05)–xCaTiO3 and ground into a fine powder for 24 h. The fine pow-
der together with the organic binder were forced through a 100-mesh sieve and
pressed into pellets of 11 mm in diameter and 5 mm in thickness. These pellets were
4
◦ min−1. The microstructure was observed with a scanning electron microscope
(SEM). The dielectric constants and the unloaded Q values were measured using
the Hakki–Coleman dielectric resonator method as modified and improved by
Kobayashi–Katoh [7,8]. The apparatus consisted of parallel conducting brass plates
and coaxial probes connected to an HP8757B network analyzer and an HP8350B
sweep oscillator. The same technique was applied to measure the temperature coef-
Fig. 3. SEM micrographs of (1 − x)(Mg0.95Ni0.05)TiO3–xCaTiO3 ceramics sintered at 1300 ◦C/4 h, (a) x = 0.02, (b) x = 0.04, (c) x = 0.06, (d) x = 0.08 and (e) x = 0.1.

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453
Fig. 4. SEM micrographs of 0.94(Mg0.95Ni0.05)TiO3–0.06CaTiO3 ceramics sintered at (a) 1250 ◦C, (b) 1275 ◦C, (c) 1300 ◦C, (d) 1325 ◦C and (e) 1350 ◦C for 4 h.
ficient of resonant frequency (ꢀf). The test set was placed over a thermostat with a
temperature range from 25 ◦C to 80 ◦C. The ꢀf value (ppm/◦C) was be calculated by
noting the change in resonant frequency (ꢁf),
at different temperatures (Fig. 2) except that second phase
(Mg0.95Ni0.05)Ti2O5 was enhanced at higher temperatures. It
showed an increase in its intensity as the sintering temperature
increased.
f2 − f1
ꢀf
=
(1)
The SEM micrographs of MNCT ceramics sintered at 1300 ◦C/4 h
are demonstrated in Fig. 3. Significant change was not observed
for grain size at different x values due to a small compositional
variation (x = 0.02–0.1). Fig. 4 shows the SEM micrographs of MNCT
ceramics with x = 0.06 at different temperatures. As the sinter-
ing temperature increased from 1250 ◦C to 1350 ◦C, the grain
size increased. Rapid grain growth was observed for specimens
at sintering temperatures above 1300 ◦C, resulting in a porous
microstructure, which would certainly affect their microwave
dielectric properties.
f1(T2 − T1)
where f1 and f2 represent the resonant frequencies at T1 and T2, respectively.
3. Results and discussion
3.1. Phases and microstructures
The X-ray diffraction patterns of the (1 − x)(Mg0.95Ni0.05
)
TiO3–xCaTiO3 (hereafter referred to as MNCT) ceramic system
sintered at 1300 ◦C/4 h with x ranging from 0.02 to 0.1 are
shown in Fig. 1. MNCT ceramics showed a mixture of a main
phase (Mg0.95Ni0.05)TiO3 and a minor phase CaTiO3. The for-
mation of MNT (ilmenite) and CT (perovskite) was due to the
Fig.
5 shows the EDS results of 0.94(Mg0.95Ni0.05)TiO3–
0.06CaTiO3 (94MNCT hereafter) ceramics sintered at 1300 ◦C for
4 h. Spot A (larger circular grain) shows that Mg and Ti were rich
but Ca was almost free, while spot B (small square grain) reveals
that Ca and Ti were rich but Mg was almost free. Spot C (needle
shape grains) is the second phase of MgTi2O5 (Mg:Ti = 1:2), which
causes the degradation in dielectric properties. Extra analysis of
spot D (smaller circular grain) demonstrated the same composi-
tion as that of spot A. These results agreed well with those from
Fig. 1.
structure and ionic size differences between Ca2+ (1.00 A) and
˚
Mg2+ (0.46 A) [9]. Moreover, a second phase (Mg0.95Ni0.05)Ti2O5
˚
(εr ∼ 13.1, Q × f ∼30,000 GHz, ꢀf ∼ −43 ppm/◦C at 1300 ◦C) was
also detected, which would lead to a degradation in the dielec-
tric properties. It was attributed to that MgTi2O5 is usually
formed as an intermediate phase and is difficult to com-
pletely eliminate from the sample prepared by mixed oxide
route when MgO and TiO2 reacts in a 1:1 molar ratio [10,11].
The intensity of the second phase decreased with increas-
ing x owing to a decrease in the compositional content (Mg,
Ni and Ti). Similar XRD patterns were obtained for specimen
The apparent densities of the MNCT system as a function of
sintering temperature are shown in Fig. 6. With increasing sin-
tering temperature, the density of the specimen increased to a
maximum at 1300 ◦C and decreased thereafter. This increase in

