High-Q microwave dielectric materials based on the spinel Mg 2TiO4

Article abstract of DOI:10.1111/j.1551-2916.2006.01271.x

Composite ceramics based on the spinel Mg₂TiO₄ were prepared by a conventional mixed-oxide route. To achieve the temperature stabilization of the dielectric constant, each of the composites was added with 7 mol% CaTiO₃. The effect of the substitution of isovalent Co for Mg on the microstructure and the microwave dielectric properties of the composite ceramics was also investigated. A maximum Q × f value of around 150-160 THz was obtained for the undoped Mg₂TiO₄, whereas a reduced Q × f value was observed for an increase in the Co concentration in the system (1- x)Mg₂TiO₄-xCo₂TiO ₄. Upon doping with 7 mol% CaTiO₃, the Q × f value passed through a maximum with increasing Co concentration. Adding ZnO-B ₂O₃ to the composite system based on Co-doped Mg ₂TiO₄ resulted in a reduction of the sintering temperature by 150° -200°C without any significant degradation in the Q × f value.

Full text of DOI:10.1111/j.1551-2916.2006.01271.x

J. Am. Ceram. Soc., 89 [11] 3441–3445 (2006)
DOI: 10.1111/j.1551-2916.2006.01271.x
r 2006 The American Ceramic Society
ournal
J
High-Q Microwave Dielectric Materials Based on the Spinel Mg2TiO4
Anatolii Belous, Oleg Ovchar, and Dmitrii Durilin
Department of Solid State Chemistry, V.I.Vernadskii Institute of General and Inorganic Chemistry, Kyiv 03680,
Ukraine
Marjeta Macek Krzmanc,w Matjaz Valant,* and Danilo Suvorov*
Advanced Materials Department, Jomef Stefan Institute, 1000 Ljubljana, Slovenia
Composite ceramics based on the spinel Mg2TiO4 were pre-
pared by a conventional mixed-oxide route. To achieve the tem-
perature stabilization of the dielectric constant, each of the
composites was added with 7 mol% CaTiO3. The effect of the
substitution of isovalent Co for Mg on the microstructure and
the microwave dielectric properties of the composite ceramics
was also investigated. A maximum Q Â f value of around 150–
160 THz was obtained for the undoped Mg2TiO4, whereas a
reduced Q Â f value was observed for an increase in the Co con-
centration in the system (1Àx)Mg2TiO4ÀxCo2TiO4. Upon
doping with 7 mol% CaTiO3, the Q Â f value passed through
a maximum with increasing Co concentration. Adding ZnO–
B2O3 to the composite system based on Co-doped Mg2TiO4 re-
sulted in a reduction of the sintering temperature by 1501–2001C
without any significant degradation in the Q Â f value.
for MgTiO3.6 For the temperature compensation of the negative
values of the tf of MgTiO3, a small amount of the paraelectric
phase Sr(Ca)TiO3, characterized by a high, positive tf, is gener-
ally introduced into the main composition.3–9 It should be noted
that, in contrast to undoped MgTiO3, the composite materials
based on MgTiO3 always display much lower Q values
(Q 5 5000–7000 at 10 GHz), which can be ascribed to the pres-
ence of low-Q Sr(Ca)TiO3, as well as to the formation of unde-
sirable crystal phases like MgTi2O5.5–7 When partially
substituting magnesium with cobalt in MgTiO3, materials with
high MW parameters have recently been obtained.4 For instance,
in the system (1Àx)Mg0.95Co0.05TiO3–xCaTiO3, the materials
with eꢀ20.3, Q Â fꢀ107 000 GHz at 7 GHz, and tf ꢀÀ22.8
ppm/1C were produced at x5 0.05.4 Moreover, the possibility of
using materials based on MgTiO3 in the low-temperature cofired
ceramic (LTCC) technique has been reported.1–9 However, the
available literature contains no data regarding the synthesis and
properties of MW dielectric materials based on Mg2TiO4.
Therefore, the objective of this study was to investigate the
formation, phase composition, microstructure, and the MW di-
electric properties of composite materials based on the system
Mg2TiO4–CaTiO3, to study the effect of the partial Co substi-
tution for Mg in this system on the structure and the properties
of sintered ceramics, as well as to evaluate the possibility of us-
ing these materials in LTCC technology when the ceramics are
doped with the glass-forming additives ZnO–B2O3.
