Characterization and microwave dielectric properties of wolframite-type MgZrNb2O8 ceramics

Article abstract of DOI:10.1016/j.jallcom.2015.09.143

Wolframite type MgZrNb₂O₈ ceramics were prepared by solid-state method and sol-gel process in this study, and the influence of sol-gel process on the sintering behavior and microwave dielectric properties of MgZrNb₂O₈

Full text of DOI:10.1016/j.jallcom.2015.09.143

Journal of Alloys and Compounds 655 (2016) 60e65
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Characterization and microwave dielectric properties of wolframite-
type MgZrNb2O8 ceramics
J.D. Guo, J.X. Bi, Q.J. Mei, H.T. Wu*
Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan, 250022, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Wolframite type MgZrNb2O8 ceramics were prepared by solid-state method and solegel process in this
study, and the influence of solegel process on the sintering behavior and microwave dielectric properties
of MgZrNb2O8 ceramics was investigated. The optimal sintering temperatures of MgZrNb2O8 ceramic for
solid-state and solegel method were 1225 ꢀC and 1175 ꢀC, respectively. For solid-state method, MZN
ceramics sintered at 1225 ꢀC with nearly full density had the microwave dielectric properties of
εr ¼ 27.08, Q$f ¼ 29,070 GHz and tf ¼ ꢁ24.31 ppm/ꢀC. For solegel process, MZN ceramics sintered at
Received 6 August 2015
Received in revised form
12 September 2015
Accepted 15 September 2015
Available online 25 September 2015
1175 ꢀC with nearly full density had excellent microwave dielectric properties of εr
¼
22.16,
Keywords:
MgZrNb2O8
Sol-gel process
Solid-state method
Microwave dielectric properties
Q$f ¼ 29,490 GHz and tf ¼ ꢁ15.99 ppm/ꢀC. Obviously, solegel synthetic route could significantly reduce
the sintering temperature and improve the microwave dielectric properties.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
via the traditional solid-state method and exhibited the microwave
dielectric properties: εr
¼
9.60, Q$f
¼
58,500 GHz and
In recent decades, microwave dielectric ceramics developed
rapidly, and have been the basis for the realization of microwave
control function and the key materials to the microwave devices
(such as resonator, phase shifter, etc.) widely used in modern
communication [1e4]. In order to meet the demand for miniatur-
ization and integration, it is increasingly needed to expand the
search for new microwave dielectric materials, which should have
appropriate dielectric constant (εr), high quality factor (Q$f), near-
zero temperature coefficient of resonant frequency (tf) [5]. At
present, many types of compounds were reported for the possible
use in microwave applications [6e10]. Among several kinds of
microwave dielectric ceramics with low dielectric constant,
wolframite-type MgZrNb2O8 (MZN) ceramics possess excellent
properties, high quality factors combined with appropriate
dielectric constant.
tf ¼ ꢁ31.5 ppm/ꢀC. As reported in the past, the major disadvantage
of solid-state method was higher sintering temperature. Solegel
process not only had advantages of high purity and homogeneous
grain size, but also lowered the sintering temperature as well as
improved the dielectric properties. However, little research
regarding preparation of MZN ceramics by the aqueous solegel
process has been reported in the literature to date. In the following
study, a wolframite MZN ceramic was prepared by solid-state
method and solegel process. The effects of solegel process on
sintering temperature, phase compositions, microstructures and
microwave dielectric properties of MZN ceramics were also inves-
tigated in detail.
2. Experimental procedure
MZN ceramics were always prepared by the conventional solid-
state method in the past reports [11,12]. For instance, Y. Cheng et al.
[11] reported that MZN ceramic was obtained at 1340 ꢀC for 4 h by
the solid-state method with εr ¼ 26, Q$f ¼ 120,816 GHz and
tf ¼ ꢁ50.2 ppm/ꢀC. In addition, S.D. Ramarao and V.R.K. Murthy [12]
also reported that the MZN ceramics could be sintered at 1500 ꢀC
High-purity oxide MgO (99%), ZrO2 (99.99%) and Nb2O5 (99.9%)
were used as raw materials to synthesize the MZN powders by the
solid state method. The raw materials were weighed and mixed
according to the formula of MgZrNb2O8. The mixed powders were
ball-milled for 8 h with distilled water in a nylon container with
ZrO2 balls. All the slurries were dried and pre-sintered at 1050 ꢀC
for 4 h to obtain single phase MZN. Then the powders were re-
milled for 6 h. Analytical-grade Mg(NO3)2$6H2O, Zr(NO3)4$5H2O,
Nb2O5, citric acid (CA) and ethylene glycol (EG) were used as raw
materials to synthesize the MZN nanopowders by the solegel
* Corresponding author.
E-mail address: mse_wuht@ujn.edu.cn (H.T. Wu).
http://dx.doi.org/10.1016/j.jallcom.2015.09.143
0925-8388/© 2015 Elsevier B.V. All rights reserved.