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C.-H. Shen, C.-L. Huang / Journal of Alloys and Compounds 472 (2009) 451–455
Fig. 7. Dielectric constant (εr) of (1 − x)(Mg0.95Ni0.05)TiO3–xCaTiO3 ceramics with
different x values as a function of sintering temperature.
3.2. Microwave properties
Fig. 7 plots the dielectric constant of MNCT system as a function
of sintering temperature. The dielectric constant increased with
increasing sintering temperature to a maximum value at 1300 ◦C,
and thereafter it decreased. The variation of εr value was mainly
consistent with that of density. Moreover, the dielectric constant
was also a function of the compositions and increased with increas-
ing CaTiO3 content since CaTiO3 possesses a much higher dielectric
constant than that of (Mg0.95Ni0.05)TiO3 (εr ∼ 170 for CaTiO3; ∼17.2
for (Mg0.95Ni0.05)TiO3). At 1300 ◦C, the εr value of MNCT ceramics
increased from 19.1 to 23.1 with as x increased from 0.02 to 0.08.
Fig. 8 shows Q × f value of MNCT system as a function of sinter-
ing temperature. The Q × f value is an important index for dielectric
ceramics at microwave frequency since a higher Q × f value corre-
sponding to a lower dielectric loss. The microwave dielectric loss is
mainly not only determined by the density but also by the lattice
vibrational modes, the pores, and the second phases [12]. The qual-
ity factor of (Mg0.95Ni0.05)TiO3 (Q × f value ∼180,000 GHz) is much
higher than that of CaTiO3 (Q × f value ∼3600 GHz) and hence it
is expected that the Q × f values will decrease as the amount of
CaTiO3 increases from 0.02 to 0.08. The Q × f value increases when
the sintering temperature increases from 1250 ◦C to 1300 ◦C. After
reaching its maximum value at 1300 ◦C, the Q × f value decreased.
The increase in Q × f value at low temperatures is due to the
increase in density as well as the uniform morphology, as shown
Fig. 5. The EDS results of 0.94(Mg0.95Ni0.05)TiO3–0.06CaTiO3 ceramics sintered at
1300 ◦C for 4 h; (a) corresponding SEM micrograph (b) typical EDS and (c) obtained
compositional ratio.
the bulk density is attributed to the grain growth resulted in a
dense microstructure, as shown in Fig. 4. The decrease in the den-
sity, however, was mainly caused by the porous microstructure
resulted from a rapid grain growth. Moreover, the density was
also related to the compositions, and increased with increasing x
since CaTiO3 (D ∼4.1 g/cm3) possesses a higher density than that
of (Mg0.95Ni0.05Ti)O3 (D ∼3.45 g/cm3). At 1300 ◦C, the density of
the MNCT ceramics increased from 3.18 g/cm3 to 3.3 g/cm3 as the x
value increased from 0.02 to 0.08.
Fig. 6. Apparent density of (1 − x)(Mg0.95Ni0.05)TiO3–xCaTiO3 ceramics with differ-
ent x values as a function of sintering temperature.
Fig. 8. Q × f value of (1 − x)(Mg0.95Ni0.05)TiO3–xCaTiO3 ceramics with different x
values as a function of sintering temperature.

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455
4. Conclusion
Microwave dielectric properties of (1 − x)(Mg0.95Ni0.05
)
)
TiO3–xCaTiO3 ceramics were investigated. For (1 − x)(Mg0.95Ni0.05
TiO3–xCaTiO3 ceramics sintered at 1250–1350 ◦C for 4 h, as
the amount of CaTiO3 (x value) increased from 0.02 to 0.1,
the dielectric constant increased from 19.1 to 23.4, the tem-
perature coefficient of resonant frequency (ꢀf) increased from
−33.5 ppm/◦C to +33 ppm/◦C, and the Q × f value decreased from
110,000 (at 8 GHz) to 72,000 (at 8 GHz). At the composition of
x = 0.06, the 0.94(Mg0.95Ni0.05)TiO3–0.06CaTiO3 ceramics sintered
at 1300 ◦C/4 h have excellent microwave dielectric properties: a
dielectric constant εr of 20.9, a Q × f value of 79,200 (at 8 GHz), and
a ꢀf value of 1.2 ppm/◦C.
Fig. 9. ꢀf value of (1 − x)(Mg0.95Ni0.05)TiO3–xCaTiO3 ceramics sintering at
1300 ◦C/4 h as a function of the x value.
Acknowledgement
This work was supported by the National Science Council of
Taiwan under grant NSC 96-2221-E-006-117.
in Figs. 4 and 6. At 1300 ◦C, a maximum Q × f value of 79,200 GHz
(at 8 GHz) was obtained for the 0.94(Mg0.95Ni0.05)TiO3–0.06CaTiO3
ceramics. The degradation of Q × f value can be attributed to inho-
mogeneous grain growth which results in a reduction in its density.
As a conclusion, the variation of Q × f is consistent with that of
density, suggesting that dielectric loss of MNCT ceramics is mainly
controlled by its bulk density.
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