I. Introduction
N the development of modern communication devices such as
filters and oscillators, materials with a low dielectric loss (high
I
quality factor Q 5 1/tan d) in the microwave (MW) range are
used. The use of high-Q dielectrics as passive MW compo-
nents—dielectric resonators, dielectric substrates, waveguides,
and antennas—makes possible a significant improvement in the
performance of communications equipment,1–3 in particular,
better sensitivity and selectivity. This is particularly relevant
with the current tendency to expand the operating frequency
ranges of MW wireless communications, for instance, up to 20–
30 GHz in VSAT, 26–38 GHz in PtP Radiolinks, or 28–40 GHz
in LMDS, and applications at frequencies as high as 60 GHz
have also been reported. A variety of microwave dielectric com-
ponents are used in equipment like Low-noise Block (LNB),
Block-Up-Converter (BUC) for LMDS, or VSAT terminals. In
this case, in addition to the high-quality factor of a material, its
price becomes an important factor. Therefore, for the above
applications, new, low-cost dielectric materials with a dielectric
constant of 10–20 and extremely low dielectric losses are
urgently required.
II. Experimental Procedure
The samples involved in this study were synthesized by the solid-
state reaction technique using high-purity MgO, TiO2, CaCO3,
CoCO3, ZnO, and B2O3. All the raw reagents were first dried.
As MgO is hygroscopic, it was additionally fired at 7001C to
avoid any water and CO2 absorption. The synthesis was carried
out in two stages: in the first stage, the binary compounds
Mg2TiO4, Co2TiO4, and CaTiO3 were produced. To do this, the
raw reagents were weighed in stoichiometric amounts and then
mixed in a ball mill in distilled water. After drying at 1001–
1501C, the powders were then homogenized. The preliminary
heat treatment (calcination) of the powders was carried out in
air in alumina crucibles at 9001–12001C.
In order to obtain the glass-forming dopant ZnO–B2O3, the
boron oxide B2O3 was dried at 1501–2001C. Both the mixing and
the milling of the ZnO–B2O3 mixture were carried out in ace-
tone, followed by heating at 6001C.
In order to produce polycrystalline samples, at the second
stage, the stoichiometric mixtures of the preliminarily synthe-
sized binary compounds were milled and homogenized in vibra-
tion mills in distilled water. After drying at 1001–1501C, the
powders were then pressed to form green pellets with a diameter
of 8–10 mm and a height of 3–4 mm, which were then sintered at
12001–15001C for 2–6 h.
Recently, the intense development of high-Q MW materials
based on low-cost raw oxides from the MgO–TiO2 system has
begun.4–7 In this binary system, two magnesium titanates, which
are good candidates for use in MW dielectrics, are known:
Mg2TiO4 and MgTiO3. Polycrystalline materials based on
Mg2TiO4 and MgTiO3 are characterized by the dielectric con-
stants e 5 14 and 16, respectively, a negative coefficient of the
resonant frequency (tf 5 À40–50 ppm/1C), and a low dielectric
loss (high Q), whose magnitude reaches Q 5 20000 at 10 GHz
P. Davies—contributing editor
Manuscript No. 21552. Received March 6, 2006; approved June 27, 2006.
*Member, American Ceramic Society.
The phase composition of the binary compounds in the
system MgO–TiO2–CaO–CoO was studied by means of X-ray
wAuthor to whom correspondence should be addressed. e-mail: marjeta.macek@ijs.si
3441

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Journal of the American Ceramic Society—Belous et al.
Vol. 89, No. 11
diffraction (XRD) analysis on a DRON-3M diffractometer
(CuKa radiation; Burevestnik, Russia). The XRD patterns
were measured in the appropriate range with a step of
D2y 5 0.021 and a time of 10 s. Both SiO2 (standard 2y) and
NIST SRM 1976—Al2O3 (certified intensity standard) were
used as internal standards. For the XRD analysis, the JCPDS
database was used as a reference for determining the peaks.
The study of the effect of the partial isovalent substitution of
Co21 for Mg21 in the spinel Mg2TiO4 on the phase composition,
the microstructure, and the MW dielectric properties was carried
out for the following systems: (1Àx) Mg2TiO4–xCo2TiO4;
0.93[(1Àx) Mg2TiO4–xCo2TiO4]À0.07CaTiO3, and [0.93[(1Àx)
Mg2TiO4–xCo2TiO4]À0.07CaTiO3]15 wt% ZnB2O4.