J.D. Guo et al. / Journal of Alloys and Compounds 655 (2016) 60e65
61
method. Firstly, a stoichiometric amount of Nb2O5 was dissolved in
hydrofluoric acid to get its ionic solution. Secondly, aqueous
ammonia was added slowly to the previous solution to obtain
precipitate. Thirdly, the precipitate was filtered and washed with
deionized water several times to remove Fꢁ ions and then dissolved
completely in citric acid aqueous solution by continuous magnetic
stirring at 300 rpm for 15 min. The whole formation process could
be formulated from Eqs. (1)e(2). Meanwhile, a stoichiometric
amount of Mg(NO3)$6H2O and Zr(NO3)4$5H2O were added to the
mixing citric solution of Nb to get the MgeZreNb solution. Finally,
ethylene was added to the mixed solution and stirred for 1 h to
form a transparent and stable sol. pH of the solution was controlled
around 4 by adding buffering agents. The resulting sol was left
evaporating at 80ꢀCe90 ꢀC until a viscous gel-like product was
formed and then subsequently calcined by a slow heating in air
atmosphere from 600ꢀC to 900 ꢀC. The calcined powders were
remilled in a polyethylene jar for 6 h using ZrO2 balls to reduce
conglobation. The dried powders synthesized by solid-state and
solegel method were mixed with high purity paraffin as a binder at
60 ꢀC, granulated and pressed into cylindrical disks of 10 mm in
diameter and about 5 mm in thickness at a pressure of about
200 MPa. These pellets were preheated at 500 ꢀC for 4 h to expel the
binder and then sintered at 1100 ꢀC ꢁ1275 ꢀC for 4 h in air at a
heating rate of 5 ꢀC/min.
temperatures ranging from 600 ꢀC to 900 ꢀC for 60 min in air at-
mosphere are shown in Fig. 1(b). Similarly, there was no significant
change for the diffraction patterns in the range of 600ꢀCe900 ꢀC,
and it can be found that the crystallization of MZN as predominant
phase took place at 600 ꢀC in view of diffraction peaks from sig-
nificant lattice planes, such as (111), (ꢁ110), and (130). The XRD
patterns of the xerogel fired at 800 ꢀC and 900 ꢀC still consisted of
predominant peaks of ZTN with sharper peaks free from any sec-
ondary phases. Therefore, according to XRD result it was indicated
that calcinations temperature of synthesizing wolframite-type
MZN phase was remarkably decreased to 600 ꢀC by the solegel
process, which was lower than the conventional mixed oxide route.
Fig. 2 shows the shrinking ratio and apparent density of MZN
samples as a function of sintering temperatures, and the optimal
sintering temperature can be got through this figure. The shrinkage
variations of the samples were characterized by the ratio of the
diametric size, and the theoretical density of MZN ceramic was
5.18 g/cm3 obtained from the crystal structure and atomic weight
[15]. As shown in Fig. 2(a), the shrinking ratio of samples prepared
by solegel process increased from 16.91% to 18.77% with the sin-
tering temperature increasing in the region of 1100ꢀCe1200 ꢀC.
And the apparent density increased from 4.32 g/cm3 to 4.86 g/cm3
with the sintering temperature increasing from 1100 ꢀC to 1175 ꢀC,
which was due to the decrease of pores and the improvement of
grains as sintering temperature increased shown in Fig. 5(a)e(c). At
1175 ꢀC, a maximum value of apparent densities reached 4.86 g/
cm3. However, when the sintering temperature was over 1175 ꢀC
the density decreased sharply, which was due to abnormal grain
growth observed in the samples.
Based on the results of sintering characteristics, it was found
that the MZN ceramic prepared by solegel process with nearly full
density was obtained at the 1175 ꢀC for 4 h. The variation of
shrinking ratio and apparent density of MZN samples prepared by
solid-state method was shown in Fig. 2(b). As shown in Fig. 2(b), it
could be easily found that the apparent density increased rapidly in
