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Synthesis and Thermoluminescent Characterization of Ceramics Materials
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7
Synthesis and Characterizations of
Ba(Mg
1/3
Nb
2/3
)O
3
Powder
Wanwilai Vittayakorn and Rachanusorn Roongtao
Department of Physics and Materials Science,
Faculty of Science, Chiang Mai University
Thailand
1. Introduction
A microwave communication system is important applications in the communication
industry such as global communication satellites, cellular phones, radar detectors and
wireless communication of the demands are rapidly rising in the present. Microwave
dielectric ceramics were applied to the operation of filters and oscillators in several
microwave systems as a small ceramic component. The general formula Ba(B’
1/3
B’’
2/3
)O
3
(B’
= Mg, Zn, Ni or Co; B’’ = Ta or Nb) ceramics are one of microwave dielectric which have
attracted a great deal of attention and are currently being used for microwave devices due to
its very high quality factors. Barium magnesium niobate (Ba(Mg
1/3
Nb
2/3
)O
3
: BMN)
compound is one of candidate materials of relative cheap and exhibits high performance
microwave dielectric. Firstly, a high dielectric constant (ε
r
= 32) is needed, so that the
materials can be miniaturized (because the size of a dielectric resonator α 1/ε
r
1/2
). Secondly,
a high quality factor (Q = 5600) is very important for radio frequency system to keep a better
selectivity and noise reduction, and a small temperature coefficient of resonance frequency
(τ
ƒ
= 33 ppm/°C) is also required so that the microwave circuits remain stable (Dias et al.,
2001; Lin et al., 2006; Lian et al., 2004; Lian et al., 2005; Chen et al., 2006; Zhong-qing et al.,
2004; Tian et al., 2009). So far, there have been only a few studies on BMN system and most
of them prepared by chemical route due to its high purity and small particle size. However,
it’s expensive and complicated technique. Therefore, in this paper, BMN were prepared by
conventional mixed-oxide technique which is the most economical and very simply consists
of wet milling the individual oxides or other compounds that decompose to the oxides
during calcining (Haertling, 1999). The characterizations of the phase formation, particle
shape and particle size of all powders were investigated and experimental results are then
discussed.
2. Experimental
The BMN powder was prepared by the convention mixed-oxide method. The reagent
grades of BaCO
3
(Fluka, >98.5% purity), MgO and Nb
2
O
5
(Aldrich, >99% purity) were
used as raw materials in this system. The raw materials were weighed and mixed by
ball milling technique with alumina balls in ethanol for 24 h. The mixtures were then dried
into mixed powder. The powder processing was shown schematically in Fig. 1.
Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications
166
BaCO
3
MgO Nb
2
O
5
Ball-milling 24 h
Drying
Calcination
BMN
Fig. 1. Preparation route for the BMN powders.
The phase characterization process for all samples was examined using X-ray diffraction
analysis (XRD) at room temperature in order to monitor phase evolution obtains calcination
conditions that result in single phase BMN powder. The particle shape and particle size of
powders were also observed using scanning electron microscopy (SEM).
3. Results and discussion
The TG-DTA curves of the BMN powders prepared by mixed-oxide method are illustrated
in Fig. 2. In the temperature rang from room temperature to ~1250°C. The TG curve shows
two distinct weight losses. The first weight loss occurs at ~300°C and the second one
between 600-1000°C. The both small endothermic and exothermic peaks are observed in the
DTA curve which is related to the first weight loss (~1.0%). These DTA peaks can be
attributed to the decomposition of the organic species from the milling process
(Wongmaneerung et al., 2006; Ananta, 2004). After the first weight loss demonstrate a
much sharper fall in specimen weight with increasing temperature from ~600-1000°C.
This precursor also exhibits a significantly larger over all weight loss (~14.2%).
Corresponding to the second fall in specimen weight by increasing the temperature up to
~1000°C the solid-state reaction occurs the formation of some crystalline phase associated
with BMN. A moderate exothermic peak at 930°C could be related to the crystallization of
BMN phase as indicated in XRD patterns shown in Fig. 3. The broad exothermic
characteristic in the DTA curve is found at the temperature range of 1000-1250°C, which has
a maximum at ~1018°C. It has been considered as the process of further crystallization in
BMN phase (Lian et al., 2004). No further significant weight loss was observed for the
temperatures above 1000°C in the TG curve, indication that the minimum firing temperature
Synthesis and Characterizations of Ba(Mg
1/3
Nb
2/3
)O
3
Powder
167
to obtain BMN compound is in good agreement with XRD results (Fig. 3). These data were
used to define the range of calcination temperatures for XRD investigation.
Fig. 2. TG-DTA curves for the mixture of BMN powders.
The XRD patterns of the BMN powder calcined in the temperature range of 800-1400°C for 4
h in air are shown in Fig. 2. The uncalcined powder show only X-ray peaks of BaCO
3
, MgO
and Nb
2
O
5
precursors, which could be matched with JCPDS file no. 05-0378 (Powder
Diffraction, 2000) 71-1176 (Powder Diffraction, 2000) and 80-2493 (Powder Diffraction, 2000),
respectively. This result confirmed that no reaction had been initiated during the milling
process. After calcination at 800°C, the crystalline phase of Ba(Mg
1/3
Nb
2/3
)O
3
was developed
accompanying with BaCO
3
, MgO and Nb
2
O
5
as separated phases. This observation agrees
well with those derived from the TG-DTA results. As the temperature increased to 900°C,
the intensity of the BMN peaks was further enhanced. Whereas the traces of minor phases of
unreacted Nb
2
O
5
could not be completely eliminated at 1100°C. This could be attributed to
the poor reactivity of niobium species (Ananta et al., 1999). The peak of precursors gradually
disappeared with increasing calcination temperature and reached to single BMN phase after
calcination at 1200°C. This perovskite BMN powder can be matched exactly with JCPDS file
no. 17-0173 for the hexagonal phase, in space group P3m1 with cell parameters of a = 5.77
pm and c = 7.08 pm (Powder Diffraction, 2000).
The morphological evolution of the calcined BMN powders was also revealed by SEM and
showed Fig. 3. In general, the particles are agglomerated and basically irregular in shape
with a substantial variation in particle size. The smallest and the biggest particle size of
powders were estimated from SEM micrographs and listed in Table 1. From the results, it is
seen that average particle size increases with increasing calcination temperature of BMN
which can be attributed to the occurrence of hard agglomeration with strong inter-particle
bond within each aggregates resulting from firing process.
Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications
168
Fig. 3. XRD patterns of BMN powder calcined at various temperatures for 4 h with heating
rates of 5°/min.