The microstructures and the phase distributions of the poly-
crystalline samples were examined by means of scanning electron
microscopy (JSM 5800, JEOL, Tokyo, Japan) using energy-
dispersive X-ray spectroscopy (EDX) and the LINK software
package (ISIS 3000, Oxford Instruments, Bucks, UK). The
dielectric characteristics of the materials, e, Q, and tf, at frequen-
cies around 10 GHz were examined using a modified dielectric
resonator method on an appropriate disk sample that resided in
an isolated waveguide section. The values of the Q factors were
additionally measured by means of the cavity-reflection method
using an HP 8719C Network Analyzer (50 MHz–13.5 GHz).
Fig. 2. X-ray diffraction patterns collected on sintered samples of the
system (1Àx)Mg2TiO4–xCo2TiO4 after firing at 14501C for 6 h; (a)
x 51, (b) x 50.02, (c) x 5 0.05, (d) x 5 0.1, (e) x 5 0.2.
These results were proved by an SEM microstructural anal-
ysis of the Mg2TiO4 sintered at 14501C, which revealed no in-
clusions of secondary phases in the matrix phase (Fig. 3(a)).
XRD studies of the sintered samples of the system (1Àx)
Mg2TiO4–xCo2TiO4 show that solid solutions (Mg1ÀxCox)2-
TiO4 with the inverse spinel structure are formed across the en-
tire range of x (Fig. 2). An increase in the cobalt concentration
results in only a slight deviation of the parameter a of the unit
cell, from 8.43 to 8.44 A, which is within the limits of the meas-
urement accuracy. An SEM investigation of the sintered sam-
ples of this system also revealed the absence of secondary phases
in the spinel matrix (Fig. 3(b)).
The microwave dielectric properties of the sintered samples
demonstrated that the dielectric constant (e) and the tempera-
ture coefficient of resonant frequency (tf) are weakly dependent
on the cobalt concentration (Table I). However, in contrast to
these findings, the magnitude of the quality factor (Q) undergoes
a sharp decline at 0oxo0.05 with increasing cobalt substitu-
tion, followed by a smoother decline at higher cobalt contents
for 0.05ox (Fig. 4(a)).
III. Results and Discussion
(1) System (1Àx)Mg2TiO4–xCo2TiO4
According to the XRD analyses, the sintered 2MgO–TiO2 and
2CoCO3–TiO2 mixtures were single phase (Figs. 1 and 2). The
Co2TiO4 phase is formed at a significantly lower temperature
(11001–11501C) than that of the Mg2TiO4 (Fig. 1). When fired at
11501C, the magnesium mixture had a noticeable amount of in-
termediate MgTiO3. It should be noted that the overlapping of
the strongest lines of MgO with those of Mg2TiO4 prevented the
identification of the residual magnesium oxide that did not react
during the heat treatment.
With an increase in the firing temperature to 13001–14001C,
the amount of MgTiO3 in the 2MgO–TiO2 mixture decreased.
However, the XRD analysis indicated that single-phase Mg2TiO4
is formed at the sintering temperature, i.e., at 14501C (Fig. 2).
Previous studies on the formation of Mg2TiO4 revealed the
partial solid solubility of MgO in the Mg2TiO4 matrix.10 Ac-
cording to the results of Petrova et al.,11 this process could be
associated with the partial thermal decomposition of Mg2TiO4,
accompanied by the formation of MgTiO3 in the following way:
Mg2TiO4-Mg212dTi1ÀdO41MgTiO3.
It should be emphasized that in the case of the stoichiometric
nominal composition corresponding to pure Mg2TiO4—in spite
of the high porosity—the samples demonstrate a Q Â f value as
high as 150 000 GHz. To the best of our knowledge, this value is
being reported for the first time here for the spinel family, and is
comparable with the Q Â f value measured in the Ba(Mg,Ta)O3
perovskites, which are widely recognized high-Q microwave di-
electrics. However, it can be clearly seen from Table I that the
main drawback of Mg2TiO4-based ceramics is the large negative
temperature coefficient of resonant frequency (tf). This can be
overcome by developing composite dielectric materials and
introducing phases with a positive temperature coefficient of
resonant frequency, for instance, CaTiO3.
The authors of Petrova et al.11 have reported that the thermal
decomposition of Mg2TiO4 becomes negligible when the tem-
perature exceeds 14001C. This results in a single-phase product
at 14001–15001C, which is consistent with our XRD data.