the temperature region of 1100ꢀCe1225 ꢀC, a saturated value of
apparent density was found to be nearly 4.87 g/cm3 at 1225 ꢀC and
the curve of diametric shrinkage ratio also showed a similar ten-
dency. So the optimal sintering temperature of MZN ceramic pre-
pared by solid-state method can be obtained at 1225 ꢀC for 4 h. By
comparison, it was concluded that MZN ceramics could be obtained
at lower sintering temperatures by the aqueous solegel process,
which was due to the fact that the specific surface energy and
driving force for sintering were increased by the decreasing particle
size and refining microstructure in solegel route.
Nb2O5 þ 10HF/2NbOF52ꢁ þ 4Hþ þ 3H2O
(1)
NbOF52ꢁ þ 5NH3$H2O þ H2O/5NH4þ þ NbðOHÞ5Y þ 2OHꢁ
þ 5Fꢁ
(2)
Phase analysis of samples was conducted with the help of a
Rigaku diffractometer (Model D/MAX-B, Rigaku Co., Japan) using Ni
filtered CuK
a
radiation (
l
¼ 0.1542 nm) at 40 kV and 40 mA set-
tings. Based on XRD analysis, the morphology and particle sizes
were examined using a transmission electron microscopy (Model
JEOL JEM-2010, FEI Co., Japan) coupled with energy dispersive X-ray
spectroscopy (EDS). A network analyzer (N5234A, Agilent Co.,
America) was used for the measurement of microwave dielectric
properties. Dielectric constants were measured using Hak-
kieColeman post-resonator method by exciting the TE011 resonant
mode of dielectric resonator by using an electric probe as suggested
by Hakki and Coleman [13]. Unloaded quality factors were
measured using TE01d mode by the cavity method [14]. All mea-
surements were made at room temperature and in the frequency of
8e12 GHz. Temperature coefficients of resonant frequency were
measured in the temperature range of 25ꢀCe85 ꢀC. The apparent
densities of the sintered pellets were measured use the Archimedes
method (Mettler ToledoXS64).
The X-ray diffraction patterns of MZN ceramics sintered at
different temperatures were illustrated in Fig. 3. It was found that
all the samples exhibited MgZrNb2O8 with the space group of P2/c
as a main crystalline phase, which was in agreement with the XRD
pattern of JCPDS No. 48e0329. And the X-ray diffraction patterns of
MZN ceramics by solid-state and solegel method were not changed
significantly in the whole temperature range, respectively. More-
over, a small amount of secondary phase was observed. For solegel
process, the secondary phase of MgNb2O6 was observed at 48ꢀe49ꢀ
in the whole range. And for solid-state method, the secondary
phase of Mg4Nb2O9 was observed at 25ꢀe26ꢀ in 1100 ꢀC, and it was
also observed at 53ꢀe54ꢀ and 63ꢀe64ꢀ in the whole temperature
range. In addition, a small peak of Nb2O5 was existed at 28ꢀe29ꢀ in
1100 ꢀC, which was due to the incomplete reaction at lower sin-
tering temperature.
3. Results and discussion
The X-ray diffraction patterns of MZN powders calcined at
900ꢀCe1100 ꢀC for 2 h are illustrated in Fig. 1(a). As shown in
Fig. 1(a), ZrO2 (JCPDS No. 83-0938) as secondary phase was
observed at 900 ꢀC, and the peak intensity of it decreased with the
calcining temperature increasing. It is obvious that powders were
not reacted completely at lower temperature. There was no sig-
nificant change in the X-ray diffraction patterns of MZN powders
with the calcining temperature increasing from 1000 ꢀC to 1100 ꢀC.
However, it was pleasantly found that the crystallization of
wolframite-structure MZN as predominant phase took place at
1050 ꢀC, which was in agreement with the XRD pattern of JCPDS No.
48-0329. The XRD patterns of MZN xerogel calcined at
SEM micrographs and EDS of samples prepared by solid-state
method from 1100 ꢀC to 1275 ꢀC for 4 h are illustrated in Fig. 4.
And the SEM micrographs and EDS for samples prepared by solegel
process from 1100 ꢀC to 1200 ꢀC are shown in Fig. 5. In both cases,
an increase in grain size and the decrease of pores were the