(2) System 0.93[(1Àx) Mg2TiO4–xCo2TiO4]À0.07CaTiO3
The addition of 7 mol% CaTiO3 to the system (1Àx) Mg2TiO4–
xCo2TiO4 resulted in a noticeable lowering of the sintering tem-
perature of the ceramics: from as high as 14201–14501C for the
undoped system to 13501–13801C for the CaTiO3-doped system.
XRD patterns collected on sintered samples of this system
differ depending on the cobalt concentration (Fig. 5). In the case
of the nominal composition 0.93Mg2TiO4–0.07CaTiO3 (x 5 0),
the ceramics without any cobalt addition were never well sinte-
red, and demonstrated a rather non-homogeneous chemical
composition with both sintered and non-sintered interlaced ar-
eas. Because of this, at x 5 0, the samples were not appropriate
for either a correct microstructural investigation or for a micro-
wave characterization. According to the consideration presented
in Section III (1), and taking into account the significantly lower
sintering temperature of the samples containing CaTiO3, in the
Fig. 1. X-ray diffraction patterns of the mixtures (a) 2MgO–TiO2 and
(b) 2CoCO3–TiO2, after firing at 11501C for 6 h, M, MgTiO3; M2,
Mg2TiO4.

November 2006
High-Q Microwave Dielectric Materials
3443
Fig. 3. SEM micrographs from the polished surface of the ceramics with the composition (1Àx)Mg2TiO4–xCo2TiO4: (a) x 5 0 and (b) x 5 0.05;
0.93[(1Àx)Mg2TiO4–xCo2TiO4]À0.07CaTiO3: (c) x50.02 and (d) x50.05; 0.93[(1Àx)Mg2TiO4–xCo2TiO4]À0.07CaTiO3 with 5 wt% ZnO–B2O3: (e) x50.02.
case x 5 0, one can associate the deteriorated sintering process
with the partial thermal decomposition of Mg2TiO4. In contrast,
the Co substitution for Mg, even at low cobalt concentrations,
enhances the sintering of composite ceramics, resulting in the
formation of a sufficiently homogeneous microstructure (Fig. 3).
According to the XRD analysis, at low substitution levels
corresponding to the ranges 0oxo0.04, the dielectric ceramic
contained only two crystal phases: the solid solution
(Mg1ÀxCox)2TiO4 and the perovskite phase CaTiO3 (Fig. 5).
This fact has also been confirmed by a detailed microstructural
Table I. Microwave Dielectric Parameters (e, sf, Q Â f ) Measured at a Frequency 10 GHz for Selected Ceramics Based
on the Spinel Mg2TiO4
Composition
TSINT (1C)
e
tf (ppm/1C)
Q Â f (THz)
Mg2TiO4
0.95Mg2TiO4–0.05Co2TiO4
0.80Mg2TiO4–0.20Co2TiO4
0.93Mg2TiO410.07CaTiO3
0.93[0.99Mg2TiO4–0.01Co2TiO4]10.07CaTiO3
0.93[0.95Mg2TiO4–0.05Co2TiO4]10.07CaTiO3
[0.93[0.98Mg2TiO4–0.02Co2TiO4]10.07CaTiO3]15 wt% ZnO–B2O3
1450
1420
1420
1420
1400
1400
1200
14
14
13
15
17
17
16
À50
À54
À60
À2
150
86
75
35
105
90
85
À5
À5
À8

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Journal of the American Ceramic Society—Belous et al.
Vol. 89, No. 11
Fig. 6. X-ray diffraction patterns collected from polycrystalline sam-
ples with the composition 0.93[(1Àx)Mg2TiO4–xCo2TiO4]À0.07CaTiO3
which additionally contained 5 wt% of preliminary synthesized ZnB2O4
(ZnO–B2O3): (a) x 50.01 and (b) x 50.05; M, MgTiO3; P, CaTiO3.
xCo2TiO4]À0.07CaTiO3 demonstrate a good temperature coef-
ficient of the resonant frequency within the ranges of À5 to À10
ppm/1C (Table I).
(3) Addition of ZnO–B2O3
When 5 wt% of the preliminarily synthesized ZnB2O4 (ZnO–
B2O3) was added to the system 0.93[(1Àx)Mg2TiO4–xCo2-
TiO4]À0.07CaTiO3, the sintering temperature decreased by
1501–2001C: in this case, well-sintered ceramics were formed at
12001–12501C. According to the XRD data the introduction of
the dopant ZnO–B2O3 did not result in drastic changes in the
phase composition of the polycrystalline materials. In the case of
a low substitution (0rxr0.04), the sintered composites contain
only two crystal phases: spinel-like solid solutions based on
Mg2TiO4 as well as the perovskite CaTiO3 (Fig. 6, pattern a).
An increase in the cobalt concentration over 0.04rx resulted in
the formation of a secondary phase MgTiO3, which was also
observed in the undoped materials (Fig. 6, pattern b).
SEM microstructural analyses of the sintered samples con-
taining a nominal 5 wt% ZnO–B2O3, in contrast to the undoped
materials, even at a low cobalt concentration (0rxr0.04), in
addition to two major crystal phases (spinel and perovskite),
indicated the presence of a third Mg-rich phase (Fig. 3(e)).
It should be noted that the addition of the dopant ZnO–B2O3,
which results in a significant (up to 1501–2001C) reduction of the
sintering temperature, does not initiate any noticeable degrada-
tion in the electrophysical parameters of the sintered material. In
particular, the magnitude of QÂ f, which in the case of the com-
position 0.93[0.98Mg2TiO4–0.02Co2TiO4]À0.07CaTiO3 is as high
as 100 000, decreases by only 15% when 5 wt% ZnO–B2O3 is
added, and is equal to 85 000 at 10 GHz. A slight decrease in the
QÂ f value, observed after the addition of 5 wt% ZnO–B2O3, is
probably due to the effect of MgO-rich inclusions in the matrix
phase. The results indicate the possibility of a further reduction in
the sintering temperature to 9001–9501C, which is required for
most LTCC applications. This could be attained by a further in-
crease in the concentration of the dopant ZnO–B2O3 that is in-
troduced into the basic composition. However, this will most
probably lead to a severe degradation in the value of QÂ f.
Fig. 4. The product Q Â f as a function of the cobalt content in the
systems (1Àx)Mg2TiO4–xCo2TiO4 (a), and 0.93[(1Àx)Mg2TiO4–xCo2-
TiO4]À0.07CaTiO3 (b).
analysis (Fig. 3). At a higher cobalt concentration (0.04rxo1),
in addition to these two main crystal phases, another phase cor-
responding to the MgTiO3 is formed (Fig. 5). Regardless of the
cobalt concentration, the amount of this phase is negligibly
small. The presence of residual MgTiO3 is clearly seen on the
SEM micrographs, even in the case of very low x (x 5 0.05). The
phase composition of the studied composites, to a large extent,
affects their microwave dielectric properties. For instance, in the
case x 5 0, the Q Â f value is as low as just 35000 GHz, which is
associated with the non-homogeneous composition. At the same
time, even a slight Co doping (x 5 0.01) results in a sharp in-
crease in Q Â f, which reaches a value as high as 100000 GHZ
(Fig. 4(b)). The highest values of Q Â f of about 90000–100000
GHz, were obtained in the case of the two-phase composites
corresponding to the ranges 0oxo0.04. A further increase
in the cobalt content, which is accompanied by the formation
of the additional phase MgTiO3, results in an almost linear
decrease in the Q Â f value (Fig. 4(b)). It should be noted that all
of the composite materials of the system 0.93[(1Àx)Mg2TiO4–
IV. Conclusions
(a) The cubic spinel structure is formed across the entire range
of Co concentration in the system (1Àx)Mg2TiO4ÀxCo2TiO4
(0oxr1).
(b) Sintered Mg2TiO4 demonstrates a Q Â f value as high as
150–160 THz, even for noticeably porous ceramics, together
with a relatively high negative temperature coefficient tf.
(c) In the system (1Àx)Mg2TiO4ÀxCo2TiO4, an increase in
Co concentration resulted in a reduction of the Q Â f value.
Fig. 5. X-ray diffraction patterns of sintered samples of the system
0.93[(1Àx)Mg2TiO4–xCo2TiO4]À0.07CaTiO3 after sintering at 14501C
for 6 h; (a) x 5 0, (b) x 50.02, (c) x 5 0.05, (d) x 50.1,. M, MgTiO3;
P, CaTiO3.

November 2006
High-Q Microwave Dielectric Materials
3445
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