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J.D. Guo et al. / Journal of Alloys and Compounds 655 (2016) 60e65
Fig. 1. XRD patterns of MZN powders at different temperature ((a) MZN powders pre-sintered at 900ꢀCe1100 ꢀC prepared by solid-state method; (b) MgeZreNb xerogel calcined at
600ꢀCe900 ꢀC prepared by solegel process).
Fig. 2. Sintering behavior of MgZrNb2O8 ceramics as a function of sintering temperatures ((a) solegel process; (b) solid-state method).
Fig. 3. XRD patterns of MZN ceramics sintered at different temperatures ((a) solegel process; (b) solid-state method).
common feature with the sintering temperature increasing, which
was consistent with the results of the apparent density shown in
Fig. 2. In Fig. 4, as sintering temperature increased to 1200 ꢀC,
porosity decreased rapidly and all pores almost disappeared at

J.D. Guo et al. / Journal of Alloys and Compounds 655 (2016) 60e65
63
Fig. 4. SEM micrographs and EDS of MZN ceramics sintered at (a) 1100 ꢀC, (b)1200 ꢀC, (c) 1225 ꢀC, (d) 1250 ꢀC, (e) 1275 ꢀC for 4 h by solid-state method and (f) EDS analysis.
1225 ꢀC on the surface of MZN samples. The dense microstructure
with low porosity and well-formed spheric-shape grains (1e2 m)
quantitative result about elementary composition. In both cases,
the ratio of Mg/Zr/Nb/O was approximately corresponding to the
formula of MZN phase.
m
was obtained at 1225 ꢀC shown in Fig. 4(c). However, when the
sintering temperature was higher than 1225 ꢀC, abnormal grain
growth appeared shown in Fig. 4(d)e(e). Abnormal grain growth
could be found as shown in Fig. 4(d)e(e) when sintering temper-
ature was over 1225 ꢀC. Similarly, MZN samples were prepared
with full density through the solegel process at 1175 ꢀC for 4 h,
which was lower than that by solid-state reaction methods. And
average grain sizes were measured approximately 50 nm at 1175 ꢀC
as shown in Fig. 5(c). By comparison, average grain size for sample
prepared by solegel process was much smaller than that by the
conventional method. In addition, EDS analysis about grains chosen
randomly from the samples prepared by solid-state method and
solegel process were also shown in Figs. 4 and 5, which presented a
Fig. 6 demonstrates the changes of εr, Q$f and tf values
depending on sintering temperatures. As shown in Fig. 6(a), with
the sintering temperature increasing from 1100 ꢀC to 1200 ꢀC, the εr
values of MZN ceramics increased firstly in the temperature region
of 1100ꢀCe1150 ꢀC, and then decreased when the sintering tem-
perature was over 1150 ꢀC. The increase at first was due to the
densification of MZN ceramics, and the decrease in dielectric con-
stant may be ascribed to the abnormal grain growth appearing at
higher sintering temperature. And the value of εr in the optimal
sintering temperature (1175 ꢀC) is 22.16, which was lower than the
dielectric constant (~27.08) of MZN ceramic prepared by solid-state
method sintered at 1225 ꢀC shown in Fig. 6(b).

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J.D. Guo et al. / Journal of Alloys and Compounds 655 (2016) 60e65
Fig. 5. SEM micrographs and EDS of MZN ceramics sintered at (a) 1100 ꢀC, (b)1150 ꢀC, (c) 1175 ꢀC, (d) 1200 ꢀC for 4 h by solegel process and (e) EDS analysis.
With the increase of sintering temperatures from 1100 ꢀC to
reported that when the relative density was higher than 90%, the
Q$f value was no longer dependent on density and porosity. That is
to say, once the prepared samples reached nearly full density, Q$f
values were mainly affected by intrinsic factors. So the decrease in
Q$f values could be attributed to the non-uniform microstructure
caused by abnormal grain growth shown in Fig. 5(d).
In addition, the presence of a small amount of secondary phase
also had a certain influence on Q$f values. According to the results
of the XRD patterns of MZN ceramics prepared by solegel process
shown in Fig. 3(a), the secondary phase of MgNb2O6 whose mi-
crowave dielectric properties were εr ¼ 21.4, Q$f ¼ 93,800 GHz and
tf ¼ ꢁ70 ppm/ꢀC sintered at 1300 ꢀC [19] was observed in the whole
temperature range. Compared to the Q$f value (~29,070 GHz) of
MZN ceramic prepared by solid-state method sintered at 1225 ꢀC
shown in Fig. 6(b), the Q$f values of MZN ceramics could be
1175 ꢀC, Q$f values increased from 13,093 GHz to 29,486 GHz and
then decreased to 28,500 GHz at 1200 ꢀC, which was in accordance
with the tendency of apparent density shown in Fig. 2. Therefore,
the optimal Q$f value of 29,486 GHz was obtained when the MZN
compounds was sintered at 1175 ꢀC for 4 h. As we all know, the
microwave dielectric loss includes not only intrinsic losses mainly
caused by the lattice vibration modes but also extrinsic losses
dominated by densification/porosity, the secondary phases, grain
sizes and oxygen vacancies [16]. Among these factors, porosity was
suggested to affect Q$f values obviously below 1175 ꢀC. The
remarkable increase in Q$f values in the range of 1100ꢀCe1175 ꢀC
was associated with the decrease of porosity according to results of
the sintering characteristic curves and SEM microstructure shown
in Figs. 2 and 5(aec), respectively. Some investigations [17,18] also

J.D. Guo et al. / Journal of Alloys and Compounds 655 (2016) 60e65
65
Fig. 6. Curves of εr, Q$G and tG values as a function of sintering temperatures for MZN ceramics ((a) solegel process; (b) solid-state method).
improved slightly by solegel process. According to Schlomann's
theory, dielectric loss tangent at microwave frequency is mainly
determined by the anharmonic terms in the crystal's potential
energy [20,21]. Dielectric ceramics have many kinds of structural or
lattice defects such as grain boundaries, voids, dislocations, point
defects and substitutional ions. Rustum Roy reported that using the
solution-sol-gel route had been developed to make a variety of
ceramic materials, which provided major advantages in lowering
sintering temperatures, refining microstructure, and controlling
morphology and final phase composition [22]. The wolframite-
structure MZN precursors were provided with well crystalline
and less lattice defects for the fabrication of ceramics by the
solution-sol-gel route in this work, which should contribute to the
improvement of Q$f values.
obtained in air atmosphere for the formation of MZN nanopowders
with well crystallinity. Moreover, MZN ceramics with nearly full
densities were obtained at 1175 ꢀC and had excellent microwave
dielectric properties of εr
¼
22.16, Q$f
¼
29,490 GHz and
tf ¼ ꢁ15.99 ppm/ꢀC. Obviously, solegel process not only had major
advantages in lowering sintering temperatures, refining micro-
structure, and controlling morphology and final phase composition
but also improved the microwave dielectric properties of MZN
ceramics.
Acknowledgments
This work was supported by the Project development plan of
science and technology of ji'nan city (No.201303061), Ji'nan City
Youth Science and Technology Star Project (No.2013035), and Na-
tional Natural Science Foundation (No. 51472108) and Study Abroad
Programs by Shandong Province Government.
Moreover, the temperature coefficient of resonant frequency (tf
)
was related to the composition, the additives and the secondary
phase. The tf values of MZN ceramics prepared by solegel process
increased to ꢁ15.99 ppm/ꢀC firstly and then decreased in the region
of 1100ꢀCe1200 ꢀC. However, tf values of MZN ceramics prepared
by solid-state method fluctuated around ꢁ30 ppm/ꢀC with sinter-
ing temperatures increasing from 1100 ꢀC to 1275 ꢀC and tf values
were ranged from ꢁ9 ppm/ꢀC to ꢁ41 ppm/ꢀC. By comparison, the tf
value was improved to ꢁ15.99 ppm/ꢀC by solegel process. Hence, it
was obvious that the improvement in the tf value was required for
applications in electronics.
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4. Conclusions
MgZrNb2O8 ceramics with wolframite type were prepared by
solid-state method and solegel process in this study, and the in-
fluence of solegel process on sintering temperature, phase com-
positions, microstructures and microwave dielectric properties of
MZN ceramics were also investigated. For solid-state method, MZN
ceramics with nearly full densities were obtained at 1225 ꢀC with
microwave dielectric properties of εr ¼ 27.08, Q$f ¼ 29,070 GHz and
tf ¼ ꢁ24.31 ppm/ꢀC. And for solegel process, MZN powders with
particle sizes of approximately 50 nm were obtained successfully. A
considerable decrease in synthesis temperature (at 600 ꢀC) was
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